[A. guy plint]_sedimentary_facies_analysis_(ias_sp(book_zz.org)

Sedimentary Facies Analysis: A Tribute to the Research and Teaching of Harold G. Reading Edited by A. Guy Plint
© 1995 The International Association of Sedimentologists ISBN: 978-0-865-42898-0
SEDIMENTARY FACIES ANALYSIS

SPECIAL P U B LI CATI O N N U M B E R 22 OF THE
INTE RNAT I O NA L ASS OCIA T I O N OF SED I M E N T O L O GISTS
Sedimentary Facies Analysis
A TRIBUTE TO THE RESEARCH AND TEACHING
OF HAROLD G. READING
EDITED BY A. GUY PLINT
bBlackwell
Science

© 1995 The International Association
of Sedimentologists
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Library of Congress
Cataloging-in-Publication Data
Sedimentary facies analysis: a tribute to
the research and teaching
of Harold G. Reading/
edited by A. Guy Plint.
p. em.
(Special publication number 22 of the
International Association of Sedimentologists)
Includes bibliographical references
and index.
ISBN 0-86542-898-0
1. Facies (Geology)
2. Sedimentation and deposition. I. Plint, A. Guy.
II. Reading, H.G. [[[. Series: Special publication
of the International Association of Sedimentologists;
no. 22.
QE651.S43 1995
552' .5-dc20 94-30445
CIP

To Harold
Photograph courtesy of Tim Barrett
We offer this collection of papers
as a token of our appreciation
for your friendship, guidance and inspiration.
In remembering your infectious enthusiasm, dedication
and sometimes daunting expectations,
we realize how deeply we were influenced
by your philosophy and attitude;
a gift that has, in no small measure, shaped the course
of our professional ]jves.
Your former students

Contents
IX Preface
XI Harold G. Reading
xm Introduction
Clastic Facies Analysis
3 Alluvial palaeogeography of the Guaritas depositional sequence of southern Brazil
Paulo S. G. Paim
17 Sedimentology of a transgressive, estuarine sand complex: the Lower Cretaceous
Woburn Sands (Lower Greensand), southern England
Howard D. Johnson and Bruce K. Levell
47 An incised valley in the Cardium Formation at Ricinus, Alberta: reinterpretation as an
estuary fill
Roger G. Walker
75 Gravelly shoreface and beachface deposits
Bruce S. Hart and A. Guy Plint
101 The return of 'The Fan That Never Was': Westphalian turbidite systems in the Variscan
Culm Basin: Bude Formation (southwest England)
Robert V. Burne
137 Depositional controls on iron formation associations in Canada
Philip Fralick and Timothy J. Barrett
157 Facies models in volcanic terrains: time's arrow versus time's cycle
Geoffrey J. Orton
Tectonics and Sedimentation
197 Coarse-grained lacustrine fan-delta deposits (Pororari Group) of the northwestern
South Island, New Zealand: evidence for Mid-Cretaceous rifting
Malcolm G. Laird
VII

vm Contents
219 Sedimentation and tectonics of a synrift succession: Upper Jurassic alluvial fans and
palaeokarst at the late Cimmerian unconformity, western Cameros Basin, northern
Spain
Nigel H. Platt
237 The use of geochemical data in determining the provenance and tectonic setting of
ancient sedimetary successions: the Kalvag Melange, western Norwegian Caledonides
Rodmar Ravnas and Harald Fumes
265 Differential subsidence and preservation potential of shallow-water Tertiary sequences,
northern Gulf Coast Basin, USA
Marc B. Edwards
Sequence and Seismic Stratigraphy in Facies Analysis
285 Seismic-stratigraphical analysis of large-scale ridge-trough sedimentary structures in
the Late Miocene to Early Pliocene of the central North Sea
Joe Cartwright
305 Millstone Grit cyclicity revisited, II: sequence stratigraphy and sedimentary responses
to changes of relative sea-level
Ole J. Martinsen, John D. Collinson and Brian K. Holdsworth
Facies Analysis in Reservoir Sedimentology
331 Productive Middle East clastic oil and gas reservoirs: their depositional settings and
origins of their hydrocarbons
Ziad R. Beydoun
355 The evolution of Oligo-Miocene fluvial sand-body geometries and the effect on hydro
carbon trapping: Widuri field, west Java Sea
Ray Young, W.E. Harmony and Thomas Budiyento
381 Index

Preface
This book stands out in the series of Special Publi
cations of the International Association of Sedimen
tologists. It is an acknowledgement of Harold
Reading's commitment to lAS, for whom he has
been Publications Secretary, General Secretary and
President successively, over the last 30 years.
Harold has not only been source and inspiration
of many of the lAS policies and activities over this
time, he has also been at the roots of 'facies sedimen
tology' as an art in itself, and as a major tool in the
broader field of geology.
More than providing his own personal contribution
to this branch of the earth sciences, Harold created a
flourishing school of teaching and research. Harold's
approach has burgeoned from Parks Road, Oxford,
to become international not only through his
students, but also through 'his book'.
The Bureau of the lAS, taking up a suggestion by
Robert Campbell of Blackwell Science, decided to
put together a scientific tribute to Harold. The
IX
Bureau considered that this would be best done
through a Special Publication on a subject in line
with Harold's work (obvious topics were clastics,
facies and depositional environments, sedimentation
and tectonics).
It is therefore most appropriate that Guy Plint,
as Editor chosen for this special publication, has
brought together a collection of original scientific
papers authored by Harold Reading's students, or
students of theirs. To honourHarold Reading's own
scientific scope, the subject chosen is broad: sedi
mentary facies analysis. The contributions contained
in this Special Publication show to what extent facies
sedimentology, as fostered by Harold Reading, is
now established as a necessary basis to any under
standing of sedimentary rocks.
PETER HOMEWOOD
/AS Publications Secretary

Harold G. Reading
Harold Reading was born in 1924 and, on leaving
school, joined the IndianArmy. This early experience
left a lasting impression and undoubtedly contributed
to Harold's later concern for international cooper
ation. He went up to Oxford in 1948, initially to read
Forestry, but his interests were diverted towards
geology and he graduated in that subject in 1951.
As an undergraduate, he visited North Norway to
investigate the Late Precambrian and Cambrian
stratigraphy of the Digermul Peninsula. This under
graduate expedition not only shed significant new
light on the stratigraphy of the area but also sowed
the seeds of a later rich sedimentological harvest.
Three years at Durham under K.C. Dunham led to a
PhD with a project that involved mapping Carbon
iferous Yoredale cycles across an area of bleak
Pennine moorland. Although the main thrust of the
study was stratigraphy and structure, the experience
of Carboniferous cyclicity was to set a further pointer
for the future.
On completion of his PhD Harold joined Royal
Dutch Shell and immediately found himself in the
contrasting field conditions of Venezuela. This multi
national, multidisciplinary environment developed
an appreciation of broader geological perspectives
and the pragmatic, though rigorous, approach to
problem solving that has characterized Harold's
career. Of particular significance was a visit to
Venezuela by Ph. Keunen who was, at that time,
actively promoting his pioneering work on turbidity
currents and their deposits. Kuenen's rigorous
approach to understanding depositional processes
struck a chord with Harold, which was to be a
cornerstone of his approach to sedimentology.
Harold returned to Oxford in 1957 as lecturer in
geology, a position that he held until retirement in
1991. When he took up his post, his teaching responsi
bilities included mapping and palaeontology and
stratigraphy. Sedimentology, as we know it, hardly
existed. Harold first revived his interests in northern
Norway through a further, largely stratigraphical
expedition to Digermul. Perhaps more importantly,
he developed his interest in sedimentary process and
environments through a relationship with Shell.
Maurits de Raaf, then Head of Geological Research
XI
at Rijswijk, arranged for Harold to investigate the
context of reported turbidites associated with English
Carboniferous deltaics in the Pennines and in south
west England.
Cooperation with de Raaf and with Roger Walker,
one of Harold's earliest research students, developed
a detailed appreciation of sedimentary structures
and their role in understanding processes, and led
to the development of the style of facies analysis
exemplified by the 1965 classic paper on the Carbon
iferous cycles of North Devon. Thereafter, Harold's
stable of research students grew rapidly as this volume
amply testifies. Until his retirement, it was unusual
for him to have fewer than five or six doctoral
students at any one time, this in addition to a full
undergraduate teaching programme and responsi
bilities in college. This formidable work load was
carried out with great conscientiousness but Harold
still had time to spare for external activities such
as his involvement with lAS and JAPEC. During
Harold's long career in Oxford, he only spent sus
tained periods away on sabbatical on two occasions,
the first in Leyden in the mid-1960s and the second
in Canada in 1972. The period in Holland led to
close cooperation with structural geologists working
in the Cantabrians, an important extension of his
interests outside Britain.
Harold's earliest students developed his early
interests, the Carboniferous deltas of Britain, and
the tillites, shallow-marine and fluvial sediments of
northern Norway. Later, the Lower Palaeozoic of
Ireland and the Carboniferous of northern Spain
were added. As students were attracted to Oxford
from around the world, the geographical spread
grew. However, geographical diversification was not
an end in itself but largely a result of Harold's
curiosity about wider controls on sedimentation,
particularly the role of tectonics. He understood
very early the implications for sedimentology of
Plate Tectonics, as exemplified by his pioneering
paper with Andrew Mitchell. Curiosity about new
geological ideas and the need to investigate their
implications for sedimentology and vice versa has
been a hallmark of Harold's geological thinking.
By some standards, Harold has not been a prolific

XII Harold G. Reading
author, although his papers are always thoughtful
and stimulating. Published evidence of Harold's
influence lies mainly in the rigour, originality and
appreciation of the wider geological perspective that
characterize many of the publications of his research
students and of second and third generation students.
Harold edited one Special Publication of the lAS
on-strike-slip mobile belts, but his most valued publi
cation is the textbook Sedimentary Environments and
Facies, initially written largely by Harold's former
students and rigorously edited to reflect the high
standards he espouses. The 3rd edition currently
occupies much of his 'retirement'.
Although this book is essentially a celebration
of Harold's scientific influence, it is important,
especially in a Special Publication of the lAS, to
acknowledge his enormous contribution to the
development of sedimentology internationally. His
unstinting efforts on behalf of the lAS, as Publi
cations Secretary, as General Secretary and as
President have already been acknowledged by the
Association itself in the granting of Honorary
Membership to Harold. It is worth remembering
that it was in no small measure due to Harold's
efforts that the Association changed from a largely
European organization to one of real international
stature. Harold's tireless efforts to meet and encour
age sedimentologists of all ages and backgrounds
around the world and his endless patience and
diplomatic skill have been well rewarded in the
healthy Association that we enjoy today. Harold
has additionally been honoured by the Geological
Society of London with the award of the Lyell Fund
and the Prestwich Medal and, most recently, by
SEPM with the award of its prestigious Twenhofel
Medal.
JOHN CoLLINSON
Shrewsbury, UK

Introduction
This volume is a very personal compilation. Unlike
previous lAS Special Publications, it is not centred
on a specific geological theme, and for that I make
no apology. Instead, my intent was to illustrate, and
celebrate, the breadth of interest, energy and inspi
ration that Harold Reading has brought to the field
of sedimentary geology.
Few would deny the depth of Harold's influence
on sedimentology, world-wide. In part, this is due to
his publications, in particular the enormously suc
cessful Sedimentary Environments and Facies,
unquestionably the cornerstone for all those who
embark on sedimentary facies analysis! Equally
important of course, has been his pivotal role in the
foundation and development of the lAS, a contri
bution acknowledged recently with honourary mem
bership of that Association.
His philosophy and attitude has of course travelled
with his graduate students, drawn from 13 countries
on six continents. Because many of these students
returned home upon completion of their work in
Oxford, and others now work and teach outside
the UK, the approach Harold fostered during their
graduate days has continued to spread. (He may not
know this, but in a geneological sense, Harold
is now a great-great grandfather to at least one
young sedimentology student who doubtless is quite
unaware of the history of the supervisory influence
that has been passed down!)
Although initially conceived as a thematic volume
with contributions to be invited from a panoply of
leading sedimentologists, two difficulties quickly
arose: first, just what was to be the theme? As
Harold has been involved in so many areas of
sedimentary geology, selection of any one topic
simply served to highlight gross neglect of another.
Secondly, it rapidly became apparent that numerous
former students were anxious to pay their own
personal tribute, and whose contributions could,
alone, easily constitute a hefty volume! Of the 34
students whom Harold guided through doctoral
theses between 1961 and 1994, 16 have authored, or
co-authored papers in this volume.
In keeping with the sentiment of this festschrift, I
took the decision to limit contributions to those
xiii
from Harold's former graduate students and their
students and co-workers, but to impose no constraint
on topic, in order to illustrate the scope of Harold's
knowledge, interest and vision. In consequence, the
contents of this book are eclectic. The collection
of papers serves to highlight the power of facies
analysis, whether the rocks be volcanogenic, bio
genic, siliciclastic, or even 'catastrophic' (mega
olistoliths!), and of course reflect the scientific
method fostered by Harold.
It is particularly appropriate that, amongst the
contributions, Ole Martinsen, John Collinson and
Brian Holdsworth offer new interpretations of
Namurian deltaic rocks in the northern Pennines,
(upon which Harold cut his sedimentological teeth),
but which, judging from referees comments, still
provide fuel for heated debate! In similar vein,
Bob Burne presents a review and discussion of the
depositional environment of the enigmatic Bude
Formation (which Harold studied in the early
1960s), but which is still subject to sharply divergent
interpretations. In a salutory lesson to us all,
Roger Walker shows how important it is, both to
separate facts from interpretations, and to ques
tion one's cherished interpretation, when he boldly
reinterprets as an incised valley fill, rocks he pro
claimed a turbidite channel deposit just nine years
ago!
As Editor of this volume, I am indebted to the
following people whose thorough reviews served
to clarify the papers, and who made my job that
much easier: Gail M. Ashley, Timothy R. Astin, T.
Christopher Baldwin, Janok P. Bhattacharya,
Charlie S. Bristow, H. Edward Clifton, Thomas C.
Connally, Edward Cotter, William R. Dupre,
Peter G. DeCelles, Frank G. Ethridge, Jill Eyers,
Stephen S. Flint, Edward C. Freshney, Robert L.
Gawthorpe, Roland Goldring, Anthony J. Hamblin,
Alan P. Heward, Phillip R. Hill, Richard N. Hiscott,
Richard S. Hyde, Elana L. Leithold, Peter J.
McCabe, Kathleen M. Marsaglia, Franco Massari,
Gerrard V. Middleton, Robert A. Morton, George
Postma, William C. Ross, Alastair H. Ruffell, Bruce
W. Sellwood, Gary A. Smith, Roger G. Walker,
James D.L. White, John A. Winchester and

XIV Introduction
Jonathon Wonham, plus two people who chose to
remain anonymous.
I am very grateful to Susan Sternberg, Edward
Wates and Julie Elliott at Blackwell Science who
provided guidance at critical phases in the prep
aration of this book. I also thank Diana Relton
(Earth Sciences, Oxford) who entered into the
clandestine spmt of this project, and provided
essential intelligence on both Harold and his former
graduate students.
A. GuY PuNT
London, Ontario

Clastic Facies Analysis

Spec. Pubis int. Ass. Sediment. (1995) 22, 3-16
Alluvial palaeogeography of the Guaritas depositional sequence
of southern Brazil
PAULO S. G. P AlM*
Earth Sciences Department, Oxford University, Parks Road,
Oxford OXI 3PR, UK
ABSTRACT
The Guaritas sequence is the uppermost stratigraphical level of the Camaqua Basin (southern Brazil)
and comprises an alluvial, deltaic and aeolian continental facies association up to 800 m thick. Facies
mapping of this unit has revealed a lateral association of tributary fans and trunk braided rivers
developed under semi-arid conditions.
Two main regions (lobes) of alluvial fan development can be discriminated and the source points of
both coincide with synforms in the nearby basement. This depositional system presents a normal down
fan facies change. An anomalous lateral change of facies within the trunk river system is interpreted as
having been inherited from pre-existing alluvial fan deposits.
The main alluvial facies comprise trough cross-stratified (74%) and horizontally bedded (7%)
sandstones, massive (16%) and tabular cross-stratified (2%) orthoconglomerates, and massive mud
stones (1%) .
Vertical aggradation of three-dimensional subaqueous dunes, followed by an upper flow regime
plane-bed phase, characterized the depositional events of the sandy areas of the alluvial system. Diffuse
gravel sheets and minor longitudinal and transverse bars were the main geomorphological features of
the gravelly alluvial reaches. Fine-grained sediments represent temporarily abandoned areas within the
braided channel network.
INTRODUCTION
The Guaritas depositional sequence constitutes the
uppermost unit of the Camaqua Basin infilling and it
is an unconformity-bounded stratigraphical unit: it
overlies older deformed molasse strata (angular
unconformity) and is covered by Permian sedimen
tary rocks of the Parana Basin.
The Guaritas sequence, about 800 m thick, is
almost always flat-lying, although, near to regional
faults some extensional reactivation has tilted the
Guaritas deposits. The available radiometric dating,
summarized in Soliani et al. (1984) and Fragoso
Cesar et al. (1984), indicates a Cambro-Ordovician
age for the deposition of the Guaritas sequence.
The Camaqua Basin is located in a NE-SW
* Permanent address: UNISINOS- Departamento de
Geologia, Av. Unisinos 950, Sao Leopoldo RS, Caixa
Postal 275, CEP 93022-000, Brazil.
3
trending tectonic structure, in southern Brazil (Fig.
1}, and evolved during the latest phases of the
Brasiliano orogenic cycle (strike-slip basins of Brito
Neves & Cordani (1991}}.
An extensional or transtensional event at the end
of the Brasiliano orogenic cycle, and the consequent
formation of intermontane basins, has been pro
posed as the tectonic setting of the Camaqua Basin
during the deposition of the Guaritas sequence
(Fragoso-Cesar et al., 1984, 1992; Beckel, 1990,
1992).
In the past decade, the Guaritas depositional
sequence has received attention from several authors
in terms of facies analysis and palaeoenvironmental
interpretation (Becker & Fernandes, 1982; Fragoso
Cesar et al., 1984; Jost, 1984; Lavina et a!., 1985;
Beckel, 1990). Generally, these papers have indi
cated continental sedimentation characterized by

4
BRAZIL
Study area
Br153
.....--_ Main roads
Major faults
Ca9apava do Sui
2 Santana da Boa Vista
P.S.G. Paim
Permo Triassic
�
L:...:!l
T Upper Vendian to Ordovician
� t-':(:�:::.,Guaritas depositional sequence
>-
.. . . •
� >:<: older molasse sequences
c::
-� Middle to Upper Proterozoic
� granites
·;;; �1 :f:f meta volcanic/sedimentary rocks
Archean to Lower Proterozoic
�
B.:::J
N
1
Scale (Km)
- - - -
6 3 0 6 12 18
Fig. I. Location map and geological setting of the Camaqua Basin. Modified from DNPM/CPRM (1987).
alluvial fan and braided alluvial plain deposits, with
associated aeolian and lacustrine sediments. A semi
arid environment has been proposed for the overall
Guaritas sequence.
The most detailed study on the depositional sys-
terns of the Camaqua Basin was presented by Lavina
et al. (1985). In this paper the alluvial facies were
related to marginal alluvial fans (channel and debris
flow deposits) associated with an axial braided
alluvial plain. Gravelly longitudinal bars and sandy

Palaeogeography of the Guaritas sequence 5
subaqueous dunes and transverse bars were the main
morphological elements attributed to the alluvial
palaeostreams.
Petrological studies by De Ros et al. (1994)
on samples from alluvial and aeolian facies of the
Guaritas sequence indicate the presence of: (i) fresh
feldspar and volcanic lithoclasts; (ii) aggregates of
hematite; (iii) oxidized grains; (iv) caliche (concen
tric interlayering of calcite and iron oxide); and (v)
silcretes. These early diagenetic features reflect arid
to semi-arid conditions during the deposition of the
Guaritas sequence.
Basin-wide facies mapping of the Guaritas se
quence carried out by the author in 1988, reinforce
previous interpretations indicating intermittent vol
canic activity and an aeolian, alluvial and deltaic
facies association (Fig. 2).
Basin-scale changes of the alluvial facies charac
teristics suggest that an objective delineation of dis
tinct alluvial subenvironments is possible. These
alluvial subenvironments, as well as a brief descrip
tion and interpretation of the main alluvial facies,
are the main subject of this paper.
The alluvial deposits will be discussed in terms of
their general features of texture, fabric, sedimentary
structures and palaeocurrent pattern on a basin
wide scale. Both the mean sedimentary facies charac
teristics and the lateral facies changes within the
alluvial system are described.
The data base includes 403 outcrop descriptions
distributed over an area of nearly 1600 km2 (see Fig.
SA). This area was subdivided into 46 equal rec
tangles (8 x 9 km) and mean values of several par
ameters were calculated for each subdivision. The
results of this approach are presented in Tables 1 &
2 and summarized in Figs 5 & 6. This approach
involves comparison of values from different strati
graphical levels. The consistent results (see Fig. 5)
throughout the basin, with sampling at several
stratigraphical levels (Fig. 2), suggest that the
palaeoenvironments were more or less stationary
throughout deposition of the Guaritas sequence.
A detailed three-dimensional facies architecture
analysis (architectural elements approach of Allen
(1983) and Miall (1985)), aiming to build up a local
alluvial model on a channel-fill scale, is part of my
ongoing studies and will be the subject of another
publication.
ALLUVIAL FACIES:
GENERAL FEATURES
To simplify terminology the lithofacies classifi
cation proposed by Miall (1977), as modified by
Miall (1978), Rust (1978) and Bromley (1991), was
adopted. Table 1 presents the main characteristics of
each sedimentary facies described in the field.
The terminology and classification scheme pro
posed by the SEPM (Society of Economic Paleon
tologists and Mineralogists) Bedforms and Bedding
Structures Research Symposium (Ashley, 1990) for
description of large-scale flow-transverse bedforms
(excluding antidunes) was adopted.
The alluvial deposits (Table 1) are sand dominated
(facies S, 81%) with a smaller amount of conglom
erates (facies G, 18%) and an insignificant amount
of pelites (facies F, 1%). Facies S is composed
mainly of medium- to coarse-grained sandstones
(41% ), with a significant proportion of pebbly to
very coarse-grained (25%) and fine- to very fine
grained (15%) sandstones.
Facies G is composed of pebbles (9%) and
granules (8%) and minor amounts of cobbles (1%).
A few boulders occur in the base of some conglom
erate beds, mainly near the eastern border of the
Camaqua Basin.
The alluvial deposits are usually arranged in fining
upward cycles bounded by fifth-order surfaces (sensu
Miall, 1988). These cycles are 0.5-4m thick and
tens of metres in lateral extent (Fig. 3), both parallel
to and perpendicular to palaeoflow, and can be
classified as laterally extensive to sheet-like deposits
following the classification of Friend et al. (1979).
The proportion of the different textural classes
within the fining upward cycles changes laterally
with increasing gravel content toward both margins.
Conglomerates (G)
Clast-supported conglomerates comprise around
18% of the alluvial facies and massive conglomerates
are the most common (Table 1). Clast-supported
conglomerates are a very common facies in the
lowermost parts of the fining upward cycles.
Massive clast-supported conglomerates (facies
Gm, Table 1) are the main lithotype of facies G and
normal grading, clast orientation and imbrication
are their most conspicuous sedimentary features.
Facies Gp is characterized by gravels (mainly
pebbles) arranged in small- to large-scale, normally
isolated, sets of tabular cross-stratification. This

6
B
P.S.G. Paim
A
10km
;·>.1 Mainly alluvial facies Q Mainly eolian facies E:f=3-g Mainly deltaic facies
� o Pre-Guaritas
t:;;:.t1 Mainly volcanic rocks
basement
LA:j Permo Triassic
Fig. 2. Three-dimensional view of Camaqua Basin and surrounding area (same region of Fig. 1): (A) topography and (B)
sketch of the Guaritas sequence facies.

Table 1. Classification and relative percentage of the sedimentary lithofacies (lithofacies code adapted from Miall (1977,
1978) and Rust (1978))
Rock type Facies code
Conglomerates (G) Gm
Gp
Gt
Gms
Sandstones (S) St
Sh
Sp
Mudstones (F) Fm
Fl
Description
Massive or crudely bedded conglomerates (cobbles,
pebbles and granules)
Small- to large-scale tabular cross-stratified
conglomerates (granules and pebbles)
Small- to large-scale trough cross-stratified
conglomerates
Massive, matrix-supported conglomerates (boulders
to granules dispersed in a muddy sand matrix)
Small- to large-scale trough cross-stratified
sandstones (pebbly to very fine-grained)
Horizontally bedded sandstones (medium to very
fine-grained)
Medium to pebbly sandstone with small- to large
scale planar cross-stratification
Massive mudstones with mudcracks
Laminated to rippled very fine sandstone to siltstone
Percentage
l6
2
74
7
Table 2. Relative percentage of trough cross-stratification also rare and occur, locally, near the eastern border
of the Camaqua Basin. The main characteristic of
this facies is its chaotic arrangement of pebbles,
cobbles and, less commonly, boulders floating in a
muddy to sandy matrix.
and horizontal lamination in each sandy textural class
Sedimentary
Texture Facies structures Percentage
Pebbly to very St Small scale 10
coarse grained Medium scale 49
Large scale 41
Sh 0
Coarse to St Small scale 10
medium grained Medium scale 43
Large scale 37
Sh 10
Fine to very fine St Small scale 15
grained Medium scale 37
Large scale 28
Sh 20
facies commonly occurs associated with facies Gm
(Fig. 3).
Facies Gt is rare, finer grained than facies Gm and
Gp and characterized by small- to large-scale trough
cross-stratification (alternations of small pebbles and
gravelly sands). This facies interfingers with facies
Gm and grades into facies St (Fig. 4).
Matrix-supported conglomerates (facies Gms) are
Sandstones
Trough cross-stratification (facies St, 74% ), in places
disrupted and/or deformed by convolution, and
horizontal bedding (facies Sh, 7%) are the main
features of the alluvial sandy deposits (Figs 3 & 4).
Planar cross-stratification (facies Sp) is rare.
Facies St is characterized by very fine- to very
coarse-grained sandstones with trough cross-bedding
(Table 1). The cross-strata are predominantly of
medium to large scale in all textural classes, but the
proportion of small-scale trough cross-stratification
increases as sandstones become finer grained (Table
2). This facies is the most common in the fining
upward cycles.
Convolute bedding is common in trough cross
stratified sandstones (facies St). Within a single
cross-stratified set, all gradations may occur from
oversteep foresets, recumbent folding to intense
deformation and even complete destruction of the
former bedding (facies Sm and Spo of Bromley,
1991). Deformation near the top of the cross-

8 P.S.G. Paim
stratified set is commonly characterized by downcur
rent oversteepening of the cross-strata (Figs 3 & 4),
and the intensity of convolution increases down the
slip-face.
Horizontal bedding (facies Sh) does not occur
associated with pebbly and very coarse-grained sand
stones and comprises 10% of the sedimentary struc
tures of medium- to coarse-grained sandstones and
20% of the fine- to very fine-grained sandstones
(Table 2). This facies is often related to the upper
most parts of the fining upward alluvial cycles (Figs 3
& 4).
Planar-tabular cross-stratified sandstones (facies
Sp) are not common in the Guaritas sequence alluv
ial deposits (Table 1). They occur as small- to large
scale sets in pebbly to medium-grained sandstones
and are normally interlayered with facies St.
Other facies
Massive mudstones are rare and commonly mud
cracks are their most conspicuous feature (facies
Fm). Very fine-grained sandstones and siltstones
(facies Fl) are also, and can be either horizontal
(Fig. 3) or, more rarely, cross-laminated (Table 1).
Both usually occur in the uppermost parts of the
fining upward alluvial cycles.
Alluvial facies: summary of general features
and interpretations
The textural aspects (Table 1) suggest that the alluv
ial facies of the Guaritas sequence represent bedload
stream deposits in which the bedload was predomi-
Fig. 3. Main alluvial lithofacies:
facies Gt,St, Sh, Spo and, in the
uppermost part of the picture,Fl,
Gm and Gp. Bar scale is 2 m long.
nantly sandy and the suspension load, if deposited,
was almost completely eroded by subsequent flood
events. This type of stream commonly has a braided
pattern characterized by low sinuosity and highly
mobile channels (Collinson, 1986). The sheet-like
geometry of the fining upward cycles enclosed by
fifth-order bounding surfaces suggests broad, shallow
channels.
In terms of the gravelly facies, the dominance of
clast-supported conglomerates (Table 1) is indicative
of gravel deposition by strong tractive flows, whereas
the finer grained material (sand and mud) was still
being carried in suspension (Rust & Koster, 1984).
Thin beds of facies Gm associated with laterally
extensive channels suggest the development of dif
fuse gravel sheets (Hein & Walker, 1977) by very
extensive and shallow sheet-floods (Collinson, 1986).
Thicker deposits of facies Gm suggest deeper and
less ephemeral flows (Rust, 1978) causing more
extensive vertical aggradation of gravel bars with
low depositional dips. These deposits commonly
have been associated with the development of longi
tudinal and/or diagonal gravelly bars (Smith, 1970;
Rust, 1972, 1978; Miall, 1977, 1978; Rust & Koster,
1984; Collinson, 1986) under high water and sedi
ment discharge (Hein & Walker, 1977).
Conglomerates with planar cross-stratification
(facies Gp) has been related to (i) two-dimensional
dune migration (transverse and/or linguoid gravel
bars of Hein & Walker (1977), Miall (1977) and
Middleton & Trujillo (1984)) as well as to (ii) later
modifications of longitudinal bars (Smith, 1970;
Rust, 1978; Enyon & Walker, 1974) in modern
alluvial gravelly reaches. The frequent occurrence of

Fig. 4. Detailed view of Fig. 3 (enlargement of its lower part): facies Gt, St, Sh, Spo and thin tabular beds of Gm. Bar scale is 2 m long.
;;,o
!:)
�
�
-§
�
-.:;,
�
"'
C)
§
;::.
s
"'
"'
.E
"'
"'
;:s
'"'
"'
'.0

10 P.S.G. Paim
isolated sets of facies Gp within deposits of facies
Gm could be explained more easily by the second
hypothesis.
Trough cross-stratified conglomerates are rare
(facies Gt) and have been associated with (i) three
dimensional dune migration, as observed by
Fahnestock & Bradley (1973) and Galay & Neill
(1967), and (ii) channel scour-and-fill structures
(Miall, 1977; Middleton & Trujillo, 1984). The same
criteria previously used to interpret facies Gp can
also be applied in this case: the solitary nature of this
facies suggests the deposition of gravel in depressions
around diffuse gravel sheets.
Matrix-supported conglomerates (facies Gms) are
also rare and represent mud- and debris-flow deposits
commonly associated with an alluvial fan setting
(Blackwelder, 1928; Bull, 1963; Hooke, 1967;
Rust & Koster, 1984; Collinson, 1986; Blair &
MacPherson, 1992).
Sandy sediments constitute the majority of the
Guaritas alluvial deposits (Table 1) and are exten
sively dominated by facies St (Table 2). Trough
cross-stratified sandstones have been related almost
invariably to migration of three-dimensional dunes
(e.g. Collinson, 1970; Williams, 1971; Harms et al.,
1975; Miall, 1977; Rust, 1978). In braided alluvial
settings these bedforms usually have been associated
with in-channel deposition (Cant & Walker, 1976,
1978; Cant, 1978; Walker & Cant, 1984). Such
repetitive sand deposits commonly are considered as
flood-stage bedforms (Williams, 1971) and are larger
in deep channels (Cant, 1978).
The association of facies St with the lower and
middle part of sheet-like fining-upward cycles
suggests this facies could be related to flood stage in
shallow channels. Subcritical climbing trough cross
strata (facies St) indicate subaqueous dune aggra
dation. Sporadic lateral accretion of these bedforms
is indicated by inclined planes (first-order bounding
surfaces of Miall (1988)) dipping perpendicular to
the dune migration direction (Paim, 1994).
The absence of third-order surfaces (except the
rare occurrence of lateral accretion surfaces) associ
ated with the subcritical climbing of the trough
cross-bedded sets (facies St) suggests rapid depo
sition of a sandy load, transported by traction plus
suspension, without macroform (sensu Jackson,
1975) development.
Deformation of trough cross-stratified sandstones
is a very conspicuous feature of the Guaritas sandy
alluvial facies. Recumbent folding in cross-bedded
sandstones commonly has been attributed to shear
stress acting on a liquefied sand bed and caused by
current drag (Allen & Banks, 1972; Doe & Dott,
1980; Owen, 1987) or by the movement of large
bedforms over an unconsolidated substrate during
high-flow stages (Plint, 1983).
Horizontal bedding (facies Sh) occurs most often
in the finest fraction of the sandy deposits (Table 2).
This textural control, associated with the occurrence
of parting lineation and scattered small pebbles and
granules near the base of the horizontally bedded
sets, indicates its origin as an upper flow regime
bedform.
Deposits with the same characteristics of facies Sh
usually have been linked to an upper flow regime
phase developed during flood stages on the channel
floor (McKee et al., 1967; Williams, 1971; Miall,
1977) or under the influence of high-velocity and low
depth flows on the top of sand-flats (Cant and
Walker, 1978; Miall, 1977; Collinson, 1986). The
common occurrence of this facies (Sh) on the upper
most parts of the fining upward cycles supports an
interpretation involving upper flow regime currents
reworking the top of the previous alluvial deposits.
Planar-tabular cross-stratified sandstones (facies
Sp) are rare. Within alluvial settings this facies
commonly has been related to slip-face advance of
two-dimensional dunes (transverse -linguoid or
lobate bars of Collinson (1970, 1986), Smith
(1970), Williams (1971), Asquith & Cramer (1975),
Miall (1977), Cant & Walker (1978) and Cant (1978);
or sand waves and straight-crested megaripples
of Smith (1970), Collinson (1986) and Miall
(1978)). Smith (1970) related the origin of the two
dimensional dunes to the development of 'deltas' in
pre-existing channel-floor depressions, whereas Cant
& Walker (1978) related them to flow expansion
at channel junctions or places where the channels
widen.
Facies Fm and Fl are not common in the alluvial
system of the Guaritas sequence (Table 1). Their
rarity and generally lenticular geometry (Fig. 3) are
suggestive of waning flood deposits settling on to
temporarily abandoned areas of the braided system
(Cant, 1978; Cant & Walker, 1978; Miall, 1978).
In general, diffuse gravel sheets and longitudinal/
diagonal bars were the main geomorphological
elements of the gravelly reaches, whereas sub
aqueous three-dimensional dunes characterized the
sandy portions of the Guaritas alluvial system.
The predominance of vertical aggradation of dunes
instead of downstream and/or lateral accretion of
more stable sandy accumulations (e.g. sand-flats)

suggests a highly variable hydrological character and
predominance of the upper part of lower flow regime
conditions within the channels.
Debris-flow and sheet-flood deposits suggest the
presence of alluvial fans within the alluvial system as
well as flashy discharge due to sporadic, but torren
tial, rainy seasons.
The above interpretations together suggest an
alluvial drainage developed under semi-arid con
ditions (large discharge fluctuations) with alter
nation of flood events and dry seasons. These
conclusions are reinforced by the aeolian associ
ation (Lavina et al., 198S) and by petrographical
evidence related to early diagenetic processes
(De Ros et a/., 1994).
ALLUVIAL FACIES:
LATERAL CHANGES
The previous section describes the pattern of alluvial
sedimentation in terms of mean regional values and,
in this way, reflects the major features of the alluvial
deposit. In the following section, spatial variation in
some sedimentary features is described and, when
possible, interpreted. To achieve this, the mean
values, per unit area, of several sedimentary par
ameters were calculated using the outcrop locations
and grid presented in Fig. SA.
Palaeocurrent pattern
The pattern of sediment transport within the entire
Camaqua Basin was calculated using the grid and
outcrops shown in Fig. SA.
In order to eliminate problems associated with the
analysis of several types and scales of sedimentary
features (Miall, 1977) mean vectors were calculated
only from trough cross-stratification. By using only
one rank of sedimentary features, difficulties related
to vector magnitude were eliminated (Allen, 1963;
Miall, 1974). In addition, dunes seem to be associ
ated with high-stage flow and, consequently, should
be good indicators of the true downstream direction
(Miall, 1977).
The distribution of the palaeocurrent vector means
(Fig. SB) indicates two major dispersal compart
ments within the alluvial system:
1 from the eastern border to the basin axis the
sedimentary transport was almost perpendicular to
the regional tectonic trend (a general mean vector of
282°, with a correlation coefficient of 0.86), reflecting
a sedimentary input towards the basin axis;
2 from the basin axis to the western border, palaeo
currents were predominantly parallel to the struc
tural trend (general mean vector of 211o, with a
correlation coefficient of 0.96).
Pattern of textural dispersion
The alluvial deposits of the Guaritas sequence are
composed primarily of sandstones (mainly facies St
and Sh), minor conglomerates (mainly facies Gm
and Gp) and trace amounts of fine-grained sediments
(facies Fm and Fl), as has been described in the
previous section. In this paper three types of alluvial
deposits are distinguished: sandy (:2: 70% sand
stones); mixed (70-30% sandstone); and conglom
eratic (::::: 30% sandstone).
Figure SC shows the percentage of sandstone (rela
tive to conglomerate) through the entire basin and
illustrates a gradual decrease from sand dominated
alluvial deposits along the basin axis (axial alluvial
sedimentation), to mixed alluvial deposits toward
both basin margins (marginal alluvial sedimen
tation). Likewise, Fig. SD presents a plan view of
the spatial changes of the percentages of coarser
grained sediments (conglomerates plus pebbly and
very coarse-grained sandstones) relative to finer
grained sediments (coarse to very fine sandstones).
A pattern quite similar to the former (Fig. SC) can
be seen. Clearly, the facies St and Sh are gradually
replaced by facies Gm towards both basin borders.
In both cases (Figs SC & SD) the only exception
to the general pattern of sediment distribution is a
NW-SE trending intrusion of coarse material in the
southeast region of the basin.
Alluvial facies: interpretation of lateral changes
Figure 6 presents an interpretation of the alluvial
palaeogeography of the Guaritas sequence based on
the lateral variations of the textural and palaeocur
rent data. This figure was constructed according to
the following considerations.
The palaeocurrents suggest the coexistence of
two distinct alluvial subenvironments (Fig. SB) with
almost orthogonal mean sedimentary transport pat
terns (282° versus 211°).
1 The first dispersal system (282°), developed in the
eastern part of the basin, is characterized by the
highestpalaeocurrent vector dispersion and by palaeo
flow almost perpendicular to the tectonic trend of

12 P. S.G. Paim
B 12 - Mean vector and number of readings per area
A
c
§>90
80-89
70-79
50 �
N
Boundary between tributary alluvial fan system
and trunk braided river system
Mean vector and number of readings of both alluvial
systems

3 46
,514 /
44ti 99
;; I
26 101
/
--
! 64
/ 32
D
60
41
100
""'-89 14
/
3 ""'-
.......... 26
5
---
-36 1
"4
</:: 800
H 30-39
0 ��29
Fig. 5. Lateral changes within the alluvial system: (A) grid and location of alluvial outcrops used to calculate palaeocurrent
and textural mean values; (B) palaeocurrent mean values per area; (C) percentage of sandstone relative to conglomerate;
and (D) percentage of coarser grained sediments (gravel plus pebbly to very coarse-grained sand) relative to finer grained
sediments (coarse to very fine-grained sand plus mud).
the basin. This subenvironment is interpreted as a
tributary alluvial fan (sensu Rust & Koster, 1984).
2 The second dispersal system (211°), represented
by palaeoflow parallel to the basin axis, by low
palaeocurrent vector disperson, and characteristic of
the western portion of the basin, is interpreted as a
trunk braided river (sensu Rust & Koster, 1984).
As Collinson (1986) stated, 'it is sometimes poss
ible to identify individual fans by the establishment
of a radial pattern of palaeocurrents over an area'.

c
1 -
2 -
Gm
Gt
S t
S h
Spo
Sm
Mudstones
Sandstones
3 - Conglomerates
Palaeogeography of the Guaritas sequence
n
o
-
Sandy alluvial deposiiS
Mixed alluvial deposiiS
Tributary fan streams
Braided trunk river streams
Reworked tributary fans
E
--- Boundary between trunk rivers and tributary fans
A Plan view of the alluvial palaeogeography
B Western margin trunk river facies association
(reworked alluvial fans)
C Axial sandy trunk river facies associalion
D Eastern margin proximal alluvial fan facies
association
E Summary of the alluvial facies
13
Fig. 6. Alluvial palaeogeography and lateral facies changes: (A) plan view of the alluvial subenvironments; (B) and (D)
vertical profiles of marginal facies association; (C) vertical profile of axial facies association; and (E) ideal vertical
arrangement of the main facies. Facies code from Miall (1978); Bromley (1991).
Here, the tributary fan mean vectors (Fig. 5B)
indicate the coalescence of two main fan lobes (Fig.
6A). The northern fan has a radius of 15 km, whereas
the southern fan has a radius of 20 km. These dimen
sions are comparable to the size of recent examples
of semi-ariel and arid alluvial fans documented by
Heward (1978). The point source of both lobes
coincides with structural lows (synforms composed
of easily erodible metapelites) in the nearby base
ment (Fig. 2). The southern lobe was more important
than the others in terms of sedimentary input, as can
be deduced both from it having penetrated furthest
into the basin (Fig. 5B) and from the major intrusion
of coarsest material from southeast to northwest
(Figs 5C & 5D) in the southeast region of the basin.
Comparison of the grain size distribution (Figs 5C

14 P.S.G. Paim
& 50) and palaeocurrent mean vectors (Fig. 5B)
reveals some obvious relationships as well as some
discrepancies.
The gradual decrease of grain size, from both
basin margins towards the basin axis (Figs 5C & 50)
can be interpreted as a consequence of lateral alluv
ial fan input. This matches the palaeoflow data of
the eastern side of the basin, but not that on the
western side (Fig. 5B).
The textural, palaeocurrent and facies data sum
marized in Fig. 6A demonstrate that the grain-size
distribution within the trunk braided river system
does not show a downcurrent fining, which is a very
common characteristic of many braided alluvial
environments (e.g. Smith, 1970; Miall, 1977, 1978;
Rust, 1978; Collinson, 1986). Instead, the trunk
rivers present a lateral change from sand-dominated
deposits near the basin axis to mixed deposits
towards the westernbasin border. Such a discrepancy
can be related to a dominant alluvial input from the
eastern border (tributary alluvial fans) causing the
development of a trunk braided river system on the
western side of the Camaqua Basin. The emplace
ment of trunk rivers in the western region could
cause a major remoulding of the alluvial fan deposits
on the western border without erasing the down-fan
fining. Some of these previous alluvial fan deposits
could be preserved and thus could explain some
atypical palaeocurrent readings made near the west
ern margin, which point to a southeasterly directed
sediment discharge.
Transitions between purely sandy or gravelly
reaches were the norm inside the alluvial system
of the Guaritas sequence. The facies are usually
arranged as fining upward cycles, bounded by fifth
order bounding surfaces, with the major facies super
imposed in the following order: Gm-Gp-St-Sh
Fm.
This characteristic vertical arrangement, reflecting
the proportion of each facies (Table 1), is illustrated
in a summary (idealized) vertical profile (Fig. 6E)
incorporating the mean values of the principal facies
observed throughout the basin. Comparison of
the summary profile (Fig. 6E) with actual sections
around the basin, and with the regional textural
variation (Figs 5C & 50), facilitates the identification
of some common marginal and axial facies associ
ations, summarized in Figure 6: profiles B and D
typify common marginal facies associations, whereas
profile C represents the axial part of the basin.
CONCLUSIONS
The alluvial deposits of the Guaritas sequence reflect
a lateral association of tributary alluvial fans and
trunk braided rivers. The alluvial fans show a down
stream decrease in mean grain size whereas the
trunk rivers present no longitudinal variation in
texture. Instead, the trunk rivers show a lateral
grain-size change that is interpreted to have been
inherited from a hypothetical alluvial fan system fed
from the western margin of the basin.
The alluvial fans are dominated by water
laid deposits and comprise two main lobes. The
source points of both alluvial fan lobes coincide with
structural lows, suggesting control by basement
topography.
Semi-arid conditions during the alluvial depo
sition are suggested by: sheet-flow and debris-flow
deposits; petrological evidence, such as fresh feld
spar and volcanic lithoclasts, interstitial hematite
aggregates, caliche and silcretes; and the association
with aeolian facies (although aeolian facies are com
mon in early Palaeozoic sequences because of the
absence of land vegetation regardless of climate).
Diffuse gravel sheets and longitudinal bars were
the main geomorphological elements in the alluvial
gravelly reaches. Subaqueous dune aggradation, fol
lowed by partial reworking of the deposit by upper
flow regime currents, characterized the sandy
reaches.
An idealized channel-fill succession is typified,
from base to top, by: (i) a horizontal to slightly
undulatory erosional surface; (ii) gravel deposits,
representing diffuse gravel sheets and longitudinal
bars (Gm), locally with avalanche faces (Gp); and
(iii) sandy deposits, consisting mainly of three
dimensional subaqueous dunes (St), rare two
dimensional dunes (Sp), and plane beds (Sh), on the
top.
ACKNOWLEDGEMENTS
Thjs study was carried out during the tenure of a
postgraduate scholarship awarded by the Research
Council of the Brazilian Government (CNPq), and
forms part of the author's D.Phil. thesis at the
University of Oxford, England, written under
the supervision of Dr H.G. Reading. Field-work
costs were supported by CNPq (Grant 413321/
88-6), Universidade do Vale do Rio dos Sinos
(UNISINOS), and Company of Research of Mineral

Resources (CPRM) of the Brazilian Government.
The author wishes to thank H.G. Reading and H.C.
Jenkyns for criticism and revision of an earlier version
of the manuscript. Later reviews by G. Plint, G.V.
Middleton, A.P. Hamblin and P.A. Allen have
enabled me to make several very useful improve
ments to the paper.
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17,1-40.

Spec. Pubis int. Ass. Sediment. (1995) 22, 17-46
Sedimentology of a transgressive, estuarine sand complex:
the Lower Cretaceous Woburn Sands (Lower Greensand),
southern England
H O W A R D D . J O H N S O N* and B RU C E K . L E V E L L t
* Department of Geology, Imperial College of Science, Technology and Medicine,
Prince Consort Road, London SW7 2BP, UK; and
t Shell UK Exploration and Production Ltd, Shell-Mex House, Strand,
London WC2R ODX, UK
ABSTRACT
A sedimentological investigation of the Lower Cretaceous Woburn Sands of southern England has been
used to develop a depositional model for a transgressive estuarine (or embayment) sand complex.
The Woburn Sands average 70 m in thickness, but are over 100 m thick in places, and infill a NE-SW
trending trough 25-30 km wide, which cuts into the western end of the NW-SE trending London
Brabant land mass. Initially, this trough was of limited extent to the northeast but opened out into a
broader shallow sea to the south and southwest. Subsequently, during the course of the major Early
Cretaceous (Aptian-Albian) transgression, this feature formed a seaway connecting two previously
separate basins (the North Sea Basin to the north and the Weald-Wessex-Channel Basin to the south).
The Woburn Sands record this overall transgressive history in the form of six main facies bodies, which
occur in five erosionally based units (from bottom to top) : (i) Orange and Heterolithic Sands; (ii) Silver
Sands; (iii) Silty Beds; (iv) Red Sands; and (vi) Transition Series.
The lowermost deposits (the Orange and Heterolithic Sands; equivalent to the Lower Woburn
Sands) display convincing evidence of tidal current deposition (e.g. bimodal-bipolar palaeocurrent
patterns, herringbone cross-bedding, clay drapes and wavy-flaser bedding) . They are further character
ized by large-scale, subhorizontal and low-angle erosion surfaces, which are interpreted as tidal channel
bases and tidal shoal accretion surfaces, respectively. The overlying sand deposits (the Silver and Red
Sands; equivalent to the Upper Woburn Sands) display similar evidence of tidal current activity but are
distinguished by overall coarser grain sizes, better sorting, lack of clay layers and the abundance of
large-scale cross-bedding. The large-scale structures in the Silver and Red Sands dip mainly towards the
south (inferred ebb direction), whereas similar structures in the exposed Orange Sands dip mainly to the
northwest (inferred flood direction).
The overall sequence is interpreted in terms of a transgressive tide-dominated estuary or embayment
model. The Lower Woburn Sands (Orange and Heterolithic Sands) were deposited in mutually evasive
ebb and flood tidal channels and intervening tidal shoals, probably in an inner estuarine environment. In
contrast, the higher energy Silver and Red Sands were deposited in the outer reaches of an estuary or
embayment where greater water depths allowed the build-up of large-scale bedforms. The southward
increase in both cross-bed set size and sand-body thickness in the Red Sands probably reflects general
southward deepening. The final element in this facies succession is the draping of the sand complex by
slowly deposited fossiliferous marine beds (Transition Series and Basal Beds of the Gault), which are
overstepped to the north by the shallow-marine muds of the Gault.
The description, interpretation and depositional model outlined here may assist in the recognition
and prediction of similar shallow marine sand bodies. This study demonstrates, for example, that thick,
high reservoir-quality sands with favourable geometries for stratigraphical traps can accumulate in
transgressive estuaries and embayments. The resulting sand complex could be expected to comprise
several erosively bounded, lenticular units displaying rapid lateral thickness and facies variations and an
upward increase in reservoir quality and sand continuity. This would contrast with the more tabular
geometry and more gradual lateral thickness and facies variations of similar tide-dominated deposits
developed in offshore/shelf environments.
17

18 H. D. Johnson and B.K. Levell
INTRODUCTION
Shallow-marine sands (i.e those deposited in water
depths of 10-200 m and ranging from inshore/sub
tidal to offshore/neritic environments) have received
less attention than most other clastic deposits and,
despite recent advances, current facies models
remain rel atively generalized.
Recent studies demonstrate that shallow-marine
sands occur in a variety of settings and owe their
variability to several factors, particularly the complex
relationships between fluctuations in water depth ,
subsidence rates, morphology of the coastal zone,
sediment supply and the hydraulic regime of the
basin, including the shoreline (e. g. Swift & Thorne,
1991). Most studies of shallow-marine sand bodies
distinguish those resulting from tide-dominated
processes and those formed m ainly by wave- and
storm-dominated processes (Johnson & B aldwin,
1986; Dalrymple, 1992; Walker & Plint, 1992). In
this context the Lower Cretaceous Woburn Sands
of southern England h as been quoted repeatedly
as a prime example of an ancient tide-dominated
shallow-marine sand complex, with the spectacular
large-scale cross-bedding interpreted as the deposits
of tidal sand waves (e. g. Bentley, 1970; de Raaf
& Boersma, 1 971; Walker, 1984; Buck, 1985;
Dalrymple, 1992). However, there h as been less
agreement on the specific type of tidal environment
preserved in the Woburn Sands and on its overall
genetic evolution. Because the resulting product is a
thick sand of high reservoir quality, it was felt that a
better understanding of the Woburn Sands would
assist in the development of stratigraphical models
of tidal sand bodies. This could aid their recognition
in the subsurface, allow comparison with the geo
metry and internal characteristics of other shallow
marine sand bodies (e. g. Exum & H arms, 1968;
McCubbin, 1969; Campbell, 1971; Spearing, 1 975)
and contribute to the development of depositional
models for hydrocarbon exploration and production.
Thiswas the background to afield study conducted
in 1979 when both authors were employed by the
Koninklijke/Shell Exploratie en Produktie Lab
oratorium (KSEPL) in Rijswijk, The Netherlands.
Subsequently, the results were presented at the
1 980 Annual Conference of the AAPG (Johnson &
Levell, 1980) and documented in an internal report
in 1 982. This l atter report forms the basis for this
paper, which is presented here for several reasons.
First, the model presented here is different in
several respects to those interpretations published
both before and since completion of our work.
Secondly, the 1 980 Abstract is clearly an inadequate
reference document, but is nevertheless quoted
by workers studying these exposures. Thirdly,
these exposures comprise sand quarries, which are
constantly changing as a result of continuing sand
extraction. Hence, new observations are frequently
made and so the data acquired during the course of
this study, despite the time lapse, are still worthy
of fuller documentation. Publishing the results of
this study will also allow our interpretation to
be more critically evaluated and will form a more
lasting contribution to the analysis of this important
geological unit.
The main aim of this paper, therefore, is to provide
a sedimentological description and to argue a
depositional model for the Lower Cretaceous
(Aptian-Albi an) Woburn Sands at Leighton
Buzzard, southern England (Figs 1 & 2). It is not our
intention to comprehensively evaluate our findings
in the context of more recent research undertaken
on the Woburn Sands, partly because much of this
remains unpublished, particularly a detailed analysis
of sedimentary structures of Buck ( 1987), a litho
stratigraphical study by Eyers (1992a) and a recent
sequence stratigraphical analysis by Wonham (1993).
GEOLOGICAL FRAMEWORK
Stratigraphic framework
The Woburn Sands have been shown through field
mapping and borehole evidence to comprise a lens
shaped sand complex up to c. 100 m thick (Fig. 3),
which infills a rel atively narrow (25-30km wide)
NE-SW trending trough (Bristow, 1963; Wyatt
et a/., 1986). The trough, which may be partly tec
tonic in origin (Eyers, 1 991), came into existence in
the Late Jurassic, but was infilled only in the Early
Cretaceous. At Leighton Buzzard, which is on the
western margin of the trough, the upper Aptian to
lower Albian Woburn Sands unconformably overlie
UpperJurassic clays and are overstepped northwards
by the Albian G ault (Fig. 4).
Ammonites and brachiopods found in the basal
phosphatic gravels of the Woburn Sands are assi gned
to the upper Aptian (nutfieldiensis zone; Casey,
1961). Although the Woburn Sands currently
exposed contain an abundant and diverse ichno-

Transgressive estuarine sand complex 19
Fig. l. Location map showing the
distribution of Lower Cretaceous
outcrops.
2'
I
fauna they do not contain a shelly fauna, possibly
because of leaching of calcium carbonate. The over
lying beds belong to the Transition Series, which
comprises a thin (1-2 m) , complex and relatively
poorly exposed succession of variable lithologies,
including the Shenley Limestone and various iron
cemented beds referred to locally as 'Carstone'
(Fig. 3). The Shenley Limestone contains lower
Albian (tardefurcata to mammillatum zone) fauna
and represents a depositional hiatus with a complex
depositional and diagenetic history (Eyers, 1992b).
The condensed lower Albian Gault (dentatus zone)
represents a northward overstep, which can be
related to the ' 108 Ma' maximum flooding surface of
Haq et a!. (1987). The paucity of datable fauna
within the Woburn Sands essentially precludes
further correlation with events elsewhere in the
Lower Greensand basin (Ruffell & Wach, 1991,
in press).
Palaeogeographical setting and depositional
environments
During the Early Cretaceous (Ryazanian to
,
LHTONBUZZARD
0' - Aptian-Albian Lower Greensand
outcrop distribution
0 50 100km
Barremian) the London-Brabant land mass formed
an intermi ttent land barrier between a shallow
marine southern North Sea Basin (the Boreal Sea)
to the north, and a freshwater Wealden Basin to the
south (Fig. 5A). This southern basin, together with
the Channel, Southwestern Approaches, Celtic Sea
and Bristol Channel Basins (Ziegler, 1988, 1990),
formed a series of mainly separate and active, fault
bounded basins in the southern British Isles, which
underwent a c. 40-million-year period of alluvial
sedimentation and the deposition of 'Wealden' facies
(P. Allen, 1981).
This period of non-marine sedimentation was
terminated by the major Aptian-Albian marine
transgression and the deposition of the Lower
Greensand Group. This was mainly a consequence
of continued sea-floor spreading and northward
extension of the Atlantic Ocean and the Early
Cretaceous eustatic sea-level rise. This resulted
in progressive, northeastward marine inundation
of the Southwestern Approaches, Channel and
Weald Basins. Simultaneously, there was also
northwestward-directed marine transgression
through the Paris Basin into southern England and

A
Heath and Reach
Old {@
Llnslade
Road
0
@
f!7' Jane's Pit
1km
Fig. 2. Location of the sand pits used in this study and the
line of the cross-section illustrated in Fig. 4.
southerly expansion throughout the southern part of
the North Sea Basin and adjacent areas (e. g. West
Netherlands, Broad Fourteens and Lower Saxony
Basins; Zeigler, 1990). The precise timing of full
marine connection between these basins is uncertain,
but most recent reconstructions show this to have
occurred by the late Aptian or early Albian, at
around the time of deposition of the Woburn Sands
and the overlying Gault. Hence, on a regional scale,
the depositional history of the Woburn Sands would
appear to be related to marine transgression that
resulted in the connection of two main intracratonic
basins.
Three main periods of marine transgression are
recorded in the Lower Greensand of southern
England, each associated with deposition of exten
sively cross-bedded tidal deposits (Bridges, 1982).
The upper Aptian to lower Albian Woburn Sands
are associated with the transgressive breaching of
the London-Brabant land ma�s (Figs SB & SC) .
However, although the Woburn Sands clearly
represent, in broad terms, a transgressive shallow
marine sand deposit, more precise environmental
interpretations have remained uncertain. The large
scale cross-bedding, extensive bioturbation and evi
dence of reversing currents has led most authors
to suggest a tidal environment (Lamplugh, 1922;
Schwarzacher, 1953; Bentley, 1970; de Raaf &
Boersma, 1971), but these authors disagree, or are
non-committal, about specific tidal subenviron
ments, with suggestions ranging from open shelf to
tidal fiat for different facies in the complex. From
these possibilities, two main depositional models
emerge for all or part of the Woburn Sands:
1 tidal shelf or seaway, such as the present-day
Straits of Dover/English Channel (e. g. Bridges,
1982);
2 tidal estuary or embayment (e. g. Johnson &
Levell, 1980).
These alternatives will be considered here in the
light of our observations. Similarly, the uncertainty
as to whether the breakthrough across the London
Brabant land mass (the 'Bedforsh ire Strait' of
Kirkaldy ( 1939)) occurred as a result of southward
or northward extension of a coastal embayment will
also be considered.
Lithostratigraphical subdivision and relationships
cThe Woburn Sands comprise up to six facies bodies,
which are readily distinguished on the basis of their
lithofacies characteristics (mainly grain size, com
position, colour, clay content, sedimentary struc
tures and bioturbation). Most of these facies bodies
are separated by major subhorizontal to low-angle
erosion surfaces, and five discontinuity-bounded
units have been defined as follows (Table 1 &
Fig. 3):
1 Transition Series (youngest);
2 Red Sands;
Fig. 3. (Opposite.) Composite vertical section through the
Woburn Sands.

LITHOSTRATIGRAPHIC NOMENCLATURE
-�--'·-· -�------,
c
"' ,.,
:0 .. :;
:;;: 0 :;
"' "'
:;; :;
(.') (.')
3: ..0
Cl-'
Carstone and Shanley
Limestone
-
u "'
<1> u
0:<1> c
<l>u "'
�� (f)
rocn u
0
<1>
u a:
�"' "'
�u
en� :=C
en"'
(f)
c
"'
5 u
.0"' c
�-g
"'
(f)
�"'
:;;Q)(f)
a. .2
a. Ui=>
c
"'
"'
"li u
..: c
:;;
"'
a.
(/)
a. E=> ::l
.0
0
3:
"' - -
u
c
"'"' u
(f) c
c "'
5 (f)
.0 c
0 3:
3: e
:;;
en
3:
0
-'
UPPER
JURASSIC
J:?/-::-1 Large-scale cross-bedding
B. Trough cross-bedding
Q Ripple cross-lamination
D Intraclasts
0- ·-:__-_-
--------
��- . "Dentatus"--
t:====-:::::-=----
-
10 --
Transition Series
"��
-----· "'"'''
��-Red Sands
''- --...Y.C
��20-
''�
�Silly Beds
''''" .,,,,,
''-'-'-'->--."."'-'-':-
�30-
-�Silver
��Sands
�-
��----------�-
40 -
-s==:Heterolithic -
Sands �L
�-
�
----�
---�..!"-....,_.
-- -- ---- �
�--.1!....,
�50-
� ..}..._
-�- �
·-··--�......__
Orange Sands
-:::'{J! -60-
-�
�
///!))})�--��---""!.
?
70-
==
=
��[�
EJ. Wavy bedding
c=J Flaser bedding
ENVIRONMENTAL SUMMARY
BURROW TYPES
Blanket shelf muds
- _____/Slow deposition/reworked, transgressive
e:i3> deposits
=
=
=M
(:==.:r>
=
==M
J
=
=
=
=
=
�
� �
� �
�
�
�
High energy, ebb dominated channel!
shoal complex (estuary mouth or open
marine, sea strait environment)
/Low energy, estuary abandonment!
transgressive deposits
High energy, ebb dominated, estuary
mouth channel/shoal complex
(= ebb-tidal delta environment)
Low-moderate energy, estuary shoal
deposits
Moderate-high energy, flood dominated,
channel-fill sands intercalated with tidal
shoal deposits
Basal transgressive deposits with
phosphate nodules and reworked faunas
� Strongly bioturbated
6:11 Shells and shell debris
1:-�;1 Low-angle erosion surfaces Q Plant debris
E;J' Concretions/nodules -& Occasional burrows

22 H.D. Johnson and B.K. Levell
Table 1. Summary of the lithofacies and reservoir characteristics of the main units within the Woburn Sands
Interval
Gault Clay
Transition
Series
Red Sands
Silty Beds
(/)
Q
z
<(
(/)
z
a:
::;:)Ill
0 Silver
;: Sands
Heterolithic
Sands
Orange
Sands
3 Silty Beds;
Lithology Physical sedimentary structures
Grey fossiliferous claystones
Iron-cemented pebbly sands (basal beds); glauconitic & phosphatic fine-coarse, partly argillaceous
sands. In-situ lenses of richly fossiliferous limestone (Shenley Lmst.). Reworked clasts of
iron-cemented sst. (Carstone) & Shenley Lmst. Rapid lateral lithological variations.
Med.-v.coarse sand
Mod.-poorly sorted.
Ferruginous with up
to 20% bv detrital
iron oxide (red
colouration) up to
2.5% bv heavy
minerals 100%
sand.
Grey-green
glauconitic & lignitic
clays, silts & f.
sands. Minor crs.,
well sorted sand
layers & lenses.
-20-40% sand.
Med.-v.crs. sand.
Well-sorted. Quartz
arenites. Minor
carbonaceous
debris. Locally
Fa-cemented e.g.
clay clasts on
erosion surfaces).
100% sand.
Fine to v.fine mod.
sorted sands.
Numerous thin clay
layers. Intraclasts of
clay & carbonac.
debris. -90% sand.
Fine to crs. mod.
sorted sands.
Scattered quartz
granules, clay
pebbles & clay
drapes. Iron oxide
cement in liesegang
rings & around clay
deposits. -95-100%
sand.
Three main types of cross-bedding:
I
0.-":
"//!/1/1/1 l
��T
11 �1
%
;; J
Giant cross-bedding with
avalanche foresets &
infilling, large scours ca 3m
deep and 1OOm wide.
Wedge-shaped sets (-2-3m
thick) superimposed on
low-angle (4-8°), S-dipping
surfaces.
III �:JtYti,;;tff -�-am0.1-4m thick tabular &
J trough cross-bedding.
Coarse sands have flat bases, large rippled or flat surfaces & internally
cross-bedded or horizontally laminated.
Fine sands occasionally show low-angle to horizontal lamination but mainly
bioturbated.
Variety of large-scale cross-bedding:
Northern area:
1 (J.t,t,l,ffZt:;Z,' l Large-scale, low-angle
��-71" (2-4°) surfaces separated
/ -rm by o.5-2m thick, tabular
'L1 1 n-:
cross-bed sets.
1-3m avalanche-type
Southern area: cross-bedding, partly filling
��:+m��
s
urs. Complex low-angle
Main structures (in order of decreasing importance) current ripple
cross-lamination, trough cross-bedding, scour & fill structures & low-angle
cross lamination. Herringbone cross-bed patterns. Abundant clay drapes
produce wavy & !laser bedding. Large-scale low-angle surfaces (dipping
-40)
Large-scale subhorizontal (1) & low-angle (2) erosion surfaces.
1
D��2
/////////////////////I I �
Large avalanche foresets fill deep scours (1-5m thick). Low angle surfaces
separated by cross-laminated, cross-bedded & bioturbated sands. Flaser &
wavy bedding/clay drapes.
Dep. environment
Muddy shelf.
Transgressive lag
deposits.
High-energy,
ebb-dominated
complex
(estuary/embayment
mouth or open marine
sea strait).
Transgressive or local
abandonment
deposit.
High-energy
ebb-dominated
embayment mouth
channel-shoal
complex (cf ebb-tidal
deltas)
Moderate-energy,
tidal shoal deposits
within an inner
estuarine/inner
embayment
?margmal to tidal
channel complex
(=Oranae Sands).
High-energy
flood-dominated tidal
channel-fill sands with
intercalated
moderate-to
high-energy tidal
shoal deposits.
4 Silver Sands;
term inology of previous workers (Table 1 & Fig. 3),
including the schemes of Wyatt et al. (1986) and the
more recent formal l ithostratigraphy of Shephard
Thorn et a/. ( 1986).
5 Orange Sands and the Heterolithic Sands
(= Lower Woburn Sands or Brown Sands).
This informal scheme generally follows that of
Bentley ( 1970) and is readily correlated with the
The vertical and lateral relationships of these units
are summarized in Figs 3 & 4, respectively. The base

Table 1. (Continued. )
Fauna and biogenic sedimentary structures Pal�'it�fe':.�ent Reservoir
characteristics
Thickness Geometry
-?Om
Ammonites, belemnites, bivalves, brachiopods. SEAL max. Sheet-like
Abundant ammonites, bivalves, belemnites, gastropods POOR to V-POOR
& oysters. Partly reworked & phosphatized. henley -partly sealing due to 1-2m Sheet-like/tabular
Lmst.= brachipods, echinoids & crustacea. cementing & argill. content
No preserved fauna (?leached) 2 main types of Unimodal to E-W lenticularbioturbation: the S-SSE 0->15m
geometry in N
I Intense, small-scale colour mottling (5mm diam.)
GOOD-pale core & darker rim. Caused by horizontal burrows
& resulting in negligible destratificatation. Very locally reduced by
minor Fe cementation.
No shale layers.
II Funnel to v-shaped burrows due to vertical animal Minor reversals
(?up Large-scale
& herringbone
1o10's m southward
escape or sediment collapse/intiII. Large burrows (1O's max.) thickening wedge
mm wide, up to -1OOmm high) caused by large bivalve patterns
in S.
or crustacaean.
No fauna observed.
POOR/SEAL Lenticular
Strongly bioturbated throughout- fine sediments -drapes irreg.
effectively destratified. Isolated structures -thin permeable sands 1-2m
surface of ilver
show dips to S probably laterally Sand.
No distinct burrow types. extensive (=Storm
-dissected cut-outlayers)
by erosional base
of the Red Beds.
Unfossiliferous.
Bimodal-bipolar
VERY GOOD
Negligible bioturbation-rare single clay-lined burrows vertically & laterally 2-15m
Tabular within
(Ophiomorpha) towards the top in some places (e.g. S-SW modes are uniform study area.
New Trees). dominant
(directions of all
major structures)
Unfossiliferous (?leached). Bimodal-bipolar Uncertain-restricted
Extensively bioturbated (ca. 10-50% of primary MODERATE to to E part of study
structures destroyed). Horizontal, slightly sinuous, POOR
<25m
area.
clay-lined burrows are the most common type & occur WSW mode Possibly interfinger
mainly in cross-laminated sands. Occasional vertical to dominant & ENE -discontinuous shale to W with Orange
oblique burrows. mode slightly layers Sands.
subordinate
Unfossiliferous (?leached). Bimodal-bipolar MODERATE to
Moderately to strongly bioturbated (30-50% of primary GOOD Uncertain,
structures destroyed) & variety of burrow types:
NW mode up to greater N-S
(i) narrow vertical tubes, (ii) sinuous subhorizontal
dominant with -distinct higher -50m continuity ct.
burrows producing colour mottling. (iii) iron-cemented
minor S-SSW permeability zones E-W.
vertical to steeply inclined burrows, (iv) subhorizontal
branching burrows Thalassinoides. (v) v-shaped mode within N-S trending
burrows.
channels.
of the lowest unit, the Orange and Heterolithic
Sands, was not seen but is thought to directly overlie
the phosphatic gravels and sands of the fossiliferous
basal beds recorded in abandoned pits (Lamplugh,
1922). The relationship between these two lower
most sands has also not been observed directly, with
either lateral interfingering or erosional contact both
possible. However, facies similarities (discussed
later) suggest that the Orange and Heterolithic Sands
are probably lateral equivalents.
The boundary between the top of the Woburn
Sands and the overlying Gault is generally poorly

2 4 H. D. Johnson and B. K. Levell
D Gault Cia'{
D Transition Series
[(g);j Red Sands (RS)
�j:j:j:j Silty Beds
- Silver Sands (SS)
� Heterolithic Sands (HS)
EO(,<�J Orange Sands (OS)
w
D Jurassic clays
-- Approx. depth of
0
10
20
30
40
50
60
present-day
exposures
North
..,.
0 2km
0
10 E:
"
20 )§
·t
30 <.)
Q)
<f)
"'
40.0
::
0
50a;
.0
<f)
60 �
Q)
70::2:
80
Fig. 4. Cross-section through the Woburn Sands illustrating the vertical and lateral relationships between the main
lithostratigraphical units (sec Fig. 2 for location) . The locations at which some of the main lithostratigraphical boundaries
can be seen arc indicated by single vertical lines. Note the dashed line indicating the approximate depth of present-day
exposures. Datum is the base of the cristatum subzone.
exposed and has not been studied here in any detail.
However, this important and richly fossiliferous
interval has been studied extensively in the past by
palaeontologists and biostratigraphe rs, who have
measured many detailed vertical profiles (e.g.
Lamplugh, 1922; Wright & Wright, 19 47; Casey,
19 61 ; Owen, 1972). These data have been incor
porated into Table 1 and Figs 3 & 4.
SEDIMENTOLOGICAL
CHARACTERISTICS
This section outlines in detail the sedimentological
characteristics of the six main facies types. The
key points of description and interpretation are
summarized in Table l .

Transgressive estuarine sand complex
A
:r:
BOREAL SEA
� Main palaeocurrent
directions
50 100km
�---
Fig. 5. Three schematic palaeogeographical maps
illustrating the transgressive history of the Lower
Cretaceous in the southern North Sea-English Channel
area, and the evolution of the 'Bedfordshire Strait' which
ultimately connected the Boreal Sea and the Wealden
Basin. Aptian-Albian outcrop shown in black
(a) Ryazanian-Valanginian; (b) Aptian: Woburn sands,
Folkcstonc sands, Hythe and Sandgate beds; (c) Albian,
Gault clay. (Based on Ziegler, 1988, 1990.)
B
25
BOREAL SEA
,'
'----'"'---..J100km

2 6 H.D . Johnson and B. K. Levell
Orange Sands
D_escription
Based on the available exposures at the time of our
field-work in 1979 (B ryant's Lane, Stone Lane and
Sheepcott quarries; Fig. 2), these sands are moder
ately sorted, fine- to coarse-grained and contain
some quartz granules, clay pebbles and wood frag
ments. The orange colour is due to widespread iron
oxide, which occurs as a cement, in Liesegang rings
and in rims around clay (e.g. clay pebbles, drapes
and burrow linings).
Large-scale erosion surfaces within the unit have
been divided arbitrarily into two types:
NE
I Subhorizontal erosion surfaces are essentially flat
and extend up to 200 m. Locally they cut down in
concave-upward scours 4- 6m deep (Fig. 6). The
erosion surfaces are spaced 5 - 10 m apart vertically
and are normally overlain by coarse lags of granules
and mud flakes.
2 Low-angle erosion surfaces occur within the units
bounded by erosion surfaces of type I and pass
laterally and down-dip into the subhorizontal erosion
surfaces (Fig. 6). They are spaced at intervals of a
few decimetres to 1 m and separate intervals with a
variable array of cross-bedding, cross-lamination and
burrows (Fig. 7).
The more deeply erosive parts of the subhorizontal
erosion surfaces are overlain by 1 -5 m thick tabular
f<---- Flood-dominated tidal channel Tidal bar -----+1
SW
metres
0
8
. Transport toN Photograph location
+----------------- 150metres----------------•
Fig. 6. An example of large-scale facies relationships in a flood tidal-channel complex in the Orange Sands. The base of the
channel is a horizontal erosion surface lined with intraformational clay clasts. A series of low-angle (4-8°) erosion surfaces
(right side of the photograph) mark the flanks of a tidal bar, which is characterized by small-scale cross-bedding and
moderate to strong bioturbation. The inclinations of the low-angle erosion surfaces (inferred bar flank surfaces) increase
laterally (to the left) and eventually pass into high-energy, channel-fill deposits displaying tabular avalanche foresets up to
4 m high. A second flood-tidal channel sequence is also exposed in the lower part of the section. The simplified field sketch
(sec Fig. 3 for legend) shows the broader relations between the bioturbated tidal bar sands and the avalanche foresets of
flood-dominated, tidal channel-fill sands (from Bryant's Lane pit) .

Transgressive estuarine sand complex 2 7
Fig. 7 . Physical and biogenic sedimentary structures i n the Orange Sands. (A) Tidal bar deposits comprising small- to
moderate-scale cross-bedding (10-SOcm thick) separated by horizontal and low-angle erosion surfaces with thin
intraformational mud-flake conglomerates, and occasional clay layers. The numerous low-angle reactivation surfaces give a
characteristic wedge-shaped appearance to the cross-bed sets (see also (D)). (B) Close-up of the central part of
(A) illustrating some details of the bioturbation. Note in particular the simple vertical burrows and a large V-shapcd
burrow (lower centre of photo). (C) Large-scale tabular cross-bedding infilling a flood tidal channel (upper half of photo).
Vertical burrows increase in density in the deeper part of the channel and arc inclined perpendicular to the forcscts. The
underlying deposits display oppositely-dipping cross-bedding, horizontal and inclined erosion surfaces and moderate
bioturbation. (D) Wedge-shaped cross-bedding with numerous reactivation surfaces, which arc occasionally overlain by
clay drapes or oppositely-dipping cross-lamination.
or wedge-shaped cross-bedding with NE-dipping
avalanche foresets separated by large-scale, low
angle erosion surfaces (Fig. 6). These low-angle
erosion surfaces may flatten up-dip into more closely
spaced, subhorizontal erosion surfaces. This is ac
companied by a change in sedimentary structures
from 1-5-m-thick avalanche cross-bedding to 0.2-
0. 7-m-thick sets of trough cross-bedding, current
ripple cross-lamination and flaserand wavy bedding.
Palaeocurrent directions from all these deposits are
variable, but there is abundant evidence of reversals,
especially in the smaller-scale structures. The larger
structures show mainly northwest-flowing palaeo
currents but with clear, subordinate reversals
(Fig. 8).
Burrowing has destroyed, on average, some 30-
50% of the primary structures and has been sub
divided into five types:
1 Narrow (c. 2 mm) vertical tubes that form a
branching network with sections 10-20 mm long.
These tubes have no clay lining and are extremely
fragile, being visible only on wind-sculpted faces.
They resemble burrows produced by polycheate
worms in modern sands of estuaries and tidal flats
(Schafer, 1972).
2 Sinuous, subhorizontal burrows producing c.
5-mm-diameter colour mottling. These burrows
occur mainly in the ripple-laminated sands.
3 Simple, vertical orsteeply inclined tubes ( c. lO mm
diameter and 50-200 mm long) with clay linings,

N
I
Red Sands
n=89
Orange Sands
n=203
N
I
Silver Sands
n=157
Heterolithic Sands
(scale x2)
n=52
Fig. 8. Palaeocurrent distributions based mainly on large
scale cross-bedding.
which are frequently the sites of iron-oxide precipi
tation (Fig. 7A & B). This type projects normal to
the bedding even when this is inclined (Fig. 7C) , and
is widespread throughout the Orange Sands.
4 Complex, subhorizontal to inclined, branching
burrow networks c. 10-40 mm in diameter and
with enlarged, bulbous junctions (Fig. 7D). Iron
cementation preserves these in three dimensions.
This type most closely resembles the crustacean
burrow system Ophiomorpha.
5 Nested cone-shaped burrows (V-shaped in two
dimensions, Fig. 7B). These appear to represent the
collapse of sedimentary lamination into 20-30-mm
wide horizontal tubes, but may also occur as iron
cemented, V-shaped laminae. They are especially
common in sands just above and below major erosion
surfaces.
Interpretation
The interbedding of deposits with opposed palaeo
current modes, evidence of rapid lateral variation in
flow regime (as shown by the intercalation of large
and small-scale structures), the range of burrow
types, and the clay drapes, are all common character
istics of high-energy, shallow-water tidal deposits
(de Raaf & Boersma, 197 1 ; N io & Yang, 1991). The
subhorizontal erosion surfaces, therefore, probably
define the bases of tidal channels and, at least in
the areas of deepest scour, these channels carried
northwestward flowing water. The low-angle erosion
surfaces are interpreted as the accretionary flanks of
in-channel bars on which current dominance was
less pronounced and low-energy structures were
preserved (e.g. Yang & N io, 1989). There is no
evidence that the preserved portions of these
bars were either emergent or suffered severe wave
activity. The Orange Sands thus represent a high
energy, tidal channel complex with mutually evasive
ebb and flood tidal currents.
Heterolithic Sands
Description
This unit comprises moderately sorted, fine to
very fine grained sands w ith numerous clay layers,
scattered clay flakes and woody detritus. The sands
are largely grey, while the clay drapes and surround
ing sands are sometimes rust-coloured due to iron
oxides.
The sands contain low-angle (c. 4°) erosion sur
faces tens of metres long that closely resemble those
of the Orange Sands, with the exception that they do
not pass downwards into channel-fill facies and are
often overlain by relatively continuous clay drapes.
The main sedimentary structures are, in order
of decreasing importance: current ripple cross
lamination (Fig. 9A) , trough cross-bedding, scour
and-fill structures and low-angle cross-lamination.
Clay drapes occur on set boundaries and foresets
within all types of cross-stratification and sometimes
produce wavy and flaser bedding (Fig. 9A & B).
Although not measured in detail , clay drape distri
bution is suggestive of tidal bundles, possibly with
neap-spring tide cycles (e.g. Visser, 1980). Evidence
of bidirectional currents is ubiquitous in all these
structures (e.g. Fig. 9A). The larger foresets
commonly have superimposed smaller sets with
reversed dips. Cross-stratification type varies over

Fig. 9. Physical sedimentary
structures in the Heterolithic Sands.
(A) Flaser bedding associated with
small-scale, current ripple cross
lamination. Herringbone patterns
are occasionally developed but
normally the southwest ebb
direction is dominant. (B) Clay
draped foresets separated by thick
clay layers, which are internally
disrupted by bioturbation.
short distances, both laterally and vertically, and
there are no progressive vertical changes in either
set thickness or grain size. Palaeocurrents are
bimodal-bipolar, with a dominant southwest directed
mode (Fig. 8).
Burrowing, which has destroyed around 10-50%
of the primary sedimentary fabric, is dominated
by sinuous, horizontal, clay-lined forms (Fig. 10).
Burrows are most common in the flaser and wavy
bedded subfacies and may be virtually absent in the
larger decimetre-scale cross-bed sets. The pre
dominance of horizontal burrows results in little
disturbance of the sedimentary structures. Less
common types are vertical to oblique and rare,
spiral clay-lined burrows (Fig. lOD).
Interpretation
The bimodal-bipolar palaeocurrent pattern, and the
assemblage and variability of sedimentary structures
in this unit suggest a shallow-marine tidal origin.
The thickness of this facies (up to 25 m was recorded
by Bentley ( 1970)) and the lack of features rep
resenting emergence (rootlets, wave-reworked sur
faces, desiccation cracks, etc.) suggest deposition in
a subtidal environment of moderate water depth
and fluctuating flow conditions. The lack of distinct
channel-fill facies and the ubiquitous presence of
low-angle inclined erosion surfaces with a constant
south-eastward dip suggests accretion on a broad
subtidal shoal. Relatively low-energy currents are

30 H.D. Johnson and B.K. Levell
Fig. 10. Biogenic sedimentary structures in the Heterolithic Sands. (A) Strong bioturbation in current ripple cross
laminated and partly ftaser bedded sands. The dominant biogenic structures are horizontal, clay-lined burrows.
(B) Moderate bioturbation mainly by horizontal, clay-lined burrows with a single inclined burrow (southeast of lens cap).
Bidirectional, cross-lamination with occasional clay-ftasers is still well-preserved. (C) Plan view of the dominant burrow
type in this facies comprising horizontal, sinuous, clay-lined burrows with back-fill laminae. (D) Isolated example of a
vertical, clay-lined burrow resembling Ophiomorpha. Background facies is ripple laminated, ftaser bedded sand.
suggested by the small scale of cross-bedding and
the predominance of ripple cross-lamination,
whereas the extensive clay drapes suggest relatively
long periods of quiet water conditions.
Silver Sands
Description
The Silver Sands consist of well-sorted, medium- to
very coarse-grained quartz arenites (previously used
as glass sands). Sooty and woody carbonaceous
matter is locally abundant, but clay drapes are
absent. These sands truncate all earlier deposits w ith
a major planar to regionally concave-upwards
erosion surface which is lined with granules and
clay flakes. The lag deposit overlying this erosion
surface is well cemented by iron oxides.
In the northwest of the area (around Heath and
Reach, Fig. 2) several pits expose up to 20 m of
Silver Sands, and major low-angle (2-4° apparent
dips) erosion surfaces with a constant southwestward
dip can be picked out throughout the unit (Fig. llA).
Between these planar to slightly undulose erosion
surfaces tabular cross-bed sets from 0.5 to 2 m thick
occur (Fig. 1 1B). The erosion surfaces terminate
abruptly down-dip, resulting in thickening of some
cross-bed sets, and formation of hanging set bound
aries. No single surface could be traced from the
top of the 15-m -thick unit to the base. There is
only occasional evidence of reversing palaeo
currents, such as at the base of the unit in Munday's
H ill quarry, resulting in an overwhelmingly domi
nant southwestward dip to all scales of foreset
(Figs 8 & 1 1).
In the extreme north of the area of the Silver

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