Brettle Et Al 2001

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The paper that caused a bit of debate as to whether tides affect deposition in the Carboniferous Pennine Basin. Prior to this there was some evidence, but it was refuted by the main players. The tidal deposits themselves occur in discrete zones - within the TST of wide valley fills, and in mouthbar systems deposited during stillstand/ early TST.

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Brettle Et Al 2001

  1. 1. Journal of the Geological Society, London, Vol. 159, 2002, pp. 379–391. Printed in Great Britain. Identifying cryptic tidal influences within deltaic successions: an example from the Marsdenian (Namurian) interval of the Pennine Basin, UK 1 1 1 2 3 M . J. B R E T T L E , D. M C I L ROY , T. E L L I OT T , S . J. DAV I E S & C . N. WAT E R S 1 Department of Earth Sciences, University of Liverpool, Brownlow Street, Liverpool L69 3GP, UK (e-mail: m.j.brettle@liv.ac.uk) 2 Department of Geology, Bennett Building, University of Leicester, University Road, Leicester LE1 7RH, UK 3 British Geological Survey, Kingsley Dunham Centre, Keyworth, Nottingham NG12 5GG, UK Abstract: Research literature abounds on the depositional processes and products associated with macro-tidal regimes, whereas there is little available literature on sediments deposited in micro-tidal regimes. This paper presents new field-based sedimentological interpretations of the Marsdenian (Namurian, Carboniferous) interval of the Pennine Basin, a basin-fill that is classically regarded as the archetypal fluvial-dominated delta system. This paper reinterprets discrete lithostratigraphic units, and suggests they were deposited under the influence of weak tidal currents. We highlight three lithofacies that contain tidally influenced deposits within the Marsdenian interval of the Pennine Basin; a cross-bedded sandstone with mud drapes and reactivation surfaces, a heterolithic ripple-laminated sandstone with muddy drapes and silty mudstone interlaminations, and a rhythmic–parallel-bedded sandstone with mudstone–siltstone interlaminations. Evidence for cryptic tidal signatures in tractionally transported and reworked sediments is qualitative, and largely dependent on the sedimentologist’s view of what constitutes a diagnostic number of tidal indicators (i.e. mud-drape couplets, reactivation surfaces). In areas away from either tractional deposition or reworking, sediments deposited from suspension are more likely to preserve indicators of tidal processes. This paper focuses on a lithofacies interpreted as a tidally influenced sand-rich delta-front mouthbar deposited from a buoyant effluent plume. Time-series analysis of laminae thickness variations in this facies implies that these variations are rhythmic. We review how the interaction of tidal currents and buoyant plume processes modifies depositional products. This model implies that the rhythmic variation observed in the Marsdenian interval is attributed to the modulation of plume deposition by tidal currents with a semi-diurnal and diurnal tidal periodicity. Keywords: Marsdenian, Pennine Basin, deltaic sedimentation, tidal currents. The characteristics of ancient macro-tidal successions have been stratigraphy, and are generally thought to represent the only well documented in the sedimentological literature (Ginsburg intervals of marine influence during deposition of the otherwise 1975; Visser 1980; Allen 1981; Siegenthaler 1982; Demowbray fluvially dominated Namurian deltaic systems (Holdsworth & & Visser 1984; Kvale et al. 1989; Williams 1989; Allen 1991; Collinson 1988). This paper, however, describes tidally influenced Nio & Yang 1991; Read 1992; Martino & Sanderson 1993; facies that occur within sandstone delta cycles (Fig. 2), suggest- Miller & Eriksson 1997; Adkins & Eriksson 1998; Greb & ing that the basin remained linked to oceanic waters for a greater Archer 1998; Fenies et al. 1999), whereas investigations of period than that suggested solely by marine bands. Evidence for a micro-tidal environments are mainly restricted to present-day tidal influence has been suggested in the Westphalian (Broadhurst examples (e.g. Pejrup 1986, 1988; Allen 1991; Hughes et al. et al. 1980; Broadhurst 1988) and Namurian (Aitkenhead & Riley 1998; Makaske & Augustinus 1998; Fenies et al. 1999; 1996; Archer & Kvale 1997) intervals. In this paper we present Johannesson et al. 2000; Hossain et al. 2001). The identification facies, time-series and stratigraphic data suggesting that tidal of tidal signatures, either macro- or micro-tidal, in shallow-water currents influenced deposition during the Marsdenian interval, marine systems is significant, as they constrain sedimentary and present models explaining the processes that may be successions within a palaeogeographical and environmental zone. responsible for deposition within such micro-tidal regimes. This is especially true when sequence stratigraphic concepts are The application of sequence stratigraphy in studying deltaic applied, where there is a potential for the identification of incised systems, namely, the identification of sand-rich incised valleys valley fills that often contain tidally influenced facies. and correlative interfluve areas, is well documented in the Upper The delta systems of the Marsdenian interval form part of the Carboniferous units of northern England (Maynard 1992; Church clastic infill of the northern Pennine Basin (Fig. 1), in which 1994; Church & Gawthorpe 1994; Hampson et al. 1996, 1997; sedimentary provenance is inferred to have been dominantly from Wignall & Maynard 1996). In addition to the identification of the north or NE (Drewery et al. 1987). The isolation of the tidal deposits, this paper places tidal deposits within a sequence Pennine Basin from oceanic water masses is suggested to have stratigraphic context. We compare Marsdenian tidal deposits with resulted in the absence of tidal currents from the Namurian existing tidally influenced valley-fill models (Allen 1991; interval of northern England (Collinson 1988). The inference that Dalrymple et al. 1992; Allen & Posamentier 1993; Zaitlin et al. tidal currents were not important has led to current interpretations 1994) and discuss the wider implications for the analysis of of Namurian deltas being based on facies models generated from deltaic successions influenced by micro-tidal regimes. river-dominated deltas. Regionally correlatable ammonoid- The data presented in this paper come from 42 borehole bearing marine bands account for a thickness of ,5% of the records (mainly written descriptions dated between 1900 and the 379
  2. 2. 380 M . J. B R E T T L E E T A L . extension during the northward-directed Rheno-Hercynian sub- duction (Gawthorpe 1987; Leeder & McMahon 1988). Extension produced a series of structurally high ‘blocks’ and basinal depocentres (Lee 1988). These were part-filled by turbidite- fronted deltaic systems during early Namurian to Kinderscoutian time, with sediment supplied from the decaying Caledonian– Appalachian Mountains (Collinson et al. 1977). The Marsdenian interval marks the onset of a period of predominantly shallow- water, mouthbar deposition. After Marsdenian time the younger Yeadonian and Westphalian intervals record eventual infill of the Pennine Basin and the development of coal-forming delta-top swamp conditions (Guion & Fielding 1988; Guion et al. 1995; Waters et al. 1996). Two mouthbar-dominated cyclothems are the focus of this paper: the lithostratigraphic units that we will show to be equivalent to the Readycon Dean Flags and Midgley– Helmshore Grit (Brettle 2001). Both cyclothems possess region- ally extensive regressive basal surfaces, with overlying tidally influenced fluvial deposits and tidally influenced mouthbar facies. Tidally influenced deposits within the Marsdenian interval Previous research suggests that fluvial processes dominated deposition in the late Namurian Pennine Basin, and that links with open oceans were tenuous except at marine band intervals (Collinson 1988). This paper describes three lithofacies that suggest tidal processes operated in addition to fluvial processes during the Marsdenian interval: (1) a cross-bedded sandstone with mud drapes and reactivation surfaces; (2) a heterolithic ripple-laminated sandstone with muddy drapes and silty mud- stone interlaminations; (3) a rhythmic–parallel-bedded sandstone with mudstone–siltstone interlaminations. Lithofacies 1: cross-bedded sandstone with mud drapes and reactivation surfaces This lithofacies comprises coarse- to fine-grained sandstone with disseminated granules and pebbles, and thin muddy drapes. This lithofacies is similar to cross-bedded sandstone lithofacies within which it is often interbedded, but is distinguished by the presence of mud drapes on toeset and foreset surfaces, and a greater abundance of reactivation surfaces within bed-sets (Fig. 4a). The drapes comprise micaceous–muddy laminae that often, but not always, occur in pairs, up to 5 mm apart. When traced, mud drapes in some cross-bed sets occur in bundles that are rhythmically spaced (Fig. 4b). Fig. 1. Map illustrating the northern Pennine Basin, Marsdenian exposure and the localities named in the text. Stratigraphic position of Interpretation: tidally influenced cross-bedded sandstone the Marsdenian interval within the Carboniferous period with marine band nomenclature is taken from Riley et al. (1993). Cross-bedded sandstone is ubiquitous within the Namurian interval of the Pennine Basin, and is often very micaceous. This facies distinguishes itself by the presence of paired muddy present), five public-access wireline well data records (from drapes, bundles of mud drapes, and reactivation surfaces suggest- petroleum exploration wells, dated from 1950 to the 1980s) and ing modulation of flow regime; potentially by either tidal from more than 90 field localities (Fig. 1), from which logged processes (Nio & Yang 1991) or rapid changes in river dis- sections, scaled field-sketches and facies analysis were con- charge. structed (Fig. 3). Either allocyclic variations in fluvial discharge or the mouth- ward progradation of the fluvial system may explain the inter- bedded relationship of this facies with the cross-bedded Pennine Basin geometry and stratigraphy sandstone. If tidal processes were responsible for the formation The geometry of the Pennine Basin during the Marsdenian of muddy drapes and reactivation surfaces, then the inertia interval was inherited from Dinantian rifting, formed by back-arc associated with the basinward flow of a body of fluvial water
  3. 3. M A R S D E N I A N C RY P T I C T I DA L S I G NAT U R E S 381 Fig. 2. Schematic cross-section through study area with current lithostratigraphic units, facies associations and stratigraphic position of Figs 4, 5, 7, 8, 9 and 10. could have suppressed any tidal signature and allowed erosion of deposited dominantly directly from suspension, but was subject sedimentary evidence for tidally influenced deposits. to frequent low-velocity currents and reworking. The variable proportions of fine sandstone to siltstone may be dependent on the proximity of the point of deposition to the locus of sediment Lithofacies 2: heterolithic ripple-laminated sandstone transport or local fluctuations in flow velocity. This facies with muddy drapes and silty mudstone represents deposition in a shallow-water mouthbar front, where interlaminations the substrate is affected by wave or current reworking in the Within parts of the Helmshore Grit, lithofacies comprising thin shallow water depths. Current-ripple laminations with bimodal parallel-laminated micaceous siltstone to fine-grained sandstone flow indicators represent periods of flow reversal during deposi- interlaminated with grey to black micaceous silty mudstone are tion. Subtle laterally discontinuous reactivation surfaces are common. These often overlie distal bayhead delta mouthbar subparallel to bedding, and are commonly truncated and over- facies, and commonly have a sharp erosive base. Sedimentary lain by ripple structures. Bayhead mouthbars form thin and structures are delicate, and comprise interlaminated fine-grained laterally extensive sheets in shallow water, where sediment entry sandstone with a muddy siltstone matrix. Asymmetric starved points (either mouthbar feeder channels or crevasse splays) are ripple cross-laminations with common flow reversals and sym- often close together (Van Heerden & Roberts 1988). The metrical wave-ripples characterize these siltstones, whereas re- interaction of sediment input from several close entry points activation surfaces are common in the sand-rich portions (Fig. 5). may provide conditions where flow reversals are generated. Proportions of sandstone to siltstone can vary, and mica is Alternatively, flow reversals may have been caused by ebb and abundant in both sandstone- and siltstone-rich components of flood tidal influences affecting the mouthbar front during this lithofacies. The ichnofauna includes Planolites isp., Curvo- deposition. The ichnofauna suggests a brackish water column lithus isp., Rosselia isp. and rare Chondrites isp. Similar (Eager et al. 1985), corroborating the interpretation of a tidal– ichnofaunal assemblages have been described throughout the marine influenced depositional environment. The input of fluvial Silesian deposits of the Pennine Basin (Eager et al. 1985), and waters into either brackish or marine waters suggests that have been attributed to brackish environments. Fragments of flocculation may have been a significant mechanism responsible carbonaceous debris are locally common. for the deposition of the clay-sized suspended fraction in such environments. Interpretation: tidally reworked bayhead mouthbar deposits Tidal winnowing of mouthbar sediment The layer-parallel, interlaminated fabric, high mica content and Whereas sediments deposited by plumes have a high preservation small size of the cross-laminations imply that this facies was potential, those in the submerged mouthbar prodelta are con-
  4. 4. 382 M . J. B R E T T L E E T A L . Fig. 3. A dip section through the R2a1–R2b3 succession (including the equivalent to the Alum Crag Grit, Readycon Dean Flags, East Carlton Grit, Woodhouse Flags and Scotland Flags) in the lower panel. Marine bands R2b1, R2b2 and R2b3 form the datum levels for each correlated delta-lobe. The logged sections are: 1, Cowloughton Clough, Cowling (SD965420); 2, Clough Beck, Keighley (SE063433); 3, Branshaw Quarry, Oakworth (SE032401); 4, Parkwood Quarries and Parkwood Brickpit, Keighley (SE065407); 5, Woodhouse Quarry, Haworth (SE062396); 6, Ponden Clough, Stanbury (SD981364); 7, Wickering Crag, Haworth (SE048372); 8, Nan Scar Clough, Oxenhope (SE039336); 9, Rag Clough, Oxenhope (SE015336); 10, Middle Moor Clough, Crimsworth Dean (SE993336); 11, Nook Quarry, Hebden Bridge (SE010275); 12, Fosters Delph Quarry, Mytholmroyd (SE022273); 13, Bare Clough, Luddenden Dean (SE018308); 14, Fulshaw Clough, Luddenden Dean (SE028301); 15, Cat-i-th well Clough, Luddenden Dean (SE042282); 16, Triangle Rail Section, Ripponden (SE045212); 17, Noah Dale Core, Rishworth (SE019218; BGS borehole SE02SW23 with gamma-ray tool data); 18, Great Clough, Scammonden (SE031147); 19, Pule Hill Section, Marsden (SE032100); 20, Leyzing Clough, Wessenden Head (SE 054064; National Geoscience Index Borehole reference number SE00NE7/BJ); 21, Wessenden Head Bore (SE062087); 22, Crowden Great Brook, Black Hill (SE083032); 23, Rake Dike, Holme Moss (SE101053); 24, Loftshaw Clough, Langsett Moor (SK170994); 25, Mouselow Quarry, Glossop (SK024952).
  5. 5. M A R S D E N I A N C RY P T I C T I DA L S I G NAT U R E S 383 Fig. 4. Lithofacies 1: cross-bedded sandstone with mud drapes and reactivation surfaces. (a) Trough cross-bedding with bed set comprising double micaceous mudstone drapes and reactivation surface (section oblique to foreset dip). Similar lithologies occur in isolated examples throughout the lower Marsdenian interval, suggesting a cryptic tidal (ebb-dominated) influence. Slab from Midgley Grit, Moselden Heights Quarry (SE044163). (b) The planar cross-bedded set in the lower half of the image has three zones with a number of muddy laminae. These could be mud drapes deposited from suspension during tidal slack water. ‘Mud-couplets’ as described by Visser (1980) are not observed, probably because the ebb regime dominates in such fluvial systems. The ebb cycle has a high potential to erode mud drapes formed during slack water and sandstones formed during subordinate current stages (i.e. the weak asymmetrical tide of Nio & Yang (1991)). Taken from Fosters Delph Quarry (SE022273) (locality 29, Fig. 3). Lithofacies 3: rhythmic–parallel-bedded sandstone stantly reworked. This implies that although flow reversals are with mudstone–siltstone interlaminations common, cyclic laminated successions are rarely preserved. Tidal processes constantly winnow the substrate surface in the sub- This lithofacies comprises fine- to medium-grained sandstone merged mouthbar front, and resuspend mud and silt into the with a mudstone matrix, and interlaminations of mudstone and water. Once suspended, landward movement of the water into siltstone. Lamination and bedding surfaces are subparallel, shallower areas during the flood cycle forces the flow to whereas beds are massive, laterally persistent across exposures accelerate the water column (Fig. 6) (Wiseman et al. 1986). The and range between 0.05 and 0.1 m in thickness (Figs. 7 and 8). subsequent ebb cycle allows suspended sediment in the shallower Rare flute and tool marks are seen on bedding planes. Trace water to return to deeper parts of the system, where it begins to fossils present in this lithofacies include Olivellites isp. and settle through the water column. The reworking of the sand Pelecypodichnus isp. bedload, in association with the settling of mud from suspension A good example of this lithofacies is seen at Kebroyd Bridge during tidal slacks, deposits an interlaminated sand–mud lithol- (SE04452120), where an exposure with subhorizontal bedding ogy. reveals cyclical variation in bed thickness on a centimetre and
  6. 6. 384 M . J. B R E T T L E E T A L . Fig. 5. Lithofacies 2: (a) heterolithic ripple-laminated sandstone with muddy drapes and silty mudstone interlaminations. (b) Slab of rock from Warland Wood Quarry (SD947202; locality 6, Fig. 3) revealing reverse ripple lamination, and a back-flow ripple, that appears to have grown up the lee slope of an older ripple. This flow reversal, along with the abundance of mud drapes, suggests a tidal influence to this facies. metre scale (Fig. 7). A repeated pairing, of 0.01 m scale, of have been interpreted as deposited by tidally influenced currents thin–thick laminae is clear (Fig. 8), and can be seen on the (Broadhurst 1988; Read 1992; Aitkenhead & Riley 1996). laminae thickness bar charts (Figs. 9, and 10b and c). On this To determine whether the laminae are truly cyclical, Fourier scale, bed thickness varies from 0.02 to 0.1 m on a c. 25–28 bed time-series analysis is used to ascertain whether the succession cycle, and individual beds commonly possess a micaceous silty contains periodic components. Fourier time-series analysis was lamination (1–5 mm thickness) on their upper surface. The base run on sections 1 and 3 by A. Archer, using the same Fourier of this lithofacies commonly lies either with a sharp contact or analysis program as was used to analyse Kinderscoutian sections erosively on the silty interdistributary bay or offshore facies. The (Fig. 10b, c and e) (Archer & Kvale 1997). The short length of upper surface is either truncated by shoaling mouthbar–distribu- the input data string, along with the apparently ‘noisy’ nature of tary channel facies, or overlain by a flooding surface and the data, implies that the output of the Fourier transform is not offshore marine–interdistributary facies. as refined as that of the Kinderscoutian sections. This is because periodicities greater than 10 laminae cannot be resolved in the sections detailed here, owing to the short length of the dataset. Interpretation: tidally influenced sand-rich mouthbar Both datasets have similar ranges of harmonic output when The dominance of layer-parallel bedding suggests this facies was compared with the Fourier transform results from laminae in the deposited from suspension. Variations in sediment grain size are Kinderscoutian deposits, and the results of the analysis from evident from the rapid spatial and temporal variations in the sections 1 and 3 share broadly similar peaks and troughs (Fig. proportion of mudstone to siltstone. This facies is typical of 10e). The harmonic wave output of sections 1 and 3 falls into sediments deposited in a mouthbar environment, where suspen- two frequency groups, of 1.9 2.6 and 3.6 4.4, suggesting an sion-deposited sands are interbedded with rare tractionally trans- output equating approximately to two and four laminae. ported sand (compare flute and tool marks). The cyclical laminae Although periodicities greater than 10 laminae cannot be thickness variation seen at Kebroyd Bridge (SE04452120) sug- resolved because of the short length of the dataset, periodicities gests a rhythmic fluctuation in the rate of suspension deposition. between the thickest laminae in sections 1 and 3 (43 and 48 It seems likely that either increased fluvial discharge or the laminae, respectively) may be invoked (Fig. 10b and c). progradation of a fluvial channel over its associated mouthbar is responsible for the presence of the large-scale internal scour Processes occurring during deposition from an effluent surfaces, rather than the subtle cyclic or paired laminae. These plume and the influence of tides on a plume broad scour features are overlain by cross-laminated sandstone, implying high-flow discharge and the input of bedload-trans- We need to consider the processes that operate within the ported sand. Similar facies have been described in the Carboni- mouthbar if we are to understand how tidal currents may ferous sequences of the Pennines and the Appalachian Basin, and influence deposition from an effluent plume. Tidally influenced
  7. 7. M A R S D E N I A N C RY P T I C T I DA L S I G NAT U R E S 385 plume processes are not as well documented as models of tidally wedge (Nemec 1995). During periods of low discharge the saline influenced duneform deposition (e.g. Visser 1980; Allen 1981). wedge is a significant feature, propagating 15–20 km upstream Variations in the amount of fluvial effluent entering the basin from the outlet source, and being displaced from the channel modulate the amount of sediment transported into the mouthbar only during periods of very high discharge (Wright & Coleman (Fig. 11a). In basins with marine waters a ‘saline wedge’ forms 1974). Within the distributary channel the relatively static nature during periods of low effluent velocity, when density difference of the saline wedge inhibits seaward bedload transport, and the between plume and basinal waters allows underflow of the saline mixing of fluvial and marine waters creates turbulence and forces the bedload into suspension (Nemec 1995). On the interface between fluvial and marine waters, waves of turbulence (internal waves) form, and these propagate mouthward. The increased turbulence entrains bedload within the water column and carries it to the mouthbar, where it is deposited (Ewing 1950; Wright & Coleman 1974). If the internal waves within the plume are stable features, the passage of alternating bodies of turbulent and non- turbulent water may also have the potential to deposit rhythmi- cally laminated sediment. As the plume passes over the shoaling mouthbar and the saline wedge, it is compressed, undergoes a hydraulic jump and becomes non-turbulent. The resultant ‘buoy- ant plume’ creates a strong density layering, which is elongated by the outward-flowing effluent, and carries sand-sized clasts into the distal mouthbar (Wright 1977). Tidal-current modulation influences the position of the saline wedge within the outlet channel and therefore inertial force of the plume (Fig. 11b). Flood tides suppress outflow as fluvial water is banked up within the channel. In the Mississippi, periods of high tide are shown to correlate with increased episodes of crevasse splaying in the upstream fluvial system. This suggests that tidal fluctuations can hold up fluvial outflow and lead to banking up of water within the channel (Andorfer 1973). De- creasing fluvial outflow forces vertical mixing within the chan- nel, and allows the saline wedge to migrate upstream (Fig. 11a; low discharge). This increases outflow turbulence, depositing sediment in proximal mouthbar, and creating a headward shift in grain-size distribution. During the ebb tide (Fig. 11b; ebb tide), Fig. 6. Processes responsible for the development of lithofacies 2. The the inertia of the outflowing plume is enhanced, creating a ebb (a, e) and flood (c) tidal currents entrain bedload, and resuspend mouthward shift in grain-size distribution. At periods of slack clay-grade material, whereas the shallowing bay area amplifies the velocity during the flood tide, further entraining and resuspending water, increased mixing of the saline wedge with fluvial waters sediment from the bay floor. Deposition of clay-grade material occurs from the distributary channel decreases effluent outflow rates, during both high (b) and low (d) slacks. This may result in a gross accelerating flocculation of clay particles and their deposition decrease in clay-grade material in the distal area, if tidal range is from suspension. The hypothetical grain-size distribution of a sufficiently high and slack water durations are short. Modified from tidally modulated low-discharge plume (Fig. 11b) has the Wiseman et al. (1986) and Nemec (1995). potential to generate the repeated thin–thick sandstone couplets Fig. 7. Lithofacies 3: rhythmic–parallel- bedded sandstone with mudstone–siltstone interlaminations. Photomosaic of Kebroyd Bridge locality (SE045213; locality 33, Fig. 3), showing unconformable base of ‘sharp based’ mouthbar. Broad internal erosive scours should be noted within the tidally influenced sand-rich mouthbar, suggesting erosion by fluvial processes during high distributary discharge (asterisks denote position of measured sections 2 and 3; see Figs. 9 and 10).
  8. 8. 386 M . J. B R E T T L E E T A L . Fig. 8. Example of lithofacies 3 at Kebroyd Bridge (SE045212). (a) Tidally influenced sand-rich mouthbar facies showing fine-grained sandstone interlaminated with micaceous laminae and silty mudstone. (Note thinning of sand-rich laminae upwards, towards the centre of picture, followed by marked thickening in the upper and lower parts.) (b) Close-up of a succession with thicker sandstone laminae. Beds occur in repeated thick–thin pairs. In some examples the thin lamina forms a thin veneer less than 1 mm thick, bounded by a thin micaceous lamina. Base of distributary Distributary channel channel sandstone increase intensity of increase in degree of bioturbation Chondrites current ripple intense 50 lamination 50 bioturbation 40 40 Marker 40 bed 30 30 Laminae number 30 20 20 20 * Fig. 9. Graphic plot of laminae thickness, for three sections at Kebroyd Bridge (see Fig. 7 for position of these sections). 10 10 10 Marker beds represent beds traced laterally Marker along the exposure; these reveal a thinning bed in the middle of section 2 by six sandstone 60 50 40 30 20 10 0 60 50 40 30 20 10 0 60 50 40 30 20 10 0 Lamina thickness (mm) Lamina thickness (mm) Lamina thickness (mm) ‘Sharp-based laminae. This may represent removal by mouthbar erosion during periods of high discharge. Kebroyd Bridge Kebroyd Bridge Kebroyd Bridge (see Figure 7) Asterisk denotes position of image in Fig. section 1 section 2 section 3 8a. with intercalated silty mudstone laminae seen in the Marsdenian extent the flood tide), with the sand-rich component being field examples (Figs. 8–10). rapidly deposited during periods of waning flow. The mud and The grain size of sediment in the fluvial channel also controls silt fractions are deposited during periods of either no or very laminae thickness characteristics in the sediment. In the case of low flow regime (i.e. during tidal slack water). the Marsdenian mouthbar facies, the sediment has a bimodal Within plume deposits, thick–thin pairs of sandstone laminae grain size, comprising very fine- to medium-grained sandstone are formed by the ebb and flood cycles within a single semi- and siltstone or silty mudstone, which is commonly rich in mica diurnal tidal cycle, with the thicker sandstone laminae interpreted flakes. Whereas the muddier component of this sediment is to have been deposited during the ebb stage (Fig. 11b). During carried in suspension, the coarser, sand-rich portion is carried by the ebb period, flow regime increases as fluvial inertia and either tractional or mixed saltation–suspension processes. By outgoing tide both flow in a downstream direction. The resultant inference, both mud and sand components are transported during higher flow regime increases the amount of entrained coarser periods of higher flow regime (i.e. the ebb tide, and to a lesser clastic sediment (typically very fine- to medium-grained sand-
  9. 9. M A R S D E N I A N C RY P T I C T I DA L S I G NAT U R E S 387 20 (a) 0 10 20 30 40 50 60 70 80 90 lamina number 60 43 lamina 40 20 (b) 0 10 20 30 40 50 lamina number 40 48 lamina 20 (c) 0 10 20 30 40 50 lamina number (d) 10 19.61 (Archer & Kvale, 1997) power spectral density 5 30.30 2.35-2.07 8.93 5.0-4.6 2.86-2.67 0 0 0.1 0.2 0.3 0.4 Fig. 10. Fourier analysis of lithofacies 3 frequency corresponding laminae cyclicity. (a) Bar chart of thickness range of sand-rich mouthbar facies laminae, from (this study) (Archer & Aitkenhead & Riley (1996). Hag Farm Kvale, 1997) (e) 20 1.9- 2.6 2.07- 2.86 Borehole (Kinderscoutian), near Keighley, 3.6- 4.4 4.6- 5.0 Yorkshire, UK. (b, c) Bar charts showing power spectral 10 thickness of mouthbar facies laminae from density 1.3 (this study) sections 1 and 3 of the Kebroyd Bridge 2.6 both =1.9 0 section (Fig. 9). Drawn to the same scale as 3.6 2.4 4.4 1.2 the bar chart of Aitkenhead & Riley (1996). -10 (d) Fourier analysis plot of data from (a) by 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 Archer & Kvale (1997). (e) Fourier analysis frequency plot of data from (b) and (c) by A. Archer Key for v) (University of Kansas), and corresponding Kebroyd Bridge Section 1 ranges between this dataset and that Kebroyd Bridge Section 3 displayed in (d). stone), and creates a seaward shift in the distribution of fine- to weaker, or subordinate tide, is formed by centripetal forces as the medium-grained sandstone. When the tide is at its lowest slack Earth rotates and pushes the ocean surface away from the Earth. water, the lack of flow allows deposition of mudstone, or more The dichotomy in tidal range during a 24 h period is known as commonly in the Marsdenian deposits, a micaceous veneer when the diurnal inequality. When this is applied to the model of viewed on laminae planes (Fig. 11b; low slack). The flood tide tidally influenced plume deposition, diurnal inequality should reworks underlying deposits, which are deposited as the tidal generate a periodicity of four, as it occurs over two semi-diurnal flow wanes, creating a thin muddy sandstone lamina that is periods. Diurnal inequality should produce a thin–thick cyclicity almost always thinner than the sandstone lamina deposited by the in alternate semi-diurnal tidal units (Fig. 12a), as the dominant ebb current. At high-tide slack water a second mudstone or a tide produces thicker sandstone laminae pairs and the subordi- micaceous lamina is deposited, completing the thick–thin, or nate tide produces thinner sandstone laminae pairs. semi-diurnal tidal cycle (Fig. 11b; high slack). Therefore, any set As the Moon rotates around the Earth, the Sun and the Moon of mouthbar deposits influenced by tidal processes may possess fall into alignment (or syzygy) or lie perpendicular (or in sandstone laminae producing repeated periodicities of two (Fig. quadrature), on a 14 day periodicity (Fig. 12b). When the Sun 11b; logs A and B). and Moon are in alignment, the tidal bulge is amplified by up to On any point of the Earth’s surface, two gravitational maxima 30%, increasing tidal ranges and creating stronger (or spring) pass in any 24 h period, creating two semi-diurnal tidal bulges of ebb and flood tides. When the Sun–Moon system is in quad- different magnitudes that are generated independently of each rature relative to the Earth, the combined gravitational forces are other. The stronger, or dominant, tide is created by the gravita- not as strong, and the resultant tidal bulge is smaller, creating tional pull of the Moon on the ocean surface, whereas the weaker (or neap) tides. Therefore, when 28 or more semi-diurnal
  10. 10. 388 M . J. B R E T T L E E T A L . Fig. 11. Processes responsible for the development of a cyclic tidal signature in heterolithic mouthbar facies (after Nemec 1995). Only during the lowest river discharges, when the saline wedge propagates headward allowing the lofting of sediment, do tidal currents enhance or suppress river outflow to the extent that it influences the rate of deposition and grain size deposited. units appear in a stacked succession of tidally influenced plume thick–thin–thick cycles over 43 and 48 laminae for sections 1 deposits, and no hiatal or erosive surfaces are present, it may be and 3 is suggestive of a spring–neap–spring cyclicity. Assuming possible to resolve a cyclical motif associated with lunar this represents a 14 day lunar cycle, the absence of a full set of precession. When deposition is influenced by a spring–neap 56 laminae may be due to reworking and erosion by fluvial cycle, periodicities of 56 laminae are expected (representing 28 processes, which can be demonstrated by the presence of scour semi-diurnal cycles deposited over 14 days). Deviations from the features (Fig. 7). Conversely, the number of days in a lunar expected cyclicity may occur in association with increased fluvial month was marginally greater in the past (c. 30.5 Æ 1.5 during discharge. In these circumstances, erosion of the substrate leads Precambrian time compared with 28.5 at the present day to stripping of laminae and the generation of discontinuities (Williams 1989), suggesting that more than 56 laminae would be within the tidal mouthbar unit. Therefore, if a plume is tidally expected within the spring–neap–spring cycle. The critical influenced, cyclicity in sandstone laminae should be observed on observation remains, however: that laminae systematically occur a semi-diurnal (two), a diurnal (four) and half-lunar monthly (56) in repeating sets of two (indicating semi-diurnal tides) and four periodicities. (indicating unequal semi-diurnal tides) with characteristic thick- ness variations. The integration of Fourier time-series analysis and the model for a tidally influenced mouthbar provides a useful Discussion: the significance of identifying cryptic tidal tool for the identification of cryptic tidal signatures within plume influences and its significance in the Carboniferous deposits. Such deposits have a higher preservation potential than sequence of the UK sediments deposited within areas where higher flow regimes The observation of systematic bed thickness variations has could either rework or erode part of the succession. suggested a tidal influence during mouthbar deposition, without The effect of tidal processes in settings dominated by macro- the necessity of identifying the presence of mudstone drapes. tidal ranges is well documented in modern deltas (Wright & Comparing results of the Fourier transform and the model for Coleman 1973, 1974; Coleman & Wright 1975; Galloway 1975; tidally influenced plume deposition suggests that laminae peri- Coleman 1981; Wiseman et al. 1986; Allen 1991), whereas the odicities of two and four represent semi-diurnal tides and subtle influence of tidal processes on areas with lesser tidal unequal semi-diurnal tides over a diurnal period. The presence of ranges is underrepresented in modern and ancient sedimentologi-
  11. 11. M A R S D E N I A N C RY P T I C T I DA L S I G NAT U R E S 389 Fig. 12. Model representing the depositional patterns expected from the tidally influenced plume during a diurnal and lunar 14 day cycle. Approximately every 24 h, any point on the Earth’s surface is influenced by two tides (a): a dominant tide created by the pull of the Moon, and a subordinate tide produced by centripetal forces that push the ocean surface away from the land. The differing tidal strengths are expressed by thinner (subordinate tide) or thicker (dominant tide) semi-diurnal units. The Moon revolves around the Earth every 28 days. During this period it either lies in alignment with the Sun (syzygy) or perpendicular to it (quadrature) (b). During alignment the combined gravitational pull of the Sun and Moon increases tidal bulges, generating a spring tide. When the Sun and Moon are at quadrature, the combined gravitational pull is less, and a weaker or neap tide occurs. Tidal ranges are greater during the spring tide, and tidally influenced mouthbar laminae occur in thin– thick packages every 56 tidal laminae (or 28 semi-diurnal laminae), equivalent to a 14 day period. cal literature. The presence or absence of a tidal regime may Conclusions have significant connotations for interpretation of sedimentary systems, regardless of the size of the tidal range. Tidal regimes The Namurian Pennine Basin has been interpreted by many have a significant effect on the coastline geometry (Wright & workers as an example of the quintessential fluvial-dominated Coleman 1973, 1974; Coleman & Wright 1975), grain-size delta. It has been suggested by most previous studies that the distribution at river mouths, or the cleaning or winnowing of Pennine Basin was geographically restricted from oceanic waters clay-grade particles from a succession (Orton & Reading 1993). and that tidal reworking had no effect on the deposition of This may be particularly true with respect to the degree of clay Namurian deltas. This paper has identified a stratigraphic interval redistribution that may occur in settings influenced by micro-tidal within the Namurian interval of the Pennine Basin that contains regimes (i.e. Fig. 6). facies that are influenced by tidal processes. We have identified Amplification of tidal range within an incised valley may facies within the Namurian interval that demonstrate tidal explain how one might expect a tidal current to be amplified processes operated, and that the Pennine Basin must therefore (Allen & Posamentier 1993); but it does not take into account have been connected to oceanic waters throughout deposition the complexity of depositional processes that occur within the and not only during marine band deposition. This new interpreta- valley; specifically, the influence of tidal processes on mouthbar tion requires that existing palaeogeographies are reassessed and plume deposits, especially when involving the mixing of saline implies that alternative depositional models should be sought for and non-saline waters in a fluvial regime that may be fluctuating the Namurian interval. in discharge (Wright & Coleman 1974; Nemec 1995; Hughes et The identification of cryptic tidal facies similar to those al. 1998a). The use of mud drapes and reactivation surfaces as described here in other basins may be useful in aiding the evidence for tidal influences is well reported (Ginsburg 1975; identification of tidally influenced systems and ascertaining the Visser 1980; Allen 1981; Siegenthaler 1982; Demowbray & true extent of palaeogeographical connectivity with oceanic Visser 1984; Nio & Yang 1991), but such structures may be water masses. absent in micro-tidal regimes or areas with low deposition rates. Within mouthbar-dominated incised valley fills the examination These findings form part of the Ph.D. thesis of the principal author. Thanks of successions deposited by plumes may reveal the presence of are due to J. Bagshaw for access to Fletcher Bank Quarry (Marshalls), A. tidal currents. Archer (University of Kansas) for running Fourier analyses on the datasets described in this paper, N. Riley and J. Macquaker for their critical Tidal deposits have been recognized in Southern North Sea reviews and A. Nuttall for assistance in the field. facies (O’Mara et al. 1999), whereas they have not in onshore outcrop (Collinson 1988). To ascertain that tidal currents influ- enced the Namurian Pennine Basin is regionally significant, as it confirms that selected onshore depositional systems form analo- Appendix: localities with examples of lithofacies 1–3 gues for Carboniferous Southern North Sea reservoirs (Hampson Examples of lithofacies 1 are observed at in the Woodhouse et al. 1997, 1999). Flags at Fosters Delph Quarry (SE022273; locality 12, Figs 3
  12. 12. 390 M . J. B R E T T L E E T A L . and 4a), the Midgley Grit at Moselden Heights Quarry, Scam- Carboniferous) deltaic sediments of the Central Pennine Basin, England. In: monden (SE043164; Fig. 4b) and the Midgley and Helmshore Curran, H.A. (ed.) Biogenic Structures, their Use in Interpreting Deposi- tional Environments. Society for Economic Paleontologists and Mineralogists, Grit at Fletcher Bank Quarries, Ramsbottom (SD805164). Special Publications, 35, 99–149. Examples of lithofacies 2 are observed in the Helmshore Grit Ewing, G.C. 1950. Slicks, surface films and internal waves. Journal of Marine at the upper part of Whittle-le-Woods Quarry (SD584217), Research, 9, 161–187. Warland Wood Quarry (SD947202), Harper Clough Delph Fenies, H., De Resseguier, A. & Tastet, J.P. 1999. Intertidal clay-drape couplets (Gironde estuary, France). Sedimentology, 46, 1–15. (SD716317) and Fletcher Bank Quarries, Ramsbottom Galloway, W.E. 1975. 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