1. Detrital Geochronology of the Nile
Matthew Simmons
Department of Earth Sciences
University College London
Student Number: 12032386
Supervisor: Dr. Pieter Vermeesch
GEOLM905: MSci Independent Project
Spring 2016
2. Abstract
This study measures U-Pb detrital zircon geochronology (DZ) of 36 samples across the entire Nile
basin, resulting in 1709 ages. Whilst the work of Garzanti et al. (2015) studied the Nile in terms of
bulk petrology and mineralogy, this project adds DZ using 3-way multi-dimensional scaling (MDS)
to find a consensus view for three provenance proxies. The DZ data shows two major peaks: a
2.4-2.9 Ga Archean peak attributed to the Tanzania Craton in the south and a broad polyphase
Pan-African cluster at 0.50-0.94 Ga from multiple sources along the course of the Nile. The influence
of the Pan-African orogeny is seen in all samples, overprinting in separate phases at 0.55, 0.65 and
0.94 Ga.
Despite providing 70% of the sediment load, the mafic volcanics of the Ethiopian Plateau (25-35
Ma) are only represented by 19 grains (1.19%) along the entire course of the Nile. This results in
the DZ analysis underestimating the input of the Blue and Atbara Nile. Mass balance calculations
using DZ age spectra indicate 20±8% total input of the White Nile, whilst Garzanti et al. (2015),
using heavy mineral and bulk petrology, calculates just 3±2%. Atbara samples are underestimated
in the DZ data, with 12±4% sediment input compared with the 36±4% estimated by Garzanti et al.
(2015). The cause of this is hypothesised to be the varying zircon fertility of major sediment sources.
The Ethiopian Plateau sediment factory produces very few zircons, making DZ studies blind to this
source. However, the Tanzania Craton has a significantly higher zircon fertility, and is thus over
represented in the DZ age spectra. Therefore, if this study had been completed with DZ alone,
the resulting sediment routing system and sand budget of the Nile would have been interpreted
differently.
This study tests a new workflow using a QEMSCAN scanning electron microscope, which allows
zircon coordinates to be exported to a LA-ICP-MS, enabling more grains to be dated in a shorter
amount of time. As a by-product of this workflow, heavy mineral information is exported, allowing
calculations of zircon concentration, in addition to information on sample grain size.
2
7. Chapter 1
Introduction
The Nile River, the longest river in the world,
drains an area of almost 3 x 106
km2
and is under-
lain by a wide range of lithologies, from Archaean
basements to Neogene sediments. This provides
an exceptional opportunity to study a sediment
routing system that covers a variety of climatic
and geomorphic regimes.
In sedimentology, provenance, derived from
the French verb provenir meaning “to come
from”, describes a study into the origin of sed-
iments and the path they took to their cur-
rent location. The provenance of a sediment is
controlled by tectonic setting (Dickinson et al.,
1983), in addition to climate, relief, and other
sedimentological factors (Dickinson, 1985). Many
different provenance tracers have been utilised
over the past half century, and these broadly clas-
sify into two categories:
1. Compositional, such as bulk petrology,
heavy mineral analysis and bulk geochem-
istry, which use a multi-mineral approach to
gain a vast amount of discrete data
2. Distributional, such as detrital zircon
geochronology, which uses a single mineral
multi-sample approach to generate continu-
ous data (Vermeesch and Garzanti, 2015)
Both single mineral and multi-mineral prove-
nance studies have advantages and disadvantages.
Multi-mineral studies create a large amount of
data, but are affected by hydraulic sorting and
dissolution effects during diagenesis. Single min-
eral studies are immune to such sedimentological
factors but are blind to certain sediment sources,
such as mafic volcanics.
Detrital zircon U-Pb geochronology has come
to the forefront of provenance studies in re-
cent years, with the age of individual grains
interpreted as the crystallisation age of the
source. The recent improvement of various tech-
niques and methods allows many grains to be
dated in a relatively short timescale, enabling
larger datasets and creating a powerful tool
for provenance studies (Thomas, 2011). Con-
sidering both distributional and compositional
datasets together forms a consensus view, which
has proved valuable in recent provenance stud-
ies (Garzanti et al., 2012; Rittner et al., 2016;
Stevens et al., 2013).
1.1 Research Aims
Nile sediment provenance has been analysed
in terms of mineralogical, geochemical and petro-
logical evidence over the past decade, although
until now the results of such studies have not
been tested with detrital U-Pb geochronology. A
review by Garzanti et al. (2015) provides a com-
prehensive overview of the chemical, petrographic
and mineralogical provenance studies, including
7
8. Detrital Geochronology of the Nile
537 heavy mineral point-counts, 438 framework
petrology samples and 230 geochemical analyses.
This study will complete the picture by adding
DZ and integrating the different proxies in a holis-
tic statistical analysis.
The highly variable geology across the Nile
catchment provides an excellent opportunity to
study the relative sensitivities of various prove-
nance proxies across different geomorphic regions.
The effect of one such sensitivity, zircon concen-
tration (the proportion of zircon within a sam-
ple due to the nature of the sediment source) is
thought to have a large impact on the Nile prove-
nance picture. It is hypothesised that detrital
zircon geochronology will underestimate the in-
put of Atbara and Blue Nile tributaries, both of
which are derived from mafic volcanics that pro-
duce minimal zircon grains and hence result in
low zircon concentration. This project will com-
pare the multi-proxy results with the detrital zir-
con data, quantifying the scale of any bias in order
to enhance future studies.
Additionally, this project will test a new
method of detrital zircon analysis, using a QEM-
SCAN scanning electron microscope to locate the
coordinates of individual grains that can then be
dated using an Laser Ablation Inductively Cou-
pled Plasma Mass Spectrometer (LA-ICP-MS).
This will present an opportunity to assess the po-
tential of the QEMSCAN as a provenance tool.
As a by-product, mineralogical compositions of
the samples are obtained. This data can be used
to calculate relative zircon concentration, using
the percentage of zircon within samples. The ef-
fectiveness of the QEMSCAN workflow will be
determined, including any corrections for varying
zircon concentration.
This report will first outline the hydraulic flow
of the Nile before reviewing the current litera-
ture in terms of Nile sediment and provinces using
geochemical and mineralogical evidence (chapter
2). The methods used during data collection and
analysis will be outlined in chapter 3, with a pre-
sentation of the results in chapter 4. This will be
followed by a discussion culminating in the full
provenance picture of the Nile.
Chapter 1. Introduction 8 Matthew Simmons, UCL
9. Chapter 2
The Nile River
2.1 Nile Hydrology
The Nile flows over 6700km northwards, cross-
ing 36 degrees of latitude including the equator
and the Tropic of Cancer and subsequently passes
through a number of climatic zones.
Adamson (1993) identified a north-westward
tilt in the lithosphere beneath Sudan and Egypt
due to magmatic upwelling, which is thought to
have initiated the initial drainage basin of the
Nile in the early Oligocene. The river flows
roughly between two continental blocks, the Pan-
African Orogen and the Saharan Metacraton
(Johnson et al., 2011), and cannot reach the sea
to the east due to the rift highlands, and so must
deviate over 2000km northwards until it reaches
the Mediterranean (Garzanti et al., 2015).
First studied by Hurst et al. (1959), the hy-
drology of the Nile is best described as a series
of different hydraulic regimes corresponding to
each tributary. Three main branches contribute
to the Nile: The Atbara, Kagera-White Nile and
Blue Nile. The southern equatorial branch, the
Kagera-White Nile, drains the rift highlands of
Burundi, Rwanda, Uganda and western Kenya.
The flow from Lake Victoria, Kyoga and Albert
is partly lost in South Sudan where the Sudd
Marshes, which cover an area of over 57,000km2
,
act as a natural filter, with only very fine parti-
cles passing through to join the main Nile flow.
The flow is restored at the northern edge of the
Sudd Marshes, whereby contribution of the Sobat
River from the east increases discharge to approx-
imately 30km3
yr−1
(Hurst et al., 1959).
The other two main branches, the Blue Nile
and the Atbara River drain the Ethiopian rift
highlands and have mean water annual fluxes of
48km3
and 11km3
respectively (Garzanti et al.,
2015). The sub-equatorial White Nile shows, in
parts, a perennial fluvial regime whereas the Blue
Nile primarily follows a seasonal regime, with
many tributaries drying out from December to
May (Hurst et al., 1959). This is illustrated by
an average flow of 15.23km3
during the month
of August, compared to the 0.32km3
in April
(Sutcliffe and Parks, 1999). A similar case is seen
in the Atbara Nile, where almost half of its 11km3
discharge also occurs in August.
North of the Atbara confluence in Sudan,
no significant tributaries contribute to the Main
Nile’s flow, with barely any rainfall across the
Sahara (Rz´oska, 1976) (Figure 2.2b). With the
White Nile flowing almost constantly throughout
the year, and the branches of the Blue and At-
bara Nile fluctuating seasonally, the White Nile
contributes 83% of low season flow, yet only a
small percentage of peak flow.
9
10. Detrital Geochronology of the Nile
Figure 2.1: (A) Summary of Nile hydrology, showing
the dominant nature of the Blue Nile. The White Nile
provides 30% of the flow, although it covers almost
half of the Nile basin in terms of area. The monthly
water flux graph demonstrates the perennial nature of
the White Nile, and the highly varying annual flow
of the Atbara and Blue Nile. (B) The sediment con-
tribution of each of the major rivers. The White Nile
contributes only 3% of the sediment, with the majority
derived from the Blue Nile. The grain size distribu-
tion of all tributaries shows the same pattern. Main
Nile sediment has a similar grain size to the Blue Nile,
while the White Nile has a smaller average grain size.
(Garzanti et al., 2015)
2.2 Nile Sedimentology
2.2.1 Sand Budget
The result of the highly productive sediment
factory of the Ethiopian Plateau is seen in the
dominant contribution of the Atbara and Blue
Nile to main Nile flow (Garzanti et al., 2015).
Figure 2.1 represents the mean annual sed-
iment flux, showing that just 3% of all sedi-
ment is derived from the White Nile, despite
draining over half of the basin. Other trib-
utaries, such as Wadi Milk to the west, con-
tribute very little to the resulting sediment load
(Garzanti et al., 2006). These results, sum-
marised in Garzanti et al. (2015), will be used in
chapter 5 to compare the effectiveness of using
detrital zircon geochronology to calculate relative
sediment budgets.
2.2.2 Sediment Provinces
A comprehensive review by Garzanti et al.
(2015) pinpointed the main anorogenic provinces
associated with this tectonic setting, which con-
sist of crystalline basements, (continental clock
and rift shoulder) large igneous provinces, (anoro-
genic volcanic (Garzanti et al., 2014) or volcanic
rift provenance) and recycling of syn-rift sedi-
ments (recycled clastic province). Compositional
studies can identify the provenance of Nile sedi-
ments, with each province having a distinct sig-
nature.
Chapter 2. The Nile River 10 Matthew Simmons, UCL
11. Detrital Geochronology of the Nile
Figure 2.2: (A) Climatic zones of the Nile using the K¨oppen-Geiger climate classification (K¨oppen, 1954, 1918), showing the large range
of climates (B) Rainfall map of the Nile river, showing rainfall of over 1500 mm in Uganda and north-western Ethiopia, with almost no
precipitation across the deserts of Egypt and Sudan. Modified from Garzanti et al. (2015)
Anorogenic Volcanic Province
Detritus from continental flood basalts represents
60-70% of the Main Nile’s suspended load, the
majority of which are derived from the Ethiopian
Plateau (Garzanti et al., 2015). Both the Atbara
and Blue Nile drain this large igneous province,
which is composed of a 3km thick succession of
alkaline to tholeiitic flood basalts, dated at 30-35
Ma (Hofmann et al., 1997; Ukstins et al., 2002).
The other sources of volcaniclastic sediment
within the Nile basin include the eastern branch
of the East African Rift in Kenya, which pro-
duces a large range of alkali olivine basalts
(Chorowicz, 2005), and the Virunga volcanoes in
north-western Rwanda, which consist of potas-
sic lavas (Rogers et al., 1992). These sources can
be distinguished by heavy mineral and geochem-
ical analysis, however it is unlikely that signals
corresponding to these volcanic provinces will be
detected during the U-Pb analysis of this study.
The mineralogical data of the Ethiopian
Plateau, Virunga volcanoes and Kenyan rift
shoulders reveals an absence of zircon, highlight-
ing the need to combine geochronological data
with compositional studies and the need to con-
sider zircon concentration when analysing age
spectra.
Continental Block
The Pan-African Orogeny represents a number of
orogenic belts in Africa formed during a major
Neoproterozoic collision. In the Nile Basin, it is
shown in the overprinting of Archean and Meso-
proterozoic basement cratons and juvenile crust.
The polyphase amphibole facies metamorphism
underlies a large area of the Nile drainage basin
Chapter 2. The Nile River 11 Matthew Simmons, UCL
12. Detrital Geochronology of the Nile
and is eroded to form metamorphiclastic sedi-
ments in all branches of the Nile. The Ouban-
guides and Mozambique orogenic belts are ex-
posed in the south and east of the Nile basin
(Bumby and Guiraud, 2005). As a result, concor-
dia diagrams of detrital zircon analysis are ex-
pected to show traces of overprinting, with mix-
ing lines intercepting concordia at approximately
650 Ma.
The Saharan Metacraton is a continental
block that has been remobilised during several
metamorphic events. It covers 5,000,000km2
of northern Africa, forming the basement rocks
of Egypt, Sudan and Libya, and is a major
source of sediment in the northern Nile Basin
(Abdelsalam et al., 2002). Sediment from the
metacraton is not only delivered fluvially; aeo-
lian sediments are also transported from the Sa-
hara in Egypt (Garzanti et al., 2015). Originally
Archean in age, in some areas the alteration of the
Pan-African orogeny has created Neoproterozoic
juvenile crust.
Pre-Neoproterozoic rocks are exposed as up-
lifted massifs within the Saharan Metacraton.
The most prominent of these inliers within the
Nile basin are the Darfur (2.1-2.2 Ga), Bayuda
(0.9 Ga) and Nubian Dessert (1.2-2.2 Ga), as
shown in Figure 2.2. The general detrital age
spectra across the entire Nile is expected to show
predominantly Neoproterozoic ages, with rare
Mesoproterozoic and Archean grains, due to the
extensive nature of the Pan-African orogeny.
The Arabian-Nubian shield is a collage of ju-
venile Neoproterozoic oceanic crust and enclaved
pre-Neoproterozoic terranes, formed 650Ma to
542Ma (Johnson et al., 2011), that forms the Red
Sea Hills. This province is covered by the Nu-
bian Sandstone, a thick sequence of carboniferous
sandstones and carbonates.
The southern area of the Nile Basin in
Uganda, Rwanda, Burundi and Tanzania erodes
the Tanzania Craton. The ages of this continental
block have been recorded between 2.4 and 3.2 Ga,
with peaks at 2.6 and 2.9 Ga (Andriessen et al.
(1985); Link et al. (2010); M¨oller et al. (2000);
Tenczer et al. (2012)).
To the north of Lake Victoria, the Watian
Granulite Complex, dated at 2.75 - 2.9 Ga un-
derlies the Aruan Gneiss complex, dated at 2.4
- 2.6 Ma, both of which make up the Western
Granulites of Uganda (Leggo, 1974) and are re-
sponsible for the two age peaks of the region. The
Watian metamorphic event reached granulite fa-
cies and was overprinted by the Aruan thermal-
tectonic event which peaked at amphibolite facies
(Appel et al., 2005).
The Tanzania craton is bounded to the west
by the Mesoproterozoic Kibara Belt, a 1500km
long orogenic zone from the Democratic Republic
of Congo (DRC) in the south, to Uganda in the
north Fernandez-Alonso et al. (2012). The sec-
tion of this elongated fold belt within the Nile
basin is termed the Karagwe-Ankolean belt, al-
though is referred to as the Kibaran during this
report. Multiple metamorphic episodes are seen
to alter sediments derived from the Tanzania
craton (Andersen and Unrug, 1984) and are ex-
pected to show Mesoproterozoic detrital zircon
ages of 1.2-1.5 Ga in the southern Nile Basin.
The basement of South Sudan is composed of
mostly pre-Neoproterozoic crust with a Neopro-
terozoic–early Cambrian tectono-thermal over-
print (Collins, 2006; Fritz et al., 2005, 2013;
Viola et al., 2008). The Ruwenzori fold belt lies
Chapter 2. The Nile River 12 Matthew Simmons, UCL
13. Detrital Geochronology of the Nile
to the north-west of the Tanzania craton and
is exposed in the southern Nile Basin. Meta-
morphosed tholeiitic lavas make up the result-
ing Buganda-Torro supergroup, dated at 2.1-2.2
Ga (Tanner, 1973). The majority of source
rocks are metamorphosed to some degree by the
Pan-African orogeny, although the Nile Basin
does contain several areas of pre-rift unmetamor-
phosed sedimentary successions. Garzanti et al.
(2015).
In the north of the Nile basin, crystalline base-
ment of the Red Sea rift shoulder is overlain by
Paleozoic to Mesozoic sedimentary strata that
are a source of zircons. Neogene syn-rift sed-
iments from the East African Rift system are,
in some locations, tectonically inverted and have
been shown to carry sediments enriched in quartz
(Garzanti et al., 2015), indicating a recycled clas-
tic province.
2.2.3 Nile Sediments
The White Nile
The White Nile is sourced in Uganda, where
it drains the Archaean gneiss-granulite complex
(Link et al. (2010)). Past Lake Victoria, the
White Nile carries sediment derived from gran-
itoids of the Tanzania craton. Garzanti et al.
(2015) identifies a quartzose sand south of Lake
Kyoga, until drainage of amphibole facies gneisses
introduces feldspatho-quartzose metaorphiclastic
sand.
The Albert Nile contains sediments recy-
cled from the Neogene deposits of the Albertine
graben. Continued drainage of high grade meta-
morphic strata promotes another introduction of
metamorphiclastic sands. A transition into an
arid climate, downstream of the Sudd in South
Sudan creates monocrystalline quartz with re-
worked caliche grains.
The Sudd, which translates from Arabic as
”dam”, filters White Nile sediment, allowing only
finer mud particles through. As a result, sed-
iments display the geochemical signature of re-
cycled quartzose sediments, and show little sign
of metamorphic basement downstream of the
marshlands. This natural barrier should be rep-
resented in this study’s detrital zircon data, with
very few Archean ages after the Sudd marsh.
The Blue Nile
Sediments of the Blue Nile are derived from three
distinct provinces. In Ethiopia, flood basalts and
rhyolitic ignimbrites contribute volcaniclastic sed-
iment. Further downstream, pre-rift sediments
including Mesozoic sandstones and carbonates are
eroded, followed by amphibole facies Neoprotero-
zoic basement of the Pan-African orogeny. Sed-
iment composition, analysed by Garzanti et al.
(2015), represent the mixed provenance, with var-
ied amounts of volcanic lithics relative to quartz
and feldspar.
Garzanti et al. (2015) also finds the geochem-
ical signature of Blue Nile sediments to be sim-
ilar to the average concentration of upper crust
(UCC), although shows signs of basaltic input.
The suspended sediment is shown to be equal to
that of the Ethiopian Traps, confirming the origin
of the suspended sediment.
The Atbara
The upper course of the Atbara river drains
the Ethiopian Plateau and as a result, its sed-
iments contain volcanic lithics with the same
mineralogical and geochemical signature of the
Chapter 2. The Nile River 13 Matthew Simmons, UCL
14. Detrital Geochronology of the Nile
Tanzania Craton
Archean Granulites
(2.6 -2.8 Ga)
Ruwenzori Fold Belt
(2.1-2.2 Ga)
Kibaran Fold Belt
(1.2-1.5 Ga)
Saharan Metacraton
Nubian Dessert Archean Crust
(2.6 Ga and 0.65 Ga)
Darfur Massif Inlier
(2.1-2.2 Ga)
Mesoproterozic crust
(1.2-1.8 Ga and 0.65 Ga)
Arabian-Nubian Shield
Juvenile Oceanic Crust
(0.87 - 0.55 Ga)
Bayuda Massif Inlier
(0.8-1.0 Ga)
Reworked Archean Crust
(2.6 Ga)
Pan-African
Fold Belts
Mesoproterozoic Metasediments
(0.87-0.55 Ga)
DA
ND
ND
BA
KB
TC
TC
OB
MB
ANS
Figure 2.3: A: Simplified geological map modified from Begg et al. (2009) and Abdelsalam et al. (2002) with published ages of East Africa
(see table 2.1), B: Simplified geological map of the Nile Basin, with general sediment provinces highlighted. ANS = Arabian-Nubian Shield,
BA = Bayuda Massif, DA = Darfur Massif, KB = Kibara Belt, ND = Nubian Dessert, MB = Mozambique Belt (Pan-African), TC =
Tanzania Craton, OB = Oubanguides orogenic belt (Pan-African) Data from Al-Shanti et al. (1979); Andriessen et al. (1985); Barth et al.
(1983); Cahen and Snelling (1966); Curtis and Lenz (1985); Deutsch and Klerkx (1977); Dixon (1981); Finger et al. (2008); Harms et al.
(1990, 1994); Key et al. (1989); Kr¨oner et al. (1987); K¨uster and Li´egeois (2001); Maboko (1995); Maboko et al. (1989); Meinhold (1979);
M¨oller et al. (2000); Pegram et al. (1976); Pin and Poidevin (1987); Ries et al. (1985); Stanley et al. (2003); Stern and Dawoud (1991);
Stern et al. (1994); Sultan et al. (1994); Tenczer et al. (2012)
Ethiopian Traps. Further downstream, the At-
bara erodes pre-rift sedimentary strata followed
by greenschist facies Neoproterozoic basement
rocks (Garzanti et al. (2015)). The majority
of sediment load is derived from the Ethiopian
Traps. Detrital zircon studies should identify the
Neoproterozoic basement, with the low zircon fer-
tility of the Ethiopian traps resulting in a misrep-
resentation of the main volcanic source.
The Main Nile
The Main Nile represents 2600km of flow from
the Atbara confluence to the Nile Delta, erod-
ing mainly basement rocks of the Arabian-Nubian
shield and Saharan Metacraton. Very little fluvial
sediment contributes to the sediment load due to
the hyper-arid conditions in northern Sudan and
Egypt. As a result, the composition of the Main
Nile is similar to that of the Blue Nile, with ad-
ditional volcanic lithics from the Atbara.
Wind blown sediment from the Sahara mixes
with the fluvial deposits in Egypt, causing quart-
zose dilution. The source of the quartz is the
Western Desert in Egypt, which contains 96%
monocrystalline quartz (Hereher, 2009).
Strong long-shore drift processes act upon
the south-eastern Mediterranean, and influ-
ence of Nile sediment on beaches can be
traced as far as Akko in northern Israel
(Rohrlicht and Goldsmith, 1984). The heavy min-
eral composition of the Nile Delta reflects this,
as shown by Stanley (1989), through signals of
Ethiopian Trap volcanics found on the beaches of
Chapter 2. The Nile River 14 Matthew Simmons, UCL
15. Detrital Geochronology of the Nile
the Levant.
2.3 Published Age Data
A large range of geochronological data has
been recorded across the Nile catchment, from
Archean granulites of the Tanzania Craton, to the
Palaeogene volcanics of the Ethiopian Plateau.
Table 1 shows the result of an extensive litera-
ture survey, listing published ages from a variety
of different methods. The locations of each study
are plotted on the geological map of Figure 2.3.
The oldest published ages are recorded in the
Tanzania craton at approximately 2.9-3.2 Ga.
Other Archean ages are recorded in fragments
of the Saharan Metacraton in Sudan and Egypt
(Fernandez-Alonso et al., 2012). The other ma-
jor peak identified in the literature is the Pan-
African polyphase orogeny, with ages of 540-
720 Ma (Appel et al., 2005; Pegram et al., 1976;
Pin and Poidevin, 1987).
Published Geochronological Data from East Africa
Reference Source Country Method Age (Ma)
Sultan (1994) Anorthositic Gabbro Egypt U-Pb 2629
Anorthositie Egypt U-Pb 2063
Arabian Sheild Egypt U-Pb 2141
Arabian Sheild Egypt U-Pb 2063
Arabian Sheild Egypt U-Pb 2141
Arabian Sheild Egypt U-Pb 634
Finger (2008) Pan African Granitoids Egypt U-Pb 622
Tenczer (2012) Tanzania Craton Tanzania U-Pb 2640
Tanzania Craton Tanzania U-Pb 2680
Tanzania Craton Tanzania U-Pb 2800
Tanzania Craton Tanzania U-Pb 2800
Moller (2000) Pan African Granitoids Tanzania U-Pb 640
Cahen and Snelling (1966) Pan African Granitoids Tanzania K/Ar 590
Andriessen (1985) Granulite Complex South Tanzania K/Ar 630
Maboko (1989) Granulite Complex Burundi Ar/Ar 635
Granulite Complex East Tanzania Ar-Ar 420
Maboko and Nakamura (1995) Uluguru Granulite Complex East Tanzania Sm/Nd 630
Uluguru Granulite Complex East Tanzania Sm/Nd 620
Pegram et al. (1976) Tebesti Granite Libya Rb/Sr 590
Klerkx and Deutsch (1977) Uweinat ring complexes Libya Rb/Sr 2620
Abdel-Monem at al (1979) Wadi Rasheid Gneiss East Egypt U-Pb 1800
Meinhold (1979) Bayuda Rahaba Gneiss Sudan Rb/Sr 870
Bayuda Kurmut Gneiss Sudan Rb/Sr 760
Bayuda Nabati Granite Sudan Rb/Sr 570
Kuster and Liegeois (2001) Bayuda Dam Amphibolite Sudan Rb/Sr 810
Continued on next page
Chapter 2. The Nile River 15 Matthew Simmons, UCL
16. Detrital Geochronology of the Nile
Published Geochronological Data from East Africa
Harms et al (1994) Nubian Desert Gneiss Sudan Sm/Nd 546
Nubian Desert Gneiss Sudan U-Pb 749
Nubian Desert Granite Sudan U-Pb 718
Nubian Desert Granite Sudan U-Pb 680
Nubian Desert Granite Sudan U-Pb 661
Stern et al. (1994) Wadi Halfa Granite Sudan Rb/Sr 653
Wadi Halfa Granite Sudan Rb/Sr 581
Wadi Halfa Granite Sudan Rb/Sr 530
Duweishat Gneiss Sudan U-Pb 2428
Duweishat Gneiss Sudan U-Pb 719
Duweishat Gneiss Sudan U-Pb 2406
Duweishat Gneiss Sudan U-Pb 1744
Stern and Dawoud (1991) Jebel Moya Charnockite Sudan U-Pb 742
Jebel Moya Granite Sudan U-Pb 744
Jebel Moya Enderbite Sudan U-Pb 739
Harms et al. (1990) Jebel Karail Gneiss Egypt Rb/Sr 673
Nubian Desert Gneiss Sudan Rb/Sr 918
Nubian Desert Granite Sudan Rb/Sr 623
Wadi Hower Migmatite Sudan Rb/Sr 565
Wadi Hower Granite Sudan Rb/Sr 686
Jebel Rahib Gneiss Sudan Rb/Sr 562
Bir Safsaf Gneiss Sudan Rb/Sr 585
Bir Safsaf Granite Egypt Rb/Sr 578
Snelling and Cahen (1984) Gebel Uweinat Libya Rb/Sr 2665
Key et al (1989) Mukogodo Migmatite Kenya Rb/Sr 1234
Lolkoitoi Gneiss Kenya Rb/Sr 622
Kotim Gneiss Kenya Rb/Sr 818
Poloi Granite Kenya Rb/Sr 826
Morupusi Granite Kenya Rb/Sr 572
Lauraki Granite Kenya Rb/Sr 582
Lolmungi Granite Kenya Rb/Sr 566
Wust (1989) Wadi Miyah Greywacke Egypt U-Pb 831
Wadi Miyah Greywacke Egypt U-Pb 2410
Wadi Miyah Greywacke Egypt U-Pb 2300
Wadi Allaqi Greywacke Egypt U-Pb 820
Wadi Allaqi Greywacke Egypt U-Pb 1460
Wadi Allaqi Greywacke Egypt U-Pb 2400
Wadi Allaqi Greywacke Egypt U-Pb 2450
Continued on next page
Chapter 2. The Nile River 16 Matthew Simmons, UCL
17. Detrital Geochronology of the Nile
Published Geochronological Data from East Africa
Pin and Poidevin (1987) Mpoko Charnockite C.A.R Sm/Nd 833
Lere Granulite C.A.R U-Pb 652
Sibut Charnockite C.A.R U-Pb 639
Kroner et al (1987) Sabaloka Granulite Sudan U-Pb 2650
Sabaloka Granulite Sudan Rb/Sr 720
Sabaloka Migmatite Sudan Rb/Sr 572
Sabaloka Granite Sudan Rb/Sr 543
Ries et al. (1985) Bayuda Abu Harik granite Sudan Rb/Sr 898
Bayuda Abu Hamed Quartzite Sudan Rb/Sr 761
Bayuda Shallai Granite Sudan Rb/Sr 549
Bayuda Deifallab Granite Sudan Rb/Sr 678
Bayuda El Kuro Metavolcanic Sudan Rb/Sr 800
Curtis and Lenz (1985) Jebel Ed Dair Granite Sudan Rb/Sr 542
Jebel Liri Granite Sudan Rb/Sr 684
Barth et al. (1983) Bayuda Nabati Granite Sudan Rb/Sr 573
Dixon (1981) Wadi Mobarak Cobble Egypt U-Pb 2300
Wadi Mobarak Cobble Egypt U-Pb 1120
Wadi Mobarak Cobble Egypt U-Pb 1860
Wadi Mobarak Cobble Egypt U-Pb 2060
Table 2.1: Published geochronological data from East Africa: Locations of all age measurements are represented on Figure 2.3.
The source rocks correlate with sources of sediment within the Nile basin, hence ages in this table are expected to be measured in the DZ
study.
Chapter 2. The Nile River 17 Matthew Simmons, UCL
18. Chapter 3
Methods
3.1 U-Pb geochronology
Found in metamorphic and acidic igneous
rocks, zircon (ZrSiO4) incorporates approxi-
mately 0.1 wt.% radioactive uranium and tho-
rium (Deer et al., 2011), and virtually no lead
during crystallisation. Its durability and mini-
mal alteration make zircon the ideal mineral for
detrital geochronology. Within zircon, 235
U, 238
U
and 232
Th form the start of the U-Th-Pb decay
series, whereby multiple α- and β emissions re-
sult in various isotopes of Pb, as represented in
equations 3.1, 3.2 and 3.3 below:
235
U → 207
Pb + 7α + 4β + 45MeV (3.1)
238
U → 206
Pb + 8α + 6β + 47MeV (3.2)
232
Th → 208
Pb + 6α + 4β + 40Me (3.3)
Multiple daughter decay chains can be regarded
as simple parent-daughter systems if the half life
of the parent is far longer than any of the inter-
mediate daughter isotopes, hence the system is in
secular equilibrium. This condition is met in each
of the three decay series of the U-(Th-)Pb system.
As a result, three chronometers can be utilised:
U-(Th-)Pb, Pb-Pb (both used in this study) and
U-(Th-)He method.
3.1.1 The U-(Th-)Pb method
Decay products of uranium include 206
Pb,
207
Pb and 208
Pb, the age equations of which are
show in equations 3.4, 3.5 and 3.6 (Holmes, 1946),
below:
t206 =
1
λ238
ln
206
Pb∗
238U
+ 1 (3.4)
t206 =
1
λ235
ln
207
Pb∗
235U
+ 1 (3.5)
t206 =
1
λ232
ln
208
Pb∗
232Th
+ 1 (3.6)
With λ238, λ235 and λ232 representing the decay
constants of 238
U, 235
U and 232
Th and Pb* repre-
senting common lead, which in zircon can be ne-
glected due to minimal non-radiogenic Pb, hence
Pb ≈ Pb*. Creating two geochronometers in one,
this method is a powerful dating tool due to an
ability to quality-check findings using different
isotopes of the same parent-daughter pair.
3.1.2 The Pb-Pb method
Similar to that of the U-(Th-)Pb system, the
Pb-Pb method uses the natural ratio of 235
U/235
U
of 137.88 to eliminate the need to measure ura-
nium. The main advantages include the insensi-
tivity to U or Pb loss, as this would not alter the
isotopic ratios of Pb. This method is rarely used
alone, with a combination of both the U-(Th-)Pb
and Pb-Pb systems commonly used.
Sections 3.2 and 3.2 detail the methodology
used to measure and analyse the isotopic ratios
of the U-Pb system.
18
19. Detrital Geochronology of the Nile
3.2 Data Collection
3.2.1 Sample Preparation
Professor Eduardo Garzanti from the Univer-
sity of Milano-Biocca provided 32 samples that
had undergone heavy mineral separation by cen-
trifuging in sodiumpolytungstate (with a density
of 2.90 g/cm3
), and recovering by partial freezing
with liquid nitrogen. The heavy mineral concen-
trates were mounted to a slide with epoxy resin
and polished, before being coated in carbon.
15 additional samples were concentrated
in non-magnetic heavy minerals before being
mounted on a slide. This was achieved by re-
moval of magnetite via a hand magnet, followed
by the use of a Franz Ferromagnetic Separator.
3.2.2 QEMSCAN Analysis
Utilised as an alternative to conventional
heavy liquid techniques, the QEMSCAN is a scan-
ning electron microscope (SEM) system, designed
to provide rapid quantitative mineral analyses for
the mining industry (Goodall et al., 2005). Dur-
ing a conventional study, individual zircon grains
would be concentrated using heavy liquid min-
eral separation, such as Di-iodomethane (DIM)
or Tetrabromoethane (TBE). This process is time
consuming and has a limited recovery of small
samples, hence would greatly restrict the poten-
tial number of zircons dated during this study.
During QEMSCAN analysis, X-ray spectra
are rapidly measured at a user-defined pixel spac-
ing and are compared against a Species Identifica-
tion Protocol (SIP), a reference database of min-
erals. This results in the qualitative elemental
composition of each pixel. The QEMSCAN soft-
ware then filters the data to eliminate unwanted
substances and stitch the collection of pixels to-
gether, creating a map of the slide. In this study,
the X-Y coordinates of all zircon grains, includ-
ing lithics that contain zircon inclusions, are ex-
ported and loaded into custom built Coordinates
Converter software (COCO) to convert to laser
stage coordinates.
The QEMSCAN is a powerful tool that is not
limited to finding zircon grains. A rich database
of measurements is recorded during every sample
run. This study made use of this otherwise excess
data to estimate relative zircon concentration and
average grain size. By using the number of pix-
els registered as zircon, and comparing to the to-
tal number of pixels (excluding that of the glass
slide) it is possible to obtain the relative propor-
tion of zircon within the slide, and the average
area of each zircon grain. Calculating the pro-
portion of zircon from heavy mineral point count
data is insufficient; zircon is represented by very
few grains as only 200-250 grains are counted per
sample. The QEMSCAN enables a precise calcu-
lation with upwards of 20,000 grains per sample.
3.2.3 Laser Ablation ICP-MS
LA-ICP-MS (Laser Ablation Inductively Cou-
pled Plasma Mass Spectrometry) is a powerful
tool used to record the relative concentrations of
elements and their isotopes. Compared to other
elemental methods, LA-ICP-MS has greater pre-
cision and speed, with each zircon taking less than
30 seconds to analyse.
LA-ICP-MS U–Pb dating was performed at
the London Geochronology Centre (LGC), us-
ing a New Wave NWR 193nm laser ablation sys-
tem coupled to an Agilent 7700 quadrupole-based
ICP–MS. Using the output coordinates from the
QEMSCAN, the location of zircons within the
slide are loaded into LA-ICP-MS software. The
mass spectrometer was set to measure 91
Zr, 206
Pb,
Chapter 3. Methods 19 Matthew Simmons, UCL
20. Detrital Geochronology of the Nile
A B
Figure 3.1: (A) Slide maps created by the QEMSCAN, showing positions of certain grains within heavy mineral separates. The coordinates
of each mineral can be exported to the LA-ICP-MS, with zircon grains located with ease. Zircon, coloured in blue, is seen in the left hand
Blue Nile sample. Rutile, coloured in red, is abundant in the right hand sample of the Albert Nile. (B) Photograph of the LA-ICP-MS
laser focused on a large zircon grain, found automatically by the QEMSCAN.
207
Pb, 208
Pb, 232
Th, 235
U and 238
U. Measurement
of zirconium ensured only the ages of zircon grains
were included in the age calculations. Electronic
mass spectrometry signals were converted to iso-
topic ratios and then ages using GLITTER pro-
cessing (see section 3.2.4)
The durability of zircon enables grains to sur-
vive metamorphic events. A zircon may be zoned
with different zones yielding different ages. These
zones can be identified before the laser spot is
focussed. However, this may lead to a sam-
pling bias, and so the centre of each zircon grain
was used for analysis. Any remaining change
in the 206
Pb/238
U ratio can be detected and fil-
tered out in the GLITTER selection software
(Griffin et al., 2008) (see section 3.2.4 below).
3.2.4 GLITTER Processing
The raw output files from the ICP-MS were
corrected for fractionation occurring within the
mass spectrometer using the GLITTER data re-
duction software (Griffin et al., 2008). This pack-
age corrects the time-resolved mass spectra for
206
Pb, 207
Pb, 208
Pb, 232
Th, 235
U and 238
U using
the sample-standard bracketing method whereby
signals are normalised to a standard, in this case
the Pleˇsovice zircon (Sl´ama et al., 2008).
The output file from the LA-ICP-MS enabled
the two separate geochronometers to be mea-
sured: 206
Pb/238
U and 207
Pb/206
Pb. As men-
tioned in section 3.1.1, this makes the U-Pb dat-
ing method robust, with a comparison of the
two methods enabling a more confident result,
and also allows indication of open system be-
haviour. Grains that have undergone metamor-
phism will show different ages for 206
Pb/238
U and
207
Pb/206
Pb methods, due to the loss of lead or
uranium. Plotting the two ratios against each
other forms a concordia curve (Wetherill, 1956),
highlighting concordant grains (zircons yielding
similar values for both systems) and discordant
grains (zircons that have undergone Pb or U loss
during exposure to an open system), a useful tech-
Chapter 3. Methods 20 Matthew Simmons, UCL
21. Detrital Geochronology of the Nile
nique given the extensive metamorphism across
the Nile basin (Andriessen et al., 1985; Dingeldy,
1999; M¨oller et al., 2000).
3.3 Data Analysis
This study created a large multivariate prove-
nance dataset, comprising not only the acquired
detrital zircon (DZ) data, but also heavy-mineral
(HM) and bulk-petrography (PT) data from 36
samples (Table 3.2). Whilst each proxy was anal-
ysed separately using the statistical technique
known as non-metric multi-dimensional scaling
(MDS), the use of 3-way MDS combining sin-
gle MDS maps, enabled interpretation of mul-
tiple samples with multiple proxies, as success-
fully used in recent studies (Stevens et al., 2013;
Vermeesch, 2013).
The first stage of a provenance study is to per-
form a sample size calculation in order to identify
what constitutes a representative sample. As the
sample size increases, the likelihood of missing
significant fractions of the detrital age spectrum
decreases. The minimum sample size needed to
include fractions as small as 5% of the total, at the
95% significance level, is 117 (Vermeesch, 2004).
Samples were combined into geographically sim-
ilar groups that together reached an appropriate
number of zircon ages, and hence a representative
sample size, although it is not possible to form ev-
ery group to the target 117 grains. The smallest
DZ sample in the dataset has 46 zircons. Using
the methods of Vermeesch (2004), the probability
at the 95% confidence level that a 10% fraction
of the whole spectra was missed is just 8%. This
worst case scenario assumes a uniform distribu-
tion, although ages of the Nile Basin generally
shows a trimodal distribution (Pan-African, Pa-
leoproterozoic or Archean ages), hence 46 grains
should be adequate to identify the three main
peaks. Nevertheless, sample size calculations for
all groups are provided in the results chapter. DZ
data was processed to remove discordant grains
that fall greater than 10% above or 15% below
the concordia curve. With two potential ages for
each grain 206
Pb/238
U and 207
Pb/206
Pb, it is nec-
essary to choose which is the“best age”. The anal-
ysis of 207
Pb/206
Pb exhibits greater imprecision
for younger zircons than the 206
Pb/238
U age. The
point at which it becomes more precise to use the
207
Pb/206
Pb age has been approximated at 1.1
Ga. Therefore, a preferred age for each grain was
calculated by using the 206
Pb/238
U below 1.1 Ga,
and 207
Pb/206
Pb for older grains. To visualise DZ
data, Kernel Density Estimators (KDE), which
are the most reliable statistical method to repre-
sent age spectra (Vermeesch, 2012) were created
for sample groups with a large number of grains.
For samples with low numbers of zircons, Cumu-
lative Age Distributions (CAD) were created as
a way of visualising age spectra without the need
for smoothing, and are produced for samples with
fewer than 60 grains. Mineralogical and geochem-
ical data taken from Garzanti et al. (2015) was
sorted to extract samples corresponding with this
study.
PT data was analysed in the form of QFL
and lithic composition ternary diagrams as in
Garzanti et al. (2015), and HM data in the form
of pie charts.
Chapter 3. Methods 21 Matthew Simmons, UCL
22. Detrital Geochronology of the Nile
Group River DZ PT HM
D1 Nile Delta x x x
D2 Nile Delta - x x
M1 Main Nile x x x
M2 Main Nile x x x
M3 Main Nile x - x
WM1 Wadi Milk x x x
GT1 Goha Tsiyon - x x
AT1 Atbara x x x
AT2 Atbara x x x
B1 Blue Nile - x x
B2 Blue Nile x x x
B3 Blue Nile x x x
B4 Blue Nile - x x
B5 Blue Nile x x x
W1 White Nile x - x
W2 White Nile x - x
SO1 Sobat x x x
BA1 Bahr el Jebel x - x
BA2 Bahr El Jebel x x x
AL1 Albert Nile x x x
V1 Victoria Nile x x x
K1 Kagera x - x
Total = 18 19 24
Table 3.1: Table of the available proxies for each group. DZ = Detrital Zircon, PT = Bulk Petrography, HM = Heavy Mineral analysis.
x = available, - = unavailable.
3.3.1 Multi-dimensional Scaling (MDS)
MDS is a dimension-reducing technique that
has been used to interpret large datasets by
other branches of science for decades, although
only recently applied to provenance studies
(Vermeesch and Garzanti, 2015). MDS plots
samples as a “map”, with similar samples plotting
close together, and dissimilar further apart. Built
within the statistical programming language R,
the Provenance package (Vermeesch, 2015) was
used to create non-metric MDS plots, by con-
verting the raw data into dissimilarity matrices.
For the distributional DZ data, the Kolmogorov-
Smirnov statistic is used. The dissimilarity of
compositional data, such as HM and PT, is cal-
culated using the Bray-Curtis distance, since the
raw data contained zero values. The position of
samples in an MDS plot is likely the contain sig-
nificant error. This error can be estimated in
MDS via the “stress value” (Vermeesch, 2013).
With that in mind, MDS maps must not be over
interpreted, but used to find relative positions of
one sample to another.
MDS maps are provided for each provenance
proxy, although to simply the interpretation fur-
ther, the maps were combined to a single plot
Chapter 3. Methods 22 Matthew Simmons, UCL
23. Detrital Geochronology of the Nile
in the form of a Generalised Procrustes Analysis
(GPA) arrangement. Each MDS plot is fit onto a
single GPA map using a combination of stretch-
ing, translation, reflection and rotation. This
enables identification of trends through multi-
sample and multi-proxy approaches, generating
a consensus view and is provided in Figure 5.4.
3.3.2 3 - Way MDS
Whilst GPA identifies trends, it does not pro-
vide any geological context. Another method of
combining the dissimilarity matrices of the three
proxies into a single plot is 3-way Individual Dif-
ferences Scaling (INDSCAL) (Carroll and Chang,
1970). Proxies are weighted differently, each
with different degrees of stretching or shrinking in
the X and Y direction (Vermeesch and Garzanti,
2015) and these source weights are plotted graphi-
cally (Figure 5.3). This information enables IND-
SCAL maps to be thought of in geological con-
text, identifying the factors that influence the
relative position of samples. That being said,
the source weights can lead to over interpreta-
tion (Borg and Groenen, 2005), and so both Pro-
crustes and 3-way MDS are used together in the
discussion.
3.3.3 Mixing Model Calculations
As a way to further quantify the DZ results
in comparison to the findings of Garzanti et al.
(2015), estimations of the relative input of the
White, Blue and Atbara Nile were calculated us-
ing a simple mixing model. Based upon over
400 ages from the Blue, and nearly 300 from
the Atbara and White, iterations of various in-
puts were compared to the observed spectra of
the Main Nile sample, M3, and quantified using a
chi squared statistic. This test assesses the good-
ness of fit between a set of observed values (Oi)
and those expected theoretically (Ei) as shown in
equation 3.7:
˜χ2
=
n
k=1
(Oi − Ei)2
Ei
(3.7)
To achieve this, age spectra was first segre-
gated into bins based on the age distributions.
100 iterations of tributary ratios, that is, the ra-
tio of the Blue to Atbara Nile, created a syn-
thetic distribution (Figure 10.4). A further ar-
ray combines 100 iterations of White Nile to each
of the calculated tributary combinations, forming
10,000 possible river input combinations. These
were then compared to the observed Main Nile
sample (M3) using the chi-squared test. Results
were plotted on a 3D surface graph (Figure 10.7),
with low points indicating the best fit between
the observed and expected, hence the best ap-
proximation of mixing proportions. The MAT-
LAB script written for this test is provided in the
Appendix, section C.
Chapter 3. Methods 23 Matthew Simmons, UCL
24. Detrital Geochronology of the Nile
Sample Group River / Lake Site Country Main Provenance
3709P D1 Delta Rosetta Egypt Anorogenic mixed
51 M1 Nile Cairo Ma’adi Egypt Anorogenic mixed
IV M1 Nile Cairo Tora Egypt Anorogenic mixed
1989S M2 Nile Qena Egypt Transitional C. Block
2401S M2 Nile Luxor bedload Egypt Anorogenic mixed
46 M2 Nile Aswan Egypt Anorogenic mixed
2408L M2 Nile Aswan Egypt Anorogenic mixed
45 M3 Nile Wadi Halfa Sudan Anorogenic mixed
2843L B2 Blue Nile Khartoum Sudan Anorogenic mixed
2843S B2 Blue Nile Khartoum Sudan Anorogenic mixed
2847S WM1 Milk Ed Debba Sudan Undissected C. Block
2852L M3 Nile Karima Sudan Anorogenic mixed
2852S M3 Nile Karima Sudan Anorogenic mixed
2899L M3 Nile Kerma Sudan Anorogenic mixed
2899S M3 Nile Karma Sudan Anorogenic mixed
2947S GT1 Abay Goha Tsiyon Ethiopia Anorogenic Volcanic
2858S AT1 Atbara Abu Ammar Sudan Anorogenic Volcanic
2873S AT1 Atbara Showak Sudan Anorogenic Volcanic
3312S AT1 Tekeze Togo Ber Ethiopia Anorogenic Volcanic
2876S B1 Rahad Hufeira Sudan Anorogenic mixed
2964L B1 Blue Nile Bambudi Ethiopia Anorogenic mixed
2964S B1 Blue Nile Bambudi Ethiopia Anorogenic mixed
2970S B1 Beles Enat Beles Ethiopia Anorogenic mixed
2976S B1 Didesa Ephrem Ethiopia Anorogenic mixed
3295S B1 Dabus Bambesi Ethiopia DCB metamorphic
2878L W1 White Nile Rabak Sudan Recycled clastic
7 W2 Lol Nyamliell South Sudan DCB metamorphic
24 SO1 Sobat Nasser South Sudan DCB metamorphic
13 BA1 Bahr el Jebel Bor South Sudan DCB metamorphic
4052S BA2 Bahr El Jebel Juba South Sudan DCB metamorphic
3690F AL1 Albert Nile Wadelai Uganda DCB metamorphic
3692S V1 Victoria Nile Murchison Falls Uganda DCB metamorphic
3641FS K1 Kagera Kasensero Uganda Continental Block
Table 3.2: Sample Table: Grouped samples named according to the river in which they are derived. The Main provenance column
provides the origin of the sediment.
Chapter 3. Methods 24 Matthew Simmons, UCL
25. Chapter 4
Results
---- W2 ----
---- B4 ----
---- B3 ----
---- B2 ----
---- GT1 ------- WM2 ----
N
M2
M2
M1
BA1
M3
(n = 169)
(n=164)
(n = 123)))
M2 AT1
AT2
(n = 146)
W1
AL1
V1
D1
(n = 127)
(n = 127)
(n = 97)
BA2
B2
B3
B5
(n = 128)
(n = 298)
(n = 46)
0 1000 2000 30000 1000 2000 3000
SO1
(n = 100)
(n=127)
(n = 104)
(n = 132)
(n = 46)
N
Age (Ma) Age (Ma)
Figure 4.1: Sample group location map, including area-normalised KDE plots for samples with sufficient zircon ages. A further KDE
plot is provided in Figure 4.2. n = number of zircon grains within each sample.
4.1 Detrital Zircon Analysis
36 heavy mineral separates were processed,
resulting in 1709 zircon ages, of which 1262
were concordant. Results were combined into
18 groups with sufficient grains to provide a rep-
resentative sample. For samples with less than 60
zircons, Cumulative Age Distributions (CAD) are
provided in Figure 4.3, for samples with greater
than 60 grains, Kernel Density Estimates (KDE)
are used (Figure 4.1 and 4.2). Peak intensities
are evaluated in terms of the percentage of grains
represented by the peak. Table 4.1 provides a
summary of DZ results, including sample size of
each age peak and primary interpretations. KDE
plots are displayed on the sample location map
(Figure 4.1) to highlight trends, whilst detailed
KDE graphs, including histograms, are provided
in Figure 4.2.
25
26. Detrital Geochronology of the Nile
10
30
20
10
30
20
10
30
20
M2 (n=146)
10
30
20
M3 (n=122)
10
30
20
W1 (n=127)
D1 (n=132)
M1 (n=69)
10
30
20
BA1 (n=161)
10
30
20
10
30
20
AL1
(n=125)
V1
(n=97)
Age (Ma)
Frequency
Main
Nile
White
Nile
Bahr el
Jebel
Albert
and
Victoria
Nile
Nile
Delta
Age (Ma)
Frequency
Atbara
Blue
Nile
Sobat
Nile
10
30
20
10
30
20
10
30
20
10
30
20
10
30
20
10
30
20
0 400 800 1200 1600 2000 2400 2800 32000 400 800 1200 1600 2000 2400 2800 32000 400 800 1200 1600 2000 2400 2800 32000 400 800 1200 1600 2000 2400 2800 32000 400 800 1200 1600 2000 2400 2800 3200
SO1 (n=100)
0 300 600 900 1200 1500 1800 2100 2400 2700 30000 300 600 900 1200 1500 1800 2100 2400 2700 30000 300 600 900 1200 1500 1800 2100 2400 2700 30000 300 600 900 1200 1500 1800 2100 2400 2700 30000 300 600 900 1200 1500 1800 2100 2400 2700 3000
Figure 4.2: KDE plots from the Nile delta in the north to the Kagera Nile in the south (left) and age spectra plotted on the same scale
from Nile tributaries: Atbara, Blue and Sobat Nile (right). Area under both plots is normalised to allow direct comparison. n = number
of zircon grains within each sample.
Group K1 shows a sharp Archean peak (2.5-
2.6 Ga), a smaller Paleoproterozoic peak (2.0-2.1
Ga) and a mid-Mesoproterozoic peak ( 1.4 Ga),
in addition to a large, broad Neoproterozoic peak
(0.55-0.9 Ga). Samples V1 and AL1, represent-
ing the Victoria and Albert branches of the Nile,
are dominated by Archean ages (2.4-2.6 Ga), rep-
resented by over 80% of all zircons (Table 4.1).
In fact, only 9.6% of all ages are below 2.0 Ga,
with a cluster at 0.55-0.65 Ga (see Appendix (E),
Table 10.2 for all U-Pb data) BA1 and BA2 of
the Bahr el Jebel river show small Archean peaks
(Figure 4.2) and a large broad Neoproterozoic
cluster (0.55-0.9 Ga), including three sharp peaks
Chapter 4. Results 26 Matthew Simmons, UCL
27. Detrital Geochronology of the Nile
Figure 4.3: Cumulative Age Distributions (CAD) for samples that contain few zircons. Without the need for smoothing, trends in smaller
samples can be seen. Samples are displayed so that moving left to right represents a decrease in latitude. For example, WM1 (light green)
is the farthest north and K1 (light blue) is the furthest south. The size of the Neoproterozoic peak is more reduced at lower latitudes.
at 0.59, 0.65 and 9.4 Ga .
The first tributary of the White Nile, the So-
bat (SO1) shows dominant Pan-African ages (0.6-
0.9 Ga) representing over 90% of zircons and a
small (6%) peak of Archean ages. In addition
to a large Pan-African peak, White Nile sample
W1 has a moderate peak (15% of the sample) at
1.1 to 1.2 Ga and few Archean ages. Sample W2
however, shows no ages above 0.9 Ga, with the
exception of a large Archean peak at 2.5-2.6 Ga.
Atbara and Blue Nile samples show very few
ages older than 1100 Ma; B2 and B5 have no
recorded ages past the Neoproterozoic (Figure
4.2). The youngest grains from the Nile basin,
dated at 27-32 Ma, are measured in the Atbara
and Blue Nile as small 2-3% peaks in samples B3,
B2, AT1 and AT2.
Main Nile samples all contain large Neopro-
terozoic peaks between 0.5 and 0.95 Ga, repre-
sented by 92%, 81% and 75% of all ages from M3,
M2 and M1 respectively. Sample M1 contains a
large Archean peak at 2.4-2.6 Ga.
4.2 Bulk Petrography
PT data plotted on a QFL diagram (Figure
4.4) shows the Atbara Nile is principally com-
posed of lithics and the Lm-Lv-Ls diagram (Fig-
ure 4.4) shows sediment is of a volcanic origin.
Main Nile sands show a progressive increase in
quartz along the river’s course (Figure 4.4, sam-
ples M1, M2 and M3), highlighting sediment mix-
ing from the Sahara.
Blue Nile sediments plot in the same areas as
Chapter 4. Results 27 Matthew Simmons, UCL
28. Detrital Geochronology of the Nile
Figure 4.4: (Left) QFL diagram, with all samples plotted, and groups labelled. Coloured envelopes represent each river. (Right) Lm-Lv-Ls
diagram, indicating the nature of the lithic content. Again, coloured envelopes represent each river. Data from Garzanti et al. (2015).
the Main Nile, with similar compositions. Sam-
ples from the Victoria and Kagera Nile plot in the
upper continental block region of the QFL clas-
sification, as expected due to the drainage of the
Tanzania Craton.
The Sobat river contains feldspatho-quartzose
samples that fall within the continental block
province, but also include feldspar-rich sands
within the basement uplift region, representing
the large-scale Pleistocene uplift of South Su-
dan. The end member sample for the Saharan
Metacraton, WM1, plots in the quartz-rich cra-
ton region of the classification as expected.
The Lm-Lv-Ls diagram (Figure 4.4) illus-
trates the metamorphic origin of the Victoria and
Kagera Nile, and the volcanic origin of the At-
bara. Blue Nile samples contain a lesser propor-
tion of volcanic lithics downstream. Main Nile
sediment contains a larger proportion of volcanic
lithics than metamorphic, indicating a greater
sediment input of the Blue and Atbara Nile.
4.3 Heavy Mineral Analysis
Heavy mineral analysis (HM) mirrors the pat-
tern seen in bulk petrography results, whereby
Main Nile sediments are influenced by Blue Nile
and Atbara compositions, confirming the domi-
nant sediment contribution from the Ethiopian
highlands (Garzanti et al., 2006).
In the southern Nile basin, samples K1 and
K2 show a contrasting HM composition com-
pared to the Albert and Victoria Nile. The en-
richment of minerals such as rutile and other
titanium oxides, in addition to chloritoid, in-
dicate drainage of the Karagwe-Ankole belt,
which contains layered Fe/Ti ultramafic intru-
sions (Fernandez-Alonso et al., 2012). This is
confirmed from the DZ results, which show a
strong peak at 1.4 Ga.
From the Victoria Nile to the Blue Nile con-
Chapter 4. Results 28 Matthew Simmons, UCL
29. Detrital Geochronology of the Nile
AT1a
AT1b
A
0 50 100km
AT2
0 100 200km
N
FLOW
FLOW
N
AL1
AT1
BA2
D1
GT1
K1
M1
M2
M3
V2
W1
W2
WM1
SO1
BA1
zircon
tourmaline
rutile
Ti.Oxides
titaniteapatite
monazite
epidote
garnet
chloritoid
staurolite
andalusite
kyanite
sillimanite
hornblende other.amphiboles
clinopyroxene
orthopyroxene
olivine
spinel
other
B1
B2
Main Nile
Atbara and Blue Nile
A
B
B1
B2
B3
B4
B5
GT1
Afar
Triangle
Main/White Nile
Current
Volcano
Atbara
Blue Nile
Legend
K2
D2
Mediterranean Sea
Clinopyroxene
Hornblende
Epidote
Olivine
Kyanite
Sillimanite
Epidote
Zircon
Rutile
Apatite
Figure 4.5: Heavy mineral pie charts from across the Nile basin. A = Full course of the Nile, B = larger scale maps of the Blue and
Atbara Nile. Data from Garzanti et al. (2015)
fluence some 1400km north, analysis (Figure 4.5)
shows a dominant metamorphic source, with sil-
liminite and kyanite representing over 25% of
heavy minerals (Garzanti et al., 2015).
Comparison between the upper White Nile
(sample W2) and the lower Main Nile (M3)
indicates a significant input from eastern trib-
utaries. Both the Atbara and Blue contain
large proportions of clinopyroxene and olivine,
resulting from the basaltic Ethiopian Plateau.
This is represented in sample M3, of which 67%
of heavy minerals are principally derived from
basalt (clinopyroxene, orthopyroxene and olivine)
(Garzanti et al., 2015).
Analysis of the Blue Nile (Figure 4.5) shows
sediment from the Ethiopian Plateau (sample
GT1) is diluted by input of southern tribu-
taries (B4, B5). These samples are derived from
a metamorphic source, containing high propor-
tions of metamorphic minerals silliminite and
epidote (Tadesse et al., 2000). The Saharan
Metacraton, represented by end member sam-
ple WM1, shows a predictably complex compo-
sition, due to the varied nature of the metacraton
(Abdelsalam et al., 2002). Mineralogy supports
the petrology yet again; volcanic and metamor-
Chapter 4. Results 29 Matthew Simmons, UCL
30. Detrital Geochronology of the Nile
D1
W1
W2
BA1
BA2
V2
K1
AT1
SO1
WM1
GT1
B2
B5
D2
V1
M1M1
M2
Heavy Mineral MDS
Stress = 8.48%
K2
AL1
B1
B3
M3
AL1
AT1
BA2
D1
GT1
V2
M1
M2
D2
W1
WM1
Bulk Petrography MDS
Stress = 5.94%
SO1
K2
V1
B4
B3
B2M3
B1
B5
Detrital Zircon MDS
Stress = 4.00%
M1
W2
BA1
SO1
M3
M2
WM1
AT1
AT2
W1
K1
AL1
V1
D1
BA2
B2
B3
B5
Nile Delta
Blue Nile
Atbara
Main Nile
Victoria/Kagera
Sobat
White Nile
Goha Tsiyon (EP)
Wadi Milk (SM)
Figure 4.6: Individual non-metric MDS maps for DZ, HM and PT data. Samples are grouped into coloured envelopes representing each
river. SM = Saharan Metacraton end-member, EP = Ethiopian Plateau end-member (See Table 3.2 for sample information)
phic accessory minerals dominate the sample in
relatively equal proportions.
4.4 MDS Maps
The DZ MDS plot has the lowest stress value
of the three proxies, at 4.00% (Figure 10.1), indi-
cating the presence of clean and confident differ-
ences between samples. The Kagera, Albert and
Victoria Nile plot far from the main cluster, which
includes all other branches except the White Nile.
The PT MDS diagram has a stress value of
5.94% (Figure 10.1), indicating a “good” fit, using
the classification of Kruskal (1978). On the PT
MDS plot, Kagera and Victoria Nile samples K2
and V1 predictably form a cluster away from the
composition of the Main Nile. These samples plot
close to the Saharan Metacraton end-member,
WM1, and White Nile sample W1. Main Nile
and Blue Nile points plot close together.
The HM MDS map has a greater stress value
at 8.48%, with only a “fair” classification (Figure
10.1), indicating that the compositions can’t be
captured well in 2D. The results of this plot, how-
ever, are somewhat predictable and group into
clusters representing each river (Figure 4.6). In
all three MDS plots, the Sobat Nile and Delta
samples consistently plot away from the main
cluster of Blue, Atbara, Main and White Nile.
Chapter 4. Results 30 Matthew Simmons, UCL
31. Detrital Geochronology of the Nile
Group River
Sample
Size
Sample Size
calc.*
Display
Age Peaks
(Ga)
Peak Intensity
(%)
Interpretation
D1 Delta 132 0.11 KDE 0.025-0.03 2% Ethiopian Flood Basalts
0.6-1.1 85% Pan-African Orogeny
1.8-2.6 11% Re-working within ANS
M1 Main Nile 69 2.90 KDE 0.6-1.2 73% Pan-African Orogeny
2.4-2.6 28% Nubian Desert
M2 Main Nile 146 0.06 KDE 0.6-01.2 81% Pan-African Orogeny
2.4-2.6 7% Re-working within ANS
M3 Main Nile 122 0.19 KDE 0.6-0.8 92% Pan-African Orogeny
2.4-2.6 7% Re-working within ANS
WM1 Wadi Milk 12 54.04 CAD 0.6-0.8 100% Pan-African Orogeny
AT1 Atbara 102 0.53 KDE 0.025-0.03 2% Ethiopian Flood Basalts
0.6-0.9 89% Pan-African Orogeny
1.8-2.3 4% Re-working within ANS
AT2 Atbara 127 0.15 KDE 0.025-0.03 2% Ethiopian Flood Basalts
0.6-1.0 85% Pan-African Orogeny
1.7-1.8 4% Re-working within ANS
2.5-2.7 9% Re-working within ANS
B2 Blue Nile 128 0.14 KDE 0.025-0.03 2% Ethiopian Flood Basalts
0.6-1.0 98% Pan-African Orogeny
B3 Blue Nile 290 0.00 KDE 0.025-0.03 3% Ethiopian Flood Basalts
0.6-1.0 95% Pan-African Orogeny
B5 Blue Nile 46 9.45 KDE 0.6-1.0 100% Pan-African Orogeny
W1 White Nile 127 0.15 KDE 0.6-0.8 69% Pan-African Orogeny
1.1-1.2 15% Neoproterozoic Basement
2.1-2.2 4% Darfur Inlier
W2 White Nile 20 35.85 CAD 0.6-0.8 62% Pan-African Orogeny
2.5-2.6 37% Archean Basement, CAR
SO1 Sobat 100 0.59 KDE 0.7-1.1 94% Pan-African Orogeny
2.5-2.6 6% South Sudan Basement
BA1 Bahr el Jebel 162 0.02 KDE 0.7-0.9 81% Pan-African orogeny
2.0-2.1 4% Ruwenzori fold belt
1.8-1.9 3% Ubarian-Usagarian Belt
2.5-2.6 11% Aruan metamorphism
BA2 Bahr el Jebel 44 10.47 CAD 0.7-0.9 57% Pan-African orogeny
2.5-2.6 43% Aruan metamorphism
AL1 Albert Nile 125 0.16 KDE 2.0-2.1 11% Ruwenzori fold belt
2.4-2.6 85% Aruan metamorphism
2.7-2.9 4% Watian Granulites
V1 Victoria Nile 98 0.66 KDE 2.0-2.1 7% Ruwenzori fold belt
2.5-2.6 88% Aruan metamorphism
2.8-2.9 5% Watian Granulites
K1 Kagera 44 10.47 CAD 0.7-0.9 11% Pan-African overprinting
1.4 30% Kibaran fold belt
2.0-2.1 23% Ruwenzori fold belt
2.5-2.6 36% Aruan metamorphism
Table 4.1: Detrital zircon geochronology results. Peak intensity is calculated as the percentage of the whole sample represented by each
peak over 1%. * Sample size calculation: Probability, at the 95% confidence level, that a 5% fraction of the age spectra has been missed.
Display indicated the method of visualisation KDE = Kernel Density Estimators, CAD = Cumulative Age Distribution.
Chapter 4. Results 31 Matthew Simmons, UCL
32. Chapter 5
Discussion
5.1 Interpretation of DZ results
Table 4.1 provides the interpretation of all ma-
jor peaks within all samples.
The Archean peak (2.5-2.6 Ga) of the Kagera
Nile samples represents the metamorphic source
of the Aruan Event. The Paleoproterozoic
peak (2.0-2.1 Ga) is attributed to the drainage
of the Ruwenzori fold belt, and the mid-
Mesoproterozoic peak ( 1.4 Ga) is most likely de-
rived from the Kibaran Belt. The broad peak
at 0.55-0.9 Ga is interpreted to represent the
polyphase Pan-African Orogeny. The absence
of Watian Metamorphic source rocks (2.9 Ga)
in the age spectra, despite a presence in the
drainage basin of the Kagera (Appel et al., 2005;
Leggo, 1974), is likely due to under-sampling;
the chances of missing a 5% fraction of the
total grains within a 44 grain sample is 92%
(Vermeesch, 2004).
The Archean ages (2.4-2.6 Ga) of the Victoria
and Albert branches of the Nile are attributed to
the Tanzania Craton. The Albert Nile shows the
only ages interpreted to be derived from the Wa-
tian Granulites, with a small peak of 2.7-2.9 Ga.
The Pan-African event, seen in all other samples
over the entire course of the Nile, is hardly rep-
resented at all, indicating near-exclusive drainage
of the Tanzania Craton. The Bahr el Jebel river
shows sediment derived from the Archean meta-
morphic strata of Uganda, in addition to strong
Pan-African peaks (0.7-0.9 Ga) due to drainage of
the Mozambique and Oubanguides orogenic belts.
Pan-African ages (0.6-0.9 Ga) dominate the
Sobat Nile, with only a small Archean peak
recorded, interpreted as a result of exposure to
an area of the Tanzania Craton in south-west
South Sudan. Pan-African ages are recorded in
the White Nile, in addition to a small peak of
Paleoproterozoic ages (2.1-2.2 Ga) interpreted to
originate from the Darfur Massif, an inlier in the
Mozambique Belt and Saharan Metacraton (Vail,
1976). A moderate peak at 1.1 Ga, represent-
ing 15% of sample W1, has no obvious origin.
Too young for the Darfur and too old to be Pan-
African, this peak is assumed to be reworked
Mesoproterozoic crust exposed upstream of sam-
ple W1.
The large sample size of B3, coupled with
the fact that the majority of grains are aged
between 0.6-1.2 Ga, provides a high resolution
insight into the polyphase nature of the Pan-
African orogeny. Three distinct peaks are ob-
served at 590, 650 and 940 Ma which are also
seen in the remaining Blue Nile age spectra, as
well as White and Main Nile samples. This
observation directly matches the results of nu-
merous past studies that have dated the Pan-
African orogeny (Dallmyer and Villeneuve, 1987;
Dingeldy, 1999; Jacobs et al., 1998; Key et al.,
32
33. Detrital Geochronology of the Nile
Figure 5.1: Concordia diagrams from the lower Nile Basin. Red mixing lines, with intercept values also in red, have consistent overprinting
ages of 0.65 Ga, with some samples showing multiple intercepts at 0.55, 0.65 and 0.94 Ga.
1989; M¨oller et al., 2000).
The Atbara and Blue Nile contain Cenozoic
ages (25-30 Ma) which are interpreted to be de-
rived from the Ethiopian Plateau. These young
zircons constitute just 2-3% of the total eastern
tributary input according to DZ result (Table
4.1). The conclusions of Garzanti et al. (2015),
however, indicate that the Ethiopian sediment
factory (Blue and Atbara Nile) inputs ∼95% of
all sediment (see section 5.3). This confirms the
initial hypothesis that DZ studies are blind to cer-
tain sediment sources.
The Main Nile age spectra shows dominant
Neoproterozoic Pan-African peaks in all samples
(M1-3). The large Archean peak (2.4-2.6 Ga)
is interpreted to be derived from the Nubian
Desert inlier of the Saharan Metacraton. Re-
worked Mesoproterozoic crust is found in certain
areas of the Arabian Nubian Shield in Egypt’s
Eastern Desert, and is seen as small peaks be-
tween 1.1 and 1.4 Ga. The age spectra of M3
closely resembles the Blue Nile, highlighting the
large sediment input of the eastern tributaries.
That being said, the results of W1, the most
northern White Nile sample do not show a dis-
tinctly different age spectra to that of the Main
Nile. Differences between samples are difficult to
see based on the KDE results; the Pan-African
ages are found everywhere and dilute the ability
to distinguish patterns in the data.
5.1.1 Concordia
Wetherhill concordia diagrams for all samples,
(see appendix, Figure 10.2) including discordant
ages, enable interpretation of episodic lead or ura-
nium loss (Wetherill, 1956). Samples that include
Archean and Paleoproterozoic ages from the Tan-
zania Craton and surrounding fold belts show ev-
idence of open system behaviour, as seen in the
form of a linear trend, annotated on the compos-
ite concordia diagram (Figure 5.1). Ages signif-
icantly above the concordia curve indicate ura-
nium loss, as perhaps seen in samples AL1 and
V1 (although this does not account for analytical
uncertainty), whilst ages below the concordia line
indicate lead loss over time. Linear regressions of
discordant mixing lines indicate an intercept of
0.65 Ga in Kagera, Victoria, Albert and White
Nile samples. Mixing lines are interpreted to rep-
resent crystallisation ages of 2.4-2.6 Ga and meta-
morphic overprinting ages of 0.65 Ga. In some
samples, such as AL1 and W2 where large sam-
ple sizes are recorded, multiple mixing lines can
Chapter 5. Discussion 33 Matthew Simmons, UCL
34. Detrital Geochronology of the Nile
50 100 150 200 500
Grain Size [µm]
BA1 - Before Sudd Marsh
W1 - After Sudd Marsh
µm65
n = 1085
n = 8957
Figure 5.2: Grain size distribution of heavy minerals (black fill, dotted line) and zircon (red fill). The zircon average grain distribution
decreases by 65µ m (38%), heavy mineral sizes decrease by an average of 55% across the Sudd marsh.
be seen, intercepting concordia at 0.55, 0.65 and
0.94 Ga. This is understood to be the result of
the Pan-African Orogeny, which was identified in
the age spectra as a polyphase event, with the
ages of the peaks matching intercept ages. That
being said, the longer the mixing line, the larger
the degree of lead loss and the greater extrapo-
lation error on crystallisation ages and intercept
values.
5.1.2 Filtering of the Sudd Marsh
Results of the detrital zircon dating also ap-
pear at first glance to highlight the Sudd Marshes
as a natural sediment dam, as suggested by
Garzanti et al. (2015). The large Archean peak
present in both V1 and AL1 with peak intensities
of over 85% are absent in samples after the sed-
iment marsh. It could be hypothesised that the
older zircons have a larger grain size and are hence
filtered out in the low energy regime of the marsh-
lands, although the size of individual zircon grains
were not tracked for this sample. However, Figure
5.2 shows the average grain size of heavy minerals
immediately before and after the Sudd Marshes,
resulting in a decrease in average zircon size of
65µm, or 38%, across the natural sediment dam.
The size distributions after the Sudd Marshes are
equivalent for all measured heavy minerals, in-
cluding zircon, and represent the filtering effect of
the marsh, which alters the distribution of grain
sizes.
5.2 MDS Maps
As seen within Figure 4.6, the DZ MDS map
shows a disparity between southern Nile samples
from the Kagera and Albert Nile (plotted in pink)
and the Main, Blue and Atbara Nile samples. K1
is the only sample to directly drain the Kibaran
fold belt, hence plots further away from the Vic-
toria and Albert Nile samples. The Bahr el Jebel
(BA1, BA2) plots as a bridge between the Tan-
zania craton derived AL1, V1 and K1 samples
and the rest of the basin, representing the increas-
ing drainage of Pan-African fold belts as the river
moves downstream (Figure 2.3b).
Chapter 5. Discussion 34 Matthew Simmons, UCL
35. Detrital Geochronology of the Nile
The Sobat Nile group, as also seen in the QFL
plot, has a more similar composition to the Albert
Nile than the White Nile, showing its main source
of sediment is to the south towards the Tanza-
nia Craton rather than the volcanics of Ethiopia.
On the two compositional MDS plots, the Blue
Nile overlaps the Main Nile. Furthermore, the
most downstream Main Nile sample is most com-
positionally similar to both Atbara and Blue Nile
samples. On the DZ MDS plot, the Blue and
Main Niles do not overlap, interpreted to repre-
sent less input of the Blue Nile in the DZ results.
Delta sample D1 plots furthest away from the
main cluster in all MDS maps. As proposed by
Garzanti et al. (2015), this sample must include
sediment from another source, such as the littoral
cell. The samples of the Kagera (K1 and K2) and
the Victoria Nile (V1) consistently plot far from
the main cluster, due to erosion of the Karagwe-
Ankole Belt and large-scale ultramafic intrusions
in southern Uganda. In terms of HM analysis,
K1, K2 and V1 are unlike the geographically sim-
ilar AL1, which lies, along with V1, close to the
White Nile samples BA2 and BA1. This is inter-
preted to represent the transition from Tanzania
Craton basement to Neoproterozoic fold belts.
White Nile samples plot close to each other
on the DZ MDS map, with the most downstream
(W2) far from Main Nile samples. This once
again highlights the dominant input of of sedi-
ment from the Blue and Atbara tributaries into
the Main Nile.
5.3 GPA and 3-Way MDS
The three separate MDS plots show the dif-
ference in results from the compositional datasets
and the distributional DZ dataset.
0.6 0.8 1.0 1.2 1.4 1.6
0.40.60.81.01.2
Source Weights
DZ
HM
PT
Figure 5.3: Source weights for the 3-way INDSCAL map, showing
the relative input of the 3 proxies.
While the compositional MDS maps appear
different, the same key conclusions can be drawn
from both of them, matching in part the work by
Garzanti et al. (2015). The DZ map, however,
shows a different arrangement of the Nile tribu-
taries. This is shown further in the 3-way Indi-
vidual Differences Scaling (INDSCAL) map, with
the source weights showing similar PT and HM
weightings and an opposite DZ weighting (Fig-
ure 5.3). The horizontal axis is more sensitive
to provenance factors, while the multi-mineral
study’s make the vertical axis sensitive to hy-
draulic and dissolution effects during diagenesis.
The Blue Nile samples consistently plot with
the Main Nile on the INDSCAL map. The Al-
bert Nile (AL1) and Victoria Nile (V1) should, ac-
cording to their DZ data alone, plot next to each
other. Separated by vertical distance in the IND-
SCAL map, a difference in compositional analysis
results in the two samples plotting apart.
The Wadi Milk sample, the end member of
the Saharan Plateau, plots away from the Main
Nile cluster. The nearest samples are that of the
White Nile. This further supports the minimal
input of the Wadi Milk into the Main Nile flow.
Chapter 5. Discussion 35 Matthew Simmons, UCL
36. Detrital Geochronology of the Nile
Figure 5.4: (Above) Non-Metric 3-Way MDS showing group configurations based on the source weightings in Figure 5.3. The horizontal
axis is sensitive to provenance factors, while the vertical axis is primarily influenced by hydraulic sorting and dissolution effects. (Below)
General Procrustes Analysis (GPA) showing the representation of the 3 separate MDS plots on a single graph, weighted equally.
The DZ data in Figure 4.6 shows a line of sim-
ilarity from White Nile samples to the Main Nile.
This would, if considered alone, imply a signif-
icant input from the White Nile. However, the
consensus view indicates no significant link be-
tween the White and Main; not one White sample
plots near a Main Nile sample in either INDSCAL
or GPA.
To think about this particular trend fur-
ther, mixing model calculations using the DZ
data alone can be compared to the mass bal-
ance in the literature. The calculations within
Garzanti et al. (2015), as shown in Figure 2.1 and
summarised in table 5.4, indicate a meagre 3±2%
input from the White Nile, 36±4% from the At-
bara and 60±4% from the Blue. Calculations
based on this studies DZ data are shown in ta-
ble 5.4, with a full methodology provided in ap-
pendix section C. Whilst the calculations contain
a wide range, the DZ results considerably under-
estimate the White Nile input by up to 27%, and
underestimate input of the Atbara by up to 30%.
The Blue Nile results are inconclusive; its input
is either overestimated or underestimated relative
Chapter 5. Discussion 36 Matthew Simmons, UCL
37. Detrital Geochronology of the Nile
Figure 5.5: Zircon concentration for each group, including averages for each major tributary. Data exported from the QEMSCAN
workflow. A large variation is seen, even within the same group of samples. Concentration is represented as an area of total grains and as
a number of grains. The difference between these two measurements gives information as the the size of zircons within the sample. Full
zircon fertility data for all samples is provided in the appendix section D, table 10.1.
to Garzanti et al. (2015), due to the low resolu-
tion of this calculation. Therefore, the DZ data
alone, in the absence of mineralogical information
would inaccurately predict the sediment routing
system of the Nile. This confirms the initial hy-
pothesis that the Atbara Nile is underestimated
and the White Nile is overestimated when using
DZ as the single provenance proxy.
5.4 Bias of Zircon Concentration
The mixing model results highlight the effect
of zircon concentration on DZ age distributions
(Figure 5.5, Appendix section D). Zircon concen-
tration is a direct result of the variable zircon
fertility of the source rocks. The overestimated
White Nile has the highest zircon concentration
at 2.57%, with over 14 times that of the Atbara
Nile, which is underestimated. The Atbara al-
most exclusively drains the Ethiopian Plateau, a
mafic volcanic source. This source is almost in-
visible to the DZ study, despite contributing 70%
of all Nile sediment. Conversely, the White Nile
drains the metamorphic Tanzania Craton, which
has a high zircon fertility, and so a greater impact
on the DZ age spectra. This bias is seen in 3-way
MDS analysis, with the White Nile plotting close
to the Main Nile on the horizontal axis (which is
sensitive to provenance factors) and further away
in the vertical direction.
Chapter 5. Discussion 37 Matthew Simmons, UCL
38. Detrital Geochronology of the Nile
Test Sediment Flux (%)
White Blue Atbara
Chi-squared (x2
) 20 ± 8 69 ± 12 12 ± 4
Garzanti (2015) 3 ± 2 60 ± 4 36 ± 4
Difference (%) +7 to +27 -6 to +25 -16 to -32
Table 5.1: Results of the chi-squared test, and direct comparison
to the findings of the compositional study by Garzanti et al. (2015).
The White Nile was overestimated and the Atbara was underesti-
mated. Blue Nile results were inconclusive due to the large range.
In addition, this bias is further demonstrated
in the amount of young zircon grains (25-30 Ma)
derived from the Ethiopian mafic volcanic source.
Despite the dominant signal in HM and PT re-
sults (Garzanti et al., 2015), these sediments rep-
resent just 1.19% (See table 10.2) of all measured
zircons across the entire course of the Nile.
5.5 The QEMSCAN Workflow
The advantage of the current workflow was
the processing of multiple samples in a relatively
short time, without the need to increase the con-
centration of zircon. In this regard, the work-
flow was successful and has the possibility to
become more efficient when coupled with addi-
tional QEMSCAN mineralogy. However, in sam-
ples with low zircon concentration, there were
not enough zircons on the slide to form a rep-
resentative sample; QEMSCAN found every zir-
con on each slide, but dilution from magnetics
and other heavy minerals prevented large num-
bers of zircons from being dated. In the second
run, removal of magnetics using a hand magnet
and Franz Ferromagnetic Separator sufficiently
concentrated zircon, allowing the QEMSCAN to
find sufficient grains per slide. In future studies,
this separation step should be added to workflow
of sediments that have low zircon concentrations
and abundant magnetics, such as those derived
from mafic volcanic sources.
A large source of error when using QEMSCAN
is the effect of variable polish depth. Having pol-
ished samples by hand, uneven slides outline the
variation in composition with resin depth (Figure
5.6B). Since only the scanned surface layer can be
seen on the output maps (Figure 5.6A), grain size
fractions below the level of polish are missed and
larger grains are polished away. This is particu-
larly important when a dependency between grain
size and age is suspected, resulting in certain age
fractions being under sampled. The Nile offers
an excellent example of this. The highly vari-
able zircon concentration, even within the same
geographical group, may indicate some degree
of sampling bias, perhaps due to the polishing
away of larger grains, and the under-sampling of
smaller zircons (Figure 5.6C).
The extra data acquired from the QEM-
SCAN has enabled analysis of zircon concentra-
tion, greatly improving the scope of this study.
In addition, the grain size information is a valu-
able tool; it was used to observe the effects of
the Sudd Marshes as a natural sediment dam. In
future studies, this vast volume of DZ data cou-
pled with grain size and mineralogical information
will enhance the understanding of large sediment
routing systems.
Chapter 5. Discussion 38 Matthew Simmons, UCL
39. Detrital Geochronology of the Nile
S S'
A
S'S
B
Shallow Polish Deeper Polish
A
Glass Slide
A
Glass Slide
A
Glass Slide
Polished Surface
E-poxy Resin
A
Glass Slide
A
Glass Slide
A
Glass Slide
Polished Surface
E-poxy Resin
C
1 mm
0.5 mm
Figure 5.6: (a) QEMSCAN map export, showing radial colour pattern. This is interpreted to represent the level of polish. The number
of black pixels of the map indicates unidentified grains. This represents grains that are partially polished away, with the QEMSCAN not
recognising the resin’s signature. (b) Zoomed in section of the map. Interestingly, the zircon concentration appears to increases radially.
It seems to show zircons being sorted by polishing effects. (c) Schematic diagram of the effect of polishing on grain size. As you polish
downwards, some grains increase in apparent size, some decrease. Note the red grain, which shows zoning. At the second level, only one
zone is exposed. In addition, the smallest grains are never fully exposed.
Chapter 5. Discussion 39 Matthew Simmons, UCL
40. Chapter 6
Conclusions
Detrital zircon results highlight sediment
sources from the Archean Tanzania Craton to the
south, the Saharan Metacraton in the north-west
and the Arabian-Nubian Shield in the north-east.
The youngest ages, derived from the Ethiopian
Plateau of 25-35 Ma, are recorded in three sam-
ples, and represent less than 1.19% (19 grains) of
the total age spectra.
Concordia diagrams show clear mixing lines
from Archean sources to intercepts of 0.6-0.9
Ma. This represents overprinting of the Tanza-
nia Craton during the Pan-African Orogeny. The
polyphase nature of this orogeny is highlighted in
the concordia plots (Figure 5.1), in addition to
the KDE plots (Figure 4.2) resulting in phases at
0.55, 0.65 and 0.94 Ma.
The 3-way MDS shows the large variation of
sources throughout this long sediment routing
system. However, the consensus view is that the
Main Nile is most similar to the Blue Nile, and
less so to the Wadi Milk sample, the end member
of the Saharan Metacraton. The White Nile plots
closer to Main Nile samples horizontally, due to
similarities in the DZ data, however plot further
apart in the vertical direction.
The DZ results underestimate the input of the
Atbara Nile. It is estimated by the use of a mixing
model that the White Nile contributes 20 ±8%
to the Main Nile bed load. Garzanti et al. (2015)
calculates that only 3±4% of sediment is derived
from the White Nile, hence the current DZ study
overestimates White Nile contribution by up to
27%. Conversely, the Atbara Nile, derived solely
from Ethiopian Plateau mafic volcanics is un-
derestimated, with just 12±4% compared to the
36±4% estimation of Garzanti et al. (2015).
Zircon concentration of the Nile varies sig-
nificantly. The White Nile (2.57%) and Victo-
ria/Albert/Kagera Nile (0.95%) have the highest
zircon concentration. The Blue and Atbara Nile
are an order of magnitude lower, at 0.05% and
0.18% respectively. The low concentration of the
Blue and Atbara Nile is responsible for underesti-
mation of the eastern tributaries. This highlights
the weakness of DZ studies; despite contributing
70% of bedload (Garzanti et al., 2015), a mafic
volcanic source is represented by very few zircons.
The new QEMSCAN workflow enabled many
samples to be processed within the scope of this
project. However, many samples with lower zir-
con fertility yielded too few zircons to provide
a representative sample. The QEMSCAN was
successfully used to measure zircon concentra-
tion within samples, record the average grain
size of a sample and to observe the effect of the
Sudd Marshes as a natural sediment dam. This
methodology will allow future studies to analyse
more zircons faster, with more information about
each grain, gaining a more comprehensive picture
of sediment routing systems.
40
41. Chapter 7
Future Work
The vast amount of data collected using the
current workflow allows for several possibilities of
future study. One such application is the tracking
of zircon grain size with age using the exported
data from the QEMSCAN. This was used to track
a single sample during the current study, although
application to all samples could be a powerful way
to understand not only provenance, but the bias
of certain grain size fractions. It would be inter-
esting to test the size to age ratio of Ethiopian
Plateau derived zircons, to see if this effected the
current study’s interpretation.
A potential further study is the utilisation of
compositional data, a by-product of the QEM-
SCAN workflow. The ability to detect heavy min-
erals (HM) during detrital geochronology with no
additional processing time would greatly enhance
the scope of provenance studies. However, the ac-
curacy of this method must first be tested against
conventional point count methods.
To improve the mass balance calculations us-
ing detrital zircon alone, samples must be anal-
ysed immediately before and after each conflu-
ence. In the current study, Main Nile samples
were only collected before and after the input of
all three tributaries making calculations inaccu-
rate. The Nile river is an ideal place to develop
such techniques. A future study would analyse
zircons from multiple size fractions before and af-
ter the input of the Blue Nile, to see if a mixing
model can match that of Garzanti et al. (2015)
and achieve a higher resolution. Moreover, the
zircon fertility bias could be corrected for within
such numerical models.
41
42. Chapter 8
Acknowledgements
Firstly, I would like to thank Dr. Pieter Vermeesch for his help and support during this study and
across my four years at UCL. His guidance enabled me to not only complete this project, but also
to learn a large range of transferable skills, and for that I am grateful.
I would also like to acknowledge Dr. Martin Rittner for the many hours spent on the QEMSCAN
and LA-ICP-MS and his assistance with data processing.
I wish to thank Eduardo Garzanti at the University of Milano-Biocca for providing the heavy
mineral separates used for U-Pb dating and access to the Nile compositional dataset. I extend my
thanks to Eduardo’s research team for the prompt preparation and delivery of the additional Blue
Nile samples.
Finally, I would like to thank my fellow UCL students, including and Ethan Petrou and George
Peters for their continued support, but mainly for making the past year enjoyable.
42
43. Chapter 9
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