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Depositional variability of an ancient distributive fluvial system: The upper member of the lower cretaceous Bima Formation, Northern Benue Trough, Nigeria
Journal Pre-proof Depositional variability of an ancient distributive fluvial system: The upper member of the lower cretaceous Bima Formation, Northern Benue Trough, Nigeria. Kachalla Aliyuda, John Howell, Musa Bappah Usman, Abdulwahab Muhammed Bello, Benjamin Maina, Usman Abubakar PII:
S1464-343X(19)30255-9
DOI:
https://doi.org/10.1016/j.jafrearsci.2019.103600
Reference:
AES 103600
To appear in:
Journal of African Earth Sciences
Received Date: 20 December 2018 Revised Date:
29 July 2019
Accepted Date: 21 August 2019
Please cite this article as: Aliyuda, K., Howell, J., Usman, M.B., Bello, A.M., Maina, B., Abubakar, U., Depositional variability of an ancient distributive fluvial system: The upper member of the lower cretaceous Bima Formation, Northern Benue Trough, Nigeria., Journal of African Earth Sciences (2019), doi: https://doi.org/10.1016/j.jafrearsci.2019.103600. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.
Depositional variability of an ancient distributive fluvial system: The upper member of the Lower Cretaceous Bima Formation, Northern Benue Trough, Nigeria. Kachalla Aliyuda, John Howell, Musa Bappah Usman, Abdulwahab Muhammed Bello, Benjamin Maina and Usman Abubakar. Keywords: Distributive fluvial system, Bima Formation, Northern Benue Trough Abstract The upper Bima Formation in the Northern Benue Trough has been interpreted as a distributive fluvial system. Previous stratigraphic studies on the Bima Formation were focussed on localized, qualitative, sedimentary facies analysis without regional context or quantitative information about sand-body scale and architecture. This study quantitatively analysed sand-body thickness variations in the upper member of the Bima Formation across the Northern Benue Trough, documented the spatial variations in channel dimensions. Photo- realistic virtual outcrops were generated for four study sites using data acquired with an unmanned aerial vehicle and processed photogrammetrically. Analysis of the virtual outcrops illustrated spatial variation in the thickness of channel infill components, a downstream decrease in proportion of multi-storey channel belt facies association (from 98% to 5%), an increase in floodplain facies association (from 0% to 13%), an increase in the proportion of isolated channel fill facies association (from 2% to 80%) and a slight decrease in average grainsizes of channel fill sandstone. These observations are interpreted to represent evidence of a distributive fluvial system (DFS) or mega-fan with the proximal part of system at Tula, Ture and Tashan Alaji to the medial part at Hinna having an increase in frequency and thickness of the flood plain facies association and abundance of isolated channel fill facies associations. This study demonstrates the regional facies variability of the
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upper Bima Formation and has provided a basis for comparison with other ancient distributive fluvial system. Highlights Images of the upper Bima Formation (B3) were acquired and processed. Multi-storey channel, isolated channel and floodplain facies were analysed. The three facies associations vary spatially in the basin typical of an ancient DFS. Variation in thickness, grainsize and facies associations is similar to other DFSs. Introduction Fluvial sandbodies can be deposited by either distributive or tributary fluvial systems. While tributary systems typically occur in upland areas with low preservation potential. Distributive fluvial systems form when rivers deposit sediment into a sedimentary basin (Weissmann et al. 2010). A study of more than 700 modern sedimentary basins shows that distributive fluvial systems account for over 90% of the fluvial deposits that occur in basins and will therefore be preserved in the future rock record (Weissmann et al. 2010, Nyberg & Howell 2015, Owen et al 2015). Over the past 10 years there have been a number of studies of DFS, initially arising from the advent of freely available remote sensing data (Google Earth etc.) for studying modern systems (Weissmann et al. 2010; Hartley et al. 2010; Davidson et al. 2013; Weissmann et al. 2015) and the translation of these concepts to the rock record (Rittersbacher et al. 2014, Owen et al. 2015; Chesley and Leier 2018; McGlue et al. 2016) Despite the recent advances there are only a few case studies that quantify changes in channel body architecture downstream within DFS as described in Owen et al. (2015 & 2017).
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Modern DFS are characterized by a radiating channel from an upstream apex. There is commonly, a downstream decrease in channel size; a downstream increase in the preservation of floodplain deposits (or fines); a downstream decrease in grain size; a downstream reduction in the degree of channel amalgamation. The first ancient DFS to be quantitatively analysed was the Salt Wash DFS of the Morrison Formation in Utah, U.S.A (Owen et al. 2015, and Chesley and Leier 2018). The interpretation of a system as a DFS is easier in the modern where the large-scale channel patterns can be observed directly. Studied modern examples include; the Taquari DFS (Assine 2005; Buehler et al. 2011) and the DFS systems in the Himalayan foredeep (Shukla et al. 2001). Modern and ancient studies highlight the variation of the different facies within the DFS in both lateral and vertical directions as shown in Owen et al., 2017. Bima Formation The Bima Formation was deposited in the Gongola and Yola sub-basins of the Northern Benue Trough during the Cretaceous, it is the oldest sedimentary succession in the basin extending towards the north-western margin of the Gongola sub-basin, and down into the Chad Basin (Fig. 1). It unconformably overlies basement rocks and is divided into three members, lower, middle and upper (Carter et al. 1963). The Bima Formation was first named by Falconer (1911). It was sourced from the granitic terrain bounding the basin (Guiraud, 1990). The thickest exposure of the Bima Formation is at the Lamurde anticline, where it is more than 3,000 m. The Lower Bima (B1) consists of coarse grained sandstone occasionally pebbled with alternating clays and shale, the middle Bima (B2) includes very coarse grained sandstones with thin clay and shale intervals. The upper Bima (B3) consists of whitish-grey well bedded, cross bedded sandstone medium to coarse and very coarse
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grained, frequently pebbly (Carter et al, 1963). The upper Bima (B3) is the focus of this study because of its regional extent within the entire Northern Benue Trough. The base and top of the Bima Formation are believed to be diachronous, however, it was suggested to be deposited from Upper Albian to Lower Turonian (Carter et al, 1963). The Bima Formation is one of the most extensively studied formations in the Northern Benue Trough. Different depositional models have been proposed for the Bima Formation, most of which are disharmonious (such as Tukur et al., 2015, Guiraud, 1990 and Carter et al, 1963). Most previous stratigraphic studies focused on localized analysis of the sedimentary facies without adequate quantitative information about sandstone-body variations. There has been little effort to understand the large-scale spatial variation within the unit. The goal of this study was to quantitatively analyse sand-body geometries in the upper member of the Bima Formation documenting the lateral variation of channel deposits, and the depositional processes associated with such deposits and to asses systematics changes in the context of the DFS paradigm. Understanding sand-body variability is crucial to deciphering hydrocarbon reservoir’s producibility (Aliyuda and Howell, 2019). Hence, Knowledge gained from this study can be applied to understanding fluvial reservoirs in the subsurface.
Study area and methods This study focused on the upper Bima Formation in the Northern Benue Trough (NBT) of Nigeria. The NBT is part of the Benue Trough (or Benue basin) which extends from the Central Benue Trough in the south to the Chad Basin in the north. It is divided into the east-
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west trending Yola sub-basin and the north-south trending Gongola sub-basin (Abubakar et al. 2008). Outcrops of the upper Bima Formation are well exposed at the four study sites across the basin (Fig. 1)The exposures are well over 200 m at two study sites (sites 1 and 4) (Fig. 2 and 3), and the overall thickness of the outcrop exposed at site 1 is in excess of 1000 m. Data were collected using a combination of field-based observation and unmanned aerial vehicle (UAV) photogrammetry (SfM). On the ground field mapping and observations were made at all four study sites (Tahan Ahaji, Ture, Tula and Hinna). These sites have the best exposures of the upper Bima Formation in the basin. Data were collected using traditional field techniques including logging, palaeocurrent directions, bed thickness, the nature of contacts, grain sizes and sedimentary structures sand: mud ratios, and total interval thickness. Data for virtual outcrops (sensu Pringle et al. 2007) were collected as aerial images of the four study sites using an unmanned aerial vehicle (DJI phantom 4 pro) with on board 20 mega pixel camera. The UAV used is equipped with GPS and records the position of every image captured. Several hundreds of images were captured from each site with over 40% overlap. 3D structure of the study sites was reconstructed using the structure from motion (SfM) photogrammetry technique (Carrivick et al. 2016). Agisoft’s Photoscan software was used for the reconstruction. The 3D model creation workflow consists of the following processes; outcrop selection, photo acquisition, photo selection, photo alignment using the recorded coordinates on the images, building dense cloud, generating 3D mesh from the dense cloud, and finally generating texture for the model. The ready-made 3D models were then exported to LIME (Buckley et al. 2019) a purpose-built software for interpreting virtual
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outcrops, to extract quantitative information such as thicknesses of beds and facies associations.
Results Lithofacies and associations Five main facies were identified in the study sites, these facies are described in Table 1. The facies are grouped into three broad facies associations: multi-storey channel belt facies association, isolated channel-fill facies association and overbank/floodplain facies association.
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Table 1: Lithofacies description Facies Lithology
Interpretation and Environment of Deposition
St
Lateral channel point bar migration deposits.
Sp
Sh
Sc
FI
Description Medium to v. coarse Trough cross sandstone occasionally bedded pebbled base, poor to pebbled moderately sorted, sandstone erosional contact. Medium, coarse to v. coarse Planar cross sandstone, occasionally bedded pebbled base, moderate to sandstone well sorted, separated by accretion surfaces. Fine to medium grained Ripple crosssandstone, moderate to bedded/horiz well sorted, with climbing ontal bedded ripple lamination, variable sandstone bed thickness (Fig. 4). Fine to medium grained sandstone, moderately Convolute sorted, convoluted beds bedded are mostly above very sandstone fine-grained sandstone and shale. Ripple laminated siltstone, Siltstone, shale purple shale and mud mudstone.
Downstream mid-channel bar migration deposits.
Planar flow deposit in Upper flow regime.
Soft sediment deformational structures.
Overbank, floodplain deposits.
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Multi-storey channel belt facies association This facies association is composed of medium to coarse grained sandstone, cream to whitish grey, with pebbled intervals, moderate to well sorted, almost entirely planar cross bedded, with very few convoluted beds (Facies – Sp; Fig. 2). Beds thickness varies (1 – 9 m), with obvious beds thickening across the channel complexes. These channel fills are separated by distinct erosion surfaces and most planar cross beds are oriented in the same direction as the accretion surfaces, suggesting deposition in downstream accreting bars in a fluvial setting. (Miall 1977, 1985). This supports Carter et al (1963) and many other works previous interpretation of the Bima Sandstone as a fluvial deposit. Downstream accretion, limited preservation of mudstone or shale, the presence of lag deposits and planar cross bedding suggest that the deposits were from a predominantly braided river system (Miall, 1977). Palaeocurrent measurements, corrected for structural dip and fold plunge for beds with structural dip exceeding 10° show a dominant radial pattern towards the northwest (Fig. 5), this spread of palaeocurrents reflect the degree of sinuosity in the depositing river. Channel bodies are stacked in sand sheet deposits which are stacked vertically and laterally forming multiple stories with almost no mud or shale intervals. Channel complexes are separated by erosional surfaces measuring over 86 meters. About 6-7 channel complexes are visible on the virtual outcrop from Tahan Ahaji (Fig. 2). Isolated channel-fill facies association This facies association is made up of lithofacies St, comprised of medium to very coarse grained sandstone with lag deposit. The deposits have trough and planar cross bedding and are poor to moderately sorted. This single-storey sand channel-fills are lensed, or wedge 8
shaped in geometry with clearly defined erosional contact with the overbank or floodplain facies association (Fig. 3). The isolated channel-fill facies association at Tula and Ture are locally separated by clay plugs representing channel abandonment. The succession shows pebbles at the base, with large scale trough cross bedding, thinning and fining upward into medium grained, planar cross bedded units mostly in smaller beds (50 - 70 cm in thickness; Fig. 6). These sedimentary structures suggest lateral migration of channel bars and bedload sheets of a fluvial channel (Keefer 1999, Friend et al. 1979 and Gibling 2006). Single-storeys range between 1-11 m in thickness and extends over 4meters. At Hinna, they are ribbonshaped, mostly asymmetric, and quite extensive in width, suggesting that most of these channel-fill were bank-attached bars that laterally migrated (Gibling 2006). Overbank/floodplain facies association This facies association consists of interbedded fine to very fine-grained sandstone, and mudstones. The sandstones are rippled laminated, creamy or whitish. Mudstone are creamy, light brown and purple (Fig. 3A and 3B). These are muds deposited out of suspension in river floodplains and overbanks during flood events. This facies association also includes crevasse-splay, abandoned channel deposits, paleosol and floodplain deposits (Demko et al, 2004). Beds vary in thickness from 0.2 - 3 m, mostly intercalated with the mudstone and fine to very fine-grained sandstone. The overbank deposits are commonly incised into by the erosional bases of the main channel sandbodies. Crevasse-splay and abandoned channel deposits were not observed in the proximal part of the system at Tashan Alaji, however, they left their imprint as erosional surfaces. At Hinna, floodplain deposits are a mixture of mud and fine grained sand.
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Fluvial architecture There is a clear variation in the distribution and the proportions of the facies associations across the four study sites. At Tashan Alhaji, the system is dominated by sandstone (98% sand) with almost no mud intervals (Fig. 2). The sandstone occurs in multi-storey channel complexes separated by erosional surfaces, with almost no floodplain facies. At Tula and Ture, the outcrops are comprised of multi-storey, laterally amalgamated sand bodies separated by overbank deposits up to 1.2 m. Channel abandonment plugs are common (Fig. 4 and 7). At Hinna, the outcrop is comprised of isolated sandstone channel bodies, which are less amalgamated, less laterally extensive sandstone bodies and clearly separated by a great proportion of floodplain deposits (Fig. 3C). The percentage of sandstone is less compared to the other study sites. This systematic variation in channel proportion and stacking patterns is typical for a DFS with Tashan Alaji, Tula and Ture occurring in the proximal portion and Hinna in the medial (Owen et al. 2015) Variation in thickness, grainsize and facies associations Thickness variation Thickness of individual channel sandstones were measured across the entire study area; these measurements were made on single traceable sandstone channel packages defined by a marked erosional contact on either older channel- or floodplain. The thickness of individual channel storeys at Tashan Alaji in the proximal part of the DFS is between 1.4-4 10
m. These storeys stack into channel complexes which are from 50 - 80 m thick. At Tula and Ture, the thickness of the channel storeys ranges from 1.5 - 17 m. At Hinna, which is 90 km down depositional dip from Tashan Alaji, the majority of deposits are single storey channels between 0.8-8 m thick. Fig. 8 shows a gradual linear decline in the thickness of single-storey sandstone packages measured across the study sites. There is a coincidental increase in the thickness of overbank mudstone deposits from Tashan Alaji with virtually no mud units thicker than 0.6 m, to Hinna with mudstones as thick as 2.5 m (Fig. 3). Grainsize variation The average grainsize of channel deposits decreases from Tashan Alaji to Hinna (Fig. 9). Sandstones at the proximal part of the system (Tashan Alaji and Tula), contain significant pebbly intervals, mostly at the base of the channels (Fig. 2B), and most beds are coarse to very coarse grained, sub-rounded with high proportion of quartz. At Hinna, the isolated channel facies are medium grained with few pebbled intervals, most sandstone units are medium grained, rounded and well sorted (Fig 3A). Distribution of facies associations Figure 10 shows the distribution of the multi-storey, isolated channel and floodplain facies association across the four study sites. At the proximal part (Tashan Alaji, Tula and Ture), there are virtually no isolated channel association, the outcrops are composed of 98-92% multi-storey facies association, with 2- 5% floodplain facies association. At the northern part of the basin (Hinna), the proportion of the multi-storey facies changes drastically to less than 10% whereas the proportion of the isolated channel facies and floodplain facies associations increases. The proportion of the isolated facies at the Hinna is 90-81%, the floodplain facies association at this site increases up to 13%. 11
Over 150 paleocurrent measurements were recorded across the four sites, measurements from each of the sites were plotted on a rose diagram and presented in Fig. 5. Palaeocurrent measurements gives a useful insight about paleo-drainage of the fluvial system. At Tashan Alaji, palaeocurrent shows a dominant radial flow towards the northwest, paleo-flow at Tula, Ture and Hinna shows a radial flow towards the northeast. Discussion Quantitative evaluation of the spatial variation in depositional facies of the continental upper Bima Formation in this study has brought to light what appears to be a distributary fluvial system in the basin. Variation in thickness of channel fill deposits across the four main study locations suggest a decrease in channel sizes as proposed by Weissman et al. (2010) and Owen et al. (2015). Outcrops at Tashan Alaji, Tula and Ture are composed of predominantly multi-storey facies associations, with very little floodplain deposits preserved, these locations are considered to be the proximal part of the DFS, similar to the Salt Wash DFS (Owen, 2015) and Huesca DFS in Spain (Hirst, 1991). There is a general decrease in thickness of in-channel components across the study area. At the proximal part of the system, the average thickness of single sandstone channel storeys at Tashan Alaji (3 m) is less than the thickness in Tula and Ture (7 m) but average thickness of channel complexes in the three sites is consistent. At Tashan Alaji, the attributes of the channel facies support a braided river deposit (Miall, 2014) joining a network of rivers flowing towards the northeast. Paleo-drainage suggests a well-defined flow towards the medial and distal part with a northeast trend which keep evolving and bifurcating downstream. At Hinna, the sandstone units are mostly isolated channel deposits with increased presence of floodplain deposits, the overbank/floodplain facies increase both in frequency, width and
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thickness along the entire section. This implies a medial portion of a DFS. The distal part of the DFS is not exposed at this location but is believed to occur further north in the sub-crop. While the distal part of the DFS which was not observed or quantified in this study, previous work such as Tukur et al, (2015) have reported the presence of the lacustrine dominated facies a few kilometres west of Hinna at Bahai, in the Gongola Basin. Conclusion The upper member of the Bima Formation in the Northern Benue Trough has been interpreted as a distributive fluvial system. Attributes and spatial variation in thickness of the channel infill components; a decrease in the proportion of multi-storey channel belt facies association downstream (from 98% to 5%); increase in floodplain facies association (from 0% to 13%); downstream increase in the proportion of isolated channel fill facies association (from 2% to 80%) and, slight decrease in average grainsize of channel fill sandstone are considered as the evidence of a distributive fluvial system. These variations imply a decrease in channel depth and downstream bifurcation of the river system from the proximal part of system at Tula, Ture and Tashan Alaji to the medial part at Hinna. Difference were noted between the thicknesses of single channel fill facies association within the amalgamated complex at Tashan Alaji and the other proximal facies at Tula and Ture. However, average thicknesses of channel complexes are similar in Tashan Alhaji, Tula and Ture which is interpreted as the proximal part of the DFS. The fluvial deposits at Tashan Alaji may have been deposited by shallower channels in a braided river system that might be connected at some point to a bigger river system trending northeast. The spatial variation of the facies associations of the Bima DFS is similar to other prominent ancient DFSs such as the Salt Wash DFS in USA and the Huesca DFS in Spain. This study has
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not identified or quantify the lacustrine facies association in the major study locations. However, the lacustrine facies association is reported to be present in the basin some few kilometres west of the Hinna. This study has added a basin-wide understanding of the upper Bima Formation which provided a basis for comparison with other ancient DFSs. The results of this study are significant because they provide a re-interpretation of the Bima Formation and because they provide a further, quantified dataset for DFS which can be used for predicting reservoir sand-body distribution in a hydrocarbon reservoirs and aquifers. Acknowledgement We are grateful to Dr. Simon Buckley for providing LIME interpretation license used for this research. The fieldwork was supported by the SAFARI Project (Grant No. RG13517-10). We greatly appreciate the comments from all the anonymous reviewers, their comments have greatly improved the manuscript.
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Figure 1: Location map of the study area, main study sites where UAV images were captured are indicated by black dots, spatial distribution of the different stratigraphic intervals are highlighted in colour. Bima Sandstone the focus of this study is in green.
Figure 2: Outcrop of the upper Bima Formation at study site 1 (Tashan Alaji), A) lower section of the exposure along a stream showing the channel packages, mostly planar cross bedded with very thin silt intervals along the lineation boundaries. B) pebbled, planar laminated bed, part of the thick intervals, this pebbled interval is present across the entire section occurring at the onset of the channel complex boundary. C) 3D SFM model of the outcrop, although somehow vegetated at close
resolution beds and structures are visible, the vertical lines represent inferred boundaries of channel complexes. The red box indicates the location of Fig.2A. D) sedimentary log of the well exposed section of the outcrop showing sedimentary structures and sand to mud ratio.
A
B
C
Figure 3: A section of the Hinna Hill (site 4), the height of the outcrop is about 340m and runs over a 1 km. A) 3D model of the section of the outcrop showing the distribution of isolated channel fill units and floodplain facies associations, there is a clear increase in thickness and width of the floodplain facies downward across the section. B) Floodplain facies association in the medial part of the DFS within the overall succession, it’s one of the thickest units of the floodplain in the lower section of the outcrop. C) The lower section of the outcrop showing an erosional contact with the overlying trough cross bedded, medium grained sandstone.
A
B
C
Figure 4: Multi-storey channel belt complexes facies association at Tula A) Stacked planar and trough bedded sandstone, sedimentary structures varies in scale from bottom to top. Top of the section is predominantly horizontal beds and rippled laminated beds. B) complete section of the outcrop about 5 m thick of stacked channel fill facies. C) Contact between the floodplain facies and the Multistorey channel belt facies association.
Figure 5: Map of the study area showing the spatial variation of the upper Bima Formation, location of the main study sites in black dots and sedimentary logs to show the variation in proportion of floodplain facies. Paleocurrent measurements are plotted in rose diagrams and were used to deduce the direction of paleo-drainage. Paleocurrent shows a radial flow trending northeast at Hinna, Tula and Ture sites, whereas a radial flow trending northwest at Tashan Alaji.
Figure 6: Sedimentary logs from each of the study sites from left to right (site 1-4) and the distance between the sites representing downstream distance along paleo-flow within the DFS. These logs show downstream increase in floodplain facies and decrease in sand to mud ratio.
A
B
Figure 7: Outcrop section of the Bima Formation at Ture, the outcrop is predominantly multi-storey channel belt facies association A) Uninterpreted digital outcrop section, B) Interpreted section multi-storey channel belt facies association, (in yellow) with about 7% floodplain facies (in light blue), most sandstone
units
are
Stacked, intermittently separated by clay plugs. Main channel complexes are separated by about 1.5m of floodplain facies.
Thickness variation Thickness (m)
20 15 10 5 26 26 27 27 27 27.5 27.5 27.5 35 36 90 90 90 90 90 90 90 90 90 90 90 90 90 90 91 91 91 91 91 91 91 91 91 91 91
0
Distance From Apex Thickness (m)
Linear (Thickness (m))
Figure 8: A plot of single sandstone unit thickness against downstream distance along the four study sites, linear trend line shows a gentle decrease in average thickness from the proximal to medial part of the system.
Variation of Grainsize 7.5 7
Average Grainsizes
6.5 6
5.5 5
4.5 4 10
15
26
27
27.5
35
36
90
91
Distance downstream (Km) Mean Grainsizes
Figure 9: Downstream variation in average grainsize of only the sandstone units, this plot shows that grainsize crudely decreases downstream from proximal to medial.
Variation of facies association 100
Percentage of Succession
90 80 70 60 50 40 30 20 10 0 TA_1
TA_2
TR_1
TR_2
TR_3
TL_1
TL_2
BH_1
BH_2
Study sites Multi-storey Facies Assoc.
Isolated channel facies Assoc.
Floodplain facies Assoc.
Figure 10: Variation of the three main facies associations identified in this study, multi-storey channel belt facies association varies from 98% in the proximal to about 5% in the medial part, isolated channel fill facies association increases from 0% in the proximal to about 79% in the medial part of the DFS, Floodplain facies association increases marginally from 2% in the proximal to 13% in the Medial part.
S1464-343X(19)30255-9
DOI:
https://doi.org/10.1016/j.jafrearsci.2019.103600
Reference:
AES 103600
To appear in:
Journal of African Earth Sciences
Received Date: 20 December 2018 Revised Date:
29 July 2019
Accepted Date: 21 August 2019
Please cite this article as: Aliyuda, K., Howell, J., Usman, M.B., Bello, A.M., Maina, B., Abubakar, U., Depositional variability of an ancient distributive fluvial system: The upper member of the lower cretaceous Bima Formation, Northern Benue Trough, Nigeria., Journal of African Earth Sciences (2019), doi: https://doi.org/10.1016/j.jafrearsci.2019.103600. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.
Depositional variability of an ancient distributive fluvial system: The upper member of the Lower Cretaceous Bima Formation, Northern Benue Trough, Nigeria. Kachalla Aliyuda, John Howell, Musa Bappah Usman, Abdulwahab Muhammed Bello, Benjamin Maina and Usman Abubakar. Keywords: Distributive fluvial system, Bima Formation, Northern Benue Trough Abstract The upper Bima Formation in the Northern Benue Trough has been interpreted as a distributive fluvial system. Previous stratigraphic studies on the Bima Formation were focussed on localized, qualitative, sedimentary facies analysis without regional context or quantitative information about sand-body scale and architecture. This study quantitatively analysed sand-body thickness variations in the upper member of the Bima Formation across the Northern Benue Trough, documented the spatial variations in channel dimensions. Photo- realistic virtual outcrops were generated for four study sites using data acquired with an unmanned aerial vehicle and processed photogrammetrically. Analysis of the virtual outcrops illustrated spatial variation in the thickness of channel infill components, a downstream decrease in proportion of multi-storey channel belt facies association (from 98% to 5%), an increase in floodplain facies association (from 0% to 13%), an increase in the proportion of isolated channel fill facies association (from 2% to 80%) and a slight decrease in average grainsizes of channel fill sandstone. These observations are interpreted to represent evidence of a distributive fluvial system (DFS) or mega-fan with the proximal part of system at Tula, Ture and Tashan Alaji to the medial part at Hinna having an increase in frequency and thickness of the flood plain facies association and abundance of isolated channel fill facies associations. This study demonstrates the regional facies variability of the
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upper Bima Formation and has provided a basis for comparison with other ancient distributive fluvial system. Highlights Images of the upper Bima Formation (B3) were acquired and processed. Multi-storey channel, isolated channel and floodplain facies were analysed. The three facies associations vary spatially in the basin typical of an ancient DFS. Variation in thickness, grainsize and facies associations is similar to other DFSs. Introduction Fluvial sandbodies can be deposited by either distributive or tributary fluvial systems. While tributary systems typically occur in upland areas with low preservation potential. Distributive fluvial systems form when rivers deposit sediment into a sedimentary basin (Weissmann et al. 2010). A study of more than 700 modern sedimentary basins shows that distributive fluvial systems account for over 90% of the fluvial deposits that occur in basins and will therefore be preserved in the future rock record (Weissmann et al. 2010, Nyberg & Howell 2015, Owen et al 2015). Over the past 10 years there have been a number of studies of DFS, initially arising from the advent of freely available remote sensing data (Google Earth etc.) for studying modern systems (Weissmann et al. 2010; Hartley et al. 2010; Davidson et al. 2013; Weissmann et al. 2015) and the translation of these concepts to the rock record (Rittersbacher et al. 2014, Owen et al. 2015; Chesley and Leier 2018; McGlue et al. 2016) Despite the recent advances there are only a few case studies that quantify changes in channel body architecture downstream within DFS as described in Owen et al. (2015 & 2017).
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Modern DFS are characterized by a radiating channel from an upstream apex. There is commonly, a downstream decrease in channel size; a downstream increase in the preservation of floodplain deposits (or fines); a downstream decrease in grain size; a downstream reduction in the degree of channel amalgamation. The first ancient DFS to be quantitatively analysed was the Salt Wash DFS of the Morrison Formation in Utah, U.S.A (Owen et al. 2015, and Chesley and Leier 2018). The interpretation of a system as a DFS is easier in the modern where the large-scale channel patterns can be observed directly. Studied modern examples include; the Taquari DFS (Assine 2005; Buehler et al. 2011) and the DFS systems in the Himalayan foredeep (Shukla et al. 2001). Modern and ancient studies highlight the variation of the different facies within the DFS in both lateral and vertical directions as shown in Owen et al., 2017. Bima Formation The Bima Formation was deposited in the Gongola and Yola sub-basins of the Northern Benue Trough during the Cretaceous, it is the oldest sedimentary succession in the basin extending towards the north-western margin of the Gongola sub-basin, and down into the Chad Basin (Fig. 1). It unconformably overlies basement rocks and is divided into three members, lower, middle and upper (Carter et al. 1963). The Bima Formation was first named by Falconer (1911). It was sourced from the granitic terrain bounding the basin (Guiraud, 1990). The thickest exposure of the Bima Formation is at the Lamurde anticline, where it is more than 3,000 m. The Lower Bima (B1) consists of coarse grained sandstone occasionally pebbled with alternating clays and shale, the middle Bima (B2) includes very coarse grained sandstones with thin clay and shale intervals. The upper Bima (B3) consists of whitish-grey well bedded, cross bedded sandstone medium to coarse and very coarse
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grained, frequently pebbly (Carter et al, 1963). The upper Bima (B3) is the focus of this study because of its regional extent within the entire Northern Benue Trough. The base and top of the Bima Formation are believed to be diachronous, however, it was suggested to be deposited from Upper Albian to Lower Turonian (Carter et al, 1963). The Bima Formation is one of the most extensively studied formations in the Northern Benue Trough. Different depositional models have been proposed for the Bima Formation, most of which are disharmonious (such as Tukur et al., 2015, Guiraud, 1990 and Carter et al, 1963). Most previous stratigraphic studies focused on localized analysis of the sedimentary facies without adequate quantitative information about sandstone-body variations. There has been little effort to understand the large-scale spatial variation within the unit. The goal of this study was to quantitatively analyse sand-body geometries in the upper member of the Bima Formation documenting the lateral variation of channel deposits, and the depositional processes associated with such deposits and to asses systematics changes in the context of the DFS paradigm. Understanding sand-body variability is crucial to deciphering hydrocarbon reservoir’s producibility (Aliyuda and Howell, 2019). Hence, Knowledge gained from this study can be applied to understanding fluvial reservoirs in the subsurface.
Study area and methods This study focused on the upper Bima Formation in the Northern Benue Trough (NBT) of Nigeria. The NBT is part of the Benue Trough (or Benue basin) which extends from the Central Benue Trough in the south to the Chad Basin in the north. It is divided into the east-
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west trending Yola sub-basin and the north-south trending Gongola sub-basin (Abubakar et al. 2008). Outcrops of the upper Bima Formation are well exposed at the four study sites across the basin (Fig. 1)The exposures are well over 200 m at two study sites (sites 1 and 4) (Fig. 2 and 3), and the overall thickness of the outcrop exposed at site 1 is in excess of 1000 m. Data were collected using a combination of field-based observation and unmanned aerial vehicle (UAV) photogrammetry (SfM). On the ground field mapping and observations were made at all four study sites (Tahan Ahaji, Ture, Tula and Hinna). These sites have the best exposures of the upper Bima Formation in the basin. Data were collected using traditional field techniques including logging, palaeocurrent directions, bed thickness, the nature of contacts, grain sizes and sedimentary structures sand: mud ratios, and total interval thickness. Data for virtual outcrops (sensu Pringle et al. 2007) were collected as aerial images of the four study sites using an unmanned aerial vehicle (DJI phantom 4 pro) with on board 20 mega pixel camera. The UAV used is equipped with GPS and records the position of every image captured. Several hundreds of images were captured from each site with over 40% overlap. 3D structure of the study sites was reconstructed using the structure from motion (SfM) photogrammetry technique (Carrivick et al. 2016). Agisoft’s Photoscan software was used for the reconstruction. The 3D model creation workflow consists of the following processes; outcrop selection, photo acquisition, photo selection, photo alignment using the recorded coordinates on the images, building dense cloud, generating 3D mesh from the dense cloud, and finally generating texture for the model. The ready-made 3D models were then exported to LIME (Buckley et al. 2019) a purpose-built software for interpreting virtual
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outcrops, to extract quantitative information such as thicknesses of beds and facies associations.
Results Lithofacies and associations Five main facies were identified in the study sites, these facies are described in Table 1. The facies are grouped into three broad facies associations: multi-storey channel belt facies association, isolated channel-fill facies association and overbank/floodplain facies association.
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Table 1: Lithofacies description Facies Lithology
Interpretation and Environment of Deposition
St
Lateral channel point bar migration deposits.
Sp
Sh
Sc
FI
Description Medium to v. coarse Trough cross sandstone occasionally bedded pebbled base, poor to pebbled moderately sorted, sandstone erosional contact. Medium, coarse to v. coarse Planar cross sandstone, occasionally bedded pebbled base, moderate to sandstone well sorted, separated by accretion surfaces. Fine to medium grained Ripple crosssandstone, moderate to bedded/horiz well sorted, with climbing ontal bedded ripple lamination, variable sandstone bed thickness (Fig. 4). Fine to medium grained sandstone, moderately Convolute sorted, convoluted beds bedded are mostly above very sandstone fine-grained sandstone and shale. Ripple laminated siltstone, Siltstone, shale purple shale and mud mudstone.
Downstream mid-channel bar migration deposits.
Planar flow deposit in Upper flow regime.
Soft sediment deformational structures.
Overbank, floodplain deposits.
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Multi-storey channel belt facies association This facies association is composed of medium to coarse grained sandstone, cream to whitish grey, with pebbled intervals, moderate to well sorted, almost entirely planar cross bedded, with very few convoluted beds (Facies – Sp; Fig. 2). Beds thickness varies (1 – 9 m), with obvious beds thickening across the channel complexes. These channel fills are separated by distinct erosion surfaces and most planar cross beds are oriented in the same direction as the accretion surfaces, suggesting deposition in downstream accreting bars in a fluvial setting. (Miall 1977, 1985). This supports Carter et al (1963) and many other works previous interpretation of the Bima Sandstone as a fluvial deposit. Downstream accretion, limited preservation of mudstone or shale, the presence of lag deposits and planar cross bedding suggest that the deposits were from a predominantly braided river system (Miall, 1977). Palaeocurrent measurements, corrected for structural dip and fold plunge for beds with structural dip exceeding 10° show a dominant radial pattern towards the northwest (Fig. 5), this spread of palaeocurrents reflect the degree of sinuosity in the depositing river. Channel bodies are stacked in sand sheet deposits which are stacked vertically and laterally forming multiple stories with almost no mud or shale intervals. Channel complexes are separated by erosional surfaces measuring over 86 meters. About 6-7 channel complexes are visible on the virtual outcrop from Tahan Ahaji (Fig. 2). Isolated channel-fill facies association This facies association is made up of lithofacies St, comprised of medium to very coarse grained sandstone with lag deposit. The deposits have trough and planar cross bedding and are poor to moderately sorted. This single-storey sand channel-fills are lensed, or wedge 8
shaped in geometry with clearly defined erosional contact with the overbank or floodplain facies association (Fig. 3). The isolated channel-fill facies association at Tula and Ture are locally separated by clay plugs representing channel abandonment. The succession shows pebbles at the base, with large scale trough cross bedding, thinning and fining upward into medium grained, planar cross bedded units mostly in smaller beds (50 - 70 cm in thickness; Fig. 6). These sedimentary structures suggest lateral migration of channel bars and bedload sheets of a fluvial channel (Keefer 1999, Friend et al. 1979 and Gibling 2006). Single-storeys range between 1-11 m in thickness and extends over 4meters. At Hinna, they are ribbonshaped, mostly asymmetric, and quite extensive in width, suggesting that most of these channel-fill were bank-attached bars that laterally migrated (Gibling 2006). Overbank/floodplain facies association This facies association consists of interbedded fine to very fine-grained sandstone, and mudstones. The sandstones are rippled laminated, creamy or whitish. Mudstone are creamy, light brown and purple (Fig. 3A and 3B). These are muds deposited out of suspension in river floodplains and overbanks during flood events. This facies association also includes crevasse-splay, abandoned channel deposits, paleosol and floodplain deposits (Demko et al, 2004). Beds vary in thickness from 0.2 - 3 m, mostly intercalated with the mudstone and fine to very fine-grained sandstone. The overbank deposits are commonly incised into by the erosional bases of the main channel sandbodies. Crevasse-splay and abandoned channel deposits were not observed in the proximal part of the system at Tashan Alaji, however, they left their imprint as erosional surfaces. At Hinna, floodplain deposits are a mixture of mud and fine grained sand.
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Fluvial architecture There is a clear variation in the distribution and the proportions of the facies associations across the four study sites. At Tashan Alhaji, the system is dominated by sandstone (98% sand) with almost no mud intervals (Fig. 2). The sandstone occurs in multi-storey channel complexes separated by erosional surfaces, with almost no floodplain facies. At Tula and Ture, the outcrops are comprised of multi-storey, laterally amalgamated sand bodies separated by overbank deposits up to 1.2 m. Channel abandonment plugs are common (Fig. 4 and 7). At Hinna, the outcrop is comprised of isolated sandstone channel bodies, which are less amalgamated, less laterally extensive sandstone bodies and clearly separated by a great proportion of floodplain deposits (Fig. 3C). The percentage of sandstone is less compared to the other study sites. This systematic variation in channel proportion and stacking patterns is typical for a DFS with Tashan Alaji, Tula and Ture occurring in the proximal portion and Hinna in the medial (Owen et al. 2015) Variation in thickness, grainsize and facies associations Thickness variation Thickness of individual channel sandstones were measured across the entire study area; these measurements were made on single traceable sandstone channel packages defined by a marked erosional contact on either older channel- or floodplain. The thickness of individual channel storeys at Tashan Alaji in the proximal part of the DFS is between 1.4-4 10
m. These storeys stack into channel complexes which are from 50 - 80 m thick. At Tula and Ture, the thickness of the channel storeys ranges from 1.5 - 17 m. At Hinna, which is 90 km down depositional dip from Tashan Alaji, the majority of deposits are single storey channels between 0.8-8 m thick. Fig. 8 shows a gradual linear decline in the thickness of single-storey sandstone packages measured across the study sites. There is a coincidental increase in the thickness of overbank mudstone deposits from Tashan Alaji with virtually no mud units thicker than 0.6 m, to Hinna with mudstones as thick as 2.5 m (Fig. 3). Grainsize variation The average grainsize of channel deposits decreases from Tashan Alaji to Hinna (Fig. 9). Sandstones at the proximal part of the system (Tashan Alaji and Tula), contain significant pebbly intervals, mostly at the base of the channels (Fig. 2B), and most beds are coarse to very coarse grained, sub-rounded with high proportion of quartz. At Hinna, the isolated channel facies are medium grained with few pebbled intervals, most sandstone units are medium grained, rounded and well sorted (Fig 3A). Distribution of facies associations Figure 10 shows the distribution of the multi-storey, isolated channel and floodplain facies association across the four study sites. At the proximal part (Tashan Alaji, Tula and Ture), there are virtually no isolated channel association, the outcrops are composed of 98-92% multi-storey facies association, with 2- 5% floodplain facies association. At the northern part of the basin (Hinna), the proportion of the multi-storey facies changes drastically to less than 10% whereas the proportion of the isolated channel facies and floodplain facies associations increases. The proportion of the isolated facies at the Hinna is 90-81%, the floodplain facies association at this site increases up to 13%. 11
Over 150 paleocurrent measurements were recorded across the four sites, measurements from each of the sites were plotted on a rose diagram and presented in Fig. 5. Palaeocurrent measurements gives a useful insight about paleo-drainage of the fluvial system. At Tashan Alaji, palaeocurrent shows a dominant radial flow towards the northwest, paleo-flow at Tula, Ture and Hinna shows a radial flow towards the northeast. Discussion Quantitative evaluation of the spatial variation in depositional facies of the continental upper Bima Formation in this study has brought to light what appears to be a distributary fluvial system in the basin. Variation in thickness of channel fill deposits across the four main study locations suggest a decrease in channel sizes as proposed by Weissman et al. (2010) and Owen et al. (2015). Outcrops at Tashan Alaji, Tula and Ture are composed of predominantly multi-storey facies associations, with very little floodplain deposits preserved, these locations are considered to be the proximal part of the DFS, similar to the Salt Wash DFS (Owen, 2015) and Huesca DFS in Spain (Hirst, 1991). There is a general decrease in thickness of in-channel components across the study area. At the proximal part of the system, the average thickness of single sandstone channel storeys at Tashan Alaji (3 m) is less than the thickness in Tula and Ture (7 m) but average thickness of channel complexes in the three sites is consistent. At Tashan Alaji, the attributes of the channel facies support a braided river deposit (Miall, 2014) joining a network of rivers flowing towards the northeast. Paleo-drainage suggests a well-defined flow towards the medial and distal part with a northeast trend which keep evolving and bifurcating downstream. At Hinna, the sandstone units are mostly isolated channel deposits with increased presence of floodplain deposits, the overbank/floodplain facies increase both in frequency, width and
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thickness along the entire section. This implies a medial portion of a DFS. The distal part of the DFS is not exposed at this location but is believed to occur further north in the sub-crop. While the distal part of the DFS which was not observed or quantified in this study, previous work such as Tukur et al, (2015) have reported the presence of the lacustrine dominated facies a few kilometres west of Hinna at Bahai, in the Gongola Basin. Conclusion The upper member of the Bima Formation in the Northern Benue Trough has been interpreted as a distributive fluvial system. Attributes and spatial variation in thickness of the channel infill components; a decrease in the proportion of multi-storey channel belt facies association downstream (from 98% to 5%); increase in floodplain facies association (from 0% to 13%); downstream increase in the proportion of isolated channel fill facies association (from 2% to 80%) and, slight decrease in average grainsize of channel fill sandstone are considered as the evidence of a distributive fluvial system. These variations imply a decrease in channel depth and downstream bifurcation of the river system from the proximal part of system at Tula, Ture and Tashan Alaji to the medial part at Hinna. Difference were noted between the thicknesses of single channel fill facies association within the amalgamated complex at Tashan Alaji and the other proximal facies at Tula and Ture. However, average thicknesses of channel complexes are similar in Tashan Alhaji, Tula and Ture which is interpreted as the proximal part of the DFS. The fluvial deposits at Tashan Alaji may have been deposited by shallower channels in a braided river system that might be connected at some point to a bigger river system trending northeast. The spatial variation of the facies associations of the Bima DFS is similar to other prominent ancient DFSs such as the Salt Wash DFS in USA and the Huesca DFS in Spain. This study has
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not identified or quantify the lacustrine facies association in the major study locations. However, the lacustrine facies association is reported to be present in the basin some few kilometres west of the Hinna. This study has added a basin-wide understanding of the upper Bima Formation which provided a basis for comparison with other ancient DFSs. The results of this study are significant because they provide a re-interpretation of the Bima Formation and because they provide a further, quantified dataset for DFS which can be used for predicting reservoir sand-body distribution in a hydrocarbon reservoirs and aquifers. Acknowledgement We are grateful to Dr. Simon Buckley for providing LIME interpretation license used for this research. The fieldwork was supported by the SAFARI Project (Grant No. RG13517-10). We greatly appreciate the comments from all the anonymous reviewers, their comments have greatly improved the manuscript.
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Weissmann G. S, Adrian John Hartley, L. A Scuderi, G. J Nichols, Amanda Owen, S. Wright, A. L Felicia, F. Holland, F. M. L Anaya., 2015., Fluvial geomorphic elements in modern sedimentary basins and their potential preservation in the rock record: a review., Geomorphology, 250, 187-219.
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Figure 1: Location map of the study area, main study sites where UAV images were captured are indicated by black dots, spatial distribution of the different stratigraphic intervals are highlighted in colour. Bima Sandstone the focus of this study is in green.
Figure 2: Outcrop of the upper Bima Formation at study site 1 (Tashan Alaji), A) lower section of the exposure along a stream showing the channel packages, mostly planar cross bedded with very thin silt intervals along the lineation boundaries. B) pebbled, planar laminated bed, part of the thick intervals, this pebbled interval is present across the entire section occurring at the onset of the channel complex boundary. C) 3D SFM model of the outcrop, although somehow vegetated at close
resolution beds and structures are visible, the vertical lines represent inferred boundaries of channel complexes. The red box indicates the location of Fig.2A. D) sedimentary log of the well exposed section of the outcrop showing sedimentary structures and sand to mud ratio.
A
B
C
Figure 3: A section of the Hinna Hill (site 4), the height of the outcrop is about 340m and runs over a 1 km. A) 3D model of the section of the outcrop showing the distribution of isolated channel fill units and floodplain facies associations, there is a clear increase in thickness and width of the floodplain facies downward across the section. B) Floodplain facies association in the medial part of the DFS within the overall succession, it’s one of the thickest units of the floodplain in the lower section of the outcrop. C) The lower section of the outcrop showing an erosional contact with the overlying trough cross bedded, medium grained sandstone.
A
B
C
Figure 4: Multi-storey channel belt complexes facies association at Tula A) Stacked planar and trough bedded sandstone, sedimentary structures varies in scale from bottom to top. Top of the section is predominantly horizontal beds and rippled laminated beds. B) complete section of the outcrop about 5 m thick of stacked channel fill facies. C) Contact between the floodplain facies and the Multistorey channel belt facies association.
Figure 5: Map of the study area showing the spatial variation of the upper Bima Formation, location of the main study sites in black dots and sedimentary logs to show the variation in proportion of floodplain facies. Paleocurrent measurements are plotted in rose diagrams and were used to deduce the direction of paleo-drainage. Paleocurrent shows a radial flow trending northeast at Hinna, Tula and Ture sites, whereas a radial flow trending northwest at Tashan Alaji.
Figure 6: Sedimentary logs from each of the study sites from left to right (site 1-4) and the distance between the sites representing downstream distance along paleo-flow within the DFS. These logs show downstream increase in floodplain facies and decrease in sand to mud ratio.
A
B
Figure 7: Outcrop section of the Bima Formation at Ture, the outcrop is predominantly multi-storey channel belt facies association A) Uninterpreted digital outcrop section, B) Interpreted section multi-storey channel belt facies association, (in yellow) with about 7% floodplain facies (in light blue), most sandstone
units
are
Stacked, intermittently separated by clay plugs. Main channel complexes are separated by about 1.5m of floodplain facies.
Thickness variation Thickness (m)
20 15 10 5 26 26 27 27 27 27.5 27.5 27.5 35 36 90 90 90 90 90 90 90 90 90 90 90 90 90 90 91 91 91 91 91 91 91 91 91 91 91
0
Distance From Apex Thickness (m)
Linear (Thickness (m))
Figure 8: A plot of single sandstone unit thickness against downstream distance along the four study sites, linear trend line shows a gentle decrease in average thickness from the proximal to medial part of the system.
Variation of Grainsize 7.5 7
Average Grainsizes
6.5 6
5.5 5
4.5 4 10
15
26
27
27.5
35
36
90
91
Distance downstream (Km) Mean Grainsizes
Figure 9: Downstream variation in average grainsize of only the sandstone units, this plot shows that grainsize crudely decreases downstream from proximal to medial.
Variation of facies association 100
Percentage of Succession
90 80 70 60 50 40 30 20 10 0 TA_1
TA_2
TR_1
TR_2
TR_3
TL_1
TL_2
BH_1
BH_2
Study sites Multi-storey Facies Assoc.
Isolated channel facies Assoc.
Floodplain facies Assoc.
Figure 10: Variation of the three main facies associations identified in this study, multi-storey channel belt facies association varies from 98% in the proximal to about 5% in the medial part, isolated channel fill facies association increases from 0% in the proximal to about 79% in the medial part of the DFS, Floodplain facies association increases marginally from 2% in the proximal to 13% in the Medial part.