Sedimentology, geochemistry and ...

Marine and Petroleum Geology 67 (2015) 663e677

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Research paper

Sedimentology, geochemistry and paleoenvironmental reconstruction of the Cretaceous Yolde formation from Yola Sub-basin, Northern Benue Trough, NE Nigeria Babangida M. Sarki Yandoka a, b, *, M.B. Abubakar b, c, Wan Hasiah Abdullah a, A.S. Maigari c, Mohammed Hail Hakimi d, Adebanji Kayode Adegoke a, e, J.J. Shirputda b, Abdulkarim H. Aliyu b a

Department of Geology, University of Malaya, 50603 Kuala Lumpur, Malaysia National Centre for Petroleum Research and Development, A.T.B.U, Bauchi, Nigeria c Geology Programme, Abubakar Tafawa Balewa University, PMB 0248 Bauchi, Nigeria d Geology Department, Faculty of Applied Science, Taiz University, 6803 Taiz, Yemen e Department of Geology, Ekiti State University, P.M.B. 5363 Ado-Ekiti, Nigeria b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 22 December 2014 Received in revised form 10 June 2015 Accepted 14 June 2015 Available online 21 June 2015

Sedimentology, organic and inorganic geochemical studies were applied on sediments of the Cretaceous Yolde formation from Yola Sub-basin, Northern Benue Trough, northeastern Nigeria with an attempt to determine the sedimentary facies, paleo-redox condition and to re-construct the palaeo-depositional environments. Eight (8) facies were identified on the basis of lithology, grain size, ichnofossils/degree of bioturbation and sedimentary structures. These facies constitute four facies associations; the FA-1 (offshore marine), FA-2 (offshore transition to lower shoreface), FA-3 (middle shoreface) and FA-4 (upper shoreface). The succession of these facies associations indicates storm and wave influenced offshore/ shoreface depositional environments for the formation. Molecular organic geochemical investigation suggests deposition under sub-oxic marine environment with major contribution of phytoplankton, bacteria and minor amount of terrigenous organic matter. XRD mineralogical assessment, and major and trace elements geochemistry revealed that the Yolde formation sediments were sourced from continental areas of passive continental margin setting and deposited in semiarid climate under suboxic shallow marine conditions related to the first marine transgression that occurred in the Benue Trough during the Late Cretaceous period. © 2015 Elsevier Ltd. All rights reserved.

Keywords: Northern Benue Trough Yola Sub-basin Yolde formation Offshore-shoreface facies Sub-oxic condition

1. Introduction The Nigerian economy is dependent almost solely on oil and gas with over 90% of its foreign earnings coming from the sector (Obaje et al., 2004). The crude oil proven reserves of approximately 37.2 billion barrels in the Niger Delta (the only sedimentary basin in Nigeria from which petroleum is exploited) is estimated to last for another 50e60 years at the current exploration rate of averagely 2 million barrels per day. Also, on a global scale, petroleum as energy source will continue to dominate other primary energy sources and

* Corresponding author. Department of Geology, University of Malaya, 50603 Kuala Lumpur, Malaysia. E-mail addresses: [email protected], [email protected] (B.M. Sarki Yandoka). http://dx.doi.org/10.1016/j.marpetgeo.2015.06.009 0264-8172/© 2015 Elsevier Ltd. All rights reserved.

is expected to account for up to 52% of energy demand in the year 2030 (for United State of America alone) under prevailing policies and regulations (AEO, 2014). These trends imply the need for sustained hydrocarbon exploration to meet up with global demands and specifically for future economic growth of Nigeria. The Yola Sub-basin is a petroleum frontier basin in Nigeria and part of a megarift system termed West and Central African Rift System (WCARS) within which several petroleum exploration successes were recorded in the neighbouring countries of the Chad (Doba, Doseo and Bongor fields), the Niger (Termit-Agadem Basin) and Sudan (Muglad Basin). Major reservoirs of hydrocarbon accumulations discovered in the WCARS are sandstones of late Albian to Cenomanian age (e.g. Bentiu Formation in the Muglad Basin and Sedigi Formation in the Termit-Agadem Basin, Lirong et al., 2013; Wan et al., 2014) and mostly of coastal marine depositional

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B.M. Sarki Yandoka et al. / Marine and Petroleum Geology 67 (2015) 663e677

environment (e.g. the Sedigi Formation). These formations are lateral equivalents to the Yolde formation of the Yola Sub-basin of the Northern Benue Trough of Nigeria (Abubakar, 2014). Also, in the adjacent Gongola Sub-basin some 33 billion cubic feet of gas was encountered in the Yolde formation from well Kolmani River-1 (Abubakar, 2006). Little however is known on the depositional conditions of this important reservoir unit (the Yolde formation). This study therefore, attempts to determine the sedimentary facies, paleo-redox condition and palaeo-depositional environment of the Cretaceous Yolde formation in the Yola Sub-basin using integrated lithofacies analysis, mineralogy, and organic (molecular) and inorganic (major and trace elements) geochemistry. The findings of this study are expected to contribute in the characterization of the Yolde formation as yet another late Albian to Cenomanian potential reservoir unit in the Yola Sub-basin part of the WCARS and will help in petroleum accumulation predictions not only in the Northern Benue Trough, but also in the entire WCARS. 2. Tectonic setting and stratigraphy The Benue Trough of Nigeria is part of the West and Central African Rift System (WCARS); a mega tectonic structure formed by the tension generated due to the separation of the African and South American plates during the Early Cretaceous (Sarki Yandoka et al., 2014). It is over 1000 km in length and exceeds 150 km in width. Its southern outcrop limit is the northern boundary of the Niger Delta Basin, while the northern outcrop limit is the southern boundary of the Chad Basin separated from the Benue Trough by an anticlinal structure termed the “Dumbulwa-Bage High” (Fig. 1). The trough is filled with up to 6000 m of Cretaceous e Paleogene sediments (Abubakar, 2014; Sarki Yandoka et al., 2014). Several authors have presented different tectonic models on the genesis of the Benue Trough (Abubakar, 2014). Most authors proposed tensional movement resulting in a rift (King, 1950) or a

graben-like structure (Stoneley, 1966). An RRF triple junction model leading to plate dilation and opening of the Gulf of Guinea was proposed by Grant (1971). Olade (1975) considered the Benue Trough as the third failed arm or aulocogen of a three armed rift system (RRR) related to the development of hotspots. Benkhelil (1982, 1989), and Guiraud and Maurin (1992) considered wrench faulting as the dominant tectonic process during the Benue Trough evolution and defined it as a set of juxtaposed pull-apart basins. The Benue Trough is geographically sub-divided into Southern, Central and Northern portions (Nwajide, 2013; Abubakar, 2014). The Northern Benue Trough is made up of two major sub-basins; the NeS trending Gongola Sub-basin and the EeW trending Yola Sub-basin (Fig. 1). The study area is located in the Yola Sub-basin of the Northern Benue Trough (Fig. 1). Most workers such as Carter et al. (1963), Offodile (1976), Benkhelil (1989), Obaje et al. (2004), Abubakar (2014), Tukur et al. (2015) among many others have described the stratigraphy of the Northern Benue Trough. The stratigraphy (Fig. 2) comprises the continental Lower Cretaceous Bima Formation, the Cenomanian transitional marine Yolde formation and the marine late CenomanianeSantonian Dukul, Jessu, Sekuliye Formations, Numanha Shales and Lamja Sandstones. The Cenomanian transitional Yolde formation, the subject of this study, was earlier recognized by Carter et al. (1963) as the transitional marine beds conformably on the Bima Formation (Fig. 2), although the boundary between the two formations has been difficult to ascertain in both Gongola and Yola Sub-basins. The lower part of the Yolde formation consists of alternating sandstones and dark-grey mudstones, while upper part consists of bedded sandstones with argillaceous intercalations. Bioturbations and trace fossils are common towards the top while groove marks are present on some beds. It was earlier interpreted as fluvial at its base and shallow marine at its upper part (Zarboski et al., 1997). Volcanic plugs of Tertiary age were reported in some of the Cretaceous

Fig. 1. (a) Generalized geological map of Nigeria showing the study area represented as Open Square (modified from Abubakar, 2006) and, b) map showing the study area and location of sedimentary logs (Google Earth Map, 2014).


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Fig. 2. Stratigraphic sequence of the Yola Sub-basin (from Sarki Yandoka et al., 2015).

formations in the Northern Benue Trough (Carter et al., 1963; Sarki Yandoka et al., 2014).

3. Field and laboratory methods Fieldwork was carried out to identify the facies of the Cretaceous Yolde formation from Yola Sub-basin, outcropped around Chegau, Bambam and Reme streams (Fig. 1). The facies description includes observation of features, such as color, sedimentary structures, texture, fossils, bioturbations, bed thickness and geometry. Ten (10) non-weathered shale samples were collected from several stratigraphic intervals. The samples were crushed to less than 200 mesh size and about 30 g was subjected to bitumen extraction using a Soxhlet apparatus for 72 h with an azeotropic mixture of dichloromethane (DCM) and methanol (CH3OH) (93:7). The extracts were separated into saturated hydrocarbons, aromatic hydrocarbons and NSO (nitrogen, sulphur and oxygen) compounds by liquid (column) chromatography. The saturated hydrocarbon

fractions were dissolved in hexane and analyzed by Gas chromatography-Mass spectrometry (GCeMS). X-ray Powder Diffraction (XRD) analysis was performed on the samples using SIEMENS D5000 X-ray diffractometer with Cu Ka radiation, run from 5 to 60 2q, with a step increment of 0.02 and a counting time of 2 s per step. The minerals were identified from the diffractograms by referencing to the ICDD Powder Diffraction File. Also, about 0.50 g of each sample was prepared for non-destructive wavelength dispersive X-ray fluorescence spectrometry (PANalyticalAxiosm AX4KW sequential XRF spectrometer) to determine the concentration of major elements. The analytical uncertainty is usually 95% analytical precision. 4. Results 4.1. Facies descriptions and interpretations Sedimentary facies is a certain volume of rock which is characterized by a set of features; such as grain size, geometry and structure that distinguish it from other rock units (Anderton, 1985). The outcrops of the Yolde formation around Chegau, Bambam and Reme streams in the Yola Sub-basin composed of mainly sandstones, siltstones and shales (Fig. 3). The sandstones are mostly buff and light brown in color and sometimes white. The shales on the other hand are light brown-grey-dark gray in color. These rock units were divided into eight (8) facies described on the basis of sedimentary structures, lithology, bioturbation and trace fossils,

and were interpreted based on the earlier works of Harms et al. (1975), Walker and Plint (1992), Bhattacharya and Walker (1992), Bhattacharya (2006, 2010), Plint (2010), Souza et al. (2012), Bressan et al. (2013), among many others. The identified facies provided an insight into depositional processes and subpaleodepositional environments of the Yolde formation (Fig. 3). 4.1.1. Facies FS: clay/shale 4.1.1.1. Description. This facies constitutes the dominant lithology in the measured sections (Fig. 3). It is characterized by dark-gray, gray or light-brown clays or shales (Fig. 4i) that are commonly moderately to intensely bioturbated and partly glauconitic. Because of their very fine-grained size, they appear homogeneous in hand specimens. Thickness ranges from 1.0 to up to 12 m (Fig. 3). The facies is associated with horizontally bedded sandstone facies (HP) and bioturbated siltstone facies (BS). 4.1.1.2. Interpretation. Fine-grained sedimentary rocks such as shales with an average grain size of less than 62.5 mm are the most

Fig. 3. Sedimentary logs of the Yolde formation showing the identified lithofacies and facies associations.


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Fig. 4. i) clay/shale facies (FS) associated with embedded horizontally bedded sandstone facies (HP); ii) horizontally bedded sandstone facies (HP); iii) bioturbated sandstone facies (BS) showing truncated nodule; iv) traces of Thalassinoides, ichnofossil on bed surface of facies BS.

commonly preserved sedimentary rocks within the earth (Macquaker and Bohacs, 2007). The fine-grained composition, bioturbation and glauconitic nature of the FS facies indicate deposition in low-energy perhaps offshore marine environment (Nichols, 2009; Di Celma et al., 2010). Similar facies were

interpreted to represent deposition below and immediately above the storm wave base in shelf (offshore) and offshore transition shallow marine environment (http://www.sciencedirect.com/ science/article/pii/S0037073809002668Pattison, 1995; Bressan and Palma, 2009; Nichols, 2009; Bressan et al., 2013).

Fig. 5. i) wave-rippled andstone facies (RC); ii) surface expression of wave ripples on facies RC; iii) bioturbated massive sandstone facies (BM); iv) hummocky cross-stratified sandstone facies (HC).

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Fig. 6. i and ii) swaley cross-stratified sandstone facies (SC); iii) diminutive trace fossils on facies SC; iv) soft sediment deformation structures on facies SC.

4.1.2. Facies HP: horizontally bedded sandstone 4.1.2.1. Description. This facies occurs as an isolated unit within the clays/shales of the Facies FS. It has a sharp basal contact and is characterized by horizontally bedded, brownish-grey, moderate-well sorted, partly glauconitic, very fine grained sandstone (Fig. 3, 4ii).

Thickness is in the range of 60e70 cm. The beds often contain evidence of bioturbation. 4.1.2.2. Interpretation. Interpreting this facies within the context of the associated Facies FS indicates deposition by storm activity

Fig. 7. Planar crossbedded sandstone facies (PC): i) showing some soft sediment deformation structures; ii) showing wedge-shaped planar crossbed sets (converging arrows identify unit sets); iii) showing bioturbated truncation surfaces indicated with arrows; iv) showing reactivation surface indicated with black arrow and low angle set-bounding surfaces indicated with white.


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Fig. 8. Mass fragmentograms of m/z 85, m/z 191 and m/z 217 of saturated hydrocarbon fractions of some studied shale samples.

related to high-energy (upper flow regime) within a rather low energy environment and is considered to represent stormgenerated offshore ridge in shallow marine environment (e.g. Nichols, 2009; Souza et al., 2012). 4.1.3. Facies BS: bioturbated siltstone 4.1.3.1. Description. This facies (Fig. 3, 4iii) is composed of highly bioturbated siltstones and clayey siltstones that are yellowish brown to light green colored and massive but occasionally rippled.

This facies is mostly glauconitic in nature and shows intensive bioturbation in the siltstones consisting of moderately sized Thalassinoides (Fig. 4iv) and Ophiomopha ichnofauna. Thickness ranges from 20 cm to 1.20 m. It is occasionally nodular (20e50 cm diameter) towards surface. 4.1.3.2. Interpretation. Based on grain size, high degree of bioturbation and glauconitic nature of the BS Facies, it could be interpreted to indicate low-energy shallow marine depositional

Table 1 Biomarker ratios based on n e alkanes, isoprenoids, terpanes, triterpanes and steranes distributions of the analysed Yolde shale sediments. Sample ID n e alkanes and acyclic isoprenoids

YOL2 YOL3A YOL3B YOL4 YOL5 YOLB

Terpanes and triterpanes (m/z191)

Steranes and diasteranes (m/z 217)

Pr/Ph Pr/n-C17 Pr/n-C18 CPI

WI

C29/C30 MC30/HC30 G/C30 Tm/Ts C24Te/C26T C23T/C24Te C27 (%) C28 (%) C29 (%) C29/C27 Diasterane/ Hop/Ster sterane

1.00 1.04 1.06 1.24 1.22 1.34

0.43 0.30 0.61 0.42 0.30 0.21

1.13 0.87 0.90 0.78 1.14 1.21

0.61 0.60 0.67 0.62 0.45 0.72

0.49 0.47 0.62 0.41 0.32 0.56

1.14 0.60 1.12 0.80 0.83 1.20

0.16 0.23 0.19 0.20 0.28 0.30

e 0.13 0.12 0.12 0.02 e

0.83 0.93 0.75 0.67 1.05 1.02

1.92 2.72 1.98 2.66 2.62 2.78

4.72 4.53 4.12 4.58 4.04 4.86

44.3 41.9 42.3 43.1 39.2 45.1

29.0 23.9 22.8 25.9 30.2 26.3

26.7 34.1 34.9 30.9 30.6 28.6

0.60 1.98 4.35 0.81 1.52 3.0 0.78 1.85 4.1 0.72 1.42 4.16 0.88 1.49 2.73 0.94 1.97 4.21 P P Pr e Pristane, Ph e Phytane,CPI e Carbon preference index (1): {2(C23 þ C25 þ C27 þ C29)/(C22 þ 2[C24 þ C26 þ C28] þ C30),Waxiness index e (n-C21-n-C31)/ (n-C15-n-C20) C29/C30: C29 norhopane/C30 hopane.Ts: (C27 18a(H)-22,29,30-trisnorneohopane) Tm: (C27 17a(H)-22,29,30-trisnorhopane)C30M/C30H: C30 moretane/C30 hopaneG/C30H: Gammacerane/C30 hopane. Diasterane/sterane ratio: C29 diasteranes/C29 regular steranes.

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process, allowing relatively fine-grained sedimentation driven by gravity settling (Buatois et al., 2002). Perhaps, it was occasionally influenced by storm activity as indicated by its nodular character n and Eynatten, 2014). (Nichols, 2009; Alva 4.1.4. Facies RC: wave-rippled sandstone 4.1.4.1. Description. This facies is characterized by wave rippled, moderate to well sorted, very fine to fine grained sandstones (Fig. 5i, ii). It is low to moderately bioturbated with thicknesses in the range of 20e60 cm (up to 1 m where amalgamated, Fig. 3). It is usually glauconitic and light green colored but deep brown on weathered surfaces (Fig. 5ii). 4.1.4.2. Interpretation. The facies contains rippled lamination inferred to be generated by waves. Similar facies were interpreted to represent low-energy wave deposition in shallow marine setting because of its glauconitic nature (Walker and Plint, 1992; Bhattacharya and Walker, 1992; García et al., 2011; Souza et al., 2012).

4.1.7.2. Interpretation. Swaley cross-stratification signifies storminfluenced shoreface processes (e.g., Duke et al., 1991; Bhattacharya, 2010). It was interpreted to represent deposition by high-energy storm due to the migration of low relief bedforms (Walker and Plint, 1992; Nichols, 2009; Dumas and Arnott, 2006; Souza et al., 2012; Bressan et al., 2013). 4.1.8. Facies PC: planar cross-bedded sandstone 4.1.8.1. Description. This facies is characterized by planar crossbedded, gray to light brown, moderate to well sorted, fine to medium grained sandstones (Fig. 3). The planar stratifications dip at 15e19 and generally occur in wedge-shaped sets (Fig. 7ii) bounded by low angle surfaces of truncations (Fig. 7iii). Thickness of individual stratifications ranges between 20 and 30 cm and coset units may reach up to 2 m (Fig. 3). The units may show reactivation surfaces (Fig. 7iv). This facies is interbedded with swaley crossstratified sandstone facies (SC) and also associated with some ripple marks and minor bioturbation especially on truncation surfaces (Fig. 3).

4.1.5. Facies BM: bioturbated massive sandstone 4.1.5.1. Description. This facies is characterized by amalgamated (occasionally thinly e medium bedded), highly bioturbated, fine to medium grained, light brown colored sandstones (Fig. 5iii). Thickness ranges from 50 cm to 3 m. It is laterally extensive and displays intense bioturbation (Fig. 3). Trace fossil assemblage includes Thalassinoides, Ophiomorpha, Planolites and Skolithos.

4.1.8.2. Interpretation. The Facies PC was interpreted to be deposited by the migration of dunes under unidirectional flow due to wave swash process in the upper shoreface or migration of ridge and runnels in the foreshore zone (McCubbin, 1981; Souza et al., 2012).

4.1.5.2. Interpretation. The lack of physical sedimentary structures on this facies due to intense bioturbation hampered interpretation of depositional processes. However, its interbedding with lowenergy facies (facies FS), ichnofossil suite and degree of bioturbation may suggest high energy depositional process within a generally low-energy depositional system (Souza et al., 2012). Similar facies were interpreted as shallow marine (lower shoreface or offshore) environments (Walker and Plint, 1992; Bhattacharya and Giosan, 2003; Bhattacharya, 2006, 2010).

Facies association is a group of faciesthat is used to define a particular sedimentary environment (Anderton, 1985; Nichols, 2009). From the description and interpretation of the facies (section 4.1), four facies associations were recognized; the FA-1 (offshore marine), the FA-2 (offshore transition to lower shoreface), the FA-3 (middle shoreface) and the FA-4 (upper shoreface).

4.1.6. Facies HC: hummocky cross-stratified sandstone 4.1.6.1. Description. This facies is characterized by hummocky cross-stratified, light brown to buff colored, well sorted, fine e medium grained sandstone (Fig. 5iv). Thickness ranges from about 50 cm to 6 m (where amalgamated) (Fig. 3). Hummocky crossstratified beds display diagnostic characteristics defined by Harms et al. (1975). This facies is moderately bioturbated (Fig. 3) and trace fossils include Ophiomorpha, Thalassinoides, Skolithos and Planolites. 4.1.6.2. Interpretation. Hummocky cross-stratifications indicate storm and wave influenced deposition (e.g. Walker and Plint, 1992; Bhattacharya and Walker, 1992; Bhattacharya, 2010). It was interpreted to represent high energy storm process with strong wave influence (Harms et al., 1975; Dott and Bourgeois, 1982; Souza et al., 2012) and generally deposited above the fair-weather wave base (Duke et al., 1991; Amir Hassan et al., 2013a, 2013b; Sakai et al., 2006). 4.1.7. Facies SC: swaley cross-stratified sandstone 4.1.7.1. Description. This facies is characterized by swaley crossstratified, gray to light brown, well sorted, fine to coarse grained sandstones (Fig. 6i, ii). It is fairly bioturbated and contains some Thalassinoides and few, mostly diminutive, trace fossils (Fig. 6iii) perhaps because of the stressed nature of the depositional environment. It is also associated with soft sediment deformation structures (Fig. 6iv).

4.2. Facies associations

4.2.1. Facies association FA-1: offshore marine The facies identified for this association are clay/shale (FS), horizontally bedded sandstone (HP) and bioturbated siltstone (BS) facies (Fig. 3). The association of these facies, their glauconitic nature, moderate to intense bioturbation and trace fossils assemblages suggest deposition in a low-energy offshore (shallow) marine environment below the storm wave base (e.g. Buatois et al., 2002; Nichols, 2009, Fig. 14.1; Bressan et al., 2013). The isolated presence of facies HP and the nodular character of the facies BS were inferred to indicate occasional influence of storm processes n and Eynatten, 2014). (e.g. Nichols, 2009; Alva 4.2.2. Facies association FA-2: offshore transition to lower shoreface The facies association FA-2 is composed of three facies that include clay/shale (FS), wave-rippled sandstone (RC) and bioturbated massive sandstone (BM) facies (Fig. 3). The clay/shale facies (FS) in this association is characteristically similar as in the facies association FA-1 (offshore marine), except that it occurs as relatively thin interbeds and is less bioturbated which suggest increase in depositional energy. This facies association could be interpreted as offshore transition to lower shoreface in view of the genetic relationship of the above facies and their interpretations (see sections 4.1.1, 4.1.4 and 4.1.5), and the stratigraphic occurrence of the association among other identified associations (Fig. 3) in an overall storm and wave influenced shallow marine (offshore to shoreface) successions (e.g. Nichols, 2009). The offshore transition to lower shoreface facies association (FA-2) points to a suspension deposits laid down below and immediately above the fair-weather wave base in a storm and


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4.2.3. Facies association FA-3: middle shoreface The facies association FA-3 consists of the wave-rippled sandstone facies (RC) and predominantly hummocky cross-stratified sandstone facies (facies HC) (Fig. 3). Generally, the sediments in this association are relatively coarser grained (fine e medium) and moderately bioturbated than that of the facies association FA-2 (oofshore transition to lower shoreface). Also, unlike in the facies association FA-2, the Clay/shale facies (facies FS) is absent from this association. The FA-3 facies association may be interpreted to indicate middle shoreface characterized by wave action in upper shoaling region near the wave-breaking zone due to high-energy and intense reworking of bottom sediments by storm waves (e.g. n and Eynatten, Walker and Plint, 1992; Souza et al., 2012; Alva 2014).

Fig. 9. Ternary diagram of regular steranes (C27eC29) indicating the relationship between sterane compositions and organic matter input (modified after Huang and Meinschein, 1979).

wave dominated environment (Walker and Plint, 1992; Storms, 2003; Bhattacharya, 2010; James and Dalrymple, 2010; Souza et al., 2012).

4.2.4. Facies association FA-4: upper shoreface The facies association FA-4 is composed of predominantly the swaley cross-stratified sandstone facies (facies SC) and the planar cross-bedded sandstone facies (facies PC) (Fig. 3). Generally, the FA4 facies association suggests low-regime tractive processes generated by waves in the wave-breaking and surf zones in an upper shoreface (e.g. Bhattacharya and Walker, 1992; Souza et al., 2012). The widespread occurrence of the swaley cross-stratified sandstone facies suggests a wave dominated siliciclastic deposits occasionally subjected to storms (Souza et al., 2012). The association may also be related to longshore bars migration in upper shoreface (e.g. McCubbin, 1981). The upper shoreface environment shows cycles of erosions and deposition (Fig. 7iii) related to changing wave conditions (McCubbin, 1981). 4.3. Molecular geochemistry

Fig. 10. Xeray diffractogram of the Yolde formation shale showing the presence of QQuartz, F- Feldspar, Py- Pyrite, I-Illite, M-Montmorillonite, Cc-Calcium carbonate, KKaolinite, G-Glauconite, Cl-Chlorite.

The normal alkanes and acyclic isoprenoids ratios were determined on m/z 85 of the GCeMS fragmentation (Fig. 8). The fragmentograms display mainly unimodal distribution and dominance of short-chain/low n-alkanes (n-C12en-C20), suggesting marine environment with high contribution of aquatic organic matter (Peters et al., 2005). Pristane is relatively higher in concentration than phytane with pristane/phytane (Pr/Ph) ratios in the range of 1.00e1.34 and pristane/n-C17 and phytane/n-C18 ratios in the range of 0.45e0.72 and 0.32e0.56 respectively (Table 1). The shales extracts also possess low to moderate waxiness index values in the range of 0.21e0.61 (Table 1), while CPI values are slightly greater than 1.0 (Table 1). The distributions of hopanoids, steroids and related compounds are commonly studied by monitoring m/z 191 and m/z 217 mass fragmentograms (Fig. 8) respectively. The identified peaks are listed in Appendix A. The hopanoids are dominated by the presence of C30-hopane, C29-norhopane, 17a (H)-trisnorhopane (Tm), and homohopanes (C31eC33), with significant amounts of tricyclic and tetracyclic terpanes (Fig. 8). The C29-norhopane/C30-hopane ratios range from 0.78 to 1.21 (Table 1). The high values of C29-norhopane/ C30-hopane ratio are commonly associated with carbonate-rich source rocks (Peters et al., 2005). The shale extracts also have varying Tm/Ts values in the range of 0.67e1.05 (Table 1), indicating a mixture of land-and aquatic-derived organic matter. The homohopane distributions are dominated by the C31 homohopane and decrease with increasing carbon number (Fig. 8). The concentration of tricyclic terpanes is much higher than that of tetracyclics in most of the analyzed samples (Fig. 9). Tricyclic terpanes are believed to have origin from algae and bacteria (e.g. Zumberge, 1987). Gammacerane is also present in very low concentration (Table 1) and is believed to have been derived from tetrahymanol in bacterivorous ciliates living at the boundary of a high salinity water layer with an et al., 1995; Ten upper layer of less saline water (Sinninghe Damste

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Table 2 Oxides of major elements (wt. %) and trace elements (ppm) concentrations of the analysed Yolde shale samples. Sample ID

YOL1 YOL2 YOL3A YOL3B YOL4 YOL5 YOLA YOLB YOLC YL11 Average

Oxides of major elements (wt. %)

Trace elements concentration (ppm)

SiO2

Al2O3

TiO2

Fe2O3

P2O5

CaO

MgO

Na2O

K2O

K2O/Na2O

V

Ni

Cu

Cr

Sr

Ba

Rb

Ga

Co

Sc

Ni/Co

V/Sc

51.2 50.4 52.6 52.9 54.8 53.6 58.3 59.0 49.8 61.3 54.4

12.6 13.9 15.6 11.5 12.8 12.7 11.8 10.2 11.4 14.1 12.7

0.77 0.79 0.76 0.68 0.67 0.76 0.92 0.65 0.64 0.81 0.75

6.51 5.82 6.21 6.34 5.67 5.86 6.47 6.18 6.62 7.12 6.40

0.35 0.16 0.39 0.15 0.20 0.15 0.20 0.18 0.23 0.13 0.21

0.44 0.51 0.45 0.56 0.62 0.56 0.34 0.41 0.37 0.38 0.74

0.59 0.57 0.60 0.62 0.56 0.61 0.65 0.62 0.58 0.66 0.61

0.22 0.23 0.30 0.28 0.22 0.21 0.25 0.23 0.27 0.31 0.25

3.12 2.98 3.32 3.11 2.96 2.87 3.10 2.98 3.21 3.40 3.1

14.2 12.9 11.1 11.2 13.5 13.6 12.4 12.9 11.8 10.9 12.5

49.1 43.2 67.8 43.6 48.9 44.6 57.9 45.2 67.7 66.5 53.5

9.0 10.4 12.7 11.2 5.4 7.8 10.2 8.9 7.9 8.4 9.2

5.21 5.17 6.12 6.11 5.60 4.23 4.12 5.03 6.30 6.22 5.4

16.4 13.8 13.4 12.6 13.6 14.0 15.9 14.1 16.4 19.2 15.0

18.6 23.1 17.6 19.0 34.6 21.0 31.0 22.9 31.6 41.1 26.1

112 114 111 119 116 131 124 98.9 118 97.9 114

32.7 27.7 34.4 37.1 45.6 32.5 36.0 35.8 42.4 38.9 36.3

12.6 13.2 21.3 16.4 14.2 15.8 11.1 12.5 18.5 10.8 14.6

1.6 2.0 2.4 2.2 1.1 1.2 2.1 1.5 1.3 1.4 1.68

3.1 2.5 3.0 2.1 2.4 3.2 4.1 2.5 4.2 3.7 3.08

5.63 5.20 5.30 5.01 4.91 6.50 4.90 5.90 6.07 6.01 5.54

15.8 17.3 22.6 20.7 20.3 13.9 14.1 18.1 16.1 17.9 17.7

Haven et al., 1988). Gammacerane index of the analyzed shales ranges from 0.02 to 0.13 and this is an evidence for the existence of low salinity stratified water column during deposition. The 17b, 21a(H)-moretane was also detected in all of the analyzed samples, though it occurs in relatively low concentrations (Fig. 8).

The steranes are another group of important biomarkers that are derived from sterols found in higher plants and algae, but rare or absent in prokaryotic organisms (Seifert and Moldowan, 1979; Huang and Meinschein, 1979; Volkman, 1986). The relative proportions of each of the regular steranes (C27, C28 and C29) can vary

Fig. 11. Facies model for the coarsening upward storm and wave-dominated facies successions of the Yolde formation based on this study.


greatly from sample to sample, depending upon the type of organic matter input to the sediment. The relative abundances of C27, C28 and C29 regular steranes and the ratios of C29/C27 regular sterane, diasterane/sterane and hopane/sterane ratios were calculated and the results were given on Table 1. The Yolde shale samples show a high proportion of C27 (39.2e45.1%) compared to C28 (22.8e30.2%) and C29 (26.7e34.9%) steranes (Table 1), reflecting a high contribution of aquatic planktonic-bacterial organic matter (Peters and Moldowan, 1993; Peters et al., 2005) as indicated by regular sterane ratio ternary diagram (Fig. 9; Huang and Meinschein, 1979) and corroborated by the low C29/C27 sterane ratios (Table 1).

673

with terrestrial detritus in high energy environments (Ross and Bustin, 2009) and mostly originates from mixed clay assemblage. Moderate calcium (Ca) concentration is explained by calcium carbonate minerals, while magnesium (Mg) is related to particularly chlorite, as well as carbonates (Spears and Zheng, 1999). High Iron (Fe) contents indicate source from pyrite, hematite or glauconite (Fig. 10). Trace element concentrations of the shale samples of the Yolde formation with several widely used geochemical ratios are listed on Table 2. The trace elements concentration show Ba, V, Rb, Sr and Ni with average values of 114.0, 53.5, 36.3, 26.1 and 9.2 ppm respectively, while Cr, Ga and Cu have average values of 15.0, 15.6 and 5.4 ppm respectively (Table 1).

4.4. Mineralogy and elemental compositions 5. Discussions The Yolde formation shale samples contain both detrital and non-detrital minerals (Fig. 10). Quartz is the most dominant mineral and other minerals include kaolinite, glauconite, montmorillonite, pyrite, illite, chlorite, hematite and calcite (Fig. 10). Glauconite is diagnostic of continental shelf environment where sedimentation of terrigenous clastics and carbonates become slow (low-energy environment) (Nichols, 2009) and interstitial waters become oxygen-deficient. This is compatible with the facies interpretation and the presence of pyrites which indicate oxygendeficiency. The oxides of major elements in the shales of the Yolde formation are dominated by SiO2 (av. 54.4%), Al2O3 (av. 12.7%), Fe2O3 (av. 6.4%), and K2O (av. 3.1%) while others (CaO%, MgO%, Na2O%, TiO2%, P2O5%, MnO %) have concentration of mostly 3.0 (oxidizing conditions), while low values of