1 Introduction
The black shale has been considered as a variety of fine-grained, organic-rich source rocks deposited in a wide range of geological settings (
Waples, 1983; Bohacs et al., 2000). The geological settings for organic-rich sediments range from freshwater-estuarine to marine and incorporate varying organic productivity, sedimentary pH values, and anoxia (
Vine and Tourtelot, 1970; Chen et al., 2013). The black color of shales in the deposited setting is not only due to the presence of high organic matter under anoxic or reducing conditions but also the presence of common microcrystalline sulfide compounds, such as FeS (black) or FeS
2 (green) and black minerals (Mn) (
Chen et al., 2013). The investigation of the bulk composition of black shales and mud shows pre-erosional source rocks, the chemistry of the depositional environment, and post-lithification processes in the reservoir and source settings (Quinby-Hunt and Wilde, 1994). The bulk composition of sedimentary rocks varies due to the physical-chemical conditions of the depositional setting (
Usman et al., 2020a,
2020b, 2020
c), such as the presence of organic matter, isotope signals, major and trace elements anomalies (
Brumsack, 2006;
Piper and Calvert, 2009; Baioumy and Ismael, 2010). The deposition of black shale is present across the globe during the Lower to Middle Jurassic era (Fig. 1). In the case of the Indus Basin, the previous researchers have addressed the subject of the conventional sedimentology and stratigraphy of Mesozoic carbonate and clastic units (
Fatmi et al., 1990;
Ahmed et al., 1997;
Abbasi et al., 2012;
Ali et al., 2019;
Iqbal et al., 2019; Mensink et al., 1988;
Mertmann and Ahmad, 1994). Besides the conventional sedimentology,
Ali et al. (2018) has studied the anoxic events in the Indus Basin, Pakistan. Overall, research about the depositional setting and paleooceanographic setting of Lower to Middle Jurassic shale in the Indus Basin, Eastern Tethys, is scarce to absent. To obtain insight into the local and regional black shales and paleoenvironmental changes, an integrated study has been conducted using the trace element anomalies, palynofacies, source rock geochemistry, and carbon and oxygen isotope (δ
13C & δ
18O) measurements. The results of this study will lead to the development of a conceptual depositional model and paleooceanographic implications of the Lower to Middle Jurassic shale of the Indus Basin.
2 Geological setting of Jurassic succession
The study area is located in the Indus Basin in Pakistan, which has been divided into the Lower, Middle, and Upper Indus Basins. Geologically, the Upper Indus Basin is a complex terrain that is further divided into Potwar and Kohat sub-basins in the east and the west, respectively (
Kadri, 1995). The Indus Basin contains late Precambrian to Quaternary sedimentary rocks (
Shah, 2009) with moderate to thick sedimentary successions (Fig. 2). The Mesozoic strata are represented by clastic and carbonate rocks such as sandstone, organic black shale, limestone, dolomite, coal, marls, and subordinate chert and laterite. The Jurassic rocks are represented by the Datta, Shinawari, and Samana Suk formations in the Upper Indus Basin with the Shirinab, Chiltan, and Mazar Drik formations in the Lower Indus Basin (
Bender and Raza, 1995;
Kadri, 1995;
Shah, 2009). The Jurassic strata largely constitute a thick succession (820 m in the Upper Indus Basin and 3000 m in the Lower Indus Basin) of clastic and carbonate sedimentary rocks (
Kazmi and Jan, 1997), and extend all along the Indus Basin from platform cover to the Kirthar-Sulaiman region, Balochistan-ophiolite, and Himalayan fold and thrust belt. In the Kirthar-Sulaiman region, they are present in the core of the anticline, whereas in Balochistan, the ophiolite and thrust belt and the Himalayan fold and thrust belt form extensive thrust blocks or sheets. A maximum thickness of approximately 3000 m is recorded in the Sulaiman-Kirthar fold-thrust belts, which consist mainly of limestones and subordinate shale of the Shirinab Formation (equal to the Ferozabad Group), Takatu/Chiltan, and Mazar Drik Formations (
Kazmi and Jan, 1997).
The deposition of the Lower Jurassic strata in the Indus Basin, Pakistan, is mainly influenced by tectonics, but the role of paleoclimate changes cannot be ignored. The tectonic rifting of Pangaea leading to the separation of the Indian Plate from the Arabian and African plates may have also affected the paleoclimate during the Triassic–Jurassic interval. This tectonic rifting during the Jurassic caused the subsidence of the west and northwest areas of the Indus Basin, forming marine deltaic deposits, i.e., the Salt Range and Kohat–Potwar plateaus (
Iqbal et al., 2019). At the time of the Early Jurassic (Hettangian to Toarcian), the shoreline of the paleo-Tethys trended approximately north–south with respect to the current configuration and was nearly parallel to the eastern flank of the Indus Basin, Pakistan (
Shah, 2009). This basin was oriented east–west with respect to the present north of the Indus Basin. The shoreline was eastward while the basin was westward, where a very shallow marine depositional system existed, represented by western parts of the Kohat Plateau and Sulaiman Range. In the Early Jurassic, a north–south oriented (with respect to the present north) belt of marginal marine depositional settings existed with a deltaic setting in the north currently occupied by the Upper Indus Basin. Clastic supply to the basin in the west and southwest was minimal to none by Zao River into the Lower Indus Basin, i.e., the Kohat Plateau and Sulaiman Range (
Shah, 2009). Therefore, conditions in the Lower Indus Basin were more favorable for carbonate production than those in the Upper Indus Basin. This difference in the depositional systems mainly controlled by the tectonics led to differences in the thickness of the organic-rich shale, with the deposition of thick organic shale intervals in the Upper Indus Basin compared to those of the Lower Indus Basin.
3 Methods and materials
The well-preserved Jurassic exposures of the Datta and Shinawari formations from the Upper sub-Indus Basin and the Chiltan Formation from the Lower sub-Indus Basin were selected for the present study. The selected stratigraphic sections from the Upper sub-Indus Basin are ACL-N Kahi village (ACL-N) in the Kala Chitta Range, Baroch Nala (BN), Chichali Nala (CN), and Gulla Khel Nala (GK) in the Surghar Ranges, Nammal Gorge section (NG) in the Salt Ranges and from Lower sub-Indus Basin Mughal Kot section (MK) in the Sulaiman Ranges (Fig. 2). The unweathered outcrop shale samples were obtained from a depth of 15–20 cm by a hand-held auger for laboratory analysis. The thickness of the formations was measured with the help of a Jacob staff, and a high-magnification Nikon camera was used to capture the field photographs. The palynofacies and palynology slides were prepared from black shale samples according to the standard procedure defined in
Wood et al. (1996). The slides were examined via transmitted light microscopy with three hundred counts following the procedure (
Steffen and Gorin, 1993). The amorphous organic matter (AOM) and palynomorphs were analyzed under epifluorescence microscopy (EF), microscopy (Nikon SMZ-25), and polarizing microscopy (Nikon OS-F12), respectively.
For trace element analysis, all samples were crushed, pulverized, and dried in an oven at 105°C for 24 h. A 0.5 g sample was dissolved in 10 mL concentrated aqua regia, which was boiled in reflux for 3 h. Then the solution was cooled and filtered through No. 42 Whatman filter paper into a 50 mL volumetric flask. The trace element analysis was performed through inductively coupled plasma-optical emission spectrometry (ICP-OES), and a few samples were examined under inductively coupled plasma-mass spectrometry (ICP-MS). The instrumental and operating conditions of ICP-OES are as follows: The RF power value is 1150 W and the pump rate is 50 r/min. The auxiliary, nebulizer and coolant of gas flow rates are 0.5 L/min, 0.7 L/min, and 12 L/min, respectively. The normal purge gas flow, radial view mode plasma, aerosol carrier has argon gas, spectrometer flushing argon gas, and the sample uptake are 2.0 mL/min, and the integration time is 5.0 s (
Naeem et al., 2011). A certified reference solution from Alfa Aesar was used for the calibration and correction factors of the ICP-OES instrument. The chosen analytical emission lines (nm) of trace elements (TM) are as follows: Aluminum (Al); Copper (Cu); Chromium (Cr); Magnesium (Mg); Manganese (Mn); Antimony (Sb); Molybdenum (Mo); Nickel (Ni); Thallium (Th); Titanium (Ti); Uranium (U); Cadmium (Cd); Vanadium (V) and Zinc (Zn). Furthermore, the stable carbon and oxygen isotopic analyses and total organic contents were analyzed using VG Isogas Prism III isotope ratio mass spectrometer with the elemental analyzer at the School of Geoscience, Wolfson Laboratory, Edinburgh, UK (UK), and Delta plus Advantage, Thermo-Fisher Scientific. The acetanilide and PACS-2 (National Research Council Canada) (
Ali et al., 2018) CaCO
3 and V-PDB: Vienna-Pee Dee Belemnite standards were used for instrument calibration.
4 Results
4.1 Trace element analysis
The results of the trace elements of the Lower to Middle Jurassic shale from the Indus Basin are shown in Tables 1–4. The samples codes such as NS, ND, BS, BD, SS, CC, GS, GD, and NG belong to Upper Indus Basin, and MG belong to Lower Indus Basin, Pakistan. Among the 50-one studied samples, these redox elements (Table 1) are represented and discussed based on the ratio and individual behavior of V, Ni, Mo, Cu, and Cr. The detrital and paleo-anoxic settings are marked by Ti & Al and Mo, Zn, Ni, Cu, Cr, and V ratios, respectively. While the abundance of organic matter is determined to correlate the individual elements such as V, Ni, and Cu with total organic content (TOC). Additionally, the enrichment factor (EF) is used to accurately determine the paleo-environmental conditions. The EF was determined from the composition of post-Archean Australian shales (PAAS) following the methods described by noteworthy literature (
Taylor and McLennan, 1985;
Wedepohl, 1971,
1991; McLennan, 2001;
Brumsack, 2006). The enriched elements are expressed by an EF greater than 1.0, whereas depleted elements have an EF less than 1.0. In the studied samples, the EF values of Al, Cu, Mn, Ni, Zn were less than 1.0, the EF values of Ti and V were equal to 1.0, and the EF values of Cr and Mo were greater than 1.0 and reached 3.0 (Table 2).
4.2 Geochemical and palynofacies analysis
Bulk carbon and oxygen isotopic study: The bulk carbon isotopic values obtained from the Lower Jurassic Datta Formation range from −2‰ to −3‰ in most of the studied samples, except for a single sample having no signal observed during laboratory analysis. Although in the Middle Jurassic Shinawari Formation, the δ
13C values of the organic matter show a negative excursion of 5‰–6‰ per mil shift (−24‰ to −28‰) in the Chichali Nala (
Ali et al., 2018), the −5‰ to −9‰ negative δ
13C excursion and one positive value had been recorded from the rest of the studied sections in the Indus Basin.
Organic geochemical analysis: The major values of TOC are 0.11% to 4.51% in the studied sections of the Indus Basin, Pakistan. TOC values ≥ 2% are categorized as high-potential source rock, and those ≤2% are known as non-high potential source rock (Table 3).
Palynofacies analysis: The Lower Jurassic Datta Formation palynofacies (DFPF) are subdivided into types of A, B, and C. The petrographic constituents of DFPF-A are composed of phytoclasts (68% to 79% with an average of 74%) (More than 10% is opaque), palynomorphs (10% to 24% with an average of 18%), and amorphous organic matter (AOM) (3% to 12% with an average of 6%). The phytoclasts are reddish-brown and black, with less internal structure, having various shapes such as elongated, circular/spherical, and blade. The color ranges of AOM are yellowish-brown, while the documented lowest fraction of tracheids and cuticles are well preserved internal structures (Figs. 3(a)–3(c)). The palynofacies DFPF-B are composed of phytoclasts (62%), amorphous organic matter (AOM) (20%), and palynomorphs (18%). The characteristics of phytoclasts are similar to the previous palynofacies (Fig. 3(f)), and the palynofacies DFPF-C are comprised of phytoclasts (85%–89%), palynomorphs (5%–10%), and amorphous organic matter (AOM) (4%–6%). The properties of phytoclasts are similar to the previous palynofacies (Fig. 3(g)). The Middle Jurassic Shinawari Formation and Chiltan Limestone palynofacies (SFPF) are represented by SFPF A and B. SFPF A is composed dominantly of phytoclasts (85%–86%), palynomorphs (10%), and amorphous organic matter (AOM) (5%), while SFPF B is represented by phytoclasts (57%), palynomorphs (33%), and AOM (10%). These palynofacies SFPF A and B are used after the studies of
Ali et al. (2019). The Middle Jurassic Chiltan Limestone has SFPF B palynofacies composed of phytoclasts (62%), palynomorphs (29%), and AOM (9%) (Fig. 3(h)).
5 Discussion
5.1 Variation of detrital flux concentration
The elemental ratios such as Ti/Al and Si/Al are considered to be the key proxies that are used to define the detrital input concentration into the basin (
Werne et al., 2002; Sageman et al., 2003;
Rimmer et al., 2004;
Li et al., 2015). In the present study, six different stratigraphic sections of the Indus Basin show no uniform distribution of Ti/Al, and most of the samples are recorded with low values of this ratio (1.96 on average). This suggests heterogeneous sources and variation in the amount of the detrital supply. Generally, Ti has been shown to be abundant in association with certain accessory minerals present in sediments. And it is present in the coarse-grain portion of fine-texture siliciclastic sediments (
Brumsack, 1986;
Calvert and Pedersen, 2007; Zheng et al., 2019). According to
Rachold and Brumsack (2001), Ti is considered as an indicator of aeolian detrital supply. The lower Ti/Al ratios reflect an enhanced Ti-depleted fluvial contribution (
Scopelliti et al., 2004). The values of Cr and Ti/Al are used to define the detrital influx into the basin (
Soua, 2011), which indicates a low amount of clastic input with low oxygen supply in the studied formations of the Indus Basin. Furthermore, the Ti/Al elemental ratio is not consistent with organic matter enrichment, i.e., low values of the Ti/Al ratio does not indicate organic-rich intervals and vice versa. Therefore, the current work suggests that the extra aeolian source of Ti is the main factor of this inconsistency and that more than one single source is involved in detrital input (Figs. 4–5 and Table 3).
5.2 Paleo redox condition
Element enrichment: The variation and wide range presence of redox-sensitive elements (Ni, V, Mo, Mn, U, Cu, Cr, Re, Cd, Sb, and Ti) in sediments reflect the ancient oceanographic setting (
Brumsack, 2006;
Tribovillard et al., 2006; Chen et al., 2013;
Pi et al., 2014;
Baioumy and Lehmann, 2017). These elements are significantly enriched in reducing anoxic depositional sediments. The enrichment in the concentration of these elements in the current studied Jurassic black shale is strong evidence of anoxic conditions (
Pi et al., 2014). In the current study, the formations are enriched in Cr and Mo, slightly enriched in Ti and V, and drained of Al, Cu, Mn, Ni, and Zn. The enrichment of Mo and V in the shales of the Lower to Middle Jurassic indicates that the deposition had taken place in oxygen-depleted water under anoxic conditions. The absence of U shows that the bacterial sulfate reduction did not take place throughout the time of deposition (
Tribovillard et al., 2006; Soua, 2011). Basically, the V and Mo enrichment and the absence of U indicated suboxic/anoxic deposition with free hydrogen sulfide (
Tribovillard et al., 2006). The anoxia of bottom water in ancient sediments has been determined from Mo enrichment when the other enriched redox-sensitive trace elements (i.e., Re and U) are absent, although the Mo enrichment may be due to boidal pyrite in the process of early diagenesis (
Tribovillard et al., 2008). In the studied strata, boidal pyrite is absent or rarely present, so the enriched value of Mo is depositional and valid in this study. According to
McManus et al. (2006) and
Zheng et al. (2000), Mo accumulation occurred under less anoxic conditions. Thus, bottom waters were in less-reducing dysoxic to anoxic conditions (Figs. 4–5 and Table 2).
Bottom water oxygenation conditions: for reliable interpretation, different ratios of trace elements are used and the individual values of various elements are not used to interpret the geological process (
Hatch and Leventhal, 1992;
Jones and Manning, 1994). The V/(V+ Ni) value is 0.75, and the V/(V+ Cr) value is 0.60 in the studied strata, supporting the anoxic/reduced state of the depositional setting (
Hatch and Leventhal, 1992;
Jones and Manning, 1994;
Rimmer, 2004; Baioumy and Lehmann, 2017). Thus, variations in the V/(V+ Ni) and V/(V+ Cr) ratios could indicate relative changes in oxygenation, with higher ratios signaling more strongly anoxic conditions of deposition. The V/Cr ratio in the studied formations is 1.22, which falls in the oxic zone, supporting the
Jones and Manning (1994) suggestion in the previous literature. The average value of V/Ni proxies in the Jurassic black shales is more than 3, indicating that deposition has taken place in a reducing setting and that the origin of these organic matters is terrestrial to marine in nature (
Galarraga et al., 2008). The V/Mo ratios range from 0.18 to 0.30, the Mo/Al ratios range from 9.47 to 3.25, and the Cr/Al ratios range from 2.67 to 2.19, which indicates that the deposition of sediments occurred in depositional conditions with depleted oxygen, such as dysoxic to anoxic settings (
Jones and Manning, 1994; Gallego-Torres et al., 2010;
Li et al., 2015). Furthermore, the level of bottom water oxygenation is indicated by the
(Cu+ Mo)/Zn relationship, in which copper behaves partly as a micro-nutrient (
Hallberg, 1982;
Calvert and Pedersen, 1993;
Szczepanik et al., 2010). The values of this proxy are enhanced under the prevailing reducing or anoxic conditions and decrease vice versa, i.e., the anoxic condition values documented for the Baltic Sea that range from 0.1 to 6 (
Hallberg, 1982). The average (Cu+ Mo)/Zn ratios for the samples from the Upper and Lower Indus Basin are 6.01 and 10.09, respectively, suggesting less-oxygenated anoxic bottom water conditions (Table 3).
Post-depositional reoxygenation: during the process of diagenesis, it is possible to develop reducing conditions by irreversible oxidation reactions in the sediments. These can restore the oxidation state after reduction in the deposited sediments, which can be related to glacial-interglacial transitions and turbidity deposits (
Lyons et al., 2003;
McManus et al., 2006). Specifically, the oxygen replenishment leads to U remobilization if oxygen enters the area where U has mainly precipitated as authigenic components. V, Cd, and Mo are less affected than U due to secondary reoxidation replenishment. Therefore, the absence of U in the studied Lower to Middle Jurassic rocks is either due to the post-depositional process or due to the absence of sulfate-reducing bacteria.
Role of anoxia: the anoxic events in Jurassic rocks featured the extensive development of euxinic and anoxic settings with subsequent deposition of organic dark black shales across the continents (
Jenkyns, 2010). The exposed Lower Jurassic strata in our study area are composed of sandstone, shale, and subordinate limestone, while the Middle Jurassic strata represent shale, limestone, and subordinate sandstone units. The units are in the form of varied multifold cycles between carbonates and shale, which are the byproducts of the Neotethys Tectonics (
Fürsich et al., 1992). The studied strata show high values of 5‰–6‰ per mil that shift from −24‰ to −28‰, suggesting a pronounced anoxic event and anoxia in the Lower to Middle Jurassic Indus Basin. The positive excursion values are reported from the gray shale in the study area, reflecting that anoxia is a significant factor in the formation of the black shale rocks.
Mineral phase: the coefficients of determination (
r2) between Mo and V, Zn, Cu, Cr, and Ni in the studied strata show positive correlations (Table 4). From the positive correlation between Mo and V, anoxic conditions are interpreted and occur in the common mineral phase (
Yang et al., 2004), and the absence of any correlation portrays different mineral phases (
Baioumy and Lehmann, 2017). To investigate the mode of occurrence or probable mineral composition of the redox elements in Jurassic strata, the correlation between sensitive elements and main rock components must be investigated. The main rock components in the black shale are total organic content (TOC), Al and Ti detrital fractions. A slightly inconsistent correlation pattern between the sensitive elements and source rock constituents has occurred. In some stratigraphic sections, there is a positive correlation and vice versa. However, in the current study, we interpreted it as the mono-bi mineral phase under the redox condition of deposition (Table 4).
Tracers for organic matter abundance and input: different trace elements have different relationships with TOC despite differing redox conditions. For example, in contrast to U and V, Ni and Cu are mainly carried to the sediments fully allied with organic matter, forming an organometallic complex (
Tribovillard et al., 2006). Subsequently, Ni and Cu can express the original occurrence of organic matter even if it partially disappeared after deposition. In this manner, Cu and Ni are better proxies for the organic matter richness than P and Ba (
Tribovillard et al., 2006). From numerous investigations of sedimentary rocks (
Tribovillard et al., 1994;
Riboulleau et al., 2003;
Algeo and Maynard, 2004;
Tribovillard et al., 2006), it has been observed that U, V, Ni, and Cu-TOC covariation patterns depend upon the nature of open and closed marine systems. A classic example of open marine systems is that in which the dissolved trace element repository of ocean water is not depleted through the process of sedimentation. In restricted basins, the flux of trace elements can be exceeded compared to the source. Therefore, an association between TOC and trace elements seems unlikely in the current study or previous research (
Algeo, 2004;
Algeo and Maynard, 2004;
Tribovillard et al., 2006). The positive and negative correlations of the Ni/Al and Cu/Al ratios with organic matter does not verify this clearly, but from the palynofacies investigation, it is clear that the deposition of sedimentary rocks of the Lower to Middle Jurassic of the Indus Basin took place in the episodes with a better influx of organic matter having mixed marine and clastic inputs (Table 4).
5.3 Depositional setting of black shale
The AOM-Phytoclast-Palynomorph ternary diagram of
Tyson (1995) plots DFPF-A palynofacies corresponding to the Type II field, which represents the marginal dysoxic-anoxic basinal setting. DFPF-B palynofacies corresponds to the Type III field that represents a heterolithic, proximal shelf. DFPF-C palynofacies corresponds to the Type I field that is useful for representing a highly proximal shallow marine shelf depositional setting. In the Shinawari Formation and Chiltan Limestone, the AOM-Phytoclast-Palynomorph ternary diagram of
Tyson (1995) plots SFPF A and B palynofacies in the Type II and Type III fields correspond to a marginal dysoxic-anoxic basinal and proximal shelf setting, respectively (Fig. 6).
5.4 Depositional model and controlling factor of high-potential source rocks
What could be the possible driving mechanism for the deposition of local, regional, or global black shale occurrences during the Lower to Middle Jurassic? This question has long been debated and is still open. The Lower to Middle Jurassic shales are reported from the Eastern Tethys (
Qiang et al., 2002;
Yang et al., 2003; Chen et al., 2013;
Srivastave and Ranawat, 2015;
Ali et al., 2018;
Ali et al., 2019), Central Tethys (
Moshrif, 1987;
Rousseau et al., 2005; Abdula et al., 2015), and Western Tethys (
Nielsen et al., 2003; Løseth et al., 2009) with current study in the Indus Basin, Pakistan, but depositional setting debates still await further research and interpretation.
The geologic aspects that controlled the deposition of organic-rich dark shale also have been under consideration and debate for a long time. Therefore, an integrated approach is undertaken to assess the question of which factors are related to the depositional setting. The palynofacies analysis, organic geochemistry, carbon and oxygen isotope analysis, and trace element analysis provide important and noteworthy implications for the formation of organic-rich dark-gray shale. The controlling factors are high preservation, pronounced anoxia, low clastic dilution with enhanced particulate sedimentary particle supply, and deposition under favorable anoxic conditions. The palynofacies analysis shows that the deposition took place under extremely anoxic conditions proximal to the distal shelf setting. In addition to the palynofacies, different elemental ratios, such as Ti/Al and Cr, indicate a low detrital supply and low oxygen levels in more stratified and stagnant water. While the V/(V+ Cr), V/(V+ Ni), V/Mo, and V/Ni elemental ratios indicate anoxic conditions despite the V/Cr ratio showing oxic conditions. The Cu/Al, Ni/Al, Mo/Al and (Cu+ Mo)/Zn elemental ratios are supported by oxygen-depleted sediments and the less-oxygenated conditions of bottom water with an influx of organic matter. The carbon and oxygen isotopic signals are fully supportive of anoxic events in water, which in the interim helped with the preservation of organic matter in shale. The studied sections show healthy values of carbon isotopes and the presence of Pliensbachian-Toarcian events in the studied strata. Globally, the strata related to anoxia show good source rocks. Therefore, the Jurassic shale in the Indus Basin is the collective result of the different processes responsible for the creation of anoxic conditions in water with the availability of the clastic and
in situ nutrient supplies and their subsequent high preservation and deposition. In the published literature, it is ascertained that these three important factors are mainly controlled by the formation of shale (
Demaison and Moore, 1980;
Tyson, 1987;
Wignall, 1994). However, it has been proven from recently updated studies that complicated multiple-control pathways most likely lead to the accumulation and preservation of organic matter in dark gray shale (
Sageman et al., 2003;
Rimmer et al., 2004;
Arthur and Sageman, 2005;
Bohacs et al., 2005;
Wang and Carr, 2013) (Fig. 7).
In contrast to the high-potential source rock, the depositional setting of the ordinary source rock or non-high potential source rock has different parameters. The elemental ratios such as Ti/Al and Si/Al are thought to be the key proxies used to define the detrital input concentration into the basin (
Werne et al., 2002; Sageman et al., 2003;
Rimmer et al., 2004;
Li et al., 2015). Moreover, due to the low value of Ti/Al in the high-potential source rock, the ordinary source rock has high values (7.82 on average) in most cases. The detrital influx, as defined by Cr and Ti/Al, into the basin (
Soua, 2011) in ordinary rock is high compared to that in high-potential source rock. The enrichment factor of redox-sensitive elements in ordinary rock is different than that in high-potential rock, i.e., most are not consistently enriched. Therefore, the lack of enrichment of Cr, V, and Mo in the ordinary shale of the Lower to Middle Jurassic indicates that the deposition has taken place in oxygenated water under oxic conditions in a low anoxic setting. According to
McManus et al. (2006) and
Zheng et al. (2000), Mo accumulation occurred under less anoxic conditions. Thus, the bottom waters were less reducing. The V/(V+ Ni) value is 0.63, and the V/(V+ Cr) value is 0.47 in ordinary source rock in the Indus Basin, which supports the oxic condition of the depositional setting in Jurassic studies according to many researchers (
Hatch and Leventhal, 1992;
Jones and Manning, 1994;
Rimmer, 2004; Baioumy and Lehmann, 2017). It has been reported that the V/Cr ratio is 1.24, and the V/Ni proxy value is 2.50, which falls in the oxic zone, suggesting a terrestrial to marine origin for this organic matter (
Jones and Manning, 1994; Galarraga et al., 2008). The (Cu+ Mo)/Zn value of 6.32 indicates the deposition that took place in a dysoxic geological setting (
Hallberg, 1982;
Jones and Manning, 1994; Gallego-Torres et al., 2010;
Li et al., 2015) (Table 4). The bulk carbon and oxygen values of the non-potential source rock started with smaller δ
13C values of −0.1‰ compared with the pronounced anoxia in the high-potential source rock of the Lower to Middle Jurassic Indus Basin. In the ordinary source rocks, the positive excursion values are reposted from the gray shale low carbon content values in the Jurassic rocks of the Indus Basin, reflecting that anoxia is a critical factor in the formation of black shale rocks.
6 Conclusions
1) The depositional setting interpreted from the palynofacies analysis shows the deposition in a shallow marine setting.
2) The trace element analysis, carbon and oxygen isotopic study, bulk geochemical analysis and palynofacies investigation of the Lower to Middle Jurassic strata reveal that the deposition of black to gray shale took place in a dysoxic to anoxic environment with pronounced oceanic anoxic events in the proximal to the distal shelf setting.
3) The controlling factor in the deposition of high potential black shale is low detrital input, feasible reducing conditions, availability of organic matter, and deposition in a viable marine setting. The low potential source rock has more clastic input, dysoxic paleoredox conditions, less anoxia, more terrestrial organic input, and low production of primary organic matter.
4) It is concluded that complex, nonlinear relations between different aspects most likely contribute to the deposition of organic material. The enhanced preservation, pronounced anoxia, less clastic dilution with enhanced particulate sedimentary particle supply, and less primary organic marine production contribute to the deposition of shale rich in organic matter.
PDF
(3054KB)
