ABSTRACT

Oil-shale beds formed under anoxic conditions that were controlled by various local, regional and global factors. The Jordanian oil shales, which were deposited during the Late Cretaceous to Eocene, are considered as an example for the interplay of these factors. Two cores of organic-rich marls were investigated and analyzed with respect to their lithology, ichnofabrics and carbonate microfacies. The first core (OS-01, 183.3 m; South Jordan) is of Late Cretaceous age, the second one (OS-23, 256.3 m; Central Jordan) is of Eocene age. Our studies revealed that the Upper Cretaceous oil shales were deposited in a shallow-water carbonate shelf. Oyster bioherms acted as physical barriers that reduced the water circulation with the open shelf, thereby causing anoxic conditions. The Eocene oil shales also accumulated on a shallow-water carbonate shelf. In this case, however, synsedimentary tectonics caused subsiding grabens and half grabens, which in turn gave way to anoxic conditions. Both deposition and richness of the Jordanian oil shales were affected by regional sea-level fluctuations and global climatic changes.

INTRODUCTION

Cretaceous to Paleogene rocks are of great importance for the economy of Jordan because they comprise significant natural resources. These resources include widely distributed Maastrichtian to Eocene organic-rich marls known as “oil shales”. Extensive geological investigations of the oil shales were carried out by the Natural Resources Authority (NRA) of Jordan (Hamarneh, 2006). Other studies focused on the origin, depositional environment, quality, and commercial development of the Jordanian oil shales (Abed, 1982; Hufnagel, 1985; Abed et al., 2005; El-Hasan, 2008; Jaber et al., 2008). Most of these studies are based on material from outcrops and boreholes in the areas of oil-shale occurrences in Jordan. Little is known about these oil shales from other localities, especially when it comes to the deep oil shales in Central and South Jordan.

As a part of an integrated research project that includes the sedimentology, biostratigraphy and geochemistry of the Jordanian oil shales, the current study investigates the lithofacies, ichnofabrics and sedimentary petrography of these oil shales. The significance of this study is that it analyzes the deeply buried oil shales of Maastrichtian–Paleocene age occurring in South Jordan, and compares them to Eocene oil shales located in Central Jordan (Figure 1). In this paper, a core from South Jordan (OS-01, 183.3 m) of Maastrichtian to Paleocene age has been investigated and compared to a previously described core from Central Jordan (OS-23, 256.3 m) of Eocene age (Ali Hussein et al., 2014a, b; Alqudah et al., 2014). These studies aim at understanding the changes in the character and origin of the Jordanian oil shales through time and space (Figure 1).

Figure 1:

(a) Map of Jordan showing the locations of the studied cores (OS-01 and OS-23), (b) Geological map of the study area of core OS-23 (compiled and modified after Al Hunjul, 1999; Smadi, 1999), (c) Geological map of the study area of core OS-01 (compiled and modified after Abu Lihie, 1985; Kherfan, 1986; Gharaibeh, 2010).

Figure 1:

(a) Map of Jordan showing the locations of the studied cores (OS-01 and OS-23), (b) Geological map of the study area of core OS-23 (compiled and modified after Al Hunjul, 1999; Smadi, 1999), (c) Geological map of the study area of core OS-01 (compiled and modified after Abu Lihie, 1985; Kherfan, 1986; Gharaibeh, 2010).

GEOLOGIC SETTING

During the Late Cretaceous and Early Paleocene the region of Jordan was situated on a shallow-marine carbonate platform adjacent to the Neo-Tethys Ocean in the northwestern part of the Arabian Plate (Figure 2a; Barjous and Mikbel, 1990; Ziegler, 2001; Sharland et al., 2001; Powell and Moh’d, 2011). The Eocene strata in the northwest part of Arabia are mainly composed of transgressive carbonate cycles. Pelagic chalks containing radiolarians and foraminifera interfinger with shallow-marine, nummulitic limestones that were deposited on a broad, shallow shelf at the northern edge of the Arabian Plate (Figure 2b).

Figure 2:

(a) Paleofacies of the Late Cretaceous to Early Paleocene of the Arabian Plate (modified after Ziegler, 2001), (b) A paleo-bathymetric map for the Middle-Late Eocene showing the different parts of the carbonate ramp. Iso-bathymetric lines in meters, the datum is the Eocene mean sea level. The coordinates are in Palestine Grid (modified after Segev et al., 2011).

Figure 2:

(a) Paleofacies of the Late Cretaceous to Early Paleocene of the Arabian Plate (modified after Ziegler, 2001), (b) A paleo-bathymetric map for the Middle-Late Eocene showing the different parts of the carbonate ramp. Iso-bathymetric lines in meters, the datum is the Eocene mean sea level. The coordinates are in Palestine Grid (modified after Segev et al., 2011).

The Syrian Arc influenced the paleotopography by forming a series of anticlines and synclines (Figure 2a). The crests of the anticlines formed a cluster of small islands, whereas the synclines formed small basins in which chert, phosphate and organic-rich sequences accumulated. The major tectonic features of the two study areas include the Zarqa-Ma’in Fault and Raha Fault (Figures 1b, c). The strata have a general 2–5° dip to the N and NE (Abu Lihie, 1985).

The two cores from South and Central Jordan (OS-01, OS-23, Figure 1) that are investigated here represent intervals from three formations, each consisting of a set of three units (Figure 3).

Figure 3:

Simplified lithological log of cores OS-01 and OS-23. The lithology and cycles of core OS-23 are modified after Ali Hussein et al. (2014b). Biostratigraphic data after Alqudah et al. (2014), biozones are calibrated to the geologic time scale (Gradstein et al., 2004). Nannos biozones = calcareous nannofossils zones, zonation UC following Burnett (1998), zonation NP following Martini (1970).

Figure 3:

Simplified lithological log of cores OS-01 and OS-23. The lithology and cycles of core OS-23 are modified after Ali Hussein et al. (2014b). Biostratigraphic data after Alqudah et al. (2014), biozones are calibrated to the geologic time scale (Gradstein et al., 2004). Nannos biozones = calcareous nannofossils zones, zonation UC following Burnett (1998), zonation NP following Martini (1970).

  • (1) The Upper Maastrichtian–Lower Paleocene Muwaqqar Chalk Marl (MCM) is represented in Core OS-01 (Figure 3a). The formation has a wide distribution in southern Jordan, but less so in Central Jordan (Bender, 1975; Powell, 1989; Andrews, 1992). It consists of chalks and marls, with concretions of chalky or microcrystalline limestone ranging in thickness from 20 cm to 1 m (Abu Lihie, 1985). Thin layers of fibrous gypsum are present within the marls. Macrofossils include bivalves, gastropods and shark teeth.

  • (2) The Lower to Middle Eocene Umm Rijam Chert Formation (URC) is represented in Core OS-23 (Figure 3b). It has the widest occurrence in both study areas. It commonly rests unconformably on the MCM. This formation consists of chalks and chalky limestones, thin-bedded chert layers interbedded with chalks and succeeded by limestones with abundant limestone concretions that are interbedded with chalk (Gharaibeh, 2010).

  • (3) The overlying Upper Eocene Wadi Shallala Chalk Formation (WSC) is also represented in Core OS-23 (Figure 3b). It covers local parts of the study area. The formation is up to 100 m thick (Gharaibeh, 2010) and consists of white chalks, marls, chalky limestones, barite concretions and glauconite in the lower part. The upper part is composed of micritic limestone interbedded with chert and chert concretions. Macrofossils include bivalves and shark teeth.

METHODS

Lithological characterization with visual logging of cored wells is considered to be the first step in any exploration campaign. This gives an initial overview of the vertical changes of the studied rocks. Sedimentary-petrographic studies are a second important approach for characterizing sediments. In the case of carbonate sediments this approach is useful since carbonate grains are normally produced in close proximity (from less than a meter to hundreds of meters) to the site of their ultimate deposition. In addition, carbonate grains are produced mainly by organisms, and thus the grains provide ecological data about the environment of formation as well as stratigraphic information of the age of the deposit (e.g. Scholle and Ulmer-Scholle, 2003). Petrographic studies enhance our knowledge about the microscopic characteristics of the investigated rocks such as texture, type of cement, compaction, porosity and preservation of the grains. This in turn helps finding the best method for developing the investigated resources. Studying the ichnofabrics of the rocks has been widely used in facies interpretations. It also sheds light on the consistency of the sediments and the dynamics of the depositional environment (e.g. Ekdale et al., 2012).

Data

Core OS-01 drilled in South Jordan has a thickness of 183.3 m and was logged visually using a logging sheet designed to collect the different parameters, with an original logging scale 1:50 (Ali Hussein et al., 2014a). Logged parameters include main lithology, texture, sedimentary structures, grain size, macrofossil content, ichnofabrics, foraminifera content, visual porosity, fractures, hydrocarbon shows, color, and key stratal surfaces.

The bioturbation index (BI) was utilized to estimate the grade of bioturbation. This index was introduced by Reineck (1963) and revitalized later by Taylor and Goldring (1993). The BI relates the degree of bioturbation to the preservation of primary sedimentary features (Knaust, 2012). In this index, each grade of bioturbation is assigned to a numerical value that clearly defines the burrow density, the amount of burrow overlap, and the sharpness of the original sedimentary fabric (Taylor et al., 2003).

A total of 20 thin sections were studied using the checklist of Wilson (1975) and Flügel (1982). The classification of limestones follows Dunham (1962) and Folk (1962). Microfacies types (MF types) were assigned to each thin section according to the standard microfacies types (SMF) model of Wilson (1975) and Flügel (1982, 2010). The data of core OS-01 were compared to previously published data of core OS-23 from Central Jordan (Ali Hussein et al., 2014a, b).

RESULTS

Lithofacies

From bottom to top core OS-01 can be divided into three main lithological units (Figure 3). Unit A (183.3–117.0 m) consists of thick chert interval at the bottom, followed by intercalations of bituminous marls, chalky marls and limestones. The overlying unit B (117.0–81.0 m) comprises intercalations of chalky marls and limestone with silicified marls near the top. This unit is characterized by the presence of four beds of limestones and bituminous marls that are rich in oysters. Unit C (81–0 m) consists of intercalations of bituminous marls and limestones.

Three lithological units (Ali Hussein et al., 2014a) can also be distinguished in core OS-23 (Figure 3). Unit A (256.3–187.1 m) comprises intercalations of bituminous marls, chalky marls and phosphate layers. Thin bands and concretions of chert and limestone appear at the top of this unit. The overlying unit B (187.1–51.7 m) consists of mudstones to wackestones of homogeneous bituminous marls and chalky marls with a few horizons of limestone concretions. Unit C (51.7–0 m) comprises intercalations of bituminous marls, chalky marls, cherts, limestones and chalky limestones. It also contains a distinctive horizon of intense bioturbation.

Ichnofabrics

Bioturbation

The bioturbated intervals in core OS-01 occur in two different patterns (Figure 4). Most common are the discrete and closely spaced bioturbated intervals, which are found throughout the entire core. The second pattern describes discrete and widely spaced intervals, which appear in the middle of unit A and at the top of unit B.

Figure 4:

Simplified lithological log of the studied cores illustrating the vertical distribution of different ichnogenera and the bioturbation index. The data of core OS-23 are from Ali Hussein et al. (2014 a, b). See Figure 3 for legend.

Figure 4:

Simplified lithological log of the studied cores illustrating the vertical distribution of different ichnogenera and the bioturbation index. The data of core OS-23 are from Ali Hussein et al. (2014 a, b). See Figure 3 for legend.

The bioturbated intervals in core OS-23 show three patterns (Figure 4; Ali Hussein et al., 2014a). The first one is marked by discrete and widely spaced intervals that appear in the lower part of unit A and at the top of unit B. The second pattern shows discrete and closely spaced intervals at the top of unit A and at the bottom of unit B. The third pattern includes the thick, highly bioturbated intervals (BI ranges between 5 and 6), which occur in two distinctive horizons of this core. The first horizon is around 25 m thick, which can be found in the middle of unit B. The second thick highly bioturbated horizon is 10 m thick that has been observed in unit C. The BI in this core ranges from 1 to 6.

Ichnogenera

Cores OS-01 and OS-23 yield four ichnogenera: Thalassinoides, Teichichnus, Chondrites, and Zoophycos (in descending abundance; Figure 4). Each ichnogenus has different distributions and abundances in the two cores. The occurrences and typical features of the four ichnogenera are summarized in Table 1.

Table 1:

Occurrences and characteristics of different ichnogenera encountered in the studied cores

IchnogeneraOccurrenceCharacteristics
ThalassinoidesDominant throughout both cores. This ichnogenus appears in many horizons. It is associated with other taxa found in this study.Large burrow compared to the other ichnogenera described in this study. The morphology of these burrows have a wide variety, from elliptical, cylindrical to semi-circular. In some cases they crosscut the core as a wide burrow with a diameter ranging from 1 to 3 cm. Thalassinoides shows a structureless fill, differentiating itself from the surrounding sediment.
TeichichnusLimited to 10 bioturbation intervals in core OS-01 and 2 intervals in core OS-23.Appears as a vertical series of tightly packed concave-up, crescentric laminae. The laminae have a width from 1 to 2 cm. They are passively filled with material derived from the surrounding sediment.
ChondritesFound in the upper unit C of both cores.A root-like burrow system that appears in the cores as an assemblage of tiny elliptical dots and strings. Vertical slices through the core truncate the numerous branching tunnels (Pemberton et al., 2001). The burrows have a size that ranges from 1 to 3 mm, while the longitudinal sections reach 2 cm. Chondrites burrows are filled by material from the surrounding sediment.
ZoophycosPresent only in the upper highly bioturbated interval of unit C in core OS-23.Appears in core sections as a thin, semi-horizontal to horizontal spreiten burrow. Individual burrows mostly have a width less than 1 cm. The filling material of these burrows are derived from the surrounding sediment.
IchnogeneraOccurrenceCharacteristics
ThalassinoidesDominant throughout both cores. This ichnogenus appears in many horizons. It is associated with other taxa found in this study.Large burrow compared to the other ichnogenera described in this study. The morphology of these burrows have a wide variety, from elliptical, cylindrical to semi-circular. In some cases they crosscut the core as a wide burrow with a diameter ranging from 1 to 3 cm. Thalassinoides shows a structureless fill, differentiating itself from the surrounding sediment.
TeichichnusLimited to 10 bioturbation intervals in core OS-01 and 2 intervals in core OS-23.Appears as a vertical series of tightly packed concave-up, crescentric laminae. The laminae have a width from 1 to 2 cm. They are passively filled with material derived from the surrounding sediment.
ChondritesFound in the upper unit C of both cores.A root-like burrow system that appears in the cores as an assemblage of tiny elliptical dots and strings. Vertical slices through the core truncate the numerous branching tunnels (Pemberton et al., 2001). The burrows have a size that ranges from 1 to 3 mm, while the longitudinal sections reach 2 cm. Chondrites burrows are filled by material from the surrounding sediment.
ZoophycosPresent only in the upper highly bioturbated interval of unit C in core OS-23.Appears in core sections as a thin, semi-horizontal to horizontal spreiten burrow. Individual burrows mostly have a width less than 1 cm. The filling material of these burrows are derived from the surrounding sediment.

Petrographic Characterization

A total of 20 (core OS-01) and 53 thin sections (core OS-23), were studied using polarized microscopy. The results of this study are summarized in Figures 5 and 6.

Figure 5:

Simplified lithological log of core OS-01 illustrating the distribution of different grains and cement types in the studied thin sections. See Figure 3 for legend.

Figure 5:

Simplified lithological log of core OS-01 illustrating the distribution of different grains and cement types in the studied thin sections. See Figure 3 for legend.

Figure 6:

Simplified lithological log of core OS-23 illustrating the distribution of different grains and cement types in the studied thin sections. The data are modified after Ali Hussein et al. (2014b). See Figure 3 for legend.

Figure 6:

Simplified lithological log of core OS-23 illustrating the distribution of different grains and cement types in the studied thin sections. The data are modified after Ali Hussein et al. (2014b). See Figure 3 for legend.

Skeletal and Non-skeletal Grains

A variety of skeletal grains was found in the studied thin sections, with benthic and planktonic foraminifera being the dominant component. Other skeletal grains include bivalves (represented by the shell fragments in Figures 5 and 6) and bone fragments. The benthic foraminifera become more dominant in the shallow parts of both cores, while the planktonic foraminifera follow no trend. There are four distinctive beds in unit B of core OS-01, where bivalves represent the major components. These beds are represented by three samples 6, 8 and 11 (core OS-01). In core OS-23 the bivalves are minor components in samples 11, 14–17, 23, 26, 29 and 38. The bone fragments are common in the lower part of both cores, where they are common in samples 1 to 3 (core OS-01) and samples 13 and 16 (core OS-23) (Figures 5 and 6). Most of the samples contain moderate- to well-preserved skeletal grains with sharp edges.

The non-skeletal grains observed in the studied samples of both cores are represented by peloids and lithoclasts (Figures 5 and 6). Samples 1 to 4, 11, 16, 19 and 20 (core OS-01), and samples 7, 8, 10, 11, 13, 16, 23 and 49 (core OS-23) contain abundant peloids (> 10%). The lithoclasts are larger than the peloids, they represent an overall minor component in the thin sections. In core OS-01 the lithoclasts are more common than in core OS-23, where they become abundant in samples 1 and 7 (core OS-01).

Types of Cements and Diagenetic Features

Three types of cements were encountered in the studied thin sections, being granular calcite, isopachous rims around the grains and drusy calcite (Figures 5 and 6). Granular calcite represents the major cement type in both cores. It is common in most of the studied samples. This cement commonly fills the foraminiferal chambers and the inter-particles void spaces. Isopachous rim cement surrounding the grains is less common in the studied cores. Generally it is associated with the peloidal facies, where it was observed in samples 1 to 4, 11 and 19 (core OS-01), and in samples 1, 8, 9, 40 and 49 (core OS-23). This cement is characterized by a single rim of microcrystalline crystals growing with equal thickness around grains such as the peloids and lithoclasts (Ali Hussein et al., 2014b). Drusy calcite cement is associated with fractures. It was not encountered in core OS-01 but it exists in samples 11, 12 and 51 (core OS-23). This cement is characterized by pore-filling calcite crystals increasing in size toward the center of the voids. Other distinctive diagenetic features are the cone-in cone structures. They occur in different depth intervals of core OS-01, samples 5 and 13, and in core OS-23, sample 35.

Microfacies Types (MF Types)

Microfacies types were assigned to each sample utilizing the grains types and their characteristics, following the standard microfacies model of Wilson (1975) and Flügel (1982, 2010) (Figure 7). The studied thin sections exhibit eight MF types, where MF types 1 to 6 are described by Ali Hussein et al. (2014a). Table 2 summarizes these MF types and describes their distribution in both cores.

Figure 7:

Simplified lithological log of the studied cores illustrating the distribution of different MF types encountered in the studied thin sections. The data of core OS-23 are modified after Ali Hussein et al. (2014b). See Figure 3 for legend.

Figure 7:

Simplified lithological log of the studied cores illustrating the distribution of different MF types encountered in the studied thin sections. The data of core OS-23 are modified after Ali Hussein et al. (2014b). See Figure 3 for legend.

Table 2:

Occurrences and characteristics of different MF types observed in the studied cores.

DISCUSSION

The investigations of the two cores OS-01 (Maastrichtian–lower Paleocene) and OS-23 (Eocene) revealed differences in lithology, ichnofabrics and microfacies between the two cores. These variations reflect a change in the depositional environment of the Jordanian oil shales.

Lithological Changes Through Time

The lithological logs of both cores show the same rock types (Figure 3): bituminous marls are the dominant lithology, intercalated with chalky marls, limestones, chalky limestones and cherts. The major lithological difference exists in the middle part of the two cores. Core OS-01 exhibits four beds (each 1–2 m thick) of bivalve-rich limestone and marls, which do not exist in core OS-23. These beds indicate the presence of oyster bioherms during the Maastrichtian time.

The middle part of core OS-23, represented by unit B (187.1–51.7 m), consists of homogeneous bituminous marls and chalky marls with a few horizons of limestone concretions. This homogeneity over a thick section reflects stability in the depositional environment in terms of energy and chemistry. This long-lived homogeneity does not exist throughout core OS-01 due to the dynamic changes in the depositional system during the Maastrichtian. These changes are represented by the frequent variations in lithologies of core OS-01.

A higher supply of reworked carbonate grains (skeletal and non-skeletal) from shallower areas signifies phases of depositional progradation, while a lower supply of these grains represents phases of depositional retrogradation. Three third-order stratigraphic cycles have been recognized in each core (Figures 3 to 7).

In core OS-01, the first progradation phase is characterized by the presence of MF-7 that contains 60–80% carbonate grains (Figure 7). This phase is followed by a retrogradation phase dominated by MF-1 and MF-3 with 7–50% carbonate grains. The second progradation phase is characterized by the occurrence of MF-1 and MF-8 with carbonate grains up to 60%, followed by the second retrogradation phase that is dominated by the presence of MF-1 with carbonate grains 7–20% and few intervals of MF-8 contains 60–80% carbonate grains. The final progradation phase consists of MF-1 and MF-4 with carbonate grains up to 50%, followed by the final retrogradation phase that is dominated by the MF-2 with 15–35% carbonate grains.

In core OS-23, the first progradation phase is characterized by the presence of MF-4 and MF-6 with carbonate grains up to 55% (Figure 7). This phase is followed by a retrogradation phase dominated by MF-1 that contains 7–20% carbonate grains. The second progradation phase is characterized by the occurrence of MF-5 with 40–55% carbonate grains, followed by the second retrogradation phase that is dominated by the presence of MF-1 with carbonate grains 7–20%. The final progradation phase consists of MF-1 and MF-4 with carbonate grains up to 50%, followed by the final retrogradation phase that is dominated by the MF-1 and MF-6 with < 20% carbonate grains.

Ichnofabrics Changes Through Time

The ichnofabrics describe all aspects of the texture and internal structure of sediments that result from bioturbation at all scales (Bromley and Ekdale, 1986). It includes ichnodiversity and the abundance together with the relationship of particular ichnotaxa and the amount of bioturbation. The ichnotaxa assemblage present in both cores are dominated by Thalassinoides; Teichichnus occurs in some intervals (Figure 4). These two ichnotaxa indicate a well-oxygenated, shallow-water setting ranging from a lower shoreface (distal position) to still, shallow upper-offshore environments slowly transitioning into the inner shelf (Pemberton et al., 2012).

The ichnodiversity and the amount of bioturbation in the sediments reflect the degree of bottom-water oxygenation, since oxygen is needed by the trace-making organisms. There is a significant difference in the distribution patterns of the bioturbated intervals throughout the two cores (Figure 4). In core OS-01 the bioturbation was found only in discrete intervals (each 1–2 m thick). These intervals are either closely or widely spaced, whereas core OS-23 exhibits continuous highly bioturbated intervals (> 10 m thick). These patterns of bioturbation reflect two different regimes (Ali Hussein et al., 2014a). The first one is characterized by a rapid onset of the bioturbation representing opportunistic colonization, lasting only for a short period of time. This regime is represented by the discrete bioturbated intervals in both cores. The second one, in contrast, is characterized by gradually increasing burrowing intensity over a long time of deposition, which indicates slow oxygenation of the bottom water. This regime is represented by the continuous highly bioturbated intervals in core OS-23.

These differences of the oxygenation regimes between the two cores, indicates a change in the mechanisms behind this oxygenation. In core OS-01 the only mechanism that played a major role in supplying the oxygen to the bottom waters were sudden events such as storm events. These enhanced the circulation of the water body and enriched bottom waters with oxygen. In addition to the aforementioned mechanism, the oxygenation of bottom waters in core OS-23 resulted from the opening of the sedimentary basin and mixing of its water with well-oxygenated marine waters. The opening of the sedimentary basin could be a result of local tectonics (Ali Hussein et al., 2014a; Alqudah et al., 2014).

Microfacies Changes Through Time

Cores OS-01 and OS-23 exhibit eight microfacies types (MF) having the numbers MF-1 to MF-8 (Figure 7). These microfacies can be correlated to eight standard microfacies (SMF), which are SMF-8, SMF-9, SMF-10, SMF-16, SMF-18, SMF-23, SMF-14, SMF-12, respectively. According to the models of Wilson (1975) and Flügel (1982, 2010) for the rimmed carbonate platform facies, these eight SMF types are found within the carbonate platform margin and interior. They are distributed between two facies zones: (1) FZ-7 that represents the open-marine zone within the platform interior, and (2) FZ-8 found in the restricted part of the platform interior (Figure 8).

Figure 8:

(a) Schematic diagram showing the distribution of the different MF types within the rimmed carbonate shelf model during deposition of the Eocene oil shales in Core OS-23. Synsedimentary tectonics caused subsiding grabens and half grabens that reduced circulation and enhanced restricted conditions. Nannofossils are largely reworked and allochthonous. (b) Schematic diagram showing the distribution of different MF types within the rimmed carbonate shelf model during deposition of the Cretaceous oil shales in Core OS-01. Rapid onset of bioturbation suggests opportunistic colonization during storm events. Shallower water conditions prevailed during the Cretaceous. Taxa are largely autochthonous. Bioherms enhanced the restricted conditions (FZ diagram redrawn from Flügel, 2010).

Figure 8:

(a) Schematic diagram showing the distribution of the different MF types within the rimmed carbonate shelf model during deposition of the Eocene oil shales in Core OS-23. Synsedimentary tectonics caused subsiding grabens and half grabens that reduced circulation and enhanced restricted conditions. Nannofossils are largely reworked and allochthonous. (b) Schematic diagram showing the distribution of different MF types within the rimmed carbonate shelf model during deposition of the Cretaceous oil shales in Core OS-01. Rapid onset of bioturbation suggests opportunistic colonization during storm events. Shallower water conditions prevailed during the Cretaceous. Taxa are largely autochthonous. Bioherms enhanced the restricted conditions (FZ diagram redrawn from Flügel, 2010).

Core OS-01 shows MF-1 to MF-4, MF-7 and MF-8, while core OS-23 has MF types from 1–6. This indicates that there were oyster reefs or bioherms during the deposition of the Maastrichtian oil shales represented by MF-8 (bivalve floatstone/rudstone). This MF-type was not found in core OS-23 (Figure 8 and Table 2).

Oil Shale Deposition

Oil-shale beds are formed in anoxic environments, where the oxygen demand in the water column exceeds the supply (e.g. Demaison and Moor, 1980; Hay, 1995). The total organic carbon content (TOC) of thermally immature sediments is always a function of three main factors: (1) organic-matter input (productivity); (2) organic-matter preservation; and (3) dilution by mineral sediment components (e.g. Tyson, 2005). In seas with a positive fresh-water balance (i.e. precipitation plus runoff exceeds evaporation) that have isolated depressions extending below the mixed surface layer, density stratification can develop between lighter, less saline surface waters and denser, more saline subsurface waters. The density stratification can restrict vertical mixing and result in anoxia of the entire subsurface water column if the organic-particle flux from the surface waters remains high as a result of nutrient input from rivers and upward mixing of the subsurface waters (Demaison and Moor, 1980; Hay, 1995).

The restricted-circulation conditions under which the Jordanian oil shales were deposited changed through time. During the Maastrichtian, a series of physical barriers developed as oyster bioherms that enhanced the restricted conditions. These bioherms are well documented for the Upper Cretaceous rocks in Jordan (e.g. Abed and Sadaqah, 1998; Powell and Moh’d, 2011). By comparing the presence of these bioherms in the middle unit B of core OS-01 with the TOC curve, it was found that the richest oil-shale interval correlates to these bioherms (Figure 9a). The Eocene oil shales of Jordan were deposited under different conditions, where the local tectonic activities played a key role. These tectonic activities are evidenced by the reworking of nannofossils of different ages present throughout core OS-23 (Figure 9b). The middle part of the core (depth interval 120–190 m) represents the richest oil shales, which corresponds to the maximum reworking of Cretaceous, Paleocene, and Lower Eocene nannofossil taxa that are allochthonous in the Middle Eocene oil shales autochthonous taxa (Alqudah et al., 2014).

Figure 9:

The richness of the oil shale represented by the TOC curves; (a) correlation of the oyster-rich beds with the TOC curve of core OS-01. (b) Correlation of the lithology and TOC curve of core OS-23 with abundances of calcareous nannofossils of different ages (from Alqudah et al., 2014) and the δ18Ocarb (Ali Hussein et al., 2014b). Global δ18Ocarb profile redrawn and simplified after Zachos et al. (2008), the biostratigraphic data after Alqudah et al. (2014), bio-zones and global δ18Ocarb profile are calibrated to the geologic time scale (Gradstein et al., 2004). TOC data were provided by the Jordanian Oil Shale Company (JOSCO).

Figure 9:

The richness of the oil shale represented by the TOC curves; (a) correlation of the oyster-rich beds with the TOC curve of core OS-01. (b) Correlation of the lithology and TOC curve of core OS-23 with abundances of calcareous nannofossils of different ages (from Alqudah et al., 2014) and the δ18Ocarb (Ali Hussein et al., 2014b). Global δ18Ocarb profile redrawn and simplified after Zachos et al. (2008), the biostratigraphic data after Alqudah et al. (2014), bio-zones and global δ18Ocarb profile are calibrated to the geologic time scale (Gradstein et al., 2004). TOC data were provided by the Jordanian Oil Shale Company (JOSCO).

Regional sea-level fluctuations played a key role in the deposition of the Jordan oil shales (Figure 10). Most of the shallowing intervals that appear in the stratigraphic cycles of cores OS-01 and OS-23 correspond to sea-level lows present also in the sea-level curve of the Arabian platform (Haq and Al-Qahtani, 2005). The middle shallowing interval in core OS-23 cannot be seen on the sea-level curve, indicating that local factors (depositional and tectonic) represent a major player in the deposition of the Jordanian oil shales. On the other hand, the sea-level curve represents an overall shallower period during the deposition of core OS-01 than core OS-23, which explains the frequent changes in lithology and oil-shale richness in core OS-01 (Figure 10).

Figure 10:

A correlation between the Arabian platform regional sea-level changes (Haq and Al-Qahtani, 2005) and the stratigraphic cycles of the two cores OS-01 and OS-23. Biotratigraphy is after Alqudah et al. (2014). Lithology and cycles of core OS-23 are modified after Ali Hussein et al. (2014b).

Figure 10:

A correlation between the Arabian platform regional sea-level changes (Haq and Al-Qahtani, 2005) and the stratigraphic cycles of the two cores OS-01 and OS-23. Biotratigraphy is after Alqudah et al. (2014). Lithology and cycles of core OS-23 are modified after Ali Hussein et al. (2014b).

During the Middle Eocene Climatic Optimum event (MECO) massive quantities of atmospheric CO2 were produced (Figure 9). These caused global warming and disturbed the overall CO2 balance leading to increased burial of organic carbon in return (Bohaty and Zachos, 2003). The top organic-rich horizon in core OS-23 (depth interval 30–50 m) corresponds to this climatic event (Ali Hussein et al., 2014b), which is evidenced as well from correlating the δ18Ocarb curve of this core with the global curve of Zachos et al. (2008).

In synthesis, the interaction between local geological setting and regional sea-level fluctuation together with the global climatic changes, controlled the depositional conditions in which the Jordanian oil shales were deposited during the Maastrichtian–Eocene.

CONCLUSIONS

The investigated cores (Maastrichtian–Eocene) revealed a change in the paleodepositional environment of the Jordanian oil shales. These changes can be seen in the lithology, bioturbated intervals and the microfacies types. Upper Cretaceous oil shales from South Jordan were deposited in a shallow carbonate shelf system. Oyster bioherms acted as physical barriers that reduced the water circulation and enhanced the anoxic conditions within the water column. The Eocene oil shales from Central Jordan were deposited on a shallow carbonate shelf but relatively deeper than the Maastrichtian one. During the Eocene, local tectonic activities controlled the opening and closing of the sedimentary basin that affected the deposition and richness of the oil shales. Throughout the period from Maastrichtian to Eocene, three key factors affected the deposition and richness of the Jordanian oil shales; namely: local geological setting, regional sea-level fluctuation and the global climatic changes.

ACKNOWLEDGMENTS

The authors would like to thank Royal Dutch Shell plc for sponsoring the project. Our thanks are extended to Susan Sawaqed and Ibrahim Al-Najjar from JOSCO for preparing and providing the sample material as well as the TOC data. GeoArabia’s Assistant Editor Kathy Breining is thanked for proofreading the manuscript, and GeoArabia’s Production Co-manager, Nestor “Nino” Buhay IV, for designing the paper for press.

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ABOUT THE AUTHORS

Mohammad A. Ali Hussein is a Geologist and Sedimentologist and obtained his MSc in Applied Geology in 2005 from The Hashemite University, Jordan. He started his professional career as a Research and Teaching Assistant in the Hashemite University. After that he worked as a Field Geologist for 7 years in the Geological Mapping Division at the Natural Resources Authority of Jordan. Mohammad recieved his PhD in 2014 from the Institute for Geology, Mineralogy and Geophysics at the Ruhr-University Bochum, Germany, where he studied the sedimentology of the Jordanian oil shales. Mohammad’s research interests include geological mapping, sedimentology, ichnology, microfacies of carbonate rocks and interpretation of paleodepositional environments working on both outcrop and cored material.

mohammad.alihussein@rub.de

Mohammad A. Alqudah is a Geologist and Paleontologist. He awarded his PhD in 2014 from the Ruhr University Bochum, Germany, where he studied the biostratigraphy of the Jordanian oil shales. He obtained his MSc in Geology in 2007 from the Yarmouk University, Jordan. Mohammad has over three years of experience in research and services in the field of exploration and production of hydrocarbons. He has worked in Schlumberger Company, Geoservices branch as Mudlogger with many clients including Saudi Aramco, TOTAL and Ras Gas. Recent research interests focus on calcareous nannofossil biostratigraphy and paleoceanography of the source rocks.

mohammad.alqudah@ruhr-uni-bochum.de

Myrna Blessenohl holds a BSc degree in Geology and recieved her MSc in 2014 from the Ruhr University Bochum, Germany. Her master’s thesis discusses lithology and depositional conditions of an oil shale core from Jordan. Myrna’s research interests include sedimentology and microfacies of carbonate rocks and interpretation of paleodepositional environments working on cored material.

myrna.blessenohl@rub.de

Olaf G. Podlaha completed his PhD on isotope stratigraphy and numerical modeling at Ruhr-University Bochum, Germany in 1995. He then spent two years as a Post-Doctoral Research Fellow of the German Science Foundation at Indiana University, Woods Hole Oceanographic Institution and Ottawa University. From 1997 onwards Olaf has worked for Shell International, first in Exploration and Production and currently as both Principal Geochemist and Team Lead for basin analysis and inversion R&D in Global Solutions. He was appointed Shell’s global subject matter expert for isotope geochemistry, kinetics and then numerical geochemistry. Since 1998 Olaf is an unpaid lecturer at the Ruhr-University Bochum. His current lectures are in petroleum geology.

olaf.podlaha@shell.com

Jörg Mutterlose is a Professor in Paleontology and was awarded his PhD in Geology and Paleontology in 1982 from the University of Hannover, Germany. He started his professional career as a Visiting Scientist in Jordan and the USA. He then participated in international research programs including ODP legs as a Micropaleontologist. He acted as a Consultant (ostracods, foraminifera, calcareous nannofossils) for various oil companies, conducting micro-sample dating, log interpretations and biostratigraphic studies. Since 1993 he is acting as the head of the paleontology group with a C3-Professor title at the Ruhr-University Bochum. He is on the editorial board of various scientific journals. Jörg’s main research interests are to use the micropaleontology and fossil record data in reconstructing the paleoceanography and paleoclimate.

joerg.mutterlose@rub.de