Sediment petrographic studies combined with carbon and oxygen stable-isotope analyses are considered to be powerful tools in deciphering the depositional and diagenetic history of carbonate rocks. These studies have been found valuable for understanding the environmental controls and global changes of climate and oceanography. Two cores of Eocene bituminous marls (OS-22 and OS-23) from central Jordan with thicknesses of 222.4 m and 256.3 m, respectively, were logged. A total of 103 thin sections were analyzed for sediment petrographic investigations. Another 295 bulk rock samples from Core OS-23 were selected for stable-isotope analysis (δ13C, δ18O). The two cores show similar lithologic variations that correspond to third-order sea-level changes. The sediment petrographic study reveals the presence of six major microfacies that range from mudstones to grainstones. The distribution of these six types of microfacies in the two cores indicates an overall shallow-marine environment in an interior carbonate-platform setting. Within this environment, conditions fluctuated between open and restricted marine. The stratigraphic variability of the carbon-isotope data of Core OS-23 reflects a highly dynamic depositional system that exhibits a variable rate of organic matter accumulation in the sediments, which can be directly linked to the interaction between primary organic-matter burial and oxidative weathering of reworked organic matter. Within the overall diagenetically controlled δ18Ocarb profile an excursion is observed for a specifically enriched organic matter interval. This excursion is believed to coincide with the Middle Eocene Climatic Optimum (MECO), suggesting that next to seawater, porewaters were influenced by this event and making it the first record of this event in the region.

The oil shales of Jordan are considered to be a major future energy source of the country, which currently depends mainly on imported oil and gas. Over the last decade, the government of Jordan represented by the Natural Resource Authority (NRA), signed numerous production sharing agreements with many national and international companies, in order to investigate and exploit these resources. These investigative operations opened the way to a huge inventory of material (i.e. cores and cutting samples) that can be studied in order to provide a better understanding of the origin of these rocks.

The oil shales of Jordan are lithologically bituminous limestones and marls. Due to the popularity of the former name, however, the term oil shale is being used in this paper. Various researchers have studied these rocks covering their geological, geochemical and economic aspects in different parts of Jordan (Bender, 1975; Abed, 1982; Hufnagel, 1985; Beydoun et al., 1994; Abed et al., 2005; Hamarneh, 2006; El-Hasan, 2008; Dill et al., 2009; Jaber et al., 2010). A few studies have investigated the sediment petrography of the Jordanian oil shales using microfacies analysis. Abed and Amireh (1983) studied the oil shales of northern Jordan and the Lajjun area recognizing one dominant bituminous biomicrite microfacies. Mihdawi and Mustafa (2007) investigated the oil shales of northern Jordan and assigned three microfacies types to these rocks, namely foraminiferal wackestone, foraminiferal packstone, and foraminiferal wackestone-packstone. Most of these studies were conducted on surface or near-surface deposits of the major oil-shale occurrences of Jordan (Figure 1).

As a part of an integrated research project that comprises geochemistry, stratigraphy and sedimentology of the central Jordanian oil shales, the current study investigates the sediment petrography. The significance of this study arises from the fact that it is the first study analyzing the deep, large and TOC-rich oil shales situated in central Jordan, where these deposits are considered to be a very promising candidate in the energy resources development of the country.

Sediment petrographic studies are essential for any resources development. They provide valuable information at a microscopic scale about the major and minor components of the studied rocks, and how these components interact within a textural framework. By providing insights into the diagenesis and related lithological property changes such as porosity, compaction and preservation of skeletal and non-skeletal grains, the findings can be upscaled to improve the understanding of the origin and alteration of the investigated rocks. Integrating petrographic results with stable-isotope data (δ13C and δ18O) and total organic carbon (TOC) data enhances the understanding of the environmental conditions that led to the accumulation of organic carbon in these marine sediments.

In this paper, two deep cores (OS-22 and OS-23) of Middle Eocene age (Alqudah et al., 2014) have been investigated. These studies aim at understanding the origin and the paleodepositional environment that controlled the deposition of the organic-rich carbonate sequence in central Jordan.

During the Cretaceous to Eocene, Jordan was located at the southern margin of the Neo-Tethys Ocean. The sedimentation during that time interval took place on a broad, shallow shelf that covered the northern edge of the Arabian Plate (Ziegler, 2001; Powell and Moh’d, 2011). A thick sequence of chalks, marls and limestones was deposited over the northern and central parts of Jordan. This sedimentary sequence was controlled by the configuration of the Neo-Tethys Ocean and its subsequent closure-related orogenic events as the African-Arabian Plate moved northwards against the Euro-Asian Plate (Barjous and Mikbel, 1990). In addition, the fluctuation in the eustatic sea level (Haq and Al-Qahtani, 2005) and the tectonic activities along the major structural features such as the Syrian Arc (Mart, 1987) were superimposed on the depositional framework.

Three formations were deposited in Jordan during the Paleogene Period, these are the Muwaqqar Chalk Marl (MCM), the Umm Rijam Chert-Limestone (URC), and the Wadi Shallala Chalk (WSC). The age assignment of the MCM and the URC formations differs between its outcrop and subsurface occurrences, with the boundary between these two formations being the top Selandian (Late Paleocene) in the outcrop studies (Powell, 1989), as opposed to it being Ypresian age (Early Eocene) in the subsurface studies (Andrews, 1992). The two investigated cores belong to the URC Formation according to both definitions (Figure 2).

According to Bender (1968) the wells are located in a structural province called ‘Block Faults of Central and Southeastern Jordan’. This province is characterized by broad epeirogenic swells and basins, which dominantly strike NW-SE and W-E (Bender, 1968, 1975; Diabat and Masri, 2005). These features reflect the interaction between major tectonic elements surrounding the area: the Sirhan Graben, Zarqa-Ma’in Fault and Siwaqa Fault movements (Figure 1). The former one is a large graben situated to the east of the study area and striking NW-SE, while the latter two structures are dextral strike-slip faults located to the south of the investigated area.

Two cores OS-22 and OS-23 from central Jordan with total thicknesses of 222.4 m and 256.3 m, respectively were investigated. They were logged lithologically using a logging sheet designed to collect the different parameters. The original logging scale was 1:50 (Ali Hussein et al., 2014). A total of 103 thin sections were investigated using a form to record the different microfacies criteria as specified by the checklists of Wilson (1975) and Flügel (1982). These criteria include: (1) relative amounts of different types of bioclastic (skeletal) grains versus different non-skeletal grain types; (2) preservation of grains; (3) texture; (4) compaction; (5) types of cement; (6) dolomite content; and (7) matrix properties. 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) of Wilson (1975) and Flügel (1982, 2010).

A total of 295 bulk rock samples from Core OS-23 were selected for stable-isotope analysis (δ13Ccarb, δ18Ocarb). These samples were powdered, the carbonate dissolved with 103% phosphoric acid at 70°C using a Gasbench II coupled to a Thermo Finnigan Five Plus mass spectrometer for online analysis at the University of Erlangen. Analytical precision (1 standard deviation) was better than 0.06‰ for both oxygen and carbon, based on repeat analysis of NBS 19 standard. All values reported here are in the standard δ-notation relative to the Vienna PDB standard.

Results were plotted against stratigraphy/depth, these curves (δ13Ccarb, δ18Ocarb) were then smoothed using a software called PAST (available online The algorithm used for smoothing is “LOWESS” (LOcally WEighted Scatterplot Smoothing; Cleveland, 1979). The smoothed curves are plotted with the 95% confidence interval based on 999 random replicates. In order to retain the structure of the interpolation, the procedure uses resampling of residuals rather than resampling of original data points (Hammer, 2012).

Total organic carbon (TOC) for 180 samples was measured using a calibrated LECO-CS-244 elemental analyzer at Newcastle University (Aqleh et al., 2013). The calibration was performed using LECO certified standard steel rings (501-506) with carbon contents 0.8 wt% and sulfur contents of 0.015 wt%. Every data point is representing an interval of homogenized crushed sample taken from a macroscopically homogeneous lithological unit. It needs to be noted that these data therefore do not represent equidistant samples.

The lithostratigraphy of the two investigated cores shows similar patterns, with intercalations of bituminous marls, cherts, phosphates, and limestones at the bottom part of both cores. The middle interval represents homogeneous bituminous marl, whereas an intercalation of limestones, cherts and bituminous marls is observed at the top of the two cores (Figures 3 and 4). Depth intervals correspond to those published in Ali Hussein et al. (2014). The integration of a petrographic study and the stable-isotope data records (δ13Ccarb, δ18Ocarb) is considered as a useful tool to better understand the depositional environment of the central Jordan oil shales.

Petrographic characterization

A total of 50 and 53 thin sections from cores OS-22 and OS-23, respectively, were studied using polarized microscopy. Details are given in the following subchapters. The results of this study are summarized in Figures 3 and 4.

Skeletal Grains

Different types of skeletal grains were found in the studied samples, with the dominant grains being benthic and planktonic foraminifera (Figure 5). Other skeletal grain types commonly present in the samples are bone fragments and bivalve shells, rarely ostracods and echinoids (Figure 6).

In both cores, eight genera of foraminifera were identified based on Loeblich and Tappan (1988) and Holbourn et al. (2013). Five of them are benthic taxa, namely; Lenticulina sp., Siphogenerinoides sp., Nodosaria sp., Gavelinella sp., Anomalinoides sp. The remaining three taxa are planktonic foraminifera; Subbotina sp., Heterohelix sp., Globigerinelloides sp.

The distribution of planktonic foraminifera follows no common trend in the cores, while there are three distinctive horizons where the benthic foraminifera become abundant (Figures 3 and 4). The lower horizon is represented by samples 17–20 (Core OS-22) and samples 20–34 (Core OS-23), the middle horizon is covered by samples 31–36 (Core OS-22) and samples 37–40 (Core OS-23), while the top horizon is restrained to sample 48 (Core OS-22) and samples 49 and 50 (Core OS-23).

The bone fragments are abundant in the bottom part of both cores and become rare toward the middle and top intervals. Bivalves are less common than bone fragments and present in the bottom and middle parts of both cores. Rare specimens of ostracods are recordable in the bottom of Core OS-23. Only two samples from Core OS-22 (29 and 30) exhibit the presence of echinoid spines.

The investigation of the skeletal grain boundaries highlighted different degrees of preservation (Figure 7). Most of the samples contain moderate to well preserved skeletal grains with sharp boundaries, whereas some samples show poorly preserved and ragged rims. These samples are 1, 2, 3, 16, 17, 18, 19, 36, 47 and 48 in Core OS-22, and samples 19, 24, 33, 45 and 47 in Core OS-23.

Non-skeletal Grains

Two different types of non-skeletal grains were encountered in the cores, peloids and lithoclasts (Figure 8). The distribution with depth of these two types of grains is illustrated in Figures 3 and 4.

Peloids are relatively small carbonate grains, ranging in size between 50 μm and 1 mm (< 500 μm on average). They are well sorted, rounded to sub-rounded, internally homogenous grains of micritic character (Flügel, 2010). Two distinctive horizons show abundant peloids (> 10%), the first one near the bottom of both cores and represented by samples 1, 6, 10, 11 and 15 (Core OS-22), and samples 7, 8, 10, 11, 13, 16 and 23 (Core OS-23). The second horizon is located at the top of the two cores in samples 44–47 (Core OS-22) and sample 49 (Core OS-23).

Lithoclasts are relatively large grains, ranging in size between 100 μm and 8 mm (> 1 mm on average). They are poorly sorted. They have angular to sub-angular grain morphology and are composed of reworked lithified carbonates. Lithoclasts are present as an overall minor component in all thin sections, selectively being more common (> 5%) in samples 1, 3, 6, 13 and 17 (Core OS-22), and samples 4, 8, 10, 11, 20, 40 and 49 (Core OS-23).

Types of Cements and Diagenetic Features

Cement represents an important record of the diagenetic history for carbonate rocks (Scholle and Ulmer-Scholle, 2003). Three types of cements were found in the studied samples of both cores, being granular calcite, grains with isopachous rims and drusy calcite (Figure 9).

Granular calcite is the major cement type in both cores as it exists in all studied samples. This cement is characterized by small pore-filling calcite crystals (up to 100 μm) exhibiting no preferred orientation. This cement fills the inter-particle void spaces and can commonly also be found inside the foraminifera chambers (Figure 9a).

Isopachous rim cement surrounding the grains is less common in the studied cores, where it can be found in samples 1, 4, 7, 13, 33, 36, 46 and 47 (Core OS-22), 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 (Figure 9b).

Drusy calcite cement is least common in both cores. It is encountered in samples 8, 36, 39 and 41 (Core OS-22), and in samples 11, 12 and 51 (Core OS-23). This cement type mainly fills the pores created by fractures in both cores (Figure 9c) and is characterized by void/pore-filling calcite crystals increasing in size toward the center of the voids.

Distinctive cone-in-cone structures were encountered in both cores. They were found between 114–115 m (Core OS-22) and 129–144 m (Core OS-23). These depths correspond to the same relative stratigraphic level. This feature appears macroscopically in the core cutting plane as dendritic cracks filled with lighter (more calcitic) sediment. Under the microscope, the calcite crystals inside those cracks show a zigzag pattern that is a diagnostic feature of the cone-in-cone structure (Figure 9d). For more details of such cone-in-cone structures in carbonate sediments we refer to Franks (1969) and Mozley (2003).

Dolomite Content

Dolomite is rare in the studied cores, with only six samples exhibiting dolomite. These samples are 1, 2, 3 and 25 (Core OS-22), and 21 and 33 (Core OS-23). All the dolomite rhombs are located in the micritic matrix, possessing clear rims in samples 1–3 from Core OS-22 and rims becoming indistinct in samples 25 from core OS-22, and 21 and 33 from Core OS-23 (Figure 10).

Microfacies (MF) Types

The combination of different grain types (i.e. skeletal and non-skeletal) and the characteristics of these grains is used to assign an MF type to each thin section following the standard microfacies model of Wilson (1975) and Flügel (1982, 2010). Six MF types were encountered in the studied thin sections (Figure 11). The distribution of these MF types is summarized in Figure 12 and their details are explained in the following sub-chapters.

Bioclastic Wackestone/Mudstone (MF-1): This MF type consists of 7–20% carbonate grains, mainly being benthic and planktonic foraminifera, with rare other skeletal grains such as bone fragments only common at the bottom of the two cores. The grains are generally well preserved with sharp rims. Non-skeletal grains (i.e. peloids and lithoclasts) are rare in this MF type (< 3%). This MF type exhibits, furthermore, a homogeneous and sometimes dark micritic matrix, with the latter reflecting enrichment in organic matter (Figure 11a). This MF type is considered to be the dominant type as 53 thin sections from both cores fall into this class. Stratigraphically it is common in the middle part of the two cores.

Burrowed Bioclastic Wackestone (MF-2): This MF type is characterized by the presence of invertebrate burrows such as Thalassinoides. It consists of 15–35% of carbonate grains, mainly benthic and planktonic foraminifera. Other skeletal grains such as bone fragments are more common than in MF-1, and concentrated in the burrows themselves (Figure 11b). Skeletal grains show moderate preservation with some of these grains being highly fragmented and exhibiting ragged rims. Non-skeletal grains are more common in MF-2 type than MF-1 type with their abundance ranging between 5–10%. The matrix has the same dark micritic character of the MF-1. This MF-2 type has limited distribution, with 14 samples throughout the two cores (Figure 12).

Bioclastic Packstone (MF-3): It consists of 40–50% of carbonate grains. Skeletal grains are the common grain type with 30–35%, including foraminifera, bone fragments and bivalves shells. These grains show low preservation states with ragged rims. The percentage of non-skeletal grains, including peloids and lithoclasts, is consequently ranging between 15–20% (Figure 11c). This MF type exhibits micritic matrix that is getting sparitic in parts and is lighter colored than in the aforementioned MF types. MF-3 is very rare in the studied cores, where only 4 thin sections from the bottom part of the two cores can be assigned to this MF type (Figure 12).

Peloidal Grainstone/Packstone (MF-4): This is the only MF type that consists of more non-skeletal grains than skeletal ones, the total grain content being 45–65%. The percentage of carbonate grains (i.e., skeletal and non-skeletal) is ranging between 30–55%, where the skeletal grain content is 10– 15% and mainly consists of benthic and planktonic foraminifera beside rare bone fragments. These grains show low preservation with fragments and indistinctive rims. The non-skeletal grains are dominated by peloids with average absolute abundances of 40% (Figure 11d). The matrix in MF-4 is inhomogeneous ranging from micritic to sparitic. Fifteen thin sections in both cores are MF-4 type, 12 of them in the bottom part of the cores and the other three are in the upper part (Figure 12).

Foraminiferal Grainstone (MF-5): This is a distinctive MF type consisting of mainly benthic foraminifera (e.g. Siphogenerinoides sp.) (Figure 11e). The average absolute contents of benthic foraminifera in the samples reaches 45%, whereas other skeletal grains are less common. The foraminifera are well preserved with sharp rims. The non-skeletal grains are very rare in this MF type with average percentage of < 5%. The matrix in MF-5 is a homogeneous micrite; it has a variable color from dark to light in different thin sections. MF-5 has a limited distribution throughout the two cores, with 8 samples belonging to this MF type (Figure 12).

Non-fossiliferous Mudstone (MF-6): This MF type is characterized by the dominance of the dark matrix and the scarcity of carbonate grains with an average occurrence of less than 2% (Figure 11f). The carbonate grains consist of a few fragments of foraminifera and lithoclasts. The matrix in this MF type is homogeneous and micritic. MF-6 can be found in the bottom and upper parts of both cores, with 11 thin sections being assigned to it (Figure 12).

Stable Isotopes: δ13Ccarb and δ18Ocarb

A total of 295 bulk rock samples (both limestones and marls) from Core OS-23 were analyzed to obtain oxygen- and carbon-isotope data (Table 1). All oxygen- and carbon-isotope data are shown in Figure 13. The stratigraphic variability of the oxygen- and carbon-isotope data of Core OS-23 is partially not correlated. The δ13Ccarb curve shows some fluctuations and a wide range of the values in the lower part (256.3–173 m). These data exhibit three distinctive negative shifts (dashed red arrows) followed by three positive shifts (dashed green arrows).

The δ18Ocarb profile shows some fluctuations and a wide range of δ18Ocarb values (± 3‰) at the bottom (256.3–173 m). Within the interval from 173 to 75 m, the δ18Ocarb values show a gradual negative trend. Above that interval a remarkable negative excursion from 75 to 18 m, with a wide range of δ18Ocarb values (± 2.5‰, Figure 13). This spread and the data ranges observed for each time are, however, close to previously published seawater oxygen-isotope curves’ uncertainty ranges determined from carbonates (Veizer et al., 1999; Zachos et al., 2008).

Petrographic Characterization and Microfacies Variability

The dominant skeletal grain types encountered in the studied cores are benthic and planktonic foraminifera. The relatively low diversity of these organisms, where some thin sections yielded one genus only (Figure 11e), suggests a restricted platform interior environment. The planktonic foraminifera have no uniform distribution pattern throughout the two cores. The bottom and top parts, however, are enriched in planktonic foraminifera suggesting a more pelagic setting (open-marine platform interior). The benthic foraminifera follow the sea-level cyclicity consistently, becoming more abundant during the low sea-level intervals (Figures 3 and 4). It remains unresolved and might be due to undersampling why in the middle part of the cores only the benthic foraminifera correlate to the sea level.

The preservation of the skeletal grains is a direct indication of the energy during deposition. The worn skeletal grains with unclear boundaries also follow the sea-level cyclicity, where they are found in thin sections corresponding to the shallow-marine intervals of the two cores.

Non-skeletal grains present in the studied samples are peloids and lithoclasts. These types of grains are dominating the shallow intervals in the bottom and the top of both cores. This suggests that these grains are detrital, derived from pre-existing rocks.

Three types of cements were encountered. These are granular calcite, isopachous rim around grains and drusy calcite. Granular calcite and drusy cements can be formed during burial diagenesis, thereby representing alteration below the zone of active seawater circulation (Scholle and Ulmer-Scholle, 2003). This type of diagenesis is the dominant one in the cores and can be found in all thin sections. Isopachous crusts around grains are a result of marine diagenesis, which can be found in few thin sections of both cores. The dolomite is considered to be another product of diagenetic processes. All the dolomite rhombs occur solely in the micrite matrix and due to their later diagenetic overprint they are absent in the dense calcitic bioclasts. This suggests a replacement controlled by the original fabric of sediment.

The six MF types found in the two cores are typical microfacies found within the carbonate platform interior according to Wilson (1975) and Flügel (1982, 2010). They are distributed between the two facies zones 7 (open-marine) and 8 (restricted, Figure 14). MF 1, 2 and 3 were deposited in open-marine platform-interior with low-energy environment below the fair-weather wave base, since no wave-related sedimentary structures (e.g. cross-bedding) have been recorded. MF 4, 5 and 6 are restricted platform-interior deposits. This environment is considered to be shallower than the previous one but less connected to the open ocean, indicated by the presence of non-laminated mud peloids of MF-4 and the low-diversity foraminiferal grainstone of MF-5.

δ13Ccarb Profile Cyclicity

Shifts in the carbon-isotopic composition of marine carbonates may be interpreted as representing shifts in the amount of organic carbon being buried. The detailed record is also controlled by the water mass and the basin connectivity with the main ocean. An increase in the amount of buried organic carbon means that 12C would be preferentially removed from sea water via primary productivity, so that the water mass itself would become isotopically heavier. Negative δ13C-shifts accordingly may indicate a decrease in the rate of carbon burial and/or enhanced oxidative weathering of once buried organic matter (Hoefs, 2009).

The long-term variations of the carbon-isotope data of Core OS-23 are composed of three cycles (Figure 13). The maximum points of the negative shifts (i.e., points where the dashed red and green lines meet on the δ13Ccarb curve) match the distribution of the MF types with MF-4 “Peloidal grainstone/packstone”. This MF type consists of microcrystalline isopachous rim cement which surrounds weathered organic matter embedded in a relatively light micritic matrix (Figures 11d and 15). This indicates a decrease in the rate of carbon burial and weathering and redeposition of old buried organic matter. The negative δ13C-shifts correspond to the shallowing part of the sea level cycles or co-occur with the point of shallowest sea level. This interval reflects a high-energy environment that allows for the formation of this type of cement and enhances the oxygenation of bottom waters. Beyond this being of value for paleo-water depth interpretation, the MF-4 represents the least mobile fraction of redeposited sediments and can thereby also be used as a proximity indicator. As such the facies does not only co-occur in shallow sea-level intervals but also indicates closest proximity of the depositional environment in comparison with other microfacies.

The carbon-isotope record itself can be furthermore interpreted to have been generated as follows. It represents a signal of primary organic matter burial with an Eocene carbon-isotope composition mixed with variable amounts of reworked organic matter of Cretaceous and/or Paleocene origin. This latter organic matter underwent different degrees of oxidation during re-deposition (Alqudah et al., 2014). The oxidation products influenced the pore-water carbon-isotope signal (dissolved inorganic carbon mixed with oxidized organic-carbon products) thereby affecting the carbon-isotope composition of the pore water.

δ18Ocarb Profile and the Middle Eocene Climatic Optimum (MECO)

The fluctuation in the δ18Ocarb curve of Core OS-23 provides hints of the temperature oscillation during the deposition of the oil shale succession. It should be noted though, that the observed isotope ratios are clearly reflecting diagenetic, i.e., pore-water chemistry. As such they are offset towards more negative values against the primary seawater signal. The spread of the isotope record is, however, similar to the published seawater curve which indicates some degree of back-coupling of pore-water and seawater (Zachos et al., 2008). For the bottom, upper middle and top part of OS-23 a 6‰ shift against the primary seawater curve can be observed. For the lower middle part of the core there is no correlatable offset. On this basis further interpretations will be given. At the bottom of the core (256.3–173 m) the curve shows a positive shift, which indicates a cooling trend that elsewhere marks the Middle and Late Eocene (Zachos et al., 2008). Whereas the original seawater shift is in the range of 1‰ indicating a 4°C cooling, the carbonates derived from diagenetic pore water amplify that signal to a more extreme shift of 3‰. They can therefore be used to interpret the change in a qualitative way as cooling but not to quantify the absolute temperature shift.

Within the interval from 173 to 75 m the δ18Ocarb profile shows a gradual negative trend, which can be interpreted as reflecting a warming period. This is, however, not fully correlatable with the seawater curve and a partial correlation with Zachos et al. (2008) should not be attempted. The primary data density of our study for this interval is considered not to be high enough to achieve conclusiveness. Above that gradual decrease in the oxygen isotopic data profile, a steep drop of the δ18Ocarb record starts at a depth of 75 m and continues for 40 m (Figure 13). It can be correlated to the high TOC content in the top interval. This steep decline reflects a sudden increase in temperature. Using the biostratigraphic data of the same core by Alqudah et al. (2014), calibrated with the geological time scale of Gradstein et al. (2004), and comparing the smoothed δ18Ocarb curve of Core OS-23 with the Cenozoic curve compiled by Zachos et al. (2008) this sharp excursion of δ18Ocarb matches one of the most prominent climatic events in the Eocene, the Middle Eocene Climatic Optimum (MECO). This event (ca. 41.5 Ma) interrupts the long-term Middle Eocene cooling trend with globally uniform 4° to 6°C warming of both surface and deep oceans within ca. 400 Kyr, as derived from foraminiferal isotope record (Zachos et al., 2001; Bohaty and Zachos, 2003; Bohaty et al., 2009; Bijl et al., 2010; Pearson, 2010). The cause of the warming is considered to be primarily due to carbon dioxide increases, since carbon-isotope signatures rule out major methane release during this short-term warming (Bohaty and Zachos, 2003). It needs to be taken into account though that the isotope record of the Jordanian cores is offset against the seawater curve and should be used for qualitative interpretation only. Nevertheless, the interesting part is the back-coupling effect that this warming event had also on the pore-waters of the oil shales. This suggests that the oil shales were not buried at that time but rather undergoing pore-water exchange with the overlying water mass. The offset supports at the same time the dominance of diagenesis on the cores lithologic framework as suggested by the microfacies studies.

The integration of petrographic analysis with the carbon and oxygen stable-isotope data is considered a powerful tool in deciphering the depositional and diagenetic history of the oil shales from central Jordan. It also helps to understand the environmental control and global changes of climate. Different types of cement prove the dominance of burial diagenesis throughout the two studied cores, accompanied by marine diagenesis in some intervals. The variability of MF types along the two cores indicates a shallow-marine environment fluctuating between restricted and open marine. The stratigraphic variability of the carbon-isotope data of Core OS-23 reflects a highly dynamic depositional system that exhibits variable rates of organic-matter accumulation in the sediments. This directly relates to the interaction between carbon burial and oxidative weathering and re-deposition of previously buried organic matter of Cretaceous or Paleocene origin. It is likely that the top TOC-rich horizon in both cores corresponds to the Middle Eocene Climatic Optimum (MECO) event, during which massive quantities of atmospheric CO2 were produced. These caused global warming and disturbed the overall CO2 balance leading to increased burial or organic carbon in return.

Taxonomic list of genera used in this study:

Anomalinoides Brotzen 1942

Gavelinella Brotzen 1942

Globigerinelloides Cushman and Stainforth 1945

Heterohelix Ehrenberg 1843

Lenticulina Lamarck 1804

Nodosaria Lamarck 1812

Siphogenerinoides Cushman 1939

Subbotina Brotzen and Pozaryska 1961

The authors would like to thank Royal Dutch Shell plc for sponsoring the project and specifically Susan Sawaqed for preparing and providing the sample material. Our thanks are extended to Adrian Immenhauser and Roman Koch for their valuable help and technical discussion as well as to Suha Aqleh and Mohammad Alalaween for the cooperation in the Jordan studies and providing the top-interval TOC, CaCO3, trace metal and biomolecular marker data. GeoArabia’s Kathy Breining is thanked for proofreading the text and Arnold Egdane, for designing the paper for press.

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. Currently he is working on his PhD in the Institute for Geology, Mineralogy and Geophysics at the Ruhr-University Bochum, Germany, studying 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 A. Alqudah is a Geologist and Paleontologist and currently is a PhD student in Ruhr University Bochum, Germany, studying 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.

Sander H.J.M. van den Boorn is a Research Geochemist with Shell Global Solutions since October 2008. He gained his MSc and PhD at Utrecht University, The Netherlands. Sander has been trained as a geologist and has subsequently specialized in geochemistry. His main responsibilities in Shell include the development and testing of (novel) geochemical technologies for hydrocarbon exploration and he acts as focal point/technical lead for a variety of external technology collaborations with universities. This includes an integrated study of the Jordan Oil Shale, linking (in) organic geochemical proxies with sedimentology and biostratigraphy. His main expertise and interests are in the field of (1) reconstructing paleodepositional environments of (organic-rich) sedimentary sequences, (2) fluid/rock interactions and (3) hydrocarbon migration and accumulation in the subsurface.

Sadat F. Kolonic obtained his PhD in Organic Sedimentology from the University of Bremen (Germany) in 2003 and began his oil industry career with EniAgip the same year. In 2004 Sadat joined Shell Exploration and Production’s Global Frontiers Team to work on various integrated subsurface evaluation projects (depositional environments) covering a wide range of (un)conventional play studies prior to moving to Jordan in 2007 as Head of Exploration for the Shell Jordan Unconventional Oil Project. Currently Sadat acts as the corporate Geochemist for Shell in Nigeria overseeing a diverse portfolio of exploration and development projects.

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.

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.