ABSTRACT

The study of trace fossils is widely used in facies interpretation. It provides a crucial tool for reconstructing depositional paleoenvironments when used in combination with other sedimentological and paleontological proxies. Here we present the first detailed study of Eocene trace fossils from Jordan. Two sections of Early to Middle Eocene age, with a total thickness of 478.7 m, from central Jordan were cored and investigated. The results of individually occurring (isolated) or co-occurring (combined) ichnofabrics and bioturbation levels, in combination with results from biostratigraphic and geochemical studies, were used for stratigraphic and paleoenvironmental reconstructions. The bioturbation index (BI) was used to classify the burrowing density versus the preservation of the original sedimentary structures. The two cores show highly variable grades of bioturbation with BI ranging from 0 to 6. Four ichnogenera were identified: Thalassinoides, Chondrites, Teichichnus and Zoophycos. Both the ichnofabrics and the variations of the BI suggest a shallow, highly dynamic depositional system with rapid changes of water depth and degree of bottom-water oxygenation.

INTRODUCTION

Jordan is facing a chronic shortage of energy resources; it depends mainly on oil and gas imported from Saudi Arabia, Iraq, and Egypt. Jordan is, however, ranked as the 8th out of 37 countries in the world with oil shale reserves (Hamarneh, 2006). Integrated studies to understand the genesis of the Jordanian oil shale are rare. They are mostly limited to in-country research efforts such as those performed under the auspices of the Jordanian Natural Resources Authority (NRA) with a focus on the commerciality of the resources. In contrast, this work is part of a set of integrated studies, which aim to understand the depositional environment of the Jordanian oil shales by using micropaleontology, biostratigraphy, sedimentology, inorganic and organic geochemistry. Within the field of sedimentology, one of the proxies that has been used for this purpose are trace fossils. These have been widely utilized in previous facies interpretations (e.g. McIlroy, 2004) providing an important tool for reconstructing the paleodepositional environment. In addition, the study of ichnofabrics gives a direct reflection of sediment consistency and sedimentary dynamics within the depositional environment (Ekdale et al., 2012).

This study is focused on the ichnology of Eocene oil shales in central Jordan. So far trace fossils associations in Jordan were studied in detail for Paleozoic rocks where various researchers analyzed the ichnology of sandstones and shale type clastic deposits. Selley (1970, 1972) deciphered the trace fossils of lower Paleozoic sandstones and shales in the southern desert of Jordan. Seilacher (1983) used trace fossils to understand the depositional environments and biogeography of the Paleozoic sandstones in southern Jordan. Mángano et al. (2013) identified 12 trace fossil assemblages in Cambrian sandstones and shales of the Dead Sea area. These assemblages provide information on the nature of the Cambrian ecosystems in the aftermath of the Cambrian radiation. Hofmann et al. (2012) reported on the paleoecologic and biostratigraphic significance of trace fossils from middle Cambrian shallow- to marginal-marine environments of Jordan. Other stratigraphic and sedimentological studies recorded the presence of trace fossils of different ages and lithofacies in various localities all over Jordan. These include Powell (1989a, b) for the Cambrian to Eocene, Amireh et al. (2001) for the Ordovician, Pufahl et al. (2003) for the Cretaceous, Abed et al. (2007) for the Cretaceous, Dill et al. (2009) for the Cretaceous and Paleogene, and Powell and Moh’d (2011) for the Cretaceous to Eocene.

The main aim of this study is to show the role of biogenic structures for reconstructions of the depositional environment of Eocene oil shales from central Jordan. For that purpose, two cores from central Jordan (Figure 1a) were thoroughly logged in order to delineate the different assemblages of ichnofabrics as well as the corresponding degree of bioturbation.

GEOLOGY

Jordan lies in the northwestern part of the Arabian Plate that in itself forms a northward protrusion of the Arabian Craton (Figure 1b). The structure and sedimentation were controlled by the tectonic evolution with respect to the configuration of the Arabian Plate (Abu-Jaber et al., 1989). During Cretaceous to Eocene times, the plate was part of the southern Neo-Tethys Ocean margin. The Neo-Tethys periodically transgressed towards the south and east, onto the margins of the Arabian Craton (Powell and Moh’d, 2011). Marine conditions prevailed during the Cretaceous to Late Eocene and ended when the region underwent uplift. It was then subjected to regional faulting, which occurred mainly along rejuvenated old faults and pre-existing zones of weaknesses (Diabat and Masri, 2005).

The study area is covered by sedimentary rocks ranging in age from the Late Cretaceous to the Eocene (Figure 1c). The oldest rocks exposed represent the upper part of the Muwaqqar Chalk-Marl (MCM) Formation, which is typically assigned to the Maastrichtian (Bender, 1975; Powell, 1989b). Lithostratigraphic formations in Jordan are, in part, diachronous. The stratigraphic extent of the MCM Formation for example, varies for the surface and subsurface occurrences (Andrews, 1992). It crops out in the southeastern parts of the study area and consists of marls and chalks, with chalky and micritic limestone concretions, gypsiferous and bituminous marl, granular phosphate and sandy limestone beds (Al Hunjul, 1999; Smadi, 1999). Well-preserved fish fossils, such as teeth and scales, as well as vertebrate bones and ammonites (Baculites sp.), were recorded by Kaddumi (2006) for the Maastrichtian MCM.

The MCM is often unconformably overlain by the Umm Rijam Chert Limestone Formation (URC) of Early to Middle Eocene age. The lower part of the URC consists of chalks, cherts and, locally, porcellanite. The middle part is composed of thin- to medium-bedded cherts alternating with white chalks, and comprises small-size chalky limestone concretions. The upper part, which is rich in bivalves and fish teeth, includes alternating beds of cherts, limestones and chalks, with large micritic limestone concretions (Al Hunjul, 1999). The overlying Wadi Shallala Chalk Formation (WSC) is of Middle to Late Eocene age. It mainly consists of chalk, marls, chalky limestones, chert nodules, phosphatic nodules, glauconite and micritic limestone. The fossil assemblages include bivalves, fish teeth, and burrows (Smadi, 1999). Pleistocene sediments form small isolated hills between wadis. These sediments consist of poorly consolidated, subrounded to subangular clasts of older rocks. Alluvial sediments characterize many wadis and consist of unsorted, friable gravels and sands.

The regional dip of the strata in the study area is towards the northeast. The area is highly faulted and transected in the south by the Zarqa Ma’in Fault, an EW-trending strike-slip fault with dextral movement, which locally has considerable vertical juxtaposition (Smadi, 1999). There are two major grabens crossing the study area. The first graben is located in the south and the other one in the northwestern part of the study area. Both cores dealt with in this study originate from wells drilled within these grabens.

METHODS

Two cores, named OS-22 and OS-23, from central Jordan with thicknesses of 222.4 m and 256.3 m, respectively, were logged. The logging scale is 1:50, and a special logging sheet was designed to collect different data types (Figure 2). It includes the main lithology, texture, sedimentary structures, grain size, macrofossil content, ichnofabrics and bioturbation index, foraminifera percentage, visual porosity, fractures, hydrocarbon shows, color, and key stratal surfaces. The different types of trace fossils were recorded using symbols within the texture and paleontology column in order to show their exact depths.

Bulk geochemical data, i.e. total organic carbon (TOC), calcium carbonate (CaCO3), total carbon and total sulfur data were acquired for selected sections in parallel studies at Newcastle University (Aqleh et al., 2013) and will be published elsewhere. Out of this dataset, a subset of 10 CaCO3 and TOC data points (in weight percent) are shown and discussed for core OS-22 because these data represent a first proxy record of the amount of bottom-water oxygenation and the production of carbonate in time, which can be correlated with the bioturbation index (BI). TOC and CaCO3 content data were measured using a calibrated LECO-CS-244 elemental analyzer. Every data point is representing an interval of homogenized crushed sample taken from a macroscopically homogeneous lithological unit.

The BI was used to evaluate the degree of bioturbation. This index was introduced by Reineck (1963), which later was revitalized by Taylor and Goldring (1993) to relate the degree of bioturbation to the preservation of primary bedding features (Knaust, 2012). In this index, each grade of bioturbation is clearly defined in terms of burrow density, amount of burrow overlap, and the sharpness of the original sedimentary fabric (Taylor et al., 2003). Each grade is allocated a numerical value and a descriptive term, so that the index can be graphically plotted on a logging form (Figure 2).

RESULTS

The core logging procedure includes the description of several visual rock properties, the most important among them for the study of trace fossils are lithofacies, ichnogenera, and bioturbation index.

Lithofacies and Fauna

The two logged cores are of Early to Middle Eocene age (Alqudah et al., 2014). With respect to changes in lithology, the cores can be subdivided from bottom to top into the three main units A, B and C.

Unit A represents the lower 65.9 m of core OS-22 (222.4–157.5 m) and the lower 69.2 m of core OS-23 (256.3–187.1 m). This unit consists of intercalations of bituminous marls, chalky marls and phosphate layers. Chert appears in thin bands or as concretions alongside concretionary limestone towards the top of this unit (Figure 3).

Unit B comprises the middle (103.9 m) of core OS-22 (53.6–157.5 m) and the middle (135.4 m) of core OS-23 (51.7–187.1 m). It is formed by mudstone to wackestone facies of homogeneous bituminous marl and chalky marl lithotypes with a few horizons of limestone concretions. A distinctive horizon of bioturbation color mottling appears in the upper part of this unit (Figure 4).

Unit C corresponds to the upper 53.6 m of core OS-22 (53.6–0 m) and the upper 51.7 m of core OS-23 (51.7–0 m). This unit exhibits intercalations of bituminous marls, chalky marls, cherts, limestones and chalky limestones. It also contains a distinctive horizon of intense bioturbation in both cores, that is (22.5–15.7m) in core OS-22 and (31.7–21.7) in core OS-23 (Figure 5).

The fossils encountered include bivalves, gastropods, bone fragments, shark teeth, fossil wood fragments and foraminifera that were found throughout the two cores. The top of unit A contains a marker horizon with a very high abundance of uniserial benthic foraminifera.

Bioturbation

The two cores exhibit intervals of varying thickness with different bioturbation intensity. A total of 156 bioturbated intervals were recorded in both cores, 77 of these intervals being represented in core OS-22 and 79 in core OS-23. The BI for these bioturbated intervals ranges from 1 to 6, and the most common BI is 4 (Figure 6).

Unit A shows discrete and widely spaced bioturbation intervals in the lower part, whereas these intervals become more closely spaced towards the top of this unit (Figure 3). The BI ranges between 1 and 3 within unit A. Unit B starts with discrete but closely spaced bioturbation intervals that become more widely spaced toward the top of the unit. This unit shows a distinctive highly bioturbated interval with color mottling (BI ranges between 5 and 6) that is around 25 m thick and can be found in both cores. The BI in unit B ranges from 1 to 6. In unit C, a distinctive highly bioturbated interval of around 7 m in core OS-22 and 10 m in core OS-23 can be observed with a high diversity of ichnogenera. This horizon can be traced across the two cores and forms a lithostratigraphic marker bed. The BI of unit C ranges from 1 to 6.

Ichnogenera

Four main ichnogenera were identified throughout the cores OS-22 and OS-23, Thalassinoides, Chondrites, Teichichnus and Zoophycos (in descending abundance). The individual ichnogenera have different distributions and abundances throughout the cores (Figures 3 to 5). It was difficult to identify the constituent burrows within the ichnofabrics in some horizons, which is due to the intense bioturbation causing repeated overprinting or the weak preservation of the biogenic structures.

Thalassinoides

In comparison to other ichnogenera identified in this study, Thalassinoides is a large burrow. The morphology of burrow sections revealed in the core sections ranges from cylindrical, semi-circular to elliptical (Figure 7a). They have no preferred orientation relative to the bedding and 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. It is considered to be the dominant ichnogenus over the two cores and can be found in many horizons associated with the other ichnogenera described in this study.

Chondrites

Chondrites is a complex, root-like burrow system that appears in 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 size of the burrows ranges from 1 to 3 mm, while the longitudinal sections reach 2 cm (Figure 7b). Its filling consists of material from the surrounding sediments. This ichnogenus is found only in the upper unit C of both cores, where it is very common.

Teichichnus

The shape of this ichnogenus in core sections is characterized by a vertical series of tightly packed concave-up, crescentric laminae (Figure 7c). The width of the laminae ranges between 1 and 2 cm and they are passively filled with material derived from the surrounding sediment. The presence of this ichnogenus is limited to 5 bioturbation intervals in both cores, and typically occurs only in units A and C.

Zoophycos

Zoophycos is a very distinctive ichnogenus that appears in core sections as a thin, horizontal to semi-horizontal spreiten burrow. Individual burrows mostly have a width less than 1 cm (Figure 7d). They are filled with material of the surrounding sediment, though mixed with a lighter colored micritic matrix showing less amorphous organic matter (thin section, core OS-23, 28.9 meters). This ichnogenus appears only in one horizon within the upper unit C and can therefore be used for the correlation of both cores taking into account the biostratigraphic results of Alqudah et al. (2014).

DISCUSSION

The amount of bioturbation together with the ichnodiversity and the abundance and relationship of particular ichnotaxa can be an expression of various environmental factors. These include oxygenation, salinity, current activity, availability of nutrients and others (Gingras et al., 2011, 2012). With respect to oxygenation, oxygen is needed by the trace-forming species, to live and burrow in the sediments. When these bioturbated intervals are separated by sediments with high organic content, this suggests a highly dynamic depositional system, in which the bottom water changes its state from oxygenated to oxygen depleted, the latter allowing for the preservation of organic matter.

The dominance of Thalassinoides, associated in some intervals with Teichichnus, suggests a well-oxygenated shallow-water setting ranging from a lower shore face distal position to still shallow but further upper offshore environments slowly transitioning into the inner shelf (Pemberton et al., 2012). Zoophycos, which is limited to one horizon in the upper unit A, is non-specific as it is found in fully marine, offshore environments (Pemberton et al., 2001) as well as nearshore environments (Knaust, 2004). Chondrites is not depth related and its presence indicates a period of low oxygenation (Bromley and Ekdale, 1984), which can be correlated with the high concentration of TOC in the upper unit (Figure 8).

The studied cores show two regimes of bioturbation. The first one is characterized by gradually increasing burrowing intensity (Figure 9a), which indicates slow oxygenation of the bottom water. This may result from the opening of the sedimentary basin and mixing of its water with well-oxygenated marine waters. This regime is recorded in the two long bioturbation intervals in the middle unit B and upper unit C in both cores. The second regime, in contrast, is characterized by rapid onset of bioturbation representing opportunistic colonization (Figure 9b), where the BI value changes from 0–6 within a 1–2 cm core interval. This regime indicates sudden events such as storm events that enhance the circulation of the water body and enrich bottom waters with oxygen. The second regime is recorded in the short bioturbation intervals throughout both cores.

In synthesis, both bioturbation regimes and assemblages of trace fossils as well as supportive data from others studies performed on these cores, including nannofossil assemblage (Alqudah et al., 2014) and inorganic geochemistry (Aqleh et al., 2013), indicate a shallow-marine, closed basin that was at times connected to the fully marine environment. Tectonics played a major role in opening and closing of this basin, as well as episodic events as represented by tempestites, which enhanced the bottom-water oxygenation (see Figure 9 and Alqudah et al., 2014).

CONCLUSIONS

The two studied cores of Eocene strata from central Jordan reflect a highly dynamic depositional environment. Underlying the latter, either tectonically induced changes of depositional regimes or regional relative sea-level fluctuations and their respective rate of change can be reconstructed using the trace fossil record. In an overall shallow-marine environment, the dominance of Thalassinoides suggests a lower distal shoreface setting. As an overprint, the ichnofabrics reflect a highly dynamic depositional system with its rapid changes in water depth and the coinciding degree of bottom-water oxygenation. The analysis of ichnofabrics and bioturbation provides a useful tool for the correlation of the two cores. The intensely bioturbated horizon in the upper unit C forms an ideal marker bed that can be easily traced across the two cores and aligned with nannofossil age assignments (Alqudah et al., 2014). These results need to be seen in context with the biostratigraphy of Alqudah et al. (2014) and will in future be combined with other proxies (inorganic and organic geochemistry).

ACKNOWLEDGEMENTS

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 Dr. Richard Porter and Dr. René Hoffmann 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. The authors would like to express a special acknowledgement to the journal reviewer Dr. Dirk Knaust, Statoil ASA, for his constructive review. GeoArabia’s Production Co-manager, Nestor “Nino” Buhay IV, is thanked for designing the paper for press.

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. 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.AliHussein@rub.de

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.

Mohammad.AlQudah@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 Podlaha works 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

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.

Sander.Van-Den-Boorn@shell.com

Sadat F. Kolonic obtained his PhD in Organic Sedimentology from the University of Bremen 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.

Sadat.Kolonic@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