Distribution of peridiniacean dinoflagellate cysts from cores of organic rich shales of the Duwi and Dakhla formations of Egypt

https://doi.org/10.1016/j.jafrearsci.2020.103892Get rights and content

Highlights

  • Peridiniacean dinocysts are recovered from the studied intervals.

  • The cored Duwi-Dakhla in Egypt yielded marine assemblage from the Abu Tartur and Quseir sections.

  • Correlatable dinocyst signatures across the belt of organic-rich shales are detected.

Abstract

The Campanian-Maastrictian black shales in Egypt comprise a significant part of the worldwide belt of organic-rich shales. These dark-colored shales stretch from the Dakhla in the Western Desert to Safaga area in the east along the Red Sea and are well known in the Middle East and North Africa. A palynological study of these shales hosted mostly in the Duwi and Dakhla formations yields a Campanian to Maastrichtian age. The extracted palynomorph assemblage from the Abu Tartur and Quseir boreholes, which is dominated by dinoflagellate cysts (dinocysts) representing a well-preserved peridinioid association (>90% of the total dinocysts), is indicative of marine origin and very high paleoproductivity. The common dinocysts are Andalusiella mauthei, Andalusiella gabonensis, Cerodinium diebelii, Senegalinium bicavatum, Senegalinium laevigatum, Palaeocystodinium golzowense and Palaeocystodinium australinum. This assemblage is comparable with the dinoflagellate cyst records of the Campanian-Maastrichtian low latitude strata in northwest Africa (Morocco and Tunisia) and southern Europe. The dinocyst distribution in the Abu Tartur and Quseir borehole sections suggests an isochronous signature during the early Maastrichtian age.

Introduction

The term “black shale” is commonly used to describe rocks that are thinly laminated, dark-colored, organic matter-enriched argillaceous sediments accumulated in oxygen-depleted environments. Being pyrite-enriched, the organic matter content of these shales is presumed to be optimally preserved as kerogen during the anoxic events in oxygen minimum zones and little agitated waters (Killops and Killops, 2005; Takashima et al., 2006; El-Shafeiy et al., 2014, 2017). Globally, the Campanian-Maastrichtian was typified by increased pCO2 from prominent magmatic activity and identified as a greenhouse time (Takashima et al., 2006). Increased burial rate was commonly associated with remarkable compositional variations in the phytoplankton and dinoflagellate community (Katz, 2005).

The organic matter-enriched black shales were deposited widely in Egypt during the Late Cretaceous. Throughout this time and on the shallow marine continental shelf, the prevailing greenhouse conditions controlled sedimentological processes and an increase in primary productivity that favored the prevalence of the anoxia and organic matter preservation (Robinson and Engel, 1993). Palynological studies proved to be very significant in providing better characterization of the black shales by using its phytoplankton constituents. Although these black shales are important hydrocarbon source rocks, few palynological studies have been published in Egypt. They include studies by Aboul Ela (1978), Schrank (1984) and El Beialy (1995) in the Egyptian phosphate belt. El Beialy (1995) studied the phosphate-bearing Duwi Formation in the Umm El Hueitat and the Hamrawein mines along the Egyptian Red Sea Coast which he dated as Campanian and Maastrichtian. He concluded that the most important dinocyst species recovered are of Maastrichtian age, although some Campanian taxa are recorded as well. A Campanian to Maastrichtian age was assigned to the studied deposits. On a global scale, the recorded Maastrichtian dinoflagellate cyst group exhibits a remarkable provincialism. His assemblage is comparable with those previously recorded in the northeast African low latitude areas (e.g. Morocco and Tunisia).

The Duwi and Dakhla formations host the belt of organic-rich shales. The Duwi Formation is a phosphorite-bearing formation with marl intercalations and chert nodules in the Red Sea region, and is rich in glauconite-bearing horizons in the AbuTartur Plateau. The overlying Dakhla Formation is typified by abundant foraminiferal shales and marls with limestone and few siltstone intercalations. The existence of organic-rich intervals within the uppermost Duwi and Dakhla units is linked to a well-known period of increased primary productivity, which enhanced the organic matter preservation in the shallow basins (Glenn and Arthur, 1990).

The present work focuses on the analysis and interpretation of the dinoflagellate cyst record and its signals within: 1) the Abu Tartur black shale sequence on the formational transition from the Duwi to the Dakhla rock units encountered in a core drilled in Liffya-Maghrabi region in 2007 (Fig. 1, Fig. 2); 2) the dark-colored shale succession in a core drilled in the Duwi Mountain-Quseir District in 2008 (Fig. 1); and 3) to correlate the dinoflagellate cyst signatures recovered along the Campanian-Maastrichtian transition in the two borehole sections.

Section snippets

Geologic and stratigraphic setting

In the Late Cretaceous age and during the late Campanian to early Maastrichtian time, the prominent erosion, which is generally associated with a substantial uplift, influenced sediment deposition in Egypt. Tectonic fluctuations and world-wide eustatic sea level variations were among the essential factors that basically governed the sedimentation of the Upper Cretaceous deposits in the country (e.g., Philobbos, 1996; El-Shafeiy et al., 2017; Mohamed et al., 2019; Deaf et al., 2020; Aboul Ela et

Material and methods

Various cores were purposely drilled in Egypt in order to evaluate the potentiality of unconventional black shales as alternative source rocks. Two of these cores were drilled in the Abu Tartur area (Western Desert) and the Quseir area (Duwi Mountain, Eastern Desert). The present study focused on these drill cores from the Maghrabi-Liffiya sector (Abu Tartur Plateau) (Fig. 2) and the Quseir area, Duwi Mountain (Fig. 3). Drilling was performed by the Ministry of Petroleum and sponsored by

Abu Tartur borehole section

Although, some of the recorded dinoflagellate cyst taxa range from Campanian and/or early Maastrichtian to Eocene and have their range tops in the Cenozoic Era, the unaltered first stratigraphic records/appearances (base ranges of these taxa) that recorded in the studied interval could be utilized effectively to proof the topmost Cretaceous age since they are core samples (Fig. 2, Fig. 3, Fig. 4).

Most of the samples preserve a well-preserved and moderately abundant dinoflagellate cyst

Palaeoecological implications

From a paleoecological point of view, it is interesting to note that increased number of peridinioid dinocysts in a dinoflagellate community may favor a low paleosalinity (Hultberg, 1986; Schrank, 1987) and/or very high paleoproductivity (Powell et al., 1990). However, the recovery of foraminiferal test linings in the Quseir section coupled with the existence of dinoflagellate cysts provides proof for a depositional phase with marine influence. The dinoflagellate cyst assemblage with its little

Conclusions

Late Cretaceous transgressions established broad continental shelves that developed in the Middle East and North Africa. This phase was associated with the deposition of a belt of black shalesin Egypt that extended from east (Quseir-Safaga) to west (Kharga-Dakhla), crossing the Nile Valley. The organic matter-rich shales were hosted in the Duwi and Dakhla formations during the Campanian-Maastrichtian transition. The palynological investigation of these units in the Abu Tartur and Quseir

Declaration of competing interest

This manuscript has not been previously published and is not under consideration in the same or substantially similar form in any other peer-reviewed media. To the best of our knowledge, no financial conflict of interest, or other, exists. We have included acknowledgements.

Acknowledgements

The authors thank their colleague, Dr. Moataz El-Shafeiy,Geology Department, Cairo University, for supporting the core samples utilized in the present study. The authors are appreciative to Dana-Gas Egypt for providingthe core samplesfor this study. The manuscript benefited from insightful comments of Editors-in Chief Damien Delvaux and Read Mapeo, and two anonymous reviewers. The authors are indebted to Associate Editor Francisca Oboh-Ikuenobe for her extensive grammatical corrections on the

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