PALYNOFACIES ANALYSES AND PALAEOENVIRONMENTS OF will be used here because it links palynofacies types

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  • PALYNOFACIES ANALYSES AND PALAEOENVIRONMENTS OF SOME

    LOWER CRETACEOUS ROCKS OF THE SIQEIFA 1X BOREHOLE,

    NORTH WESTERN DESERT, EGYPT

    Magdy S. Mahmoud* , , Mohamed A. Masoud**, Mohamed A. Tamam**, and

    Miran M. Khalaf**

    * Assiut University, Faculty of Science, Geology Department, Assiut, Egypt

    ** Sohag University, Faculty of Science, Geology Department, Sohag, Egypt  Corresponding author e-mail: magdysm@yahoo.com

    ABSTRACT

    A detailed palynofacies analysis was carried out on 56 ditch-cutting samples

    obtained from the lower Cretaceous of Siqeifa 1x borehole, north Western

    Desert, Egypt. The main aim of this study is to identify depositional

    palaeoenvironments in details, reconstruct vegetation cover and to infer

    palaeoclimate conditions. Three palynofacies types were recognised;

    palynofacies type (PF-1) corresponds to the lower-middle Alam El Buieb

    Formation (Berriasian-Barremian). This is deposited in a deltaic (prodelta)

    subenvironment during a Berriasian-early Barremian regression episode, and

    the lower upper Alam El Buieb deposited in an inner shallow marine

    environment during a partial regain of a late Barremian-Aptian transgression,

    under prevailing dysoxic-anoxic to suboxic-anoxic conditions. The second

    palynofacies (PF-2) represents the uppermost Alam El Buieb, Alamein, and

    Dahab formations (late Barremian-Aptian), where the uppermost Alam El-

    Buieb Formation was deposited in a distal bar of a prograding delta,

    accumulated during a minor local regression. However, the carbonate of the

    Alamein Formation and the shale of the Dahab Formation was deposited in a

    saline lagoon environment developed during a partial regain of the local early

    Aptian marine transgression. Suboxic-anoxic to dysoxic-anoxic conditions are

    interpreted to prevail during deposition of the PF-2. Third palynofacies (PF-3)

    represents the Kharita Formation (Albian), where the lower Kharita was

    deposited in a lagoon setting, while the upper Kharita was deposited in a

    deltaic environment due to a major marine regression, under dysoxic-suboxic

    conditions. Local pteridophyte vegetation on low lands near the borehole and

    conifers on relatively dry hinterlands is interpreted to thrive under a regional

    warm and relatively dry palaeoclimate. Possible seasonal dry periods may be

    developed during deposition of the uppermost Alam El Buieb, Dahab and

    Alamein formations.

    THE SEVENTH INTERNATIONAL CONFERENCE

    ON THE GEOLOGY OF AFRICA

    P-P VI-33 – VI-58 (NOV. 2013) ASSIUT-EGYPT

    magdysm@yahoo.com

  • VI-34 Palynofacies Analyses and Palaeoenvironments of Some Lower Cretaceous Rocks...

    1. INTRODUCTION

    The lower Cretaceous in the north Western Desert exhibits a major regressive

    phase indicated by clastics (Said, 1990). Neocomian-Barremian sedimentation

    represents a regressive phase and marginal marine sandstone and shale of the Alam

    El Buieb Formation were deposited. The Aptian witnessed a transgressive phase,

    which brought a shallow sea over the north Western Desert, while the carbonate

    unit of the Alamein Formation was deposited. The Albian time is represented by

    another regressive phase, when the north Western Desert was receiving the fluvial

    (mainly coarse sands) sediments of the Kharita Formation coming from the south.

    However, in the extreme north, specifically at the Matruh Basin, sedimentation of

    fine silt and shale was dominant (Said, 1990).

    Combaz (1964) introduced the term palynofacies as “the total complement of

    acid-resistant particulate organic matter recovered from sediments by

    palynological processing techniques”, and based on this term palynofacies analysis

    was used to interpret palaeoenvironments. However, Tyson (1995) provided the

    most recent and widely used definition of the palynofacies term as “the total

    particulate organic matter assemblage contained in a body of sediment thought to

    reflect a specific set of environmental conditions, or to be associated with a

    characteristic range of hydrocarbon-generating potential”. The later term can be

    used in palaeoenvironments interpretation as well as in source rock evaluation, and

    will be used here because it links palynofacies types to sedimentary sequences.

    Palynofacies analysis is interested with changes in the relative abundance of various

    types of sedimentary organic matter as palynomorphs, phytoclasts, and amorphous

    organic material (AOM). Because palynofacies analysis is closely linked to

    sedimentology, it can thus used in identifying palaeoenvironmental and

    hydrographic parameters such as distance from shoreline, hydrodynamic energy in

    the water column, salinity, and oxygen regime (e.g. Tyson, 1995; Batten, 1996).

    Several palynological investigations have dealt with the palaeoenvironmental

    interpretations of the Egyptian Cretaceous rocks (e.g. Abdel-Kireem et al., 1996,

    Ibrahim, 2002; Zobaa et al., 2013). However, the use of detailed palynofacies

    analyses in interpreting palaeoenvironmental settings still lacking. Mahmoud and

    Deaf (2007) studied the palynostratigraphy of the lower Cretaceous succession

    penetrated by the Siqeifa 1x well borehole northern Western Desert, Egypt. The

    present work aims to study the palynological facies of the Siqeifa 1x with more

    detailed analyses upon the percentage distribution of the palynological organic

    matter (POM) assemblages to infer the palaecological settings in terms of

    depositional palaeoenvironments, palaeoclimate, and palaeovegetation covers.

    2. MATERIAL AND METHODS

    The present study is based on fifty-six ditch cutting samples were collected from

    the lower Cretaceous of the Siqeifa 1-X borehole, Matruh Basin, northern Western

    Desert (Fig. 1). The samples were processed by standard (HCl-HF) palynological

    preparation technique, without oxidation or ultrasonic treatments. The palynological

    residue was sieved through 10 μm nylon sieves. Three to five permanent slides were

    prepared using glycerin jelly as a mounting medium. It should be noted that neither

    ultrasonic nor oxidation (nitric acid) treatments were carried out, where these

    analyses would results in oxidation and inevitable destroyment of the plant debris

  • Magdy S. Mahmoud, Mohamed A. Masoud, Mohamed A.Tamam, and

    Miran M. Khalaf

    VI-35

    and the palynomorphs. All slides and residues are stored and catalogued in the

    Geological Museum, Geology Department, Faculty of Science, Sohag University,

    Egypt.

    Figure 1. Geographic map of Egypt showing the location of the Siqeifa 1-X well

    (After Abdel Kireem et al., 1993)

    Palynological slides were examined using the transmitted light microscopy

    Olympus BX41 Microscope at X 200 and X 500 magnifications in order to:

    establish semi quantitative analysis of the POM; determine the palynofacies types

    and the palaeoenvironmental interpretations. The photomicrographs are published

    in Mahmoud & Deaf (2007). For full reference to taxa refer to Mahmoud & Deaf

    (op. cit.). Each slide was counted for its POM content, in which the first 200

    particles were categorized as abundant >35 %, frequent 16-35 %, common 5-15 %

    and rare

  • VI-36 Palynofacies Analyses and Palaeoenvironments of Some Lower Cretaceous Rocks...

    Figure 2. Lithologic column of the Siqeifa 1x borehole (After Mahmoud & Deaf, 2007)

    3.1 Alam El Bueib Formation

    This formation is composed of sandstone with frequent shale interbeds in its

    lower part and occasional limestone in its upper part, but in the Siqeifa 1-X well, the

    formation is mainly composed of thick shale unit intercalated with very minor

    streaks of sands, and shows more sandy facies at its bottom and top. This shale unit

    was commonly referred to as the Matruh shales or the Mersa Matruh Formation of

    Norton (1967). Its type section is the interval from 3927 to 4297 m of the Alam El

    Bueib-1 well (Hantar, 1990). This formation ranges in age from Barremian to

    Aptian (Hantar, 1990). WEPCO (1970) referred to this unit in the investigated well

    as: no information, top of Umbaraks Sands, and top of Matruh Shale. Based on the

    palynology work of Mahmoud and Deaf (2007) and lithologic description provided

    above, the thick 869 m (2850 ft) clastic sequence confined between depths 3463 and

    2594 m (11950-9100 ft) can be identified as the Alam El Bueib Formation and

    given a Berriasian-Barremian age. This formation is belived to be deposited under

    shallow marine conditions with more continental influence towards the south

    (Hantar, 1990).

    3.2 Alamein Formation

    This unit consists mainly of hard dense brown dolomite with a few thin shale

    interbeds at its base and top (Hantar, 1990), but in the Siqeifa 1-X well, it is

    composed of chalky limestone intercalated with minor shale streaks. It rests

    conformably over the Alam El Bueib Formation and underlies the Kharita

  • Magdy S. Mahmoud,

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