Sedimentology and facies development of an Archaean shelf: carbonate platform transition in the Kaapvaal Craton, as deduced from a deep borehole at Kathu, South Africa

ABSTRACT The Coffee Group of Upper Cretaceous age within the Desha Basin is composed of two major lithologies; a light to dark marlstone and a series of white, fine to medium grained siltstones and sands. There are two source areas for the sands; the Sharkey platform to the south and the southern Appalachian Mountains to the east. The presence of hydrocarbons have been described at the outcrop and in subsurface cuttings and cores. Depositional environments in the shallow shelf consist of lagoons, barrier island bars, offshore bars and surge channel deposits. Southwest regional dip of approximately 40 ft/mi (7.5 m/km) is reflected on all upper Cretaceous horizons. Cross sections, isopachous and structure maps of various sand intervals depict depositional environments and the regional geology. Entrapment of oil is postulated in up dip portions of these shallow shelf sands similar to fields in southern Arkansas.

Outcrop

Marl

Lithology

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Sedimentological and ichnological characteristics of Dur At Tallah siliciclastic rock sequence, and their significance in the depositional environment interpretation of tidal-fluvial system (Upper Eocene, Sirt Basin, Libya)

HAL (Le Centre pour la Communication Scientifique Directe) (2013)

Ashour Abouessa

Dur At Talah sedimentary sequence, located at the southern side of the Sirt Basin in central Libya, is composed of 150 m thick of mainly siliciclastic rocks. The importance of this sequence is linked to the importance of the Sirt Basin as one of large hydrocarbon reservoirs in Libya. The sequence is also an excellent site for vertebrate fossils of Late Eocene, the age of the sequence. Previous studies, though very limited compared to the importance of this area, are focused on its paleontological content. Sedimentology received only scant attention before this project. This thesis is an outcrop based study in which the focus is given to the sedimentary and biogenic (trace fossils) structures, aiming at defining and interpreting depositional facies which building up the sequence. The study is mainly based on field data which are analyzed on the light of related published literature and on the comparison with modern sedimentary environments. Results of facies analysis have led to splitting the entire sequence into three genetically related intervals. The oldest, we called the New Idam Unit (around 80m), is composed of very fine sandstones to mudstones. New Idam Unit is unconformably overlain by the Sarir Unit (around 50m), composed of medium grained cross bedded sandstones (the lower 25-30 m) changes up to very coarse and microconglomeratic sandstone (the upper 20-30 m). Thus, the Sarir Unit is split into the lower Sarir Subunit and upper Sarir subunit. The New Idam Unit presents both classical and unusual sedimentary and biogenic indicators that attribute this unit to estuarine depositional environment. It starts with outer estuarine (the lower 35 m) and ends up with inner estuarine (the upper 45 m). Maximum flooding surface is located in between. Above this surface the fluvial indicators increase and tidal indicators decrease, thus providing clue for basinward (North) migration of the shoreline. The lower Sarir subunit which was previously interpreted as fluvial deposits, preserves multi-scale sedimentary structures that undoubtedly belong to tidal processes. This is especially evidenced at the lower part of the lower Sarir Subunit (LLS). Fluvial indications over dominates the tidal ones in the upper part of the lower Sarir (ULS). Due to this configuration the whole lower Sarir subunit is interpreted as shallow marine, deltaic, depositional system, occurred during sea level ¨normal¨ regression. This time, maximum flooding surface is located between the LLS and ULS. The lower Sarir subunit is terminated by subaerial unconformity, with evidences of subaerial exposure preserved at the top of the ULS. These are intruded by the upper Sarir subunit which presents clear evidences of strictly fluvial environment of deposition. The deposits of the upper Sarir subunit record the low stand system tract part of the Dur At Talah sequence. In addition to the outlined results, the sequential pattern of the depositional events is suggested for the entire sequence of Dur At Talah. This study provides a valuable information regarding the depositional and sequential aspects of the Sirt Basin during the late Eocene, it also provide an unique case study for the better understanding of the shallow marine tidal deposits.

Siliciclastic

Outcrop

Sequence (biology)

Stratigraphic unit

Sequence Stratigraphy

Ichnology

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Integrated petrophysical and sedimentological study of the Middle Miocene Nullipore Formation (Ras Fanar Field, Gulf of Suez, Egypt): An approach to volumetric analysis of reservoirs

Journal of African Earth Sciences (2017)

Mohamed M. Afife Emad S. Sallam Mohamed Faris