Sedimentology, Sequence stratigraphy and Spatial-temporal Patterns of the late Paleocene Succession, western Sirt Basin, Libya

Abstract

In the western Sirt Basin, sedimentation during the Late Paleocene was characterized broadly by shallow-marine carbonates, local build-ups and deeper-water shales and marls on the Dahra Platform and in the Dor al Abid/Zallah Troughs. Seven lithofacies and eleven associated microfacies have been recognised within the Selandian/Thanetian carbonate succession in the study area, and these range from mud-supported carbonates to grain-dominated facies. The Dahra Formation on the Dahra Platform was deposited on a homoclinal carbonate ramp with inner, mid and probably outer ramp facies, each with distinctive sub-facies and microfacies. The similarity of facies and associated microfacies throughout the Dahra Formation suggest deposition under similar conditions throughout the east and west Dahra Fields on the Dahra Platform, and these persisted during deposition of the Zelten and Harash Formations. The Mabruk Member, which consists mainly of shallow-water carbonates bounded above and below by deeper-marine shale and marl, accumulated in lagoonal and reefal environments, probably in a rimmed-shelf setting. Different types of diagenetic alteration occurred at various stages in the Paleocene sedimentʼs history including dissolution, cementation and compaction. Primary and secondary types of porosity were developed within the studied rocks, particularly in the Dahra Formation, where the depositional facies, diagenesis and the pattern of carbonate cycles played an important role in porosity creation and preservation.
The Selandian/Thanetian succession is dominated by a regressive trend, especially on the Dahra Platform, which resulted in the development of shallowing-up cycles. Numerous key surfaces have been documented with characteristic funnel-shaped and bell-shaped log patterns, based on which a number of depositional sequences have been recognized. These sequences comprise both transgressive systems tract and highstand systems tract and are commonly defined by transgressive surfaces, particularly on the Dahra Platform. The possible lack of reef-building organisms along with tectonic subsidence and/or significant sea-level rise resulted in the development of a drowning unconformity on the top of the Mabruk Member.
The thickness of the entire Paleocene succession is generally thicker in the trough areas and thinner over the platform. The latter is dominated by uniform and monotonous strata with a significant thickness of shallow-marine carbonates, suggesting relative stability in tectonics, sea-level and climate. The high temperature recorded from the aqueous inclusions in the Thanetian section is possibly due to the passage of hydrothermal fluids from deeper parts of the area; the high thermal conductivity of carbonates, along with the Mid-Late Tertiary uplift and erosion may also have been involved. The overall similarity of the Paleocene palaeotopography suggests that differential subsidence and sea-level fluctuation were the dominant process that controlled the spatial and temporal variation of the Paleocene facies.