Facies and sequence stratigraphy of the tamet formation (middle eocene), eastern sirte basin, Libya

Abstract

The middle Eocene Tamet Formation on the eastern side of the Sirte Basin, is largely shallow-marine carbonate platform-to-basin transition formed along passive margin. The stratigraphy and deposition of this formation were largely controlled by eustatic processes of superimposed short-term and long-term of relative sea-level fluctuations. Tamet sediments were mostly deposited in subtidal environments, which ranged from above fairweather- to below storm wave-base. Intrinisic processes such as storm and wave redeposition and reworking may have acted to inhibit aggradation into the zone of peritidal sedimentation. Estimates of water depth during deposition range from a few metres to a few tens metres maximum. There are three major plat formal facies associations in the transition from deep subtidal to hypersaline deposits. These facies associations are defined and interpreted on the basis of their constituent microfacies and depend on their palaeogeographic setting on the platform. The spatial distribution of the complete spectrum of the facies associations suggest that deposition took place under low-energy conditions, as a stacked prograding homoclinal ramp. Ten microfacies types have been distinguished, and their vertical interrelationships reflect metre-scale, shallowing-upward subtidal cycles, which are considered as the basic building blocks of the Tamet ramp. Two different types of subtidal cycle have recognised in the study area. Open-marine subtidal cycles are present along the outer through inner ramp within the transgressive and most of the highstand deposits. They are characterised by relatively deep subtidal microfacies at the base, gradationally overlain by shallow subtidal microfacies. Hypersaline subtidal cycles are present only upon the Cyrenaica Platform and predominated during the late phase of highstand deposition; they are composed of dolomitised shallow subtidal microfacies, capped by anhydrite. The middle Eocene across this area is not a single carbonate ramp but rather an amalgamation of stacked ramps. Facies associations and cycles within the Tamet Formation have allowed the recognition of three depositional sequences separated by stratigraphic transitional zones. Each sequence represents a prograded ramp. The development of a sequence framework is based on the metre-scale cycle architecture and maintaining microfacies interpretations. Most of the sequences are interpreted as transgressive-highstand deposits. Each transgressive ramp is typically characterised by an aggradational patterns of relatively deep subtidal mud-rich carbonates deposited in a catch-up depositional system and episodically affected by storm events. Away from the ramp-margin, the transgressive facies change and stratigraphically thin into lagoonal facies deposited under keep-up conditions. The subsequent highstand ramp begins with an aggradational geometry but finally shifts into a distinct progradational pattern. The highstand cycles cover a broader area than that occupied by the transgressive sediments and are made up of mud-poor packstones reflecting a keep-up depositional system. The Cyrenaica Platform at this time was occupied by a very shallow and hypersaline sea. Carbonate sedimentation was shut off and replaced by precipitation of shallow-water evaporites; associated with this was dolomitisation, marking the end of sequence. Several lines of evidence suggest that the magnitude of the middle Eocene sea-level fluctuations on the eastern Sirte Basin were relatively low. First, if the magnitude of the oscillations had been greater, then the sea-level falls would potentially have lead to formation of major sequence boundaries on the Tamet sequence upper surfaces. Second, if the magnitude was larger, then the rapid sea-level rises would have caused drowning of the ramp or domination by "catch-up" style of subtidal deposits. However, the Haq et al. (1987) sea-level chart in some circumstances may require modification, at least in terms of magnitudes and eustatic sea-level rises.