The Jurassic-Lower Cretaceous Cabao Formation of Northern Libya: A Large Scale Tidal Siliciclastic Sand Dune Complex Fringing the Southern Tethyan Margin

Jean-Loup Rubino¹, Francois Lafont¹, Ali Sbeta², and Herbert Eichenseer^3
1Clastic Sedimentology, Total, Pau, France
²Sedimentology and Stratigraphy, Al Fateh University, Tripoli, Libya
³Exploration Div., Total E&P Libya, Tripoli, Libya

The Cabao Formation of Upper Jurassic to Lower Cretaceous age unconformably overlies the shallow marine Oxfordian Shakshuk Fm. along the Jabal Nefusah in Northwestern Libya. It includes numerous stratigraphic cycles.

Basal units which are well exposed and laterally extensive are sand rich deposits. They are often considered as fluvial in origin because of reddish to yellowish colours, and the predominance of large-scale cross-bedding in coarse material. It is in fact related to a shallow marine environment as indicated by the presence of turtle plates and shark teeth.

A detailed facies analysis and section measurements along a 100 km long East-West transect allowed to clearly establish the influence of tidal processes. Several criteria have been documented, including sigmoidal dune foresets, bidirectional current patterns, tidal couplets, upward coarsening sequences and well exposed lateral facies changes from sandy dune core to heterolithic dune bottomsets visible in large quarries.

The whole succession is truncated by a tectonically enhanced unconformity (Austrian event) with a major emergence surface and the development of a weathering profile below the Kiklah Formation of Albian age.

The persistence of the same facies over numerous depositional sequences and the constant palaeocurrent pattern tend to suggest a sustained palaeogeographic setting: probably an elongated embayment that was structurally controlled along the southern Tethyan margin, and where tidal current enhancement allowed the formation of sand dunes complexes.

The northward extension of the system is poorly documented because of subsequent Tertiary erosion whilst the westward extension toward Tunisia is limited first because of lateral facies change toward intertidal area and then to an emerged high. In term of facies, the succession shows many similarities with the UK Lower Green Sand.

AAPG Search and Discovery Article #90078©2008 AAPG Annual Convention, San Antonio, Texas