Lateral Facies Variations of Upper Cretaceous Carbonate Ramp Deposits, Jebel Nefusah, Northwest Libya

Peter M.A. Driessen*, Jean-Louis Cappelli, and Laurent Lambert
Total, France
*[email protected]

The Upper Cretaceous (Cenomanian) platform carbonates of Jebel Nefusah, northwestern Libya, were deposited as part of a regional prograding low-angle ramp system. These deposits are well-exposed in the Jebel Nefusah area but are poorly documented in the literature. Strata in this area are relatively undeformed making this system ideal for the study of lateral facies variation. The Cenomanian Stage is a major cyclic transgressive event over the regional unconformity that overlies the Aptian-Albian fluvio-deltaic sandstones of the Chicla Formation. The system contains an extensive oolitic to rudist-rich member that serves as an alternative analogue for Middle Eastern reservoir-prone facies. Field stops at 17 localities and five detailed sections over an area of 200 km form the base of a stratigraphic correlation panel, including the stacking pattern and depositional context of the recognised members. A geological model is proposed showing three third-order systems tracts during Cenomanian platform evolution. The first unit (less than 40 m thick) consists of inner ramp, tidally-influenced shallowing upward sequences. The second unit is characterised by progradational, inner-ramp oolitic shoal (40 m thick), which pass laterally into the mid- to outer ramp bioclastic, rudist boundstone and rudstone facies. This facies is a regionally developed (more than 200 km wide) member, 4–8 m in thickness. The two units are known as the highly dolomitised Ain Tobi Formation. A third regressive unit, the Yefren Formation, reaching 80 m in thickness, is formed by restricted inner-ramp marls with inter-bedded evaporitic gypsum layers. The depositional environment corresponds to a supra-tidal to sabkha setting. The architecture and geometry of the Cenomanian passive ramp system was controlled by eustatic sea-level changes rather than localised, abrupt tectonic events.

AAPG Search and Discovery Article #90077©2008 GEO 2008 Middle East Conference and Exhibition, Manama, Bahrain