ABSTRACT
The Lower Carboniferous Mrar Formation is exposed extensively along the southern margin of the Ghadames basin in northwest Libya. The basal part of the Mrar forms the cap rock and possible hydrocarbon source for many of the producing reservoirs in the underlying Tahara sandstones. Furthermore, the Mrar itself is known to contain significant gas shows southwest of the outcrop, associated with some oil potential.
The Mrar Formation was deposited in a deltaic environment which developed on the northern part of the stable Saharan platform. The formation is composed of 15 coarsening-upward sequences which grade from shale to sandstone, and it incorporates 19 subenvironments, ranging from deltaic and delta plain to open-marine shelf, beach, and tidal flat. The deltaic complex appears to have prograded consistently westward and was initially dominated by fluvial processes. During middle Mrar deposition, the delta became progressively more influenced by shallow-marine processes, the fluvially introduced sand being reworked into bars. This stage of deposition was terminated by a widespread marine transgression which diverted the discharge regime 62 mi (100 km) northward. The higher Mrar sequences reflect progressive southward migration of the diverted discharge regime, with successive cycles becoming more proximal in character. The upward passage from bars into beach and barrier sands results from development within a wavedominated delta system which continued to migrate south, eventually reintroducing fluvial sedimentation into the upper sandstones of the outcrop sections. The Mrar Formation is capped by a distinctive algal Collenia Zone thought to be formed during a period of widespread delta abandonment.
The Mrar Formation provides a unique opportunity to study a delta system which built out into a slowly subsiding basin and progressively changed from being fluvial to wave dominated. Several features distinguish it from most documented examples. It is, therefore, considered to provide a valuable model for deltaic systems developed on relatively stable cratonic areas. In particular, the fluvially dominated sequences of the lower Mrar are represented by relatively thin and unusually extensive sheet sandstones preserved within thick prodeltaic shales.
