Early Diagenetic Controls on Allochthonous Carbonate Debris Flows--Examples from Egyptian Lower Eocene Platform-Slope: ABSTRACT

Abstract

Many of the models that have been proposed for the origin of carbonate debris flows are based upon examples from ancient carbonate slope settings. Few ancient slope environments have been described, however, where submarine cementation processes were prevalent. This early diagenetic phenomenon, common on many modern carbonate slopes and platforms, can control the mechanisms by which debris flows are generated, as well as the ultimate form of the beds they produce. Within the Duwi Trough of central Egypt, a basin to slope facies transition is preserved within the lower Eocene chalks and limestones of the Thebes Formation. Basinal facies are characterized by sequences of laminated chalks and limestones with thin, intercalated horizons of nodular limestone and limestone hardgrounds. Carbonate platforms developed on structural highs adjacent to the basin and periodically shed bioclastic detritus downslope in the form of fine-grained, skeletal turbidites. Nodular limestones and hardgrounds, that formed upslope, were in places dislocated and reworked into the basin as submarine debris flows. Individual debris flow beds preserved within the lower slope and basinal facies can be traced over 50 km down the trough axis and several kilometers laterally. > Nodular conglomeratic debris found within the flows range from 30 to 300 cm in thickness and are mud supported. Unlike most ancient debris flow breccias, larger clasts are unusually uniform in size and well rounded. This is not a reflection of textural maturity but a result of the primary nodular morphology of these clasts. Channels and basal scour features are poorly developed in these beds owing to the cemented (hardground) nature of the basin-floor during debris flow deposition. Sites of nodular limestone bed dislocation are not recognized within the slope facies. Neither large-scale rotational slides, nor slump structures associated with translational slides are developed. The proposed mechanism for the detachment of these nodular horizons is one of relatively shallow decoupling of the early-cemented surface layer from the underlying, unconsolidated sediment. This process would have been accelerated by the presence of high pore fluid pressures owing to an impermeable surface layer (hardground) and loading, resulting from both sedimentation and cementation. Where submarine cementation was a continuous process, as on the upper slope, and uninterrupted sequences of nodular limestone were developed, sediments were stable and debris flows were not generated. End_of_Article - Last_Page 995------------