Sedimentologic and facies evidences reveal a marine environment for the Gercus Formation. Facies analysis and associated sedimentary structures including graded beddings decide turbidity origin of the rocks. Marine environment is supported by the identifying glauconite and fossils types reported for the first time.
The formation composed of seven lithotypes; shale/claystone, mudstone, sandstone, carbonate, conglomerate, breccias and debris flow, which are arranged in repeated cycles of mixed siliciclastic-carbonate turbidites in a range of gravity-flow regime. The Gercus successions are grouped into four facies associations confirming marine depositional systems, these are (from bottom to top); slump siliciclastic-calciturbidites (dolomite/shale dominated), proximal siliciclastic-calciturbidites (dolomite/sand dominated), distal siliciclastic-calciturbidites (sand/mud dominated) and slope siliciclastic turbidites (sand/clay dominated) respectively.
Petrographic analysis of sandstone units show predominant of lithic fragments, most of it are carbonate with subordinate tuffaceous fragments, chert, chalcedony, volcanic ash, metamorphic and detrital iron oxides grains, with noticeable grains of glauconite. Varieties of marine fossils are identified includes planktonic bivalves and benthic forams of cool water, which support the deeper marine environment. Petrographic examination of carbonate units reveal skeletal grains of benthic and planktonic forams, stromatolite, planktonic bivalves, corals and algae, with non-skeletal grains of chert, chalcedony, tuffaceous fragments, volcanic ash, and volcanic bubbles.
Petrography, lithofacies and lithostratigraphic analysis of the Gercus Formation suggest deposition in developed marine environment, mainly effected by gravity-flow turbidity currents, and displays successive submarine fans of high density turbulent currents in deeper margins. Mixed siliciclastic-carbonate cycles were deposited in intervals of weaning of turbulent currents. Based on clast type and size, it seems likely that a weakly turbulent to laminar gravity-flow phase was present when the flow event entered the basin at the end part of the fan. A change in flow behavior may have led to deposit sand-rich unit with ‘turbidite’ characteristics, which was subsequently grades upwards to clay-dominated unit.
This paper presents new details of lithostratigraphic subdivisions and associations of the Gercus Formation in Koi Dokan area, and new suggested marine environment of deposition. The previous workers suggest continental and probably mixed with deltaic environments in the upper part.