Subtidal, flat-pebble conglomerates from the Early Ordovician Mungok Formation, Korea : origin and depositional process

Abundant flat-pebble conglomerate beds occur in the Early Ordovician Mungok Formation. They are characterized by sharp and erosive base, variable intraclast lithologies, poor grading and bioclastic to peloidal matrices. Associated lithofacies with flat-pebble conglomerate include flaser rock, ribbon carbonate, and marlstone to shale. Sedimentation rate of the Mungok Formation seemed to be very low. Occassional minor storms delivered carbonate sediments to deeper subtidal zones of the Mungok platform. Based on sedimentary structures and textures, petrographic features and relationships with associated lithofacies, the Mungok flat-pebble conglomerate is interpreted to have been formed in subtidal environments by processes of early submarine cementation of precursor sediments and storm reworking. Early submarine cementation in the Mungok sea floor occurred in two ways. Early concretionary cementation at just below water-sediment interface prevailed under fair-weather condition resulting in lime nodule formation. Planar-bedded calcareous layers such as peloidal or bioclastic limestone, or lime mudstone layers formed during intermittent minor-storm events and also were cemented early on the sea floor during fair-weather condition. Early cemented lime nodules and limestone layers were eroded and redeposited as intraclasts during a major storm event. Bioclasts and peloids form the matrices of flat-pebble conglomerates. Bioclastic matrices were derived in situ by storm winnowing of finer sediments, whereas peloidal matrices were transported from shallower environments by storm-driven currents.