Submarine Debris-Flow Deposits from the Ordovician Mane Ting Formation in Tethyan Black Mountain Basin, Central Bhutan

The rocks in the basin range in age from Early Ordovician to Devonian. Proximal sedimentation began with the deposition of an orthoquartzite-conglomerate lithofacies of the Nake Chu Formation. Rapid sedimentation coupled with quicker subsidence initiated a period of instability that led to the deposition of the distal facies sediments of the Mane Ting Formation characterised essentially by interbanded quartz wacke and shale with an input of andesite. A number of conformable interbedded para-conglomerate beds, identified as submarine debris flow deposits, occur within this Mane Ting Formation.

Lithic fragments ranging in size from 1 mm to 2 m and upwards constitute one modal class and are of diverse types, including quartz wacke, shale, quartzite and chert and limonitised clasts. Monocrystalline quartz, muscovite, siderite and feldspar in combination with the matrix define the second mode with size ranging from 1/4 to l/200 mm. The bimodality is interpreted to be due to mixing of the coarse sand and gravel component with the matrix.

Intact load structures and convolute folds in rip-up clasts and rafts, very poor sorting with high mud matrix, angularity of the constituent fragments. chaotic and often vertical orientation of the platy clasts and the presence of mud balls, armoured sand grains and shale-coated clasts strongly suggest transportation within a high density, cohesive debris-flow.