Architecture and facies pattern of a sublacustrine fan, Jharia Basin, India

A small sublacustrine fan deposit has been identified within a lacustrine succession belonging to the Barren Measure Formation (Middle Permian) of the Lower Gondwana deposits of Jharia Basin, India. The stratigraphic position of the fan and its structural relationship with the associated sediments suggest that the east-west aligned elongate trough formed in the central part of the basin experienced intrabasinal normal faulting of limited lateral extent, and accumulation of elastic input within the associated hangingwall syncline led to formation of this deposit, Three major framework components can be identified within this deposit: (a) sheet-like sandstone aprons, (b) channel-fill sandbodies and (c) lensoid massive beds of pebbly sandstone. The laterally extensive sheet-like bodies of sandstone, covering almost the whole of the older sediments, appear to be the product of deposition from unconfined sediment-laden flows during episodic flood events, The proximal part of the deposit is dominated by the successive deposition of sandstone aprons with a few channel plug sandbodies produced through rapid sedimentation from channelised sediment-laden turbulent flows. The distal part, on the other hand, is mainly represented by the stacked channel-fills with alternate sandstone aprons. Following the deposition of bedded sandstones, as the slope of the depositional surface reduced, turbulent flows with relatively less grain concentration extended towards the ambient waterbody through channels incised into the preexisting unconsolidated sediments. During this process of incision, the flows gradually became hyperconcentrated. The channels were ultimately filled-in by concordant bedsets in approximate conformity with the shape of the channel, either through bedload deposition or suspension fall-out from these submerged currents. A few massive channel-fills, indicating emplacement of liquefied sandflows, caused by slope instability during high-flood event, are also present. Isolated channel-fills show lateral uniformity in facies pattern while confluent channels produced laterally variable composite facies association with a distinct zone of interference. Lensoid mass flow deposits with distinct upslope slide scarps represent the event of slope failure as a result of cannibalisation during low water stand. These slide lenses are present in different levels in association with either the sandstone aprons or the channel-fills showing high degree of randomness in their order of appearance, The consistently developed round-bottomed geometry of the channels and the nature of channel-fills strongly advocate in favour of their subaqueous origin. Absence of any subaerial character within the sandstone aprons also suggests their emplacement under subaqueous condition. The thick stacks of channel-fill sandbodies were developed as a consequence of vertical channel aggradation under continued hangingwall subsidence, and the upward declining trend in vertical aggradation suggests gradual fall in the rate of this subsidence. Subsequently, the whole block subsided and went under deepwater condition due to lacustrine transgression and the fan was buried under deepwater carbonaceous shale. (C) 2002 Elsevier Science B.V. All rights reserved.