Lagoon-tidal flat sedimentation in an epeiric sea: Proterozoic Bhander Group, Son Valley, India

The Bhander Group, the uppermost stratigraphic unit of the Proterozoic Vind hyan Supergroup in Son Valley, exhibits in its upper part a 550 m thick, mu ddy siliciclastic succession characterized by features indicative of deposi tion in a wave-affected coastal, lagoon-tidal flat environment suffering re peated submergence and emergence. The basic architecture of the deposit is alternation of centimetre- to decimetre-thick sheet-like interbeds of coars er clastics (mainly sandstone) and decimetre-thick mudstones. The coarser i nterlayers are dominated by a variety of ripple-formed laminations. The pre served ripple forms on bed-top surfaces and their internal lamination style suggest both oscillatory and combined flows for their formation. Interfere nce, superimposed, ladder-back and flat-topped ripples are also common. Syn sedimentary cracks, wrinkle marks, features resembling rain prints and adhe sion structures occur in profusion on bed-top surfaces. Salt pseudomorphs a re also present at the bases of beds. The mudstone intervals represent susp ension settlement and show partings with interfaces characterized by synsed imentary cracks. It is inferred that the sediments were deposited on a coas tal plain characterized by a peritidal (supratidal-intertidal) flat and eva porative lagoon suffering repeated submergence and emergence due to storm-i nduced coastal setup and setdown in addition to tidal fluctuations. The 550 m thick coastal flat succession is surprisingly devoid of any barri er bar deposits and also lacks shoreface and shelfal strata. The large area l extent of the coastal flat succession (c. 100,000 km(2)) and its great th ickness indicate an extremely low-gradient epeiric basin characterized by a n extensive coastal Bat sheltered from the deeper marine domain. It is infe rred that the Bhander coastal flat was protected from the open sea by the B undelkhand basement arch to the north of the Vindhyan basin, instead of bar rier bars. Such a setting favoured accumulation of a high proportion of ter rigenous mud in the coastal plain, in contrast to many described examples f rom the Proterozoic. Copyright (C) 2001 John Wiley Br Sons, Ltd.