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

Abstract

The Bhander Group, the uppermost stratigraphic unit of the Proterozoic Vindhyan Supergroup in Son Valley, exhibits in its upper part a 550 m thick, muddy siliciclastic succession characterized by features indicative of deposition in a wave-affected coastal, lagoon±tidal¯at environment suffering repeated submergence and emergence. The basic architecture of the deposit is alternation of centimetre-to decimetre-thick sheet-like interbeds of coarser clastics (mainly sandstone) and decimetre-thick mudstones. The coarser interlayers are dominated by a variety of ripple-formed laminations. The preserved ripple forms on bed-top surfaces and their internal lamination style suggest both oscillatory and combined¯ows for their formation. Interference, superimposed, ladder-back and¯at-topped ripples are also common. Synsedimentary cracks, wrinkle marks, features resembling rain prints and adhesion structures occur in profusion on bed-top surfaces. Salt pseudomorphs are also present at the bases of beds. The mudstone intervals represent suspension settlement and show partings with interfaces characterized by synsedimentary cracks. It is inferred that the sediments were deposited on a coastal plain characterized by a peritidal (supratidal±intertidal)¯at and evaporative lagoon suffering repeated submergence and emergence due to storm-induced coastal setup and setdown in addition to tidal¯uctuations.The 550 m thick coastal¯at succession is surprisingly devoid of any barrier bar deposits and also lacks shoreface and shelfal strata. The large areal extent of the coastal¯at succession (c. 100,000 km 2 ) and its great thickness indicate an extremely lowgradient epeiric basin characterized by an extensive coastal¯at sheltered from the deeper marine domain. It is inferred that the Bhander coastal¯at was protected from the open sea by the Bundelkhand basement arch to the north of the Vindhyan basin, instead of barrier bars. Such a setting favoured accumulation of a high proportion of terrigenous mud in the coastal plain, in contrast to many described examples from the Proterozoic.

Microalgal mediation of ripple mobility

“…Many such examples display evidence of microbial or adhesive structures, implying that these played a part in their preservation (e.g. Chakraborty, 2001;Parizot et al , 2005;Noffke et al , 2006). For ripple preservation to occur, a likely precursor scenario prior to burial by sediment is that ripple mobility ceases either due to cementation, in the case of carbonate sands (Taylor and Illing, 1969), or by some other method such as algal mat formation (Neumann, 1970), or by a combination of such processes.…”

“…Many such examples display evidence of microbial or adhesive structures, implying that these played a part in their preservation (e.g. Chakraborty, 2001; Parizot et al. , 2005; Noffke et al.…”

Proterozoic intracontinental basin: The Vindhyan example

“…The upper part is dominated by medium-grained sandstones. The lower part represents deposits of storm-influenced tidal flat system, whereas the upper sandstone-dominated part contains braidplain and eolian strata (Singh 1980;Chakraborty 2001;Bose et al 1999;Chakraborty and Chakraborty 2001).…”

“…This formation begins with oolitic and stromatolitic limestones followed by shales with stringers of sandstone and siltstone. A lagoonal environment has been inferred (Singh 1980;Chakraborty 2001;.…”

“…The formation consists of sandstones with interlayers of reddish mudstone. The depositional system is considered as a tide-storm influenced coastal flat (Bose and Chaudhuri 1990;Chakraborty 2001). Lakheri Formation (Dolni Limestone??)…”