The controlling factors of lateral variability in coastal marine sequence building pattern

The creation and obliteration of accommodation space are controlled primarily by the local changes in sea level. The sedimentation rate, on the other hand, played a pivotal role in determining the sequence stratigraphic trend during filling up of that accommodation space. The current study involving the two basal members of the Kundargi Formation, Bagalkot Group, Kaladgi Basin, provides an opportunity to highlight the importance of sedimentation rate in sequence building. Quantification of the actual sedimentation rate is beyond question in such a Proterozoic succession due to lack of accurate age data; however, their qualitative variations can be traced laterally across the basin. The study initiates with facies analysis that revealed four distinct facies associations, incorporating 25 facies. Three-dimensional facies variability suggests palaeogeographic realm shifted from fluvial to open marine shoreface, along depositional dip, through an intervening locally developed estuary. The shoreface assemblage gradually passes upwards into deeper water fines demarcating gradual deepening of the basin. Bounded between an unconformity and a transgressive lag, the fluvial interval represents a lowstand (LST) product. Study area, located near the paleoconfluence, received higher sedimentation, which resulted in progradation of the bay-head delta over estuarine fines during the ‘quasi-standstill’ period in-between two faster periods of relative sea-level rises. While the first episode of faster sea-level rise produced the estuary, the second one led to the submergence of the estuary and complete marine inundation of the entire area. However, away from the paleoconfluence, sediments, deposited during the quasi-standstill period, were eroded during the younger period of faster sea level rise leaving amalgamated bodies of beach conglomerates. Present study reveals spatial variability in the rate of net sedimentation played the pivotal role in shaping sequence architecture.