A transgressive storm- and fair-weather wave dominated shelf sequence: Cretaceous Nimar Formation, Chakrud, Madhya Pradesh, India

Abstract

The lithologic assemblage of the Cretaceous Nimar Formation in the area around Chakrud, Madhya Pradesh, India, has been classified in five marine facies: (A) polymictic conglomerate resting on granite basement, largely representing a lag deposit; (B) coarse-grained pebbly sandstone with normal and reverse coarse-tail grading and also frequency grading; (C) sandstone, with alternate hummocky cross-stratified and small-scale wave-rippled units; (D) mudstone; and (E) limestone. Facies C bears the definite stamp of alternations between storm- and fair-weather waves. Probable parameters of the fair-weather waves have been determined from the grain size and morphometric parameters of the wave ripples. The depth of the generation of the wave corresponds to a modern mid-shelf region. Facies B shows gradational contact with facies C and was deposited shoreward above fair-weather wave base. Bed shear stresses calculated from variable grain-size parameters within individual cross-laminae indicate that the nature of the current that deposited the pebbly sands was similar to that of storm currents. It has been postulated that storm ebb surges initially moved as fluid gravity flows pushing ahead the bed loads, and eventually turned into density currents farther offshore carrying most of the sediment in suspension. The other two facies, D and E, were deposited largely below storm wave base. The overall fining-upward sequence reflects gradual deepening of the basin. Local aberrations in the facies sequence as represented by short-phase coarsening-upward sequences suggest temporary reversals of this trend.