Physical and biological changes across the Jurassic/Cretaceous Boundary in northwestern Kutch Basin

Abstract

In the northwestern part of the Kutch Basin, Lower Cretaceous shales and trigonid-bearing / bioturbated/barren sandstones, overlie the glauconitic Upper Tithonian Virgatosphinctes-bearing "Umia Ammonite Bed" (138±20 Ma). During the lithofacies studies of this sequence, lithology, physical and biogenic sedimentary structures, bed geometry, palaeocurrent and fossil contents were given special attention. Laboratory investigations of heavy minerals, rock fragments and clay minerals revealed two distinct assemblages, one for the lower (Jurassic) and other for the upper (Cretaceous) part of the sequence. Lower part of the sequence is rich in low energy sedimentary structures, biogenic structures of sediment and deposit feeder, heavies and rock fragments of the metamorphic province and clay minerals of marine origin. On the other hand, the upper part is rich in high energy sedimentary structures, biogenic structures of suspension feeders, heavies and rock fragments of the granitic province and clay minerals of fresh to brackish water origin. These differences are of vital importance in delineating major changes such as shoreline orientation, changing provenance, relative sea level and tectonics during Jurassic/Cretaceous period in the northwestern Kutch Basin.