Early Eocene Subaerial Erosional Valleys in Cambay Basin, India

Abstract

Anomalous thickness of the Kalol Formation, a clastic wedge of middle Eocene age, have been traced on the topographic high reflecting the Mehsana horst in the central part of the north Cambay basin. Subsurface data and isopach studies indicate that the Kalol sequence occupies ancient erosional valleys in the Cambay Shale bedrock. These erosional valleys trend north-south, are more than 15 km long, 1 to 1.5 km wide, and up to 120 m deep. Downslope, the stream channel bifurcates and the width of the anastomosing channel system is 3.5 km. The slope of the walls of the valley ranges from 3 to 18° and is generally steep in the south. A major factor contributing to the formation and deep excavation of the erosional valleys is thought to be the slow uplift of the fault-controlled Mehsana horst which is situated in the middle of the basin floor. As a result of the uplift a structural depression, roughly circular in outline, was formed around the slopes reflecting the horst along which the major rivers of the basin started flowing. In one of the river systems, where sand input was limited, freshwater flood basins and lagoonal swamps were formed in which coal-forming vegetation flourished. These basins and marshes, in turn, acted as sand traps. As a result of reduced sediment-carrying competence of the river system, some of the river flow was diverted along new distributaries. Deep erosion on the slopes reflecting the orst is believed to have been caused by one of the tributaries of the major river system. This erosion was aided by the slow uplift and initial declivity of the Mehsana topographic high. The tributaries may have cut across the slowly rising horst as antecedent drainage. The stream-channel distributary system, which excavated the erosional valleys during the early Eocene, deposited deltaic Kadi sediments in a shallow sea south of the North Kadi area. The erosional valleys, which were in the nature of estuaries of Cambay Gulf, were filled with sandstone, siltstone, shale, and coal of the Kalol Formation during the middle Eocene. Geometry, age of gorge erosion and sedimentary fill, thickness of the channel fill, and the presence of the regional blanket which can be correlated with the channel fill are similar to other ancient submarine valleys. However, the cross section and slope of the walls of the valley and the sandstone to coal lithology of the channel fill characterize the channels on an emergent surface and are in contrast with other submarine valleys.