Palaeocurrent Analysis of Quaternary Terrace Gravels in the Northern Part of the Cachar Fold Belt, NE India

ABSTRACT A sequence of about 20 m thick Quaternary terrace gravels overlies deformed Neogene clastics across a pronounced unconformity in the northern part of the Cachar fold belt. These gravels are predominantly clast-supported towards north, becoming more matrix supported towards south and spread over a narrow ENE-WSW trending belt of about 60 km X 20 km between the Barail range of hills towards north and the Barak River towards south. Mean grain size of the gravels varies between 54 to 195 mm with a discernible southward decrease in size. Here we present the first report on palaeocurrent analysis of the late Quaternary terrace forming gravels in the fold belt region of Assam Arakan sedimentary basin based on outcrop scale observations and measurements of clast imbrications, and correlate them with the present river regime. The mean palaeo flow direction is found to be towards SSE, with an overall variation between 93 and 233° in different terrace levels, while the variance and vector strengths respectively range from 144 to 1124 and 86-98. Compared to this, the present river regime shows a general flow direction towards SSW, has higher flow variance but lower vector strength with values ranging from 1101 to 4819 and 57 to 91, respectively. It is inferred that the terrace gravels were deposited by south-flowing tributaries of the Barak River, namely, Chiri, Madhura, Jatinga, Larang and Kalanchara, at their mountain exit from the Barail range of hills, during a phase of river rejuvenation, with a possible activation of an offshoot of the Dauki-Disang thrust. No significant changes in the direction of fluvial sediment transport have taken place during the Late Quaternary in the northern front of the Cachar fold belt.