Abstract: Analyses of Regional Gravity and Magnetic Anomalies across the Southern Part of the Bengal Basin, Bangladesh

Dean M. Kleinkopf

Most of Bangladesh is underlain by deltaic and alluvial deposits of Quaternary age, though small areas of Tertiary sedimentary rocks are exposed near the northern and eastern borders. Knowledge of subsurface geology depends largely upon geophysics and deep boreholes. In earlier studies done in cooperation with the Geological Survey of Bangladesh and with

Petrobangla, combined interpretations of digital gravity and aeromagnetic anomaly maps, using anomaly enhancement techniques, provided new insights in geologic framework investigations, mineral and energy resources exploration for Gondwana coal and natural gas, and natural hazards mitigation. The work reported here is focused on gravity and magnetic modeling and interpretations of geologic framework and structure of the southern part of the Bengal basin, south of lat. 24° N., where seismic and borehole data are scarce. The extension of deep structures offshore into the Bay of Bengal is examined. The structural regime probably developed in the middle Tertiary from stresses produced by the northward- and northeastward-directed collision of the Indian plate with the Asian plate.

The deep part of the Bengal basin, the Bengal foredeep, lies between the Tripura-Chittagong fold belt on the east and the Bogra shelf and the Calcutta-Mymensingh hinge zone on the west. The Bengal foredeep exhibits broad, northeast-trending gravity anomalies (10-15 mGal) and wide magnetic gradient zones (50-70 nT), which reflect deeply buried lithology and structure. Examples are the Faridpur trough, the Barisal gravity high, and the Hatia trough, which extends into the Bay of Bengal and is believed to contain the thickest sedimentary deposits, northwest of Cox's Bazar. Modeling suggests that these anomaly sources are deep structures, reaching depths that exceed 15 km in places, and are rooted in Archean crystalline rocks, or perhaps (in the case of the Hatia trough) in oceanic crust. South of Khulna, magnetic anomaly contours become distinctly east-west; an east-trending positive anomaly extends more than 100 km to the east. Then, near Patuakhali, it gives way to an east-northeast-trending magnetic trough punctuated by a series of three closed negative anomalies. This pattern suggests a deep structure that may extend beneath the north-northwest trending Tripura-Chittagong fold belt.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994