Tectono-sedimentary evolution of the Gondwanan Satpura Basin of central India: evidence of pre-Trap doming, rifting and pal˦oslope reversal

Abstract

The Mesozoic Gondwanan Satpura Basin of central India, comprising an approximately 1300 m thick sequence of the Pachmarhi, Denwa and Bagra Formations, was subjected to at least three major tectonic events. These events are manifested by tectonic dislocation, marginal uplifts, basin subsidence and deformation, as well as by stratigraphical disposition, lithofacies assemblage, and palaeoslope and pal˦ocurrent patterns. The first tectonic event is manifested by the onset of Early Triassic Pachmarhi sedimentation, which is marked in the basal part by a sudden increase of conglomeratic, pebbly, gritty to coarsegrained cross-bedded sandstone. This contrasts with the underlying fine elastics of the Late Permian Bijori Formation. The stratigraphical relationship and lithofacies, together with pal˦ocurrent and petrographic data, reflect tectonic uplift in the source area to the southeast of the Satpura Basin during or prior to the deposition of Pachmarhi Formation. The pebbly coarse sandy facies of the Pachmarhi Formation represents a braided river assemblage, overlain by a meandering river facies of the Denwa Formation, with river systems flowing dominantly from southeast to northwest. The progressive change in lithofacies and grain size upward from Pachmarhi to Denwa implies that the source area became peneplained and that the basin stabilised. During the prolonged gap of non-deposition, following the Mid-Triassic break in sedimentation after deposition of the Denwa Formation, a second tectonic event resulted in the widespread faulting and uplift of Permo-Triassic Gondwana sediments and basement rocks, respectively, to the south and north of the Narmada-Son Lineament Zone of Peninsular India. A third tectonic event is manifested by Late Jurassic-Early Cretaceous Bagra conglomerate and sandstone-shale facies in the northern part of the Satpura Basin. This formation, which unconformably overlies the Precambrian, and Permian and Triassic Gondwana formations or abuts against faulted contacts, represents proximal and distal facies of an alluvial fan deposit in a rifted (pull-apart?) basin with uplifted highlands to the north. This tectonism, representing the termination of continental Gondwana sedimentation, preceded the widespread eruption of the Deccan Traps (65 Ma) after the break-up of India from Antarctica. As a consequence, a northward sloping, peninsular craton was tilted southward and small rift basins developed along peripheral parts to the north, west and along the east coast of Peninsular India. To the north of the study area, the doming before the Deccan volcanism and tectonic movement along the Narmada-Son Lineament caused uplift of the Palaeoproterozoic Mahakoshal/Bijawar terrane. This uplift was accompanied by a reversal of palaeoslope. Consequently, north to south pal˦currents followed the deposition of the Pachmarhi and Denwa Formations, as borne out by the alluvial fan-braided complex of the Bagra Formation.