Large soft-sediment deformation structures (SSDS) in the Permian Barren Measures Formation, Pranhita-Godavari Valley, India: potential link to syn-rift palaeoearthquake events

Abstract

Records of palaeoearthquakes in sedimentary rocks are often debated due to the potential confusion in distinguishing seismic versus aseismic trigger mechanisms causing liquefaction. The present paper documents some unique soft-sediment deformation structures (SSDS), characterized by their extremely large size, complex morphology and preservation in coarse-grained pebbly sandstone. The SSDS are present in the Permian Barren Measures Formation, a syn-rift depositional unit within the Lower Gondwana succession in the Pranhita-Godavari Valley, eastern Peninsular India. The ~ 210 m thick succession is represented by stacked fining-up retrogradational cycles, characterized by coarse-grained trough cross-stratified pebbly sandstones near the base and fine-grained heterolithic sandstones–mudstones at the top. Each cycle signifies a change from fluvial- to tidal-influenced depositional systems. Coarse-grained pebbly sandstone beds near the base of each cycle record most SSDS, including complexly deformed layers, pseudonodules, load and flame structures, various water-escape structures like vertical/inclined sediment columns (sedimentary dykes) and contorted beds, and syn-sedimentary faults. The deformed beds are underlain and overlain by the undeformed beds. Complexly deformed SSDS are often sharply truncated at the top by undeformed beds manifesting syn-sedimentary character, which signifies that deformation took place just after deposition of the affected beds, but before deposition of the overlying beds while sedimentation was continuous. Facies analysis reveals the absence of processes like storms/pounding waves, slumps, rapid dumping (massive beds), impact shaking, volcanisms, tsunami waves or sediment gravity flows in the study area, thus negating their possibility as triggering agents for the liquefaction. The complex nature and large size of the deformation structures imply extensive liquefaction near the sediment–water interface. In addition, the deformed beds comply with most of the criteria of typical seismites. In half-graben type Gondwana basins, such seismites can be linked to palaeoearthquakes, which signify the phases of syn-rift fault reactivation, basinal sagging and associated accommodation changes.