204 / 2019-06-19 20:04:41

Large soft-sediment deformation structures (SSDS) in Permian Barren Measures Formation, Pranhita-Godavari Valley, India –unambiguous clue to syn-rift palaeoearthquake events

Large SSDS; Seismites; Fluvial-tidal; Syn-rift; Gondwana; Permian

Identifying and deciphering records of palaeoearthquakes in sedimentary rocks are often debated due to confusion in distinguishing seismic vs. aseismic trigger mechanisms causing liquefaction. The present paper documents some unique SSDS, characterized by their (i) extremely large size, (ii) complex meta-depositional character and (iii) preservation in coarse-grained pebbly sandstone, which signifies definite clue to identify earthquake-induced liquefaction. The SSDSs are present in Permian Barren Measures Formation, a syn-rift depositional unit within the Lower Gondwana succession in the Pranhita-Godavari valley, peninsular India. The ~210 m thick succession is represented by stacked fining-up retrogradational cycles, characterized by coarse-grained pebbly trough cross-stratified sandstone near base and fine-grained, heterolithic sandstone-mudstone near top. Each cycle signify change from fluvial to tide-influenced depositional systems. Coarse-grained pebbly sandstone beds near the base of each cycle record most SSDS, including complex convoluted strata, pseudonodules, load and flame structures, various water escape structures like vertical/inclined sediment columns and contorted beddings, and syn-sedimentary faults. The deformed beds are underlain and overlain by undeformed beds. Complexly deformed SSDS are often sharply truncated at the top by undeformed beds manifesting meta-depositional character. Facies analysis reveals absence of processes like storms/pounding waves, slumps, rapid dumping (massive beds) or sediment gravity flows in the study area, thus negating their possibility as triggering agent for the liquefaction. The complex nature and large size of the deformation structures imply extensive and quick liquefaction near sediment-water interface. Such large amount of liquefaction in coarse-grained sandstone can be caused by intense seismic shocks only. In addition, the deformed beds comply with most of the criteria of typical seismites. So, coarse-grained pebbly sandstone beds containing such unusually large, complex, meta-depositional SSDS are identified as definite seismite layers. In half-graben type Gondwana basins, such signatures of paleo-earthquakes signify phases of syn-rift fault reactivation and associated accommodation changes.