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.
Highlights
The goal of this study is to examine the possible link between the inferred liquefaction processes to palaeoearthquake events
4.1 Facies associations and depositional settings The Barren Measures Formation in the Godavari River section is represented by two predominant facies
The truncated, syn-sedimentary nature of these structures signifies that the trigger for liquefaction was confined at or near to the sediment–water interface before the deposition of the overlying bed
Summary
Soft-sediment deformation structures (SSDS) formed by deformations in sediments or sedimentary rocks before significant diagenesis/lithification (Owen et al 2011), are reported from sediments/sedimentary rocks of varied depositional environments, including, glacial, eolian, lacustrine, fluvial, alluvial, tidal flat, deltaic, estuarine, shore, shelf and deep marine slopes (Seilacher 1969; Rossetti 1999; Rodríguez-Pascua et al 2000; Obermeier et al 2005; Moretti et al 2016). Individual trigger mechanism does not leave any characteristic morphological/structural imprints during or after liquefaction, attempts to link these trigger mechanisms with seismic shocks (‘seismite’ layers), leading to the identification of palaeoearthquake events in the rock record, are very common (Seilacher 1969, 1984; Lowe 1975; Seth et al 1990; Pratt 1994; Owen 1995; Bhattacharya and Bandyopadhyay 1998; Owen and Moretti 2011; Moretti and Van Loon 2014; Van Loon 2014). Such indiscriminate attempts have led to a lot of terminology confusion, diluting the actual aim/goal of identifying these structures
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