142 / 2022-02-26 18:57:54

Seismically induced soft-sediment deformation structures (SSDS) from the early Miocene Khari Nadi Formation, western Kutch basin of India: evidence of a tectonic uplift triggering an ichno-sedimentological regime change

sand volcano,seismite,ichnology,Khari Nadi Formation,Kutch basin,Miocene

Post-burial deformation of unconsolidated sediments leads to the development of distinctive sedimentary structures called the soft-sediment deformation structures (SSDS). The sand volcanoes and their associated array of faults can truly exemplify the SSDS, which particularly indicate the paleo-earthquake activities if found along a lithostratigraphic horizon, thereby, defining the concept of “seismite”. The Kutch basin has been tectonically active throughout since the onset of eastern Gondwana rifting and the extensional tectonics reversed (tectonic inversion) later in the Cenozoic due to the convergence of Indian and Eurasian continents. A stratum-bound series of sand volcanos from the Khari Nadi Formation are exposed along the Khari River trail in and around Aida. The sand volcanoes are exposed along the transition horizon separating the shallow-marine below and the non-marine deposits above, which manifest their process-sedimentary characteristics in terms of the sedimentological and ichnological evidences.

The studied interval consists of four layers, from bottom to top: viz., fine-grained sand (layer I; thickness of 60 cm), shale (layer II; thickness of 85 cm), medium-grained sand (layer III; variable thickness between 20 to 45 cm), fine-grained sand (layer IV; thickness of more than 3.5 m). The layer I and II shows the preservation of marine trace-fossil assemblages defined by Psilonichnus isp., Thalassinoides isp., Ophiomorpha annulata, and Planolites isp. with highly localized overprinting of deep-tier continental trace fossil Vondrichnus isp. coming from the layer IV. The layer III is a swaley-cross-stratified sand bed intermediately bioturbated with Ophiomorpha nodosa and in this layer, the sand volcanoes occur. The base of overlying layer IV is sharp and conformable. This surface is flat away from the sand volcanoes and irregular, wherever it veneers over the volcanoes. The layer III preserves both the marine (Ophiomorpha nodosa) and overprinted continental (Vondrichnus isp.) ichnofossils. Several small-scale synsedimentary faults have been observed surrounding the central/axial feeders of sand volcanoes, resembling caldera-like structures. Interestingly, the faults crosscut the O. nodosa burrows and the O. nodosa also crosscut the faults; the fabric definitely indicates their simultaneous syndepositional origin and the coeval heterogenous occurrence of both liquefaction (the sand volcano extrusion and undulated layers III-IV contact) and brittle deformation (synsedimentary faults). The first generation of O. nodosa predates the faults, therefore is disrupted by the latter. The second generation of O. nodosa burrows were produced after the faulting and liquefaction event because it disrupts the faults. Furthermore, the entire sequence of events occurred during the sedimentation of layer III. The layer IV contains an intricate three-dimensional network of termite nest (Vondrichnus isp., Termitichnus isp., and possibly Microfavichnus isp.) and root traces within the channel-fill fine-grained semi-consolidated sand and is devoid of any marine trace fossil. The termite colonization surface is not exposed, and it probably lies along the channel top. The deep-tier Vondrichnus burrows are found to penetrate all the way to layer I, and therefore, are present within the sand volcano as well.

Hence, it is proposed that the development of sand volcanoes can be attributed to a seismic event resulting in a tectonic upliftment and the consequent shallowing of the basin, since the layer III horizon containing them marks the transition from the marine to continental sedimentary regime – pointing towards an allogenic change for the basin. The observed evidences appear to contradict to previous works that suggested that there has been a continuous rise in the sea level form the beginning of sedimentation of the Khari Nadi Formation up to the middle of the overlying Chhasra Formation. However, these two different phenomena likely belong to two different sequence-stratigraphic hierarchy with the tectonic uplift being a lower-order (or higher-resolution) phenomenon within the Khari Nadi Formation sequence.