Styles and origin of post- and syn-depositional structures of metagreywacke-argillite strata (Goa Group), and formation of Bouma sequence, West Coast of India

Abstract

Study of the sedimentary structures helps to interpret the depositional process of siliciclastic sediments. Sedimentary structures are classified based on the morphology, formation process, sediment rheology, deformation mechanism and relative timing of sedimentation. In this study, the structures are categorized as Primary depositional, Diagenetic, Soft Sediment Deformation Structures (SSDS) and Deformational Structures. Laminations, dropstone, graded beddings, and cross-bedding are primary structures formed during sedimentation and among the diagenetic structures, liesegang rings were identified. The other structures such as convolute, flame and load, ball and pillow (pseudonodules), slump fold and syn-sedimentary fault are classified as SSDS and are reported for the first time. The dykes and shear zone are the intrusive and deformational structures, respectively. In our study, the regional geology and structural data suggest a deltaic environment with a turbiditic condition of deposition for the formation of the SSDS. In a deltaic environment the SSDS were formed due to rapid deposition of sediments by suspension, their disruption due to liquefaction, and movement of sediment in a water-logged state. The processes were controlled by slope of the basin, gravity controlled density currents and differential compaction. In this environment of high sediment supply, fluctuating water level, slope instability and sediment density there resulted an in situ deformations of the sediments and formation of the SSDS. We have correlated the typical Bouma sequence with the various structures including the SSDS that are associated with the metagreywacke-argillite strata in the study area. The strata with 4 units (A-D) reveal a typical Bouma sequence which was influenced the low-density turbidity currents. With time, the deep basin sediment depositional environment progressively changed to a shallow environment.