Palaeogeographic reconstruction of a fluvio-marine transitional system in Narmada rift basin, India——Implications on Late Cretaceous global sea-level rise

摘要

Rising sea-levels in tectonically active epicontinental basins often lead to varied depositional settings and palaeogeography, mostly influenced by the net accommodation resulting from mutual interference of the extent and nature of landward encroachment by the sea and the net sedimentation. The Cenomanian Nimar Sandstone Formation, Bagh Group, Narmada rift basin, uniquely portrays the effect of sea-level rise within an intra-cratonic setting and attributes to the corresponding palaeogeographic changes in west-central India. An integrated sedimentological–sequence-stratigraphic study of the broadly fining-upward Nimar Sandstone Formation(thickness~ 20–30 m) depicts the actual nature of changeover from a fluvial to a marine-dominated transitional depositional setting. Detailed sedimentological study reveals total seventeen facies, grouped in five facies associations, viz., the channel-fill facies association(FA-1), the overbank facies association(FA-2), the fluvial-dominated fluvio-tidal facies association(FA-3), the tide-dominated fluvio-tidal facies association(FA-4), and the shoreface facies association(FA-5). Overall facies architecture indicates a west-to-eastward marine encroachment, resulting in stacking of three distinct palaeo-depositional conditions:(i) an initial fluvial system with channel and overbank, changing into a tideinfluenced fluvial bay-head delta in the inner estuary, followed by(ii) marine encroachment leading to a tidedominated central estuary with inter-to sub-tidal settings, and finally,(iii) with further intense marine encroachments, a wave-reworked open shore condition in the outer estuary zone. The overall fining-up succession with a systematic change from fluvial to marine-dominated depositional systems points to a landward shift of the shoreline, signifying a major transgressive event correlated to the Cenomanian global sea-level rise. Characteristic stratal stacking patterns point to four coarsening-and fining-up hemicycles, embedded within the major transgressive succession. These high-frequency cycles attest to the varied interplay of sedimentation, tectonics and sea-level changes, and the resultant net accommodations. A palaeogeographic model is proposed based on the high-frequency transgressive–regressive hemicycles, which envisages the evolution of the depositional environments in relation to the Cenomanian eustatic rise in the intra-cratonic riftogenic fluvio-marine transitional basinal setup.