Sedimentary Record of Forced Regression Along The Margin of Kutch Basin: Terminal Cenozoic Succession (Sandhan Formation), Western India
The sedimentation of Cenozoic successions of Kutch took place in passive margin sag-basin over the stable continental shelf, primarily controlled by relative sea-level fluctuations vis-à-vis siliciclastic supply/carbonate production. The basin-wide two-tier unconformity bounded clastic dominated Sandhan Formation is deposited in shallow marine (~135m, lower part) to fluvial environment (~157m, upper part). The discontinuous exposures along cliff/banks of Kankawati River (type section) and Kharod River provided an excellent opportunity for detailed sedimentological and sequence stratigraphic analysis. The fluvial forced regressive sediments are very rare in rock records because of subsequent transgression and erosion. The total 11 facies were identified (Miall, 1985 classification) viz. Gm, Gmm, Gp, Gt, Sh, Sp, St, Sm, Fsm, Fl, and P are grouped into three Facies Association (FA): 1. Channel and Channel Fill FA; occurs at the base, characterized by Gm, Sh and Sp facies with vertically and laterally amalgamated stacked tabular sheet sandstone bodies with concave upward erosional base, individual sandstone sheet shows the fining upward trend with channel-lag. 2. Sandy and Gravel Bar FA; characterized by fining upward tabular sandstone bodies (Sh and Sp) overlain by the cycle of coarse poorly sorted massive gravel fining upward to trough cross-stratified sandstone. Towards downstream section multistoried stacked sheet of trough cross-stratified gravel beds are overlain by the planar stratified gravel bed (Gm, Gt and Gp). 3. Overbank Fines and Floodplain FA; characterized by Fl, Fsc, and P elements and by 5-6m thick massive deposit of lithofacies P in the upper part of the succession, dominated by abundant root penetration structures and extensive pedogenic features with calcrete/ferricrete layers, indicates a major break in sedimentation and sequence boundary. The Sandhan Formation is characterized by the wave-dominated TST overlain by normal regression/progradation of HST followed by the forced regressive deposit of FSST. The FSST is bounded at the base by a basal surface of forced regression, characterized by cobble/pebble horizons followed by abundant fluvial channel lags occurring at the top of HST. The low dipping shelves are very sensitive to sea-level changes and subjected to inundate/expose a large part of the shelf in small fluctuations. The abrupt fall in the sea-level exposed a large part of the continental shelf and older HST prism provided the sufficient slope for the braided fluvial system to develop. The relative sea-level never reached to its previous extent which leads the preservation of forced regressive deposits. The final withdrawal of shoreline shifted the depositional milieu westward and sedimentation/basin closed at the onland part of Kutch.
Keywords: Sandhan Formation, Falling Stage Systems Tract, Channel and Channel Fill, Sandy and Gravel bar, Overbank Fines and Floodplain.