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Finding the Sequence Missing Link: Recognition of the RST in Clastics and Carbonates in Outcrop, Borehole and Seismic Stratigraphy


The sea level sequence paradigm for shallow water marine clastic and carbonate cycles classically includes the tripartite classification: HST, LST, and TST. For deeper water sediments, the FSST (Falling Stage Systems Tract) of the early stage LST is included. The time equivalent of the FSST on the shelf has gone largely unrecognized owing to the susceptibility of subsequent subaerial erosion of such deposits of uncemented sediments when sea level falls. However, the recognition of downstepping shelf-edge carbonate buildups concomitant to the early stage FSST would be facilitated by the potential preservation of early cemented or organically bound previously shallow water carbonates as sea level falls. 3D seismic sequence stratigraphy of Pleistocene Sunda Shelf coastal plain clastics in the Gulf of Thailand calibrated to radiocarbon sea levels that during falling sea level the low-gradient coastal fluvial plain is characterized by erosion or non-deposition, while the fluvial sediments and channel geometries in the incised valley and tributary valleys formed in a synchronized fashion and left terraces in response to changes in climate and/or gradient and/or mechanical load grain size with downstepping of the base level. Fluvial terrace levels along region's larger rivers can be correlated and are interpreted to represent RST fluvial deposition in Gulf of Thailand. Supporting outcrop evidence comes from the present day observed terraces on major U.S. coastal rivers. An analogous seismic analysis of Leonardian carbonate buildup deposition on the Midland Basin rim calibrated to the global sea level curve also reveals definitive evidence for the RST. In carbonate depositional systems, owing to biolithite development and/or enhanced shallow water cementation, the falling sea level forced regression carbonates are well preserved and produce generally higher gradients and comprise broader facies zones than can be achieved by clastic settings. Supporting outcrop and subsurface evidence comes from the Permian strata of West Texas and New Mexico with geomorphic and full spectrum decreasing API gamma ray stacking patterns. Subsurface well controls demonstrates that clastic fluvial RSTs make favorable reservoir targets as they tend to be sealed by and sourced by the ensuing TST's and carbonate buildup RST's have heightened porosities owing to a decreased influence by intense vadose associated diagenetic cementation in comparison to HST's.