The classic tripartite parasequence set scheme is widely used in coastal plain sequence stratigraphic interpretations which respond to a relative sea level cycle. The Highstand System Tract (HST) is immediately followed by a Lowstand Systems Tract (LST). The ensuing Transgressive System Tract (TST) accompanies the flooding stage, but there is no corresponding parasequence set during a fall in relative sea level fall. In theory a Regressive Systems Tract (RST), would be produced during a relative seal level fall, where it would erode and replace portions of the upper HST and would lie beneath in time (though not in the stratal record) the ensuing LST and would be characterized by offlap and basinward shift in facies (e.g. the FSST distal clastic wedges descrbed by Plint and Nummedal, 2000). However, as preserved deposition is minimal during the fall, an RST would be less well-preserved in a deltaic setting. The purpose of this study is to demonstrate that the RST which responds to a drop in base level indeed can be found in ramp coastal plain settings, where they are represented as fluvial terraces. The example comes from offshore Thailand.


The neglect of the RST in stratigraphic interpretation can be attributed in part to the initial evolution of the systems tract concept and subsequent confusion of this concept in different theories concerning the effects of base level changes upon depositional sequences (Pigott et al., 2011).

First, owing to the difficulty of recognition of offlapping strata in early 2D seismic sections, the subdivision of sequences into the classic component systems tracts was first presented by Posamentier and Vail (1988) where a highstand was immediately followed by lowstand. The limited recognition of sediments deposited during relative sea-level fall led to the early representation of relative sea-level fall as 'instantaneous' and the asymmetry of the relative sea-level curve was later attributed to incomplete preservation of the sedimentary record with the classical definition of prograding, aggrading, and retrogradational system tracts which were then defined as the parasequence sets termed HST, LST, and TST (van Wagoner et al., 1988).

Second, in consideration of a complete sea level cycle, as several previous workers have addressed the sea-level falling stage in different theories (Catuneanu, 2007), there exists a general lack of a unanimous agreement on the division of base level falling from onset to the end. There are four opinions: (1) early LST fan (Haq, 1987; Posamentier, 1988; and Galloway, 1989; etc.); (2) Late HST (vanWagoner 1995, etc.); (3) FSST (Hunt and Tuck, 1995, etc.); (4) RST (Embry, 1993, etc.).

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