Abstract The Chenier Plain of southwestern coastal Louisiana is a Holocene strand plain composed of wooded beach ridges (cheniers) and intervening mudflat grassy wetlands. The mudflats form as prograding tidal flats along the open, but low-energy Gulf of Mexico coast; cheniers form from winnowing of sand and shells from the mudflats by waves during transgression. Mudflats are deposited when a Mississippi River delta lobe is nearby to the east, and cheniers are formed when distributaries switch to a more distant location farther east. All of the cheniers have formed within approximately the past 3000 yr or less and are progressively younger toward the present coastline. Spits are attached to the cheniers at estuaries; they grow westward in response to the dominant longshore currents. Currently, mudflats are prograding in Vermillion Parish to the east, while cheniers form in eastern Cameron Parish along with some regressive beach ridge development in western Cameron Parish. This coast is microtidal with low wave energy. A high rate of subsidence as well as sea-level rise characterizes the Chenier Plain, which is subject to increased wave energy and mud transport every year during many cold-front passages and periodic storm surges associated with tropical cyclones of much lower frequency. Major storm surges can inundate the entire Chenier Plain, wreaking havoc on human settlements.

Abstract The early work of Fisk (1944; 1956) on the Mississippi River demonstrated that glacio-eustatic lowering of sea level caused the incision of fluvial systems into the exposed continental shelf. During maximum glaciation sea level withdrew to or near the shelf margin, resulting in discharge of coarse sediments onto the outer shelf and upper slope. On the exposed coastal-plain/shelf-surface, floodplain deposits accumulated and soil profiles formed. As continental glaciers began to melt, rising sea level submerged the fluvial systems, forming estuaries that eventually were filled with sediments as the sea transgressed. Frazier (1974) expanded these concepts into a model of depositional episodes that defined the timing and relative importance of progradation, aggradation, and marine transgression within a glacio-eustatic cycle. Maximum progradation, accompanied by fluvial downcutting, occurs during sea level withdrawal. Aggradation lags slightly behind progradation both in time and space; progradational units are thickest offshore, but aggradational units thicken landward. Marine transgression caused by relative sea level rise terminates the cycle. Sediments deposited during such a cycle make up a depositional sequence. In the uppermost late Quaternary sediments of the southwestern Louisiana continental shelf, three separate depositional sequences are recognized on high-resolution seismic-reflection profiles. Within these sequences are four sets of buried channels. The oldest channels are at subsurface depths of > 75 m (246 ft) at mid-shelf. They are designated as pre-Wisconsinan and are presumed to mark the Illinoian low- stand of sea level. These channels are recorded only in certain areas of the continental shelf where penetration was sufficient

Abstract Dome-like structures are the most conspicuous features recorded on the seismic-reflection profiles of the Louisiana continental shelf. Some of these structures lie at depth with no surficial expression, some were truncated to the sea floor by erosion when the shelf was exposed during lowstands of sea level, and others form topographic highs on the sea floor. Along the shelf margin and upper slope topographic expression varies from nearly circular to oblong and ridge-like. Progressively down the slope the uplifts increase in number, height, and expanse. The origin of these structures throughout the coastal region of the Gulf of Mexico has long been attributed to salt diapirism. The distribution of salt deposits in the northern Gulf of Mexico region and of diapirs by structural type are shown by Martin (1978, figure 4). A discourse on theories of salt dome origin and historical development of current theory is beyond the scope of this report. The data strongly suggest, however, that the key element causing diapirism in the northern Gulf of Mexico is the plasticity of deep-lying salt activated by differential sediment loading. Difference in specific gravity between salt and sediments probably is a secondary factor in the origin of salt diapirism. For further reading on salt diapirism the reader is referred to Hal- bouty (1979), and to Braunstein and O'Brien (1968), who edited a symposium of 21 papers on the general subject of diapirism, including a comprehensive review of salt structures of the Gulf of Mexico basin by Murray (p. 99-121). A

Abstract In addition to ancient deltaic and fluvial deposits, other features along the shelf margin also document former shoreline positions during lowstands of sea level. The most obvious of these are erosional surfaces near the shelf edge and carbonate reefs that formed in shallow waters seaward of the shoreline. The locations of both types of features and the profiles selected to illustrate them are shown on Figure 6-1. Three types of erosional features are illustrated by the profiles in this chapter: (1) truncated diapiric uplifts, (2) wave-cut terraces on the outer shelf, and (3) a buried submarine trough. In addition, the significance of ancient fluvial channels as indicators of sea level history is summarized.