ABSTRACT Subsidence is one of the major processes driving coastal land loss in Louisiana. In an attempt to better understand local conditions influencing this process two continuously cored borings were taken in the Terrebonne Parish wetlands of the Lafourche delta complex. These borings were of sufficient length to completely penetrate sediments of the thick Holocene Mississippi River valley fill. One boring, 140 ft (42.6 m) long, was collected in western Terrebonne Parish under US Corps of Engineers support while the eastern Terrebonne Parish boring, 200 ft (61 m) long, was sponsored by the US Geological Survey. X-ray radiography revealed that the sediments of each boring were composed of cyclic depositional events. Each cycle basically incorporates a coarsening upward sedimentary sequence and may be described as a parasequence within the Holocene systems track. However, a wide range of sedimentary environments are represented from brackish/marine bay sediments to once exposed natural levees of major channels. Geochemical testing, particularly of carbonates and sulfides, radiocarbon dating of appropriate materials, and cone penetrometer profiling of the eastern Terrebonne boring site provided an initial data base for evaluation of effects of lithology and diagenesis on subsidence.

ABSTRACT Louisiana is experiencing land loss rates of over 75 km 2 per year. One component in the land loss is marsh deterioration caused by insufficient sedimentation to keep pace with high rates of relative sea-level rise. Sediment deposition depends upon both the opportunity for sediment transport onto the marsh surface and the availability of suspended sediment for deposition. Factors affecting opportunity and availability are examined, including the role of storms and riverine flooding. Some sediment is still being deposited in Louisiana’s coastal marshes but it may not be sufficient to prevent further coastal land loss.