ABSTRACT

Late Pleistocene fluvial entrenchment, cyclic deposition during the Holocene rise in sea level, and primary consolidation characteristics of valley-fill facies are key factors in understanding subsidence and land loss in the Mississippi River delta plain. Recent investigations complemented with seven continuous borings (to >60-m [197-ft] length) have emphasized the importance of Holocene sedimentation history, sediment type, early diagenetic changes, and consolidation characteristics of the Holocene sediment column. Radiocarbon dating of organic units in borings indicates a low rate of subsidence (~9 cm [3.5 in]/100 yr) on the flanks of the alluvial valley where the Holocene sediments are thin and a high rate of subsidence (>40 cm [15.6 in]/100 yr) where these sediments are the thickest. Types of sediment and modes of Holocene deposition are also important. Upward-coarsening cycles of various dimensions (small crevasses to delta lobes) are nested in the valley-fill deposits. They reflect rapid depositional pulses separated by organic-rich deposits and surfaces of erosion associated with delta-lobe and subdelta abandonment. Individual facies of the Holocene valley fill contain unique pore space and thereby affect primary consolidation.

Siderite and pyrite occur in delta-front and salt-marsh deposits where salt- and fresh-water environments interface. The following factors account for a significant proportion of delta-plain subsidence and land loss: (a) thickness of Holocene deposits, (b) facies type and stacking characteristics, (c) primary consolidation characteristics of fundamental facies units, and (d) early diagenetic history.