Subsidence in the Mississippi River Delta--Important Influences of Valley Filling by Cyclic Deposition, Primary Consolidation Phenomena, and Early Diagenesis

Abstract

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.