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

Harry H. Roberts, Gerald J. Kuecher, Alan M. Bailey

Late Pleistocene fluvial entrenchment, cyclic deposition during the Holocene rise in sea level, and primary consolidation characteristics of the fundamental valley-fill facies units are key factors in understanding subsidence and land loss in the Mississippi River delta plain. Recent investigations complemented with 17 continuous borings (to >60 m 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/100 yr) on the flanks of the alluvial valley where the Holocene sediments are thin and a high rate of subsidence (>100 cm/100 yr) where these sediments are the thickest Types of sediment and modes of Holocene deposition are also important. Coarsening-upward 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 (ravinements) associated with delta-lobe and subdelta abandonment. Individual facies of the Holocene valley fill contain unique diagenetic and geotechnical properties. Syndepositional diagenetic minerals occlude pore space and thereby affect primary consolidation. The greatest diversity and frequency of occurrence of diagenetic products occurs in the slowly deposited facies, especially freshwater swamp deposits. However, siderite and pyrite occur in delta-front and salt-marsh deposits where salt and fre hwater environments interface. Primary consolidation of Holocene deposits occurs primarily in the first few meters of the sediment column (>90% in the top 10 m). Results indicate that sands accumulate rapidly, do not readily consolidate, and form loads that may deform underlying prodelta clays that tend to be underconsolidated and have high pore-water contents. Both these clays and highly organic deposits (peats and near peats) are subject to drastic volume changes upon loading. These factors--thickness of Holocene deposits, facies type and stacking characteristics, and primary consolidation characteristics of fundamental facies units and, to a lesser extent, diagenetic history--account for a significant proportion of delta-plain subsidence and land loss.

AAPG Search and Discovery Article #90983©1994 GCAGS and Gulf Coast SEPM 44th Annual Meeting, Austin, Texas, October 6-7, 1994