Abstract: Water Clarity Near Oil Production Platforms on Louisiana Continental Shelf and Source of Turbid Bottom Water Layer

George M. Griffin

Water clarity and particulate matter in the South Timbalier outer continental shelf area off central Louisiana were studied from 1972 to 1974 to determine whether or not oil production platforms produce a detectable impact on turbidity of continental shelf waters. Turbidity of surface waters varied seasonally from a mean of 0.8 mg/l in autumn 1972, to 5.3 mg/l following the Mississippi River flood of 1973. Mid-depth waters were the clearest, typically containing 0.15 mg/l. A turbid bottom water layer persisted throughout the period; turbidity in this layer ranged from 1.5 to > 10 mg/l. No differences in turbidity existed between the area of numerous production platforms and an undeveloped control area.

The clay minerals suspended in the surface, mid-depth, and bottom water layers were quantitatively similar to those in the Mississippi River, and dissimilar to the cohesive surface of the shelf sediments. Therefore, the bottom turbid layer is not generated by erosion of the cohesive bottom muds, but by the settled residuum of Mississippi River suspended sediment. The turbid bottom water layer is actively by-passing the shelf at present. A process model is outlined for the transport path of the suspended river sediment from the turbid surface plume, through the mid-depth water, into the turbid bottom layer; there it migrates toward shore, probably in a series of resuspensive pulses, merges with a surf-generated littoral turbid layer, and ultimately is trapped by vegetation in the progr ding chenier plain of southwestern Louisiana. Offshore production platforms appear to have no detectable effect on this process or on turbidity of the central Louisiana shelf.

AAPG Search and Discovery Article #90965©1978 GCAGS and GC Section SEPM, New Orleans, Louisiana