Facies Interpretations in Mississippi Delta Borings

A study has been made of samples cut from borings drilled by the Corps of Engineers between New Orleans and the Chandeleur Islands and extending to depths of 60 to 180 feet in the now-abandoned St. Bernard subdelta of the Mississippi River. Interpretations of depositional fades are based on criteria obtained in the study of modern deltaic sediments in and around the presently forming birdfoot delta. The Foraminifera were particularly useful as environmental indicators, but grain-size and coarse-fraction analyses also were diagnostic in many places. It was possible to correlate many of the facies between borings. The resulting section shows the transgression of Holocene (Recent) marine sediments over an eroded Pleistocene delta. During this initial marine encroachment there was deposition of relatively thin shelf sands containing abundant shells and Foraminifera. Later the Mississippi River shifted its locus of deposition, and the St. Bernard subdelta prograded across the shallow shelf. During this stage the rate of deltaic sedimentation was greater than the rate of rise of post-Pleistocene sea level, resulting in an apparent regressive facies formed during a eustatic transgression. Subsidence of the subdelta andits abandonment by the Mississippi River have permitted the sea to attack and rework the seaward portions with the attendant formation of discontinuous barrier islands and shallow- water sound deposits.