Identifier

etd-04092013-134044

Degree

Doctor of Philosophy (PhD)

Department

Oceanography and Coastal Sciences

Document Type

Dissertation

Abstract

The Holocene Mississippi River Delta is arguably the world’s most thoroughly researched deltaic system. However, much of this research has occurred predominantly within the present-day subaerial delta, whereas comparatively little research has been attempted downdip within the offshore, more difficult-to-reach parts. This study advances our understanding of the Holocene delta by examining deltaic sediments within the Trinity-Tiger Shoals Complex region, located ~ 30 – 40 km offshore of central Louisiana. Specifically, this study addresses two questions. Which delta complex prograded into this offshore region? Do the overlying transgressive deposits reflect that predicted by the prevailing transgressive depositional systems model? To answer these questions, this study uses an integrated data set consisting of ~ 1,350 km of high-resolution subbottom sonar profiles and various sedimentological data gleaned from 60 vibracores, including 22 AMS radiocarbon dates. Results from this study depict an intricate stacking arrangement of 12 distinct depositional units. Near the base of the stratigraphic framework above antecedent topography are the remnants of a middle Holocene Mississippi Delta complex. Prodelta and delta-front facies along with the partially-preserved southwest trending distributary system that fed these depositional environments are recognized. Furthermore, analysis of the regressive component reveals an offsetting, overlapping, and stacked arrangement of four delta lobes and one subdelta. Five AMS radiocarbon dates (~ 4,820 yr BP to 5,980 yr BP) measured from these regressive sediments confirm a Maringouin Delta Complex origin. A ~ 4,000 yr hiatus ensued following the abandonment of the Maringouin Delta Complex, as indicated by 17 AMS radiocarbon dates measured from the overlying transgressive component. During this hiatus, erosional shoreface retreat produced a prominent ravinement surface along the entire top boundary of the Maringouin Delta. It was not until ~ 1,000 yr BP that significant amounts of sediments began migrating into the study area from the east, which have remained, at least for the time being, mostly preserved. Among these recent post-Maringouin depositional units are a detached down-drift subaqueous delta related to the Lafourche Delta Complex as well as the most-recent sandy deposits of Trinity Shoal and an unnamed shoal. This new understanding of post-Maringouin deposition is considered incompatible with the prevailing transgressive depositional systems model, particularly stage 3 of that model.

Date

2013

Document Availability at the Time of Submission

Release the entire work immediately for access worldwide.

Committee Chair

Roberts, Harry

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