Establishing the Depositional History within the Trinity-Tiger Shoal Complex Region: A Transgressed Delta Complex of the Early-to-Middle Holocene Mississippi Delta

The Mississippi Delta is arguably the world’s most studied deltaic system. Nevertheless, its Holocene geochronology below the delta complex scale, particularly for the earlier deposits, is surprisingly poorly developed. Previous research focusing on the delta’s depositional history has predominantly occurred within the present-day subaerial delta, whereas comparatively little research has been attempted downdip within the offshore deltaic deposits that comprise the transgressed early-to-middle Holocene delta (i.e. Maringouin and Teche Delta Complexes). Such inconsistency is consequent to logistical challenges in accessing offshore deposits. This study advances the Mississippi Delta’s early-to-middle Holocene geochronology by examining deltaic sediments within the Trinity-Tiger Shoal Complex region, which presently resides ~ 30-40 km offshore of central Louisiana. Based on its geographic position, some studies have argued that Trinity Shoal is the transgressed shoreline expression of the Maringouin Delta Complex, whereas other studies have suggested a Teche origin based on its bathymetric position. However, neither argument is based on radiocarbon dating. We gathered ~ 1,350 km of high-resolution subbottom sonar profiles (chirp seismic) and 60 vibracores across the study area. Cores have been logged, imaged, X-rayed, and described, and discrete samples (i.e. Mulinia lateralis and foraminifera) are being radiocarbon dated. All data are being integrated, and an exhaustive depositional model is being developed. Preliminary results depict partial elements of two conformable deltaic parasequences: prodelta and delta-front facies, along with a southwest trending distributary system that fed these regressive depositional environments, underlie distinct sand shoals and other transgressive sediments. Twelve radiocarbon dates measured thus far from the transgressive component illuminate an intricate aggregation of relatively recent deposits (hitherto < ~ 1,500 cal yr BP). Based upon the prevailing model for the evolution of transgressive depositional systems in abandoned delta complexes, these relatively recent deposits could support the hypothesis that the underlying regressive sediments are of Teche origin. However, two radiocarbon dates obtained from the prodelta environment were measured no younger than ~ 5,600 cal yr BP, which suggests that these regressive deltaic deposits are actually a part of the final progradational stage of the Maringouin Delta Complex.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California