Print this pageReinterpretation of a Mobile River Shelf Edge Delta (Lagniappe Delta): The Important Influence of Climate and the Loop Current
ROBERTS, R. FILLON, B. KOHL, and J. SYDOW
Preceding the latest Pleistocene glacial maximum (approx. 18 ky BP) the Mobile River entrenched a fluvial delivery system and established a shelf edge deltaic depocenter to the east of the modem Mississippi River delta lobe. Over 4500 km of high resolution 2-D seismic data, 3-D seismic imagery, and 4 continuous borings through this delta complex have provided data for determining the seismic stratigraphic architecture, lithostratigraphic and biostratigraphic relationships, and environmental conditions under which this delta complex was deposited. Deltaic deposits are organized into two major lobes both of which exhibit prograding clinoform internal geometries and a classical coarsening upward sedimentary sequence entrenched by fluvial channels containing a coarse sand-to graveliferous fill. Late falling stage foraminiferal faunas from the Main Pass 303 boring (western lobe) record deposition under low salinity-to-fresh water conditions, while those from the Main Pass 288 boring (eastern lobe) record post-glacial maximum deposition in a marginal marine setting dominated by frequent episodes of normal salinity sea water interpreted as Loop Current intrusions. Hemispheric climate during glacial build-up, faunal data, and numerical oceanographic models suggest that late falling stage conditions were dominated by frequent northwesterly frontal passages and a general eastward flow at the shelf edge which helped maintain low salinity water of Mississippi origin along the coast. Following onset of vigorous temperate deglaciation persistent southeasterlies replaced northwesterlies as the glacial high pressure cell broke down. Our faunal data support this transition which pushed the low salinity coastal band westward allowing warm saline water from Loop Current intrusions to invade shelf edge environments. Absence of a broad shallow shelf caused by lowered sea levels effectively inhibited seasonal shelf watermass cooling thus promoting warm water temperatures in the prodelta zone of low stand deltas.