The modern Mississippi drainage system has an area of over 3.3 million square kilometers, an order of magnitude greater than the area of the Rio Grande or Brazos drainage basins (Figure 1). The Mississippi drainage basin includes areas that have been glaciated during the last several glacial cycles. These two factors imply that the Mississippi responds to variable sea level and ice cover in the northern hemisphere that is different than that of the Rio Grande or Brazos systems. This study will attempt to isolate and map a single lobe of the Mississippi from the last highstand, determine the sediment type which constructed the delta and the timing of that construction and the timing of that construction and then compare this to other systems. The size of the deltas created by the abundant data available on the Louisiana continental shelf give the control needed. Preliminary results show a progression from a sandy delta in the beginning of the highstand, early stage 5 (Figure 2), to a muddy delta in the late stages of falling sea level, late stage 3 (Figure 2).

In the fall of 1997 approximately 300 km of high-resolution seismic data was collected on the R/V Lone Star (Figure 3). This data was collected to the north of a dataset which has been donated by Texaco that includes approximately 2,400 km of high-resolution, analog seismic data. Twenty-eight platform borings within the study area have been donated by Fugro-McClelland; many more borings as well as tens of thousands of boring descriptions are available. To the southwest of the study area a thesis has been completed (Figure 3) that has mapped a muddy delta that prograded to the west during the middle to late stages of falling sea level (Sarzalejo, 1993). Sarzalejo (1993) called this system the Western Louisiana delta since there was an uncertain source location. To the north of the study area Coleman and Roberts (1991) have mapped a large delta system based on platform boring descriptions. They assumed that the delta was older than stage 5; since sea level was low during the period of time they assigned to the delta, which sits on the inner shelf, we have assumed that it may in fact be the early stage 5 delta. The dataset for this study falls between the Coleman and Roberts (1991) delta and that mapped by Sarzalejo (1993); this study aims to connect the two study areas, to determine if one delta lobe can be followed from the Coleman and Roberts (1991) delta, through the study area, and connect with the Western Louisiana delta.

Preliminary examination of the seismic data shows a large delta system delta which downlaps onto a flooding surface; this surface can be correlated to Sarzalejo's (1993) stage 5 maximum flooding surface. The extent and orientation of this delta is controlled in large part by the existence of many diapirs on the outer shelf, but a progradation can be mapped form NE to SW. The seismic facies show an overall progradation from sandy, mouth-bar facies in the north, to the more dominant distal, prodelta facies in the south (Figure 4). If the sand-rich delta mapped by Coleman and Roberts (1991) to the north is continuous with the mud-rich delta in the study area, then a change in sediment type occurred as sea level fell, and the drainage basin became glaciated.

During the late highstand (Stage 3) the western Louisiana fluvial system constructed a sizable delta on the outer shelf, originally mapped by Suter and Berryhill (1985). Sarzalejo (1993 studied this delta in detail using a close grid of high resolution seismic data and platform borings from the area. Her work showed that the delta was a fluvial-dominated one, with a large feeder channel that branched into several distributaries that fed four large sandy mouth bars. Seismic profiles from the area show the main feeder channel (Line 17), distribuary channels with their chaotic seismic facies incising into acoustically layered muddy prodelta deposits (Line 18), and Line 16 shows to offlapping delta lobes. Upon reaching the shelf margin, the delta was forced to prograde to the west behind a nearly continuous salt ridge at the shelf break. When the shoreline reached the shelf break (Stage 2 lowstand), the delta was abandoned and the upper slope experienced massive failure (Line HI-18)

For a more detailed discussion see Wellner et al (2004).

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