Holocene Stratigraphic Architecture of a Sand-Rich Shelf and the
Origin of Linear Shoals
Authors: Mcbride Randolph A. , Anderson Laurie C. , Tudoran Andrei , Roberts Harry H. 
Published: November 2010

Late Pleistocene and Holocene geology of the northeastern Gulf of Mexico shelf offshore Alabama and northwest Florida was investigated using 47 vibracores, foraminiferal and macrofaunal assemblages, and bathymetric data. The morphologic and stratigraphic signatures of the last rise of eustatic sea level were examined along this passive continental margin characterized by low subsidence. Major shelf features include shore-oblique sand ridges, mid-shelf linear shoals, and shelf-edge deltas. Surficial shelf sediments consist of &gt;90% sand, &lt;2.7% mud, and &lt;2% granules and fine in a westerly direction from medium to fine sand. The sharp boundary that separates these two surficial sand types (Apalachicola and Mobile subprovinces) is identified for the first time in this study.

Six facies and two erosional surfa ces characterize the shelf stratigraphy. Facies 1 is a Pleistocene soil horizon. This facies is truncated by a major erosional unconformity (Type 1 sequence boundary [SB]) that was created by subaerial exposure during the last sea level lowstand and during the bay ravinement process (flooding surface [FS]) of the ensuing transgression (FS/SB). Fine-grained estuarine deposits (Facies 2, 3, or 4 [lower transgressive systems tract]) overlie the unconformity. Facies 3 or 4 are truncated by a shoreface ravinement diastem (flooding surface) and are overlain by a marine shell-bed (Facies 5; lower shoreface). Facies 5 grades into Facies 6, a quartz sand with open marine foraminifera that re presents a shelf sand sheet Facies 5 and 6 comprise the upper transgressive systems tract, which is up to 5.5 m thick.

The mid-shelf is characterized by two long (30-120 km), narrow (&lt;6 km), shore-parallel to subparallel sand shoals that average 4 m thick. North Perdido Shoal is located 15-25 km offshore at the 20-25-m isobath, whereas South Perdido Shoal lies 20-70 km offshorc at approximately the 35-m isobath. Both shoals trend southwest-northeast. The linear shoals are not in situ or degraded barriers (Stubblefield et al., 1984a, b), offshore shelf-ridge (bar) complexes (Tillman and Martinsen, 1984, 1987; Gaynor and Swift, 1988), or lowstand/transgressive incised shoreface deposits (Bergman,1994; WalkerandWiseman,1995) because the sediments that comprise the shoals Jie above the shoreface ravincment diastem, and open marine species dominate the foraminiferal and molluscan assemblages. Although shelf morphology is similar to modern barrier island geo morphology, shelf morphostratigraphy is related to transgressive and post-transgressive processes. Shoal form and orientation are dictated by underlying transgressive topography (escarpments) that was cut into the Pleistocene substrate during the post-glacial transgression. During transgression, erosional shoreface retreat produced a trailing sand sheet that draped the transgressive topography. Consequently, 1) the linear nature of the shoals is derived from their formation along the shoreface (i.e., depositional strike) at lower stands of sea level during an overall transgression; 2) sediment transport from the present shoreline across the shelf appears to have little influence on shoal development; 3) the interplay between relative sea-level changes and sediment supply caused translation of the shoreface profile, thus dictating the position of the linear shoals; and 4) post-transgressive reworking and subaqueous landward migration in response to storm processes are integral parts of shoal evolution.