| Summary: | Sidescan Sonar, chirp sonar sub-bottom profiles, and grab samples were collected on the north-central Gulf of Mexico continental shelf as part of an interdisciplinary study mapping juvenile red snapper habitat. Demarcation of essential fish habitat for juvenile red snapper (Lutjanus campechansis) in the Gulf of Mexico is considered critical for effective management of this valuable species. The first goal of this study was to map and describe the geology of this region. The second goal was to attempt to relate variations in geology to juvenile red snapper abundance and distribution.
Sidescan mosaics were created for ten polygons, ranging in size from 2 to 20 km<sup>2</sup> on the inner to middle shelf south of Mississippi-Alabama, in water depths of 17-40 m. Geological observations delineated three contrasting seabed types: (1) linear to patchy shell regions on the inner-middle shelf, (2) muddy sand sheets on the middle shelf, and (3) prodeltaic muds in the southwest of the study area, marking the eastern extent of recent shelf deposits from the modern Mississippi delta. The shell ridges stand 1-3 m above the surrounding seabed, and may extend 200 m across. They are composed of > 50% CaCO<sub>3</sub>, including shell fragments from both estuarine and marine taxa, and contrast sharply with adjacent muddy sands containing minor shell. Radiocarbon dating of shell material, along with the geological characteristics of the ridges suggests that they are remnants of Holocene coastal environments.
This region was previously described as either an extension of the MAFLA sand sheet or a transitional zone between the MAFLA sands and prodelta muds (Ludwick, 1964). In the present study, we identified a range of geological features of estuarine, shoreface, and wholly marine origin. The diversity of deposits described records a wide range of geological processes active from early-middle Holocene to recent time. The integration of geological observations with coordinated biological observations reveals that geologic structures and sediment composition on the northern Gulf of Mexico continental shelf are major controls on the distribution of juvenile red snapper (Patterson et al, in press), and record both coastal depositional histories and open-shelf processes active during Holocene transgression.
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