Bottom Morphology and Shallow Subbottom Sedimentary Structures on U.S. Continental Shelf Between Cape Hatteras, North Carolina, and Jacksonville, Florida: ABSTRACT

Abstract

During the period July 1978 to 1980, a survey of the U.S. South Atlantic shelf was made to determine the occurrence and distribution of biologically productive bottom areas and bottom and shallow subbottom geologic features of potential hazard to petroleum exploration and production. Methods of data acquisition included the use of high-resolution seismic systems, sidescan sonar, towed television, and minisub. Shelf sediments are largely composed of 2 to 5 m of Quaternary sands and silty clays with mixtures of shell that were deposited in fluvial, paralic, and shallow-marine environments. Greater sediment thicknesses (up to 40 m) are present in buried river channels, tidal inlets, and cut-and-fill structures associated with meandering tidal streams. Surficial sediments exhibit a wide range of bed-form type and maturity. Ripples of less than 0.5 m wavelength and megaripples of 0.5 to 1 m wavelength are common across the shelf. Crest orientation is generally north-south. Algal growth along the crests of many of the smaller bed forms as well as abundant bioturbation and tracks indicate bottom stability except during storms. Sand waves up to 100 m long are common, particularly within the 25-m isobath and are present in discrete fields 3 km or more wide with north-south crest orientation. They are less common, less sharply defined offshore and usually underlie smaller bed forms of different orientation. These features may have resulted from infrequent major storms and, because of rising sea level, may now be below the effect of most s orm-generated waves. Although the mobility of these large bed forms is inferred by the presence of buried reefs within some of the sand bodies, the time and rate of movement are unknown. Many areas of the shelf exhibit irregular, low-relief bed forms that appear to be in advance stages of deterioration from intense bioturbation. Several distinct, acoustcally hard reflectors present in the shallow subbottom over much of the shelf represent indurated strata which on outcrop provide substrate for the development of reef and hardground communities. These temperate bioherms are primarily populated by sponges and octocorals and, on the basis of topographic expression, are classified as low-relief hardgrounds (> 0.5 m), moderate-relief reefs (up to 2 m), and high-relief reefs (up to 15 m). Low-relief hardgrounds are the most widely distributed of the three types but are the least productive in terms of biomass. Moderate- and high-relief reefs occur most commonly off North and South Carolina and along most of the shelf break. Substrate ranges from Pleistocene to Miocene in age and locally contains significant perc ntages of phosphate grains. The total area of shelf occupied by reefs and hardgrounds is estimated to be less than 10%. Outcrop patterns reflect both local bottom processes and regional structural framework. In the middle to outer shelf off the Georgia coast, a relatively narrow zone of large channels and numerous cut-and-fill structures occur in Quaternary deposits just landward of a shallow buried Miocene high. These features are very similar to sedimentary structures associated with present-day barrier islands, tidal inlets, and estuaries and strongly suggest a sea-level stillstand near the end of the last Pleistocene regression or early in the present transgression. End_of_Article - Last_Page 1663------------