Randolph A. McBride

Origin, evolution, and distribution of shoreface sand ridges, Atlantic inner shelf, U.S.A.

Abstract A computer mapping system was employed to document the location of 259 shoreface-attached and detached sand ridges in water Although shoreface-attached sand ridges appear to have several different modes of formation, a two-step process for the development of most shoreface-attached and detached sand ridges along U.S. Atlantic barrier island and cape coastlines is proposed: (1) sand is deposited as ebb-tidal deltas or river deltas along the lower shoreface and/or inner continental shelf prior to or during transgression, followed by (2) further transgression, which reworks the deltaic sand bodies into linear sand ridges at the base of the shoreface by shelf processes. The best-developed shoreface sand ridge fields along the U.S. Atlantic shelf lie adjacent to shorelines characterized by all of the following: (1) transgression, (2) mixed energy, wave-dominated barrier islands, and (3) laterally migrating tidal inlet systems. Tidal inlet systems are natural sediment sinks that capture sand carried by longshore sediment transport. Ebb-tidal delta deposits associated with these migrating tidal inlets provide the initial sand body for the development of shoreface-attached sand ridges. The oblique orientation and linear form of shoreface-attached sand ridges appear to be a function of shoreline transgression, lateral inlet migration, and wave reworking of ebb-tidal delta deposits concentrated along an ebb-tidal delta retreat path. Shelf processes act as modifying agents in the evolution of sand ridges during and after ebb-tidal delta deposition. In general, shoreface-attached sand ridges are poorly developed or absent along eroding headlands, spits, and barrier island shorelines characterized by naturally stabilized tidal inlets. These latter inlets and shoreline types are dominated or influenced by one or all of the following: (1) antecedent topography, (2) higher tidal ranges, (3) larger tidal prisms, (4) lower wave energies, or (5) finer grain sizes. The coupling of shoreline and shallow marine sedimentary processes during a transgression is critical to the origin, evolution, and distribution of shoreface sand ridges in the study area. Modern shoreface-attached sand ridges are also known to occur in different coastal and shelf settings where large amounts of sediment were supplied to the shoreface and inner shelf during, or immediately before, transgression. Commonly observed vertical and lateral interrelationships of Holocene shoreface-attached sand ridges and tidal inlets or distributary channels have important ramifications for the development of shelf sandstone facies models. In addition, the geometric relationship documented in this study provides predictive petroleum and hard mineral exploration models of the spatial and temporal distribution of shoreface and shelf sand ridges.

GEOMORPHIC RESPONSE-TYPE MODEL FOR BARRIER COASTLINES : A REGIONAL PERSPECTIVE

Based on quantitative documentation of historical changes in shoreline position between 1847 and 1991, eight geomorphic response-types were established for classifying megascale changes along barrier coastlines: (1) lateral movement, (2) advance, (3) dynamic equilibrium, (4) retreat, (5) in-place narrowing, (6) landward rollover, (7) breakup, and (8) rotational instability. Long-term (decades to centuries) monitoring of shoreline position over a spatial scale of 10 to 100 km provides a scientific basis for documenting process-response relationships that shape regional coastal morphodynamics. Although megascale shoreline change studies often are lacking, this type of information is critical for developing realistic research and management strategies regarding form/process relationships in coastal depositional systems. The spatial distribution of geomorphic response-types is delineated along the barrier coastlines of Louisiana, Mississippi, and southern Georgia/northern Florida. At megascale, the rate of relative sea level rise along these barrier coastlines appears to be one of the major factors controlling the occurrence of geomorphic response-types; however, sediment supply exerts significant influence on shoreline response as well.

Reduced seasonality of Holocene climate and pervasive mixing of Holocene marine section: Northeastern Gulf of Mexico shelf

The large porcelaneous foraminifers Cyclorbiculina compressa, Parasorites orbitolitoides , and Peneroplis proteus are conspicuous in death assemblages from Holocene marine sediments of the Alabama and Florida panhandle shelf. The species inhabited the northeastern Gulf of Mexico in the Holocene (ca. 6.4–1.9 ka) but do not live in the region today. These foraminifers require warm, clear waters, and thus are important paleoclimatic and paleoenvironmental indicators. They apparently were derived from sublittoral seagrass habitats and indicate reduced seasonality in the region during the middle to late Holocene. In addition, a mixed foraminiferal fauna, a hydrodynamically reworked macrofaunal assemblage, and stratigraphic disorder in foraminiferal 14 C dates indicate extensive reworking of the entire Holocene marine transgressive package. Evidence that mollusks are indigenous but large foraminifers are transported supports the generalization that out-of-habitat transport of macrofauna is negligible in most marine settings.

Taphonomic and paleoenvironmental evidence of Holocene shell-bed genesis and history on the northeastern Gulf of Mexico Shelf

Subsurface shell beds that are common on the eastern Alabama/western Florida Panhandle shelf provide paleoenvironmental and taphonomic data that demonstrate a dynamic depositional history with reactivation and amalgamation to the base of the Holocene marine transgressive package. In most of the ten shell beds we examined, shallow-marine mollusks, large soritid foraminifera, and cupularid bryozoans are common. Bioclasts occur in all preservation states, but a majority are pristine or only slightly altered. Normal grading, and concave-up, stacked, and random fabrics are common. In three of these shell beds, a relict estuarine component characterized by poorly-preserved Chione cancellata can be detected. Three other shell beds contain 1 of 2 estuarine molluscan assemblages. One assemblage is characteristic of fine-grained and the other of coarser-grained substrates. In all estuarine shell beds, bioclast preservation ranges from excellent to mixed, and bioclasts have random fabrics and locally are normally graded. Most molluscan remains are indigenous and assemblages range from within-habitat time-averaged to environmentally condensed. Shell beds are thick (up to about 75 cm) and overlie bay or shoreface ravinement surfaces, indicating that shell beds first accumulated as coarse transgressive deposits, and are composite concentrations modified by multiple events. Shell beds, however, are amalgamated. Only fabrics reflecting the final modifying events are preserved, and evidence of previous accumulation processes are obliterated. For marine shell beds especially, these final agents were, and possibly continue to be, highenergy events such as storms (winter cold fronts, hurricanes) or currents associated with Loop Current eddies. Episodically high sedimentation rates associated with these events may have acted as a buffer, preventing a long history of reworking and exposure (thus reducing shell alteration) typical of transgressive lags. Our results indicate that: 1) within-habitat time-averaged and environmentally condensed assemblages can be distinguished by combining taphonomic and environmental data, even when the ranges of environmentally disparate species overlap, 2) transgressive-lag deposits can be composed of well-preserved bioclasts, and 3) transgressive lags can be reworked and amalgamated but still be recognizable on the basis of their stratigraphic context.

Late Holocene record of community replacement preserved in time-averaged molluscan assemblages, Louisiana chenier plain

Late Holocene relict shorelines of the southwestern Louisiana chenier plain contain molluscan assemblages that vary greatly in taxonomic composition and bioclast preservation. Taxonomic composition varies with ridge age: older ridges are oyster rich, whereas younger ridges are dominated by infaunal bivalves. Taphonomic features can be separated into those caused by biostratinomic and those caused by pedogenic processes. Pedogenic alteration generally increases as ridge age increases, whereas biostratinomic alteration reflects the prevalence of reworked bioclasts in assemblages. These molluscan assemblages are extensively time averaged, causing temporal overcompleteness of depositional units (i.e., amount of time averaging for bioclasts within a unit is much greater than the time it took for that unit to form). Chenier-plain progradation over the last 3,000 years both caused and preserved the observed trend in community composition. This trend was caused by community replacement related to changing substrate stability and by changes in the source of reworked bioclasts, both of which operated in response to progradation. Net progradation also allowed this trend to be preserved because time averaging occurred episodically and shorelines were effectively separated into discrete generations. Although coastal deposits are not typically viewed as ideal sites for high-resolution paleoenvironmental studies, millennial-scale community trends can be detected in this setting.

Geophysical Techniques for Evaluating the Internal Structure of Cheniers, Southwestern Louisiana

ABSTRACT Historically, Louisiana chenier plain studies have consisted of surficial mapping, sediment analyses, cores, and direct observation of limited exposures from borrow pits. Ground penetrating radar (GPR), a shallow geophysical technique, provides high resolution profiles of contrasts in electrical permittivity that can be correlated with sedimentary layers in sand and gravel depositional environments not effected by salt water. Reconnaissance vertical electrical soundings (VES) were performed at several sites to determine the applicability of GPR on the chenier plain. Based on resistivity data, three field sites were targeted for GPR profiling. GPR profiles were collected using a Geophysical Survey Systems, Inc. Subsurface Interface Radar 10A unit with a 500MHz antenna on a grid designed to assess three-dimensional variability in subsurface characteristics of each chenier. Topographic corrections were applied for each processed profile to adjust for surface elevation differences that effect reflection orientation. At selected chenier locations, subsurface reflections reveal a complex internal structure within these clastic deposits. However, at the interface with adjacent or subsurface fine-grained deposits, clay and/or conductive pore water limit penetration. Electrical resistivity proved to be a valuable reconnaissance tool for determining applicability of GPR, and VES models correlated directly with vibracore data. Preliminary interpretations of radar profiles suggest cheniers evolve through multiple processes in different environments. Radar facies observed in dip-oriented profiles at two sites are described as landward stepping foresets and may be interpreted as transgressive washover deposits. Bedset orientation of strike-oriented profiles indicates a westward component of migration. At a third site landward and seaward dipping reflections, interpreted as upper shoreface and washover deposits, indicate shifting shoreline position related to transgression and regression associated with variations in sediment supply.

IMPLEMENTING THE SHORELINE CLEANUP ASSESSMENT TEAM PROCESS IN THE GULF OF MEXICO

ABSTRACT Louisiana State University (LSU) and Woodward-Clyde Consultants are working with state and federal agencies, and industry through the LSU Gulf of Mexico Oil Spill Research and Response Program to establish a shoreline cleanup assessment team program (SCAT) in the Gulf of Mexico. Each SCAT team consists of a coastal geomorphologist and ecologist (and archaeologist when appropriate), as well as representatives from the responsible federal, state, and private agencies. This cooperative effort is aimed at identifying oil spill impact and interagency coastal resource concerns and recommendations, and developing a cleanup strategy based on interagency cooperation and concurrence within a systematic and standardized framework. The SCAT program provides interagency coordination, SCAT preparedness, spill drill participation, interagency training, geographic information systems services, monitoring, and routine aerial videotape surveys. It also offers technical support to the decision-making process within...

SUBSIDENCE-CORRECTED ELEVATIONS OF PALEOSHORELINES (CHENIER RIDGES) OF THE MISSISSIPPI RIVER CHENIER PLAIN, USA: IMPLICATIONS FOR FORCING MECHANISMS

Chenier ridges along the Louisiana Chenier Plain formed over the past 3,000 years as a function of sediment supply, incident wave/current energy, and relative sea-level change. Spatial variations in ridge elevations suggest that geologic-scale processes, such as sea-level changes and variations in sediment supply, produce distinct morphologies. The exact process by which ridge morphologies evolve is not well understood, primarily due to complexities associated with upland sediment sources (i.e., variations in magnitude and composition of sediment sources) and redistribution along the coast. Relative sealevel changes add another level of uncertainty regarding geomorphic evolution of coastal ridge deposits. Subsidence-corrected chenier ridge elevations for four primary ridge trends (paleoshorelines) along the Louisiana Chenier Plain illustrate a maximum elevation difference of approximately 1 m over a maximum period of about 1,700 years, possibly reflecting short-term variations in relative sea level.

Abstract: Holocene Sedimentary Framework of Transgressions and Regressions Along the Chenier Plain, Southwestern Louisiana

ABSTRACT The coastal zone of southwestern Louisiana is the product of regressions and transgressions in response to complex interactions among marine, riverine, aeolian, and storm processes. The two primary components of the chenier plain, a marginal deltaic environment, are sand/shell ridges, or relict beaches, and extensive inter-ridge marshes. Previous studies have interpreted fine-grained prograding deposits of the chenier plain to be the result of high sediment loads from the Mississippi River when the primary discharge pathway is to the west. Conversely, cheniers are thought to have formed as the result of delta lobe switching of the Mississippi River from west to east, causing decreased sediment supply to the chenier plain and enabling wave activity to erode deposits along the shoreline and concentrate sand/shells into ridges. Although information exists regarding the general sedimentary framework of the study area, questions remain regarding the timing of deposition and its relationship to switching of the Mississippi River and distributaries, sediment source, and spatial variations in sedimentary facies as a function of sediment source and environment of deposition. Approximately 30 vibracores (up to 9 m long) were obtained along the central chenier plain that extend from the upper continental shelf (5 m water depth) north to the Holocene-Pleistocene margin of the coastal plain. Most cores contain the entire Holocene record of sediment deposition and penetrate the pre-Holocene surface as a basal unit. At least six distinct sedimentary facies characterize subsurface deposits, ranging from clean sandy/shelly units (1 to 4 m thick) to interbedded sandy muds (up to 4 m thick) to fine laminated clay (1.5 to 3 m thick). Depth to the pre-Holocene surface shallows northward and varies between 5 and 7 m near the present coastline. Spatial variability in the thickness and extent of specific units varies depending on sediment source and antecedent topographic control. A transition from predominantly estuarine to marine macrofossil assemblages (north to south) is accompanied by an increase in the extent of relict beach deposits, some ranging up to 4 m thick and 200 m wide. Although the Mississippi River has provided important input of fine-grained material to the chenier plan, local sediment sources, including the concentration of sand and shell from eroding marsh/mudflat deposits during transgressions, have contributed substantially to chenier evolution. End_of_Record - Last_Page 472-------

LCGISN GeoSpatial Search Program; user manual

A brief narrative summary of the data set. The abstract should provide information about the sources and processing of the data set. The abstract, along with a statement of the purpose or intended use of the data, provides a complete overview of the data. access The process of obtaining data from or placing data into storage in a computer. Access Constraints Restrictions and legal prerequisites for accessing the data set. These include any access constraints applied to assure the protection of privacy or intellectual property, and any special restrictions or limitations on obtaining the data set. Access Instructions Instructions on the steps required to access the data set. access time A measure of the time interval required for the data to be retrieved from storage and delivered to the user. Indicating the size of the file (in kilobytes) provides the user with an idea of the relative access time. accuracy The closeness of the results of observations, computations, or estimates to true values or values accepted as being true. Accuracy relates to the quality of a result and is distinguished from precision, which relates to the quality of the operation used to obtain the result. accuracy assessment Comparison of a classification to geographical data assumed to be true. Usually, the assumed-true data are derived from ground truthing. accuracy report A list of the percentages of accuracy computed from the error matrix in a classification accuracy assessment. ACSM American Congress on Surveying and Mapping. address See Internet Protocol (IP) address and Universal Resource Locator (URL). ADP Automated data processing. ADPE Automated data processing equipment. AEC Army Environmental Center. aerial photograph Generally, any photograph of the terrain taken with a camera mounted in an aircraft. Aerial photography is called simply Photography in LCGISNs data base. aerial photographs, composite Aerial photographs made with a camera having one principal lens and two or more surrounding and oblique lenses symmetrically placed. The several resulting photographs may be rectified in printing to permit assembly as verticals with the same scale.