Surficial sediment character of the Louisiana offshore continental shelf region: A GIS Compilation
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Nomenclature describing sediment texture distributions is important to geologists and sedimentologists because grain size is the most basic attribute of sediments. Traditionally, geologists have divided sediments into four size fractions that include gravel, sand, silt, and clay, and classified these sediments based on ratios of the various proportions of the fractions. Definitions of the fractions have long been standardized to the grade scale described by Wentworth (1922), and the size data compiled in this report conform to these definitions. Specifically, according to the Wentworth grade scale (PDF version) gravel-sized particles have a nominal diameter of 2 mm; sand-sized particles have nominal diameters from <2 mm to >62.5 µm; silt-sized particles have nominal diameters from <62.5 µm to >4 µm; and clay is < 4 µm.
Although several classification schemes have been adopted to describe the approximate relationship between the size fractions, most sedimentologists use one of the systems described either by Shepard (1954) or Folk (1954, 1974). The original scheme devised by Shepard (1954) utilized a single ternary diagram with sand, silt, and clay in the corners to graphically show the relative proportions among these three grades within a sample. This scheme, however, does not allow for sediments with significant amounts of gravel. Therefore, Shepard's classification scheme was subsequently modified by the addition of a second ternary diagram to account for the gravel fraction (Schlee, 1973). The system devised by Folk (1954, 1974) is also based on two triangular diagrams, but it has 21 major categories, and uses the term mud (defined as silt plus clay). The patterns within the triangles of both systems differ, as does the emphasis placed on gravel. For example, in the system described by Shepard, gravelly sediments have more than 10 percent gravel; in Folk's system, slightly gravelly sediments have as little as 0.01 percent gravel. Folk's classification scheme stresses gravel because its concentration is a function of the highest current velocity at the time of deposition, together with the maximum grain size of the detritus that is available; Shepard's classification scheme emphasizes the ratios of sand, silt, and clay because they reflect sorting and reworking (Poppe and others, 2005).
Although most source data sets in this compilation (see the Data Catalog) contain raw grain size data, several provide only verbal descriptions of the sea-floor character. Some of these verbal descriptions are somewhat detailed, such as in the lithologic descriptions file from the USGS Continental Margin Program; others are quite abbreviated, as in the one-word descriptors supplied with the NOAA Hydrographic Database. Furthermore, most source data sets contain sediment classifications that were assigned by scientists as part of the original study. These word-based data sets have been related to numeric values for inclusion in the usSEABED data sets. Users are encouraged to review the Data Dictionary section and the usSEABED Web site for a thorough explanation.
Most of the samples compiled in this report were collected using some type of grab sampler, but some were obtained by coring or with dredges. When core samples are included, or when changes in the sediment type with depth are present in a grab sample, only the analysis from the uppermost sediment type was used when mapping surficial sediment distributions. Similarly, samples collected with chain dredges are probably texturally biased and care must be taken with the use of this data.
The USGS has traditionally defined surficial samples as those sediments collected from the interval 0-2 cm below the sediment/water interface. Although many of the samples in this compilation conform to this standard, some of the studies did not define this interval or reported intervals with slightly greater bottom depths (e.g. 0-5 cm). Concerned users should consult the original source references or the metadata files provided in this report.
Plotting routines (Matlab based) for the Shepard and Schlee classifications mentioned above, are available in the For Educators section of this publication. These routines allow users to plot their own ternary diagrams.
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