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The Atlantic continental shelf of the United States provides an excellent laboratory for application of statistical techniques designed to augment the megascale distribution of spatially oriented geologic data.
Multivariate methods have been applied to data from over 2,000 surface-sediment grab samples in a baseline study conceived both to discern regional trends of textural and mineralogic parameters and to evaluate the relative usefulness of these statistical strategies. The data include textural information on mean grain size, sorting, skewness, and various mineralogic constituents, whereas the statistical techniques employed include cluster, ordination, and trend surface analysis.
Distinct sedimentary facies were first determined by Q-mode cluster analysis, based on the similarity among individual samples utilizing Sorenson's coefficient as the similarity index. To interpret the relations among sedimentary facies, the clusters were plotted in n-dimensional space by Q-mode ordination, employing a highest dissimilarity criterion. The method suggests, for example, that the sediments from the shelf south of Cape Hatteras can be classified on the basis of water depth and calcium carbonate. The sedimentary facies include: a low CaCO3 inner-shelf facies, an outer-shelf facies with variable CaCO3 content, a shelf-break facies, an biogenic upper-slope facies, and a lower-slope facies. Plotting the spatial distribution of these clusters shows the tr nd toward low CaCO3 content in sediments on the shelf off Georgia, thereby emphasizing the inverse relation between clastic influx and CaCO3 content.
As would be expected, trend surface analysis ignores local variations in sedimentary parameters in favor of extracting the regional gradient. The trend from fine sand nearshore to coarser sand on the shelf below wave
base is consistent. Strong regional trends are also present for the calcium carbonate fraction and the aragonite/calcite ratio on the shelf south of Cape Hatteras. The existence of a nearshore zone of present sediment reworking, longshore clastic transport, and skeletal comminution is implied by the low CaCO3 values along the coastline.
Generally, the combination of cluster, ordination, and trend surface analysis proves to be an excellent strategy for the extraction of sedimentary trends, particularly where local variations obscure the regional gradient.
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