Abstract:

Shelf-margin reefs and reef sand aprons are important geomorphic and facies elements associated with many isolated platforms today and in the stratigraphic record. Although it is generally appreciated that shelf-margin reefs and reef sand aprons have formed at or near breaks in slope at the rims of platforms throughout geologic time, possible environmental causes for variation in their sizes are not well understood. To provide better constraints for assessing possible reasons for such variability in ancient systems, this study relates local, regional, and global patterns of variation in the seascape attributes of Holocene shelf-margin reef and reef sand apron systems to a suite of potential environmental controls.

Landsat data from more than 60 isolated carbonate platforms and atolls from the Caribbean Sea, Indian Ocean, South Pacific Ocean, Central Pacific Ocean, South China Sea, and Indonesian Seas were investigated by digitization and spatial analysis of facies belts, and compared with regional environmental parameters. The results of this analysis suggest that: (1) Reefs cover from < 1 to 55% of the spatial extent of lithotopes on platforms (mean  =  20%), whereas reef sand aprons represent between 0 and 65% (mean  =  20%). (2) The spatial extent of reefs is inversely related to platform size, but reef sand aprons show no comparable trend. (3) Widths of reefs and reef sand aprons are variable within and among platforms, but widths from all platforms and from platforms within different regions are characterized by log-normal size-frequency distributions. (4) Widths of these facies belts vary as a function of environmental factors such as swell period, swell height, tidal amplitude, latitude, and margin orientation relative to direction of wind and wave approach.

These data indicate the presence of trends in widths of facies belts within different regions that correlate with regional oceanographic patterns. Nonetheless, marked variability is evident on individual platforms, among platforms within each region, and globally; these variations are not related to any of the parameterized environmental variables. This observation are interpreted to reflect the influences of unknown (and in some cases, unknowable) spatial and historical contingencies such as the depth to bedrock, character of earlier Holocene sea-level change, or changes in physical oceanographic state (waves or tides). The collective influence of these parameters, each of which individually may have the potential to invoke a deterministic response, may be reflected in reef and sand aprons with log-normal width-frequency distributions, interpreted to reflect multiplicative random effects. Collectively, however, these results suggest that accurate, a priori prediction of the details of the sizes of analogous facies bodies in the stratigraphic record may be impossible, especially in the deep geologic past, where biologic and physical controls are even less well constrained.

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