Abstract

Many of the larger Silurian mounds throughout the midwestern United States consist predominantly of thinly bedded carbonate units inclined at angles as great as 45°. Because these values exceed those for reasonable estimates of angles of repose, two opposing end-member models for their origin have been suggested, the ramifications of which affect evaluation of carbonate sources and modes of deposition. One implies that steeply sloping units exhibit primary inclinations, are composed largely of autochthonous carbonate debris held in place by benthic organisms and/or synsedimentary cement, and that deposition took place in relatively deep, low-energy marine settings. The other holds that present steep slopes are the result of differential compaction, that most carbonate is allochthonous, and that deposition took place in relatively shallow, high-energy marine environments.

The Pipe Creek Junior quarry in east-central Indiana exposes as much as 45 m of such clinothems. Paleomagnetic signatures, inclinations of geopetal cavity fillings, and the density and geometry of vertical extension fractures which transect inclined units record 13° of postdepositional steepening. Restoration of inclined units to their initial slopes requires initial depositional inclinations of no more than 32°.

Postdepositional compaction at Pipe Creek Junior was increasingly important in finer-grained downslope lithofacies; grainstone units shortened 3%, grainstone-mudstone units shortened 30%, and mudstone units shortened 58%. Compaction mechanisms, however, varied in different lithofacies. Dewatering resulted in 50% compaction of mudstone and 13% compaction of grainstone-mudstone units. Pressure solution, although less important than dewatering, was most intense in grainstone-mudstone lithofacies where shortening was at least 17%; dissolution thinned mudstone and grainstone units 8% and 3%, respectively.

Data on unit rotation and unit shortening require that axes of flank-bed rotation correspond approximately to the position of the present quarry rim, suggesting that only minor amounts of Silurian section have been removed during Devonian and Pleistocene erosion. These relations, in conjunction with data on over-all complex geometry and flank-bed lithologic features, indicate that the Pipe Creek Junior complex is not truly reefal, in that it consists almost entirely of allochthonous biotic debris, derived from horizontal wave-swept bench surfaces and deposited on laterally prograding, bench-margin slopes at angles of repose. Other large midwestern Silurian carbonate buildups may have formed in similar settings.

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