New Insights into Shallow-Marine Carbonate Sedimentary Records from Cratonic Interior versus Continental Margin Settings
Brady, Mara E.
Over Phanerozoic time scales, sedimentary records from the tectonically-stable interiors of continents versus their subsiding margins are dramatically thinner and assumed to be relative incomplete, with more numerous gaps caused by erosion and non-deposition. However, this assumption need not be true for the shorter time scales over which the preserved sedimentary record accumulated on the continental interior. This field-based study compares Middle-Upper Devonian sedimentary and fossil records from Iowa (cratonic interior) and Nevada (continental margin) to evaluate whether the cratonic record is (1) miniaturized, i.e. thinner, but equally complete; (2) comparable in thickness and quality where the sedimentary record is preserved, but certain portions of the record are notably absent, i.e. omitted or truncated; or (3) so invariably preserved that the stratigraphic packages and skeletal concentrations are qualitatively different compared to the contemporaneous continental margin record.
This study finds that, where preserved, comparable but thinner lithofacies and meter-scale cycles can be recognized in Iowa. Nonetheless, stratigraphic omission of meter-scale cycles, and to a less extent, lithofacies, does occur. However, the absence of depositional record in Iowa relative to Nevada is likely due to suppression of the subtidal carbonate factory, rather than more frequent subaerial exposure. In both Iowa and Nevada, taphonomic characterization of skeletal concentrations (i.e. calcirudites) reveals a surprising lack of thick, densely packed, laterally extensive, and highly time-averaged shell beds, even at major non-depositional hiatuses (where low bioclast dilution would favor their formation). These results are consistent with inherently low bioclast input rates characterizing Paleozoic seafloors. Taken together, these findings have implications for Phanerozoic trends in stratigraphic stacking patterns, the thickness and abundance of coarse bioclast deposits, and the distribution and connectivity of petroleum system elements.
AAPG Search and Discovery Article #90162©2013 Pacific Section AAPG, SPE and SEPM Joint Technical Conference, Monterey, California, April 19-25, 2013