Forward Modeling of Peritidal Facies in Icehouse Successions, with Outcrop Evidence from the Orogrande Basin, NM: Preservation Potential and Sequence Stratigraphic Implications
Metre-scale peritidal cyclothems, characterised by tidal flat laminites, are distinctive components of early and mid Paleozoic successions and many Mesozoic platform carbonates. However, they are rare throughout the rest of the Paleozoic and Mesozoic, and especially rare in the Cenozoic (possibly due to bioturbation related to the spread of mangroves). In the absence of peritidal cyclothems Upper Palaeozoic carbonates are dominated by characteristically asymmetric sub-tidal cycles truncated by abnormal sub-aerial exposure of sub-tidal facies.
The accepted interpretation for this distribution is icehouse large-amplitude high-frequency eustatic sea-level oscillations forcing deposition in catch-up mode, preventing extensive deposition of peritidal facies. In contrast, low-amplitude low-frequency eustatic oscillations during greenhouse periods probably allowed development of extensive, keep-up stacked cyclic sequences containing significant thicknesses of peritidal sediment.
This study uses one-dimensional numerical forward modeling to identify the major controls on development and preservation of peritidal facies. Using published data on rates and amplitudes of sea-level change, our modelling shows that identifiable but relatively thin intertidal caps do form even during icehouse intervals. However, estimates of erosion associated with subaerial exposure during the glacial lowstands suggest that such deposits have very low preservation potential based on their sequence stratigraphic position during deposition.
Our modelling also suggests that during icehouse periods peritidal facies are most likely to be deposited and preserved on carbonate ramps rather than shelves due to differences in topographic relief. This interpretation is supported by Pennsylvanian strata from the Orogrande Basin, where peritidal facies are absent from cycles on the steeper eastern shelf, but present along the low-gradient western margin. Where preserved, peritidal facies are unlikely to be solely indicative of a reduction in the amplitude and rate of sea-level change. Instead it is likely that they record the base-level minima for a particular oscillation of relative sea-level.
This finding has important implications for the role of preservational processes in developing accurate facies models from limited modern analogues and could be a useful predictive exploration tool for understanding facies distributions on icehouse carbonate ramps.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009