Two-Dimensional Forward Modeling of Lower Ordovician Platform Carbonate Sequences (Beekmantown Group, Central Appalachians): The Search for High-Frequency Autocycles

DUNN, P. A., and R. K. GOLDHAMMER, Exxon Production Research Co., Houston, TX, and L. A. HARDIE and C. T. NGUYEN, Johns Hopkins University, Baltimore, MD

Platform carbonates of the Beekmantown Group are organized into several onlap-offlap facies tongues that record four third-order depositional sequences (each about 150 m thick; 3 myr duration). A dip-oriented, 2-D facies mosaic (600 m thick; 275 km wide) displays a lateral facies zonation that, along a given time line, consists of (1) tidal flat facies composed of peritidal fifth-order cycles (average 3 m thick; 50-100 kyr duration); (2) inner shelf facies composed of shelf-lagoon grainstones-packstones; (3) outer shelf facies composed of Renalcis-Girvanella thrombolites and grainstones recording a shelfal patch reef setting; and (4) shelf margin facies dominated by Epiphyton framestone mounds and grainstones recording a shelf-edge reef-shoal complex. Facies stratigraphy equates to se uence stratigraphy as follows: transgressive phase of each third-order tongue = transgressive systems tract (TST); regressive phase of a tongue = highstand systems tract; locus of facies reversal points within a regressive tongue = sequence boundary; locus of facies reversal points within a transgressive tongue = maximum flooding surface. Sequence boundaries are stratigraphically conformable with no evidence for erosion or downward shift in facies.

Third-order accommodation changes, as revealed by systematic stacking patterns of thickening and thinning fifth-order cycles, are correlated across strike using megafossil zonation. Time series analyses detected no peritidal cycle rhythms suggestive of Milankovitch forcing. This, coupled with the progradational nature of individual cycles, suggests a Ginsburg autocyclic mechanism linked to onshore sediment transport and seaward progradation of tidal flats. The 2-D internal facies architecture of a sequence and cycle stacking patterns have been simulated via computer utilizing an autocyclic mechanism for fifth-order cycle development triggered by third-order accommodation changes.

AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)