ABSTRACT: Cycle and Sequence Dynamics of Lower Ordovician Platform Carbonates of the Great American Bank (USA): Constraints from Inverse and Two-Dimensional Forward Modeling

GOLDHAMMER, R. K., P. A. DUNN, and P. J. LEHMANN, Exxon Production Research Co., Houston, TX

Lower Ordovician platform carbonates of the Gondwana passive margin (the Great American Bank of the USA) contain several third-order depositional sequences (each 75-200 m thick, 2-5 m.y. duration) correlated from the Diablo platform (El Paso, TX), southeast to the Diablo arch (west Texas), northeast into the Ardmore basin (Arbuckle Mountains, Oklahoma) and to the Appalachian basin (central Pennsylvania and western Maryland). Correlation utilized biostratigraphy and high-frequency cycle stacking pattern analyses (Fischer plots). Owing to late Paleozoic structuring of the Gondwana margin, present-day exposures occur in updip shelfal positions and lack internal stratal geometries across depositional strike. Thus, sequences and systems tracts are identified by the onlap-offlap third-order facies tongues, the distribution of siliciclastics that straddle third-order sequence boundaries, and via vertical stacking patterns of depositional subfacies and higher-frequency fifth-order cycles (~1-5 m thick; 50-100 k.y. duration). Typically, sequence boundaries are stratigraphically conformable with no evidence for erosion or downward shift in facies.

The origin of fifth-order cycles has been investigated utilizing time series analyses (autocorrelation and maximum entropy spectra) and we conclude that a strict allocyclic, Milankovitch-driven glacio-eustatic mechanism, or a tectonic-reversal model are alone insufficient to account for the origin of fifth-order cyclicity. This, coupled with the progradational nature of peritidal cycles, suggests a Ginsburg-type 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. The origin of the third-order sequences appears to b eustatic based on biostratigraphically constrained correlation of Fischer plots for all sections.

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)