Comparative Cycle Stratigraphy of Cordilleran and Appalachian Late Cambrian Passive Margins: Field and Computer-Modeling Study

David A. Osleger, J. Fred Read

Superimposed scales of cyclicity can be recognized within the Upper Cambrian carbonate sequences of the early Paleozoic Appalachian and Cordilleran passive margins. Fifth-order (10,000-100,000 years) small-scale (1-10 m) shoaling-upward cycles are evident throughout both miogeoclinal sequences. The Upper Cambrian of the Appalachians is dominated by peritidal cycles consisting of a grainstone lag overlain by cryptalgal bioherms and ribbon rocks capped by laminated dolomites. The flat-topped platform that developed behind the rimmed shelf margin favored the development of broad tidal flats that rapidly responded to even small fluctuations in sea level. In contrast, the Upper Cambrian miogeoclinal section of the Utah-Nevada passive margin has a ramp morphology and is dominated by deep to shallow subtidal cycles, few of which show laminite caps. The greater slopes on the Cordilleran ramp favored the development of facies belts from deep to shallow subtidal but seldom formed extensive tidal flats. Response to sea level fluctuations was less pronounced in the Utah cycles as indicated by their generally greater average thickness and periodicity. All of the small-scale cycles have average periodicities between 60,000 and 120,000 years.

Subsidence modeling has revealed synchronous deviations from model cooling curves for both passive margin sequences which suggests a eustatic control on the development of third-order (1-10 m.y.) cycles. Fischer plots (cycle thickness corrected for linear subsidence plotted against time) can be used to determine long-term sea level fluctuations and can be checked against relative sea level curves determined for each section based on relative water depth estimates. Third-order sea level events, ranging from 2 to 8 m.y., correlate well between the geographically separate localities, further supporting a eustatic origin. Two-dimensional computer modeling of third-order cyclic sequences allows for an estimation of the probable parameters involved in their development. The model incorporat s tectonic and rotational subsidence, platform flexure, isostatic compensation for sediment and water loading, linearly interpolated water depth-dependent sedimentation rates, platform slope and width, and complex superimposed scales of sea level fluctuations to generate synthetic sequences similar to the actual sequences in the field.

AAPG Search and Discovery Article #91024©1989 AAPG Pacific Section, May 10-12, 1989, Palm Springs, California.