SEQUENCE STRATIGRAPHY OF CAMBRO-ORDOVICIAN PASSIVE MARGIN CARBONATES, U.S. APPALACHIANS

The Cambro-Ordovician passive margin carbonates, Appalachians, were influenced by 2 major depocenters, one in Pennsylvania and one in Tennessee, which controlled thickness trends. Late Early to Middle Cambrian rifting generated an intrashelf basin in the Tennessee depocenter and the Rome trough. The carbonates were initiated as a ramp on the Early Cambrian siliciclastic shelf, but this developed into a high relief tropical rimmed shelf by the late Early Cambrian, and remained as such until the end of the Late Cambrian, when the shelf was converted back to a ramp during incipient collision. The carbonate succession contains 4 supersequences, ranging from 600 to 1200 m maximum thickness. These generally are capped by major, although relatively subtle unconformities. The supersequences commonly contain 3 to 6, 3rd order sequences recognized by accommodation plots of carbonate-cycle thickness, and by quartz sandy cycles near sequence boundary zones. Early Cambrian parasequences are relatively thick compared to later cycles and may reflect longer cycle periods and perhaps higher amplitude sea level changes than later peritidal parasequences (cycles). Subtidal, 1-5 m parasequences in the intrashelf basin are subwave-base-shale to storm reworked calcisiltite to high energy, subtidal storm reworked skeletal grainstone/flat pebble conglomerates, that lack microbial mounds/heads. They appear to have formed under moderate amplitude, high frequency sea level changes, and stratified to non-stratified water columns. Peritidal cyclic carbonates dominate the shelf successsions and are upward shallowing carbonates with a locally developed, transgressive, thin grainstone/intraclast lag, overlain by thin bedded or burrowed pellet limestones and microbial heads, overlain by laminated tidal flat carbonates, and capped by a sharp disconformity, or rarely a carbonate regolith. Lack of burrowing of the transgressed tidal flat caps (even though overlying beds commonly are quite burrowed) supports the idea that these carbonates were lithified rocks prior to the next cycle transgression. They appear to have formed by high frequency, low amplitude sea level changes, compatible with greenhouse climate, together with autocyclic processes.