Cambro-Ordovician Passive Margin, U.S. Appalachians

J. F. Read

The passive margin carbonates developed over Early Cambrian shelf clastics. Shelf depocenters, regional arches, and domes controlled sediment thicknesses and directions of progradation. Initially, an Early Cambrian bank-fringed ramp developed. By Middle Cambrian, it had formed a reef-rimmed margin fringed by thick periplatform talus; this rimmed shelf persisted through the Cambrian. Middle Cambrian rifting formed the Rome Trough inboard of the passive margin; red beds were deposited over much of the shelf. With subsequent sea level rise, a huge intrashelf shale basin formed in Tennessee-Alabama. Both the Rome Trough and intrashelf basin had ceased to exist by Late Cambrian. The rimmed shelf evolved into a ramp during initial collision in the Early Ordovician. During Middl Ordovician collision, a widespread unconformity formed, the ramp foundered, and its leading edge was deformed.

At the formation level, the stratigraphy reflects third-order (2 to 10 m.y.) sea level fluctuations. At a smaller scale, platform facies are mainly 1 to 5 m thick, peritidal carbonate cycles, that reflect 20,000 to 100,000 year sea level fluctuations with less than 10 m amplitude. Slopes/cycle were 1 to 2 cm/km. Subsidence rates were 2 to 10 cm/1,000 years (mature passive margin rates). These rates tended to allow the shelf to track sea level fall during regressions; thus widespread evidence of subaerial emergence and regolith development is rare. However, dolomitization during these phases was intense. Regionally widespread, thick (30 to 300 m) subtidal sequences that punctuate the cyclic shelf sequence may be due to higher amplitude sea level fluctuations coupled with longer term (1 to 10 m.y.) sea level rise. Long-term sea level regressions periodically caused widespread deposition of eolian and coastal clastics over the shelf at tops of carbonate cycles. Intrashelf basin "grand cycles" reflect third-order transgressions coupled with basin shallowing under higher frequency sea level fluctuations.

AAPG Search and Discovery Article #91038©1987 AAPG Annual Convention, Los Angeles, California, June 7-10, 1987.