Development of Cyclic and Noncyclic Sequences in Appalachian Carbonates
J. F. Read
Cambrian-Ordovician peritidal cyclic carbonates result from small-scale (few meter), glacio-eustatic oscillations characterized by rapid rise, superimposed on slow subsidence (5-10 cm/1,000 years), coupled with a lag time and variable depth-dependent sedimentation rate. Thin subtidal-based cycles result from low lag time, long periods (100,000 years) and/or low amplitude of sea level oscillation. Thick subtidal-based cycles result from longer lag times (few thousand years), higher amplitudes of sea level oscillation, and/or short periods (about 40,000 years). These controls, coupled with climate and position on the rimmed shelf or ramp, largely govern facies makeup within cycles. Conformable contacts between cycles indicate that sea level fall rates roughly equaled or wer less than subsidence. Erosional contacts between other cycles indicate that sea level fall rates exceeded subsidence rates.
Upward-shallowing sequences of thick (tens to hundreds of meters) noncyclic subtidal units grading into peritidal cycles reflect incipient drowning of the platform (to depths of 10-30 m), as a result of third-order sea level rises (periods of 2-10 Ma), on which are superimposed 20,000-100,000-year sea level oscillations. This causes coastal onlap and long-term landward retreat of peritidal facies. It is followed by gradual offlap of cyclic peritidal dolomites and sandstones during long-term sea level falls, during which time unconformities are developed at the margins of the platform.
Disconformity-bounded cycles (e.g., Mississippian) lack tidal-flat caps, and have well-developed caliche beds. They reflect high-amplitude sea level oscillations, which inhibit progradation of tidal flats out from cratonic shorelines, and allow development of deep vadose profiles because sea level fall greatly exceeds subsidence rate.
AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.