Three types of cycles are recognized for the Pennsylvanian rocks of the Appalachian basin: 1) autocycles (river avulsion and shifts in supply); 2) sub-regional allocycles (tectonism within the basin or uplift of parts of the orogenic belt; and 3) regional and eustatic allocycles (regional tectonism or eustasy). Autocycles occur within minor allocycles and are interpreted as sediment-supply shifts accompanying river avulsions within relatively fixed-drainage basins. Minor allocycles of coal measures commonly are 18 to 30 meters in thickness and contain extensive paleosols, coal, marine and/or freshwater limestone beds. Current estimates of allocycle durations vary greatly depending on the time scale used. Minor allocycles probably are glacio-eustatic and shoreline T-R shifts range from 32 to >800 km. Intermediate allocycles consist of several minor allocycles either in progradational, aggradational or retrogradational sets with relatively more transgressed units serving as the boundary units and are 90 to 115 m thick. Major allocycles are bounded by extensive transgressive units, are about 300 m thick, and approximate the Lower, Middle, and Upper Series of Pennsylvanian. These major allocycles are subregional, reflect basin tectonism shown by shifts in the seaway during the Pennsylvanian, and do not correlate with worldwide sea level curves, suggesting the basin's response to thrust-sheet loading during the Alleghanian orogeny. Cycles in paleoclimate were significant in influencing the lithic response within the allocycles (Cecil, 1990).
Figures & Tables
The collected volume begins with a brief perspective by one of the conveners, followed by articles in order of increasing stratigraphic age. Eustatic sea-level changes and tectonic warpings of basins are competing mechanisms for explaining many stratigraphic patterns. The model for sea-level changes should be developed first for a basin, since it is allocyclic and leads to a series of time bands in the strata. The residual effects should then be modeled for tectonic patterns affecting the depositional processes. Doing the reverse limits time constraints on the tectonic warping models and will blur the resolution of detailed time surfaces in the strata. Case histories of situations with both tectonic warping and time surfaces marked by sea-level events will lead to improved interpretations of earth history.