Abstract

Regressive sediments of the Pennsylvanian and Permian major cycle, predominantly clay and sand, were deposited as a westward-advancing embankment hundreds of feet high with unda (shelf), clino (slope) and fondo (deep bottom) elements. Within this mile-thick section occur tens of large cycles, each a few hundred feet in thickness, which contain seemingly hundreds of small cycles some tens of feet thick, the range of thicknesses appearing to be continuous. The cycles are expressed most characteristically as intercalations of limestone in the upper clino and outer unda facies, but in some cycles dolomite and evaporites replace much of the unda limestone.

The “typical” cycle began when a rise in relative sea level abruptly terminated the arrival of clay and sand, thereby clearing the water and enabling lime-secreting organisms to thrive and contribute most of the sediment deposited over the drowned undaform. As the terrestrial drainage system regraded to the higher base level a new small-scale embankment of clay and sand advanced out onto the drowned undaform, and sometimes clear across the undaform-edge limestone belt to produce the “complete normal” small cycle. However, renewed transgression interrupted many of the cycles before this stage was attained, so that a number of limestone bodies hundreds of feet thick were able to accumulate over drowned undaform-edges, giving rise to the large cycles.

Lowering of sea level is indicated less frequently. Moderate regression led to extensive deposition of coal, or of evaporites and red beds, over the emergent undaform. Greater emergence led to erosion of valleys, with deposition of the resulting detritus over the shallowed fondoform.

The changes in relative sea level responsible for the cycles are attributed largely to slow subsidence of the Eastern shelf as a result of regional westward tilting at a fairly uniform rate, upon which was superimposed numerous fluctuations caused by waxing and waning of the Gondwana continental glaciers. Each significant fluctuation in sea level shifted abruptly most sedimentary environments, which then tended slowly to return to their pre-existing pattern. Thus the cycles represent similar sequences of events, and not necessarily uniform intervals of time.