Abstract: Distinguishing Allocyclic and Autocyclic Causes of Parasequence-level Cyclicity--Lessons from Deltaic Strata of the Upper Cretaceous Ferron Sandstone, Central Utah
Thomas A. Ryer, John A. Dewey Jr., Thomas H. Morris
Parasequences constitute the smallest-scale genetic units at which transgressive-regressive cyclicity in deltaic units can be effectively analyzed. Parasequences are commonly regarded as resulting from fluctuations of sea level but in deltaic environments, progradation, abandonment, and subsequent marine reworking of deltas or sub-delta lobes can produce bodies of sediment that have all the characteristics of parasequences.
We can distinguish two basic types of parasequences on outcrops of the Ferron Sandstone in the Indian Canyon-Willow Springs Wash-Coyote Basin are& Four of the five parasequences identified there accumulated along wave-dominated segments of the coast and represent the first parasequence type. Substantial landward shifts of facies occur across the marine-flooding surfaces that underlie each of the wave-dominated parasequences. The surfaces may record rises of eustatic sea level, in which case the parasequences associated with them are of allocylic origin. Alternatively, they may record reduction of the supply of sediment delivered by the long-shore-drift system, which could have been caused either by autocyclic processes acting at a distance or by allocyclic processes (e.g. differential subsidence). One of the five parasequences is distinctly different and represents the second type. It constitutes a sub-delta lobe that prograded at a high angle to the general shoreline trend and includes meanderbelt and distributary channel facies. Although the surface that separates this parasequence from the preceding one is easily defined, there is no evidence of associated trangression of the shoreline. The surface, instead, marks an abrupt change of shoreline facies from wave- to river-dominated. This parasequence is clearly of autocyclic origin.
AAPG Search and Discovery Article #90959©1995 AAPG Rocky Mountain Section Meeting, Reno, Nevada