Abstract: Models of Shallow-Marine Cyclic Sedimentation, Western Interior Cretaceous, North America
Thomas A. Ryer
Deposits of Cretaceous age in the Western Interior of North America long have been noted for extensive intertonguing of marine and nonmarine strata. Cyclicity is at several different orders of magnitude. Both asymmetric and symmetric models of deposition have been utilized by various authors to explain cyclic repetition of sequences of rock types. The symmetric model can be applied only at high orders of magnitude, becoming unworkable when cyclicity is analyzed closely. Sediments deposited along the western margin of the Cretaceous seaway display repetition of progradational sequences separated by nondepositional or erosional (ravinement) disconformities. Absence of depositional sequences recording marine transgression is attributed to the process of shoreface erosion and to marked reduction of the quantity of sediment introduced to shallow-marine environments by rivers during periods of rising relative sea level. Nearly contemporaneous strata of the Frontier Formation in north-central Utah and of the Ferron Sandstone Member of the Mancos Shale in central Utah contain repetitions of disconformity-bounded progradational sequences that demonstrate the general applicability of the asymmetric model to deposits of both nondeltaic and deltaic shorelines, respectively.
AAPG Search and Discovery Article #90968©1977 AAPG-SEPM Annual Convention and Exhibition, Washington, DC