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Abstract: Proposed Revisions to Parasequence-Set Nomenclature of the Upper Cretaceous Ferron Sandstone Member of the Mancos Shale, Central, Utah

Paul B. Anderson, Thomas A. Ryer

The Ferron contains an Upper Cretaceous clastic wedge with lower forward stepping and upper aggradational to backward stepping parts. Most of the marine section consists of wave-modified to river-dominated deltaic deposits. The forward stepping part of the Ferron was previously divided into two delta-front or genetic units (Kf-1 and Kf-2). Numerous parasequences are mapped in both of these units. Regrouping of the parasequences and revision of one earlier correlation results in two and possibly three proposed parasequence sets.

Kf-Last Chance consists of a series of marine beds in the southern outcrop area along the Limestone Cliffs. The unit was formerly part of the No. 1 and rapidly pinches out in a seaward direction (NE). Its landward edge is not exposed, and its origin is enigmatic.

Kf-1 parasequence set lies above the Kf-Last Chance. The landward edge is not exposed. The seaward pinch-out of the parasequence set is between I-70 and Miller Canyon, approximately 20 miles from the most landward exposures.

Kf-2 parasequence set has its landward edge exposed in Willow Springs Wash. Near the seaward feather-edge of the underlying Kf-1, Kf-2 dramatically thickens (filling the available accommodation space) and adds an additional parasequence for a total of four. The lowest parasequence is generally thin with the exception of erratic thickening associated with slump and growth fault features. This is the result of initial progradation onto a relatively unstable substrate. In the other three parasequences, delta-front facies have been replaced locally by fluvial systems.

AAPG Search and Discovery Article #90959©1995 AAPG Rocky Mountain Section Meeting, Reno, Nevada