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Abstract: Major Redefinition of Morrison Formation and Upper San Rafael Group on the Colorado Plateau

Orin J. Anderson, Spencer G. Lucas

The Morrison Formation in southeastern Utah and northwestern New Mexico is redefined to reflect the original concept, the inferred depositional environments, and an emphasis on lithostratigraphy. Morrison strata are accordingly assigned to two mappable lithostratigraphic units, in place of the five unworkable, overlapping units previously recognized. The two of the five which we retain as Morrison subdivisions are--the basal Salt Wash Member, trough crossbedded quartzose sandstones and conglomerates with interbedded muddy siltstones and minor volcanic detritus; and an upper unit, the Brushy Basin Member, dominated by smectitic claystone. In so doing we (1) restrict the Recapture Member to pre-Salt Wash gypsiferous, fine-grained sandstones and siltstones that intertongue w th the Bluff Sandstone and are herein identified as the Recapture Member of the Bluff; (2) recognize that the Westwater Canyon Member of Gregory (1938) is a synonym of Lupton's (1912) Salt Wash Member, so the name Westwater Canyon is abandoned; and (3) assign the Tidwell Member to the Summerville Formation because its thin, parallel bedded, gypsiferous strata are lithologically more similar to Summerville strata than to Morrison strata. The resulting two-member Morrison Formation can be readily correlated into the San Juan Basin of New Mexico where strata long termed Westwater Canyon Member are reassigned to the Salt Wash Member and the so-called Recapture Member strata are a paleosol or cumulative pedon developed on the Zuni Sandstone. Salt Wash and Brushy Basin strata can also be recog ized along the Front Range and onto the southern High Plains, although in this area the Salt Wash is locally thin or absent. A regional unconformity marks the base of the redefined Morrison Formation, expressed by either a paleosol or scour surface. Uranium mineralization in the Salt Wash may have been influenced locally by a permeability break at its base afforded by the paleosol. Dilute groundwater was thus forced to move slowly and laterally through organic-rich (humate) traps enhancing the efficiency of this concentration mechanism.

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