New Mexico Geological Society Annual Spring Meeting — Abstracts

As originally defined in the Colorado Front Range, the Morrison Formation consists of variegated shale or claystone, largely smectite (altered volcanic ash), with lesser sandy shale and thin sandstones. As currently used on the Colorado Plateau it includes diverse lithologic units which cannot be traced eastward into the type area.

Subdivision of the Morrison on the Plateau dates from Gilluly and Reeside (1928) who used Lupton's (1914) informal name, Salt Wash sandstone member, to designate a basal Morrison conglomeratic sandstone unit. The overlying shale-dominated beds were left as undivided Morrison Formation. The correlation from the Front Range was based on similarity of fauna and lithology, although equal emphasis was given to a possible correlation with the Lower Cretaceous Cloverly Formation. Ten years later Gregory subdivided Morrison strata in southeastern Utah into the Recapture Shale (a lateral facies of the Salt Wash), the Westwater Canyon Sandstone Member, and the overlying Brushy Basin Shale. The Westwater was described asa quartzose sandstone with chert and quartzite conglomeratic stringers, occupying a stratigraphic position close to that of the Salt Wash, somewhat lower in the section than has generally been recognized in regional correlations. Later workers have recognized volcanic pebbles in the Westwater, but not specifically in the type area.

We thus contend that in the western type area all members except the Brushy Basin are largely pre-volcanic and likely to be all or in part older than the type Morrison Formation. Moreover the quartzose character of the lower sandstones suggests affinities to the San Rafael Group, although genetically there is a contrast. A dilemma exists, however, because neither lithology nor genesis have been applied consistently to placement of the basal Morrison contact. Recent USGS work by Condon and Peterson (1986) has advocated an "unrecognized time boundary" as the base of the Morrison Formation. Adhering to the definition of a formation is more appropriate (mappability), and hence we would place the contact on the basis of lithic characteristics. This, unfortunately, is not an easy task because of great lateral variability of these strata and the failure of many previous workers to recognize this sequence as transitional between San Rafael Group and Morrison Formation.

Recent work on Haystack Mountain has indicated that a two phase transition exists. The sandstone facies in the lower part are generally well-sorted high-sphericity quartzose sandstone very similar to San Rafael Group lithologies, but contrasting in lithogenesis. The sandstone facies in the upper part consist of a mixture of the quartzose facies plus a more angular, poorly sorted lithic fraction containing chert, quartzite, and volcanic fragments. Each of these might be recognized as members of a pre-Morrison unit in the San Juan Basin. The base of the Morrison thus becomes the base of the local Westwater Canyon Member, which may not be the same unit as type Westwater Canyon in SE Utah. If further detailed work establishes these as two separate units, consideration will have to be given to applying Smith's (1954) name, Prewitt Sandstone Member, to strata previous called Westwater Canyon Member.