LITHOLOGIC CRITERIA FOR LOCATING THE J/K BOUNDARY WITHIN THE NONMARINE LOWER CEDAR MOUNTAIN FORMATION, SAN RAFAEL SWELL, UTAH

The Jurassic/Cretaceous (J/K) unconformity in central Utah represents a major nonmarine sequence boundary that previous workers have placed at either the base or the top of the Buckhorn Conglomerate Member of the Cedar Mountain Formation. The absence of paleontological data leaves lithologic criteria as the only means to determine the position of the boundary. Sedimentological and petrographic data from 28 measured sections from the base of the Cedar Mountain Formation in the type area show a complex succession of interbedded fluvial and lacustrine sediments containing a discontinuous but prominent horizon of nodular carbonate underlain by variable amounts of silica that we interpret as a pedogenic calcrete and an associated groundwater silcrete.

The carbonate consists of coalesced nodules containing rhizoliths and is capped by a highly indurated calcic horizon; both the nodules and the indurated horizon contain multiple generations of brecciated intraclasts that range in size from micro- to macroscopic. The episodes of brecciation and recementation that make up the thick indurated masses are interpreted as a stage 5 to 6 pedogenic calcrete. The silcrete contains length-slow chalcedony and typically occurs with a few meters of the base of the calcrete as isolated to coalesced vugs and fracture fillings or as laminar horizons that vary from horizontal to vertical. In the study area the silica is best developed within a lacustrine carbonate but it is also present within the sandstones and conglomerates of the uppermost Buckhorn Member. The silica is interpreted to be a groundwater silcrete based on macroscopic morphology and the presence of glaebules and capping textures in thin section. Calcretes with multiple generations of brecciation have been interpreted by others to be the product of carbonate accumulation over time spans up to millions of years. The combination of the calcrete and the silcrete indicates the presence of a long-term exposure surface that we interpret to be the J/K boundary above, not below, the Buckhorn Conglomerate.