Paper No. 15
Presentation Time: 9:00 AM-6:30 PM
ALLUVIAL ARCHITECTURE, PALEOSOLS, VERTIC FEATURES, AND PRELIMINARY CHEMOSTRATIGRAPHY OF THE YELLOW CAT MEMBER (LOWER CRETACEOUS) OF THE CEDAR MOUNTAIN FORMATION IN GRAND COUNTY, UTAH, USA
Three alluvial storeys in the Yellow Cat Member (YCM) at Poison Strip, Utah exhibit widespread vertic paleosols. A decimeter-scale organic δ13C profile collected through this 18.3 m section includes values of -28.9 to -25.2‰ VPDB, suggesting a Barremian age preceding stratigraphically-higher excursions related to OAE 1a. Alluvial architectural elements in the study area are, in descending order of abundance: (1) massive, reddish overbank claystones to muddy very fine sandstones with pervasive pedogenic overprints; (2) thinly bedded, laminated, or rippled to bioturbated or massive fine sandstones; and interbedded, thin weakly fissile shales producing thin sheets or extensive lenses; and (3) laterally restricted, massive fine sandstones with faintly discernible channel forms and wings. The upper storey exhibits lateral transitions in paleosol texture and horizonation, the distribution of sand-filled desiccation cracks, and the homogenization of primary sedimentary structures. All three storeys show deep (as much as 450 cm), very well-developed sets of very large pedogenic slickensides forming “bowls” of 3 to 7 m in apparent width, as well as intervening “peaks” at slickenside intersections. Very large, roughly discoidal, palustrine carbonate nodules within paleosols—and particularly in the uppermost story—are frequently tilted, folded, or even sheared along very large slickensides, indicating that could not have formed after extensive pedoturbation occurred. Abrupt lateral termini of units of architectural element #2 (above) are striking features at the bases of the paleosols. Most of these informally termed “turnups” curve upward by at least one bed-thickness above the horizontal part of the element. “Turnups” appear to follow the trends of upward-curving very large slickensides, indicating that they were gently deformed at shallow depths by soil thrusting. Comparatively slow rates of aggradation and seasonally dry climates during YCM times promoted deep pedoturbation and cracking and the homogenization or localized deformation of sands, muds, and palustrine carbonate sediments. Vertisol microrelief likely contributed to the configuration of the basal PSS contact, and in underlying storeys it may have formed contemporaneously with sedimentation.