SEQUENCE ARCHITECTURE OF A LOW LATITUDE, HETEROZOAN CARBONATE RAMP COMPLEX: THE PERMIAN “CONFUSION SHELF” IN NORTHERN UTAH AND ADJACENT AREAS, USA

Heterozoan carbonate ramp complexes developed not only in cool water environments of high latitudes and on upwelling effected shelves, but also in some epeiric basins of low latitude. Bio- and lithostratigraphic correlations are difficult because of a reduced faunal spectrum and a monotonous carbonate lithofacies. Carbonate microfacies analysis, however, recovers the sequence stratigraphic architecture of system. The “Confusion Shelf” was a shallow, extreme wide, fringing carbonate ramp bordering the southern margin of the semi-enclosed, mid-Permian Phosphoria Sea. Oxygen deficiency in the basin center with black shales and phosphorites resulted from thermohaline stratification rather than upwelling. The carbonates are composed of bioclastic packstones including biograinstones, mudstones, and bindstones. The faunal spectrum is principally reduced to crinoids, bryozoans, and brachiopods. Most foraminifers, including all fusulinids, calcareous algae, corals and calcareous ooids are absent. The carbonates intertongue with evaporites and siliciclastics on the upper ramp. The microfacies analyses identify three, surface-bound, third-order stratigraphic sequences that roughly correlate with the Grandeur/Kaibab, Plympton, and Gerster Formations. Different internal organization of the sequences defines two types. The first and third sequence is laterally and vertically composed of monotonous bioclastic carbonates. The T/R profile of the preserved sediment suite displays abrupt rise and fall of sea-level. Only the uppermost ramp produced some intertidal mudstones and evaporites. The monotonous facies is not able to record parasequences. The laterally and vertically diverse second sequence, however, shows a zonal facies distribution and a symmetrical, low angle T/R profile. Systems tracts are present by microfacies evidence, but lack bounding surfaces and lateral stacking. Low-order cyclicity indicates parasequences. The early TST of the middle and lower ramp is composed of black shales above phosphorite concentrates. Rare bioclastic bars developed on the upper ramp, but grainstones are completely cemented. The shift between sequence types was controlled by slightly changing dip of the ramp.