ABSTRACT: Interaction between Paleoclimate and Changes in Relative Sea Level and Their Effects on Fluvial Sedimentation: An Example from the Cretaceous of Southern Utah

MCCABE, PETER J., U.S. Geological Survey, Denver, CO, and KEITH W. SHANLEY, Shell Development Co, Houston, TX

An unconformity within the Straight Cliffs Formation separates Upper Turonian from Lower Coniacian strata in the Kaiparowits Plateau of southern Utah. Our stratigraphic interpretations suggest this sequence boundary was cut during a fall in relative sea level resulting in a basinward shift in facies tracts and deposition of the Ferron Sandstone. Below the unconformity, strata consist of shoreface and fluvial sandstones, floodplain mudstones, and thin coals; the maximum grain size of these strata is medium sand. Strata overlying the unconformity are interpreted as a valley-fill complex with fluvial sandstones overlain by estuarine strata. These strata are much coarser and contain pebbles up to 2.5 cm in diameter.

Paleohydrological considerations indicate that, if the rivers maintained similar discharge patterns throughout the cycle of sea level change, gradients must have been at least an order of magnitude greater during the lowstand than at the previous highstand. However, observed facies geometries over 60 km of depositional dip within the Kaiparowits Plateau do not appear to support such a change in gradient. It appears, therefore, that discharge patterns, and local climate, varied through time. Other evidence of climatic variation includes the development of red paleosols below the unconformity and the prominent white color of the basal fluvial sandstones above the unconformity, which may reflect early postdepositional weathering. Furthermore, mid-Cretaceous coals in the Kaiparowits Plate u are interpreted to have developed in raised mires within 40 km of coeval shorelines, suggesting a maritime influence on precipitation patterns.

We suggest that during sea level lowstands the climate became more seasonal as the maritime influence decreased. The coarser grain sizes above the sequence boundary, therefore, may reflect a decrease in chemical weathering and a flashier discharge pattern, in addition to the increase in gradient caused by lowering of sea level.

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)