Sedimentary Provenance, Transport, and Mixing of Cretaceous Fluvial and Marginal Marine Strata in the Straight Cliffs Formation, Southern Utah: Insights from Detrital Zircon Geochronology

Tyler Szwarc and Cari L. Johnson
University of Utah

Cretaceous strata of the Straight Cliffs Formation in southern Utah document fluvial to shallow marine deposition along the margin of the Western Interior Seaway. High sediment supply and prolonged subsidence in this part of the foreland basin allowed over 250 meters of Coniacian-Santonian sediment to accumulate, but the sources of this basin fill are poorly known. In order to better understand the sedimentary provenance of the Straight Cliffs Formation, 24 detrital zircon samples were collected from several prominent stratigraphic units throughout the Kaiparowits Plateau in southern Utah. Detrital zircons derived from fluvial sandstones (n=1004) yield primarily Paleoproterozoic ages near 1.7 Ga (65% of all fluvial zircons) and 1.4 Ga (13%). These ages, in addition to outcrop paleocurrent measurements, suggest the majority of sediment was sourced from Proterozoic intrusive igneous bodies exposed in the Mogollon highlands of central Arizona, and was transported northward in an axial river system. Additionally, Mesozoic zircons (2%) indicate these rivers drained Cordilleran volcanic arc sources. Subordinate populations of Grenville (6%), Paleozoic (2%) and Archean (2%) aged zircons were transported into the basin via minor tributaries draining the Sevier thrust belt to the west. Ages from shallow marine sandstones (n=425) provide evidence for the mixing of fluvial sediments with shoreface sands that were transported from the north by longshore currents. The dominant ages are still Paleoproterozoic (1.7 Ga, 42% of all shallow marine zircons; 1.4 Ga, 10%), but a prominent Grenville aged peak (1.1 Ga, 17%) with moderate Paleozoic (5%), Neoproterozoic (4%), and Archean (4%) ages indicate longshore drift transported Sevier thrust belt sediment southward and mixed it with the Mogollon and arc-derived sediment. Zircons from intertidal deposits (n=187) yield a combined age signature of the fluvial and shoreface rocks, signaling mild mixing of fluvial and shoreface sediments by tidal processes. This study demonstrates that an axial drainage system draining the Mogollon highlands was the dominant sediment transport mechanism in this part of the basin; despite the proximity of the Sevier thrust belt, the Mogollon highlands remained the primary sediment source for the Straight Cliffs Formation during Coniacian-Santonian time.

AAPG Search and Discovery Article #90169©2013 AAPG Rocky Mountain Section 62^nd Annual Meeting, Salt Lake City, Utah, September 22-24, 2013