Abstract: Coarse-Grained Gilbert Deltas: Relationships to Pleistocene Climate and Tertiary Uplift at the Easte of Lake Bonneville, Northern Utah

Mark R. Milligan, Marjorie A. Chan

Coarse-grained Gilbert deltas were described geomorphically over one hundred years ago by G. K. Gilbert. Detailed sedimentologic and stratigraphic studies of gravel pit faces near Big Cottonwood Canyon, American Fork, and Brigham City indicate that the deltas are the result of the complex interplay of climate and glaciation, tectonics, and lake level history.

The formation of Gilbert-type deltas requires coarse-grained sediments supplied from steep canyons draining directly into a standing body of water. Active uplift along the Wasatch Fault created the necessary condition of steep canyons. Uplift along the Wasatch fault began about 15 Ma and continues at a rate of about 1 m per 1000 years (average for last 14,000 years). The interplay between climate and basin physiography brought the lake shore within close proximity of the Wasatch Front. The lake level highstand occurred about 15,000 years ago. Glaciation reached a maximum about 19,000 years ago.

Two end member coarse-grained delta systems are distinguished: transgressive deposits at the Bonneville level (maximum high) showing crude horizontal stratification; and regressive deposits at the Provo level showing well-developed foresets. At the mouth of Big Cottonwood Canyon sedimentation rates overwhelmed the rate of transgression, and extensive horizontal subaerial glacial outwash deposits resulted at the Bonneville level. At the mouth of American Fork Canyon less-extensive glaciation provided lower sedimentation rates to build a Bonneville transgressive delta dominated by horizontal subaqueous gravel. Reworked Bonneville-level deltaic sediments built extensive foresets in the Provo-level deltas, as found in the exposures near Big Cottonwood Canyon.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994