SEDIMENTATION IN PALEOLAKE GRAFTON, ZION NATIONAL PARK AREA, UTAH

Paleolake Grafton was the largest of a series of small lakes that developed in the Zion National Park area during the Quaternary Period. These short lived lakes were caused by landslides and lava dams that periodically blocked rivers and washes. Paleolake Grafton was formed when lava flows from Crater Hill dammed the Virgin River valley near the ghost town of Grafton, Utah. The lake had an area of approximately 26 square kilometers (Hamilton, 1979).

The sediments of paleolake Grafton in the study area are approximately 100 meters thick and unconformably capped by coarse river gravel. Only the upper 20 meters of sediment is being considered here. The study area is east of Coalpits Wash just south of Zion NP boundary.

Sediments are dominated by thinly bedded mud layers up to 40 cm thick. Lenses of secondary selenite are dispersed throughout. Decimeter scale layers of thin to medium bedded, coarse to medium sand occur between the mud layers. The poorly sorted, poorly consolidated sand is composed mostly of rounded to subrounded quartz with basalt clasts in a silt to fine sand matrix. Sand layers are characterized by scouring, channel filling, small scale cross bedding, and fining upward sequences. Types and distribution of sediments suggest climatic conditions similar to today.