THE AGE, DISTRIBUTION, AND DEPOSITION OF HOLOCENE TERRACE DEPOSITS IN HOP VALLEY, ZION NATIONAL PARK, UTAH

Holocene lakes that formed when landslide dams blocked the drainage occur throughout Zion National Park. One of these lakes is represented by the Hop Valley terrace deposits, which are associated with Virgin Anasazi pictographs. Two radiocarbon dates are available for Hop Valley, one at the south end of the valley of 2,640 years and one at the north end of the valley of 670 years. To better constrain the age of the terrace deposits a number of samples were collected that are currently being dated with C-14 AMS. Previous workers interpreted the Hop Valley terrace sediments as being deposited in a lake of standing water, produced by a landslide at the northern end of the valley. However detailed stratigraphic and sedimentological observations did not yield any evidence for a body of standing water. By comparing the terrace sediments to modern stream environments the following 7 facies were identified; 1) broad unconfined stream channel, 2) bioturbated flood plain, 3) flash flood, 4) paleosol, 5) eolian, 6) colluvium, and 7) pond. Topographic maps, aerial photographs and GPS were used to map the distribution of the terrace deposits. Particular attention was paid to the elevation of the terraces relative to the current drainage and elevation of the slide deposits. It was found that the elevation of the terraces decreased downstream and define a smooth longitudinal profile. These data were used to support the interpretation that the terrace sediments were deposited by alluvial processes rather than in a lake. The results of this study will be used in the development of interpretive material on the recent geologic history of Zion National Park and to make resource management decisions regarding the Archeological significance of these deposits. This project was funded by the “Geologist in the Parks” program, sponsored by the Association of Women Geologist, in cooperation with the National Park Service.