THE AGE OF SOIL FORMATION ON HOLOCENE TERRACE DEPOSITS IN HOP VALLEY, ZION NATIONAL PARK, UTAH

Holocene terrace deposits in Hop Valley, Zion National Park, Utah are associated with Virgin Anasazi pictographs. The following 7 facies were identified within the terrace deposits; 1) broad unconfined stream channel, 2) bioturbated flood plain, 3) flash flood, 4) paleosol, 5) eolian, 6) colluvium, and 7) pond. Base level for the terrace deposits was controlled by a landslide dam at the north end of the valley. A prominent paleosol characterized by irregular blocky peds, organic rich horizons, root penetration, and insect burrows, occurs within the deposits. The paleosol has a high magnetic susceptibility, loss on ignition and clay content, relative to sediments of the other 6 facies. The deposits occur above a peat deposit dated at BC 640. C¹⁴ dates of charcoal associated with the paleosol indicate it formed AD 1522 (± 94). A C¹⁴ date of charcoal within the soil A-horizon at the top of the section yielded an age of AD 1676 (± 37). The deposits represent a period of aggradation after BC 640, followed by paleosol development around AD 1500 which corresponds to the regional prehistoric period of arroyo cutting. This was followed by another period of aggradation and soil formation. Sometime after AD 1676 erosion occurred dissecting the terraces. This episode may correspond to the regional historic period of arroyo cutting which began after the 1880's.