Mike Lowe

Reevaluation of Holocene faulting at the Kaysville site, Weber segment of the Wasatch fault zone, Utah

The 1978 Kaysville, Utah, trench excavated by Swan and others (1980) across a large graben of the Weber segment of the Wasatch fault zone was reexcavated in 1988 to reevaluate the timing and nature of Holocene faulting. Relogging of the trench reveals evidence for five or six faulting events younger than the Provo phase of Lake Bonneville (circa 13,000 14C years B.P.). Geometric reconstruction of net vertical offset in the last three events suggests a variation in coseismic vertical displacement at this site, ranging from a net of 1.4–3.4 m per event. The three latest faulting events occurred at shortly before 0.6–0.8 ka, 2.8 ± 0.7 ka, and circa 3.8–7.9 ka. Earlier events cannot be directly dated because older graben-fill sediments yielded thermoluminescence ages older than the time of deposition, and some scarp-derived colluvial wedges beneath the trench floor were not exposed. The two younger faulting events we recognize at Kaysville correlate reasonably well with faulting events on the same segment 25 km north near East Ogden, Utah, at circa 0.8–1.2 ka and 2.5–3.0 ka (Forman and others, 1991), whereas the earlier Kaysville event is significantly older than the earliest (3.5–4.0 ka) event dated at East Ogden. The 3.5–4.0 ka ground rupture recognized at East Ogden may have died out at a subsegment boundary between the two trench sites within the 61-km-long Weber segment.