NEW AGE CONTROL FROM THE WASATCH FAULT MEGATRENCH OF 1999

The “megatrench” excavated in 1999 across the Salt Lake City segment of the Wasatch fault zone yielded evidence for 7-8 paleoearthquakes since the transgression of Lake Bonneville ca. 20 ka. In 2000 I dated 14 radiocarbon samples from the megatrench and proposed a preliminary chronology. This paper presents a refined paleoearthquake chronology based on 11 additional radiocarbon samples dated in 2001, and on age estimates from quantitative pedology. The megatrench exposed evidence for five paleoearthquakes younger than an early Holocene alluvial fan dated at ca. 8-10 ka (calibrated millennia before present). The later four of these events (ca. 1.3 ka, 2.4 ka, 3.9 ka, 5.3 ka) had been recognized in previous trenching studies on Holocene fans. An additional post-fan paleoearthquake occurred at the megatrench site at ca. 7.5 ka. An ambiguous colluvial wedge interstratified with fan gravels, plus unconformities on fault blocks, suggest a paleoearthquake between 8.8 and 9.8 ka during fan deposition. Evidence for the previous earthquake consists of a subaqueous debris flow and soft-sediment deformation, indicating it occurred immediately before the Bonneville Flood (ca. 17.2 ka) when the site was submerged. Thus, we see no evidence for earthquakes between ca. 17.2 ka and 9.8 ka, a span of 7.4 ky, or roughly five times as long as the mid-late Holocene average recurrence interval (1.4 ky). One possibility is that the early Holocene fan eroded away all preexisting colluvial wedges. However, two thick soil profiles underlie the basal Holocene fan and are not visibly eroded. The soils show no evidence of scarp-derived colluviation, even adjacent to faults, nor of incremental tilting during the 7.4 ky time span. A similar early Holocene aseismic interval is interpreted from the 1991 Brigham City trench site, where half of the trench sites were never subjected to Holocene fan erosion or deposition. The rate of soil clay accumulation during the latest Pleistocene-earliest Holocene is 5-10 times that of the post-Bonneville average.