FourierĪnalysis of that sequence failed to identify evidence of tides. Where thicknesses could be determined with adequate precision from field photography. Only one sequence, in lower Milpitas Wash (California), was identified Southern Bouse Formation (south of Blythe, California) so that thicknesses could be measuredįrom the photographs. ToĮvaluate the tidal interpretation, we attempted to photograph several laminated sequences in the Spectral analysis of lamination thicknesses in continuous sequences of laminated sediments. Tidal cycles, if present, should be detectable by Fourier (2017) to represent the spring-neap tide cycleĭuring sediment deposition in an estuary. Marl and claystone interpreted by O’Connell et al. The Pliocene Bouse Formation in the lower Colorado River trough locally contains laminated Today,Įphemeral or seasonal drainages continue to deeply erode alluvial fans and bajadas on all sides of the ranges San Pedro Rivers and other streams integrated the drainage of southeastern Arizona with that of the Gila andĬolorado Rivers, their tributaries eroded headward into the alluvial fill of these formerly closed basins. The sea and held shallow lakes (playas) that fluctuated in size with changes in climate. Many of these sediment-filled basins had no drainage outlets to The rising ranges and transported eroded boulders, cobbles, gravel, sand and clay to aprons of sediment - alluvialįans and bajadas - in nearby subsiding basins. Some of these crustal blocks were uplifted toįorm ranges other blocks subsided as much as 2.4 mi (4 km) to form deep basins. North America into blocks, separated by steeply dipping faults. Results from a period of extension from about 15 to 5 million years ago that broke the crustal rocks of western Most of these ranges and intervening basins trend northwest-southeast and are part of the huge geologicīasin and Range Province that extends from southern Oregon to central Mexico. Make up the Basin and Range geologic province of southeastern Arizona. Mountains, together with over a dozen other ranges, that are either lower in elevation or more difficult to access,
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Rita, Tucson, Galiuro, Pinaleño, Chiricahua, Mule, Huachuca, and Whetstone Mountains (Figure 1). The order that they are presented in this book, these island ranges are: Santa Catalina, Rincón, Tortolita, Santa The average slip rate during the past 300,000 to 700,000 years is between 0.015 to 0.04 m/ky (ky means thousands of years).Įleven island-like mountain ranges tower thousands of feet above adjacent basins in southeastern Arizona. The magnitude of the youngest surface-rupturing earthquake on the fault was probably 6.6 to 6.9, which may be characteristic of previous surface ruptures on the fault.
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The Bellemont fault is an active fault capable of producing large earthquakes. This fault data was integrated into several alternative probabilistic assessments of seismic hazard in the Flagstaff area. We augmented this study with a reconnaissance survey of other faults in the area that have been active during the past 5 million years. Much of our effort was devoted to a detailed paleoseismologic investigation of the Bellemont fault west of Flagstaff. In this report, we provide a perspective on seismic hazard in the Flagstaff area based primarily on geologic assessments of the potentially active faults in the area. Geologic investigations of young faults provide information about large prehistoric earthquakes in an area, which can be used with the historical seismic record to evaluate the potential for damaging earthquakes. Historical seismic activity and the geologic record of young faulting both suggest that there is significant seismic hazard in the Flagstaff area.