Distal Floodplain Research Articles

Depositional control of aquifer characteristics in alluvial fans, Fresno County, California: Summary

Three coalescing alluvial fans, the high alluvial fans of the San Joaquin and Kings Rivers and the intervening compound alluvial fan of intermittent streams (Page and LeBlanc, 1969), and their distal flood plains from the surficial and shallow subsurface deposits (100 m thick) of central Fresno County (Fig. 1). These deposits extend from the foothills of the Sierra Nevada to the axis of the San Joaquin Valley 58 km away. Physically they resemble the smaller, semiarid-region alluvial fans found in western Fresno County (Bull, 1964a, 1964b), but they lack the extensive debris-flow (mud-flow) deposits found in the western fans. The central Fresno County alluvial fans consist of vertical and lateral accretion deposits derived from meandering streams. Within these fan—flood-plain deposits, aquifer properties change in a specific downfan sequence. From available data on sediment in surface exposures, cores and well logs, and ground-water information, a description of the relationship between the observed aquifer characteristics and the depositional system of the fans is given here. GEOLOGIC SETTING The Fresno alluvial fans are part of a continuum of fans along the east side of the San Joaquin Valley. The present fans are the most recent of a sequence of Cenozoic sedimentary deposits that in the axis of the San Joaquin Valley are more than 4,000 m thick (Hackel, 1966). They contain Pleistocene and Holocene deposits of lacustrine, marsh, and fluvial origins. Much of the sedimentary and depositional history of this sequence is the result of the Pleistocene climate and associated glaciations. These valley-fill sediments provide a record of tectonism and erosion of the Sierra Nevada during Late Cretaceous to Paleocene time and during renewed uplift in late Miocene—early Pliocene time. This renewed uplift was responsible for the present elevation and tilting of the Sierra Nevada and for the depth of incision of the major rivers into the batholith (Bateman and Wahrhaftig, 1966).