THE SOUTHERN SIERRA NEVADA FOOTHILLS BEDROCK PEDIMENT

A bedrock landscape present in the western Sierran Foothills between 36°N and 37°N is here interpreted to be an exhumed Late Cretaceous to early Tertiary pediment. Based on field reconnaissance, the pediment landscape is divided into three main geomorphic zones: a sub-horizontal bedrock pediment surface; a multi-scale set of bedrock tors (~ 1 meter scale relief), monadnocks and hilly ranges (~ 100 meter scale relief); and the transition between these two zones marking the slope to pediment plane transition, lacking lithologic change. Relief of hill slopes adjacent to the modern pediment locally exceeds 500 m, which serves as a minimum constraint on paleo-relief above the Late Cretaceous pediment surface. Analysis of water-well logs indicates that the pediment extends at least several kilometers west of the Foothills in the San Joaquin Valley subsurface, where it is overlain by 100 m or less of middle to late Pleistocene glacial outwash fan deposits. Lithology of the bedrock pediment generally is granitoid, but locally includes pendants of ultramafic rocks of the Kings-Kaweah ophiolite belt. Apatite U-Th-Sm/He thermochronometric data from 10 samples collected from this landscape document 74.6 Ma +/- 7.4 Ma rapid exhumation of the bedrock. The pediment surface is overlain by Eocene Ione Formation at Little Table Mountain in the north and by Eocene Walker Formation near Fountain Springs in the south. These relations indicate a Late Cretaceous to Eocene age for the pediment. Where exposed as part of the pediment landscape, individual monadnocks of ophiolite are covered with a Si and Fe-rich rind of highly weathered and indurated rock, which possibly developed during the oxic (tropical) Eocene climate and weathering regime in this part of California. These thermochronometric, geomorphic, and stratigraphic data define a roughly range-parallel joint pre-Eocene relict landscape and Late Cretaceous apatite U-Th-Sm/He isochronal swath that extends along strike at least from Fountain Springs to Little Table Mountain. This relict landscape correlates with a set of surfaces that mimic both the western shoulder of a regional orogenic plateau called the Nevadaplano, which characterized the U.S. Cordillera in Late Cretaceous to mid-Cenozoic time, and the basal Late Cretaceous nonconformity beneath the adjacent Great Valley.