COMPOSITION OF MODERN AND CRETACEOUS SAND(STONE) DERIVED FROM THE SIERRA NEVADA, CALIFORNIA, USA, WITH IMPLICATIONS FOR CENOZOIC AND MESOZOIC UPLIFT AND DISSECTION

The California Sierra Nevada (SN) represents the roots of a primarily Cretaceous magmatic arc that is presently being dissected. The Cenozoic magmatic arc built on top of the Cretaceous arc has been dissected concurrently with northward migration of the Mendocino triple junction, south of which the magmatic-arc is inactive. A north-to-south transect from the modern Cascade arc along the SN, therefore, acts as a proxy for evolution from an active magmatic arc to a transitional arc to a dissected arc to uplifted basement.

Discriminant analysis of Cascade and SN alluvial sand defines four compositional groups and clearly distinguishes volcaniclastic undissected-arc and plutoniclastic uplifted-basement end members. Two intermediate compositional groups are primarily volcaniplutoniclastic and metamorphiplutoniclastic. The former group represents Dickinson’s (1985) “transitional arc;” the latter represents “dissected arc.” Sand composition in the southern SN reflects significant Cenozoic uplift, resulting from Basin and Range extension (footwall uplift of the western rift shoulder), possibly enhanced by mantle-lithosphere delamination. High concentrations of quartz and feldspar, and extreme depletion of aphanitic lithic fragments are typical at the southernmost SN; this extreme depletion characterizes Dickinson's “basement uplift.”

Cretaceous sandstone petrofacies of the Great Valley Group (GVG) of western California document dissection of the Cretaceous magmatic arc, as well as erosion of residual orogenic highlands formed during the latest Jurassic Nevadan orogeny (analogous to modern Taiwan). When Cretaceous petrofacies data are entered into the discriminant analysis as unknowns, they are classified into the modern Cascade/SN groups and a group from Taiwan. The results confirm that the lowest GVG petrofacies represent erosion of the Nevadan suture belt and the undissected post-Nevadan arc. Increasing dissection of the arc occurred during the Cretaceous, but most samples represent "transitional arc," indicating that even by the latest Cretaceous (Laramide) cessation of arc activity in California, dissection of the magmatic arc was less than what we see today in the central and southern SN. No Cretaceous samples have compositions equivalent to the "basement uplift" of the southern SN.