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Composition of Modern Sand from the Sierra Nevada, California, U.S.A.: Implications for Actualistic Petrofacies of Continental-Margin Magmatic Arcs

¹ Department of Earth and Space Sciences, University of California, Los Angeles, California 90095-1567, U.S.A.; ringer{at}ess.ucla.edu
² Department of Earth and Space Sciences, University of California, Los Angeles, California 90095-1567, U.S.A.

The Sierra Nevada of California represents the roots of a long-lived magmatic arc (primarily Cretaceous) that is presently being dissected as the range is uplifted, beginning in the south and progressing northward. This dissection is occurring concurrently with northward migration of the Mendocino triple junction, south of which magmatic-arc activity is absent, and north of which magmatic-arc activity continues. A north-to-south transect along the Sierra Nevada represents transitions of active magmatic arc to transitional arc to dissected arc, and in the southern Sierra Nevada, to uplifted basement. Thus, analysis of composition of modern sand derived from these separate parts of the Sierra Nevada provides a test for actualistic sand(stone) petrofacies of magmatic arcs at all stages of dissection.

Rigorous statistical analysis of the composition of modern sand from the Sierra Nevada characterizes stages of magmatic-arc dissection. Discriminant analysis of point-count data from Sierra Nevada modern sand defines four compositional groups and distinguishes clearly between undissected-arc and uplifted-basement end members. Two intermediate compositional groups reflect the complex geologic history of the Sierra Nevada. Volcanic and plutonic rocks dominate one intermediate group; metamorphic and plutonic rocks dominate the other. The former group exhibits characteristics of Dickinson's (1985) "transitional arc," whereas the latter is equivalent to "dissected arc." No single compositional parameter is diagnostic of the degree of magmatic-arc dissection, although volcanic-lithic content clearly differentiates between "undissected" and "basement" groups. Regional compositional trends are evident when recalculated point-count data are plotted against latitude.

Sand composition in the southern Sierra Nevada reflects significant Cenozoic uplift of the batholith, resulting from Basin and Range extension. High concentrations of quartz and feldspar, and extreme lithic depletion are typical at the southernmost Sierra Nevada; this extreme depletion characterizes "basement uplift."

The primary drainage divide of the Sierra Nevada forms a rough compositional boundary between eastern and western groups. Potassium-feldspar content best distinguishes these groups, with higher values in the east.

Rigorous confidence regions in ternary diagrams yield results that are similar, but not identical, to hexagonal fields of compositional variation. Arithmetic means, associated with hexagonal fields, are not identical to geometric means, associated with logratio confidence regions. Definition of sand(stone) provenance groups, in general, depends on both data-acquisition methods (e.g., Gazzi–Dickinson method) and statistical methods (e.g., Weltje method), so that direct comparison of data analyzed using different methods is unwarranted.