DETRITAL ZIRCON AGE PROVENANCE OF LATE PRECAMBRIAN TO EARLY CAMBRIAN SEDIMENTS FROM THE SAN BERNARDINO MOUNTAINS, CALIFORNIA

In the San Bernardino Mountains of southern California a discontinuous to semicontinuous succession of quartzites and phyllites records North America’s transition from a rifted to a passive margin in the Eocambrian. Known in part as the Big Bear Group (Cameron, 1981), this succession illustrates how provenance patterns change in tandem with the evolving margin. Using the SHRIMP-RG to measure the age distributions of detrital zircon populations, provenance patterns were determined for several units in the Neoproterozoic Big Bear Group, and overlying Cambrian units, the Wood Canyon Formation and the Zabriskie Quartzite. From oldest to youngest, age spectra all displayed a trimodal distibution with peaks at ~1.0-1.2 Ga., ~1.3-1.5 Ga., and ~1.6-1.8 Ga. These populations vary relative to one another, giving each unit a distinct pattern. Minor age populations appear as a distinguishing characteristic as well. The Zabriskie for example, has a number of 2.6-3.4 Ga. grains, a subpopulation entirely absent in the underlying Wood Canyon Formation. The Wood Canyon Formation is distinguished by an abundance of 1.0-1.2 Ga. grains whereas the Zabriskie has but little 1.0-1.2 Ga material compared to its much larger 1.7-1.8 Ga grain population. The basal unit, the Wild Horse quartzite differs from these upper units in having relatively equal proportions of 1.0-1.2 Ga., 1.3-1.5 Ga., and 1.6-1.8 Ga. material. Regardless of relative proportions, the age populations do not bear perfect resemblance to nearby basement sources. Widespread cratonic or exotic sources are necessary to create these provenance patterns. The nature of this source and how it fed sediment to the margin is an important factor in determining North America’s role in late Precambrian continental arrangements.