Implications of Palaeozoic phosphorites in the northern Sierra Nevada range

Abstract

Marine phosphorites are known to occur within multiply deformed Ordovician/Silurian and Upper Devonian rocks of the northern Sierra Nevada range, California. Although the phosphatic sediments constitute a small percentage of total lithologies in the area, they provide unique information regarding the age and palaeogeographical setting of these rocks. The mode of occurrence of the Sierran phosphorites and their association with black shale and chert suggest that they may have formed along a continental margin which experienced strong equatorial or trade wind belt upwelling. Consideration of worldwide plate reconstructions for the early to middle Palaeozoic shows that the present western margin of North America was one of the few favourable sites for phosphate formation during this time period. Thus, the presence of marine phosphorites within Palaeozoic rocks of the northern Sierra Nevada is consistent with arguments which suggest that these rocks were deposited near the western margin of North America. Although considerable margin-parallel translations cannot be ruled out for these rocks, they do not appear to constitute a far-travelled microplate derived from a distant, trans-oceanic site.