Abstract

Dating of lower Paleozoic cherts is now possible through advancements in radiolarian biostratigraphy, permitting greater refinement of structural and stratigraphic models than has previously been possible. A detailed study of the type section of the Ordovician Vinini Formation, Vinini Creek, Roberts Mountains allochthon, Nevada, has yielded anomalously young Devonian ages. These ages were unanticipated and indicate that structural imbrication exists in a classic and previously well dated section of basinal strata. Previous ages of cherts were inferred from their stratigraphic position relative to graptolite-bearing shales. New mapping and biostratigraphy reveal considerable structural interleaving of Ordovician and Devonian strata within the allochthon, and many of the cherts appear to have structural bases. The Roberts Mountains allochthon, considered to be allochthonous but not far traveled, displays a highly complex structural history, including out of sequence thrusting and/or overprinting by later contractional deformation having a similar transport direction. This deformation was previously unrecognized because of the lack of overlap sequences in critical areas and the paucity of internal age control. Because the Roberts Mountains allochthon is not considered to be exotic, but rather has undergone only a modest amount of transport, this study shows that convergent margins with high degrees of structural imbrication do not require large amounts of transport. A biostratigraphic approach that examines all lithologies, coupled with detailed observations of field relations, is required for the construction of detailed and accurate structure sections in early Paleozoic orogenic belts.

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