LATE TRIASSIC GASTROPOD FAUNAS OF WESTERN NORTH AMERICA - A USEFUL TOOL IN TERRANE ANALYSIS

Late Triassic gastropods are locally common to abundant in shallow-water strata of accreted terranes and para-autochthonous rocks of western North America. Despite their abundance, they have only recently been the subject of detailed taxonomic study. Gastropods are excellent tools in terrane analysis because certain groups (notably pleurotomarioideans and trochoideans) have lecithotrophic larvae, allowing for only limited geographic dispersal (typically being restricted to only one or several terranes). In contrast, other gastropod groups such as certain elements of the neritimorphs and caenogastropods have planktotrophic larvae capable of much broader geographic dispersal (occurring in many terranes), making them excellent tools in interterrane correlation.

Middle Norian gastropods from central Nevada and Sonora share many species and belong to a single biogeographic province (which we previously named the Nevada-Sonora Province). These rocks are considered para-autochthonous, representing a setting near to or a part of the North American craton. The presence of the same fauna in both areas lends support to the concept of the Mojave-Sonora megashear. The Wrangellia terrane of southern Alaska and the Wallowa terrane of the eastern Oregon and western Idaho share a very similar, nearly identical, early Norian gastropod fauna of high diversity (50+ species), suggesting that they were positioned near one another, possibly conterminous as formerly suggested, in the tropics of the Panthalassa Ocean. In contrast, the Alexander terrane of southeast Alaska has a strikingly dissimilar early Norian gastropod fauna, with only one species, Spinidelphinulopsis whaleni Blodgett, Fryda, and Stanley, shared in common with the Wrangellia and Wallowa terranes. Late Norian gastropods are very similar between the Alexander, Chulitna, and Farewell terranes of southern Alaska, sharing species such as Chulitnacula alaskana (Smith), Andangularia wilsoni Blodgett, and Neritaria nuetzeli Blodgett, suggesting close physical proximity between these three terranes during Late Triassic time. Our future collaborative work should focus on further study of the taxonomy and paleobiogeographic affinities of Late Triassic gastropods from these and other western North American terranes, as well as additional materials from Peru.