UPPER TRIASSIC GASTROPOD FAUNA FROM SOUTHERN ALASKA AND ITS IMPLICATIONS FOR TERRANE ACCRETION

Diverse Norian age gastropod faunas are known from four accreted terranes (Alexander, Wrangellia, Chulitna, and Farewell) of southern Alaska. For the most part, gastropods from these terranes are quite distinct at the species level and represent new taxa.

Taxa having lecithotrophic larval development, notably the more primitive groups such as the pleurotomarioideans tend to be endemic and are restricted to only one or several terrane(s). In contrast, some taxa characterized by planktotrophic larvae, such as certain elements of the neritomorphs and caenogastropods, tend to be broadly distributed and are found in many terranes.

A diverse (50+ species) early Norian gastropod fauna is present in the uppermost part of the Chitistone Limestone of the Wrangellia terrane. Over 60% of the species are also known from coeval strata in the Wallowa Mountains of eastern Oregon (Wallowa terrane). In contrast, only one species, Spinidelphinulopsis whaleni, is shared in common between the early Norian gastropod faunas of the Alexander and Wrangellia terranes of southern Alaska. The very close similarity among the gastropods of the Wrangellia and Wallowa terranes, as well as gross stratigraphic similarities supports the earlier view of the Wallowa terrane as representing the southernmost extension of Wrangellia. The abundance of evaporite deposits and paucity of fossil wood in Upper Triassic strata of Wrangellia suggest that this terrane was located in a warm arid position somewhere on the eastern side of the Panthalassa Ocean.

Late Norian gastropods are very similar between the Alexander, Chulitna, and Farewell terranes, sharing species such as Chulitnacula alaskana (Smith), Andangularia wilsoni, and Neritaria nuetzeli and other taxa. These close faunal ties (also noted among the corals, brachiopods and bivalves) suggest close physical proximity between these three terranes during Late Triassic time in a warm humid position (as suggested by the abundance of fossil wood in the Alexander terrane) also on the eastern side of the Panthalassa Ocean.