CAMBRIAN TRILOBITES OF SIBERIAN BIOGEOGRAPHIC ASPECT FROM SOUTHWESTERN ALASKA: IMPLICATIONS FOR UNDERSTANDING THE PROVENANCE OF THE FAREWELL TERRANE

A small trilobite fauna from an unnamed limestone unit in the Nixon Fork subterrane of the Farewell terrane, southwestern Alaska, mostly comprises taxa having a Siberian biogeographic aspect. Biostratigraphically, the fauna compares most favorably with taxa characteristic of the Toyonian Stage as used in Siberia, although species in the fauna range from the Botomian through the Amgaian. All trilobites identified to species level in the fauna have been previously reported from Siberia. At the generic level, however, many of the taxa were rather widespread among continents that were in low paleolatitudes. This suggests that the Nixon Fork subterrane, which contains pagetiid, dorypygid, and anomocarid trilobite genera, had access to the open ocean during Toyonian time.

Determination of the provenance of the Farewell terrane has been based on various lines of geologic evidence, and has been subject to changing interpretation as evidence has accumulated. Previous interpretation of the Farewell terrane as a native Laurentian terrane, based largely on stratigraphic evidence, led to an earlier conclusion that Cambrian trilobites from the Nixon Fork subterrane represented a deep, cool-water assemblage with faunal links to Siberia. However, much recently acquired information, primarily Ordovician to Devonian biogeographic data, suggests that the Farewell terrane represents instead a continental margin succession that rifted from the Siberian paleocontinent. Of the two alternative hypotheses, Cambrian trilobite data tend to accord better with the interpretation that the Nixon Fork subterrane represents a fragment of the Siberian paleocontinent.