REEXAMINING THE STRATIGRAPHIC AND PALEONTOLOGIC DEFINITION OF WRANGELLIA

The concept of tectonostratigraphic terranes originally assisted verification of plate tectonics by providing numerous remnant examples of past geologic histories. These entities, defined by their unique tectonic and stratigraphic histories, comprise most of western North America and provide the best source for understanding the evolution of the North American continental margin. Renewed scrutiny of defined terranes with advances in stratigraphy and paleontology has revealed that many of the original terranes need correction or redefinition.

Wrangellia occupies the geographically disparate areas of Vancouver Island, the Queen Charlotte Islands, and southern Alaska, and was one of the earliest recognized terranes. Wrangellia is defined by similarities in the Upper Triassic paleomagnetic results and gross stratigraphic section, including faunas, position of large-scale unconformities, and overall geologic history. The base of the terrane, as defined, was originally a Pennsylvanian to Permian volcanic arc sequence that may have been deposited on oceanic crust.

However, Vancouver Island no longer approaches the definition of Wrangellia and is dissimilar to the published stratigraphy of Alaskan Wrangellia. To contrast, on Vancouver Island the stratigraphy begins with Silurian to Devonian sea floor volcanism overlain by Mississippian to Permian carbonate and siliciclastic deposits. While Pennsylvanian to Permian volcanic arcs are preserved in Alaska, there is no arc volcanism in the Pennsylvanian to Permian of Vancouver Island. Furthermore, Paleozoic faunas from Vancouver Island differ substantially from those in Alaskan Wrangellia. The Triassic succession, as defined, has gross stratigraphic similarities, but little precise control exists. Recent data also suggest that portions of the Triassic type section of Wrangellia are incorrectly dated.

Based on updated stratigraphy from Vancouver Island and the lack of detail for Alaskan Wrangellia, the concept of Wrangellia requires reexamination. Until proof exists for a relationship between the two geographically unconnected areas, Vancouver Island and the Queen Charlottes should be relegated to the previously named Vancouver Terrane.