UPDATES TO THE LATE PALEOZOIC HISTORY OF THE ALEXANDER TERRANE

New data from the Alexander Terrane significantly changes interpretations of its Late Paleozoic history.

The Pennsylvanian succession includes unnamed Lower Pennsylvanian bioclastic limestone, Middle Pennsylvanian volcanic debris flows and carbonate of the Saginaw Formation volcanic member, and Middle Pennsylvanian slope, silty carbonate deposits of the Saginaw silty and cherty limestone members. Shallow-water carbonate crops out northeast of the slope deposits, indicating an approximate southwest trending depositional slope during the Pennsylvanian.

The Pennsylvanian rocks are unconformably overlain by immature sandstone and conglomerate with rugose coral bioherms of the Halleck Formation. Wave ripples, channel scour at the base of conglomerates, and reworked Pennsylvanian conodonts indicate abrupt shallowing and erosion preceding this unit. Conodonts from the Halleck Formation are a mixed Mesogondolella spp. and Streptognathodus spp. Fauna; an Asselian (earliest Permian) age fauna previously found only in the southern Ural Mountains of Russia.

These Pennsylvanian and lowermost Permian rocks are unconformably overlain by Middle Permian (Wordian) carbonate of the Pybus Formation; most of the lower Permian is absent. Middle Permian Pybus rocks also overlie more deformed Mississippian rocks of the Cannery Formation to the east. The abrupt shallowing recorded within the Halleck and the overlap of diachronous units by the Pybus suggests tectonic uplift near the Carboniferous-Permian boundary followed by transgressive onlap of the Pybus. Late Triassic rocks in the region show a depositional slope opposite to that of the Pennsylvanian assemblage, further supporting tectonic reorganization during the Permian. Some of the Pybus was also found to be Late Permian (Wuchiapingian) and Early Triassic (Smithian).

Brachiopods and conodonts in the Halleck and the Pybus generally indicate cool-water settings similar to the Pennsylvanian and Permian of northwestern Pangea. However, the Late Permian conodont Clarkina at one Pybus locality indicates warm-water conditions more closely related to the Tethys, suggesting a shift in temperature for the Alexander Terrane during the Permian. This mixed paleobiogeographic signature agrees with that found in Mississippian foraminifers for the terrane.