PETROLOGIC, FOSSIL, AND DETRITAL ZIRCON DATA FROM DEVONIAN-TRIASSIC STRATA ON ST. LAWRENCE ISLAND, ALASKA: LINKS TO NORTHWESTERN ALASKA AND EASTERN RUSSIA

Lithologic and U-Pb detrital zircon (DZ) data from Devonian-Triassic strata on St. Lawrence Island (SLI), Alaska, suggest ties to allochthonous successions in the western Brooks Range, Alaska, as well as to rocks in eastern Russia. Two successions of Triassic and older strata were delineated by Patton and Csejtey (1980) on SLI: one mainly carbonate rocks and the other mainly graywacke turbidites. Petrologic, fossil, and DZ data from archival USGS samples illuminate the affinities of these rocks.

The carbonate succession consists of ~1300 m of Devonian dolostone with local corals and brachiopods, overlain by >400 m of fossiliferous Mississippian limestone that contains beds of quartz±carbonate arenite in its lower part. The youngest unit is ~120 m of shale, chert, and limestone that contains daonellid, halobid, and monotid clams of Triassic age in its upper part. Fossils and lithofacies of these strata correlate well with the Devonian Baird Group, Mississippian Utukok and Kogruk Fms., and Triassic Otuk Fm. The overall succession is most similar to coeval strata of the Kelly River allochthon in the western Brooks Range, >600 km to the NE.

Distinctive arkosic rocks, locally rich in calcareous clasts, late Paleozoic fossils, and perthitic feldspar, make up the lower part of the SLI graywacke succession. DZ U-Pb ages from a plagioclase-rich sample of this lithofacies are virtually all 2.06 Ga. Similar feldspathic rocks with this characteristic DZ signature are the Nuka Fm., which is exposed sparsely throughout the Brooks Range and is the hallmark of the Nuka Ridge allochthon.

The main part of the graywacke succession is poorly sorted heterolithic sandstone associated with gabbro and chert containing Triassic radiolarians. A DZ spectrum from a lithic-rich sample of this lithology has a youngest peak at 232 Ma (Late Triassic) and a major peak at 305 Ma (Pennsylvanian). This age distribution is similar to those from sandstone interbedded with the Otuk Fm. on Cape Lisburne, NW Alaska, as well as from Triassic turbidite sequences in eastern Russia (Chukotka and Wrangel Island; Miller et al., 2006).

Our data thus suggest the presence on SLI of strata equivalent to upper allochthons of the Brooks Range (Kelly River and Nuka Ridge) in fault(?) contact with lithic-rich Triassic turbidites likely derived from the present-day NW.