A PRELIMINARY REPORT ON THE THE STRATIGRAPHY AND BIOSTRATIGRAPHY OF THE PERATROVICH FORMATION AND LADRONES LIMESTONE

The earliest geologic studies of the Paleozoic systems on Prince of Wales Island were restricted to shoreline exposures of the thickly vegetated islands near Craig, Alaska. Outcrops recently exposed by logging and quarrying on Madre de Dios and Shelikof Island have revealed features, not previously described, within the Peratrovich Formation and Ladrones Limestone. Measured sections were made on the interior of Madre de Dios Island and along the southern shoreline of Shelikof Island. Samples were collected at various stratigraphic intervals for faunal and facies analysis.

The Peratrovich Formation on Madre de Dios and Shelikof Island ranges from rhythmically bedded chert and micrite in the lowermost part of exposed sections to massively bedded fossiliferous micrite. Micrite sampled from these beds exhibit characteristics indicative of a semi-protected depositional environment. Exposures along the southern shore of Shelikof Island contain the conodonts Mestognathus sp., Bispathodus utahensis, and Cavusgnathus sp., which are Viséan (Late Mississippian) in age and are characteristic lagoonal fauna described by von Bitter and Sandberg (1986). The upper portion of this section consists of thickly-bedded limestone, containing lenses of cross-bedded fossiliferous packstone and grainstone. This suggests that shallow water facies are more prevalent in southern sections of the Peratrovich Formation than previously reported.

A discrete volcaniclastic unit overlies the lower Peratrovich Formation on Shelikof Island. The Carboniferous deposits of the Alexander terrane have been previously observed to be devoid of volcaniclastic input, which is in contrast to those Carboniferous deposits of the Wrangellia terrane.

The Ladrones Limestone is poorly exposed at Madre de Dios, Shelikof, and the Ladrones Islands. Its contact with the underlying Peratrovich Formation is covered on the Madre de Dios and Shelikof Island. The conodonts Adetognathus unicornis and Gnathodus bilineautus bilineautus were recovered from the Ladrones Limestone. Conodont zonations suggest that the unit is Serpukhovian (Late Mississippian) in age. Previous workers have not placed the Ladrones Limestone within the Mississippian and have suggested that it is entirely Pennsylvanian.