Permian Tethyan Fusulinina from the Kenai Peninsula, Alaska

Journal of Paleontology, Nov 1997 by Stevens, Calvin H, Davydov, Vladimir I, Bradley, Dwight

ABSTRACT-Two samples from a large, allochthonous limestone block in the McHugh Complex of the Chugach terrane on the Kenai Peninsula, Alaska contain species of 12 genera of Permian Fusulinina including Abadehella, Kahlerina, Pseudokahlerina?, Nankinella, Codonofusiella, Dunbarula, Parafusulina?, Chusenella, Verbeekina, Pseudodoliolina, Metadoliolina?, Sumatrina?, and Yabeina, as well as several other foraminiferans and one alga. The assemblage of fusulinids is characteristically Tethyan, belonging to the Yabeina archaica zone of early Midian (late Wordian) age. Similar faunas are known from the Pamirs, Transcaucasia, and Japan, as well as from allochthonous terranes in British Columbia, northwestern Washington, and Koryakia in eastern Siberia.

INTRODUCTION

TWO-SAMPLES bearing Permian fusulinaceans were collected from a large limestone block in the McHugh Complex along the eastern wall of Petrof Glacier, about 3.5 km north of its terminus along the southeast coast of the Kenai Peninsula in the Seldovia quadrangle (Figure 1). These fossils are important in that they show that the block in which they occur was derived from a shallow-water, tropical, western Pacific, Permian, carbonate platform prior to being incorporated into the subduction complex, along with other blocks of vastly different ages and environments.

The McHugh Complex of south-central Alaska is part of a vast subduction complex of Mesozoic age-the Chugach terrane-exposed along the margin of the Gulf of Alaska. It is named for exposures near Anchorage (Clark, 1972) which consist of a tectonic melange of greenstone, chert, argillite, graywacke, and conglomerate, plus rare blueschist, gabbro, ultramafic rocks, and, the focus of this paper, limestone. Equivalent rocks on Kodiak Island are assigned to the Uyak Complex (Connelly,1978). In the USGS 1:250,000-scale Seldovia quadrangle, where the McHugh Complex has been studied in greatest detail, it has an outcrop width of 40 km (Bradley and Kusky, 1992), and consists of a deformed argillite matrix that encloses blocks from several kilometers across down to hand-sample size and smaller of the rock types listed above.

Most of the fossil control for the McHugh Complex has come from radiolarian chert. Intensive sampling of chert from the Seldovia quadrangle has disclosed ages ranging from Ladinian (Middle Triassic) to Albian-Aptian (mid-Cretaceous) according to C. Blome (personal commun., 1994). The span of ages from the Seldovia quadrangle is consistent with, but more complete than, ages previously reported from the Anchorage quadrangle (Nelson et al., 1987), the Valdez quadrangle (Winkler et al., 1981), and Kodiak and adjacent islands (Connelly, 1978). One sample from Kodiak Island yielded cryptoceptic Nassellarna forms (Connelly, 1978; identification by E. Pessagno), originally considered Paleozoic; however, the lower range of nasselarians is now known to be in the Triassic (B. Murchey, personal commun., 1996).

Blocks of limestone, including those containing the fusulinaceans described here, are scattered throughout the Chugach terrane. Most of the dated blocks are Permian in age, but one from a conglomerate in the McHugh Complex in the Anchorage quadrangle has yielded conodonts with a possible age of late Meramecian to early Morrowan (Late Mississippian to Early Pennsylvanian). Nelson et al. (1986) suggested that this clast could have been derived from the Strelna Formation of the Wrangellia terrane. In the Seldovia quadrangle, the distribution of limestone blocks shows no regional pattern. Blocks range in size from subequant lozenges a few centimeters across, to fragments of formations 50-100 m thick and 100-200 m long. Typically, the limestone blocks are surrounded by phacoidally cleaved argillite; none has yet been found in stratigraphic contact with adjacent rocks. Some limestones occur in long strings of boudins, suggesting extensional disruption of initially continuous layers.

In addition to the Permian fusulinid-bearing limestone block that is the focus of this paper, two other limestone blocks in the Seldovia quadrangle have yielded Permian conodonts of Tethyan affinity; both conodont faunas are Wuchiapingian (Late Permian) in age (B. Wardlaw and A. Harris, written commun., 1994). One of these blocks (93APH204 in Figure 1) is less than 1 km from, and on strike with, the fusulinid-bearing limestone (93ADw41 in Figure 1). Clark (1972) previously reported the presence of schwagerinid fusulinids and the Tethyan neoschwagerinid Cancellina in the Chugach Mountains near Anchorage. On Kodiak Island, Connelly (1978) reported one limestone block in the Uyak Complex (part of the Chugach terrane) as containing a mid-Permian Tethyan fossil assemblage including Neoschwagerina, Cancellina?, Codonofusiella, Colaniella, Pachyphloia, Nodosinella, and a dasycladacean alga, and another block containing interbedded tuff and yielding Neoschwagerina (identifications by G. Wilde).

FOSSIL ASSEMBLAGE

Permian fossils have been recovered from two small samples from the large limestone block in the McHugh Complex at the locality on the east side of Petrof glacier (Figure 1). The diversity of sample 93ADw4lA is low, consisting of nine recognized species. Two species of Yabeina and one species of Nankinella represent the fusulinids. In addition, at least five species of smaller foraminifers probably belonging to the genera Lunucammina and Agathamina and one species of alga, probably Mizzia, are present. The second sample, 93ADw4lB, contains the foraminiferan Pachyphloia and a much richer assemblage of Fusulinina including species of the genera Abadehella, Kahlerina, Pseudokahlerina?, Codonofusiella, Dunbarula, Parafusulina?, Chusenella, Verbeekina, Pseudodoliolina, Metadoliolina?, Sumatrina?, and Yabeina. Species of all of these genera are briefly described except for the form assigned to Pseudokahlerina? because it is represented by only one very poorly preserved and oriented specimen. It is differentiated from Kahlerina, however because the axial length apparently exceeds the diameter. Thin sections of all specimens have been retained in the Museum of Paleontology, Department of Geology, San Jose State University.