IMPLICATIONS OF MIXED BIOGEOGRAPHIC AFFINITY OF A COASTAL FLORA AND FAUNA IN THE EARLY PERMIAN DALL BASIN, ALASKA, USA

Metamorphic and sedimentary evidence proposed recently by Bradley et al. suggests an important event in the early coalescence of Alaska during the Pennsylvanian/Permian called the Browns Fork orogeny. This orogeny resulted from the collision of the Farewell Terrane with inboard rocks now to the west. As a result, ~1500m of conglomerate with interbedded siltstones and sandstones called the Mt. Dall Conglomerate accumulated in the foreland Dall Basin.

Macrofossil floral and faunal remains from the finer-grained parts of the unit indicate an Early Permian age. Lenticular bedding, scours and cobble imbrications in massive and intermediate scale beds suggest a braided-stream environment. The biogeographic affinities of fossil collections are of particular importance in paleogeographic reconstructions of the Farewell Terrane because paleomagnetic data are sparse throughout the terrane’s entire Paleozoic record. In the Mt. Dall, floral impressions assignable to the leaf genera Zamiopteris, Angaropteridium, and Rufloria suggest ties to the northern Pangean Angaran floral realm of the Late Paleozoic, which was originally erected on the distinct floras of the Russian Platform, Siberia and adjacent regions. The occurrence of more North American forms of the fern Pecopteris, however, suggests that the Mt. Dall flora overall has a mixed phytogeographic affinity with Euramerican representatives of more southerly paleolatitudes. The co-occurrence of the brachiopod Stenoscisma cf. hueconianum with this flora also provides a marine link with Permian North America.

Biogeographic data from this locality support the geology-based hypothesis of a close proximity between the Farewell Terrane and North America in the Late Paleozoic, even considering the occurrence of seemingly exotic Angaran floral elements. This temperate latitude flora extends across Siberia and other northern Pangean regions in the Permian as evidenced by microflora. Terranes adjacent or attached to North America at these paleolatitudes are most likely within the same general climate regime and should be expected to contain macrofloras of temperate character. The affinities of fossil leaf genera in the Dall Basin are therefore more latitudinally than longitudinally informative in the paleogeographic evolution of the Farewell Terrane.