RECOGNITION OF A NEW ANGARAN-TYPE FLORA AND FACIES VARIATION IN THE LATE PALEOZOIC MT. DALL CONGLOMERATE, FAREWELL TERRANE, ALASKA

New fieldwork in the Pennsylvanian/Permian Mt. Dall Conglomerate in the Farewell Terrane of central Alaska has yielded not only new specimens and species from a unique Angaran-type flora in North America but also has provided insights into facies level differences in floral composition in braided stream settings. The Mt. Dall assemblage is important biogeographically, being the only late Paleozoic flora known within a 2000 km radius. Its study enhances our understanding of Pangean (Angaran) phytogeography and paleogeographic reconstructions of western North America. The Farewell Terrane of central Alaska, USA is represented by the White Mountain sequence in the early - middle Paleozoic and the Mystic sequence in the late Paleozoic - Jurassic. Marine biotas preserved within the Cambrian through Devonian strata tie this terrane biogeographically with the Siberian and North American Craton and their respective taxon specific biozones. In this research, the first extensive collection of a late Pennsylvanian/early Permian flora from the Mt. Dall Conglomerate was taken from an alpine locality in Denali National Park and Preserve, AK. Major floral constituents of this formation are Pecopteris spp., Zamiopteris spp., cordaites, and numerous rhizomorphs in paleosols. Their affinity is with the Angaran flora, and is based mainly on comparisons with the Siberian Platform. These taxa are found with different frequency throughout the spatially variable braided stream deposits. My fieldwork indicates that on a 1-meter vertical scale, the Mt. Dall Conglomerate is an interfingering of facies lenses representing migrations of channels within a braided stream setting. Larger-scale (tens of meters) source sediment variations and overall grain-size changes provide mappable members within the 1500 m section exposed on Mt. Dall. Fine-grained fossiliferous horizons are interbedded with cobble to boulder conglomerate through the section. Zamiopterids and pecopterids predominate in organic-rich sediments while cordaites leaves, rhizomorph horizons, and woody fragments are more abundant in coarser-grained facies. This study shows the importance of facies sampling completeness, and further work on this flora’s place in paleogeographic context will enhance our understanding of the Pangean terrestrial biosphere.