PALEOBOTANY AND PALEOECOLOGY OF THE EARLY PALEOGENE ARKOSE RIDGE FORMATION, TALKEETNA MOUNTAINS, ALASKA

The Arkose Ridge Formation in south-central Alaska comprises variable mudrock to conglomerate sedimentary lithologies that were deposited into the Matanuska Valley - Talkeetna Mountains basin during late Paleocene to early Eocene time. Lithofacies analysis of a >780 m thick stratigraphic section at Box Canyon west of Boulder Creek suggests rapid accumulation in alluvial and lacustrine depositional environments influenced by episodic volcanic eruptions. A laminated siltstone and shale lithofacies association exposed ~643 m above the base of the section exhibits well-developed varves and is interpreted to represent deposition in ponded water. This localized horizon preserves ~20 eudicot and magnoliid leaf morphotypes, four conifer shoot morphotaxa including Metasequoia and Glyptostrobus, parallel-veined monocot leaves, thalloid fragments of the bryophyte Ricciopsis, and mold/cast fossils of sphenopsid axes. Other floral remains include fruits similar to Macginicarpa (Platanaceae) and well-preserved seed fossils assignable to Cruciptera, Metasequoia, Paleosecuridaca, and other winged-fruit forms. The fine preservation of all elements indicates minimal out-of-habitat transport of these remains. Evidence for insect damage during leaf life appears in the form of two leaf galling damage types and at least three hole feeding damage types. Necrolysis of leaf margins and skeletonization is rare among collected specimens.

Leaf physiognomic analysis on the assemblage of broadleaf fossils indicate that warm to cool temperate climate conditions prevailed at the time of deposition. Entire (smooth) leaf margins are characteristic of seven of the 20 discrete dicot morphospecies (p = 0.35). Leaf size analysis indicates moderate to wet climatic conditions. These results are concordant with other leaf margin analyses conducted on similar age assemblages in the nearby Paleocene/Eocene Chickaloon Formation. With ongoing geochronological analyses of tuff and lava interbeds in the Arkose Ridge Formation, the assemblage described here will provide much needed paleoclimatic, paleoenvironmental, and paleobiogeographic data from subpolar latitudes during a global hothouse climate phase.