LOWLAND AND COASTAL VEGETATION IN AN ANCIENT ISLAND SETTING: THE PALEOFLORA OF THE EARLY JURASSIC TALKEETNA VOLCANIC FORMATION, TALKEETNA MOUNTAINS, ALASKA

Fossil plant remains in upper strata of the Early Jurassic Talkeetna Volcanic Formation in the southern Talkeetna Mountains, Alaska reflect a diverse terrestrial flora living in an evolving island arc depositional setting offshore from North America. Moderately well-preserved leaf, cone, and wood fossils occur as compressions/impressions in volcaniclastic siltstones and fine sandstones that are interbedded with coarser volcaniclastic sediments, breccias, airfall deposits, and lava flows. Collections from three field sites in the unit (Hicks Creek, East Boulder Creek, and Sheep Mountain) indicate that ephemeral cycadophyte-dominated forests grew during subaerially exposed phases in what was a rapidly accumulating and mostly marine succession of basin fill.

The overall paleofloral collection is composed of predominantly benettitaleans (e.g. Otozamites), conifers (e.g. Pagiophyllum, Brachyphyllum), sphenopsids (Equisetites?), and ferns (e.g. Sagenopteris, Cladophlebis). These are commonly preserved as articulated compound fronds or shoots indicating autochthonous or parautochthonous deposition. Characteristics exhibited by these floral elements (e.g. coriaceous leaves) have been suggested to be consistent with plants living in xerothermic conditions as well as halophytic habits with rooting into saline soil. This, combined with the fact that the flora described here variably co-occurs with bivalve assemblages and is found in an overall unit rich in marine invertebrate fossils indicates that the plant macrofossil-rich facies were deposited in lowland and potentially coastal terrestrial settings. Paleobiogeographic affinities of the Talkeetna Volcanic Formation’s fossil plant taxa suggest a subtropical paleogeographic location of the basin (and the Peninsular Terrane) during Early Jurassic time with year-long frost-free conditions. Comparisons are made with other Early Jurassic paleofloras in Alaska (Paule Bay) and globally.