PALYNOLOGY AND TEPHROCHRONOLOGY: AN OLIGOCENE (~26 MA) TO PLIOCENE (~3.8 MA) RECORD OF FLORA CHANGE FROM THE EASTERN ALASKA RANGE FRAMED BY HIGH PRECISION RADIOMETRIC DATES OF NEIGHBORING TEPHRA LAYERS

The Oligocene to Pliocene strata of the eastern Alaska Range (EAR) provide a unique record of flora changes within a tectonic context because of: 1) Transpressive Denali fault wedge-top basin Oligocene to Pliocene strata; 2) Tephra deposits for geochronological control from the Wrangell Arc; 3) Thermochronology and basin analysis constraints on tectonic and fault slip activity.

The Oligocene (~26 Ma)-early Miocene (~20 Ma) strata of the EAR consist of conglomerate and interbedded sandstone and mudstone and rare tephra with a thickness of ~549 m. The late-Miocene (~6 Ma)-early Pliocene (~5 Ma) strata of the eastern Alaska Range consist of lacustrine strata with numerous (>20) tephra with a thickness of 100 m. The late-Miocene (~5 Ma)-early Pliocene (~3.8 Ma) strata of the eastern Alaska Range consist of alluvial fan strata with many discrete tephra layers (>5) with a thickness of 464 m.

The Oligocene early Miocene strata have hickory, hemlock, cypress and pine palynomorph, an assemblage that bares some resemblance to mid-latitude swamp forests found in the eastern US, such as the Mississippi Lowlands. The late-Miocene early Pliocene strata have cypress and pine palynomorphs and abundant algal zygospore. The late-Miocene early Pliocene strata have a similar assemblage to modern (spruce and alder) with the exception of the presence of hemlock palynomorphs.

Oligocene-early Miocene strata to the west (Healy area) have more diverse warm loving flora, but lack the presence of cypress. A palinspastic restoration along the Denali fault places the late-Miocene early Pliocene in a paleo-releasing bend fitting a swamp or marsh-type setting fitting the abundant algal zygospore present and the late-Miocene early Pliocene strata with similar flora to present along a transpressive uplift fault segment.

In this framework Oligocene-Pliocene flora in the EAR appear to be dominated by mountain building and crustal blocks being translated out of a releasing bend and into an area of transpressive uplift and not world-wide climatic events. Though the influence of late Cenozoic cooling likely plays a role in the general trend from ~26 Ma to ~3.8 Ma in less flora diversity. Overall palynology data sets need to be viewed in terms of local tectonically controlled environments before being used for biostrat and regional climate reconstructions.