Paleoenvironment and paleoecology of a Late Paleocene high-latitude terrestrial succession, Arkose Ridge Formation at Box Canyon, southern Talkeetna Mountains, Alaska

Abstract

Paleogene sedimentary rocks of the Arkose Ridge Formation (Talkeetna Mountains, Alaska) preserve a record of a fluvial–lacustrine depositional environment and its forested ecosystem in an active basin among the convergent margin tectonic processes that shaped southern Alaska. An ~800m measured succession at Box Canyon indicates braid-plain deposition with predominantly gravelly deposits low in the exposure to sandy and muddy facies associations below an overlying lava flow sequence. U–Pb geochronology on zircons from a tuff and a sandstone within the measured section, as well as an Ar/Ar date from the overlying lava constrain the age of the sedimentary succession to between ~59Ma and 48Ma. Fossil plant remains occur throughout the Arkose Ridge Formation as poorly-preserved coalified woody debris and fragmentary leaf impressions. At Box Canyon, however, a thin lacustrine depositional lens of rhythmically laminated mudrocks yielded fish fossils and a well-preserved floral assemblage including foliage and reproductive organs representing conifers, sphenopsids, monocots, and dicots. Leaf physiognomic methods to estimate paleoclimate were applied to the dicot leaf collection and indicate warm temperate paleotemperatures (~11–15±~4°C MAT) and elevated paleoprecipitation (~120cm/yr MAP) estimates as compared to modern conditions; results that are parallel with previously published estimates from the partly coeval Chickaloon Formation deposited in more distal depositional environments in the same basin. The low abundance of leaf herbivory in the Box Canyon dicot assemblage (~9% of leaves damaged) is also similar to the results from assemblages in the meander-plain depositional systems of the Chickaloon. This new suite of data informs models of the tectonostratigraphic evolution of southern Alaska and the developing understanding of terrestrial paleoecology and paleoclimate at high latitudes during the Late Paleocene–Early Eocene greenhouse climate phase.