FLORAL REMAINS IN THE PALEOGENE ARKOSE RIDGE FORMATION, SOUTH-CENTRAL ALASKA: CLIMATIC AND PALEOENVIRONMENTAL IMPLICATIONS

Collections of plant fossils have been recovered from newly studied sites within the >1700 m thick Paleocene-Eocene Arkose Ridge Formation (southern Talkeetna Mountains, Alaska). Compression/impression leaf, seed, and axis (stem/shoot) fossils are well-preserved in laminated siltstones and fine sandstones. Conifer foliage includes Glyptostrobus and Metasequoia shoots among other cupressaceous forms. Broadleaf fossil of the Juglandaceae, Betulaceae, Salicaceae, and Magnoliaceae are abundant and show details of leaf venation and margin character. Other remains include Equisetites axes, cycad fronds, and the first reported occurrence of a palm frond (Sabalites?) in the formation. The coarser tan-grey sandstone lithofacies at the “Grey Ridge” locality west of the Chickaloon River contain a flora of palms, Metasequoia, Glyptostrobus, cycads, and four different leaf morphotypes in fine-grained sandstone and siltstone. The majority of these fossils were preserved in strata interpreted as being deposited in lacustrine environments. Collections from the “Box Canyon” locality east of the Chickaloon River include a higher diversity of more than nine different magnoliid and eudicot leaf morphotypes, Acer? samaras, and Metasequoia in light gray sandstone and laminated/varved siltstone. The majority of these fossils were preserved in strata interpreted as lacustrine and fluvial overbank environments. Very few leaves in the entire assemblage exhibit evidence of insect herbivory.

The high quality of preservation indicates that these assemblages are parautochthonous with minimal out of habitat transport. Habitats of nearest living relatives in conjunction with fluvial-lacustrine sedimentary data suggest that this flora represents a moderately diverse floodplain forest community. Simply the occurrence of some fossil leaf morphotypes, and preliminary leaf physiognomic analysis on the composite assemblage, suggest at least warm temperate paleotemperatures during the time of Arkose Ridge Fm. deposition. Preliminary investigation of this plant fossil assemblage may help in testing models of middle to high latitude paleoclimate as well as the assemblage’s relationship with the coeval Chickaloon Formation paleoflora to its southeast across the Castle Mountain Fault in the Matanuska Valley.