Abstract Detail

The Miocene vegetation and environment of Western North America

Zaborac-Reed, Stephanie [1].

The Complex History of the Thorp-Bristol Floral Deposit Near Teanaway, Washington.

A transported landslide block is wedged between thick, tilting layers of rhyolitic ash, lahars, and conglomerates at a road cut off the Thorp Highway near Teanaway, between Cle Elum and Ellensburg, Washington. Assigned to the middle Miocene Ellensburg Formation, these deposits overlie an undated cross-bedded layer and unconformably underlie the middle Pleistocene-aged Swauk Prairie member of the Kittitas Drift glacial deposit, which grades roughly parallel with the Yakima River. The small flora contained in this landslide block, called Thorp-Bristol, is the westernmost flora associated with the Ellensburg Formation. The specimens were examined to determine their systematic placement and for climate analysis. The flora includes East Asian and Southeastern United States hardwood elements such as Ginkgo, Liquidambar, Platanus, Ulmus, Quercus (leaves and acorns), members of the Betulaceae and Salicaceae, and Comptonia, as well as others. It also contains fossils bearing a strong resemblance to Poaceae (grass) glumes and Podocarpus leaves. Ginkgo is thought to have disappeared from Washington around 14Ma, and is therefore an important indicator of age. ³⁹Ar/⁴⁰Ar radiometric dating of the matrix is underway to determine the proper age of the flora and the significance of the presence of Ginkgo, as well as determine the site’s relationship to various local floras, as the plants do not corroborate well for assignment into the nearby Ellensburg Flora (12-10Ma). Some of the identified taxa evaluated are cosmopolitan in their climatic tolerances (Salix, Ginkgo, Typha) as long as water is present, but other identified taxa have specific summer-wet climatic requirements (Liquidambar, Platanus, Ulmus) and lend support to studies which conclude that climate in this region was substantially warmer and wetter during the Middle Miocene than today. The site is also geologically interesting due to the implications of the overlying Kittitas Drift, the uplifting Cascade Range, and the overall history of the Columbia River Basalt Group in the Yakima River valley.

1 - University of Washington, Biology, Box 351800, Seattle, WA, 98195, USA

Keywords:
Columbia River Basalt Group
Ellensburg Formation
landslide
Ginkgo.

Presentation Type: Symposium or Colloquium Presentation
Session: C2
Location: Salmon/Boise Centre
Date: Tuesday, July 29th, 2014
Time: 3:00 PM
Number: C2007
Abstract ID:409
Candidate for Awards:None