Abstract Detail

The Miocene vegetation and environment of Western North America

Leopold, Estella B. [1], Mustoe, George [2], Zaborac-Reed, Stephanie [1].

Paleobotanical evidence for the post-Miocene uplift of the Cascade Range.

Paleobotanical evidence indicates that the rain shadow east of the Cascade Range in central Washington did not develop until after ~6-8 Ma. Seven floras between southern Idaho and western Washington verify that a summer-wet climate with warm winters persisted along the central Pacific Northwest region during the mid and late Miocene. In contrast, younger floras from the Snake River Plains, Idaho, indicate that a strong orographic rain shadow was in place by 3.2 to 3.3 Ma, as well-dated pollen sections record a ~30-50% decrease in annual precipitation. Jaccard similarity coefficients calibrated for comparing fossil and modern floras establish that the mixed hardwood conifer forest floras such as the Miocene Wilkes and the Ellensburg floras in Washington and the Trapper Creek/Poison Creek floras along the Snake River in Idaho were quite similar to the present-day broad-leaved hardwood forests of southeastern USA, forests that commonly contain Taxodium (bald cypress) and/or Nyssa cf. aquatica (sweet gum). Such forests predict a mean annual temperature (MAT) of 14-15°C and require a high rainfall (>1000mm annually and >100mm in July). In contrast, at the Miocene-Pliocene transition the fossils signal an important change. Jaccard coefficients demonstrate that the semi-arid Pliocene forests and vegetation of the Snake River Plains, Idaho, were very similar to the modern vegetation of the area. The Pliocene and early Pleistocene pollen data suggest the xeric pine forests of Idaho were associated with dominant plants such as Ephedra, Artemisia, Sarcobatus, and other dryland shrubs that require only low rainfall, and connote a lower MAT (8-10°C). Independent evidence from the fish fauna and tortoises at White Bluffs (Upper Ringold Formation, 4.5 Ma) indicate a frost-free winter climate. However, in younger Pliocene beds (3 – 2.8 Ma) of the Glenns Ferry Formation of Idaho, cold-water fishes indicate that cold month temperatures regularly reached -8° C and a highly seasonal climate had developed. Other evidence that the Cascade Range was developing comes from the discovery of Pliocene (Repenning’s Taunton-age local fauna dated at 3.1 Ma) faunal remains of the Upper Ringold Formation perched on the north side at the east end of Saddle Mountain. This finding suggests Pliocene uplift had begun by 3.1 Ma.

1 - University of Washington, Biology, Box 351800, Seattle, WA, 98195, USA
2 - Western Washington University, Geology, 516 High Street, Bellingham, WA, 98225, USA

Keywords:
Cascade Range
Neogene flora
Paleoclimate
uplift.

Presentation Type: Symposium or Colloquium Presentation
Session: C2
Location: Salmon/Boise Centre
Date: Tuesday, July 29th, 2014
Time: 5:15 PM
Number: C2016
Abstract ID:422
Candidate for Awards:None