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

Abstract

Five Neogene floras in southern Washington and northwest Oregon indicate that the uplift of the Cascade Range occurred after ϳ8-6 Ma. Miocene floras west of the range (Wilkes and Faraday floras) resemble paleofloras of correlative age to the east (Rattlesnake Hills, Palouse Falls, and lower Ringold). The statistical similarity of Miocene floras east and west of the Cascade Range axis demonstrates that a similar mesic, warm climate across the Pacific Northwest existed during the Middle and Late Miocene. These floras represent mixed hardwood-conifer forests that commonly contained Taxodium (bald cypress) and other mesophytic taxa. Based on Jaccard similarity coefficients calibrated with climatic data from modern plant communities, these paleofloras indicate a climate that was summer-wet, unlike that of the Pacific Northwest today. The annual precipitation was >100 cm, and the estimated mean annual temperatures of the fossil sites were 12-13°C. The Miocene floras indicate that the development of a Cascade rain shadow in eastern Washington did not develop until after the deposition of the lower Ringold Formation in eastern Washington. That conclusion is reinforced by a well-documented climate and sediment sequence in the Snake River Valley, Idaho. There, well-dated pollen sections record a sharp decrease (by ϳ30%-50%) in the annual precipitation regime after the Late Miocene Banbury Basalt and Poison Creek formations and before ϳ3.4 Ma. The existence of extensive late Ringold sediments uplifted on the north side of Saddle Mountain indicates that the structural lifting of the Cascade Range had begun by 3.

The Bruneau Woodpile: A Miocene Phosphatized Fossil Wood Locality in Southwestern Idaho, USA

“…Low elevation forests bore a resemblance to the modern broadleaved hardwood forests of north central USA. Annual precipitation was in the range of 1200-1300 mm, with a mean annual temperature of approximately 13.5 • C [5]. Analyses of fossil pollen show Pinus as the dominant conifer, but Abies, Picea, and Tsuga were present.…”

“…The region typically receives less than 250 mm of rain each year, with temperatures that range from −34 • C in winter to 43 • C in summer [5].…”

“…Analyses of fossil pollen show Pinus as the dominant conifer, but Abies, Picea, and Tsuga were present. Deciduous trees were abundant and taxonomically diverse [3,5]. The Pickett Creek leaf and pollen deposit [6] of the Chalk Hills Formation documents a flora with a high incidence of summer-dry tolerant species, including white oaks, evergreen oaks, pine and legumes, similar to the wood types found in the Bruneau Woodpile assemblage.…”

“…By the mid Pliocene, the development of the Cascade Range rain shadow [5] and global climatic change [21] profoundly altered the flora and fauna of the Snake River Plain. Temperate forests were replaced by pine/oak woodlands with areas of grassland.…”

“…The Pleistocene brought increased aridity to the Snake River Plain. By 1.3 Ma, annual rainfall had decreased to approximately 250 mm, and the landscape became dominated by sagebrush (Artemisia), saltbush (Sarcobatus) and grasses, i.e., semi-desert sagebrush/bunchgrass steppe flora that flourishes in modern times [5] ( Figure 3). …”