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Abstract Detail

The Miocene vegetation and environment of Western North America

Dunn , Regan Elizabeth [1].

A middle Miocene phytolith record from western North America, the Mascall Formation of eastern Oregon.

The John Day Basin in eastern Oregon contains over 2100 meters of volcaniclastic strata that were deposited on a heterogeneous landscape between 45-7 Ma. Paleobotanical records here document global and regional climatic change from the west coast of North America. The Mascall Formation, which is 350 meters in thickness, spans from <16 to 13 Ma. It rests on top of the Picture Gorge Basalt Group and was deposited during and after the Middle Miocene Climatic Optimum (17-14 Ma). The lower Mascall consists of diatomaceous lacustrine deposits that contain a diverse warm temperate megaflora with eastern North American and eastern Asian elements. The rest of the Mascall Formation consists of tuffaceous alluvial and floodplain deposits dominated by well-developed paleosols classified as alfisols, andisols, and vertisols; these strata preserve no megafloras. In contrast, phytoliths (plant silica) are well-preserved throughout the entire section, providing a high-resolution record of vegetation from this sequence. Phytoliths tend to record information about grass-tree dynamics and other aspects of vegetation type that is often complimentary to other paleobotanical data. Consistent with paleosol evidence, comparisons of forest-indicator phytoliths versus grass phytoliths suggest that these middle Miocene landscapes were mostly forested, and the diversity of wetland or aquatic type phytoliths point to the existence of extensive wetlands. The occurrence of palm and ginger phytoliths indicate warm and humid conditions. Grasses were only present in low abundances based on phytolith assemblages, mainly in the form of C3 forest grasses (bamboos) and open habitat grasses (pooids). Phytolith composition is relatively stable throughout the section, indicating little climatic or hydrologic change during this time. The forest indicator-dominated phytolith assemblages of the Mascall Formation stand in stark contrast to coeval phytolith assemblages known from Idaho, Montana, and the central Great Plains that point to dominantly open habitat grassland ecosystems. This suggests a high degree of spatial variability in vegetation types across a longitudinal gradient and potentially adds information about the nature and timing of orographic effects on grassland expansion in the Northern Great Plains.

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1 - University Of Washington, Box 353010, Seattle, WA, 98195, USA

Middle Miocene Climatic Optimum
Mascall Formation.

Presentation Type: Symposium or Colloquium Presentation
Session: C2
Location: Salmon/Boise Centre
Date: Tuesday, July 29th, 2014
Time: 2:00 PM
Number: C2003
Abstract ID:657
Candidate for Awards:None

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