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USGS Open-File Report 94-023

Mid-Pliocene Vegetation, Environment, And Climate In The Western Interior Of The United States

Robert S. Thompson
U.S. Geological Survey, Denver, CO 80225
Tectonic basins in the western interior of the United States contain thick sedimentary sequences that preserve records of long-term climatic changes. However, the same forces that built these basins led to mountain building and the development of rain-shadows that impose local signals on the records of climatic variations. The modern climate in the northern portion of this region is dominated by winter precipitation associated with the westerlies, while the southeastern sector receives significant summer rainfall from subtropical monsoonal sources. Except for higher mountains, the entire region is semi-arid to arid, due to the rain-shadows of major mountain masses and to the distance from the oceans. Seasonal temperature variations are extreme, especially under the highly continental climate of the northern interior.

Figure 1. Map of sites mentioned in text.
This figure is available as a GIF, PICT, or TIFF (line-art) image.
Vertebrate paleontological and paleobotanical data suggest that temperature extremes were less than today during much of the Pliocene, and paleohydrological and palynological records from this region (figure 1) indicate that conditions were also much wetter. Lacustrine conditions occurred in now-arid settings at Searles Lake (Smith, 1984) and other sites in southern California and adjacent Nevada (e.g. Amargosa Desert -- Hay et al., 1986; Hoover, 1989); central Arizona (Verde Valley -- Nations et al., 1981), and southern Arizona (St. David Formation -- Wang et al., 1991; Safford and San Simon Valleys -- Gray, 1961; Tomida, 1987); central Utah; and southern Idaho and adjacent Oregon and Wyoming. The most detailed chronology is from Searles Lake, where sedimentary and geochemical data indicate sustained moisture from 3.2 to 2.6 Ma, followed by aridity until 2.0 Ma. Deuterium isotopes from calcite veins at Furnace Creek, California (near Death Valley -- Winograd et al., 1985) indicate that high Pliocene moisture levels continued until the last million years, when (presumably) the uplift of the Sierra Nevada and Transverse ranges blocked the incursions of moisture from the Pacific into the interior. Geological studies from the high central Sierra Nevada suggest that as recently as 3 Ma this mountain mass was significantly lower than today (Huber, 1981), and much of the modern aridity of the West must be tied to the development of regional rain shadows since the mid Pliocene.

In southwestern Idaho, the Glenns Ferry Formation appears to represent a deep (>300 m?) long-lived Pliocene lake in the modern Snake River drainage. Pollen data from a ~3.7 to ~3.4 Ma section at the eastern margin of this system (Leopold and Wright, 1985) record a near-modern flora with a sequence of from steppe to forest and back to steppe. Sedimentological variations between lake-margin sands and lignites suggest shorter term, lower-amplitude climatic fluctuations also occurred that are not reflected in the pollen record.

Farther west, a record from the deeper water sediments from near the town of Bruneau, in the east-central part of the Glenns Ferry Formation, apparently covers portions of the period from ~3.0 to ~2.48 Ma (Thompson, 1992). Very few exotic Tertiary elements are present in this diagram, and the vegetation fluctuates between three forest periods and three steppe periods before the lake becomes shallow (still within Gauss Chron). Unlike at Fossil Gulch, no sedimentological variations from this deeper- water environment correspond with the pollen fluctuations. Younger pollen assemblages from the Bruneau Formation at this site indicate a cold steppe environment sometime in post-Olduvai Matuyama time.

The undated Glenns Ferry pollen assemblages from a well site in the town of Vale, Oregon, contain palynological assemblages similar to the forest periods at the Bruneau site, and the longer (but also undated) Mountain Home Idaho core records a forest to steppe oscillation similar to those in the Bruneau core. The West Weiser outcrop, near the western extent of the Glenns Ferry Formation domain, is presumably >4 Ma and is strongly dominated by conifers and contains more Tertiary elements than sediments post-dating this time.

To the east of the Glenns Ferry deposits, the INEL (Idaho National Engineering Laboratory) 2-2a core from southeastern Idaho contains episodic lacustrine deposits separated by thick sequences of basalts. As with the Glenns Ferry palynological data, this site records fluctuations between steppe and forest dominance through the Pliocene. Assemblages thought to date ~4 Ma are dominated by conifers and are more diverse than younger pollen spectra (Thompson, 1991). Pollen assemblages dated from ~3.0 Ma are dominated by steppe taxa, which give way to conifers by ~2.9 Ma. Spectra dated to ~2.4 Ma contain low abundances of coniferous and other arboreal taxa and resemble late Pleistocene glacial-age pollen assemblages from southern Idaho. Coniferous taxa are somewhat more abundant by 2.0 to 1.8 Ma, but did not recover their pre-2.4 Ma levels.

Figure 2. Changes in the relative abundance of conifers through the Pliocene and earliest Pleistocene in southern Idaho and adjacent Oregon.
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Figure 2 illustrates the long-term Pliocene/early Pleistocene vegetation history of southern Idaho and adjacent Oregon: diverse early Pliocene coniferous forests with Tertiary elements gave way to modern steppe dominance over this period (presumably due in part to developing rain shadows). However, this trend was not monotonic, and apparent Milankovitch band high-amplitude fluctuations were recorded in forest/ steppe cycles. With the exception of the apparent glacial-age steppe period (~2.4 to ~2.0 Ma), the palynological record suggests that conditions were wetter than today through the Pliocene and early Pleistocene in this region.

The information presented here provide only a sketch of Pliocene conditions in the western interior. Much more work is required to segregate the influences of global climate change from the regional aridification caused by mountain-building through the late Neogene and Quaternary.

References

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