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.
- This figure is available as a
GIF,
PICT,
or
TIFF (line-art) image.
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
- Gray, J.,1961, Early Pleistocene paleoclimatic record from Sonoran Desert, Arizona: Science v. 133, p. 38-39.
- Hay, R.L., Pexton, R.E., Teague, T.T., and Kyser, T.K., 1986, Spring-related carbonate rocks, Mg clays, and associated minerals in Pliocene deposits of the Amargosa, Nevada and California: Geological Society of America v. 97, p. 1488-1503.
- Hoover, D.L., 1989, Preliminary description of Quaternary and late Pliocene surficial deposits at Yucca Mountain and vicinity, Nye County, Nevada: U.S. Geological Survey Open-File Report 89-359. 45 p.
- Huber, N.K., 1981, Amount and timing of late Cenozoic uplift and tilt of the central Sierra Nevada, California Ñ evidence from the upper San Joaquin River Basin: U.S. Geological Survey Professional Paper 1197. 28 p.
- Leopold, E.B. and Wright, V.C., 1985, Pollen profiles of the Plio-Pleistocene transition in the Snake River Plain, Idaho: in Smiley, C.J. (ed.), Late Cenozoic History of the Pacific Northwest: American Association for the Advancement of Science, Pacific Division. Pp. 323-348.
- Nations, J.D., Hevly, R.H., Blinn, D.W., and Landye, J.J., 1981, Paleontology, paleoecology, and depositional history of the Miocene-Pliocene Verde Formation, Yavapai County, Arizona: Arizona Geol. Society Digest v. 13, p. 133-149.
- Smith, G.I., 1984, Paleohydrologic regimes in the southwestern Great Basin, 0Ñ3.2 my ago, compared with other long records of "global" climate: Quaternary Research v. 22, p. 1-17.
- Thompson, R.S., 1991, Pliocene environments and climates in the western United States. Quaternary Science Reviews v. 10, p. 115-132.
- Thompson, R.S., 1992, Palynological data from a 989-ft (301-m) core of Pliocene and early Pleistocene sediments from Bruneau, Idaho. U.S. Geological Survey Open-File Report 92-713. 28 p.
- Tomida, Y., 1987, Small mammal fossils and correlation of continental deposits, Safford and Duncan Basins, Arizona, USA. National Science Museum, Tokyo. 135 p.
- Wang, Y., Cerling, T.E., Smith, G.A., Geissman, J.W., Quade, J., Lindsay, E.H., and Bowman, J.R., 1991, Climatic and ecologic changes during the Pliocene and early Pleistocene in southeastern Arizona: stable isotopic records from the St. David Formation. Geological Society of America Abstracts with Programs, 1991 Annual Meeting, San Diego. P. A301.
- Winograd, I.J., Szabo, B.J., Coplen, T.B., Riggs, A.C., and Kolesar, P.T., 1985, Two-million-year record of deuterium depletion in Great Basin ground waters: Science v. 227, p. 519-522.
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