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Show Owen K. Davis Department of Geosciences University of Arizona Tucson, Arizona 85721- 0077 LATE NEOGENE ENVIRONMENTAL HISTORY From June, 1978 through December, 1980, Amoco Production Company oversaw the drilling of 15 wells into the sediments of Great Salt Lake from floating platforms. Several of these wells exceeded 3 km depth, and the cumulative thickness of sediment drilled exceeded 26 km. These data have been provided to the University of Arizona Department of Geosciences as part of a cooperative research agreement with Amoco Production Company. The upland vegetation recorded in the pollen diagrams is desert from the late Miocene onward. Contemporary analogs for the fossil samples are abundant from 5.5 Ma onward ( Fig. 1). The pollen spectrum is dominated by xerophytic Sarcobatus and Ephedra pollen during the late Miocene (< 5 Ma), and is replaced by Chenopodiaceae- Amaranthus, and Artemisia during the Pliocene and early Pleistocene ( 5 - 0.75 Ma) as the Chenopodiaceae and Compositae ( Asteraceae) diversify and spread through the global flora. Artemisia and Pinus percentages abruptly increase in the late Pleistocene ( 0.75 Ma). The antiquity of desert vegetation in the eastern Great Basin conflicts with the hypothesis that deserts expanded during the last few million years in response uplift of the Sierra Nevada ( Axelrod, 1957; Axelrod and Bailey, 1969). Three noteworthy events -- 3.7, 2.6, and 0.75 Ma - punctuate the gradual evolution of the desert flora. The 3.7 Ma event is best represented in the Bridge and South Rozel wells, but the 2.6 Ma event is better represented in the East Gunnison and Indian Cove wells. The 3.7 Ma event is characterized in the Bridge Well ( Fig. 1) by high percentages of Chenopodiaceae-/ 4mara/ 7tf?£ 7S ( warm climate ) from 3980- 3682 Ka ( 1698- 1607 m) followed by high percentages of Artemisia, Pinus, Abies, and Figure 1. Percentage Pollen Diagram for Bridge Well, Great Salt Lake. |
