Paleoecology of an Early Pleistocene Lake on the High Plains of Texas
Published:January 01, 1969
Roger Y. Anderson, Douglas W. Kirkland, 1969. "Paleoecology of an Early Pleistocene Lake on the High Plains of Texas", Paleoecology of an Early Pleistocene Lake on the High Plains of Texas, Roger Y. Anderson, Douglas W. Kirkland
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The early Pleistocene (Blancan) Rita Blanca lake deposits in the north-western part of the Panhandle of Texas occupy a deflation basin cut through the caprock of the Pliocene Ogallala Group. During its maximum development, Rita Blanca lake had an elliptical shape with a north-south length of about 6 miles and a breadth of about 3.5 miles.
The lower exposures of the Rita Blanca deposits contain an areally restricted clay bed with a maximum observed thickness of 65 inches that contains abundant fossils representing the lacustrine and riparian environments, and a plains community of mixed sagebrush and grass. The clay bed also contains two types of cyclic lithology: (1) an alternation of light- and dark-colored clay zones on the scale of centimeters and decimeters, and (2) couplets of light- and dark-colored laminae with an average thickness of 0.9 mm.
The light-colored laminae are rich in calcium carbonate, quartz silt grains, and ostracodes; the dark-colored laminae are composed principally of illite and organic matter. The laminae can be correlated with little change over 300 yards, and changes in the proportion of the constituents are responsible for the larger scale cyclic bedding.
The microstratigraphic position and associations of the ostracodes Lim-nocythere, Cyprideis, and Candona, deciduous leaves of oak (Quercus) and willow (Salix), Artemisia pollen, pupae and adults of Chironomidae (midges), and carbon and oxygen isotope ratios demonstrate that the light-colored high-carbonate laminae were deposited in the summer and the dark-colored low-carbonate laminae were deposited in the winter and spring.
A saline temporary hard-water lake is suggested by sediment composition and by four kinds of fossils that customarily reflect a saline or alkaline environment but are not necessarily restricted to it (Ruppia maritima, ditch grass; Fundulus, killifish; Limnocythere and Cyprideis, ostracodes; a species of Chironomus, midge). The entire fossil assemblage, with the exception of fungal spores, was transported to site of deposition in the profundal zone of the lake. The absence of a benthonic fauna, the distinct laminations and the saline character of the water suggests meromixis.
The continuous 1,400-year varve series was sampled on a time-series basis and the long-term associations subjected to regression, Fourier, and bi-spectral analysis. Clay is negatively associated with calcium carbonate for intervals longer than 60-70 years, whereas the association is positive for short-term changes — which is interpreted to mean that some calcium carbonate was washed in with the clay but the long-term changes in carbonate were the result of carbonate precipitation in the lake. Harmonic analysis of couplet thickness revealed a moderately well-defined 22-year period that was also present in the carbonate fraction but not in the clay fraction.
Seasonal and long-term associations of clay, silt, carbonate, organic matter, fungal spores, and ostracodes parallel each other, and the negative long-term association of clay and carbonate is interpreted as intervals of cool-moist climate alternating with intervals of warm-dry climate. The fossils and lithologic changes suggest a climatic regime with cooler temperatures and with the same or less moisture than the present area receives today, but with most of the moisture arriving in the cool season rather than in the warm season.