Paleoenvironmental reassessment of the 1.6-million-year-old record from San Agustin Basin, New Mexico
Vera Markgraf, J. Platt Bradbury, R. M. Forester, W. McCoy, G. Singh, and R. Sternberg


Early recognition of potential Pleistocene paleoenvironmental records from the arid Southwest came from dry basins and playas in the Basin and Range and Colorado Plateau provinces (Meinzer, 1922). Fossil beach ridges, wave-cut cliffs, and thick sedimentary sequences provided evidence of extensive ancient lakes in these basins during past pluvial periods that were tentatively correlated with the glacial ages.

The San Agustin Basin in western New Mexico (Lat. 34°N, Long. 108°W) and its fossil-lake features have been described in detail by Bryan (1926) and Powers (1939). In the years 1955 and 1958-59, two cores, Oberlin 1 (200 m depth) and Oberlin 2 (600 m depth) were taken near the deepest part of the basin (fig. 1). Sediment texture and pollen were analyzed and yielded a continuous record down to about 300 m (Clisby and Sears, 1956; Clisby and Foreman, 1957; Foreman and others, 1959; Sears, 1961). However, the detailed data were never published. A radiocarbon date of 27,000 yr from the upper 10 m of the section and the appearance of Tertiary pollen below 280 m suggested a long record that extended to the Pliocene. With the exception of the last glacial/interglacial cycle (Wisconsin-Holocene), the paleoenviron-mental analysis failed to define the anticipated earlier cycles of alternating glacial and interglacial environments. The last cycle was characterized by pine/spruce woodland during glacial times which changed to saltbush/grassland by about 10,000 yrs ago. Below 40 m pine/sage-brush/grassland dominates the record, but gradually sagebrush/grass-land becomes dominant below 100 m; spruce pollen was always sparse of the San Agustin Basin to woodland elevations only during the late Pleistocene (Foreman and others, 1959). Although this interpretation seems unlikely, it cannot be rejected because few comparative early Pleistocene paleoenvironmental data are known in this region. On the other hand, the static pollen record could be the result of variable sampling density, poor pollen preservation, and the extreme extraction techniques applied at that time.

Given the importance of such a long continental record and refined dating and preparation techniques, it becomes appropriate to reassess the San Agustin Plains record as follows: 

(1) Determine the age of the record by paleomagnetic analysis (in-clination) of the available original core samples.

(2) Determine the continuity of sedimentation in the basin by cal-culating sedimentation rates of the section using different dating techniques such as radiocarbon, amino-acid racemization, and paleomagnetic data.

(3) Determine the paleoenvironment region as recorded in the pollen assemblages and the paleolimnology as recorded by ostracods and algal assemblages (diatoms, Pediastrum, and Botryococcus). For this purpose we collected and analyzed a third sediment section (auger, fig. 1) extending back to about 20,000 years ago. This interval encompasses the two extreme climate modes (the glacial and interglacial modes), whose paleontologic character should provide a standard for interpreting the earlier record. Comparison of the new paleoenvironmental data with the original results should also provide some insight into the problems of the earlier study.


  1. Markgraf, Vera; Bradbury, J. Platt; Forester, R. M.; McCoy, W.; Singh, G.; Sternberg, R., 1983, Paleoenvironmental reassessment of the 1.6-million-year-old record from San Agustin Basin, New Mexico, in: Socorro region II, Chapin, C. E., New Mexico Geological Society, Guidebook, 34th Field Conference, pp. 291-297.

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