Circular 144—Caliche and clay mineral zonation of Ogallala Formation, central-eastern New Mexico

By J. C. Frye, H. D. Glass, A. B. Leonard, and D. D. Coleman, 1974, 16 pp., 2 tables, 7 figs., 1 appendix.

Clay mineral zones, lithology of rocks, and physiographic setting determine climatic influences and correct age of zones. The top 4–6 ft of the capping caliche was developed by events of the Pleistocene and Holocene. The Ogallala Formation is widespread in the central and southern Great Plains, comprising the deposits at or near the surface in much of central-eastern New Mexico. The uppermost part of the formation generally consists of a zone containing a very high percentage of calcium carbonate, variously called "cap rock", "lime rock", and caliche. This capping zone of carbonate has its thickest development in central-western Texas and eastern New Mexico, becoming thinner northward across western Kansas, eastern Colorado, and western Nebraska. The origin of this conspicuous, widespread, and distinctive rock has been controversial; first considered to be a caliche resulting from surficial processes of soil formation; and described as the complex product of multiple generations of soil formation, brecciation of the caliche zone, and of recementation.

Recently, studies of caliche and its origin have been made in the adjacent High Plains of Texas and eastern New Mexico, and chemical data on eastern New Mexico caliche has been presented. Studies of caliche, and radiocarbon dating of caliches, have been made in the Rio Grande valley to the west of this area. Several weeks of field work in central-eastern New Mexico during the summers of 1971 and 1972 led to a regional study of clay minerals in Pleistocene deposits, collection of data on clay minerals in the Ogallala Formation, and review of the late Cenozoic stratigraphy and molluscan paleontology of the region. During the course of this work, questions arose concerning the origin, age, and clay mineral content of the extensive caliche deposits. During the summer of 1973, six previously studied sections of Ogallala Formation were sampled with emphasis on the predominantly carbonate zone in the uppermost 10 ft. Caliche samples were also collected from several other Ogallala localities, and from Pleistocene sections for comparative purposes.

Clay mineral compositions permit the zonation of the Ogallala Formation in central-eastern New Mexico. The clay mineral assemblage of the lowest part is dominated by montmorillonite; above is a zone of progressive upward increase of attapulgite; and above this, a thin zone dominated by sepiolite and attapulgite. Zones 1 through 3 are interpreted as representing progressive desiccation through Pliocene time. At the top of the formation, above Zone 3, are two thin zones characterized by abundant carbonate. Zone 4 is again dominated by montmorillonite with lesser amounts of illite and kaolinite, but no attapulgite or sepiolite. Zone 5 at the top, the pisolitic limestone, contains weathered montmorillonite, illite, kaolinite, and locally chlorite. These uppermost zones are interpreted as representing the modifying effects of Pleistocene deposits of several ages, and in the capping zone of the Ogallala. These caliches were studied by their clay mineral assemblages, and by radiocarbon dating. Radiocarbon dates on Pleistocene caliches ranged from 11,250 to 31,700 B.P.; on the pisolitic limestone, at the top of the Ogallala Formation, from 27,160 to 35,000 B.P.; and on caliches from 2–10 ft below the top of the Ogallala, from 30,880 to 43,100 B.P. The radiocarbon dates are apparent ages and do not indicate the time of initial deposition of the caliche. The dates reflect modifications of the calcium carbonate by events during late Pleistocene and Holocene time.