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

Abstract

Clay mineral compositions permit the zonation of the Ogallala Formation (Pliocene) in central-eastern New Mexico. The clay mineral assemblage of the lowest part is dominated by montmorillonite (Zone I); above is a zone of progressive upward increase of attapulgite (Zone 2); and above this, a thin zone dominated by sepiolite and attapulgite (Zone 3). 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 events. Caliche, displaying a wide range of physical appearance, occurs on 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 (Zone 5), at the top of the Ogallala Formation, from 27,160 to 35,000 B.P.; and on caliches from 2 to 10 ft below the top of the Ogallala (Zones 2-4), 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.