Stable isotope geochemistry of the caprock caliche

Abstract

Recent studies have shown that stable carbon and oxygen isotope analyses of soil carbonate (caliche) are useful in paleoenvironmental reconstructions. The Ogallala Formation of the Southern High Plains contains widespread caliche profiles, making it an ideal target for such studies. The "caprock caliche" (early Pliocene) is an erosion-resistant soil carbonate layer that marks the top of the Ogallala Formation. Three sections of the "caprock" were chosen for this study. Samples were taken from outcrops near Crosbyton in Crosby County, Texas, near Lamesa in Dawson County, Texas, and near Ragland in Quay County, New Mexico. The Crosby County profile was the primary section used in this study. Samples were taken in 10 em intervals along vertical transects through the nodular, massive, and laminated zones. The o13C values from this profile range from -4.3 to -7.2o/oo for the carbonate and -21.7 to -26.1%o for the organic matter. These values suggests that C3 flora was favored in this region at the time of formation of the "caprock" caliche, with a slight trend towards increased c4 flora. Similar results are seen at Lamesa and Ragland. Oxygen isotope analyses of the Crosby County section yield 0180 values ranging from -4.4 to -5.6%o, with a standard deviation of 0.3%o. This suggests that either relatively uniform climatic conditions existed in the region when the "caprock" formed, or that the oxygen isotopes are not sensitive enough to show the temperature change necessary to cause the slight variation in vegetation.