Degradation of Pedogenic Calcretes in West Texas

Abstract

Calcretes (petrocalcic horizons) are widespread indurated accumulations of carbonate in arid and semiarid regions of the world. Many studies have detailed the formation of these calcretes; however, litde direct attention has been given to their degradation. The primary objective of this study was to evaluate the continuity of calcrete on the Llano Estacado as a step toward understanding processes of calcrete degradation across the region. The Llano Estacado is a tableland spanning eastern New Mexico and West Texas with a well-developed calcrete, known as the caprock, exposed as an escarpment around most of its perimeter. We excavated several deep pits along a transect between the caprock on the eastern margin of the Llano Estacado and a similar outcrop on a playa slope. We documented degradation features in the calcrete and compared the extent of degradation with a calcrete in a more arid region (Diablo Plateau). Our findings show a lack of continuity of the calcrete on the Llano Estacado even in a relatively short transect (220 m) between the two outcrops. Individual calcrete fragments showed evidence of partial dissolution. These observations were confirmed in road cuts and pits excavated for road material across the area. Geomorphic position and surface slope control water movement across the landscape and apparendy explain both the variation in the presence of degradational features and the distribution of calcrete fragments in the transect. Observations from the more arid Diablo Plateau suggest that calcrete degradation is widespread. The degradation of calcrete on the Llano Estacado makes the caprock unreliable in some areas as a stratigraphic marker in distinguishing the Ogallala and Blackwater Draw Formations.