The Ogallala and Gatuna Formations in the southeastern New Mexico region: A progress report

_ The Ogallala Formation in the Southern High Plains section (Great Plains province) of southeastern New Mexico includes alluvial, eolian and playa-lake deposits and pedogenic calcretes of late Miocene and early Pliocene age (about 4-12 Ma). Beneath the Llano Estacado, it forms an almost continuous cover on rocks of Mesozoic age, is locally more than 400 ft ([20 m) thick and is a major aquifer. In the Pecos Valley section of the Great Plains and along the Portales Valley through the western Llano Estacado, fine- to coarse-grained elastics of late Miocene to middle Pleistocene age locally form thick fills (>1000 ft, 300 m) in large solution- subsidence depressions. These features are aligned along segments of the ancestral Pecos and Brazos Valleys and are underlain by evaporites of Late Permian age. Some of these deposits have always been included in the Ogallala Formation; but in the lower Pecos Valley area (Roswell, NM to Pecos, TX), correlative depression and valley fills have been mapped variously as "older alluvium, quartzose conglomerate, valley-fill alluvial deposits," and as the Gatuňa Formation. Gatuňa-Ogallala chronologic and nomenclature problems have not yet been resolved in that area; however, it is clear that an ancestral "lower" Pecos fluvial system has existed since late Miocene time near the present valley position between the Roswell (artesian) and Delaware Basins. In the sediment source area west of the Great Plains, Ogallala and Gatuňa correlatives are discontinuous, commonly thin and only locally aquifers. The oldest deposits include piedmont fan alluvium, pediment veneers and valley and basin fills. They record semiarid climatic conditions, prior epeirogenic uplift and volcanism and ongoing Basin-and-Range tectonism in a broad area extending southward from the Southern Rocky Mountains through the Sacramento section of the Basin and Range province. Significant uplift of mountain fault blocks occurred along the Rio Grande rift margin in the western part of the (sediment) source region. The facies-distribution patterns of both the Ogallala and Gatuňa Formations are quite complex west of the Southern High Plains. The oldest units may form basal fills of structural basins, solution-subsidence depressions, or stream valleys, or they may be preserved as piedmont alluvium capping high divides and tablelands, with younger deposits occurring as inset valley fills. Rising western highlands not only contributed runoff and sediment to the High Plain depositional system but also had a major influence on regional climate. The occurrence of prominent zones of secondary-carbonate accumulation in paleosols of the High Plains eolian cover indicates increasingly dry and more continental conditions in late Cenozoic time. Episodic deflation of alluvial plains prograding eastward and southeastward from mountain and piedmont source areas also produced eolian sediments that are a significant component of the Ogallala Formation and overlying Plio-Pleistocene deposits of the Llano Estacado area.