The upper Cenozoic Gatuna Formation of southeastern New Mexico

Abstract

The Gatuňa Formation of southeastern New Mexico has been studied in the field for two landfill projects and the Waste Isolation Pilot Plant project. Shafts, drilling and field mapping reveal the distribution, thickness and sedimentary features of the unit in an area where it was poorly known or assigned to other units. The Gatuňa is at least 300 ft thick in the study area. The formation was deposit ed in the north and east as elastic beds ranging from conglomerates to laminar claystones. Fining upward cycles are common, though depositional features and facies associations are consistent with braided river/stream environments, not meandering rivers. Laminar and thinly bedded siltstones to claystones were deposited in floodplain to playa environments. Pedogenic features superimposed on many fining upward cycles include soil fractures, slick ensides, MnO2, illuviated clay, bioturbation, probable ped structures and desiccation cracks. The upper Gatuňa more consistently includes pedogenic development. Beds of poorly indurated "orange" sand, consisting of rounded and well-sorted grains, are interpreted as eolian deposits. From southern Nash Draw to Orla, th e Gatuňa is fine-grained and gypsiferous, including displacive crystals and probable subaqueous deposits. These outcrops represent low energy environments, including playas, which were near local base level. The age of the upper Gatuňa is reasonably constrained by the Lava Creek B ash (0.6 Ma) within the Gatuňa along Livingston Ridge. The age of basal deposits is poorly or not constrained. An ash within probable Gatuňa near Orla, TX, is about 13 Ma based on both radiometric and geochemical data. The Gatuňa represents an important piece of the geological history of southeastern New Mexico. Further studies could include efforts to better determine the age of the formation; to obtain paleontological data; and to map Gatuňa structural relationships to older and younger beds in detail to determine the timing of and spatial evidence for, dissolution of evaporites and collapse of overlying beds, including the Gatuňa. INTRODUCTION AND OBJECTIVES The Gatuňa Formation was investigated during the past few years in a broad area around Clayton Basin, Nash Draw and Pierce Canyon (Fig. 1). Here we report some geological information about the Gatuňa developed through two unrelated projects. A project for the Waste Isolation Pilot Plant (WIPP) was designed (1) to better understand Gatuňa lithofacies and their distribution, depositional environments and climatic implications, and (2) to understand the implications of the thickness, attitude and facies distribution for the timing and location of dissolution of underlying Permian rocks. Dissolution of Permian evaporites affected the hydrologic properties of the overlying units, including the Culebra Dolomite Member of the Rustler Formation. The distribution of Gatuňa facies may indicate possible patterns of recharge to and discharge from, the underlying units during the Pleistocene. The Gatuňa has also been recovered in cores obtained both at a potential site for a new landfill (Sand Point) in Eddy County and at the Loving landfill site that is now closed (Fig. 1, J). These cores provide additional vertical and lateral control on the distribution of the Gatuňa and its facies. The data will also be used as part of an application for a permit for a new landfill (Powers and Magee, this volume) and as part of the application for closure of the Loving landfill. STATUS AND AGE OF THE GATUŇA FORMATION The Gatuňa was first named and generally described by Lang (in Robinson and Lang, 1938) as an assemblage of terrestrial deposits that began to fill the Pecos River valley after maximum erosion. The formation was named for Gatuňa Canyon, about 17 mi north-northeast of the WIPP site (Fig. 1). Robinson and Lang (1938) originally spelled Gatuňa using a tilde. We continue this original usage even though many subsequent maps and reports have avoided using the tilde. A Gatuňa reference section, measured by Bachman (1974) in the type area at Gatuňa Canyon, is about 54.5 ft thick. We also measured and described a composite section at Gatuňa Canyon. It differs from Bachman's description because of our choice of location and because Gatuňa lithofacies vary laterally; we observed less conglomerate. Several of our sections can be usefully considered additional reference sections, but the Gatuňa is variable enough that it is not particularly useful to designate