Quaternary history and landscape development of some tributary drainage basins north of Chaco River

Topography in the northeastern tributary watersheds to the Chaco River is characterized by either subplanar alluvial surfaces or badland topography. The Alamo and Ah-shi-sle-pah watersheds contain extensive badland topography, whereas the Coal Creek and Tsaya basins are characterized by broad alluvial surfaces. Correlation of geomorphic surfaces between the drainage basins allows comparison of the responses each basin had to baselevel falls. The two oldest geomorphic surfaces extended beyond present-day drainage divides; drainage divides and the resulting proportions of sandstone and mudrock bedrock lithologies were inherited during the incision of these surfaces. Three younger geomorphic surfaces formed during periodic downcutting of the drainages. The character of drainage-basin evolution during regional dissection is a function of initial basin shape, relative relief of the drainage basin, distribution and proportion of mudrock to sandstone bedrock units, and distribution and preservation of sandy surficial deposits. Sandstone outcrop, elongate low-relief morphology, and accumulation of extensive sandy surficial deposits reduced sediment yield and downcutting in the Tsaya basin. Extensive badland development in the Ah-shi-sle-pah basin was augmented by the dominance of mudrock bedrock in the basin, a high drainage-basin relief ratio, and minimal preservation of sandy surficial deposits. INTRODUCTION SURFICIAL GEOLOGY The subtle topography in the northern tributaries to the Chaco River is interrupted locally by regions characterized by bedrock outcrop, high erosion rates and a badland topography. The Alamo, Coal Creek, Tsaya and Ah-shi-sle-pah drainage basins north of the Chaco River display a wide range of landforms and surficial processes characteristic of the region. The purposes of this paper are to describe the geomorphic history of the region and to discuss those geomorphic factors that have influenced drainage-basin evolution and the present distribution of the region's distinct landforms. The northern tributary drainage basins to the Chaco River are oriented across the strike of northeast-dipping Cretaceous and Paleogene sandstones, mudrocks, coal and minor conglomerate in the southwestern San Juan Basin (Fig. 1). Bedrock formations crop out at similar distances upstream from the Chaco River in each of the drainage basins. Although similar bedrock units crop out in each basin, variable thicknesses of bedrock units results in different proportions of sandstone-tomudrock among the study basins (Table 1). Many of the low relief, vegetated landforms in the northern tributary drainage basins of the Chaco River are capped by alluvium that is part of a topographically stepped Pleistocene and Holocene sequence (Fig. 4). Regional correlation of these deposits allows comparison of the differing downcutting histories of the basins. These alluvial deposits range in topographic position from present-day drainage divides to terraces to valley floors (Fig. 5). These landforms and deposits represent a series of geomorphic surfaces, which are defined herein as subplanar topographic units, and associated alluvial deposits, that grade to a given baselevel. Surlicial deposits and geomorphic surfaces were correlated between study basins by soil development and analysis of topographic LANDSCAPES Landscapes of the northeastern tributary region of the Chaco River reflect erosional and depositional processes affected by bedrock control, alluvial sedimentation and periodic eolian activity. The upper and lower portion of the study basins are characterized by canyons, cuestas and mesas where resistant sandstone units control local relief. Along the outcrop belt of the Kirtland and Fruitland Formations, the landscape is characterized by either subplanar alluvial surfaces or badland topography (Fig. 2). This area, in the central portion of each study watershed, contains the most varied landforms among the study drainage basins because of the susceptibility to erosion of the mudrock-dominated bedrock. The central portions of the Ah-shi-sle-pah and Alamo basins contain extensive badland topography and incised alluvial slopes. In contrast, the Kirtland and Fruitland Formations are mostly mantled by alluvial and eolian sediment in the Coal Creek and Tsaya basins; badland topography is limited to local areas (Table I; Figs. 2 and 3). Whereas the Ah-shi-sle-pah and Alamo basins are undergoing widespread badlands erosion, Tsaya Wash and Coal Creek are discontinuous and currently depositing sediment. The distribution of badlands in the northern Chaco River drainage area is determined by a number of factors, including the local proportion of mudrock to sandstone bedrock, deposition of sandy mantles by fluvial and eolian processes, the amount of headcutting in any given stream, and the incision history and form of the drainage system (Wells, 1983; Smith, 1983a, b). TABLE I. Morphometric and areal characteristics of selected northern tributary drainage basins, Chaco River watershed. Alamo Coal Creek Tsaya Ah-shi-sle-pah