Relationship between extensional tectonic style and the paleoclimatic elements at Laguna El Fresnal, Chihuahua Desert, Mexico

A multi-disciplinary (geomorphology, ecology, sedimentology, geology, gravi metry, ground penetrating radar, and direct current resistivity) study esta blished a relation among the subsurface structure of Laguna El Fresnal, the geomorphic units, and the vegetation. According to gravity data, Laguna El Fresnal is an asymmetric fault bounded basin typical of the Basin and Rang e province and the Rio Grande rift. The geomorphic units along the eastern and western sides of El Fresnal basin show marked differences. The western slope is characterized by a variety of sedimentary deposits and geomorphic features, such as alluvial fans, debris flows, braided stream deposits, she et flood deposits, erosional surfaces, paleosols and fluvial paleosystems o f dendritic and parallel types. In contrast, the eastern slope has only aeo lian deposits covering structures of alluvial origin. The vegetation also s hows this asymmetry. Along the western slope vegetation is composed, from t he highest to the lowest elevations, of Juniperus ssp,, yucca, Echinocereus ssp., and Prasopis ssp.; whereas, on the eastern slope, the vegetation is mostly composed of small Prosopis ssp. associated with Larrea tridentata co mmunities. The Juniperus ssp. vegetation represents remnants of an environm ent (Upper Pleistocene age?) more humid than at present. The stratigraphy o f the morphological units was tentatively established. Three depositional l obes (QI, QII, and QIII), and three generations of alluvial fans (QIV, QV, and QVI) were defined. Silty-clayey sediments of aeolian origin (loess) fro m the basin constitute unit QVII. The structure of the first 3-4 m of units QVII and QI were established using a ground penetrating radar. Vertical el ectric soundings indicate at least 40 m of clay sediments beneath the plain . The contrasting differentiation between geomorphic units and vegetation a t both slopes of the basin indicates that the western border of the basin i s or was uplifting. (C) 1999 Elsevier Science B.V. All rights reserved.