Sedimentologic and geomorphic evidence for seesaw subsidence of the Santo Domingo accommodation-zone basin, Rio Grande rift, New Mexico

Abstract

Basin analysis of facies patterns and basin geomorphology reveals a complex subsidence pattern for part of the Rio Grande rift in north-central New Mexico, United States. The Santo Domingo basin study site is located in the accommodation zone between the east-tilted northern Albuquerque basin and the west-tilted Espanola basin half-graben. Upper Miocene through lower Pleistocene basin-fill sediment records a longitudinal, south-flowing, ancestral Rio Grande and flanking piedmont stream systems. Chronology of deposition is provided by 40Ar/39Ar geochronological study of lava flows and pyroclastic deposits interbedded with the sediment. Down-to-the-east faults concentrated in the western basin illustrate increasing displacement toward the north and merge with structures forming the western margin of the Espanola basin. Down-to-the-west faults in the central and eastern basin exhibit southward-increasing displacement and become the principal structures along the eastern margin of the northern Albuquerque basin. The latitudinal overlap of these fault systems in the Santo Domingo basin gives the appearance of a symmetrical graben with anticlinal bedding attitudes. Analysis of the distribution of sedimentary facies, high-level geomorphic surfaces, and Pleistocene terraces indicates, however, that the basin subsided in a seesaw fashion rather than as a symmetrical graben. During the late Miocene and early Pliocene, the course of the ancestral Rio Grande was deflected to the west, suggesting strong westward tilting of the basin. In late Pliocene and Pleistocene time, however, eastward tilting brought the river into the eastern basin. At that time, an east- sloping pediment was eroded across west- dipping strata in the western basin. Pediment gravel and tuff present as unconformity-bounded strata in the western basin correlate with continuously aggraded conformable basin-fill strata to the east. A return to westward tilting since middle Pleistocene time is suggested by unpaired terraces consistent with net westward migration of the Rio Grande, lack of post–middle Pleistocene motion on faults in the eastern basin, and numerous fault displacements of the pediment gravel in the western basin.