ABSTRACT

The Miocene Tesuque Formation is a > 2,000m-thick sedimentary unit filling most of the Espanola basin, an asymmetrical basin which is part of the Rio Grande continental rift in north-central New Mexico (U.S.A.). It is composed of conglomerate, sandstone, mudrock, and limestone of nonmarine origin, with numerous interbedded ashfall-tuff layers. The sedimentation pattern of the Tesuque Formation was determined by the interaction of two distinct paleodrainage systems deriving from the eastern and northeastern margins of the Espanola basin during the rifting process. The two systems generated two lithosomes, which can be discriminated in outcrop by means of paleocurrent study, sandstone and conglomerate petrology, and sedimentary facies analysis. Their surficial expressions are called rovinces A and B. Province A has westward paleocurrents and a mixed plutonic and medium-to-high-grade metamorphic provenance. Province B has SSW-directed paleocurrents and significant volcanic and sedimentary detrital inputs. Both provinces represent deposition in a dynamic alluvial-fluvial-lacustrine environment. Nevertheless, sedimentary facies of province A are representative of an ephemeral environmental regime, whereas those of province B seem to represent a more perennial regime.

The basin fill, as a whole, is organized in upward fining megasequences averaging about 200 m in thickness. The gradual passage from proximal to more distal environments of deposition is documented within several megasequences. The lower part of these megasequences is composed of braidplain deposits, whereas the upper (and finer) part is characterized by sandflat deposits, lacustrine carbonates and nodular calcretes.

Although the eastern margin of the Espanola basin (contributing the detritus of the Tesuque Formation) represents the relatively unfaulted side of a half graben, the documented upward fining megasequences are comparable to similar megasequences derived from tectonically active basin margins. This suggests that sedimentation style on a shallow ramp along the tectonically inactive side of a half graben can be determined by tectonism of the master fault system along the opposite (deeper) side of the basin.