Joy D. Griffin¹, Richard T. Buffler¹
(1) The University of Texas at Austin, Austin, TX

Abstract: Alluvial architecture and tectonic setting of the Mangas conglomerate in the Mangas Graben, Tyrone mine area, southwestern New Mexico

The Mangas Graben, located southwest of Silver City in southwestern New Mexico in the vicinity of the Tyrone copper mine, is situated at the triple junction of the Rio Grande rift, the Colorado Plateau, and the Basin and Range. The study area forms a northwest trending down-dropped block between the Big Burro Mountains to the southwest and the Little Burro Mountains to the northeast. Volcanism and erosion of surrounding highlands filled the Mangas Graben during the late Neogene, probably Plio-Pleistocene, depositing as much as 800m of Mangas conglomerate. The Tyrone mine provides excellent exposures of the conglomerate making it an ideal location to perform a study of rift sedimentation.

The conglomerate was syn-depositional with half-graben formation along the northwest trending Mangas fault. Uplift of the bounding Burro Mountains controlled sedimentation patterns creating localized punctuated influxes of matrix-supported, angular, poorly sorted, poorly stratified sands and conglomerates as alluvial fans. The processes controlling deposition of the fans were debris flows with reworking by sheet flooding. A second episode of dip-slip movement postdating deposition is recorded by northwest trending faults cutting the Mangas. Two ash beds intercalated with the conglomerate when dated will put an age constraint on deposition as well as tectonism.

The Mangas conglomerate is in stratigraphic contact with the highly productive copper porphyry system of Tyrone and is locally host to copper oxides. In addition to its economic importance in mineral exploration, a depositional study of the conglomerate is significant for developing models of rift sedimentation, particularly for hydrocarbon exploration.

AAPG Search and Discovery Article #90914©2000 AAPG Annual Convention, New Orleans, Louisiana