THE BEARTOOTH-MOJADO CONNECTION: USING MID-CRETACEOUS SEDIMENTARY ROCKS TO UNDERSTAND THE TECTONIC HISTORY OF SOUTHWEST NEW MEXICO

Mid-Cretaceous quartzarenites in the Bisbee basin of southwestern New Mexico record the transition from Late Jurassic-Early Cretaceous rifting to the onset of Laramide shortening in the latest Early Cretaceous. Detrital zircon analyses and measured sections indicate a correlation between late Albian quartzarenites on opposite sides of the relict Bisbee rift basin.

Exposed in the Burro Mts., along the northern flank of the basin, the Beartooth Formation is a 30-meter-thick section of quartzarenite and conglomerate representing a tidal-marginal marine environment. Until this study, the age of the Beartooth Formation remained poorly constrained, but detrital zircon analyses indicate a maximum depositional age of 100 Ma. The Mojado Formation is a 1245-meter-thick sequence of marginal marine quartzarenite in the Little Hatchet Mts. that represents renewed subsidence of the Bisbee basin in the late Albian. The conformable contact with the underlying, middle Albian U-Bar Formation and the 99 Ma detrital zircon grains from the upper part of the Mojado Formation are evidence of rapid subsidence that exceeds the rate of thermal subsidence, indicating tectonic influence on basin formation. Additionally, an air-fall tuff in the lowermost strata of the overlying Mancos Formation, which represents a marine transgression, is ~ 97 Ma.

Foreland basins that form after relict rift basins are poorly understood. The Cenomanian strata are characteristic of a foreland basin system, with the Mojado Formation deposited in the foredeep and the Beartooth Formation deposited on the forebulge. Rather than attributing the increased accommodation in the late Albian to thermal subsidence of the Bisbee rift basin, an alternative model of a flexural origin for the Mojado-Beartooth section is supplied by the asymmetric geometry of those strata. A foreland basin origin for the uppermost Albian strata in turn provides evidence for earliest Laramide shortening in southwestern New Mexico near the beginning of the Late Cretaceous.