Detrital Sanidine Ages from The Upper Jurassic Brushy Basin Member of The Morrison Formation and Overlying Strata, Sandoval County, New Mexico
— Spencer G. Lucas, Matthew T. Heizler, Karl E. Karlstrom, Edward L. Simpson, and Julia Ricci

The Morrison Formation, a world-famous Jurassic stratigraphic unit in the western United States, consists of three members in
New Mexico (ascending): the Salt Wash, Brushy Basin, and Jackpile Members. This study focuses on 40Ar/39Ar ages of detrital sanidines
(DS) from the Brushy Basin and Jackpile Members in New Mexico, near the San Ysidro area in Sandoval County. The formation’s stratigraphy
is characterized by sandstone-dominated and mudrock-dominated units. The research used six DS samples, two from the uppermost
Brushy Basin Member, three from the Jackpile Member, and one from the basal Oak Canyon Member of the Dakota Sandstone, to determine
the maximum depositional age (MDA) and to characterize provenance. Maximum depositional age results for the Brushy Basin Member
are Late Jurassic, at ≤150.39 ± 0.41 and ≤148.86 ± 0.31 Ma, for two samples that are a combination of integrated dates from low-resolution
age spectrum and total fusion analyses; this is similar to prior 40Ar/39Ar dates of 147–155 Ma from Utah. These are also analytically
indistinguishable from U-Pb zircon dates (149–150 Ma) from Colorado. The youngest MDA of the Jackpile Member samples comes from
the stratigraphically highest sample and is ≤151.39 ± 0.15 Ma, which does not resolve whether the Jackpile is Late Jurassic or Early Cretaceous.
The lowest Dakota Sandstone had a single youngest grain at 213.4 ± 0.2 Ma, much older than the 40Ar/39Ar ash age of 98.1 ± 2.4
Ma from this unit elsewhere. Provenance changes markedly upsection based on the distribution of DS dates: The Brushy Basin Member
contains dominantly 160–190 Ma Jurassic grains whereas the Jackpile Member and Dakota Sandstone contain dominantly 250–300 Ma
Permian–Triassic grains. This is consistent with a disconformity between the Brushy Basin and Jackpile Members and northerly fluvial
transport of Jackpile Member sands across a regional sub-Cretaceous unconformity in southern Arizona and New Mexico. Furthermore, the
overall correspondence of DS dates between the Jackpile Member and the Dakota Sandstone versus less correspondence with the Brushy
Basin could support a Cretaceous age for the Jackpile Member. These findings demonstrate the power of DS dating, especially age spectrum
analysis, for refining Mesozoic chronostratigraphy.