Jurassic stratigraphy in the Hagan Basin, north-central New Mexico
Spencer G. Lucas, Orin J. Anderson, and Clarence Pigman
Jurassic strata exposed in the Hagan basin are assigned to four formations (ascending): Entrada, Todilto, Summerville and Morrison. The Entrada Sandstone disconformably overlies the Petrified Forest Formation of the Upper Triassic Chinle Group and consists of two members also recognized in west-central New Mexico: (1) the medial silty member, as much as 20 m of mostly reddish brown sandstone and siltstone overlain by the (2) upper sandy member, as much as 29 m of mostly yellowish gray trough-crossbedded sandstone. The Todilto Formation disconformably overlies the Entrada Sandstone and consists of two members named here: (1) the Luciano Mesa Member (limestone member of previous usage), as much as 6 m of mostly gray kerogenic limestone, overlain by (2) the Tonque Arroyo Member (gypsum member of previous usage), as much as 61 m of mostly white gypsum/ anhydrite. The Summerville Formation conformably(?) overlies the Tongue Arroyo Member of the Todilto Formation and is as much as 57 m of cyclically bedded red and gray sandstone, mudstone and siltstone. The Salt Wash Member of the Morrison Formation disconformably overlies the Summerville Formation and is as much as 75 m of mostly grayish yellow sheets of fluvial sandstone and conglomerate with some interbeds of olive and brown mudstone. The Salt Wash Member grades upward into the Brushy Basin Member of the Morrison Formation, as much as 128 m of mostly variegated olive, gray and brown bentonitic mudstone/siltstone. The Jackpile Member of the Morrison Formation overlies the Brushy Basin Member and is as much as 62 m of mostly light gray and pale orange, trough-crossbedded sandstone. Cretaceous strata of the Dakota Formation disconformably overlie the Jackpile Member of the Morrison Formation in the Hagan basin. The Hagan basin Jurassic section aids confident lithostratigraphic correlation of Jurassic rocks on the southern Colorado Plateau to the west (San Ysidro) with Jurassic rocks on the western edge of the southern High Plains to the east (Romeroville). This correlation indicates almost total continuity of the Jurassic section from the Colorado Plateau to the southern High Plains with little change in thicknesses or facies. This continuity casts doubt on some recent interpretations of Jurassic depositional systems on the Colorado Plateau because these interpretations fail to consider the entire depositional basin.
- Lucas, Spencer G.; Anderson, Orin J.; Pigman, Clarence, 1995, Jurassic stratigraphy in the Hagan Basin, north-central New Mexico, in: Geology of the Sante Fe Region, Bauer, Paul W.; Kues, Barry S.; Dunbar, Nelia W.; Karlstrom, K. E.; Harrison, Bruce, New Mexico Geological Society, Guidebook, 46th Field Conference, pp. 247-255.