Jurassic strata in east-central New Mexico and their regional significance
Spencer G. Lucas and Lee A. Woodward

Abstract:

The Jurassic section in east-central New Mexico is as much as 330 m thick and consists of Middle and Upper Jurassic strata of the San Rafael Group (Entrada, Todilto and Summerville formations) and overlying Morrison Formation. The Entrada Sandstone is as much as 53 m of eolianite sandstone assigned to the lower, Slick Rock Member and the upper, Exeter Member, which is an upper tongue of the Entrada that postdates Todilto deposition. The Todilto Formation is 2-7 m of organic-rich limestone of the Luciano Mesa Member that disconformably (J-3 unconforrnity?) overlies the Slick Rock Member of the Entrada. Summerville Formation strata are up to 40 m of repetitively-bedded sandstone and siltstone that disconformably overlie the Todilto and grade into/intertongue with the Exeter Member. The Morrison Formation disconformably (J-5 unconformity) overlies the Summerville and consists of a basal Salt Wash Member (up to 50m of arkosic sandstone/conglomerate) and an upper, Brushy Basin Member (up to 150 m of smectitic claystone); locally, a 10 15-m-thick, trough-crossbedded sandstone at the top of the Morrison may be an equivalent of the Jackpile Member to the west.

The absence of the basal, Dewey Bridge Member of the Entrada Sandstone indicates that east-central New Mexico was landward of marine influence of the Cannel seaway during the Bajocian-early Callovian, and that the 1-2 unconformity at the Entrada base represents a longer hiatus in east-central New Mexico than it does to the west. Todilto deposition took place in a paralic salina with little or no direct connection to the coeval Curtis-Sundance seaway. Summerville tidal flat facies extended across northern New Mexico, much of Colorado and Utah, northeastern Arizona and adjacent areas during the Callovian-Oxfordian transition, and graded southward and southeastward into colian dunes. The J-5 unconformity at the base of the Salt Wash Member of the Morrison Formation represents a significant tectonic reorganization of the Jurassic depositional systems. Deposition of the Brushy Basin Member of the Morrison Formation did not take place in a single, large lake (Lake T'oo'dichi'), but instead on a vast floodplain dotted with smaller lakes. 

Regional eastward-trending piercing lines between the Colorado Plateau and east-central New Mexico can be defined by stratigraphic truncations and depositional pinchouts in Jurassic strata. They only allow 5-20 km of right slip between the eastern margin of the Colorado Plateau and the craton in post-Jurassic time. Arguments that 60-170 km of right slip occurred here during the Laramide orogeny (Late Cretaceous- Paleogene) or Neogene thus are untenable.


Citation:

  1. Lucas, Spencer G.; Woodward, Lee A., 2001, Jurassic strata in east-central New Mexico and their regional significance, in: Geology of the Llano Estacado, Lucas, Spencer G.; Ulmer-Scholle, Dana S., New Mexico Geological Society, Guidebook, 52nd Field Conference, pp. 203-212.

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