New Mexico Geological Society Annual Spring Meeting — Abstracts

Petrographic and stratigraphic characteristics of Upper Devonian and Lower Mississippian strata in the Big Hatchet Mountains were examined. The Upper Devonian Percha Shale rests unconformably on the Upper Ordovician Montoya Formation and is overlain by strata referred to the upper part of Sabin's (1957) Portal Formation. To date, this is the furthest east the Portal has been observed. Devonian rocks record a shallowing up sequence going from quiet, uncirculated water that deposit ed the lower Percha (Ready Pay-Box Members) to high energy grainstanes deposited at or above wave base in the Portal. The Portal yielded Upper Devonian (Famennian) conodonts indicative of Sandberg's (1976) shal low water polygnathidicriodid biofacies.

Mississippian strata, represented by the Escabrosa Group, contain Early Mississippian (Osagean) conodonts at the base. The Devonian-Mississippian contact is placed at the base of the first cliff. Lower Mississippian strata record two Osagean cycles of submergence and emergence. Encroachment began in early Osagean time (isosticha-upper crenulata Zone) and deposited the basal oolitic grainstones of the Bugle Member of the Keating Formation. A shallowing up sequence followed culminating in the deposition of high energy grainstones of the upper Bugle. The end of Bugle time is marked by a second submergence (lower typicus-anchoralis latus Zone) depositing argillaceous wackestones. This deepening continued into the basal Witch Member. Regression began within the Witch with the sequence going from fine grained mudstones to high energy grainstones with intraclasts. This shallowing sequence continues into the lower Hatchita Formation.