New Mexico Geological Society Annual Spring Meeting — Abstracts

Upper Paleozoic strata in the southern Cooke's Range at Fryingpan Canyon (NW 1/4 sec. 17, T21S, R8W) have previously been assigned to the Pennsylvanian Magdalena Group and the Lower Permian Abo Formation. Recent re-examination of these strata indicates they encompass three lithostratigraphically distinct units: (1) 18 m of cherty limestone and shale; (2) 25 m of interbedded shale, marine limestone and limestone-cobble conglomerate; and (3) 100 m of red-bed mudstone, arkosic sandstone and limestone/siliceous-pebble conglomerate overlain by the Lower Cretaceous Mojado Formation ("Sarten Sandstone"). The base of unit 1 is composed of thin-bedded fossiliferous limestone containing abundant fusulinaceans, overlain by cherty limestone composed of gray limestone nodules embedded in brownish cherty matrix, and interbedded brownish to reddish, cherty shale. Near its base, unit 1 yields the Desmoinesian (DS 1 or 2, Strawn) fusulinaceans Beedeina ex. gr. euryteines (Thompson) and Wedekindellina ex. gr. euthysepta (Henbest). Unit 1 is thus best assigned to the Madera Group as a Desmoinesian correlative of the Nakaye and Gray Mesa formations to the north and northeast. Unit 2 does not yield age diagnostic fossils but is interbedded shallow marine limestones (thin nodular limestone beds rich in crinoidal debris in the lower part, fossiliferous nodular limestone in the middle and upper part), limestonepebble conglomerates, red-bed siliciclastics and reddish mudstones containing abundant pedogenic limestone nodules. Unit 2 represents a transitional facies of thin limestones deposited during short, shallow marine episodes, and fine-grained red beds, mainly of pedogenic origin. This sequence is strikingly similar to the Bursum Formation, to which we assign it. Unit 3 is the Wolfcampian Abo Formation composed of red mudstones and siltstones with intercalated conglomerates, sandstones and pedogenic horizons. The presence of Bursum strata directly above Desmoinesian limestones suggests a profound hiatus between units 1 and 2 equivalent to at least Missourian time. Indeed, the MaderaI Bursum contact in Fryingpan Canyon is marked by a 3-m thick pedogenic calcrete at the Bursum base directly overlying cherty marine limestone at the Madera top, indicative of a profound unconformity. The presence of a Late Pennsylvanian unconformity in the southern Cooke's Range is further evidence of a regional tectonic event(s) in the Orogrande basin during Missourian time.