New Mexico Geological Society Annual Spring Meeting — Abstracts

An apparently complete section that encompasses the Pennsylvanian-Permian boundary is exposed west of Robledo Mountain (SW ¼ sec. 34, T21S, R1W). Thompson (1954. Univ. Kansas Pal. Contrib.) described this section as Virgilian Fresnal Group overlain by Wolfcampian strata of the Bursum Fm?, Bursum Fm. and Hueco Ls. Subsequent workers either reiterated that Bursum strata are present at Robledo Mountain or stated that the Hueco rests directly on Virgilian strata. Our detailed measured section through the supposed Bursum interval at Robledo Mountain indicates that there is no lithologic basis for identifying Bursum Formation there, and that most, if not all, "Bursum" strata should be assigned to the Hueco. Thompson's assignment of strata at Robledo Mountain to the Bursum was purely biostratigraphic in which Bursum referred to Wolfcampian strata below the lowest local occurrence of the fusulinid Schwagerina.

We measured 56.5 m of strata at the base of the Hueco, beginning on top of a several m thick cliff of massive limestone. The lower part (0 -29 m) is composed of gray, bedded limestones, nodular limestones and intercalated reddish marly shale with limestone nodules. Bioclastic wackestones and bioclastic peloidal wackestones are the most abundant microfacies; subordinate are peloidal grainstones and wackestones, oncolithic wackestones and fusulinid wackestones. Nodular limestones are composed of bioclastic peloidal wakestones, grainstones and fusulinid wackestones/packstones. The upper part (29 - 56.5 m) consists of thin to thick bedded gray limestones and two nodular limestone beds. The dominant microfacies is again bioclastic wackestone; subordinate are bioclastic grainstone/packstone, pelodal wackestone, algal wackestone and boundstone formed of algal crusts. The limestones contain bioclastic remains of brachiopods, bivalves, gastropods, echinoderms, bryozoans, calcareous algae (phylloid algae Epimastopora, rarely Archaeolithoporella, ?Anthracoporella, Ejlugelia), fusulinids smaller foraminifers (mostly calcivertellid forms, also Bradyina sp., B. lucida, B. magna, Climacammina, Diplosphaerina, Endothyra, Syzrania bella, Syzrania sp., Tetrataxis, Tuberitina), ostracods, Tubiphytes, rare serpulids and trilobite fragments. Most limestones contain a diverse biota indicating a shallow, normal marine depositional environment within the photic zone (presence of algae). A few samples (e.g. sample RM 37) are characterized by a low diversity biota pointing to a restricted shallow marine environment. The dominance of muddy carbonate textures (different types of wackstones) indicate low-energy water conditions near or just below wave base. Grainstones, which formed under higher-energy water conditions (shoal environment), are rare.