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Bulletin 125—Petrography and depositional environments of the Lower Ordovician El Paso Formation

By R. E. Clemons, 1991, 68 pp., 34 tables, 49 figs., 2 appendices.

The El Paso Formation is one of many time-correlative units deposited on the North American craton in shallow Early Ordovician (Canadian) seas. It crops out in most mountain ranges from southeast Arizona to west Texas. Field examination, sampling, and petrographic study of 38 stratigraphic sections from southeastern AZ, across NM, to Van Horn, TX, indicate the El Paso was deposited in shallow waters near a shoreline to the north. The southwestern edge of the North American craton had subsided gradually throughout most of Canadian time, receiving the up-to-420-m-thick El Paso carbonate blanket. This paper presents: (1) descriptions of the sedimentary structures, textures, and compositions of the El Paso at 38 locales; (2) possible origins of dolomitization; and (3) an interpretation of the depositional environments.

Outcrops of the El Paso Formation at 54 locales were investigated and compared with sections described in the literature. From the investigations and published descriptions, representative sections at 38 locales were chosen for detailed examination and for sampling for petrographic study. Because a study of the Mescal Canyon was already underway, the less well-documented Ram Gorge section was chosen as the representative section in the Big Hatchet Mountains. The Scenic Drive section and the El Paso type section were sampled in the southern Franklin Mountains to check for variations over short lateral distances.

Sections were measured with a Jacob's staff and were sampled at varying intervals. Whether limestone portions were sampled at 0.5 m or 10 m intervals depended on outcrop lithology. That is, if a limestone section appeared uniform in composition, then it was sampled every 10 m; whereas if the lithology changed, each variation in composition was sampled to ensure that all lithology in the section would be represented. The Capitol Dome and Victorio Canyon sections were sampled at 2–3 m intervals as part of another project. From the samples taken from the 38 locales, more than 1,200 thin sections were examined and most were point counted; from the Capitol Dome and Victorio Canyon sections, thin sections were examined, but only about one third were point counted. Thin sections were described using the classifications of Folk and Dunham and were point counted using grain-solid measurements and a 300-point grid.

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