Abstract

Rocks of the Montoya dolomite of Late Ordovician age and the Fusselman dolomite of Silurian age, exposed in the Silver City region of SW. New Mexico, show marked similarities in lithology, sequence, and thickness to rocks of the same formations as reported farther E. The sections in the Silver City region, described here in detail for the first time, are at Bear Mountain, NW. of Silver City; at Lone Mountain, SE. of Silver City; and near Georgetown, NE. of Silver City. The Montoya dolomite is divisible into the same 4 units recognized farther E. in New Mexico and Texas, and the same names are applied to them: from the base up they are the Cable Canyon sandstone member, 11-28 ft. of coarse-grained dolomitic sandstone; the Upham member, 52-91 ft. of massive dolomite; the Aleman cherty member, about 75 ft. of distinctive interlaminated dolomite and chert; and the Cutter member, about 200 ft. of massive dolomite. The Cutter member is separated by a generally sharp and distinct lithologic break from the overlying Fusselman dolomite, which is composed of about 100-300 ft. of gray massive dolomite. The contact is marked by 2 lithologic contrasts: the Cutter is light gray and dense, and the Fusselman dark gray and vuggy. Fossils collected from the Cutter member at Bear Mountain are of Late Ordovician age. Pentameroid brachiopods near the middle of the Fusselman dolomite in its thickest occurrence - at Lone Mountain - are of Middle or Late Silurian age, but corals near the base of the Fusselman suggest that the lower part may be of Late Ordovician age. The subdivision and nomenclature of the Montoya and Fusselman have had an involved history since the 2 formations were originally differentiated and named by Richardson in 1908. The major developments in this history, especially those since Kelley and Silver's work in 1952, are reviewed and are summarized diagrammatically, in order to provide a background for the discussion of the Montoya and Fusselman at Silver City. Most workers have now accepted Kelley and Silver's proposal to raise the Montoya to group status and to consider its subdivisions, which are valid lithogenetic units, as formations. Reference to the Montoya as a formation in this paper reflects the current usage of the U.S. Geological Survey but does not preclude group status of the Montoya in areas where its component members are so exposed as to fulfill the other requirement of a formation, that of mappability.

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