Upper Ordovician and Silurian strata of the Sacramento Mountains of south-central New Mexico consist largely of dolomite and are easily separable into four distinctive lithologic units. These were classified by Darton as two members of the Montoya limestone (Upper Ordovician), and two members of the Fusselman limestone (Middle Silurian). In this report these four rock units, in order of decreasing age, are referred to as the lower and upper member of the Montoya formation, the Valmont dolomite (new name), and the Fusselman(?) formation. A detailed stratigraphic section representative of these units in the Sacramento Mountains is presented. The only unconformity recognized in the Sacramento Mountains within these four rock units occurs between the Valmont and the Fusselman ?) formations. This disconformity is indicated by abrupt lithologic change; by the local occurrence of basal detrital zones of quartz, glauconite, and dolomite pebbles in the overlying Fusselman(?) formation; and by truncation and channeling of the uppermost Valmont strata.

The Montoya formation is essentially as outlined by Darton. It is 190-225 feet thick and consists of two intergrading members: a lower member (80-100 feet) of massive, dark-weathering dolomite that contains little chert, and an upper member (85-125 feet) of olive to light gray, thin-bedded, distinctively cherty dolomite. Quartz sandstone forms the basal few inches to 12 feet of the lower member, and commonly grades upward into the overlying dolomite. Fossils indicate Upper Ordovician age.

The Valmont dolomite is remarkably uniform lithologically and easily recognized in much of south-central New Mexico. It consists of 150-225 feet of light gray weathering, thin- to medium-bedded, sublithographic dolomite that contains little chert. A few feet of argillaceous dolomite from 40 to 70 feet above the base divides the formation into two members. Fossils from the lower member are Upper Ordovician, those from the upper member are indeterminate as to Upper Ordovician or Silurian.

The Fusselman(?) formation consists of resistant, gray to brown, cherty dolomite up to 100 feet thick. A fauna of about 25 genera identified by Arthur L. Bowsher includes Pentamerus parvulus, Dalmanella edgewoodensis, Camarotoechia? cf. C. festinata, and Loxoplocus fasciata, and is similar to many forms from the Lower Silurian (Alexandrian) Edgewood limestone of Missouri. Diagnostic Middle Silurian (Niagaran) fossils have not been recognized in the Sacramento Mountains, although known in parts of the type section of the Fusselman. It appears probable that Lower Silurian strata once extended from Missouri and southern Oklahoma to southern New Mexico. Some of the strata now correlated with the Fusselman in southern New Mexico and West Texas may be a part of these older deposits.

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