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Silurian Outcrops of Southcentral and Southwestern New Mexico
Silurian strata crop out in many isolated mountain ranges of southcentral and southwestern New Mexico and of westernmost Texas. All of these strata are classified as the Fusselman Dolomite; they consist largely of gray, finely to medium crystalline, massive, and but slightly porous, tan-weathering dolomite. The Fusselman Dolomite is 1000 or more feet thick in the southernmost outcrops of southcentral New Mexico and 600 to 800 feet thick in the type area of the Franklin Mountains of westernmost Texas. Lower, middle, and upper members at the type section are herein designated the Chamberino, Flag Hill, and Crazycat Members of the Fusselman Dolomite. It thins northward at a uniform rate over a distance of 75 to 100 miles to an irregular eroded knife-edge in the northern Sacramento, San Andres, and Caballo Mountains, and the Sierra Cuchillo. On the west along a north-south trend west of the Victorio Mountains and Silver City, the Fusselman Dolomite was more abruptly truncated by erosion that, like the northward deepening erosion, took place during Late Silurian to Late Devonian time. The Fusselman Dolomite persists eastward into the subsurface of southeastern New Mexico and western Texas as a unit several hundreds of feet in thickness.
The Fusselman Dolomite overlies thin-bedded, sublithographic to finely crystalline dolomites of Late Ordovician age (Cutter Dolomite) throughout the outcrop region of southern New Mexico and westernmost Texas. The contact is everywhere a sharp, erosional break representing a gently undulatory regional disconformity.
The Fusselman Dolomite is overlain unconformably by various Late Devonian rock units in southwestern New Mexico and by Middle (?) Devonian strata in the southernmost part of southcentral New Mexico. In the subsurface farther to the southeast (McGlasson, 1967) the Fusselman Dolomite is overlain by "Upper Silurian" carbonates and shales or by Devonian or younger Paleozoic strata overlying regional unconformities.
Pervasive dolomitization has obscured or obliterated most of the fossils of the Fusselman Dolomite. The fauna is dated as Early and Middle Silurian age. Brachiopods, corals, crinoids, and stromatoporids are the most commonly recognized fossils. Some limestone and a few small bioherms occur locally in the Franklin Mountains of westernmost Texas.
The Fusselman Dolomite in southern New Mexico was deposited on a broad shelf northwest and west of the Tobosa Basin, which formed an embayment of broader Silurian seas farther to the south and east. The northern limit of Fusselman deposition could have been tens or hundreds of miles north of the present eroded edge. Westward, Silurian strata may have once been continuous with similar dolomites in Nevada. Most strata of the Fusselman Dolomite appear to represent shallow marine deposits, but some are suggestive of intertidal to supratidal deposition. Detailed sedimentologic and paleontologic studies of the Fusselman Dolomite are needed to adequately interpret the depositional environments and to better establish both the local and regional correlations.
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