The Montoya Group (Middle and Upper Ordovician) of west Texas and New Mexico consists of several carbonate types including crinoidal calcarenite with a microcrystalline matrix, crinoidal calcarenite with sparry calcite cement, calcilutite with abundant chert, chert-free calcilutite, shell limestones, calcirudites, and completely or partly dolomitized equivalents of the former.

Dolomitization and silicification are volumetrically important. Recrystallization and (or) replacement obscure much of the original texture and fossils. Dolomitization begins with the formation of sporadic small crystals, which increase in number until a complete dolomitic mosaic results. Coarse fossil debris becomes progressively reduced in texture as the process advances to completion. Texture and degree of silicification are not related in many places. Fluctuating silica supply in the sea water is strongly indicated. Intervals of abundant chert are separated by less cherty strata.

Study of the unaltered rocks show considerable range in environment from high-energy, shallow-water to low-energy, deep-water conditions. Montoya sedimentation can be compared with correlative strata deposited during the Middle and Upper Ordovician submergence of the North American continent. Montoya deposition in the Cincinnatian is estimated to be 320 feet in 10 million years--very slow when compared with 4,500 feet in 10 million years for the Cayugan autogeosyncline of Michigan (Kay, 1951).

Present lithologic and stratigraphic data support rejection of a disconformity within the Montoya Group, although several minor erosional breaks occur.

End_of_Article - Last_Page 532------------