Circular 59—Glauconite in the
Cambrian-Ordovician Blis Formation near
Silver City, New Mexico

By by D. W. Lewis, 1962, 30 pp., 2 figs.

The Bliss Formation is composed principally of arenaceous dolomites and sandstones and has been divided into six members, from the base: lower quartzite, lower hematitic sandstone, thin-bedded glauconite, middle quartzite, middle calcareous sandstone, and upper glauconite. The thin-bedded glauconite and upper glauconite members are the most variable in thickness and lithology, both vertically and laterally, and contain most of the glauconite in the formation.

The glauconite is derived from fecal pellets of vagrant organisms in a shallow marine environment distant from the shore. Deposition took place under conditions of varying currents and turbulence. The term perigenic is proposed to describe materials that originated near its position of final incorporation into the sediments. The glauconite is commonly replaced by carbonate minerals, hematite, and limonite, and more rarely by silica. Light- and dark-green varieties of glauconite are distinguished; the lighter variety probably represents incomplete glauconization of clay minerals in the fecal pellets. No substantial difference was apparent between the glauconite of the different members.

This study of the Bliss Formation in the Silver City area of New Mexico describes the genetic environment of glauconite, based on occurrences in closely associated beds of differing rock types but of similar post-depositional history. The Bliss Formation is a marine carbonate-rock series, about 190 ft thick, with interbedded quartzites and siltstones and with widely distributed hematite and glauconite.

Glauconite, in the original meaning of the word, was a globular, lobate, commonly shiny, blue-green minerals aggregate, usually less than 1 mm in size, and rich in iron and potassium. At the present, glauconite is a field term for small, subspherical, sometimes green, earthy pellets which often include chlorite and montmorillonite clay minerals, although true glauconite is similar to illite. In this usage, glauconite is the common green mineral in the Bliss Formation of the Silver City area.