Investigation of the sources of quartz grains of the Bliss Formation (Cambro-Ordovician), Silver City Area, New Mexico

The Cambro-Ordovician Bliss Formation of the Silver City area. New Mexico is composed of approximately 190 feet of arkose, orthoquartzite, hematitic sandstone and glauconitic, arenaceous carbonate, resting nonconformably upon Precambrian granitic and metamorphic rocks, which are believed to be the source of most of the quartz grains in the Bliss. Three classes of quartz grains are distinguished: (1) polycrystalline (2) undulatory (monocrystalline strained) (3) nonundulatory (monocrystalline unstrained). The purpose of this investigation was: (1) to determine whether or not clastic quartz grains can be related to their source rocks, and (2) to examine some current ideas concerning quartz grains, such as the hypothesis that undulatory grains are preferentially destroyed during transportation. Sixty-eight samples of basement rocks, their outcrop detritus, and sedimentary rocks were collected from outcrops over a distance of 17 miles, and 102 thin sections of these samples were studied. Quartz grain size parameters were determined from sieve analyses and from thin section measurements. Ten parameters were tabulated for each of 5,964 quartz grains. Size, extinction type, abundance of bubble trains, roundness, inclusions, and polycrystallinity were found to be the most significant. Polynrystalline quartz grains are rare in the Bliss (5.0 to 0.4 percent), but relative proportions of these grains in the various members permitted local correlation. Survival of these grains is attributed mainly to the interlocking nature of individual crystal boundaries. Stretched, sutured polycrystalline grains, presumed to be metamorphic, are virtually absent. Orthoquartzites of the Bliss, in the fine sand size, have an average percentage (29-3) of nonundulatory quartz grains which is less than that of the local granite (39.0 percent), and is much less than that of the metamorphic rocks (67.O percent). Percentages of relatively stable nonundulatory quartz grains are therefore not an index of sandstone maturity in this area. Faulting of ...