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



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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 undetermined but "substantial" displacement within 100 feet of two sample sites did not significantly alter the percentage of nonundulatory quartz grains. Although the average width/length ratio of quartz grains in the Bliss (0.66) is almost identical with those of the schist (0.64) and metaquartzite (0.65) and is quite unlike that of the granite (0.48), a preponderance of evidence points to a granitic origin for most Bliss quartz grains. It is apparent that form can not be relied upon to determine provenance in this area. In the local metaquartzite, interstitial growth of muscovite, subsequently altered in part to sillimanite, resulted in peripheral penetration of adjacent quartz grains by these crystals. This suggests that sedimentary quartz grains which have peripheral concentrations of acicular or irregular inclusions have been derived from a metamorphic source. After taking into account the size of the quartz grains and postdepositional history, percentages of nonundulatory quartz in the Bliss indicate a granitic rather than a metamorphic source for most of the quartz grains in this sedimentary unit. This conclusion is supported by evidence such as the presence of granite pebbles in the basal arkose; types of polycrystalline quartz grains; percentage of quartz grains without bubble trains; inclusions, and heavy minerals.