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Nodular chert from the Arbuckle Group, Slick Hills, SW Oklahoma: a combined field, petrographic and isotopic study
Author(s) -
GAO GUOQIU,
LAND LYNTON S.
Publication year - 1991
Publication title -
sedimentology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.494
H-Index - 108
eISSN - 1365-3091
pISSN - 0037-0746
DOI - 10.1111/j.1365-3091.1991.tb01876.x
Subject(s) - geology , petrography , diagenesis , quartz , chalcedony , geochemistry , sponge spicule , dolomite , anhydrite , paleontology , mineralogy , gypsum
Nodular chert from the middle and upper Arbuckle Group (Early Ordovician) in the Slick Hills, SW Oklahoma, was formed by selective replacement of grainstones, burrow fillings, algal structures, and evaporite nodules. Chert nodules are dominantly microquartz with minor fibrous quartz (both quartzine and chalcedony), megaquartz, and microflamboyant quartz. Lepisphere textures of an opal‐CT precursor are preserved in many (especially in finely‐crystalline) chert nodules. The δ 18 O values of microquartz chert range from +23.4 to + 28.8 0/00 (SMOW), significantly lower than those of Cenozoic and Mesozoic microquartz chert formed both in the deep sea and from near‐surface sea water. The δ 18 O values of chert decrease with increasing quartz crystal size. Silicification in the Arbuckle Group occurred during early diagenesis, with the timing constrained by the relative temporal relationships among silicification, burial compaction, and early dolomite stabilization. Silica for initial chert nucleation may have been derived from both dissolution of sponge spicules and silica‐enriched sea water. Chert nucleation appears to have been controlled by the porosity, permeability, and organic matter content of precursor sediments. This conclusion is based on the fact that chert selectively replaced both porous grainstones and burrows and algal structures enriched in organic matter. Growth of chert probably occurred by a maturation process from opal‐A(?), to opal‐CT, to quartz, as indicated by the presence of opal‐CT precursor textures in many chert nodules. Although field and petrographic evidence argues for an early marine origin for chert in the Arbuckle Group, the light δ 18 O values are inconsistent with this origin. Meteoric resetting of the δ 18 O values of the chert during exposure of the carbonate platform best explains the light δ 18 O values because: (i) the δ 18 O values of chert nodules decrease with decreasing δ 18 O values of host limestones, and (ii) chert nodules from early dolomite, which underwent more extensive meteoric modification than associated limestones, have lighter δ 18 O values than chert nodules from limestones. Increasing recrystallization of chert nodules by meteoric water resulted in progressive 18 O depletion and (quartz) crystal enlargement.