ABSTRACT

Oxygen isotopic composition of nodules and beds of replacement chert of the Edwards Group, from outcrop and subsurface of Central Texas, ranges between +30.2 and +33.9 relative to Standard Mean Ocean Water (SMOW). D/H ratios vary between -26 and -86 relative to SMOW. Although only 8 samples from the subsurface (maximum depth 3,400 m) and 25 from outcrop have been completely analyzed to date, burial to approximately 3.5 km (approximately 130°C) does not appear to have affected the 0¹⁸, but may have affected slightly the D of these microquartz cherts. Other forms of silica from both outcrop and subsurface, including megaquartz filling molds resulting from the dissolution of evaporite nodules, replacement megaquartz, and replacement chalcedony, are depleted in 0¹⁸ relative to the chert nodules, with values as light as +25.5.

Microquartz chert probably formed from opal C-T precursors and replaced carbonate sediments which formed and were altered in a variety of environments ranging from hypersaline through meteoric. The variation in the oxygen isotopic composition of waters responsible for diagenesis of these rocks must have exceeded by at least a factor of 2 the observed 4 range in 0¹⁸ of these cherts. Possible contamination of chert by later generations of silica (depleted in 0¹⁸) cannot be excluded as contributing to the observed range and, if present, would further limit the possible range in isotopic composition of water forming the chert. In addition, the 0¹⁸ of Edwards chert is virtually identical to that of Cretaceous chert from European chalk which formed on open marine shelves. On this basis, silicification under a wide variety of salinity-temperature conditions is ruled out. Contemporaneous silicification under evaporitic conditions is ruled out by calibration with all available silica-water temperature scales. Silicification must have occurred either contemporaenously from solutions having a 0¹⁸ close to SMOW or in the shallow subsurface at elevated temperature and from 0¹⁸ enriched water.

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