Abstract: Hydrogen and Oxygen Isotopic Composition of Cretaceous (Edwards Group) Chert, Central Texas

Lynton S. Land

The oxygen isotopic composition of nodular replacement chert from the Edwards Group, Central Texas, in outcrop and in the subsurface, ranges between +30.2 and +34.0 per mil relative to SMOW. Deuterium/hydrogen ratios range between -52 and -76 per mil. Although only 8 samples from the subsurface (maximum depth 3,400 m) and 20 from outcrop have been analyzed completely, burial to approximately 3.5 km, at a temperature of approximately 120°C, appears to affect neither the ^dgrO¹⁸ nor the ^dgrD of these microquartz cherts. Other forms of silica in both outcrop and the subsurface, including megaquartz that fills molds resulting from the dissolution of evaporite nodules, replacement megaquartz, and replacement chalcedony, are depleted in O¹⁸ relative o the chert nodules, with values as light as +25.5 per mil.

The microquartz chert nodules probably formed from opal C-T precursors in a variety of diagenetic environments ranging from hypersaline through meteoric, and they reflect variations in the temperature and isotopic composition of the water in these environments. The variable salinities associated with the diagenesis of these rocks easily can account for the observed 4 per mil range in O¹⁸, although the possibility of contamination by later generations of silica cannot be excluded as contributing to the observed range.

AAPG Search and Discovery Article #90967©1977 GCAGS and GC Section SEPM 27th Annual Meeting, Austin, Texas