Sulfate reduction as a cause of depleted isotopic carbon values from calcite in the middle Frio Formation, South Texas

Conference · Wed Jan 01 04:00:00 EST 1992 · Geological Society of America, Abstracts with Programs; (United States)

OSTI ID:5932058

Grigsby, J D ^[1]

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1. Ball State Univ., Muncie, IN (United States). Dept. of Geology

Sulfate reduction of SO[sub 4][sup 2[minus]] enriched pore waters in fluvial deposits of the middle Frio Formation (FF) in South Texas resulted in the precipitation of pore-filling calcite with depleted delta C-13 values. Precipitation of elemental sulfur and pyrite was closely associated with this event. These occurrences indicate that sulfate reduction can be an important process in middle FF diagenesis. Detailed petrographic and isotope analyses of core samples from a 95-ft thick interval of the middle FF has found that calcite precipitated in two stages. The initial stage of precipitation (cal 1) is preserved in mudstones and siltstones of overbank deposits. It is micritic, Fe-poor, and commonly associated with root traces. Stable isotope analysis of calcite in several siltstone samples indicates that delta O-18 ranges between [minus]4.5--[minus]6.8[per thousand] (PDB) and that delta C-13 ranges between [minus]8--[minus]20.6[per thousand] (PDB). The second stage of calcite precipitation (cal2) is Fe-poor, occurs as pore-filling sparry cement, replaces cal 1 in siltstones and mudstones, and is closely associated with the precipitation of elemental sulfur and pyrite. Stable isotope analysis indicates that delta O-18 ranges between [minus]6.7--[minus]7.9[per thousand] (PDB) and delta C-13 ranges between [minus]17--[minus]27[per thousand] (PDB). Cal 1 is interpreted to have formed during pedogenesis from meteoric water having a delta O-18 between [minus]3--[minus]4[per thousand] (SMOW). Assuming that sulfate reduction occurred between 40 and 50 C, and using [minus]7.3[per thousand] as the average delta O-18 value, cal 2 is interpreted to have precipitated from water having a delta O-18 of [minus]2--0[per thousand] (SMOW) at a depth less than 1,000 m. The gradual replacement of cal 1 in siltstones and mudstones by cal 2 during burial is recorded by the linear trend that exists between delta O-18 and delta C-13 from these samples.

• OSTI ID: 5932058
• Report Number(s): CONF-921058--
• Conference Information: Journal Name: Geological Society of America, Abstracts with Programs; (United States) Journal Volume: 24:7
• Country of Publication: United States
• Language: English

Related Subjects

58 GEOSCIENCES
580000* -- Geosciences
CARBON 13
CARBON ISOTOPES
DEVELOPED COUNTRIES
DIAGENESIS
ELEMENTS
EVEN-EVEN NUCLEI
EVEN-ODD NUCLEI
GEOLOGIC HISTORY
GEOLOGIC STRUCTURES
GEOLOGY
ISOTOPE RATIO
ISOTOPES
LIGHT NUCLEI
MINERALS
NONMETALS
NORTH AMERICA
NUCLEI
OXYGEN 18
OXYGEN COMPOUNDS
OXYGEN ISOTOPES
PETROGRAPHY
PYRITE
SEDIMENTARY BASINS
STABLE ISOTOPES
SULFATES
SULFIDE MINERALS
SULFUR
SULFUR COMPOUNDS
TEXAS
USA