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The AAPG/Datapages Combined Publications Database

AAPG Bulletin


Volume: 69 (1985)

Issue: 2. (February)

First Page: 297

Last Page: 298

Title: Provincial Variations in Cap Rock Source Materials of Gulf Coast Salt Domes: ABSTRACT

Author(s): Harry H. Posey, Stephen D. Hurst, J. Richard Kyle, Peter E. Price, Arnold R. Taylor

Article Type: Meeting abstract


Isotopes of strontium, carbon, and oxygen are used to model hydrocarbon, brine, and meteoric fluid interactions during cap rock evolution. Provincial isotopic variations occur between older salt domes of the east Texas (ETx) and northern Louisiana (NLa) basins and the younger domes of the Texas-Louisiana (Tx-La) coastal basin. ETx and NLa cap rocks exhibit normal, mid-Jurassic seawater values (87Sr//86Sr = 0.7068 to 0.7076), very wide ^dgr13C ranges (-5 to -49 per mil PDB) and ^dgr18O values (-6 to -11 per mil PDB) that are slightly lighter than Tx-La (-4 to -10 per mil). Tx-La domes yield remarkably high 87Sr/86Sr ratios (0.7073 to 0.7100), and their ^dgr13C values (-8 to -41 per mil) have means which are 5 to 15 per mil heavier than ETx and NLa domes.

Detailed studies of the Hockley dome (Tx-La basin) reveal chemical diversity not recognized in domes farther inland. Anhydrite from the salt stock (mid-Jurassic Louann evaporites) mixed with two separate strontium sources during calcite formation. Calcites near the dome's center formed from an intermediate Sr ratio fluid (87Sr/86Sr ^cong 0.7090), which, based on heavier than average ^dgr13C values, was enriched in CO2 relative to CH4; peripheral calcites evolved from a high Sr ratio fluid (87Sr/86Sr ^cong 0.7105) with a lower CO2/CH4 ratio.

High 87Sr/86Sr ratios in other Tx-La anhydrite cap rocks compared with normal mid-Jurassic type values in ETx and NLa cap rocks suggest

End_Page 297------------------------------

that the Tx-La basin was periodically isolated from normal seawater during Louann deposition; radiogenic fluids, derived either from local red beds or from meteoric waters, equilibrated with seawater prior to anhydrite precipitation.

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