Evidence of elevated pressure and temperature during burial of the Salem Limestone in south-central Indiana, USA, and its implications for surprisingly deep burial

• Published in: Sedimentary geology 2001-09, Vol.143 (3), p.245-257
• Main Author: Ambers, Clifford P.
• Format: Article
• Language: English
• Subjects: Diagenesis; Fluid inclusions; Illinois Basin; Indiana; Salem Limestone; Stylolitization
• ISSN: 0037-0738; 1879-0968
• DOI: 10.1016/S0037-0738(01)00091-4

A minor, normal fault related to compaction of the grainstone shoal facies of the Salem Limestone in south-central Indiana provides an unusual opportunity to test the pressure and temperature of both faulting and associated stylolitization. Syn-deformational sphalerite occurs in voids along the fault where it intersects an organic-rich shale parting in the sand flat facies overlying the grainstone. The sphalerite contains fluid inclusions that can be used for microthermobaric measurements. Most fluid inclusions in the sphalerite are demonstrably cogenetic with the host sphalerite and of the two-phase aqueous type common in Indiana, although many contain petroleum and others contain gas. Crushing tests in kerosene indicate that the aqueous inclusions contain dissolved methane in varying amounts as high as 1000 ppm. Microthermometry shows that late sphalerite growth, late fault movement, and late stylolitization all occurred as conditions approached 108°C and 292 bars. This pressure is in accord with a normal, basinal, geothermal gradient of 32.5°C/km that would produce the observed temperature under hydrostatic conditions at a burial depth of 2.7 km using an average fluid density of 1.1 g/cm 3. These results serve as a reminder that fluid inclusions in diagenetic minerals hold important temperature and pressure information regarding burial diagenesis of Paleozoic rocks across the North American midcontinent. Detailed study of dissolved gases in fluids trapped in disseminated sphalerite that is common across the midcontinent could help resolve the enigma of sedimentary rocks with high thermal maturity exposed at the surface across the region.