107-8 Quartz in geodes in the Fort Payne Chert: Evidence for deep burial and hydrothermal conditions

Session: Sedimentary Geology Division/SEPM Student Research Poster Competition: Dynamics of Stratigraphy and Sedimentation

Presenting Author:

Authors:

Abstract:

The Mississippian Fort Payne Chert in northeastern Alabama contains megaquartz-lined geodes previously interpreted to have formed from low-temperature surface fluids ^[1] or during shallow burial diagenesis at ~40 °C ^{[2, 3]}. Burial history models and regional paleomagnetic data, however, suggest the area experienced temperatures significantly higher than ~40 °C during Pennsylvanian–Permian burial ^{[4, 5]}. To constrain temperature and fluid conditions of megaquartz precipitation, we conducted petrographic and fluid inclusion analyses on ten primary fluid inclusion assemblages (FIAs) from a geode sampled at the Kirbytown, Alabama locality. Fluid inclusion microthermometry included homogenization temperatures (T_h), final ice melting temperatures (Tm_ice), and eutectic temperatures (T_e). Pressure corrections were applied using a 26 °C/100 bar thermobaric gradient. T_h values range from ~92 °C to ~115 °C in FIAs 1–6, increase to ~130 °C in FIA 7, drop to ~115 °C in FIA 8, and rise again through FIA 9 (~120 °C) to a measured peak of ~150 °C in FIA 10. Tm_ice values from –2.7 °C to –1.5 °C and T_e values near –21 °C indicate low-salinity NaCl solutions (~2.6–4.5 wt.% NaCl eq.) consistent with connate marine origins. These data support an interpretation of progressive megaquartz precipitation during Pennsylvanian–Permian burial under pressure-temperature conditions of ~100 °C to ~140 °C and ~300 to 400 bars, with later-stage quartz growth influenced by localized hydrothermal heating reaching ~140 °C to ~180 °C and ~400 to ~600 bars. These results directly contradict earlier models invoking shallow burial or low-temperature diagenetic formation and instead record a history of deep burial conditions and hydrothermal influence.

[1] Bassler, R.S. (1908) The formation of geodes with remarks on the silicification of fossils. U.S. Museum National Proceedings. 35, 133-154.

[2] Milliken, K.L. (1979) The silicified evaporite syndrome- Two aspects of silicification history of former evaporite nodules from southern Kentucky and northern Tennessee. Journal of Sedimentary Petrology. 49, 1, 245-256.

[3] Nolte, R.A., Benson, D.J. (1998) Silica diagenesis of Mississippian carbonates of northern Alabama. Gulf Coast Association of Geological Societies Transactions. 48, 301-310.

[4] Simonis, A. F. (2014) Burial history and thermal maturity of the Chattanooga Shale, northwestern Alabama. Gulf Coast Association of Geological Societies Transactions. 64, 355-370.

[5] Hodych, J. P., Patzold, R. R., Buchan, K. L. (1985) Chemical remanent magnetization due to deep-burial diagenesis in oolitic hematite-bearing ironstones of Alabama. Physics of the Earth and Planetary Interiors. 37, 261-284.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-8522

© Copyright 2025 The Geological Society of America (GSA), all rights reserved.