--> Post-Devonian Diagenesis of Middle Ordovician Carbonate Buildups (Alabama, U.S.A.): Implications for the Timing of Fluid Migration in the Alleghanian Foreland Basin, by K. J. Tobin, K. Srinivasan, K. R. Walker, C. I. Mora, and S. G. Goldberg; #90986 (1994).

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Abstract: Post-Devonian Diagenesis of Middle Ordovician Carbonate Buildups (Alabama, U.S.A.): Implications for the Timing of Fluid Migration in the Alleghanian Foreland Basin

Kenneth J. Tobin, Krishnan Srinivasan, Kenneth R. Walker, Claudia I. Mora, Steven G. Goldberg

Ordovician buildups at Red Mountain Expressway (RME) and Tidwell Hollow (TWH) record diagenesis associated with the Alleghanian orogeny. Diagenetic phases include (oldest to youngest): (1) blocky calcite in non-selective voids, (2) planar dolomite around stylolites, (3) baroque dolomite in primary voids and late veins, and (4) blocky calcite in late veins.

Blocky calcite post-dates Ordovician meteoric cement. Fe and Mn <400 ppm and MgCO3 <1.00 mol% suggest precipitation from meteoric fluids. This is supported by 87Sr/86Sr = 0.70847 to 0.70869 (= Ordovician seawater). Blocky calcite has a narrow range of ^dgr13C (0.1 to -0.9^pmil) with variable ^dgr18O (-5.2 to -11.4^pmil), suggesting water-rock interaction distal from recharge. Only shallow burial occurred before the Mississippian (<150 m) so pre-Mississippian precipitation of blocky calcite with such negative ^dgr18O is unlikely. Two precipitation settings are thus suggested: (1) post-Devonian meteoric waters depleted by orographic effects, or (2) deep meteoric fluids below now unpreserved Pennsylvanian overburd n.

Non-ferroan to ferroan, planar dolomite (^dgr13C = 0.1 to 0.7^pmil, ^dgr18O = -1.4 to -6.1^pmil) is associated with stylolites and forming at a burial depth >500 m (late Mississippian at RME and Pennsylvanian at TWH).

Ferroan baroque dolomite (^dgr13C = 0.5 to 0.6^pmil ^dgr18O = -5.9 to -7.2^pmil) post-dates planar dolomite, and records migration of basinal brines into the succession. The strontium isotopic composition of baroque dolomite is the same as Ordovician seawater, suggesting high water/rock ratios. Baroque dolomite, then, probably formed during the Pennsylvanian. Late veins filled with baroque dolomite and blocky calcite with depleted isotopic composition (^dgr13C = -1.1^pmil, ^dgr18O = -2.8^pmil) cross cut earlier phases and formed during Alleghanian overthrusting. Thus, basinal fluid migration associated with deformation is post-Mississippian, contrasting with Mississippian fluid migration documented by others.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994