Petrographic and geochemical characterization of the Lower Ordovician Ellenburger Group, west Texas




Kupecz, Julie A.

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Dolomites within the Lower Ordovician Ellenburger Group can be broadly subdivided into two groups, those which predate pre-Middle Ordovician karstification (termed "early-stage" dolomite), and those which postdate pre-Middle Ordovician karstification (termed "late-stage" dolomite). Pre-Permian karstification provides the upper limit on the age of precipitation of late-stage dolomite. Early-stage dolomites comprise the most extensive and volumetrically abundant dolomite type within the Ellenburger. However, because early-stage dolomites approach stoichiometry and have low total Sr, they are interpreted to represent a stabilization phase of an even earlier dolomite generation. Regional analyses indicate that 𝛿¹³C data range from -0.6 to -3.5 o/oo PDB while 𝛿¹⁸O data are highly variable (ranging from -2.4 to -8.8 o/oo PDB). Data from karst breccias suggest that much of this geochemical variation is due to late diagenetic modification in which 𝛿¹⁸O values represent a physical mixture of two end-members: early-stage dolomite and a late-stage replacement dolomite. The variation between end-members represents a gradation from partial to complete recrystallization of early dolomite. Cathodoluminescence petrography supports the interpretation of partial to complete replacement of early-stage dolomite and progressive depletion in 𝛿¹⁸O. Luminescent dolomite which modifies earlier non-luminescent dolomite can be correlated to late-stage post-karstification dolomite cement. This suggests that fluids which precipitated late-stage dolomite cement are responsible for modification of earlier dolomite. Late-stage dolomite is comprised of a "precursor" and three major generations. The volumetrically most abundant type replaces the grainstone facies; this is followed by two generations of pore-filling dolomite cement. Megaquartz, interpreted to be cogenetic with late-stage replacive dolomite, yields homogenization temperatures of 85 ± 6 °C. Based on temperature/water plots of replacement dolomite and quartz, extrapolated temperatures of replacement dolomite range from 60 - 110 °C. Late-stage replacive dolomite and dolomite Cement-1 yield 𝛿¹⁸O values from -3.2 to -9.2 o/oo (PDB) and ⁸⁷Sr/⁸⁶Sr values from ~0.708 to 0.709, similiar to or slightly lower than that of Ordovician seawater (~0.709). Dolomite Cement-2, in contrast, yields 𝛿¹⁸O values from -7.2 to -10.3 o/oo and ⁸⁷Sr/⁸⁶Sr from ~0.710 to 0.713. ⁸⁷Sr/⁸⁶Sr data suggest rock-buffering relative to Sr within late-stage replacive dolomite (and a retention of a Lower Ordovician seawater signature), while cements became increasingly fluid-buffered. A contour map of 𝛿¹⁸O from replacement dolomite suggests a regional trend consistent with derivation of radiogenic fluids from the Ouachita Orogenic Belt. The timing and direction of fluid migration associated with the Ouachita Orogeny are consistent with the timing and distribution of late-stage dolomite. It is hypothesized that reactive fluids initiated as Pennsylvanian pore fluids, derived from basinal shales. The ⁸⁷Sr/⁸⁶Sr ratio of the fluids evolved from a Pennsylvanian seawater signature to radiogenic values; this evolution is due to the increased temperature of the system and a concomitant evolution in pore-water geochemistry in the dominantly siliclastic sediments. The source of Mg for late-stage dolomite is interpreted to be from the dissolution of early-stage dolomite by reactive basinal fluids