Vitrinite reflectance analysis and the mean diameter of metamorphosed quartz, suggest that the eastern Ouachita region, Arkansas, USA reached the lower greenschist facies, regardless of burial depth. An increase in “crystallinity” may explain that the maturation is from the exhumation of the rocks from a greater depth. Shale and sandstone samples collected from outcrops along the crest of the Benton Uplift exhibit illite and chlorite crystallinities that yield higher indices compared to younger rocks adjacent to this crustal-scale anticlinal structure. Illite crystallinity d(001) of air-dried, fine (<0.2 μm) authigenic clays (Δ°2θ (CuKα) = 0.417° to 0.875°) yield a range of conditions from diagenetic temperature to lower anchizone temperatures up to ∼200 °C. Air-dried, fine authigenic chlorite clays (Δ°2θ (CuKα) = 0.259° to 1.570°) yield a wide range of diagenetic to metamorphic conditions that span the diagenetic zone through epizone, which indicate a maximum temperature slightly above ∼300 °C. These results are in contrast to prior thermal maturation data. No additional heat sources, such as from pluton emplacement, were identified by major areas of anomalous high maturation based on crystallinity. Regionally, the illite and chlorite crystallinity increases toward the central axis of the Benton Uplift. Exhumation of the rocks from greater depth is all that is required to explain the illite and chlorite crystallinity data.

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