Abstract

Secondary Fe- and Mn-oxides are locally common near faults and fractures within sandstones of the Colorado Plateau in the form of cements, concretions, and fracture-fill material. However, little is known about how and why these oxides formed, and less is understood about when they formed. In this study, we integrated field observations, detailed scanning electron microscope and petrographic observations, and (U-Th)/He and 40Ar/39Ar dating to better understand the formation of Fe- and Mn-oxides from three fault zones in Flat Iron Mesa, Utah. Most Fe- and Mn-oxide (U-Th)/He ages range from 0.50 to 3.4 Ma, which are much younger than 40Ar/39Ar ages of 25–20 Ma from a previous study. Current 40Ar/39Ar ages on a Mn-oxide sample from this study yielded a plateau age of 3.6 ± 0.08 Ma. 4He/3He diffusion data from Fe- and Mn-oxides are consistent with the presence of multiple diffusion domains with varying He retentivity. Predicted fractional radiogenic He retention over 3.6 m.y. at near-surface temperatures for the bulk samples, with domain proportions of the diffusion experiments, is ∼90% for Fe-oxide and ∼45% for Mn-oxide. The multidomain behavior exhibited by the oxides and the variability observed in (U-Th)/He ages among aliquots are consistent with variable amounts of diffusive loss of He within dated aliquots. Using 4He/3He data and He bulk ages, step-age plots indicate a high fractional release plateau at ca. 3.6 Ma, concordant with 40Ar/39Ar dating and most of the oldest (U-Th)/He ages observed. Taken together, these results are most consistent with formation of Flat Iron Mesa Fe- and Mn-oxides near the surface (<0.5 km) at 3.6 Ma, due to hydrologic changes, fault activity, or both. Our data and interpretations suggest that erosional exhumation rates over ∼1–4 m.y. time scales may be variable in the central Colorado Plateau, possibly driven by local effects, and they are not spatially uniform over large regions.

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