The siliciclastic Cutler Group comprises predominantly red bed strata that accumulated across large regions of the Colorado Plateau. The proximal (to the Uncompahgre Plateau) region of the Cutler Group is unique as it records subaerial exposure of complex subsurface fluid dynamics reflected in strata with either intercalated red-green and red-bleached coloration. The change from red to green or bleached colors corresponds with removal of iron oxides in bleached layers and retention of some Fe(II) in phyllosilicates and epidote in green layers, both likely through the action of one or more reactive fluid events (Hullaster et al., 2023). Permian Cutler strata onlap fractured and faulted Precambrian basement of the Uncompahgre Front, leading to a poorly constrained hydrologic system within Precambrian-hosted Unaweep Canyon. To better understand the origin and significance of color variegations in the proximal Cutler Formation, I investigated the changes in clay minerals and their associated chemistry and texture as it relates to the different colored samples. To achieve this, I integrated x-ray diffraction (XRD), scanning electron microscopy (SEM) with energy dispersive x-ray spectroscopy (EDS), with zircon (U-Th)/He (ZHe), and apatite fission-track (AFT) low-temperature thermochronology in order to provide a better separation of event(s) and a temporal framework for the greening and bleaching evident in the proximal Cutler Formation.

The data are consistent with green and bleaching alteration taking place during the Cenozoic, perhaps through the action of distinct processes related to the movement of basinal reducing fluids upwards along faults. Alteration of abundant biotite to secondary clay minerals led to distinct chemical signatures in the green and bleached layers, and red sample secondary products that overlap with both green and bleached populations. Green samples contain clays depleted in Mg, slightly enriched in Al, and with Fe preserved. Phyllosilicate alteration in bleached samples is characterized by Fe loss followed by Fe+Mg loss and Al enrichment. Interlayer K is replaced mostly with Ca in green and red samples and Na + Ca in bleached samples. The modern clay mineral alteration assemblage is consistent with the temperatures experienced by the samples post-maximum burial. Associated thermal history modelling of the AFT and ZHe data yields (1) late Jurassic-early Cretaceous maximum burial to ~150 – 200 ºC (~6 – 8 km); (2) cooling during the Eocene – Oligocene (~40 – 30 Ma) with green samples cooling to 40 ºC and 70 ºC in red); (3) beginning at ~27 Ma both samples were reheated to 80 ºC by 4 Ma; and finally (4) Pliocene (~4 Ma) rapid cooling to surface temperatures. I associate these two periods of cooling with regional exhumation associated with the Laramide uplifts and incorporation of the Colorado river headwaters, respectively. This study contributes to our understanding of the evolution of the regional fluid dynamics in the post-Permian Cutler system.