NANOSCALE CHEMISTRY AND MINERALOGY OF AUTHIGENIC CLAYS IN THE PERMIAN CUTLER FORMATION AS INDICATORS OF FLUID ALTERATION AND PALEOCLIMATE

Abstract

The Permian Cutler Formation onlaps Precambrian basement at the northeastern edge of the Paradox Basin, western Colorado. Regionally, bleached and green colors in redbed Cutler strata are associated with a complex history of fluid alteration events, leading to the formation of related Cu and U deposits. In particular, the Cutler Formation adjacent to the uplift-bounding fault zones of Uncompaghre Plateau was influenced by fault-based fluid alteration and also records important paleoclimatic information. We hypothesized that the chemistry of the authigenic clay minerals reflects the chemical conditions of the fluid alteration event in green and bleached layers while red layers may retain climatic and burial diagenetic signatures. We determined whole rock and clay-fraction mineralogy of a suite of red, bleached, and green rock samples. Then, we selected a subset of samples for further nanoscale investigation of individual grain/aggregate chemistry via field-emission scanning electron microscopy (FE-SEM), (scanning) transmission electron microscopy (S/TEM), electron diffraction, and energy-dispersive X-ray analysis (EDXA). Most of the samples contain smectite and illite. Smectite is more abundant in green samples, and illite more abundant in red and bleached samples. Our results show that (1) the green samples contain dioctahedral smectites where Al>Mg, (2) the bleached rock samples contain illite and smectite expected to be dioctahedral according to EDXA analyses, and (3) red rock contains corrensite, illite, and likely dioctahedral smectite with Al>Fe. Further investigations linking nanoscale clay chemical and mineralogical analyses with sedimentological and geological field relationships will aid in unravelling the complex fluid and climatic signatures.