Middle Jurassic magmatism: the volcanic record in the eolian Page Sandstone and related Carmel Formation, Colorado Plateau

Widespread volcanic components in the eolian Page Sandstone and coeval and overlying Carmel Formation (fluvial, eolian, sabkha, restricted marine) in south-central Utah and adjacent Arizona document extensive Middle Jurassic volcanic activity in the adjacent Cordilleran arc. Bentonites accumulated during brief to possibly extensive lulls in sedimentation. Those with pure composition suggest little to no reworking and rapid accumulation, probably in one ash fall. X-ray diffraction (XRD) analyses reveal that the bentonites are predominantly authigenic, long-range ordered, mixed-layered illite/smectite. Bentonites in underlying and overlying formations occur as randomly interstratified illite/smectites with no more than 60% I. Mineralogy of Page bentonites is not produced by regional burial diagenesis, but is a product of local, near-surface diagenetic environment. These bentonites document geochemical alteration of volcanic ash by highly saline, potassic waters in a cyclic wet/dry hydrologic regime. -from Authors