Widespread volcanic components in the eolian Page Sandstone and coeval and overlying Carmel Formation (fluvial, eolian, sabkha, restricted marine) in southcentral Utah and adjacent Arizona document extensive Middle Jurassic volcanic activity in the adjacent Cordilleran arc. The most unusual of these volcanic components consist of pure to sandy bentonite fill within trough-shaped eolian scours or super scoops encased in cross-stratified eolian dune deposits. More than 200 of these deposits have been identified in the Page Sandstone near Page, Arizona. Also present are several regional but irregular and discontinuous, thin (several cm) bentonite beds associated with sandy sabkha and eolian sandsheet deposits. Volcanic components are also present as detrital sand- to boulder-size grains in both the Page Sandstone and Carmel Formation. Most of the identified clasts consist of rhyolite tuff fragments. Sand-size clasts occur in fluvial and eolian deposits and cobbles to boulders occur in bedload and mudflow deposits associated with ephemeral stream channels. 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. Volcanic clasts were eroded and transported from extensive rhyolite-tuff aprons in and adjacent to the arc, northward, onto alluvial and coastal plains of the study area. A sharp, northward decrease in grain-size of the largest boulders documents that some volcanic aprons extended to within several kilometers of present Jurassic outcrops. X-ray diffraction (XRD) analyses reveal that the bentonites are predominantly authigenic, long-range ordered, mixed-layered illite/smectite (80%-90% illite, Reischeweite 1-3; Reischeweite is explained in the "Clay Mineralogy" section). 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. Regional stratigraphic analysis of the Page Sandstone and the coeval and overlying Carmel Formation document the following tectonic events in the region: (1) lower and middle Page-long-term, sporadic accumulation of volcanic ash on a stable, arid coastal plain with increasing fluvial activity late in this episode; (2) upper Page- increased volcanic activity and local rapid subsidence (development of foredeep adjacent to arc uplift); and (3) uppermost Page, overlying Carmel-strong influx of volcanic detritus and reworked sedimentary material. These events document the well-known Middle Jurassic ignimbrite flare-up (ca. 170 Ma) and strong, possibly localized uplift near the southern margin of the Colorado Plateau.
|Number of pages
|Special Paper of the Geological Society of America
|Published - Jan 1 1995
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