Gondwana, though extant for approximately one-half billion years, is now present as fragments across much of the globe. Following an assembly during the latest Protero zoic into the early Phanerozoic, the megacontinent has gradually fragmented to its current dispersed pattern. Paleozoic fragmentation, primarily on its north and west margins, formed a series of ribbon-shaped continents that collided with southern Laurasia and generated major orogenic events. Meanwhile, much of its southern and eastern margin was the site of subduction and associated Cordilleran-style tectonics. Mesozoic and Paleogene rifting completed the fragmentation, sending continents northward to generate the Alpine-Himalayan mountain chain from Spain to China. Much of Gondwana flirted with the South Pole throughout the Paleozoic, and several major glacial episodes resulted. The largest and most extensive of these was the late Paleozoic ice age; the consequences of this event dominated global geology for nearly 100 m.y. and orchestrated the greatest cyclic stratigraphic record in Phanerozoic history.

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. 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.

Abstract Standing astride the Utah-Arizona border with bronzed shafts of sandstone, the buttes and pinnacles of Monument Valley rise above the high desert floor of the central Colorado Plateau. The valley is tucked into the apex of the southern end of the Monument Upwarp, a large asymmetrical anticline bounded on its southern and eastern margins by the Comb Ridge monocline (Fig. 1). As recently as 35 years ago, no paved roads served the remote region, today three U.S. highways (U.S. 160 running east-west, and U.S. 163 and U.S. 191 running north-south), provide access to the region. But the best way to see Monument Valley is via the back roads that provide access to some spectacular backcountry. Monument Valley is located within the Navajo Indian Reservation. Travel on paved and main dirt roads is generally unrestricted. However, travel in some areas is restricted and a permit is required for rock collecting or extensive geological study. Inquiries should be addressed Navajo Nation, Window Rock, Arizona 86515. Monument Valley and vicinity is covered by geologic maps of various scales 1:62,500 (Witkind and Thaden, 1963), 1:96,000 (Baker, 1936), 1:250,000 (U.S.G.S. Iseries, 345, 629, 744, 1003), and 1:500,000(Geological Maps of Utah and Arizona). Witkind and Thaden (1963) have provided the most comprehensive geological report of the region.