Abstract

Mature quartz-rich sandstone interbedded with Mesozoic magmatic-arc strata in southern Arizona has been previously considered correlative with the eolian Pleinsbachian to Toarcian Navajo Sandstone on the Colorado Plateau. New U-Pb zircon data from the Mount Wrightson Formation in the Santa Rita Mountains and from the strata of Cobre Ridge in the Pajarito Mountains and in the Arivaca area in southern Arizona support the hypothesis that sand derived from the Navajo Sandstone probably did gain access to the magmatic arc and further indicate that sands extending from younger eolian units on the Colorado Plateau are probably present as well.

The Mount Wrightson Formation accumulated from ∼190 Ma to 170 Ma, corresponding to late Pleinsbachian to Aalenian time (time scale of Harland, 1990). Isotopic ratios from zircon fractions from the lower member of the Mount Wrightson Formation define a discordia line with lower intercept of 184 ± 8 Ma. The lower member is approximately coeval with the 188 ± 2 Ma Piper Gulch monzonite, dated by Asmerom and others (1990), which intrudes it. The middle and upper members of the formation yield somewhat less precise dates that indicate deposition between 183 ± 2 Ma and ≈170 Ma. Quartz-rich sandstone occurs in all three members, though most abundantly in the middle and upper members, and in many places contains sedimentary structures diagnostic of eolian sedimentation. Our results suggest that sands related to Lower Jurassic Navajo ergs and Middle Jurassic Temple Capergs gained access to the arc.

The age of the strata of Cobre Ridge is ∼170 ± 5 Ma. Eighteen fractions taken from three samples of the tuff lie in a cluster near concordia. A best-fit age of 169 Ma is based on near concordance of one fraction and on model discordia trajectories using assumed upper intercepts. Sandstone horizons interbedded within the strata of Cobre Ridge in many cases contain sedimentary structures diagnostic of eolian sedimentation and, although quartz rich, in general have more volcanic detritus than sandstones in the Mount Wrightson Formation. The interval between 165-175 Ma corresponds to the Aalenian and Bajocian stages and is within the time span represented by the Temple Cap and Page sandstones of southern Utah and northern Arizona. Sandstone interstratified with the strata of Cobre Ridge may be correlative with either of those units. The possibility cannot be discounted, however, that unconsolidated or weathered Navajo-equivalent sands from the upper member of the Mount Wrightson Formation were recycled into the strata of Cobre Ridge.

These results support the models (Busby-Spera, 1988; Riggs and Busby-Spera, 1990, 1991), suggesting that during Early to Middle Jurassic time the magmatic arc in southern Arizona was low standing and did not constitute an impediment to the south and southwestward movement of continental sediments. We suggest that much of the region between the Colorado Plateau and the arc was topographically low, and that tectonic and volcanic basins within the arc provided traps for windblown mature sands for as long as 35 m.y. We speculate that Early to Middle Jurassic tectonic subsidence within the magmatic arc may have been more continuous than subsidence on the Colorado Plateau, thus providing a depocenter in which major tectonically induced unconformities that characterize the Mesozoic record in the back arc are masked by ongoing volcanic activity.

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