Abstract

Stratigraphic and geochronologic data from three regionally extensive Mesozoic stratigraphic units exposed in the Palen Mountains, California, provide constraints on (1) the duration of supermature craton-derived sedimentation within the Jurassic magmatic arc, (2) the timing of cessation of Jurassic magmatism in southeastern California, and (3) plate-margin scale correlation of siliciclastic units that record the initiation of Gulf of Mexico rift-related sedimentation. These three regionally extensive units occur as a 7.5-km-thick conformable succession in the Palen Mountains, consisting dominantly of (1) eolian quartzose sandstone (upper member of the Palen formation), (2) silicic volcaniclastic rocks and associated plugs and domes (Dome Rock sequence), and (3) terrigenous sedimentary rocks (McCoy Mountains Formation).

Our data constrain the age of the eolian quartz arenite (upper Palen formation) to Middle to latest Early Jurassic time (Toarcian to Bathonian) because its upper part is locally interbedded with the basal Dome Rock sequence, which we have dated as 174 ± 8 Ma (U-Pb zircon). This age for the eolian quartz arenite suggests that potentially correlative Middle Jurassic cratonal rocks on the present-day Colorado Plateau include the Page Sandstone, eolian parts of the Carmel Formation, and the Temple Cap Sandstone, all of southwestern Utah and northern Arizona. Correlation with the Navajo Sandstone can not be precluded, although this seems unlikely because one must assume the maximum error on both the U-Pb zircon age and the uncertainties on the stage boundaries to obtain an overlap of only 1 m.y.

Our stratigraphic data also document a locally interfingering contact between the Dome Rock sequence and the overlying McCoy Mountains Formation, the chronostratigraphic extent and tectonic significance of which are controversial. We have dated three units along and below this contact: (1) a green ignimbrite concordantly overlain by (2) a pink ignimbrite capped by a pink nonwelded tuff, and (3) a pink plug-dome complex that intrudes the pink ignimbrite and the dome talus of which interfingers with the pink tuff. The dome talus also interfingers with maroon siltstone, a unit that is regionally typical of the basal McCoy Mountains Formation. The U-Pb zircon dates of the green ignimbrite and the pink ignimbrite overlap within error (155 ± 8 Ma and 162 ± 3 Ma, respectively). U-Pb data for zircon fractions from the plug-dome complex do not further constrain the age of the upper Dome Rock sequence, due to complex inheritance and lead loss, but are consistent with the other dates. Taken together, these data demonstrate that the lowermost McCoy Mountains Formation is late Middle to early Late Jurassic (Bathonian to Oxfordian) in age.

A late Middle to early Late Jurassic age for the lower unit of the McCoy Mountains Formation supports the previous interpretation that development of the extensional-transtensional McCoy basin (southeastern California and western Arizona) and the Bisbee basin (southeastern Arizona) was broadly coeval, within the limits of sparse isotopic data in southeastern Arizona, and was the result of rifting during opening of the Gulf of Mexico. We suggest that the much more abrupt volcanic to sedimentary transition in southeastern California, relative to that recorded in southern Arizona, may reflect a greater component of strike-slip faulting relative to extension in a regionally transtensional regime. We speculate that this resulted from along-strike variation in plate-margin orientation.

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