A LATE CRETACEOUS DEPOSITIONAL AGE FOR SOME METACONGLOMERATES PREVIOUSLY INTERPRETED AS UPPER JURASSIC IN NORTHERN SONORA

Detrital zircon data clarify age relationships of metaconglomerate successions from several localiites in northern Sonora that were of great interest to César Jacques-Ayala. Upper Jurassic stratigraphic designations based on field relationships need modification because Late Cretaceous zircon is now confirmed in units that structurally underlie strata previously correlated to the Lower Cretaceous Bisbee Group.

The tectonostratigraphy of the Magdalena core complex comprises: 1) schistose Jurassic rhyolite porphyry, 2) stretched gray conglomerate with rhyolite, quartz arenite, and quartzite cobbles, 3) green polymict stretched conglomerate, 4) metasandstone, pelitic schist, and marble. Two sandstone samples from gray conglomerate of Arroyo Amolares display prominent age maxima at 173 Ma (N = 107 of 126) and 174 Ma (N = 102 of 122). The deeper sample yielded 3 Late Cretaceous zircons (YSG = 86 Ma) and preserves a minor mode at 127 Ma. The shallower sample yielded a single Late Cretaceous grain (83 Ma). A few Paleozoic and Proterozoic grains occur in both samples. Cumaro sandstone in the overlying green conglomerate yielded 43% Cretaceous grains with a prominent mode at 89 Ma (YSG = 82 Ma), and lesser modes at 151 and 171 Ma (N = 100). Compared with the gray conglomerate, the Jurassic mode is subdued (22%) and a Triassic to Proterozoic component accentuated (36%).

Stretched conglomerate of Sierra El Batamote is exposed in an overturned anticline, juxtaposed against Bisbee Group strata of Sierra El Chanate. Sandstone from the anticline core (N =100) has a prominent age mode at 114 Ma, with lesser modes at 147 and 165 Ma. Stratigraphically higher sandstone of Arroyo El Charro (N = 100) displays a maximum at 163 Ma and a minor mode at 90 Ma. Proterozoic components in these samples comprise 11% and 18%, respectively.

A sandstone sample from Sierra Las Jarillas (N = 74) has a major age mode at 163 Ma, a minor mode at 118 Ma, two Late Cretaceous grains at 86 and 74 Ma, and three Proterozoic grains.

These results show that the conglomerates did not form in Late Jurassic pull-apart basins. Instead, they represent parts of widespread Late Cretaceous fluvial systems that tapped local Jurassic arc sources and received detritus from heterogeneous distal sources prior to profound structural and metamorphic overprint related to the Laramide orogeny.