SEDIMENT SOURCES RECORDED IN CRETACEOUS METACONGLOMERATES OF NORTHERN SONORA AND A POSSIBLE CONNECTION TO THE CORDILLERAN FORELAND

Metamorphic rocks of the Altar and El Batamote complexes in northern Sonora contain sparse to abundant young detrital zircon (DZ) grains with Cretaceous ages. The young grains indicate that the sedimentary protoliths can be divided into three age groups: (1) Aptian–Albian (~120–100 Ma); (2) Cenomanian–Santonian (~100–82 Ma,); and (3) Campanian–early Maastrichtian (~80–71 Ma). Unmetamorphosed strata of these age ranges in northern Sonora include the Lower Cretaceous Bisbee Group, Upper Cretaceous El Chanate Group, defined originally by César Jacques-Ayala, and Cabullona Group, respectively.

Metaconglomerate intervals of each age group contain abundant silicic and intermediate volcanic clasts; quartzite and quartzarenite clasts dominate locally. Prominent DZ age modes generally become younger upsection, but most samples contain a dominant Jurassic (~173–155 Ma) mode indicating a major source in the Jurassic magmatic arc or SLIP. Most group 1 samples have a prominent Lower Cretaceous (~114 Ma) mode, some group 2 samples contain a prominent early Late Cretaceous (~97–90 Ma) mode, and group 3 samples contain a major younger Cretaceous (~80 Ma) mode. Sediment represented by these modes was derived from early and late stages of the Peninsular Ranges arc, and Laramide igneous rocks, respectively. Group 3 samples contain subordinate Proterozoic modes (~1.71–1.68 and 1.44 Ga) that may signal uplift of SW Laurentian basement sources in the foreland.

Strata correlative with groups 1 and 3 present in southern Arizona and SW New Mexico pertain to the Bisbee Group and Laramide synorogenic units such as the Fort Crittenden and Ringbone formations. Strata of group two are generally absent at a pre-Laramide unconformity in intermontane basins of the border region. However, correlative Turonian–Santonian strata rich in volcanic lithic fragments underlie Laramide strata of intermontane basins in south-central New Mexico. These units, the Tres Hermanos and Crevasse Canyon formations, have prominent age modes at ~171–150 Ma and 98–91 Ma, similar to those of age group 2. The compositional similarity of these units suggests a direct connection between early Late Cretaceous deltaic systems of the southern Cordilleran foreland basin and synorogenic conglomerates of the Altar and El Batamote complexes of northern Sonora.