PROVENANCE OF CRETACEOUS THROUGH MID-TERTIARY STRATA OF THE FOUR CORNERS REGION: INSIGHTS FROM DETRITAL ZIRCONS IN THE SAN JUAN BASIN, NEW MEXICO AND COLORADO

Cretaceous through mid-Tertiary strata of the Four Corners region provide an excellent record of changes in sediment provenance from Sevier thin-skinned thrusting through the formation of Laramide block uplifts and intra-foreland basins. During the ~125-50 Ma timespan, the San Juan Basin (SJB) was flanked by the Sevier belt to the west, the Mogollon highlands rift shoulder to the southwest, and was influenced by (~75-50 Ma) Laramide tectonism, ultimately preserving a >6000’ sequence of continental, marginal-marine, and off-shore marine sediments. In order to evaluate the influences these tectonic features had on sediment delivery to the area, a total of 3228 U-Pb laser analyses have been completed on 32 detrital zircon (DZ) samples. The DZ results indicate four stratigraphic intervals with internally consistent age peaks: 1) Lower Cretaceous Burro Canyon Formation, 2) Turonian Gallup Sandstone through Campanian Lewis Shale, 3) Campanian Pictured Cliffs SS through Fruitland Fm, and 4) Campanian Kirtland SS through Eocene San Jose Fm. Statistical analysis of the DZ results, in conjunction with paleocurrent data, reveals three distinct changes in sediment provenance. The first transition, between the Burro Canyon Fm and the Gallup SS, reflects a change from predominantly reworked sediment from the Sevier thrust front, including uplifted Paleozoic sediments and Mesozoic eolian sandstones, to a mixed signature indicating both Sevier and Mogollon derivation. Deposition of the Pictured Cliffs SS at ~75 Ma marks the beginning of the second transition and is indicated by the spate of depositional age zircons, likely derived from the Laramide porphyry copper province of southern Arizona and/or southwestern New Mexico. The third change in provenance occurs during the deposition of the Campanian Kirtland Fm and is interpreted to reflect the unroofing of surrounding Laramide basement blocks and a switch to local derivation. At this time, sediment entering the SJB was largely being generated from the nearby San Juan Mountains to the NNW, including uplift associated with early phases of Colorado Mineral Belt magmatism. Thus DZ spectra in the SJB document the transition from initial reworking of the Paleozoic and Mesozoic cratonal blanket to unroofing of basement-cored uplifts and Laramide plutonic rocks.