Search for: All records

Abstract Sanidine dating and magnetostratigraphy constrain the timing of integration of the lower Colorado River (southwestern United States and northern Mexico) with the evolving Gulf of California. The Colorado River arrived at Cottonwood Valley (Nevada and Arizona) after 5.24 Ma (during or after the Thvera subchron). The river reached the proto–Gulf of California once between 4.80 and 4.63 Ma (during the C3n.2r subchron), not at 5.3 Ma and 5.0 Ma as previously proposed. Duplication of section across newly identified strands of the Earthquake Valley fault zone (California) probably explains the discrepancy. The data also imply the start of focused plate motion and basin development in the Salton Trough (California) at 6–6.5 Ma and relative tectonic stability of the southernmost part of the lower Colorado River corridor after its integration. After integration, the Colorado River quickly incised through sediment-filled basins and divides between them as it also likely excavated Grand Canyon (Arizona). The liberated sediment from throughout the system led to deposition of hundreds of meters of Bullhead Alluvium downstream of Grand Canyon after 4.6 Ma as the river adjusted to its lower base level. 

Abstract Trilobites appeared and diversified rapidly in the Cambrian, but it is debated as to whether their radiations and extinctions were globally synchronous or geographically restricted and diachronous. The end of the early Cambrian is a classic example—it has traditionally been defined by the extinction of olenellid and redlichiid trilobites and the appearance of paradoxidid trilobites. Here we integrate the global biostratigraphy of these three trilobite groups with high-precision tuff and tandem detrital zircon U-Pb age constraints to falsify prior models for global synchronicity of these events. For the first time, we demonstrate that olenellid trilobites in Laurentia went extinct at least 3 Ma after the first appearance of paradoxidids in Avalonia and West Gondwana (ca. 509 Ma). They also disappeared before the extinction of redlichiids and prior to the base of the Miaolingian at ca. 506 Ma in South China. This indicates that these three trilobite groups (paradoxidids, olenellids, and redlichiids) and their associated biotas overlapped in time for nearly 40% of Cambrian Epoch 2, Age 4. Implications of this chronological overlap are: (1) trilobite transitions were progressive and geographically mediated rather than globally synchronous; and (2) paleontological databases underestimate the diversity of the early Cambrian. This ∼3 Ma diachroneity, at a critical time in the early evolution of animals, also impacts chemostratigraphic and paleoclimatic data sets that are tied to trilobite biostratigraphy and that collectively underpin our understanding of the Cambrian Earth system. 

Abstract We applied tandem U-Pb dating of detrital zircon (DZ) to redefine the Tonto Group in the Grand Canyon region (Arizona, USA) and to modify the Cambrian time scale. Maximum depositional ages (MDAs) based upon youngest isotope-dilution DZ ages for the Tapeats Sandstone are ≤508.19 ± 0.39 Ma in eastern Grand Canyon, ≤507.68 ± 0.36 Ma in Nevada, and ≤506.64 ± 0.32 Ma in central Arizona. The Sixtymile Formation, locally conformable below the Tapeats Sandstone, has a similar MDA (≤508.6 ± 0.8 Ma) and is here added to the Tonto Group. We combined these precise MDAs with biostratigraphy of trilobite biozones in the Tonto Group. The Tapeats Sandstone is ca. 508–507 Ma; the Bright Angel Formation contains Olenellus, Glossopleura, and Ehmaniella biozones and is ca. 507–502 Ma; and the Muav Formation contains Bolaspidella and Cedaria biozones and is ca. 502–499 Ma. The Frenchman Mountain Dolostone is conformable above the Muav Formation and part of the same transgression; it replaces McKee’s Undifferentiated Dolomite as part of the Tonto Group; it contains the Crepicephalus Biozone and is 498–497 Ma. The Tonto Group thickens east to west, from 250 m to 830 m, due to ∼300 m of westward thickening of carbonates plus ∼300 m of eastward beveling beneath the sub-Devonian disconformity. The trilobite genus Olenellus occurs in western but not eastern Grand Canyon; it has its last appearance datum (LAD) in the Bright Angel Formation ∼45 m above the ≤507.68 Ma horizon. This extinction event is estimated to be ca. 506.5 Ma and is two biozones below the Series 2–Miaolingian Epoch boundary, which we estimate to be ca. 506 Ma. Continued tandem dating of detrital grains in stratigraphic context, combined with trilobite biostratigraphy, offers rich potential to recalibrate the tempo and dynamics of Cambrian Earth systems.