William R. Dickinson’s research while affiliated with University of Arizona and other places

Cretaceous through Eocene 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 ca. 125-50 Ma timespan, the San Juan Basin was flanked by the Sevier thrust belt to the west, the Mogollon highlands rift shoulder to the southwest, and was influenced by (ca. 75-50 Ma) Laramide tectonism, ultimately preserving a > 6000 ft (> 2000 m) sequence of continental, marginal-marine, and offshore marine sediments. In order to decipher the influences of these tectonic features on sediment delivery to the area, we evaluated 3228 U-Pb laser analyses from 32 detrital-zircon samples from across the entire San Juan Basin, of which 1520 analyses from 16 samples are newly reported herein. The detrital-zircon results indicate four stratigraphic intervals with internally consistent age peaks: (1) Lower Cretaceous Burro Canyon Formation, (2) Turonian (93.9-89.8 Ma) Gallup Sandstone through Campanian (83.6-72.1 Ma) Lewis Shale, (3) Campanian Pictured Cliffs Sandstone through Campanian Fruitland Formation, and (4) Campanian Kirtland Sandstone through Lower Eocene (56.0-47.8 Ma) San Jose Formation. Statistical analysis of the detrital-zircon results, in conjunction with paleocurrent data, reveals three distinct changes in sediment provenance. The first transition, between the Burro Canyon Formation and the Gallup Sandstone, 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 Sandstone at ca. 75 Ma marks the beginning of the second transition and is indicated by the spate of near-depositional-age zircons, likely derived from the Laramide porphyry copper province of southern Arizona and southwestern New Mexico. Paleoflow indicators suggest the third change in provenance was complete by 65 Ma as recorded by the deposition of the Paleocene Ojo Alamo Sandstone. However, our new U-Pb detrital-zircon results indicate this transition initiated ~8 m.y. earlier during deposition of the Campanian Kirtland Formation beginning ca. 73 Ma. This final change in provenance is interpreted to reflect the unroofing of surrounding Laramide basement blocks and a switch to local derivation. At this time, sediment entering the San Juan Basin was largely being generated from the nearby San Juan Mountains to the north-northwest, including uplift associated with early phases of Colorado mineral belt magmatism. Thus, the detrital-zircon spectra in the San Juan Basin document the transition from initial reworking of the Paleozoic and Mesozoic cratonal blanket to unroofing of distant basement-cored uplifts and Laramide plutonic rocks, then to more local Laramide uplifts.

Uranium-lead (U-Pb) geochronology and Hafnium (Hf) isotope geochemistry of detrital zircons of the Harmony Formation of north central Nevada provide new insights into the tectonic evolution of the Late Paleozoic western Laurentian margin. Using laser-ablation inductively coupled plasma mass spectrometry, 10 arenite samples were analyzed for U-Pb ages, and 8 of these samples were further analyzed for Hf isotope ratios. Three of the sampled units have similar U-Pb age peaks and Hf isotope ratios, including a 1.0-1.4 Ga peak with εHf values of +12 to -3 and a 2.5-2.7 Ga peak with εHf values of +7 to -5. The remaining seven samples differ significantly from these three, but are similar to one another; having age peaks of 1.7-1.9 Ga with εHf of +10 to -20 and age peaks of 2.3-2.7 Ga with εHf of +6 to -8. The data confirm the subdivision of the Harmony Formation into two petrofacies: quartzose (Harmony A) and feldspathic (Harmony B). The three samples with 1.0-1.4 and 2.5-2.7 Ga peaks are the Harmony A, which originated in the central Laurentian craton. The other seven samples are the Harmony B, which originated in eastern Alberta-western Saskatchewan, north of the Harmony A source. We propose that all Harmony Formation strata were deposited near eastern Alberta and subsequently tectonically interleaved with Roberts Mountains allochthon strata. We interpret that the entire package was tectonically transported south along the western Laurentian margin and then emplaced eastward onto the craton during the Late Devonian-Early Mississippian Antler orogeny.

New field mapping and laboratory studies of Paleoproterozoic rock units around Little Chino Valley in central Arizona clarify the timing of magmatism, deformation, and sedimentation in part of the Yavapai tectonic province and yield new insights into sources of sands and weathering environments. Mafic lavas, calc-silicate rocks, and pelitic and psammitic strata in the Jerome Canyon area west of Little Chino Valley were deposited, deformed, and intruded by the 1736 ± 21 (2σ) Ma Williamson Valley Granodiorite. U-Pb geochronologic analysis of detrital zircons from a sample of psammitic strata yielded a maximum depositional age of ca. 1738 Ma. Approximately 25% of the detrital-zircon grains were derived from a ca. 2480 Ma source, as previously identified in Grand Canyon schist units. Kolmogorov-Smirnov statistical comparison of the Jerome Canyon detrital-zircon analyses with Grand Canyon schist analyses indicates that three of the 12 samples analyzed by Shufeldt et al. (2010) are statistically indistinguishable from the Jerome Canyon sample at the 95% confidence level and supports the concept that the Jerome Canyon sequence and Paleoproterozoic schists in the eastern and western Grand Canyon are part of the same tectonostratigraphic terrane. The Del Rio Quartzite on the northeast side of Little Chino Valley, previously considered an outlier of Mazatzal Quartzite, consists of poorly sorted quartz arenite, pebbly quartz arenite, and conglomerate deposited in a braided-stream environment. Microscope examination of 32 thin sections stained for potassium and calcium failed to identify any feldspar, mica, or mafic silicate grains. Similarly, conglomerate clasts consist entirely of vein quartz and less abundant argillite and jasper. A rock unit interpreted as a paleosol beneath the Del Rio Quartzite contains no surviving minerals except quartz, some of which is embayed and rounded as in corrosive saprolitic soils. U-Pb geochronologic analyses of detrital zircons from the 1400-m-thick Quartzite indicate maximum depositional ages of ca. 1745 Ma for the base and ca. 1737 Ma for the top. The unit is folded but is unaffected by the penetrative deformation and metamorphism that affected other Paleoproterozoic volcanic and sedimentary strata in the area, and it is probably significantly younger. We infer that the physically immature but chemically super-mature Del Rio Quartzite was deposited during a time of extreme weathering during a hot, humid climate with exceptionally high atmospheric CO2 concentrations and associated corrosive rainwater rich in carbonic acid.

Small-scale excavation was undertaken at the Malakai site on the small island of Nimowa, located in the Louisiade Archipelago, Massim region, Papua New Guinea. This is the first excavation to be reported in detail from the archipelago, with the Malakai site providing insight into cultural practices on the island and pottery exchange in the southern Massim region. A stratified deposit was revealed with dense cultural material, first inhabited from 1350 to 1290 cal. BP, with a subsequent period of settlement within the last 460–300 cal. years. Pottery, shell, and stone artifacts were recovered, as well as human skeletal remains in a primary burial context, which contributes to understanding regional patterns of prehistoric mortuary activity. It is argued that Nimowa was already part of an exchange network that encompassed many of the southern Massim islands when the Malakai site was first occupied. There is increased diversity in the number of vessel forms in later prehistory, but with remarkable continuity in the decorative motifs over time, suggesting some degree of regional social cohesion in the southern Massim. It appears that the northern Massim islands were not a major supplier of pottery to Nimowa. The implications for the prehistory of the wider region are subsequently discussed.

Detrital zircon U-Pb geochronology and Hf isotope geochemistry provide new insights into the provenance, sedimentary transport, and tectonic evolution of the Roberts Mountains allochthon strata of north-central Nevada. Using laser-ablation inductively coupled plasma mass spectrometry, a total of 1151 zircon grains from six Ordovician to Devonian arenite samples were analyzed for U-Pb ages; of these, 228 grains were further analyzed for Hf isotope ratios. Five of the units sampled have similar U-Pb age peaks and Hf isotope ratios, while the ages and ratios of the Ordovician lower Vinini Formation are significantly different. Comparison of our data with that of igneous basement rocks and other sedimentary units supports our interpretation that the lower Vinini Formation originated in the north-central Laurentian craton. The other five units sampled, as well as Ordovician passive margin sandstones of the western Laurentian margin, had a common source in the Peace River Arch region of western Canada. We propose that the Roberts Mountains allochthon strata were deposited near the Peace River Arch region, and subsequently tectonically transported south along the Laurentian margin, from where they were emplaced onto the craton during the Antler orogeny.

Essential features of the previously named and described Miocene Crooked Ridge River in northeastern Arizona (USA) are reexamined using new geologic and geochronologic data. Previously it was proposed that Cenozoic alluvium at Crooked Ridge and southern White Mesa was pre-early Miocene, the product of a large, vigorous late Paleogene river draining the 35-23 Ma San Juan Mountains volcanic field of southwestern Colorado. The paleoriver probably breeched the Kaibab uplift and was considered important in the early evolution of the Colorado River and Grand Canyon. In this paper, we reexamine the character and age of these Cenozoic deposits. The alluvial record originally used to propose the hypothetical paleoriver is best exposed on White Mesa, providing the informal name White Mesa alluvium. The alluvium is 20-50 m thick and is in the bedrock-bound White Mesa paleovalley system, which comprises 5 tributary paleochannels. Gravel composition, detrital zircon data, and paleochannel orientation indicate that sediment originated mainly from local Cretaceous bedrock north, northeast, and south of White Mesa. Sedimentologic and fossil evidence imply alluviation in a low-energy suspended sediment fluvial system with abundant fine-grained overbank deposits, indicating a local channel system rather than a vigorous braided river with distant headwaters. The alluvium contains exotic gravel clasts of Proterozoic basement and rare Oligocene volcanic clasts as well as Oligocene-Miocene detrital sanidine related to multiple caldera eruptions of the San Juan Mountains and elsewhere. These exotic clasts and sanidine likely came from ancient rivers draining the San Juan Mountains. However, in this paper we show that the White Mesa alluvium is early Pleistocene (ca. 2 Ma) rather than pre-early Miocene. Combined 40Ar/39Ar dating of an interbedded tuff and detrital sanidine ages show that the basal White Mesa alluvium was deposited at 1.993 ± 0.002 Ma, consistent with a detrital sanidine maximum depositional age of 2.02 ± 0.02 Ma. Geomorphic relations show that the White Mesa alluvium is older than inset gravels that are interbedded with 1.2-0.8 Ma Bishop-Glass Mountain tuff. The new ca. 2 Ma age for the White Mesa alluvium refutes the hypothesis of a large regional Miocene(?) Crooked Ridge paleoriver that predated carving of the Grand Canyon. Instead, White Mesa paleodrainage was the northernmost extension of the ancestral Little Colorado River drainage basin. This finding is important for understanding Colorado River evolution because it provides a datum for quantifying rapid post-2 Ma regional denudation of the Grand Canyon region.