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ABSTRACT Using the youngest detrital-zircon date(s) of a sedimentary deposit to constrain its maximum depositional age (MDA) is a widespread and growing application of geochronology. Most MDA studies analyze zircon grains at random, but this strategy can be costly and inefficient in cases where the youngest age group is only a small fraction of the population. We propose that handpicking sharply faceted zircon grains will increase the likelihood of encountering first-cycle zircon that have not undergone significant sedimentary transport, thus producing MDA estimates that are closer to the depositional age. We evaluate this procedure by conducting intra-sample comparisons of randomly selected versus handpicked zircon separates from 30 samples analyzed via laser-ablation–inductively coupled plasma–mass spectrometry (LA–ICP–MS). Our results show that handpicking zircon produces an overall shift towards younger ages in comparison to their randomly analyzed counterparts by an average of ∼ 406 Myr. In randomly analyzed separates, only 1.6% of grains were within 5 Myr of an independent estimate of the MDA, while handpicked separates contained 14.2%, an approximately nine-fold increase. However, handpicking can also lead to selection of older grains if they have been minimally transported, as with one handpicked Mesozoic sample that yielded 81% of ∼ 1.1 Ga zircon interpreted to be derived from a local granitic source. Handpicking is most effective in samples where young, sharply faceted grains are diluted by older, rounded grains, as with one sample that exhibited an ∼ 18-fold increase in the proportion of near-depositional-age zircons relative to its counterpart where grain selection was random. Because handpicking zircon imparts a severe bias on the resulting U–Pb age distribution, we recommend that two separate aliquots be used for quantitative provenance characterization through random analysis and MDA analysis through handpicking. 

Abstract New whole‐rock major and trace element geochemistry from the Leka Ophiolite Complex in Norway is presented and compared to the geochemical evolution and proposed tectonomagmatic processes recorded in the Izu‐Bonin‐Mariana system. These data demonstrate that the Leka Ophiolite Complex formed as forearc lithosphere during subduction initiation. A new high‐precision zircon U‐Pb date on forearc basalt constrains the timing of subduction initiation in the “Leka sector” of the Iapetus Ocean to 491.36 ± 0.17 Ma. The tectonomagmatic record of the Leka Ophiolite Complex captures only the earliest stages of subduction initiation and is thereby distinct from some other Appalachian–Caledonian ophiolites of similar age. The diversity of Appalachian–Caledonian ophiolite records may represent differing preservation and exposure of a variable forearc lithosphere. 

ABSTRACT Current investigations into the Albian–Cenomanian sedimentary record within the Western Interior have identified multiple complex tectono‐sedimentary process–response systems during the ongoing evolution of North America. One key sedimentary succession, the upper Cedar Mountain Formation (Short Canyon Member and Mussentuchit Member), has historically been linked to various regionally and continentally significant tectonic events, including Sevier fold‐and‐thrust deformation. However, the linkage between the Short Canyon Member and active Sevier tectonism has been unclear due to a lack of high‐precision age constraints. To establish temporal context, this study compares maximum depositional ages from detrital zircons recovered from the Short Canyon Member with that of a modified Bayesian age stratigraphic model (top‐down) to infer that the Short Canyon Member was deposited atca100 Ma, penecontemporaneous with rejuvenated thrusting across Utah [Pavant (Pahvant), Iron Springs and Nebo thrusts]. These also indicate a short depositional hiatus with the lowermost portion of the overlying Mussentuchit Member. The Short Canyon Member and Mussentuchit Member preserve markedly different sedimentary successions, with the Short Canyon Member interpreted to be composed of para‐autochthonous orogen–transverse (across the Sevier highlands) clastics deposited within a series of stacked distributive fluvial fans. Meanwhile, the muddy paralic Mussentuchit Member was a mix of orogen–transverse (Sevier highlands and Cordilleran Arc) and orogen–parallel basinal sediments and suspension settling fines within the developing collisional foredeep. However, the informally named last chance sandstone (middle sandstone of the Mussentuchit Member) is identified as an orogen–transverse sandy debris flow originating from the Sevier highlands, similar to the underlying Short Canyon Member. During this phase of landscape evolution, the Short Canyon Member – Mussentuchit Member depocentre was a sedimentary conduit system that would fertilize the Western Interior Seaway with ash‐rich sediments. These volcaniclastic contributions, along with penecontemporaneous deposits across the western coastal margin of the Western Interior Seaway, eventually would have lowered oxygen content and resulted in a contributing antecedent trigger for the Cenomanian–Turonian transition Oceanic Anoxic Event 2. 

Miocene strata of the Claremont, Orinda, and Moraga formations of the Berkeley Hills (California Coast Ranges, USA) record sedimentation and volcanism during the passage of the Mendocino triple junction and early evolution of the San Andreas fault system. Detrital zircon laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) age spectra indicate a change in sedimentary provenance between the marine Claremont formation (Monterey Group) and the terrestrial Orinda and Moraga Formations associated with uplift of Franciscan Complex lithologies. A sandstone from the Claremont formation produced a detrital zircon chemical abrasion–isotope dilution–thermal ionization mass spectrometry (CA-ID-TIMS) maximum depositional age of 13.298 ± 0.046 Ma, indicating younger Claremont deposition than previously interpreted. A trachydacite tuff clast within the uppermost Orinda Formation yielded a CA-ID-TIMS U-Pb zircon date of 10.094 ± 0.018 Ma, and a dacitic tuff within the Moraga Formation produced a CA-ID-TIMS U-Pb zircon date of 9.974 ± 0.014 Ma. These results indicate rapid progression from subsidence in which deep-water siliceous sediments of the Claremont formation were deposited to uplift that was followed by subsidence during deposition of terrestrial sediments of the Orinda Formation and subsequent eruption of the Moraga Formation volcanics. We associate the Orinda tuff clast and Moraga volcanics with slab-gap volcanism that followed the passage of the Mendocino triple junction. Given the necessary time lag between triple junction passage and the removal of the slab that led to this volcanism, subsidence associated with ca. 13 Ma Claremont sedimentation and subsequent Orinda to Moraga deposition can be attributed to basin formation along the newly arrived transform boundary. 

The Dadeville Complex of Alabama and Georgia (southeastern United States) represents the largest suite of exposed mafic-ultramafic rocks in the southern Appalachians. Due to poor preservation, chemical alteration, and tectonic reworking, a specific tectonic origin for the Dadeville Complex has been difficult to deduce. We obtained new whole-rock and mineral geochemistry coupled with zircon U-Pb geochronology to investigate the magmatic and metamorphic processes recorded by the Dadeville Complex, as well as the timing of these processes. Our data reveal an up-stratigraphic evolution in the geochemistry of the volcanic rocks, from forearc basalts to boninites. Our new U-Pb zircon crystallization data—obtained from three amphibolite samples—place the timing of forearc/protoarc volcanism no later than ca. 467 Ma. New thermobarometry suggests that the Dadeville Complex rocks subsequently experienced deep, high-grade metamorphism, at pressure-temperature conditions of ~7 kbar and ~760 °C. The data presented here support a model for formation of the Dadeville Complex in the forearc region of a subduction zone during subduction initiation and protoarc development, followed by deep burial/underthrusting of the complex during orogenesis. 

Understanding the effects of climatic upheavals during the Early to Late Cretaceous transition is essential for characterizing the tempo of tectonically driven landscape modification and biological interchange; yet, current chronostratigraphic frameworks are too imprecise, even on regional scales, to address many outstanding questions. This includes the Mussentuchit Member of the uppermost Cedar Mountain Formation, central Utah (southwestern United States), which could provide crucial insights into these impacts within the Western Interior Basin of North America yet remains imprecisely constrained. Here, we present high-precision U-Pb zircon dates from four primary ash beds distributed across ~50 km in central Utah that better constrain the timing of deposition of the Mussentuchit Member and the age of entombed fossils. Ages for ash beds are interpreted through a combination of Bayesian depositional age estimation and stratigraphic age modeling, resulting in posterior ages from 99.490 + 0.057/–0.050 to 98.905 + 0.158/–0.183 Ma. The age model predicts probabilistic ages for fossil localities between the ashes, including new ages for Moros intrepidus, Siats meekerorum, and several undescribed ornithischian dinosaur species of key interest for understanding the timing of faunal turnover in western North America. This new geochronology for the Mussentuchit Member offers unprecedented temporal insights into a volatile interval in Earth’s history. 

The geologically rapid appearance of fossils of modern animal phyla within Cambrian strata is a defining characteristic of the history of life on Earth. However, temporal calibration of the base of the Cambrian Period remains uncertain within millions of years, which has resulted in mounting challenges to the concept of a discrete Cambrian explosion. We present precise zircon U–Pb dates for the lower Wood Canyon Formation, Nevada. These data demonstrate the base of the Cambrian Period, as defined by both ichnofossil biostratigraphy and carbon isotope chemostratigraphy, was younger than 533 Mya, at least 6 My later than currently recognized. This new geochronology condenses previous age models for the Nemakit–Daldynian (early Cambrian) and, integrated with global records, demonstrates an explosive tempo to the early radiation of modern animal phyla.