Thomas Farrell’s research while affiliated with Boston College and other places

We present a refined global Furongian (late Cambrian) time scale derived through the application of Bayesian age modeling, using an integrative assemblage of conditioning likelihoods (age constraints) including U-Pb zircon maximum depositional ages in the Steptoean positive isotopic carbon excursion (SPICE) reference section in Smithfield Canyon (Utah, USA) and nearby McPherson Canyon (Idaho, USA); Re-Os geochronology from the SPICE-bearing interval of the An-drarum-3 core (Scania, Sweden); and new high-precision chemical abrasion-isotope dilution thermal ionization mass spectrometry U-Pb zircon tuff ages from Avalonian Wales. We embed these radioisotopic ages within a novel probabilistic treatment of biozones to establish temporal constraints on rock accumulation rates in the Great Basin (USA), the duration of the SPICE event, and Laurentian trilobite biozones correlated to the global Cambrian time scale. Results reveal a beginning of 494.5 (+0.7/−0.6) Ma and an end of 487.3 ± 0.08 Ma for the Furongian Epoch, representing a reduction of the traditional late Cambrian by ∼30% and an extension of the Ordovician by nearly half a million years. Furthermore, the SPICE is confined to a duration of 2.6 (+0.9/−0.8) m.y. Our new approach to integrating faunal succession into Bayesian age modeling can help to constrain rock accumulation rates and possible hiatuses in sections with limited radioisotopic ages. Additionally, it offers a robust calibration tool for further refining the numerical calibration of the geologic time scale, for testing hypotheses about the rates of trilo-bite evolution and extinction, for evaluating causes of the SPICE, and for constraining pa-leoclimatic conditions including atmospheric O 2 levels.

We present a refined global Furongian (late Cambrian) time scale derived through the application of Bayesian age modeling, using an integrative assemblage of conditioning likelihoods (age constraints) including U-Pb zircon maximum depositional ages in the Steptoean positive isotopic carbon excursion (SPICE) reference section in Smithfield Canyon (Utah, USA) and nearby McPherson Canyon (Idaho, USA); Re-Os geochronology from the SPICE-bearing interval of the Andrarum-3 core (Scania, Sweden); and new high-precision chemical abrasion−isotope dilution−thermal ionization mass spectrometry U-Pb zircon tuff ages from Avalonian Wales. We embed these radioisotopic ages within a novel probabilistic treatment of biozones to establish temporal constraints on rock accumulation rates in the Great Basin (USA), the duration of the SPICE event, and Laurentian trilobite biozones correlated to the global Cambrian time scale. Results reveal a beginning of 494.5 (+0.7/−0.6) Ma and an end of 487.3 ± 0.08 Ma for the Furongian Epoch, representing a reduction of the traditional late Cambrian by ∼30% and an extension of the Ordovician by nearly half a million years. Furthermore, the SPICE is confined to a duration of 2.6 (+0.9/−0.8) m.y. Our new approach to integrating faunal succession into Bayesian age modeling can help to constrain rock accumulation rates and possible hiatuses in sections with limited radioisotopic ages. Additionally, it offers a robust calibration tool for further refining the numerical calibration of the geologic time scale, for testing hypotheses about the rates of trilobite evolution and extinction, for evaluating causes of the SPICE, and for constraining paleoclimatic conditions including atmospheric O2 levels.

Multiple studies have applied zoned garnet geochronology to place temporal constraints on the rates of metamorphism and deformation during orogenesis. We report new high-resolution isotope dilution−thermal ionization mass spectrometry Sm-Nd isochron ages on concentric growth zones from microstructurally and thermodynamically characterized garnets from the Betic Cordillera, southern Spain. Our ages for the garnet core (13.64 ± 0.31 Ma), mantle (13.41 ± 0.37 Ma), and rim (13.34 ± 0.45 Ma) indicate rapid garnet growth and are consistent with published garnet ages interpreted to reflect high-pressure metamorphism in the region. Thermodynamic analysis indicates garnets grew during subduction at ∼1.5−2.0 GPa and 570−600 °C. The core to rim duration of spiral garnet growth was just a few hundred thousand years. While other zoned garnet studies have shown similar rapid growth in subduction zone settings, this is the first documentation of such rapid growth of a spiral garnet. Combining this garnet growth duration with the magnitude of spiral inclusion trail curvature, we compute a strain rate of ∼10−13 s−1, an order of magnitude faster than all previous spiral garnet studies. We interpret that these spiral garnets recorded a rapid pulse of deformation and strain during the final stages of subduction and incipient exhumation.

The late Cambrian (Furongian Epoch) was one of the most dynamic times in Earth history; yet it remains one of the most poorly dated epochs in the Geologic Time Scale 2020 (GTS2020). This is due to the lack of precise depositional ages in strata that can be linked to global timescale boundaries. Because of this, the ages of internal stage boundaries and the timing, tempo and duration of changes to the Earth system during this time remain largely conjectural, including the prominent Steptoean Positive Isotopic Carbon Excursion (SPICE), and the potential link to trilobite mass extinctions, oceanic anoxia, and atmospheric oxygenation. The development of Bayesian age models has allowed for the incorporation of diverse datasets to improve temporal constraints on biological and geochemical processes preserved in the rock record. In stratigraphic sections where radioisotopic constraints are sparse, Bayesian age models allow for the novel integration of other age constraints, such as the occurrence of fossils, to fill this gap. Here we integrate recently reported U-Pb maximum depositional ages, Re-Os geochronology, and two new high-precision CA-ID-TIMS U-Pb zircon depositional ages linked to biostratigraphy from Wales, UK (Ogof-ddû: 490.097 ± 0.15 Ma and Bryn-llin-fawr: 487.292 ± 0.08 Ma), and trilobite biozonation to condition a Bayesian age model through a well-characterized upper Miaolingian through Furongian section at Smithfield Canyon, northern Utah, USA. Our modeling results place temporal constraints on the duration of the SPICE event and Laurentian trilobite biozones correlated to the global Cambrian GTS2020. This work represents the most rigorous attempt to numerically quantify the Furongian Epoch with an updated base age of the 494.5 +0.67/-0.58 Ma and a base age of the Ordovician Period of 487.3 ± 0.08 Ma, a ~30% reduction in the duration of the Furongian Epoch from the GTS2020. The duration of the SPICE is constrained to 2.3 ± 0.7, shorter than previous estimates of 3-4 Myr, updating hypotheses related to the cause(s) and consequences of this excursion. Finally, we show that our new approach of incorporating faunal succession into Bayesian age modeling is a useful means of constraining accumulation rates and possible hiatuses in sections with few radioisotopic ages. Abstract ID#: 393532 Meeting

Early Paleozoic tectonic subsidence in the Nopah Range reference section of eastern California (Levy and Christie-Blick, 1991, GSAB) has been re-evaluated in light of changes to the Cambrian-Ordovician timescale. The estimated time of onset of thermal subsidence (τ0) based conservatively on implied subsidence rates as a function of the square root of time since τ0 is updated from the late Precambrian-early Cambrian (590-545 Ma; timescale of Harland et al., 1982) to Terreneuvian–Cambrian Series 2 (538.8-505.5 Ma, and perhaps later still; International Commission on Stratigraphy, June, 2023, updated). This constraint is consistent with U-Pb dating of detrital zircons in the syn-extensional Sixtymile Formation of the Grand Canyon, suggesting that continental rifting continued until <508.6 Ma. An early phase of basin inversion associated with the Butte fault and predating deposition of the Tapeats Sandstone (>505.5 Ma and <506.4 Ma) is taken to indicate a change in regional stress within the upper crust. That is consistent with tentative placement of a comparable regional stratigraphic discontinuity within the Zabriskie Quartzite of California (~506.6 Ma, assuming linear extrapolation of age with stratigraphic thickness below the base of the Miaolingian). The misfit of R1 (tectonic subsidence + eustasy) with respect to a best-fit exponential curve provides a measure of the late Cambrian eustatic maximum. However, a eustatic change as large as 261 m during the Miaolingian (a span of 11.15 Myr) is arguably too rapid for a tectonic mechanism and an interval lacking evidence for continental glaciation. We suggest that some fraction of the misfit may relate to a tectonic phenomenon not accounted for by the best-fit exponential. Given a β factor of ~1.5-2.0, yet only modest evidence for upper crustal extension during deposition of the middle Wood Canyon Formation–lower Zabriskie Quartzite (basal Cambrian; ~538.8-506.6 Ma), we infer that thermal subsidence was driven by extension preferentially in the lower crust and upper mantle. Such deformation may have continued into the Miaolingian, after upper crustal extension had ceased. The Noonday Dolomite–Johnnie Formation (early Ediacaran; 635 Ma to ~580 Ma) is thought to represent an earlier phase of passive margin development following Cryogenian rifting.

The Steptoean Positive Isotopic Carbon Excursion (SPICE) is a prominent +4–5‰ shift in the Cambrian δ13C record used for global chronostratigraphic correlation. The onset of this excursion is traditionally placed at the base of the Pterocephaliid trilobite biomere (base of the Furongian Series). Recent studies have documented local controls on the expression of the SPICE and emphasize the need for chronostratigraphic standards for these complex biogeochemical signals. We build upon prior work in western Laurentia by integrating δ13C and biostratigraphy with high-precision isotope dilution U-Pb detrital zircon maximum depositional ages that are coincident with the onset, peak, and falling limb of the SPICE. Our study provides the first useful numerical age constraint for the onset of the SPICE and the Laurentian trilobite biozones and requires revision of the late Cambrian geologic time scale boundaries by several million years.

High‐resolution microstructural analysis of porphyroblast inclusion trails integrated with Sm‐Nd garnet geochronology has provided new insight into the tectonic history of the Betic‐Rif orogen. Three principal age groups of porphyroblasts are demonstrated with distinctly oriented inclusion‐trails. Inclusion‐trail curvature axes or “FIA” (Foliation Inflexion/Intersection Axes) are shown to represent “fossilized” crenulation axes from which a succession of different crustal shortening directions can be deduced. The regional consistency of microstructural orientations and their geometric relationship with multiple sets of macroscopic folds reveal the composite character of the Gibraltar Arc formed by a superposition of different folding directions and associated lineations. Bulk‐garnet ages of 35–22 Ma obtained from five micaschist samples of the Alpujarride‐Sebtide complex (ASC) and of 35–13 Ma from four micaschists of the Nevado‐Filabride complex (NFC) allow to deduce NNE‐SSW directed shortening in the Late Eocene changing to NW‐SE shortening in the early Oligocene, alternating with suborthogonal NE‐SW shortening during the Miocene. These directions can be related to a major swing in the direction of relative Africa‐Iberia plate‐motion known from kinematic modeling of magnetic seafloor anomalies, and subsequent dynamic interference between plate convergence and suborthogonal “tectonic escape” of the Alboran Domain. Coupled to previously established P‐T‐t paths, the new garnet ages support a common tectono‐metamorphic evolution of the ASC and NFC as laterally equivalent orogenic domains until, in the Miocene, the second became re‐buried under the first.

Early draft of article with the same title published in October 2022