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1. Episodic Late Eocene to Recent Extension in the vicinity of the Ruby Mountains and East Humboldt Range, Elko County, Nevada

   https://doi.org/doi: 10.1130/abs/2021CD-363293  

   McGrew, Allen J. ; Metcalf, James M. ( May 2021 , Abstracts with programs Geological Society of America)

   null (Ed.)

   The Ruby Mountains, East Humboldt Range and Wood Hills (REHW) of Elko County Nevada, one of the classic metamorphic core complexes of the Cordillera, preserves a protracted and episodic record of both ancient and modern crustal extension that has only recently been unraveled based on its thermochronometrically constrained cooling history. Extension began during the Late Eocene synchronously with a major pulse of intermediate to felsic magmatism preserved locally by plutonic rocks intruded into the REHW and regionally by widespread Late Eocene to early Oligocene volcanism (“the ignimbrite flare-up”). The Eocene-Oligocene event accommodated at least 15 km of extension concentrated in the northern half of the complex and associated with deposition in the Elko Basin to the west, a relatively thin (~1 km), broad sequence of Late Eocene lacustrine and related strata that contrasts with the younger sedimentation patterns represented by the narrower, thicker (up to 4+km), coarse clastics of the Miocene Humboldt Basin. Though locally significant, the Eocene-Oligocene extensional phase appears not to have been associated with broadly distributed regional extension, again contrasting with Miocene and younger events. The initial phase of extension slowed or halted by the mid-Oligocene, after which extension re-accelerated in the latest Oligocene to early Miocene (~25 – 21 Ma), correlative with deposition of a coarse clastic and lacustrine sequence known as the Clover Formation. This extensional phase propagated farther south than the earlier phase along the full length of the REHW. Extension likely slowed again between ~21 Ma and ~17.5 Ma, after which it abruptly re-accelerated through the Middle Miocene to ~10 Ma, synchronous with deposition of the thick, coarse clastics of the Humboldt Formation. Middle Miocene extension likely initiated with crustal-scale heating marking the impingement of the Yellowstone hot spot in NW Nevada. Sometime after 10 Ma, the interior of the core complex was transected by east-dipping normal faults that today define the steep eastern face of the Ruby Mountains and East Humboldt Range; these face west-dipping normal faults along the west flank of the Pequop Mountains and Spruce Mountains. Extension continues today at a rate of ~1 mm/yr as represented by the 2008 MW 6.0 Wells Earthquake. 

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2. Conifer wood assemblage dominated by Podocarpaceae, early Eocene of Laguna del Hunco, central Argentinean Patagonia

   https://doi.org/10.3897/phytokeys.156.54175  

   Pujana, Roberto R. ; Wilf, Peter ; Gandolfo, Maria A. ( August 2020 , PhytoKeys)

   null (Ed.)

   During the early Eocene, Patagonia had highly diverse floras that are primarily known from compression and pollen fossils. Fossil wood studies from this epoch are scarce in the region and largely absent from the Laguna del Hunco flora, which has a highly diverse and excellently preserved compression assemblage. A collection of 26 conifer woods from the Laguna del Hunco fossil-lake beds (early Eocene, ca. 52 Ma) from central-western Patagonia was studied, of which 12 could be identified to genus. The dominant species is Phyllocladoxylon antarcticum , which has affinity with early-diverging Podocarpaceae such as Phyllocladus and Prumnnopitys . A single specimen of Protophyllocladoxylon francisiae probably represents an extinct group of Podocarpaceae. In addition, two taxonomic units of cf. Cupressinoxylon with putative affinity to Podocarpaceae were found. Diverse Podocarpaceae taxa consistent with the affinities of these woods were previously reported from vegetative and reproductive macrofossils as well as pollen grains from the same source unit. Some of the woods have galleries filled with frass. Distinct growth ring boundaries indicate seasonality, inferred to represent seasonal light availability. Growth ring widths suggest that the woods came from mature trees, whereas the widths and types of some rings denote near-uniform temperature and water availability conditions. 

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3. New physaloid fruit‐fossil species from early Eocene South America

   https://doi.org/10.1002/ajb2.1565  

   Deanna, Rocío ; Wilf, Peter ; Gandolfo, Maria A. ( November 2020 , American Journal of Botany)

   Solanaceae is a scientifically and economically important angiosperm family with a minimal fossil record and an intriguing early evolutionary history. Here, we report a newly discovered fossil lantern fruit with a suite of features characteristic of Physalideae within Solanaceae. The fossil comes from the early Eocene Laguna del Hunco site (ca. 52 Ma) in Chubut, Argentina, which previously yielded the only other physaloid fruit fossil,Physalis infinemundi.

   The fruit morphology and calyx venation pattern of the new fossil were compared withP. infinemundiand extant species of Solanaceae.

   Physalis hunickeniisp. nov. is clearly distinct fromP. infinemundiin its fruiting calyx with wider primary veins, longer and thinner lobes, and especially in its venation pattern with high density, transverse tertiary veins; these features support its placement in a new species. In comparison with extant physaloid genera, the calyx venation pattern and other diagnostic traits reinforce placement of the new fossil, likeP. infinemundi, within the tribe Physalideae of Solanaceae.

   Both species of fossil nightshades from Laguna del Hunco represent crown‐group Solanaceae but are older than all prior age estimates of the family. Although at least 20 transoceanic dispersals have been proposed as the driver of range expansion of Solanaceae, the Patagonian fossils push back the diversification of the family to Gondwanan times. Thus, overland dispersal across Gondwana is now a likely scenario for at least some biogeographic patterns, in light of the ancient trans‐Antarctic land connections between South America and Australia.

    

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4. “Missing links” for the long-lived Macdonald and Aragohotspots, South Pacific Ocean

   https://doi.org/10.1130/G48276.1  

   Buff, L. ; Jackson, M.G. ; Konrad, K. ; Konter, J.G. ; Bizimis, M. ; Price, A. ; Rose-Koga, E.F. ; Blusztajn, J. ; Koppers, A.A.P. ; Herrera, Santiago ( January 2021 , Geology)

   null (Ed.)

   The Cook-Austral volcanic lineament extends from Macdonald Seamount (east) to Aitutaki Island (west) in the South Pacific Ocean and consists of hotspot-related volcanic islands, seamounts, and atolls. The Cook-Austral volcanic lineament has been characterized as multiple overlapping, age-progressive hotspot tracks generated by at least two mantle plumes, including the Arago and Macdonald plumes, which have fed volcano construction for ~20 m.y. The Arago and Macdonald hotspot tracks are argued to have been active for at least 70 m.y. and to extend northwest of the Cook-Austral volcanic lineament into the Cretaceous-aged Tuvalu-Gilbert and Tokelau Island chains, respectively. Large gaps in sampling exist along the predicted hotspot tracks, complicating efforts seeking to show that the Arago and Macdonald hotspots have been continuous, long-lived sources of hotspot volcanism back into the Cretaceous. We present new major- and trace-element concentrations and radiogenic isotopes for three seamounts (Moki, Malulu, Dino) and one atoll (Rose), and new clinopyroxene 40Ar/39Ar ages for Rose (24.81 ± 1.02 Ma) and Moki (44.53 ± 10.05 Ma). All volcanoes are located in the poorly sampled region between the younger Cook-Austral and the older, Cretaceous portions of the Arago and Macdonald hotspot tracks. Absolute plate motion modeling indicates that the Rose and Moki volcanoes lie on or near the reconstructed traces of the Arago and Macdonald hotspots, respectively, and the 40Ar/39Ar ages for Rose and Moki align with the predicted age progression for the Arago (Rose) and Macdonald (Moki) hotspots, thereby linking the younger Cook-Austral and older Cretaceous portions of the long-lived (>70 m.y.) Arago and Macdonald hotspot tracks. 

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5. Laramide evolution of the San Juan Basin, New Mexico and Colorado: Paleocurrent and detrital-sanidine age constraints from the Paleocene Nacimiento and Animas formations

   https://doi.org/10.1130/GES02072.1  

   Cather, S. ; Heizler, H. ; Williamson, T.E. ( October 2019 , Geosphere)

   Understanding the tectonic and landscape evolution of the Colorado Plateau−southern Rocky Mountains area requires knowledge of the Laramide stratigraphic development of the San Juan Basin. Laramide sediment-transport vectors within the San Juan Basin are relatively well understood, except for those of the Nacimiento and Animas formations. Throughout most of the San Juan Basin of northwestern New Mexico and adjacent Colorado, these Paleocene units are mudstone-dominated fluvial successions intercalated between the lowermost Paleocene Kimbeto Member of the Ojo Alamo Sandstone and the basal strata of the lower Eocene San Jose Formation, both sandstone-dominated fluvial deposits. For the Nacimiento and Animas formations, we present a new lithostratigraphy that provides a basis for basin-scale interpretation of the Paleocene fluvial architecture using facies analysis, paleocurrent measurements, and 40Ar/ 39Ar sanidine age data. In contrast to the dominantly southerly or southeasterly paleoflow exhibited by the underlying Kimbeto Member and the overlying San Jose Formation, the Nacimiento and Animas formations exhibit evidence of diverse paleoflow. In the southern and western part of the basin during the Puercan, the lower part of the Nacimiento Formation was deposited by south- or southeast-flowing streams, similar to those of the underlying Kimbeto Member. This pattern of southeasterly paleoflow continued during the Torrejonian in the western part of the basin, within a southeast-prograding distributive fluvial system. By Torrejonian time, a major east-northeast–flowing fluvial system, herein termed the Tsosie paleoriver, had entered the southwestern part of the basin, and a switch to northerly paleoflow had occurred in the southern San Juan Basin. The reversal of paleoslope in the southern part of the San Juan Basin probably resulted from rapid subsidence in the northeast part of the basin during the early Paleocene. Continued Tiffanian-age southeastward progradation of the distributive fluvial system that headed in the western part of the basin pushed the Tsosie paleoriver beyond the present outcrop extent of the basin. In the eastern and northern parts of the San Juan Basin, paleoflow was generally toward the south throughout deposition of the Nacimiento and the Animas formations. An important exception is a newly discovered paleodrainage that exited the northeastern part of the basin, ∼15 km south of Dulce, New Mexico. There, an ∼130-m-thick Paleocene sandstone (herein informally termed the Wirt member of the Animas Formation) records a major east-flowing paleoriver system that aggraded within a broad paleovalley carved deeply into the Upper Cretaceous Lewis Shale. 40Ar/ 39Ar dating of detrital sanidine documents a maximum depositional age of 65.58 ± 0.10 Ma for the Wirt member. The detrital sanidine grains are indistinguishable in age and K/Ca values from sanidines of the Horseshoe ash (65.49 ± 0.06 Ma), which is exposed 10.5 m above the base of the Nacimiento Formation in the southwestern part of the basin. The Wirt member may represent the deposits of the Tsosie paleoriver where it exited eastward from the basin. Our study shows that the evolution of Paleocene fluvial systems in the San Juan Basin was complex and primarily responded to variations in subsidence-related sedimentary accommodation within the basin. 

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