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1. Detrital sanidine 40Ar/39Ar dating confirms <2 Ma age of Crooked Ridge paleoriver and subsequent deep denudation of the southwestern Colorado Plateau

   https://doi.org/10.1130/GES02319.1  

   Heizler, Matthew T. ; Karlstrom, Karl E. ; Albonico, Micael ; Hereford, Richard ; Beard, L. Sue ; Cather, Steven M. ; Crossey, Laurie J. ; Sundell, Kurt E. ( February 2021 , Geosphere)

   Abstract Crooked Ridge and White Mesa in northeastern Arizona (southwestern United States) preserve, as inverted topography, a 57-km-long abandoned alluvial system near the present drainage divide between the Colorado, San Juan, and Little Colorado Rivers. The pathway of this paleoriver, flowing southwest toward eastern Grand Canyon, has led to provocative alternative models for its potential importance in carving Grand Canyon. The ∼50-m-thick White Mesa alluvium is the only datable record of this paleoriver system. We present new 40Ar/39Ar sanidine dating that confirms a ca. 2 Ma maximum depositional age for White Mesa alluvium, supported by a large mode (n = 42) of dates from 2.06 to 1.76 Ma. Older grain modes show abundant 37–23 Ma grains mostly derived ultimately from the San Juan Mountains, as is also documented by rare volcanic and basement pebbles in the White Mesa alluvium. A tuff with an age of 1.07 ± 0.05 Ma is inset below, and hence provides a younger age bracket for the White Mesa alluvium. Newly dated remnant deposits on Black Mesa contain similar 37–23 Ma grains and exotic pebbles, plus a large mode (n = 71) of 9.052 ± 0.003 Ma sanidine. These deposits could be part of the Whitemore »Mesa alluvium without any Pleistocene grains, but new detrital sanidine data from the upper Bidahochi Formation near Ganado, Arizona, have similar maximum depositional ages of 11.0–6.1 Ma and show similar 40–20 Ma San Juan Mountains–derived sanidine. Thus, we tentatively interpret the <9 Ma Black Mesa deposit to be a remnant of an 11–6 Ma Bidahochi alluvial system derived from the now-eroded southwestern fringe of the San Juan Mountains. This alluvial fringe is the probable source for reworking of 40–20 Ma detrital sanidine and exotic clasts into Oligocene Chuska Sandstone, Miocene Bidahochi Formation, and ultimately into the <2 Ma White Mesa alluvium. The <2 Ma age of the White Mesa alluvium does not support models that the Crooked Ridge paleoriver originated as a late Oligocene to Miocene San Juan River that ultimately carved across the Kaibab uplift. Instead, we interpret the Crooked Ridge paleoriver as a 1.9–1.1 Ma tributary to the Little Colorado River, analogous to modern-day Moenkopi Wash. We reject the “young sediment in old paleovalley” hypothesis based on mapping, stratigraphic, and geomorphic constraints. Deep exhumation and beheading by tributaries of the San Juan and Colorado Rivers caused the Crooked Ridge paleotributary to be abandoned between 1.9 and 1.1 Ma. Thermochronologic data also provide no evidence for, and pose substantial difficulties with, the hypothesis for an earlier (Oligocene–Miocene) Colorado–San Juan paleoriver system that flowed along the Crooked Ridge pathway and carved across the Kaibab uplift.« less
2. Climate and ecology in the Rocky Mountain interior after the early Eocene Climatic Optimum

   https://doi.org/10.5194/cp-17-2515-2021  

   Stein, R.A. ( October 2021 , Climate of the past)

   As atmospheric carbon dioxide (CO2) and temperatures increase with modern climate change, ancient hothouse periods become a focal point for understanding ecosystem function under similar conditions. The early Eocene exhibited high temperatures, high CO2 levels, and similar tectonic plate configuration as today, so it has been invoked as an analog to modern climate change. During the early Eocene, the greater Green River Basin (GGRB) of southwestern Wyoming was covered by an ancient hypersaline lake (Lake Gosiute; Green River Formation) and associated fluvial and floodplain systems (Wasatch and Bridger formations). The volcaniclastic Bridger Formation was deposited by an inland delta that drained from the northwest into freshwater Lake Gosiute and is known for its vast paleontological assemblages. Using this well-preserved basin deposited during a period of tectonic and paleoclimatic interest, we employ multiple proxies to study trends in provenance, parent material, weathering, and climate throughout 1 million years. The Blue Rim escarpment exposes approximately 100 m of the lower Bridger Formation, which includes plant and mammal fossils, solitary paleosol profiles, and organic remains suitable for geochemical analyses, as well as ash beds and volcaniclastic sandstone beds suitable for radioisotopic dating. New 40Ar/39Ar ages from the middle and top of the Blue Rimmore »escarpment constrain the age of its strata to ∼ 49.5–48.5 Myr ago during the “falling limb” of the early Eocene Climatic Optimum. We used several geochemical tools to study provenance and parent material in both the paleosols and the associated sediments and found no change in sediment input source despite significant variation in sedimentary facies and organic carbon burial. We also reconstructed environmental conditions, including temperature, precipitation (both from paleosols), and the isotopic composition of atmospheric CO2 from plants found in the floral assemblages. Results from paleosol-based reconstructions were compared to semi-co-temporal reconstructions made using leaf physiognomic techniques and marine proxies. The paleosol-based reconstructions (near the base of the section) of precipitation (608–1167 mm yr−1) and temperature (10.4 to 12.0 ∘C) were within error of, although lower than, those based on floral assemblages, which were stratigraphically higher in the section and represented a highly preserved event later in time. Geochemistry and detrital feldspar geochronology indicate a consistent provenance for Blue Rim sediments, sourcing predominantly from the Idaho paleoriver, which drained the active Challis volcanic field. Thus, because there was neither significant climatic change nor significant provenance change, variation in sedimentary facies and organic carbon burial likely reflected localized geomorphic controls and the relative height of the water table. The ecosystem can be characterized as a wet, subtropical-like forest (i.e., paratropical) throughout the interval based upon the floral humidity province and Holdridge life zone schemes. Given the mid-paleolatitude position of the Blue Rim escarpment, those results are consistent with marine proxies that indicate that globally warm climatic conditions continued beyond the peak warm conditions of the early Eocene Climatic Optimum. The reconstructed atmospheric δ13C value (−5.3 ‰ to −5.8 ‰) closely matches the independently reconstructed value from marine microfossils (−5.4 ‰), which provides confidence in this reconstruction. Likewise, the isotopic composition reconstructed matches the mantle most closely (−5.4 ‰), agreeing with other postulations that warming was maintained by volcanic outgassing rather than a much more isotopically depleted source, such as methane hydrates.« less
3. Detrital zircon geochronology and provenance of glaciogenic strata of the Middle Carboniferous San Eduardo Formation, Calingasta-Uspallata Basin, NW Argentina

   https://doi.org/10.1130/abs/2019AM-337940  

   Malone, J. E. ( September 2019 , Geological Society of America Abstracts with Programs)

   The Calingasta-Uspallata Basin preserves a near continuous sequence of glaciomarine deposition from the middle to late Carboniferous, represented by five separate formations. Correlation between these formations have been achieved using index marine invertebrates, which also provides some implications for max-depositional ages. However, no isotopic dating analyses have been sought in this basin to further constrain the age of deposition or provide a source of provenance for sediments. The San Eduardo formation near the El Leoncito Astronomical Complex, San Juan Province, Argentina, was deposited within the Calingasta—Upsallata Basin on the western margin of the Proto-precordillera during the late Mississippian to early Pennsylvanian. This succession preserves a complete sequence of proximal glaciomarine, nearshore, and fluvial systems deposited at the beginning of the late Paleozoic ice age. Samples were collected from various stages throughout the sequence for detrital zircon U-Pb geochronology to determine sediment provenance as a way of isolating different glacier sources. Results indicate multiple stages of glaciation, with at least three distinct source areas. The lowermost stage includes locally sourced basement and recycled underlying Silurian, represented by similar Famatinian (500-460 mya) and Grenville peaks (1250-1000 mya) peaks, where the Grenville source likely originating from the Western Sierras Pampeans, which would representmore »a breaching of the Proto-precordillera from the east. The middle stage shows a population distinct unto itself, with a peak during the Mississippian (330-360 mya). A volcanic island arc was situated along the Andean margin during the late Paleozoic, likely resulting in the influx of Carboniferous aged volcanic sediments. The lower most stage shows relations based on K-S results to formations within the Paganzo basin to the northeast, likely serving as the outwash of these distant glaciers through braided fluvial systems. This study will expand upon current chronologic knowledge within the Calingasta-Uspallata basin and will be supported by sandstone petrology and mineralogic composition, pebble counts and composition of dropstones.« less
4. Cretaceous to Miocene magmatism, sedimentation, and exhumation within the Alaska Range suture zone: A polyphase reactivated terrane boundary

   https://doi.org/10.1130/GES02014.1  

   Trop, J. ; Benowitz, J. ( June 2019 , Geosphere)

   The Alaska Range suture zone exposes Cretaceous to Quaternary marine and nonmarine sedimentary and volcanic rocks sandwiched between oceanic rocks of the accreted Wrangellia composite terrane to the south and older continental terranes to the north. New U-Pb zircon ages, 40Ar/39Ar, ZHe, and AFT cooling ages, geochemical compositions, and geological field observations from these rocks provide improved constraints on the timing of Cretaceous to Miocene magmatism, sedimentation, and deformation within the collisional suture zone. Our results bear on the unclear displacement history of the seismically active Denali fault, which bisects the suture zone. Newly identified tuffs north of the Denali fault in sedimentary strata of the Cantwell Formation yield ca. 72 to ca. 68 Ma U-Pb zircon ages. Lavas sampled south of the Denali fault yield ca. 69 Ma 40Ar/39Ar ages and geochemical compositions typical of arc assemblages, ranging from basalt-andesite-trachyte, relatively high-K, and high concentrations of incompatible elements attributed to slab contribution (e.g., high Cs, Ba, and Th). The Late Cretaceous lavas and bentonites, together with regionally extensive coeval calc-alkaline plutons, record arc magmatism during contractional deformation and metamorphism within the suture zone. Latest Cretaceous volcanic and sedimentary strata are locally overlain by Eocene Teklanika Formation volcanic rocks withmore »geochemical compositions transitional between arc and intraplate affinity. New detrital-zircon data from the modern Teklanika River indicate peak Teklanika volcanism at ca. 57 Ma, which is also reflected in zircon Pb loss in Cantwell Formation bentonites. Teklanika Formation volcanism may reflect hypothesized slab break-off and a Paleocene–Eocene period of a transform margin configuration. Mafic dike swarms were emplaced along the Denali fault from ca. 38 to ca. 25 Ma based on new 40Ar/39Ar ages. Diking along the Denali fault may have been localized by strike-slip extension following a change in direction of the subducting oceanic plate beneath southern Alaska from N-NE to NW at ca. 46–40 Ma. Diking represents the last recorded episode of significant magmatism in the central and eastern Alaska Range, including along the Denali fault. Two tectonic models may explain emplacement of more primitive and less extensive Eocene–Oligocene magmas: delamination of the Late Cretaceous–Paleocene arc root and/or thickened suture zone lithosphere, or a slab window created during possible Paleocene slab break-off. Fluvial strata exposed just south of the Denali fault in the central Alaska Range record synorogenic sedimentation coeval with diking and inferred strike-slip displacement. Deposition occurred ca. 29 Ma based on palynomorphs and the youngest detrital zircons. U-Pb detrital-zircon geochronology and clast compositional data indicate the fluvial strata were derived from sedimentary and igneous bedrock presently exposed within the Alaska Range, including Cretaceous sources presently exposed on the opposite (north) side of the fault. The provenance data may indicate ~150 km or more of dextral offset of the ca. 29 Ma strata from inferred sediment sources, but different amounts of slip are feasible. Together, the dike swarms and fluvial strata are interpreted to record Oligocene strike-slip movement along the Denali fault system, coeval with strike-slip basin development along other segments of the fault. Diking and sedimentation occurred just prior to the onset of rapid and persistent exhumation ca. 25 Ma across the Alaska Range. This phase of reactivation of the suture zone is interpreted to reflect the translation along and convergence of southern Alaska across the Denali fault driven by highly coupled flat-slab subduction of the Yakutat microplate, which continues to accrete to the southern margin of Alaska. Furthermore, a change in Pacific plate direction and velocity at ca. 25 Ma created a more convergent regime along the apex of the Denali fault curve, likely contributing to the shutting off of near-fault extension- facilitated arc magmatism along this section of the fault system and increased exhumation rates.« less
5. Late Cretaceous angiosperm woods from the McRae Formation, south-central New Mexico, USA: Part 2

   https://doi.org/DOI: 10.1086/695503  

   Estrada-Ruiz, E. ; Wheeler, E.A. ; Upchurch, G.R. ; Mack, G.H. ( January 2018 , International Journal of Plant Sciences)

   Premise of research. Over the past 3 decades, angiosperm woods have been reported from the Campanian to the Maastrichtian of southern Laramidia, including Coahuila and Chihuahua, Mexico; Big Bend National Park, Texas; and the San Juan Basin, New Mexico. Recent investigations of the upper Campanian (76.5 to >72.5 Ma) Jose Creek Member of the McRae Formation, south-central New Mexico, indicate an abundance of well-preserved silicified woods, representing one of the most diverse Cretaceous wood floras in the world. In this report, we describe four new angiosperm wood types. Methodology. The fossil woods described here were collected from the upper Campanian of south-central New Mexico, along the northeastern flank of the Caballo Mountains and in the adjacent Cutter Sag, and were studied using thin sections. The potential affinities of these McRae woods were determined by comparison with fossil and extant woods. Pivotal results. The woods reported here comprise one magnoliid and three eudicots with varying levels of comparability to extant taxa. Laurinoxylon rennerae sp. nov. belongs to Lauraceae and has a combination of features found in multiple extant genera variously referred to as Cinnamomeae Nees, Laureae Maout & Decaisne, or Lauroideae Burnett/core Lauraceae. Turneroxylon newmexicoense gen. et sp. nov. is amore »eudicot with many similarities to Dilleniaceae but differs in having narrower rays. Mcraeoxylon waddellii gen. et sp. nov. has a suite of features seen in several families of Malpighiales, Myrtales, and Oxalidales. McRae angiosperm wood type 1 has a suite of features found in genera of Dilleniales, Ericales, and Malpighiales. Conclusions. All wood types, with the exception of M. waddellii, have minimum axis diameters of 110 cm (12–50 cm), indicating that they represent trees. This reinforces previous evidence for the presence of small to large angiosperm trees in the Jose Creek Member and underscores the importance of woody angiosperms in vegetation of the southern Western Interior during the Campanian-Maastrichtian.« less