Downhill from Austin and Ely to Las Vegas: U-Pb detrital zircon suites from the Eocene–Oligocene Titus Canyon Formation and associated strata, Death Valley, California

ABSTRACT In a reconnaissance investigation aimed at interrogating the changing topography and paleogeography of the western United States prior to Basin and Range faulting, a preliminary study made use of U-Pb ages of detrital zircon suites from 16 samples from the Eocene–Oligocene Titus Canyon Formation, its overlying units, and correlatives near Death Valley. The Titus Canyon Formation unconformably overlies Neoproterozoic to Devonian strata in the Funeral and Grapevine Mountains of California and Nevada. Samples were collected from (1) the type area in Titus Canyon, (2) the headwaters of Monarch Canyon, and (3) unnamed Cenozoic strata exposed in a klippe of the Boundary Canyon fault in the central Funeral Mountains. Red beds and conglomerates at the base of the Titus Canyon Formation at locations 1 and 2, which contain previously reported 38–37 Ma fossils, yielded mostly Sierran batholith–age detrital zircons (defined by Triassic, Jurassic, and Cretaceous peaks). Overlying channelized fluvial sandstones, conglomerates, and minor lacustrine shale, marl, and limestone record an abrupt change in source region around 38–36 Ma or slightly later, from more local, Sierran arc–derived sediment to extraregional sources to the north. Clasts of red radiolarian-bearing chert, dark radiolarian chert, and quartzite indicate sources in the region of the Golconda and Roberts Mountains allochthons of northern Nevada. Sandstones intercalated with conglomerate contain increasing proportions of Cenozoic zircon sourced from south-migrating, caldera-forming eruptions at the latitude of Austin and Ely in Nevada with maximum depositional ages (MDAs) ranging from 36 to 24 Ma at the top of the Titus Canyon Formation. Carbonate clasts and ash-rich horizons become more prevalent in the overlying conglomeratic Panuga Formation (which contains a previously dated 15.7 Ma ash-flow tuff). The base of the higher, ash-dominated Wahguyhe Formation yielded a MDA of 14.4 Ma. The central Funeral Mountains section exposes a different sequence of units that, based on new data, are correlative to the Titus Canyon, Panuga, and Wahguyhe Formations at locations 1 and 2. An ash-flow tuff above its (unexposed) base provided a MDA of 34 Ma, and the youngest sample yielded a MDA of 12.7 Ma. The striking differences between age-correlative sections, together with map-based evidence for channelization, indicate that the Titus Canyon Formation and overlying units likely represent fluvial channel, floodplain, and lacustrine deposits as sediments mostly bypassed the region, moving south toward the Paleogene shoreline in the Mojave Desert. The profound changes in source regions and sedimentary facies documented in the Titus Canyon Formation took place during ignimbrite flareup magmatism and a proposed eastward shift of the continental divide from the axis of the Cretaceous arc to a new divide in central Nevada in response to thermal uplift and addition of magma to the crust. This uplift initiated south-flowing fluvial systems that supplied sediments to the Titus Canyon Formation and higher units.

Late Pleistocene glacial chronologies and paleoclimate in the northern Rocky Mountains

The geologic record of mountain glaciations is a robust indicator of terrestrial paleoclimate change. During the last glaciation, mountain ranges across the western U.S. hosted glaciers while the Cordilleran and Laurentide ice sheets flowed to the west and east of the continental divide, respectively. Records detailing the chronologies and paleoclimate significance of these ice advances have been developed for many sites across North America. However, relatively few glacial records have been developed for mountain glaciers in the northern Rocky Mountains near ice sheet margins. Here, we report cosmogenic beryllium-10 surface exposure ages and numerical glacier modeling results showing that mountain glaciers in the northern Rockies abandoned terminal moraines after the end of the Last Glacial Maximum around 17–18 ka and could have been sustained by −10 to −8.5 °C temperature depressions relative to modern assuming similar or drier than modern precipitation. Additionally, we present a deglacial chronology from the northern Rocky Mountains that indicates while there is considerable variability in initial moraine abandonment ages across the Rocky Mountains, the pace of subsequent ice retreat through the Lateglacial exhibits some regional coherence. Our results provide insight on potential regional mechanisms driving the initiation of and sustained deglaciation in the western U.S. including rising atmospheric CO2 and ice sheet collapse.

Notes on Stenochiini Kirby, 1837 genera and species from western North America (Coleoptera: Tenebrionidae)

The tribe Stenochiini Kirby, 1837 comprises six genera in North America with most species occurring in the tropical and temperateregions of the continent. Only two species in the genus Strongylium Kirby, 1818 have previously been reported from west of theContinental Divide in the United States from Arizona and New Mexico and no members of the tribe have been reported from the stateof Sonora, Mexico. We here report Strongylium tenuicolle (Say, 1826), known to be widely distributed east of the Rocky Mountains,from west of the Continental Divide for the first time from both Arizona and New Mexico. We similarly report the first records ofboth Strongylium apache Triplehorn and Spilman, 1973 and Strongylium atrum Champion, 1888 from Sonora. Oploptera chamelensis(Doyen, 1990) was previously known only from the type series from Jalisco, Mexico and is here reported from Sonora, which thereby extends the known range of this genus significantly. To promote consistency in generic recognition, we propose the transfer of Oploptera simplicicollis (LeConte, 1878) New Combination from Strongylium for the species distributed across the southeastern United States. Species diagnoses are given, and generic boundaries are discussed along with the expected diversity of the Sonoran Desert region.

The spatial extent of hydrological and landscape changes across the mountains and prairies of Canada in the Mackenzie and Nelson River basins based on data from a warm-season time window

East of the Continental Divide in the cold interior of Western Canada, the Mackenzie and Nelson River basins have some of the world's most extreme and variable climates, and the warming climate is changing the landscape, vegetation, cryosphere, and hydrology. Available data consist of streamflow records from a large number (395) of natural (unmanaged) gauged basins, where flow may be perennial or temporary, collected either year-round or during only the warm season, for a different series of years between 1910 and 2012. An annual warm-season time window where observations were available across all stations was used to classify (1) streamflow regime and (2) seasonal trend patterns. Streamflow trends were compared to changes in satellite Normalized Difference Indices. Clustering using dynamic time warping, which overcomes differences in streamflow timing due to latitude or elevation, identified 12 regime types. Streamflow regime types exhibit a strong connection to location; there is a strong distinction between mountains and plains and associated with ecozones. Clustering of seasonal trends resulted in six trend patterns that also follow a distinct spatial organization. The trend patterns include one with decreasing streamflow, four with different patterns of increasing streamflow, and one without structure. The spatial patterns of trends in mean, minimum, and maximum of Normalized Difference Indices of water and snow (NDWI and NDSI) were similar to each other but different from Normalized Difference Index of vegetation (NDVI) trends. Regime types, trend patterns, and satellite indices trends each showed spatially coherent patterns separating the Canadian Rockies and other mountain ranges in the west from the poorly defined drainage basins in the east and north. Three specific areas of change were identified: (i) in the mountains and cold taiga-covered subarctic, streamflow and greenness were increasing while wetness and snowcover were decreasing, (ii) in the forested Boreal Plains, particularly in the mountainous west, streamflows and greenness were decreasing but wetness and snowcover were not changing, and (iii) in the semi-arid to sub-humid agricultural Prairies, three patterns of increasing streamflow and an increase in the wetness index were observed. The largest changes in streamflow occurred in the eastern Canadian Prairies.

Lightning Occurrence and Casualties in U.S. National Parks

National park visitors travel primarily to view natural features while outdoors, however visits often occur in warmer months when lightning is present. This study uses cloud-to-ground flashes from 1999-2018 and cloud-to-ground strokes from 2009-2018 from the National Lightning Detection Network to identify lightning at the 46 contiguous United States national parks larger than 100 km2. The largest density is 6.10 flashes km-2 y-1 within Florida’s Everglades, and the smallest is near zero in Pinnacles National Park.The six most-visited parks are Great Smoky Mountains, Grand Canyon, Rocky Mountain, Zion, Yosemite, and Yellowstone. For these parks, lightning data are described by frequency, location and time of year and day. The four parks west of the Continental Divide have most lightning from 01 July to 15 September, and 1100 to 1900 LST. Each park has its own spatial lightning pattern dependent on local topography.Deaths and injuries from lightning within national parks have the same summer afternoon dominance shown by lightning data. Most casualties occur to people visiting from outside the parks’ states. The most common activities and locations are mountain climbing, hiking, and viewing canyons from overlooks.Lightning Fatality Risk, the product of areal visitor and CG flash densities, shows that many casualties are not in parks with high Risk, while very small Risk indicates parks where lightning awareness efforts can be minimized. As a result, safety advice should focus on specific locations where lightning-vulnerable activities are engaged by many visitors such as canyon rims, mountains and exposed high-altitude roads.

Fortuna Forest Reserve, Panama: Interacting Effects of Climate and Soils on the Biota of a Wet Premontane Tropical Forest

The Fortuna Forest Reserve and adjacent upland areas of the Palo Seco Reserve in western Panama support some of the most extensively studied lower and premontane tropical forests in the world. The forests of Fortuna are among the most diverse in Central America and are therefore of exceptional significance for the preservation of regional biodiversity. This volume brings together more than 50 years of research on the climate, geology, soils, and major plant groups of Fortuna. Spanning the Continental Divide at around 1,000 m above sea level, some parts of the reserve receive more than 6,000 mm of annual rainfall, although there is considerable variation in cloud cover and seasonality. Soil fertility also varies markedly, reflecting the complex regional volcanic geology. The resulting gradients of climate and fertility across the reserve shape the composition, structure, and diversity of plant communities. A network of 12 one-hectare plots at Fortuna contains more than 400 species of trees greater than 5 cm diameter at breast height and reveals extensive compositional turnover across the reserve. One tree species, <i>Oreomunnea mexicana</i>, forms monodominant stands in otherwise species-rich forests, while forests on extremely infertile soils are dominated by the canopy palm <i>Colpothrinax aphanopetala</i> and include the tropical conifer <i>Podocarpus oleifolius</i>. There are also almost 400 species of bryophytes, almost 300 species of ferns and lycophytes, 31 species of palms, 80 species of bromeliads, and more than 200 species of orchids. Many species of ectomycorrhizal fungi identified from fruiting bodies are new to science. Overall, results from Fortuna highlight the remarkable diversity of plants that occur in montane forests and the extent to which their communities are structured by gradients of climate and soil fertility. The chapters in this volume provide a foundation for further research and exploration in this fascinating region.

Fortuna Forest Reserve, Panama: Interacting Effects of Climate and Soils on the Biota of a Wet Premontane Tropical Forest

The Fortuna Forest Reserve and adjacent upland areas of the Palo Seco Reserve in western Panama support some of the most extensively studied lower and premontane tropical forests in the world. The forests of Fortuna are among the most diverse in Central America and are therefore of exceptional significance for the preservation of regional biodiversity. This volume brings together more than 50 years of research on the climate, geology, soils, and major plant groups of Fortuna. Spanning the Continental Divide at around 1,000 m above sea level, some parts of the reserve receive more than 6,000 mm of annual rainfall, although there is considerable variation in cloud cover and seasonality. Soil fertility also varies markedly, reflecting the complex regional volcanic geology. The resulting gradients of climate and fertility across the reserve shape the composition, structure, and diversity of plant communities. A network of 12 one-hectare plots at Fortuna contains more than 400 species of trees greater than 5 cm diameter at breast height and reveals extensive compositional turnover across the reserve. One tree species, <i>Oreomunnea mexicana</i>, forms monodominant stands in otherwise species-rich forests, while forests on extremely infertile soils are dominated by the canopy palm <i>Colpothrinax aphanopetala</i> and include the tropical conifer <i>Podocarpus oleifolius</i>. There are also almost 400 species of bryophytes, almost 300 species of ferns and lycophytes, 31 species of palms, 80 species of bromeliads, and more than 200 species of orchids. Many species of ectomycorrhizal fungi identified from fruiting bodies are new to science. Overall, results from Fortuna highlight the remarkable diversity of plants that occur in montane forests and the extent to which their communities are structured by gradients of climate and soil fertility. The chapters in this volume provide a foundation for further research and exploration in this fascinating region.