First posted May 30, 2024 For additional information, contact: Alaska Science Center staffU.S. Geological Survey4210 University Dr.Anchorage, AK 99508Alaska Mineral ResourcesAlaska Science Center In this paper, we provide new information on the stratigraphy and paleoflora of the Sheep Creek volcanic field in the Alaska Range that bolsters our understanding of a key interval in the tectonic, paleoclimate, and paleoenvironmental history of the northern Cordillera. Although the distribution and basic stratigraphy of these rocks have been previously reported, here we document the stratigraphic context of recently dated igneous rocks and paleosols ranging from the Paleocene–Eocene boundary to the early middle Eocene, describe a more complete fossil leaf flora from the succession, and place the Sheep Creek volcanic field in its regional tectonic context of ridge subduction and slab window migration in central Alaska.

We tested the sensitivity of paleoclimate estimates to different resolutions of leaf morphotype distinction by grouping one paleofloral collection three ways and comparing the results. We examined the different morphotype resolutions using a recently collected Eocene (34 Ma) fossil leaf assemblage from Florissant Fossil Beds National Monument Collection Site 9 (CS-9, Florissant, Colorado), an exposure that yields one of the most diverse sets of dicotyledonous angiosperm ('dicot') morphotypes yet analyzed from a single site within this well-studied formation. We grouped this collection using what are herein referred to as 'Splitter' (number of morphotypes n = 129), 'Intermediate' (n = 96), and 'Lumper' (n = 53) morphotyping approaches, which differed in their degree of delineating foliar morphotypes. Leaf Margin Analysis (LMA) and Leaf Area Analysis (LAA) were used to obtain paleotemperature and paleoprecipitation estimates from the morphotype datasets and Climate Leaf Analysis Multivariate Program (CLAMP) was employed to examine further parameters. We perform an additional paleoclimatic analysis of CS-9 in which we divide the site into three stratigraphic sections, and finally we perform an analysis on sample collections above and below a volcanic event horizon within CS-9. Whole collection results across the three morphotype resolutions indicate a mean annual temperature between 10.8 and 12.5 degrees C, and annual precipitation between 61.8 and 130.7 cm at the time of deposition. The 'Splitter,' 'Intermediate,' and 'Lumper' approaches provide very similar estimates by all methods of analysis and suggest that morphotype resolution specificity has very little effect on climatic results using these leaf physiognomic techniques.

The Nutzotin basin of eastern Alaska consists of Upper Jurassic through Lower Cretaceous siliciclastic sedimentary and volcanic rocks that depositionally overlie the inboard margin of Wrangellia, an accreted oceanic plateau. We present igneous geochronologic data from volcanic rocks and detrital geochronologic and paleontological data from nonmarine sedimentary strata that provide constraints on the timing of deposition and sediment provenance. We also report geochronologic data from a dike injected into the Totschunda fault zone, which provides constraints on the timing of intra–suture zone basinal deformation. The Beaver Lake formation is an important sedimentary succession in the northwestern Cordillera because it provides an exceptionally rare stratigraphic record of the transition from marine to nonmarine depositional conditions along the inboard margin of the Insular terranes during mid-Cretaceous time. Conglomerate, volcanic-lithic sandstone, and carbonaceous mudstone/shale accumulated in fluvial channel-bar complexes and vegetated overbank areas, as evidenced by lithofacies data, the terrestrial nature of recovered kerogen and palynomorph assemblages, and terrestrial macrofossil remains of ferns and conifers. Sediment was eroded mainly from proximal sources of upper Jurassic to lower Cretaceous igneous rocks, given the dominance of detrital zircon and amphibole grains of that age, plus conglomerate with chiefly volcanic and plutonic clasts. Deposition was occurring by ca. 117 Ma and ceased by ca. 98 Ma, judging from palynomorphs, the youngest detrital ages, and ages of crosscutting intrusions and underlying lavas of the Chisana Formation. Following deposition, the basin fill was deformed, partly eroded, and displaced laterally by dextral displacement along the Totschunda fault, which bisects the Nutzotin basin. The Totschunda fault initiated by ca. 114 Ma, as constrained by the injection of an alkali feldspar syenite dike into the Totschunda fault zone.These results support previous interpretations that upper Jurassic to lower Cretaceous strata in the Nutzotin basin accumulated along the inboard margin of Wrangellia in a marine basin that was deformed during mid-Cretaceous time. The shift to terrestrial sedimentation overlapped with crustal-scale intrabasinal deformation of Wrangellia, based on previous studies along the Lost Creek fault and our new data from the Totschunda fault. Together, the geologic evidence for shortening and terrestrial deposition is interpreted to reflect accretion/suturing of the Insular terranes against inboard terranes. Our results also constrain the age of previously reported dinosaur footprints to ca. 117 Ma to ca. 98 Ma, which represent the only dinosaur fossils reported from eastern Alaska.

The Chickaloon Formation in south-central Alaska contains rich coal deposits dated very close to the Paleocene-Eocene boundary, immediately beneath which occur dispersed nodules of amber along with abundant remains of Metasequoia, dicots, and monocots. The nodules are small (less than 10 mm in length), nearly 10,000 of which were screened, yielding several inclusions of fungi and plant fragments, but mostly terrestrial arthropods: 29 specimens in 10 orders and 13 families. The fungi include resinicolous hyphae and a dark, multiseptate hyphomycete. Plants include wood/bark fragments and fibers, and the microphylls of a bryophyte (probably a moss, Musci). Among the arthropods are arachnids: mites (Acari: Oribatida), Pseudoscorpionida, and the bodies and a silken cocoon of spiders (Araneae). Insecta include Blattodea, Thysanoptera, Hemiptera (Heteroptera and Aphidoidea), Coleoptera (Dermestidae: Megatominae), Trichoptera, Diptera (Chironomidae: Tanypodinae), and Hymenoptera (Formicidae: Formicinae). Nymphal aphids predominate (65% of the arthropod individuals), which were probably feeding on the source tree, likely Metasequoia. There is a bias in preservation toward small arthropods (mean body length 0.75 mm) that are surface-dwelling (nonwinged) stages and taxa. Chickaloon amber contains the most northerly fossil records of pseudoscorpions, thrips, Dermestidae, and Cenozoic ants and mites, so the deposit is contributing unique data on high-latitude paleodiversity of the Paleogene hothouse earth.

This guide provides outcrop descriptions for a one-day field trip leaving from Anchorage, Alaska and ending at Sheep Mountain, Alaska. The field trip route provides opportunities to observe outcrops representing three bedrock successions that record the tectonic development of southern Alaska: (1) Lower to Middle Jurassic igneous and metamorphic rocks comprising the Talkeetna arc, one of the most complete accreted oceanic arc crustal sequences known world-wide, totaling 7 km of section; (2) A >3,800 m thick succession of Middle Jurassic–Upper Cretaceous marine forearc basinal deposits that record accretion of the Talkeetna arc against the former continental margin; and (3) a >2,900 m thick succession of Paleocene–Oligocene alluvial-fluvial strata and Eocene gabbro and rhyo-dacite km-scale intrusions emplaced in remnant forearc depocenters partly coeval with spreading ridge subduction and near-trench magmatism. These successions were uplifted and partly exhumed during flat-slab subduction of the Yakutat terrane beneath south-central Alaska during Oligocene–Holocene time. Outcrop descriptions emphasize sedimentary strata and igneous intrusions exposed near the Glenn Highway. Key stops include Upper Cretaceous marine mass flow deposits with diverse sedimentary structures, Paleocene–Eocene coal-bearing strata with abundant plant fossils, mid-Eocene rhyo-dacite plugs and gabbro sills that are hundreds of meters thick, and a cross-section through the seismogenic Castle Mountain fault zone. Field guides for linked trips during this meeting describe correlative strata exposed outboard (south) of the Matanuska forearc strata in the Chugach accretionary prism in the Chugach Mountains and the Kenai Peninsula.

Paleogene sedimentary rocks of the Arkose Ridge Formation (Talkeetna Mountains, Alaska) preserve a record of a fluvial–lacustrine depositional environment and its forested ecosystem in an active basin among the convergent margin tectonic processes that shaped southern Alaska. An ~800m measured succession at Box Canyon indicates braid-plain deposition with predominantly gravelly deposits low in the exposure to sandy and muddy facies associations below an overlying lava flow sequence. U–Pb geochronology on zircons from a tuff and a sandstone within the measured section, as well as an Ar/Ar date from the overlying lava constrain the age of the sedimentary succession to between ~59Ma and 48Ma. Fossil plant remains occur throughout the Arkose Ridge Formation as poorly-preserved coalified woody debris and fragmentary leaf impressions. At Box Canyon, however, a thin lacustrine depositional lens of rhythmically laminated mudrocks yielded fish fossils and a well-preserved floral assemblage including foliage and reproductive organs representing conifers, sphenopsids, monocots, and dicots. Leaf physiognomic methods to estimate paleoclimate were applied to the dicot leaf collection and indicate warm temperate paleotemperatures (~11–15±~4°C MAT) and elevated paleoprecipitation (~120cm/yr MAP) estimates as compared to modern conditions; results that are parallel with previously published estimates from the partly coeval Chickaloon Formation deposited in more distal depositional environments in the same basin. The low abundance of leaf herbivory in the Box Canyon dicot assemblage (~9% of leaves damaged) is also similar to the results from assemblages in the meander-plain depositional systems of the Chickaloon. This new suite of data informs models of the tectonostratigraphic evolution of southern Alaska and the developing understanding of terrestrial paleoecology and paleoclimate at high latitudes during the Late Paleocene–Early Eocene greenhouse climate phase.

New collections of a leaf compression-impression paleoflora preserved in fluvio-lacustrine sediments of the upper Chickaloon Formation, south-central Alaska, United States, provide leaf physiognomic climate estimates for the early Eocene in southern Alaska and rare data on plant-insect interactions from a subarctic setting. Thirty-nine broadleaf angiosperm morphotypes occur in a parautochthonous assemblage along with Metasequoia shoots and trunks, compressions of a diverse suite of seeds, monocotyledonous aquatic plants, freshwater gastropods, and inclusion-bearing dispersed amber. Leaf-character derived mean annual temperature estimates (11-14.6 degrees C) are significantly warmer than Alaska at present and indicate warm temperate conditions at the time of deposition. Leaf-derived mean annual precipitation estimates of similar to 110-160 cm/annum are comparable to those from similar-age paleofloras in Arctic Canada and indicate wetter conditions than nearly coeval paleofloras further south in the North American mid-latitudes. Leaf herbivory is rare in the Chickaloon assemblage (similar to 9% of leaf fragments) as compared to other, lower latitude Eocene assemblages, but exhibits four of the main leaf-damage guilds (hole feeding, margin feeding, surface feeding, and skeletonization). These data provide a rare glimpse at a high-latitude terrestrial forested ecosystem during a global hothouse climate phase and thus have implications in understanding how biogeographic patterning and ecological systems respond to non-analog, warm high-latitude environmental conditions.

The Paleocene-Eocene Chickaloon Formation of southern Alaska represents rapid deposition in a floodbasin setting, contains abundant and diverse fossil plant material, and spans the Paleocene-Eocene Thermal Maximum (PETM). Sedimentologic, lithofacies, and chemostratigraphic analyses of exposures of the Chickaloon Formation were conducted in order to provide a contextual framework for this, late Paleocene-early Eocene high-latitude sedimentary succession and to test if hyperthermal conditions are recorded in the unit.At the main study area in the Evan Jones Coal Mine, strata represent an alluvial sequence dominated by floodplain, channel sandstone, crevasse lobe, crevasse channel, and mire deposition. Floodplain environments are represented by medium-gray, commonly nodular and/or carbonate-cemented, shaly siltstone and centimeter-scale thick sheet-sandstone interbeds. Most channel sandstone deposits are interpreted to be crevasse channels and are generally small (2-5 m thick), rarely coarser than medium-grained sandstone, and sand grains are commonly coated with Fe-oxides. Carbonaceous shale and coal are localized and indicate a high-rate of organic carbon accumulation in the area. delta C-13 analyses of bulk organic carbon from the succession at the Evan Jones Mine are internally consistent with this interpretation; they yield uniformly negative values that are typical of continental organic matter (21.7 parts per thousand to 28.5 parts per thousand; X = 25.4 parts per thousand). Fossil plant material is also very common and includes leaves, wood, seeds, and resins from broad leaf angiosperms, conifers, ferns, and sphenopsids. Considered together, lithologic and paleontologic data suggest that these strata represent a floodbasin environment; the observed crevasse channels probably originate from a main channel located within several kilometers of the depocenter. Ash-fall tuffs punctuate the studied sequence and contain abundant reworked or recycled zircons, indicating that the depocenter was proximal to a volcanic source. Across all of these Chickaloon depositional environments, a significant negative isotopic shift (-5.8 parts per thousand.) occurs, which we hypothesize to represent a short-lived perturbation associated with a global hyperthermal event. (C) 2011 Elsevier B.V. All rights reserved.