• A comparative study of contrasting structural styles in the range-front region of the northeastern Arctic National Wildlife Refuge, northeastern Brooks Range, Alaska

      Hanks, Catherine Leigh; Stone, David B.; Crowder, R. Keith; Keskinen, Mary J.; Watts, Keith W.; Ave Lallemant, Hans G.; Mull, C. G. (1991)
      The range front of the northeastern Brooks Range in the Arctic National Wildlife Refuge (ANWR) is defined by anticlinoria cored by a 'basement' complex of weakly metamorphosed sedimentary, volcanic and intrusive rocks. These anticlinoria are interpreted to reflect horses in a northward-propagating regional duplex between a floor thrust at depth in the 'basement' complex and a roof thrust near the base of the cover sequence. Lateral variations in the geometry of these range-front anticlinoria reflect changes in lithology and deformational style of both the 'basement' and its cover. Two distinct structural geometries are displayed along the range front of northeastern ANWR. To the east, the large range-front anticlinorium is interpreted to reflect multiple horses of Cenozoic age within the stratified, slightly metamorphosed sedimentary and volcanic rocks of the pre-Mississippian 'basement'. During Cenozoic thrusting, these mechanically heterogeneous rocks deformed primarily via thrusting and related folding with minor penetrative strain. The Mississippian and younger cover sequence shortened via both thrust duplication and detachment folding above a detachment in the Mississippian Kayak Shale. In contrast, to the west the pre-Mississippian rocks consist primarily of the mechanically homogeneous Devonian Okpilak batholith. The batholith was transported northward during Cenozoic thrusting and now forms a major topographic and structural high near the range front. The batholith probably shortened during thrusting as a homogeneous mass via penetrative strain. Because the Kayak Shale is thin to absent in the vicinity of the batholith, Mississippian and younger rocks remained attached to the batholith and shortened via penetrative strain and minor imbrication. These two range-front areas form the central portion of two regional transects through northeastern ANWR. General area-balanced models for both transects suggest that the amount of total shortening is governed by the structural topography and the geometry of the basal detachment surface. While the structural topography of northeastern ANWR is reasonably well-constrained, the geometry of the basal detachment is not. Given a range in reasonable basal detachment geometries, shortening in both transects ranges from 16% to 61%. Detailed balanced cross sections based on subsurface and surface geologic data yield 46-48% shortening for both transects.
    • A Detailed Structural Analysis Across A Regional Unconformity, Forks Of The Canning River, Franklin Mountains, Northeastern Brooks Range, Alaska

      Ziegler, Jennifer Ann (1989)
      Structural analysis on the northern flank of the 'Franklin Mountains anticlinorium,' northeastern Brooks Range, Alaska, addressed the geometry and sequence of structures, and the deformational mechanics of the Franklinian and Ellesmerian sequences, which are separated by a sub-Mississippian unconformity. The anticlinorium is comprised of two horses of Franklinian sequence rocks in a Cenozoic north-vergent duplex thrust system. South-dipping pre-Mississippian slaty cleavage may have been a plane of preferred failure during ramp formation. Above the unconformity, the Kekiktuk Conglomerate remained attached to pre-Mississippian rocks, deforming with them beneath a roof thrust in the Mississippian Kayak Shale. Increased shear stress and overburden pressure beneath overthrust Franklinian sequence rocks may have led to local detachment near the unconformity surface. Above the Kayak Shale, progressive detachment folding and thrust faulting occurred in the Lisburne and Sadlerochit Groups as a result of emplacement of the two underlying horses. <p>
    • An Investigation Into Argon-40/Argon-39 Radiogenic Dating And X-Ray Analysis Of Shales And Clays From Northern Alaska

      Munly, Walter Campbell; Layer, Paul (2004)
      In this thesis I develop a new 40Ar/39Ar dating technique for measuring ages and estimating cooling histories of potassium-bearing minerals within shales and clays. To overcome problems of argon recoil, small shale or clay flakes (possessing diameters less than 2 mm) were encapsulated within microampoules under vacuum. Encapsulation ensures that argon that recoils from tiny crystals during irradiation in a nuclear reactor cannot escape and will therefore be detected during 40Ar/39Ar laser step-heating. The step heating method is effective in differentiating between, and dating neoformed and detrital illite components. My use of this method has revealed a significant age difference across the Toyuk thrust, Brooks Range, northern Alaska. Devonian shales from south of the thrust yield relatively flat age spectra indicative of a younger illitic component, and argon retention ages around 225 Ma. Shale samples from north of the thrust yield staircase age spectra indicative of a detrital illite component, and older retention ages (233--391 Ma). Modeling of these spectra across the Toyuk also suggests that argon loss and subsequent cooling occurred at about 140 Ma. Retention ages across the Toyuk thrust may reflect differences in depths of tectonic burial, or differing ages of crystallization of neoformed illite during initial deposition and burial. 40Ar/39Ar age spectra from the Colville basin, North Slope, Alaska, illites are generally dominated by detrital illite, yielding high temperature step-heat ages up to 475 Ma. Illite crystallization ages from the NPRA (National Petroleum Reserve in Alaska) range between 205 and 225 Ma. These ages are older than depositional ages and therefore suggest that this illite was transported from outside the Colville basin, perhaps from the Brooks Range. The Colville basin samples also reveal argon loss at ~45 Ma. Paleocene samples from the Exxon Alaska State A-1 well yield illite crystallization ages of about 205 Ma, and argon loss ages around 40 Ma. X-ray diffraction of the Colville basin samples indicates the presence of multiple clay phases, including detrital and neoformed illite. This complex mineralogy precluded estimating when the host shales were within the oil generation window.
    • Application Of Argon-40/Argon-39 Chronostratigraphy To Geologic Problems In Yellowstone Caldera And Cook Inlet Basin

      Dallegge, Todd A.; Layer, Paul (2002)
      The 40Ar/39Ar dating method is a useful tool in addressing problems concerning stratigraphic distribution of rock units. This tool is used where previously K-Ar-dated units are problematic, and where further chronostratigraphic control is needed for lavas of the Central Plateau Member (CPM) of the Plateau Rhyolite in Yellowstone National Park and volcanic ash dispersed in the sedimentary units of the Kenai Group in Cook Inlet Basin. Sanidine 40Ar/39Ar ages for CPM rhyolite lavas indicate that xenocrysts, excess argon, and incomplete degassing affected the K-Ar age interpretations. With the 40Ar/39Ar dating method these effects are removed, revealing four apparent eruptive episodes at approximately 135, 122, 107, and 90 ka. The 40Ar/ 39Ar eruption ages are 10's ky younger than U-Th ages from zircon indicating significant residence time of the CPM magma chamber prior to eruption. 40Ar/39Ar dating using multiple analytical runs and statistical assessment of the data produced satisfactory results from the potassium-poor, plagioclase-bearing, tephras of the Kenai Group. The results indicate that excess argon, argon loss, and xenocrystic contamination exist. The chronostratigraphic framework indicates that faulting offsets strata of widely different ages causing repetition of the stratigraphy. Age data indicate that the Sterling and Beluga Formations are time-equivalent strata representing lateral facies variations rather than distinct time-stratigraphic formations. Based on crosscutting relations and structural folding, the dated horizons suggest that structural deformation is no older than early Pliocene in age. Based on the structural complexities affecting the Kenai Group strata, the previously measured coal resource estimates for the Kenai Peninsula are overestimated by 23% in the Clam Gulch area and 50% in the Diamond Creek area. Desorbed gas contents, ranging between 66 and 123 scf/t (daf) from cuttings collected from the Beaver Creek and Kenai Gas Fields, are applied to coal volumes to estimate coalbed methane gas-in-place. CBM in-place estimates suggest values of 93 Tcf for the Kenai Peninsula and 272 Tcf for Cook Inlet Basin. Hypothetical recovery rates suggest 4.7 and 13.6 Tcf for the Kenai Peninsula and Cook Inlet Basin, respectively. The hypothetical CBM recovery rates could continue natural gas supplies to the Anchorage area for an additional 60 years beyond the estimated shortened supply date of 2011.* *This dissertation is a compound document (contains both a paper copy and a CD as part of the dissertation). The CD requires the following system requirement: Adobe Acrobat.
    • Beach ridge geomorphology of Kotzebue Sound: Implications for paleoclimatology and archaeology

      Mason, Owen Kenneth (1990)
      Beach ridges occur on all continents and record the horizontal addition of shoreface beyond the reach of storms. Improved cartographic methods in the nineteenth century allowed British historians to link shoreline changes with abandoned villages. This scientific trajectory was paralleled in the Bering Strait region from the 1880's to the 1930's. In the 1950's J. L. Giddings formalized "beach ridge archaeology" as a survey strategem using relative position to infer relative cultural chronology in northwest Alaska. Modern researchers use archaeological dates and data to document past climates or environments. At Cape Espenberg, on Seward Peninsula, my use of archaeological, stratigraphic, pedological, granulometric and photogrammetric data allows the delineation of 4000 years of coastal evolution. Four chronostratigraphic units are distinguished, using archaeological dates as minimum age assignments. Dune ridges formed in discrete intervals: 3300 to 2000 BP and from 1200 BP to the present; while low, berm ridges are predominant 4000-3300 and from 2000-1200 BP. The two different types of ridges correspond to variable climatic conditions: dune ridges formed after higher storm surges and winter winds while the lower berm ridges are related to less intense storm surges. Coastal dunes at Cape Espenberg are soon altered by plant succession processes with distance from the beach. As primary dunes are eroded, a complex blowout topography results. Erosional processes in blowouts were monitored during 1987-1989, revealing substantial vertical changes, up to 10 cm of erosion per yr. These rapid changes have considerable influence on archaeological site stability. Studies of the gravel ridge systems confirm the proxy storm record apparent in the coastal dunes atop the beach ridges on the Seward Peninsula. The geoarchaeological methodology allows correlations between depositional units within nine of the principal beach ridge and chenier complexes of northwest Alaska. The onset of deposition was at 4000-3500 BP. The complexes at Cape Espenberg and Choris Peninsula contain elevated, broader transgressive ridge sets 3300-2000 BP and from 1100-200 BP, connected with increased storm activity in the North Pacific. Erosional disconformities between successive sets of beach ridges occur at Cape Krusenstern at ca. 3000 BP and before 2000 BP. Between 2000-1000 BP extensive progradation occurred at nearly all complexes, indicating that less stormy conditions predominated.
    • Carboniferous Lisburne Group Carbonates Of The Porcupine Lake Valley: Implications For Surface To Subsurface Sequence Stratigraphy, Paleogeography, And Paleoclimatology

      Mcgee, Michelle Marie; Whalen, Michael T. (2004)
      This study utilizes high-resolution stratigraphy and sequence stratigraphy to document the response of the Carboniferous Lisburne Group carbonate platform during a change from greenhouse to icehouse conditions. The Lisburne Group in northern Alaska represents a laterally extensive carbonate ramp deposited on a passive continental margin during a greenhouse to icehouse transition. The Lisburne Group is subdivided into the Mississippian Wachsmuth and Alapah Limestones and Mississippian to Pennsylvanian Wahoo Limestone. I have identified six depositional sequences and corresponding systems tracts within the Lisburne Group based on bounding surfaces, cycle stacking patterns, and lateral lithofacies relationships. The Wachsmuth Limestone (Sequences I and II) is comprised of relatively thick cycles that have a "layer cake" stacking pattern that records minor migration of facies. Cycles in the uppermost Wachsmuth Limestone and the Alapah Limestone (Sequences III and IV) are thick, less "layer cake"-like, have deep water tongues at the base, and record significant migration of facies. Cycles in the uppermost Alapah Limestone and the Wahoo Limestone (Sequences V and VI) are thin, juxtapose deep water over shallow water facies, and record significant migration of facies. An unconformity marked by paleosols and karst features has been documented near the Mississippian-Pennsylvanian boundary in the Wahoo Limestone. I interpret the distinct change in cycle stacking patterns between the Mississippian and Pennsylvanian Lisburne Group to be related to changes in Paleoclimate. I interpret Sequences I through IV to be deposited during a greenhouse and transitional climate; whereas, Sequences V and VI were deposited during an icehouse climate.
    • Cenozoic tectono-thermal history of the southern Talkeetna Mountains, Alaska: multiple topographic development drivers through time

      Terhune, Patrick J.; Benowitz, Jeffrey; Freymueller, Jeffrey; Gillis, Robert (2018-08)
      Intraplate mountain ranges can have polyphase topographic development histories reflecting diverse plate boundary conditions. We apply ⁴⁰Ar/³⁹Ar, apatite fission track (AFT) and apatite (U-Th)/He (AHe) geochronology-thermochronology to plutonic and volcanic rocks in the southern Talkeetna Mountains of Alaska to document regional magmatism, rock cooling and inferred exhumation patterns as proxies for the deformation history of this long-lived intraplate mountain range. High-temperature ⁴⁰Ar/³⁹Ar geochronology on muscovite, biotite and K-feldspar from Jurassic granitoids indicates post-emplacement (~158-125 Ma) cooling and Paleocene (~61 Ma) thermal resetting. ⁴⁰Ar/³⁹Ar whole rock volcanic ages and AFT cooling ages in the southern Talkeetna Mountains are predominantly Paleocene-Eocene, suggesting that the Range is partially paleotopography that formed during an earlier tectonic setting. Miocene AHe cooling ages within ~10 km of the Castle Mountain Fault suggest ~2-3 km of vertical displacement that also contributed to mountain building, likely in response to the inboard progression of the subducted Yakutat microplate. Paleocene-Eocene volcanic and exhumation ages across interior southern Alaska north of the Border Ranges Fault System are similar and show no N-S or W-E progressions, suggesting a broadly synchronous and widespread volcanic and exhumation event that conflicts with the proposed diachronous subduction of an active west-east sweeping spreading ridge beneath south-central Alaska. To reconcile this, we propose a new model for the Cenozoic tectonic evolution of southern Alaska. We infer that slab breakoff sub-parallel to the trench and subsequent mantle upwelling drove magmatism, exhumation and rock cooling synchronously across south-central Alaska and played a primary role in the development of the southern Talkeetna Mountains.
    • Characterization And Interpretation Of Volcanic Activity At Redoubt, Bezymianny And Karymsky Volcanoes Through Direct And Remote Measurements Of Volcanic Emissions

      Lopez, Taryn M.; Cahill, Catherine; Dehn, Jonathan; Newberry, Rainer; Simpson, William; Werner, Cynthia (2013)
      Surface measurements of volcanic emissions can provide critical insight into subsurface processes at active volcanoes such as the influx or ascent of magma, changes in conduit permeability, and relative eruption size. In this dissertation I employ direct and remote measurements of volcanic emissions to characterize activity and elucidate subsurface processes at three active volcanoes around the North Pacific. The 2009 eruption of Redoubt Volcano, Alaska, produced elevated SO2 emissions that were detected by the Ozone Monitoring Instrument (OMI) satellite sensor for over three months. This provided a rare opportunity to characterize Redoubt's daily SO2 emissions and to validate the OMI measurements. Order of magnitude variations in daily SO2 mass were observed, with over half of the cumulative SO2 emissions released during the explosive phase of the eruption. Correlations among OMI daily SO2 mass, tephra mass and acoustic energies during the explosive phase suggest that OMI data may be used to infer eruption size and explosivity. From 2007 through 2010 direct and remote measurements of volcanic gas composition and flux were measured at Bezymianny Volcano, Kamchatka, Russia. During this period Bezymianny underwent five explosive eruptions. Estimates of passive and eruptive SO2 emissions suggest that the majority of SO2 is released passively. Order of magnitude variations in total volatile flux observed throughout the study period were attributed to changes in the depth of gas exsolution and separation from the melt at the time of sample collection. These findings suggest that exsolved gas composition may be used to detect magma ascent prior to eruption at Bezymianny Volcano. Karymsky Volcano, Kamchatka, Russia, is a dynamic volcano which exhibited four end-member activity types during field campaigns in 2011 and 2012, including: discrete ash explosions, pulsatory degassing, gas jetting, and explosive eruption. These activity types were characterized quantitatively and uniquely distinguished using a multiparameter dataset based on infrasound, thermal imagery, and volcanic emissions. These observations suggest a decoupling between exsolved volatiles and magma at depth. I propose that variations in magma degassing depth influence the flux and proportions of decompression- and crystallization-induced degassing, as well as conduit permeability, can explain the variations in volcanic activity.
    • Characterization Of The Dat Zone, Eastern Alaska Range, Alaska: A Calcic Iron (Copper-Gold) Skarn Prospect

      Athey, Jennifer Erin; Newberry, Rainer (1999)
      I studied a Cu-Fe-Au prospect in the eastern Alaska Range, Alaska, informally known as 'the DAT Zone'. Previous workers disagreed about the prospect's genesis; my work conclusively shows it to be a calcic iron skarn deposit with late quartz-pyrite-gold mineralization, broadly coeval with the host Pennsylvanian island arc-related volcanic and associated sedimentary rocks. A sericite-quartz-pyrite altered shear yields a $\sp{40}$Ar/$\sp{39}$Ar age similar to that of a nearby quartz monzodiorite pluton (300 Ma $\pm$ 1 Ma). The skarn consists of andradite extensively retrograded to ferrotremolite, quartz, magnetite, hematite, calcite, and chalcopyrite. Based on alteration pseudomorphs, the prograde assemblage was clinopyroxene with 20-40 %Hd, wollastonite, and andradite. The main-stage fluids were approximately 600$\sp\circ$C, moderately oxidized, and extremely saline ($>$60% NaCl equivalent). This fluid was clearly different from those of volcanogenic massive sulfide, metamorphic, or epithermal deposits. The lack of explosive textures is inconsistent with the DAT Zone as a porphyry copper deposit. <p>
    • Coseismic Deformation Of The 2001 El Salvador And 2002 Denali Fault Earthquakes From Gps Geodetic Measurements

      Hreinsdottir, Sigrun; Freymueller, Jeffrey T. (2005)
      GPS geodetic measurements are used to study two major earthquakes, the 2001 MW 7.7 El Salvador and 2002 MW 7.9 Denali Fault earthquakes. The 2001 MW 7.7 earthquake was a normal fault event in the subducting Cocos plate offshore El Salvador. Coseismic displacements of up to 15 mm were measured at permanent GPS stations in Central America. The GPS data were used to constrain the location of and slip on the normal fault. One month later a MW 6.6 strike-slip earthquake occurred in the overriding Caribbean plate. Coulomb stress changes estimated from the M W 7.7 earthquake suggest that it triggered the MW 6.6 earthquake. Coseismic displacement from the MW 6.6 earthquake, about 40 mm at a GPS station in El Salvador, indicates that the earthquake triggered additional slip on a fault close to the GPS station. The MW 6.6 earthquake further changed the stress field in the overriding Caribbean plate, with triggered seismic activity occurring west and possibly also to the east of the rupture in the days to months following the earthquake. The MW 7.9 Denali Fault earthquake ruptured three faults in the interior of Alaska. It initiated with a thrust motion on the Susitna Glacier fault but then ruptured the Denali and Totschunda faults with predominantly right-lateral strike-slip motion unilaterally from west to east. GPS data measured in the two weeks following the earthquake suggest a complex coseismic rupture along the faults with two main regions of moment release along the Denali fault. A large amount of additional data were collected in the year following the earthquake which greatly improved the resolution on the fault, revealing more details of the slip distribution. We estimate a total moment release of 6.81 x 1020 Nm in the earthquake with a M W 7.2 thrust subevent on Susitna Glacier fault. The slip on the Denali fault is highly variable, with 4 main pulses of moment release. The largest moment pulse corresponds to a MW 7.5 subevent, about 40 km west of the Denali-Totschunda fault junction. We estimate relatively low and shallow slip on the Totschunda fault.
    • Depositional Environments Of The Late Cretaceous (Maastrichtian) Dinosaur-Bearing Prince Creek Formation: Colville River Region, North Slope, Alaska

      Flaig, Peter Paul; McCarthy, Paul J. (2010)
      The Prince Creek Formation contains first-order meandering trunk channels, second-order meandering distributary channels, third-order fixed anastomosed(?) distributary channels, crevasse splays, levees, lakes, ponds, swamps, paleosols, and ashfall deposits. Trampling by dinosaurs is common. Most deposition occurred on crevasse splay-complexes adjacent to trunk channels. Rhythmically-repeating coarse-to fine-grained couplets in inclined heterolithic stratification suggest tidal-influence in channels. Cumulative to compound soils similar to Entisols, Inceptisols, and potential acid sulfate soils formed on levees, point bars, crevasse splays, and on the margins of lakes and swamps. Frequent overbank flooding is evidenced by silt and sand dispersed throughout paleosol profiles and fluctuations with depth in several molecular ratios. Drab colors, organics, siderite, depletion coatings, and zoned peds indicate waterlogged, anoxic conditions while ferruginous and manganiferous features, insect and worm burrows, and rare illuvial clay coatings and infillings suggest drying and oxidation of some soils. Repeated wetting and drying is tied to fluctuating river discharge. Marine influence is evidenced by jarosite, pyrite, and gypsum which become increasingly common up-section near the contact with the shallow-marine Schrader Bluff Formation. Pollen includes Peridinioid dinocysts; algae; projectates; Wodehouseia edmontonicola; pollen from lowland trees, shrubs and herbs; Bisaccates; fern and moss spores; and fungal hyphae and indicates that all strata are Early Maastrichtian and that sediments become progressively younger to the north. 40Ar/39Ar analysis of a tuff returned an age of 69.2 +/- 0.5 Ma. World-class dinosaur bonebeds are encased in muddy overbank alluvium overlying floodplains. No concentration of bone was found in channels. Bonebeds are laterally extensive except where truncated by distributaries. At the Sling Pont, Liscomb, and Byers bonebeds alluvium encasing bone exhibits a bipartite division of flow and a massive mudstone facies containing flow-parallel plant fragments that "float" in a mud matrix suggesting deposition by fine-grained hyperconcentrated flows. Exceptional floods driven by seasonal snowmelt in the Brooks Range increased suspended sediment concentrations, generating hyperconcentrated overbank flows that killed and buried scores of juvenile dinosaurs occupying this high-latitude coastal plain. This unique killing mechanism likely resulted from fluctuating discharge tied to extreme seasonality brought about by the near polar latitude of northern Alaska in the Late Cretaceous.
    • Depositional Environments Of The Lower Triassic Ivishak Formation In The Northeastern Brooks Range, Alaska

      Harun, Nina (1994)
      The Lower Triassic Ivishak Formation in the northeastern Brooks Range represents a progradational wave-dominated delta system. Eight distinct lithofacies were recognized based upon 27 measured sections in the Sadlerochit and Shublik Mountains and at Leffingwell and Bathtub ridges. These lithofacies record a series of progradational episodes beginning in the Kavik Shale, and continuing upward into the Ledge Sandstone Member, and the overlying Fire Creek Siltstone Member. Isopach and correlation diagrams indicate that the paleoshoreline was located in the Sadlerochit and northern Shublik Mountains area, roughly parallel to the present trend of the mountains front. The major dispersal centers were located in the Katakturuk River area of the western Sadlerockit Mountains and in the Marsh Creek area in the eastern Sadlerochit Mountains to Fire Creek area in the eastern Shublik Mountains. The Ivishak Formation contains texturally mature, very fine- to coarse-grained, light gray to light gray-blue, sublitharenite to chert arenite with an average framework: matrix:cement:porosity ratio of $\rm F\sb{75}:M\sb{<1}:C\sb{24}:P\sb1$. The Ledge Sandstone Member was derived from a recycled sedimentary provenance, with an ultimate metamorphic provenance. Compaction and late silica cementation resulted in major porosity destruction. <p>
    • Detachment folds of the northeastern Brooks Range, Alaska: A basis for geometric and kinematic models of detachment folds

      Homza, Thomas Xavier; Wallace, Wesley K.; Hanks, C. L.; Layer, P. W.; Shapiro, L. (1995)
      Detachment anticlines are defined by mechanically competent rock layers and form both by internal deformation of an adjacent weak layer and detachment above a lower competent unit. This study is important for: (1) Other fold-and-thrust belts. Detachment folds are probably very common in fold-and-thrust belts worldwide, but they are rarely recognized as such and are commonly mistaken for other fold-types. This is partly because a rigorous general model for detachment folds that allows for changes in detachment depth and for fixed-arc length kinematics is lacking in the geologic literature. A general detachment fold model is presented here that: (a) is based on observations of natural folds in the northeastern Brooks Range of Alaska; (b) does not assume constant detachment depth or hinge-migration kinematics and; (c) allows quantification of non-plane strain. The folds observed can be modeled kinematically as fixed-hinge buckle folds, whereas the fold geometry and distribution of strain indicators in each fold precludes the migrating-hinge kinematic interpretation that is common in published models. Layer-parallel shortening, initial fold asymmetry, initial stratigraphic thickness of the incompetent unit, and the nature of rheological gradations each predictably influence fold evolution. This study suggests a general scenario for the evolution of a typical detachment fold. The area defined by a detachment anticline increases rapidly during early stages of folding and this is accompanied by a decrease in depth to detachment beneath synclines and the formation of fixed-hinge parasitic and disharmonic folds. This trend continues until the interlimb angle of the primary fold reaches 90$\sp\circ$. Increased shortening requires volume-loss in the core and/or an increase in detachment depth beneath the fold. Finally, depending on the rheology of the system, the fold may lock and/or be truncated by a thrust fault. (2) Regional tectonics. The western part of the northeastern Brooks Range is mostly a passive-roof duplex, but this study shows that forward-propagating deformation occurred at various structural positions. (3) Economics. Detachment folds may form petroleum traps that require a treatment different than that for fault-bend or fault-propagation folds. Detachment fold traps may exist beneath the coastal plain of the Arctic National Wildlife Refuge.
    • Eruptive Stratigraphy And The Transport And Deposition Of Pyroclastic Material From The Caldera-Forming Eruption Of Volcan Ceboruco, Nayarit, Mexico

      Browne, Brandon Lanquist; Gardner, James (2001)
      The ~1000 A.D. eruption of Volcan Ceboruco produced 3--4 km<super>3</super> of rhyodacitic to dacitic magma erupted as the Jala Pumice, forming a 3.7-km-wide caldera. The tephra sequence consists of alternating Plinian fall and pyroclastic surge deposits, and a series of lithic-rich pyroclastic flow deposits. The latter are known as the Marquesado and North-Flank Pyroclastic Flow deposits. Stratigraphic evidence and results of a comprehensive analysis of accidental lithic and pumice populations constrain the timing of caldera collapse, and the emplacement of the pyroclastic flow deposits to the end of the Jala Pumice. Mass accumulation calculations of particles from fall layers display sedimentation patterns consistent with deposition from dilute and turbulent currents with low particle concentration. Pyroclastic flow and surge deposits likely resulted from density-stratified currents, with a basal region of relatively high particle concentration, and an upper dilute layer that transports particles in turbulent suspension. <p>
    • Evidence For A Northern Transitional Continental Margin Flora In The Cretaceous (Campanian To Maastrichtian) Matanuska Formation, Talkeetna Mountains, Southcentral Alaska

      Reid, Sabra Louise; Fowell, Sarah J. (2007)
      The Late Cretaceous Matanuska Formation contains shallow and deep marine sediments and nonmarine sediment derived from the Talkeetna volcanic island arc. The sediment accumulated in the Matanuska Seaway, a tectonically active basin on the southern margin of southcentral Alaska. The Matanuska Seaway was contemporaneous with the Cretaceous Western Interior Seaway (CWIS) of North America. Angiosperm pollen taxa from the CWIS have been used to date units and reconstruct both paleolatitude and paleoclimate (Nichols and Sweet, 1993). Comparison of pollen taxa from the CWIS to assemblages from the Matanuska Formation reveals that outcrops at Mazuma Creek, Granite Creek, Syncline Mountain, and Slide Mountain are Late Maastrichtian, while Hicks Creek outcrops are Campanian. During the Late Maastrichtian, the Matanuska Seaway was located south of 75&deg; N latitude. The presence of ash layers overlain by low-diversity palynofloras with relatively high proportions of spores indicates the presence of volcanic recovery floras within the Matanuska Formation. Palynofloral composition and diversity of the climax vegetation suggest that the Matanuska paleoflora is a northern, transitional, continental margin flora that shares taxa with Late Cretaceous Pacific Rim floras of the Russian Far East and Japan and continental margin floras of western North America. When combined with coeval assemblages from the Alaska Peninsula, southcentral Alaskan palynofloras of the Late Cretaceous contain the most diverse assemblages of Aquilapollenites group taxa known from the North Pacific Rim. The Matanuska Seaway thus represents a coastal dispersal corridor where floras of the North Pacific Rim and western North American mingled. Within Alaska, the diversity of Late Cretaceous Aquilapollenites group taxa increases from north to south. Comparison of the Matanuska Formation palynoflora with assemblages from the interior Lower Cantwell Formation and the Arctic Prince Creek Formation reveals the presence of a north-south paleoecological and paleoclimatic gradient during the Campanian-Maastrichtian. The Matanuska Formation paleoflora suggests that the paleoclimate of southcentral Alaska was warm and humid. Assemblages from the interior Lower Cantwell Formation paleoflora are indicative of a warm, dry paleoclimate, while palynofloras from the Arctic Prince Creek Formation paleoflora record a cooler, more temperate paleoclimate on the North Slope.
    • Evolution of late Cretaceous-early Tertiary depositional sequences in the Beaufort-Mackenzie Basin, Canada

      Myers, Mark D. (1994)
      The Maastrichtian to Lower Eocene rocks of the Fish River and Aklak sequences of Arctic Canada's Beaufort-Mackenzie basin were deposited during the northward migration of a fold and thrust belt across a north-facing passive margin. Detailed outcrop analysis performed in this study has resulted in delineation of the depositional processes, environments and history of systems tracts contained within the Fish River sequence. The Fish River sequence contains Maastrichtian to Lower Paleocene rocks of the Tent Island and Moose Channel Formations of the Fish River Group. This 1900 m thick terrigenous clastic succession consists of channelized conglomerate, sandstone and siltstone overlain by mudstone and capped by a large scale coarsening-upward package of interbedded mudstone, sandstone and conglomerate. Regional unconformities separate the Fish River Group from both the underlying shale of the Cenomanian to Turonian Boundary Creek Formation and the overlying conglomerate, sandstone, mudstone, and coal of the Paleocene to Eocene Aklak Member of the Reindeer Formation. The Fish River Group records a systematic vertical succession of depositional environments including submarine canyon, slope to outer shelf, prodelta, delta front and lower and upper delta plain. The succession of paleoenvironments, regional stratigraphic correlations and relationships, thickness and age control, and bounding unconformities suggest that the Fish River Group records the evolution of a complete type-1, second order, depositional sequence. The Cuesta Creek Member at the base of the Fish River sequence consists of stacked fining-upward channel fills consisting of conglomerate, sandstone and siltstone. The channel fills contain a complex assemblage of high and low density turbidites, current deposits, debris flows, slumps, and slide blocks. These deposits record the complex fill history of a coarse-grained lowstand submarine canyon system. The internal stratigraphic organization of the Fish River sequence records alternating periods of uplift and subsidence in the Beaufort-Mackenzie basin. The scale and timing of these episodes, combined with the environments of deposition and lithologic character of the rocks, suggests that the driving mechanism was alternating episodes of subsidence due to thrusting followed by flexural rebound.