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At least 415 records · Page 23Linked to original sources

USGS2011

Playa-lake sedimentation and organic matter accumulation in an Andean piggyback basin: The recent record from the Cuenca de Pozuelos, NW Argentina

Expansive playa‐lake systems situated in high‐altitude piggyback basins are important and conspicuous components of both modern and ancient cordilleran orogenic systems. Extant playa lakes provide vital habitat for numerous endemic species, whereas sediments from these deposystems may record signals of climate change or develop natural resources over geological time. Laguna de los Pozuelos (North‐west Argentina) provides the opportunity for an actualistic sedimentological and geochemical assessment of a piggyback basin playa lake in an area of critical interest for understanding Quaternary palaeoclimate dynamics. Silty clays and diatom ooze are the dominant playa‐lake centre microfacies, with concentrations of total organic carbon and biogenic silica commonly exceeding 1·5 wt% in this sub‐environment. Elemental and stable isotopic analyses point to a mixed organic matter composition in the playa‐lake centre, with substantial contributions from algae and transported aquatic macrophytes. Bulk sediment and organic mass accumulation rates in the southern playa‐lake centre approach 0.22g cm −2 year −1 and 2.89 mg cm −2 year −1 , respectively, indicating moderately rapid deposition with negligible deflation over historic time. Playa margin facies contain higher percentages of fragmented biogenic carbonate (ostracods and charophytes) and inorganically precipitated aragonite crusts due to seasonal pumping and evaporation of ground water. Organic matter accumulation is limited along these heavily bioturbated wet and dry mud flats. Fluvial–lacustrine transitional environments, which are key waterbird habitats, are either silty terminal splay (northern axis) or sandy deltas (southern axis) containing highly oxidized and partially allochthonous organic matter. Modern analogue data from Laguna de los Pozuelos provide key insights for: (i) environmental reconstructions of ancient lake sequences; and (ii) improving facies models for piggyback basins.

Cuenca de Pozuelos↗

USGS1999

The origin and paleoecologic significance of the trace fossil Asteriadtes in the Pennsylvanian of Kansas and Missouri

The trace fossil Asteriacites , recorded in Cambrian to Recent shallow- and deep-marine facies, is traditionally interpreted as the resting trace of asterozoans. Well-preserved specimens of A. lumbricalis are abundant in Pennsylvanian (Upper Carboniferous) shallow- and marginal-marine siliciclastic deposits of eastern Kansas and western Missouri. Detailed morphologic analysis of these specimens suggests that they record the activities of mobile epifaunal ophiuroids. Evidence of a brittle star (ophiuroid) producer rather than sea star (asteroid) is provided by (1) trace-fossil morphologic features reflecting the anatomy of the producer (e.g., well-differentiated central structure, slender vermiform arms) and ophiuroid burrowing technique (e.g., proximal arm expansion, arm branching), and (2) mode of occurrence (e.g., gregarious behavior, horizontal and vertical repetition). Vertical and horizontal repetition produces complex aggregates of A. lumbricalis that are interpreted either as escape structures (fugichnia) or as feeding structures, respectively. Ophiura texturata is proposed as a modern analogue for the A. lumbricalis producer, based on inferred life habit and feeding behavior. Asteriacites lumbricalis is present in two different intertidal trace-fossil assemblages. The first assemblage is characterized by high diversity and records tidal flats developed outside of embayments under normal marine conditions. The second assemblage consists of A. lumbricalis together with a few other ichnotaxa and represents a depauperate association that developed in restricted tidal flats within an embayment or estuarine setting. This challenges the conventional view of Asteriacites as a normal-marine salinity indicator. Some echinoderms, and particularly asterozoans, penetrate and inhabit modern environments of depressed salinity. The presence of Asteriacites in Pennsylvanian marginal-marine facies of Kansas and Missouri provides evidence that ophiuroids had adapted to brackish-water conditions by the late Paleozoic.

Kansas, Missouri↗

USGS2024

Deformation-induced graphitization and muscovite recrystallization in a ductile fault zone

A suite of slate samples collected along a 2 km transect crossing the Lishan fault in central Taiwan were evaluated to assess the role of ductile deformation in natural graphitization at lower greenschist facies metamorphic conditions. The process of natural aromatization, or graphitization, of an organic precursor is well established as a thermally driven process; however, experimental studies have shown that the energy provided by deformation can substantially reduce the activation energy required for graphitization. This study provides a natural example of deformation-induced graphitization. A strain gradient approaching the Lishan fault was established by scanning electron microscope imaging and X-ray diffraction analysis of phyllosilicates and quartz that showed an increase in the strength of slaty cleavage development via dissolution-precipitation processes. Thermal conditions were constrained to be near isothermal using calcite-dolomite geothermometry. Raman spectroscopic results from carbonaceous material, including D1-full width-at-half-maximum (FWHM), G-FWHM, Raman band separation (RBS), and a lesser-known vibrational mode B 2g -FWHM, showed robust linear trends across the same sampling transect. However, the G-FWHM parameter showed a trend opposite of that expected from thermally driven graphitization. The Raman results are interpreted to reflect a strain-driven reduction in graphite crystallite size (decrease in G-FWHM) but improvement in structural ordering in individual coherent domains. A multiple linear regression with an R 2 value of 0.92 predicts the graphite D1-FWHM values from the XRD-derived ratio of muscovite populations and muscovite microstrain, demonstrating the concomitant recrystallization of silicates and carbonaceous material across the strain gradient, despite the disparate processes accommodating the deformation. This study demonstrates the role of deformation in natural graphitization and provides a new perspective on the use of graphite as a geothermometer in strongly deformed greenschist facies rocks.

Journal of Metamorphic Geology↗

USGS2025

Finding the hidden orogeny – The Proterozoic polymetamorphic history of northern New Mexico

Pressure–temperature–time-deformation histories provide key constraints on orogenic processes but can be affected by later overprinting. This is exemplified in the Proterozoic orogenic belts of southwestern Laurentia where competing tectonic models involve either a single progressive Mesoproterozoic event, the Picuris orogeny, or a polyorogenic history that also includes the ~1.65 Ga Mazatzal orogeny. We address this controversy with structural analysis and petrochronology in the type locality of the Picuris orogeny. Xenotime and monazite domains associated with the greenschist-facies axial planar fabric of early F 1 folds yield 1644 ± 11 Ma (xenotime) and 1641 ± 15 Ma (monazite) ages and 450°C–482°C temperatures recording metamorphism and shortening associated with the Mazatzal orogeny. This Palaeoproterozoic greenschist-facies assemblage was overprinted by higher grade, protracted (1470–1350 Ma) tectonism that included the Picuris orogeny. Our results document the complex polyphase crustal assembly of Laurentia and highlight how petrochronology can effectively see through higher grade overprints to identify a more complete orogenic evolution.

New Mexico↗

USGS1970

Geological history of the western North Pacific

A considerable portion of the abyssal floor of the western North Pacific was already receiving pelagic sediment in late Jurassic time. Carbonate sediments were later replaced by abyssal clays as the basin deepened and bottom waters became more aggressive. The resulting facies boundary, which can be recognized on seismic profiles, is broadly transgressive; it ranges in age from mid-Cretaceous in the western Pacific to Oligocene in the central Pacific. Cherts are encountered at and below the major facies boundary and appear to have been formed by postdepositional processes.

Science↗

USGS2017

Element partitioning in magnetite under low-grade metamorphic conditions – A case study from the Proterozoic Belt Supergroup, USA

The distribution and partitioning of elements in igneous rocks is well established for various melt –(fluid) –solid pairs and provides important insights into the petrogenesis of these rocks. Studies of the partitioning behavior of elements under metamorphic conditions are scarce and commonly focus on high-grade metamorphic facies. Little is known about the partitioning behavior of elements under low-grade metamorphic conditions. Greenschist-facies metasedimentary rocks of the North American Belt Supergroup host magnetite that displays equilibrium features with co-existing mineral phases such as quartz and carbonate. Magnetite is an ideal target for geochemical investigations because it can incorporate a large number of cations and is sensitive to changes in temperature, oxygen fugacity, pressure, whole-rock composition, and cooling trends. Whole-rock major and trace element analyses have been undertaken on representative samples from Belt Supergroup metasedimentary rocks using X-ray luorescence. Electron microprobe and laser ablation ICP-MS were used to obtain major and trace element concentrations for magnetite hosted in these rocks. Stable-isotope geothermometry of magnetite–quartz and magnetite–carbonate pairs constrain metamorphic temperatures to ca. 390°C. Partition coefficients (D) for magnetite–matrix pairs presumably reflect equilibrium at these low-grade metamorphic conditions. Except for Mn and Ni, which show comparable partition coefficients, the calculated values are one to two orders of magnitude lower than those for igneous magnetite. Aluminum displays the lowest calculated partition coefficient with a value of 0.006 and Ni and Fe the highest values with 6.3 and 20.9, respectively. Of the elements that commonly occur in spinel-group minerals, two groups can be distinguished: (1) elements that preferentially partition into the host rock (D<1): Al, Mg, Pb, and Ti and (2) elements that show a preference to partition into magnetite (D>1): Zn, Mn, Cr, V, Ni, and Fe.

European Journal of Mineralogy↗

USGS2000

Stratigraphy, correlation, depositional setting, and geophysical characteristics of the Oligocene Snowshoe Mountain Tuff and Creede Formation in two cored boreholes

Core descriptions and geophysical logs from two boreholes (CCM-1 and CCM-2) in the Oligocene Snowshoe Mountain Tuff and Creede Formation, south-central Colorado, are used to interpret sedimentary and volcanic facies associations and their physical properties. The seven facies association include a mixed sequence of intracaldera ash-flow tuffs and breccias, alluvial and lake margin deposits, and tuffaceous lake beds. These deposits represent volcanic units related to caldera collapse and emplacement of the Snowshoe Mountain Tuff, and sediments and pyroclastic material deposited in the newly formed caldera basin, Early sedimentation is interpreted to have been rapid, and to have occurred in volcaniclastic fan environments at CCM-1 and in a variery of volcaniclastic fan, braided stream shallow lacustrine, and mudflat environments at CCM-2. After an initial period of lake-level rise, suspension settling, turbidite, and debris-flow sedimentation occurred in lacustrine slope and basin environments below wave base. Carbonate sedimentation was initially sporadic, but more continuous in the latter part of the recorded lake history (after the H fallout tuff). Sublacustrine-fan deposition occurred at CCM-1 after a pronounced lake-level fall and subsequent rise that preceded the H tuff. Variations in density, neutron, gamma-ray, sonic, and electrical properties of deposits penetrated oin the two holes reflect variations in lithology, porosity, and alteration. Trends in the geophysical properties of the lacustrine strata are linked to downhole changes in authigenic mineralology and a decrease in porosity interpreted to have resulted primarily from diagenesis. Lithological and geophysical characteristics provide a basis for correlation of the cores; however, mineralogical methods of correlation are hampered by the degree of diagenesis and alteration.

Colorado↗

USGS1962

Tuscaloosa formation in Tennessee

Late Cretaceous Tuscaloosa Formation occurs as discontinuous remnants that cap many of the ridges in the Western Highland Rim. Typically the formation consists of well-rounded, poorly sorted chert gravel which is trimodal in size distribution. The gravel fraction (mode 15 to 40+ mm) consists of Devonian and Mississippian chert and a small percentage of sandstone pebbles. The medium sand fraction (mode 0.5 mm) consists mainly of angular to well-rounded chert grains developed by attrition during transport. Well-rounded and frosted quartz grains also are present. The fine fraction (mode 0.15 mm) consists of clay, authi-genic (?) mica, and quartz. At its eastern limit the Tuscaloosa is locally well sorted and contains quartz pebbles and a large proportion of quartz sand. Also present in the same area are well-sorted, heavy-mineral-bearing sands and bimodal (0.04, 0.2 mm) siltstone, which contains sand-sized pellet aggregates and fragmentary plant fossils. The finest fraction (less than 0.044 mm) of both eastern and western facies of the Tuscaloosa consists of 60-80 per cent quartz, 5-30 per cent kaolin, and 5-30 per cent montmorillonite, all of which are present in Devonian and Mississippian bedrock. Minor exotic constituents include volcanic(?) glass and heavy minerals. The Mississippian chert gravel in the Tuscaloosa is of local origin, but the Devonian chert was transported from a western source. Other components from a western source are sandstone pebbles and frosted sand grains, both of which probably were derived from Cambrian or Ordovician formations that cropped out on the Pascola arch, an eastward-sloping extension of the Ozark dome. Quartz pebbles, heavy minerals, and some of the angular quartz sand present at the eastern edge of the Tuscaloosa may have been derived from Pennsylvanian sandstone and conglomerates that cropped out north and south of the Pascola arch. Most of the Tuscaloosa Formation is believed to be of nonmarine origin, deposited on the eastward-sloping flank of the Pascola arch. The eastern facies of the Tuscaloosa is believed to be partly marine in origin, the exotic components having been swept in by longshore currents. During deposition of the Tuscaloosa , the Ozark dome and the Cincinnati arch were connected by the Pascola arch, which is now deeply buried beneath the Mississippi Embayment, At that time the Pascola arch stood structurally about 3000 feet higher than at present, and its structural shape and dimensions were comparable to the present Nashville dome. During deposition of the Tuscaloosa the Nashville dome was structurally about 1000 feet lower than at present, and its crest probably was submerged beneath the sea.

Tennessee↗

USGS1969

Glacial sedimentology of the Precambrian Gowganda Formation, Ontario, Canada

The Gowganda Formation is part of the thick Huronián sequence of Precambrian sedimentary rocks that crop out in central Ontario from Lake Superior to Quebec. Although it has long been considered to be glacial, recent work on submarine slump and turbidite deposits has reopened the question of its origin. This study was made to determine its origin and paleogeography. Till-like conglomerates, varved argillites, and abundant dropstones characterize the Gowganda and provide strong evidence for a glacial origin. Pebble fabric parallel to regional paleocurrents, rare striated and grooved pavements, and abundant unweathered detritus are also in harmony with a glacial origin. Local thin-bedded sandstones contain flame structure, graded bedding, contorted bedding, and rippled tops, suggesting deposition by turbidity flows. Association of these sandstones with varved argillites and rafted stones indicates that Gowganda turbidites are glaciolacustrine. Along the southern margin of the Gowganda, at Whitefish Falls, thick, laterally continuous till-like conglomerates contain internal stratification indicative of subaqueous deposition. Argillites that lack varved structure and associated silty limestones suggest a glacial marine environment. In the Cobalt region the apparent long axes of pebbles in conglomerates show a predominant north-south alignment. In the Bruce Mines-Elliot Lake region, the orientation of the long axes of pebbles is more variable. Differences in orientation are related to stratigraphic position. Ripple cross-lamination in sandstones that are interbedded with argillite indicates south-trending paleocurrents. Distribution of varved argillite, silty limestone, and probable marine argillites suggests a northern fresh-water facies and a southern marine facies. Abundant plutonic pebbles and feldspar indicate that the Gowganda ice sheets eroded a northern plutonic terrane and deposited much of their sediment load on the underlying Huronian sediments to the south.

Ontario↗

USGS1985

The Dunbar Gneiss-granitoid dome: Implications for early Proterozoic tectonic evolution of northern Wisconsin

The Dunbar dome in northeastern Wisconsin is a critical structural feature in the early Proterozoic Penokean orogen. It provides exposures of gneisses (Dunbar Gneiss) that structurally underlie the voluminous metavolcanic rocks of northeastern Wisconsin, and exposures of abundant granitoid rocks ranging from tonalite to granite. The granitoid rocks cut both the gneisses in the core and the supracrustal (cover) metavolcanic rocks and were emplaced essentially along the core-cover boundary. The Dunbar Gneiss is calc-alkaline and was derived from volcanic and intrusive rocks of intermediate composition. The various intrusive rocks have calcic, calc-alkaline, and alkali to alkali-calcic compositions, and they progress with time to more SiO 2 and K 2 -rich compositions. U-Pb zircon ages indicate that accumulation of the layered rocks in the core and cover, deformation and metamorphism, and intrusion of the granitoid rocks spanned a relatively short time, ∼1865–1835 Ma. We interpret the dome as being a large-scale, fold-interference structure resulting from polydeformation modified by diapirism. Northeast-oriented folds (F 3 ) and a related mylonitic foliation (S 3 ), nearly confined to the dome, are superposed on northwest-oriented folds (F 2 ) that developed during regional deformation. In the core-cover boundary, these structures are obliterated by a zone of intense deformation—a mylonitic foliation and a steeply plunging stretching lineation—as much as 500 m wide, which we interpret as resulting from diapirism. Metamorphic zoning is concentric: amphibolite facies in inner parts of the mantle and greenschist facies in the outer part of the mantle. The Wisconsin magmatic terrane, as represented by the rocks in the Dunbar dome, differs from the epicratonic, early Proterozoic sedimentary-volcanic sequence (Marquette Range Supergroup) in Michigan, to the north, in stratigraphy, structure, and volume and composition of igneous rocks. Whereas the basalts in northern Michigan are compositionally similar to continental rift basalts, the volcanic rocks in the Dunbar dome have over-all island-arc compositional affinities. The over-all calc-alkaline compositions of the intrusive rocks are similar to those in magmatic arcs formed at convergent plate-margin settings. Accordingly, we interpret the Wisconsin magmatic terrane as an oceanic-arc complex that was sutured to the North American continent during development of the Penokean orogen. Similar interpretations based on broad regional observations have been proposed previously.

Michigan, Wisconsin↗

USGS1990

Pliocene-Pleistocene growth of the Rio Ebro margin, northeast Spain: A prograding-slope model

Pliocene-Pleistocene marine deposits overlying Messinian evaporites off northeast Spain are composed of five distinct seismic facies. These facies are interpreted to represent deposition in five adjacent settings: distal (isolated) basin, basin plain, base of slope,slope, and shelf. Contrary to previous studies, a submarine-fan style of deposition is not evident. Distal-basin deposits are characterized by thin, wavy, parallel beds draping the sea floor. Basin-plain deposits are similar but also have flat-lying beds and some low-relief channels. Base-of-slope deposits are dominated by stacked, overlapping, elongate channel/levee deposits that have concave-upward bases and convex-upward tops. Slope sequences are thick accumulations of overbank deposits laterally interrupted by large upward-climbing canyon fill. The shelf component is composed of overlapping and stacked delta lobes. The composite sediment body is a prograded, upward-shallowing, offlap sequence initiated and controlled by the influx of terrigenous sediment from the Rio Ebro during the Pleistocene. This style of deposition is common in the rock record, and our model may be more appropriate as a modern analogue for some rocks than are conventional shelf-canyon-fan models of margin sedimentation.

Ebro River, Gulf of Valencia, Mediterranean Sea↗

USGS1993

40Ar/39Ar thermochronology and Alleghanian development of the southernmost Appalachian Piedmont, Alabama and southwest Georgia

40 Ar/ 39 Ar age spectra of hornblende, muscovite, and microcline, and total fusion ages of biotite from metamorphic rocks of the Inner Piedmont, Pine Mountain, and Uchee belts are reported. Mineral cooling ages from the eastern part of the Inner Piedmont are as follows: hornblende, 320 Ma; muscovite, 296 Ma; biotite, 293 Ma; and microcline (diffusional release patterns) Tmax = 267 Ma, Tmin = 234 Ma. A 347 Ma hornblende spectrum from the highest Inner Piedmont structural level sampled is the oldest date determined and implies earlier passage of this level through the 500 °C isotherm. Most release spectra from Pine Mountain belt units are discordant with little or no apparent geologic meaning. Modified saddle-shaped release patterns for hornblende indicate extraneous argon with a maximum age of ∼358 Ma. Muscovite from the Pine Mountain belt cover sequence is 286 Ma (plateau age), and one from the underlying Grenville basement is 277 Ma (correlation age), indicating cooling below the 350 °C isotherm. Plateau ages on Uchee belt rocks are as follows: hornblende, from 297 to 288 Ma; muscovite, 285 Ma; biotite, 276 Ma; and microcline Tmax = 261 Ma, Tmin 230 Ma. Muscovite fish from a Bartletts Ferry fault zone phyllonite have a plateau age of 283 Ma. The 40 Ar/ 39 Ar results combined with other geologic data indicate that (1) a large part of the southern and Inner Piedmonts of Alabama and southwest Georgia experienced a late Paleozoic amphibolite-facies thermal and deformational event contemporaneous with the Alleghanian orogeny observed in the foreland; (2) the tectonic development of this event, characterized by initial crustal thickening followed by right-slip and normal-slip movements, is grossly similar to that described for the amphibolite-facies Alleghanian belt in the eastern Piedmont of South Carolina and Georgia; and (3) extensional movements along the flanks of the Pine Mountain window occurred between ca. 277 Ma and the Late Triassic-Early Jurassic and thus may reflect latest Alleghanian extensional collapse or Mesozoic rifting.

Alabama, Georgia↗

USGS1986

Crater Lake, Oregon: A restricted basin with base-of-slope aprons of nonchannelized turbidites

The basin floor of Crater Lake (10-km diameter, 600-m water depth) is covered by up to 75 m of sediment–gravity-flow deposits interbedded with mud. In the upper units (8 m (thick), sand and gravel layers with numerous wedging, strong seismic reflectors characterize the base-of-slope aprons at the basin margin. These layers evolve to turbidites of mainly thin, fine-grained, basin-plain type, characterized by numerous flat and weak seismic reflectors in the central basin floor. Many individual debris-chute sources funnel sediment to base-of-slope aprons: there, coarse-grained parts of the sediment–gravity flows deposit nonchannelized beds attributed to the F, A, B turbidite facies. While traversing the base-of-slope aprons, flows evolve to sheet-flow turbidity currents that deposit D-facies beds over the central basin floor. These processes and patterns of deposition characterize small siliciclastic basins without channelized submarine fans and are common in carbonate basins of all sizes.

Geology↗

USGS1992

Upper Pleistocene turbidite sand beds and chaotic silt beds in the channelized, distal, outer-fan lobes of the Mississippi fan

Cores from a Mississippi outer-fan depositional lobe demonstrate that sublobes at the distal edge contain a complex local network of channelized-turbidite beds of graded sand and debris-flow beds of chaotic silt. Off-lobe basin plains lack siliciclastic coarse-grained beds. The basin-plain mud facies exhibit low acoustic backscatter on SeaMARC IA sidescan sonar images, whereas high acoustic backscatter characteristic of the lobe sand and silt facies. The depth of the first sand-silt layer correlates with relative backscatter intensity and stratigraphic age of the distal sublobes (i.e., shallowest sand = highest backscatter and youngest sublobe). The high proportion (>50%) of chaotic silt compared to graded sand in the distal, outer-fan sublobes may be related to the unstable, muddy, canyon-wall source areas of the extensive Mississippi delta-fed basin slope. A predominace of chaotic silt in cores or outcrops from outer-fan lobes thus may predict similar settings for ancient fans.

Mississippi Fan↗

USGS2008

Devonian brachiopods of southwesternmost laurentia: Biogeographic affinities and tectonic significance

Three brachiopod faunas discussed herein record different depositional and tectonic settings along the southwestern margin of Laurentia (North America) during Devonian time. Depositional settings include inner continental shelf (Cerros de Los Murcielagos), medial continental shelf (Rancho Placeritos), and offshelf continental rise (Rancho Los Chinos). Ages of Devonian brachiopod faunas include middle Early (Pragian) at Rancho Placeritos in west-central Sonora, late Middle (Givetian) at Cerros de Los Murcielagos in northwestern Sonora, and late Late (Famennian) at Rancho Los Chinos in central Sonora. The brachiopods of these three faunas, as well as the gastropod Orecopia, are easily recognized in outcrop and thus are useful for local and regional correlations. Pragian brachiopods dominated by Acrospirifer and Meristella in the "San Miguel Formation" at Rancho Placeritos represent the widespread Appohimchi Subprovince of eastern and southern Laurentia. Conodonts of the early to middle Pragian sulcatus to kindlei Zones associated with the brachiopods confirm the ages indicated by the brachiopod fauna and provide additional information on the depositional setting of the Devonian strata. Biostratigraphic distribution of the Appohimchi brachiopod fauna indicates continuous Early Devonian shelf deposition along the entire southern margin of Laurentia. The largely emergent southwest-trending Transcontinental arch apparently formed a barrier preventing migration and mixing of many genera and species of brachiopods from the southern shelf of Laurentia in northern Mexico to the western shelf (Cordilleran mio-geocline) in the western United States. Middle Devonian Stringocephalus brachiopods and Late Devonian Orecopia gastropods in the "Los Murcielagos Formation" in northwest Sonora represent the southwest-ernmost occurrence of these genera in North America and date the host rocks as Givetian and Frasnian, respectively. Rhynchonelloid brachiopods (Dzieduszyckia sonora) and associated worm tubes in the Los Pozos Formation of the Sonora allochthon in central Sonora are also found in strati-form-barite facies in the upper Upper Devonian (Famennian) part of the Slaven Chert in the Roberts Mountains allochthon (upper plate) of central and western Nevada. Although these brachiopods and worm tubes occur in similar depositional settings along the margin of Laurentia in Mexico, they occur in allochthons that exhibit different tectonic styles and times of emplacement. Thus, the allochthons containing the brachiopods and worm tubes in Sonora and Nevada are parts of separate orogenic belts and have different geographic settings and tectonic histories. Devonian facies belts and faunas in northern Mexico indicate a continuous continental shelf along the entire southern margin of Laurentia. These data, in addition to the continuity of the late Paleozoic Ouachita-Marathon-Sonora orogen across northern Mexico, contradict the early Late Jurassic Mojave-Sonora megashear as a viable hypothesis for large-magnitude offset (600-1100 km) of Proterozoic through Middle Jurassic rocks from California to Sonora.

Special Paper of the Geological Society of America↗

USGS2013

Geophysical constraints on Rio Grande rift structure and stratigraphy from magnetotelluric models and borehole resistivity logs, northern New Mexico

Two- and three-dimensional electrical resistivity models derived from the magnetotelluric method were interpreted to provide more accurate hydrogeologic parameters for the Albuquerque and Española Basins. Analysis and interpretation of the resistivity models are aided by regional borehole resistivity data. Examination of the magnetotelluric response of hypothetical stratigraphic cases using resistivity characterizations from the borehole data elucidates two scenarios where the magnetotelluric method provides the strongest constraints. In the first scenario, the magnetotelluric method constrains the thickness of extensive volcanic cover, the underlying thickness of coarser-grained facies of buried Santa Fe Group sediments, and the depth to Precambrian basement or overlying Pennsylvanian limestones. In the second scenario, in the absence of volcanic cover, the magnetotelluric method constrains the thickness of coarser-grained facies of buried Santa Fe Group sediments and the depth to Precambrian basement or overlying Pennsylvanian limestones. Magnetotelluric surveys provide additional constraints on the relative positions of basement rocks and the thicknesses of Paleozoic, Mesozoic, and Tertiary sedimentary rocks in the region of the Albuquerque and Española Basins. The northern extent of a basement high beneath the Cerros del Rio volcanic field is delineated. Our results also reveal that the largest offset of the Hubbell Spring fault zone is located 5 km west of the exposed scarp. By correlating our resistivity models with surface geology and the deeper stratigraphic horizons using deep well log data, we are able to identify which of the resistivity variations in the upper 2 km belong to the upper Santa Fe Group sediment

New Mexico↗

USGS2014

Late Devonian–Mississippian(?) Zn-Pb(-Ag-Au-Ba-F) deposits and related aluminous alteration zones in the Nome Complex, Seward Peninsula, Alaska

Stratabound base-metal sulfide deposits and occurrences are present in metasedimentary rocks of the Neoproterozoic and Paleozoic Nome Complex on south-central Seward Peninsula, Alaska. Stratabound and locally stratiform deposits including Aurora Creek (Zn-Au-Ba-F), Wheeler North (Pb-Zn-Ag-Au-F), and Nelson (Zn-Pb- Cu-Ag), consist of lenses typically 0.5–2.0 m thick containing disseminated to semimassive sulfides. Host strata of the Aurora Creek and Wheeler North deposits are variably calcareous and graphitic siliciclastic metasedimentary rocks of Middle Devonian or younger age based on detrital zircon geochronology; the Nelson deposit is within Ordovician–Devonian marble (Till et al., this volume, Chapter 4). Deformed veins such as Quarry (Zn-Pb-Ag-Ba-F) and Galena (Pb-Zn-Ag-F) occur in a unit composed mainly of marble and schist; fossil and detrital zircon data indicate that this unit contains rocks of Ordovician, Silurian, and Devonian age. None of these Zn- and Pbrich deposits or occurrences has spatially associated metavolcanic or intrusive rocks. All were deformed and metamorphosed to blueschist facies and then retrograded to greenschist facies during the Jurassic and Early Cretaceous Brookian orogeny. Disseminated Cu-rich deposits including Copper King (Cu-Bi-Sb-Pb-Ag-Au) and Wheeler South (Cu-Ag-Au) occur in silicified carbonate rocks and have textures that indicate a pre- to syn-metamorphic origin. The Zn- and Pb-rich sulfide deposits and occurrences consist mainly of pyrite, sphalerite, and/or galena in a gangue of quartz and carbonate. Minor minerals include arsenopyrite, chalcopyrite, magnetite, pyrrhotite, tetrahedrite, barite, fluorite, and chlorite; gold and electrum are trace to minor constituents locally. Sphalerite is uniformly unzoned and commonly aligned in the dominant foliation. These textures, together with the presence of folded layers of barite at Aurora Creek and folded sulfi de layers at Wheeler North, indicate that mineralization in the stratabound deposits predated deformation and metamorphism. Electron microprobe (EMP) analyses of the carbonate gangue show three major compositions comprising siderite, ankerite, and lesser dolomite. The Cu-rich deposits differ in containing chalcopyrite and bornite in a quartzose matrix. Altered wall rocks surrounding the Zn- and Pb-rich deposits and occurrences have aluminous assemblages composed of muscovite + chloritoid + siderite + chlorite + quartz ± tourmaline ± ilmenite ± apatite ± monazite. Muscovite within these assemblages and in sulfide-rich samples is phengitic and locally enriched in barium; chloritoid at Aurora Creek is enriched in zinc. Minor minerals including pyrite, sphalerite, galena, chalcopyrite, barite, and hyalophane occur as fine-grained disseminations. These altered rocks vary from small lenses a few meters thick to large zones tens of meters in thickness that extend along strike, discontinuously, for 4 km or more. Whole-rock geochemical analyses of the altered rocks from deposit-proximal and deposit-distal settings reveal generally lower SiO 2 /Al 2 O 3 ratios and higher Fe 2 O 3 T /MgO ratios compared to those of unaltered clastic metasedimentary rocks of the Nome Complex and of average shale or graywacke. The deposit-proximal samples are also characterized by anomalously high Zn, Pb, Hg, and Sb, relative to the unaltered metasediments. These data, together with mass change calculations, suggest that the aluminous rocks formed as replacements of permeable graywacke in semi-conformable alteration zones, beneath the seafloor contemporaneously with Zn-and/or Pb-rich sulfide mineralization. Exposures of all three stratabound Zn-Pb deposits show evidence of deformation and recrystallization that occurred in a largely brittle deformational regime. This evidence includes small faults and veins that cut foliation and localized zones of breccia. Sulfide minerals, fluorite, quartz, chlorite, and carbonate minerals crystallized within these structures, which probably formed during Cretaceous deformation of the Nome Complex. Previous studies of the Zn-Pb(-Ag-Au-Ba-F) deposits and occurrences have invoked models of epigenetic veins, volcanogenic massive sulfides (VMS), or carbonate- replacement deposits (CRD). In contrast, our field and laboratory data (including sulfur isotopes; Shanks et al., this volume) suggest that these Zn- and/or Pb-rich deposits represent different levels of sediment-hosted, seafloor-hydrothermal systems, with stratabound and locally stratiform deposits such as Aurora Creek and Wheeler North having formed on the seafloor and/or in the shallow subsurface like many sedimentary-exhalative (SEDEX) deposits worldwide. The deformed veins such as Quarry and Galena are interpreted to have formed deep in the subsurface, possibly as feeders to overlying SEDEX deposits such as Aurora Creek. Formation of all of the Zn- and Pb-rich deposits and occurrences took place during episodic rifting of the continental margin between the Ordovician and Mississippian(?). Regional relationships are consistent with at least some of the deposits having formed in Late Devonian–Mississippian(?) time.

Alaska↗

USGS2021

Perspectives on the paleolimnology of the late Eocene Florissant lake from diatom and sedimentary evidence at Clare’s Quarry, Teller County, Colorado, USA

The late Eocene Florissant Formation in central Colorado is a rich and diverse continental Lagerstätte yielding well-preserved fossil assemblages from lacustrine and fluvial facies. This investigation focused on the lacustrine facies at Clare’s Quarry and used biotic and abiotic evidence to characterize aspects of the lake and processes that resulted in the accumulation and preservation of the host rock and its fossils. Autecology of modern analogs representing the fossil diatom taxa was used to augment sedimentary data in characterizing the lake, propose peripheral habitats within the catchment area, and suggest a terrestrial source for mudstone units. The sedimentary and stratigraphic record at the study site reveals a lake with sufficient depth to allow bottom waters to remain isolated and anoxic for long periods. Sediments that accumulated in the lake produced distinct lacustrine lithofacies that are interpreted as representing at least three modes of origin: stable lake, pyroclastic, and mud turbidite sedimentation. Slow, suspension settling of fine clays and volcanic ash into a moderately deep, stable lake resulted in laminated shales. These laminated shales contain frustules of diatoms from planktic and benthic lake habitats; diatoms transported into the lake from streams and wetlands; fish, mollusks, ostracods, and insects; and plants from marginal and upslope environments. Intermittent volcanic eruptions produced air-fall ash and granular tuff that accumulated as interbeds within the lake shales. Periods of stable lake sedimentation were frequently interrupted by rapid influxes of suspended fine clays, perhaps as mud-dominated turbidites that prograded into the lake at intervals of high runoff triggered by climatic, volcanic, or tectonic events.

Colorado↗