SEARCH · Search USGS

Results for “Facies”

Search indexed USGS publications on groundwater, aquifers, geologic maps, mineral resources and earthquakes. Explore source records by subject and place.

SourceResearch connectionFrom yearThrough year

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 235 records · Page 13Linked to original sources

USGS2005

Stratigraphy and sedimentology of a dry to wet eolian depositional system, Burns formation, Meridiani Planum, Mars

Outcrop exposures of sedimentary rocks at the Opportunity landing site (Meridiani Planum) form a set of genetically related strata defined here informally as the Burns formation. This formation can be subdivided into lower, middle, and upper units which, respectively, represent eolian dune, eolian sand sheet, and mixed eolian sand sheet and interdune facies associations. Collectively, these three units are at least 7 m thick and define a “wetting-upward” succession which records a progressive increase in the influence of groundwater and, ultimately, surface water in controlling primary depositional processes. The Burns lower unit is interpreted as a dry dune field (though grain composition indicates an evaporitic source), whose preserved record of large-scale cross-bedded sandstones indicates either superimposed bedforms of variable size or reactivation of lee-side slip faces by episodic (possibly seasonal) changes in wind direction. The boundary between the lower and middle units is a significant eolian deflation surface. This surface is interpreted to record eolian erosion down to the capillary fringe of the water table, where increased resistance to wind-induced erosion was promoted by increased sediment cohesiveness in the capillary fringe. The overlying Burns middle unit is characterized by fine-scale planar-laminated to low-angle-stratified sandstones. These sandstones accumulated during lateral migration of eolian impact ripples over the flat to gently undulating sand sheet surface. In terrestrial settings, sand sheets may form an intermediate environment between dune fields and interdune or playa surfaces. The contact between the middle and upper units of the Burns formation is interpreted as a diagenetic front, where recrystallization in the phreatic or capillary zones may have occurred. The upper unit of the Burns formation contains a mixture of sand sheet facies and interdune facies. Interdune facies include wavy bedding, irregular lamination with convolute bedding and possible small tepee or salt-ridge structures, and cm-scale festoon cross-lamination indicative of shallow subaqueous flows marked by current velocities of a few tens of cm/s. Most likely, these currents were gravity-driven, possibly unchannelized flows resulting from the flooding of interdune/playa surfaces. However, evidence for lacustrine sedimentation, including mudstones or in situ bottom-growth evaporites, has not been observed so far at Eagle and Endurance craters. Mineralogical and elemental data indicate that the eolian sandstones of the lower and middle units, as well as the subaqueous and eolian deposits of the Burns upper unit, were derived from an evaporitic source. This indirectly points to a temporally equivalent playa where lacustrine evaporites or ground-water-generated efflorescent crusts were deflated to provide a source of sand-sized particles that were entrained to form eolian dunes and sand sheets. This process is responsible for the development of sulfate eolianites at White Sands, New Mexico, and could have provided a prolific flux of sulfate sediment at Meridiani. Though evidence for surface water in the Burns formation is mostly limited to the upper unit, the associated sulfate eolianites provide strong evidence for the critical role of groundwater in controlling sediment production and stratigraphic architecture throughout the formation.

Earth and Planetary Science Letters↗

USGS1999

Organic geochemistry in Pennsylvanian tidally influenced sediments from SW Indiana

Tidal rhythmites are vertically stacked small-scale sedimentary structures that record daily variations in tidal current energy and are known to overlie some low-sulfur coals in the Illinois Basin. Tidal rhythmites from the Pennsylvanian Brazil Formation in Indiana have been analyzed sedimentologically, petrographically, and geochemically in order to understand the character and distribution of organic matter (OM) preserved in an environment of daily interactions between marine and fresh waters. The concentration of organic matter (TOC) ranges from traces to 6.9% and sulfur rarely exceeds 0.1% in individual laminae. Angular vitrinite is the major organic matter type, accounting for 50-90% of total OM. The C/S ratio decreases as the verfical distance from the underlying coal increases. A decreasing C/S ratio coupled with decreases in Pr/Ph, Pr/n-C17, Ph/n-C18 ratios and a shift of carbon isotopic composition towards less negative values suggest an increase in salinity from freshwater in the mudflat tidal rhythmite facies close to the coal to brackish/marine in the sandflat tidal rhythmite facies further above from the coal. Within an interval spanning one year of deposition, TOC and S values show monthly variability. On a daily scale, TOC and S oscillations are still detectable but they are of lower magnitude than on a monthly scale. These small-scale variations are believed to reflect oscillations in water salinity related to tidal cycles.Tidal rhythmites are vertically stacked small-scale sedimentary structures that record daily variations in tidal current energy and are known to overlie some low-sulfur coals in the Illinois Basin. Tidal rhythmites from the Pennsylvanian Brazil Formation in Indiana have been analyzed sedimentologically, petrographically, and geochemically in order to understand the character and distribution of organic matter (OM) preserved in an environment of daily interactions between marine and fresh waters. The concentration of organic matter (TOC) ranges from traces to 6.9% and sulfur rarely exceeds 0.1% in individual laminae. Angular vitrinite is the major organic matter type, accounting for 50-90% of total OM. The C/S ratio decreases as the vertical distance from the underlying coal increases. A decreasing C/S ratio coupled with decreases in Pr/Ph, Pr/n-C17, Ph/n-C18 ratios and a shift of carbon isotopic composition towards less negative values suggest an increase in salinity from freshwater in the mudflat tidal rhythmite facies close to the coal to brackish/marine in the sandflat tidal rhythmite facies further above from the coal. Within an interval spanning one year of deposition, TOC and S values show monthly variability. On a daily scale, TOC and S oscillations are still detectable but they are of lower magnitude than on a monthly scale. These small-scale variations are believed to reflect oscillations in water salinity related to tidal cycles.

Organic Geochemistry↗

USGS2021

Late Pleistocene baldcypress (Taxodium distichum) forest deposit on the continental shelf of the northern Gulf of Mexico

Approximately 13 km south of Gulf Shores, Alabama (United States), divers found in situ baldcypress ( Taxodium distichum ) stumps 18 m below the ocean surface. These trees could have only lived when sea level fell during the Pleistocene subaerially exposing the tectonically stable continental shelf. Here we investigate the geophysical properties along with microfossil and stratigraphical analyses of sediment cores to understand the factors that lead to this wood’s preservation. The stumps are exposed in an elongated depression (~100 m long, ~1 m deep) nested in a trough of the northwest–southeast trending Holocene sand ridges and troughs with 2–5 m vertical relief and ~0.5 km wavelength. Radiocarbon ages of the wood were infinite thus optically stimulated luminescence (OSL) dating was used to constrain the site’s age. Below the Holocene sands (~0.1–4 m thick), separated by a regional erosional unconformity, are Late Pleistocene mud-peat (72±8 ka OSL), mud-sand (63±5, 73±6 ka OSL), and palaeosol (56±5 ka OSL) facies that grade laterally from west to east, respectively. Foraminiferal analysis reveals the location of the terrestrial-marine transitional layer above the Pleistocene facies in an interbedded sand and mud facies (3940±30 (1σ) 14 C a BP), which is part of a lower shoreface or marine-dominated estuarine environment. The occurrence of palaeosol and swamp facies of broadly similar ages and elevation suggests the glacial landscape possessed topographic relief that allowed wood, mud and peats to be preserved for ~50 ka of subaerial exposure before transitioning to the modern marine environment. We hypothesize that rapid sea-level rise occurring ~60 or ~40 ka ago provided opportunities for local flood-plain aggradation to bury the swamp thus preserving the stumps and that other sites may exist in the northern Gulf of Mexico shelf.

Alabama, Florida, Louisiana, Mississippi↗

USGS2024

Deep-water first occurrences of Ediacara biota prior to the Shuram carbon isotope excursion in the Wernecke Mountains, Yukon, Canada

Ediacara-type macrofossils appear as early as ~575 Ma in deep-water facies of the Drook Formation of the Avalon Peninsula, Newfoundland, and the Nadaleen Formation of Yukon and Northwest Territories, Canada. Our ability to assess whether a deep-water origination of the Ediacara biota is a genuine reflection of evolutionary succession, an artifact of an incomplete stratigraphic record, or a bathymetrically controlled biotope is limited by a lack of geochronological constraints and detailed shelf-to-slope transects of Ediacaran continental margins. The Ediacaran Rackla Group of the Wernecke Mountains, NW Canada, represents an ideal shelf-to-slope depositional system to understand the spatiotemporal and environmental context of Ediacara-type organisms' stratigraphic occurrence. New sedimentological and paleontological data presented herein from the Wernecke Mountains establish a stratigraphic framework relating shelfal strata in the Goz/Corn Creek area to lower slope deposits in the Nadaleen River area. We report new discoveries of numerous Aspidella hold-fast discs, indicative of frondose Ediacara organisms, from deep-water slope deposits of the Nadaleen Formation stratigraphically below the Shuram carbon isotope excursion (CIE) in the Nadaleen River area. Such fossils are notably absent in coeval shallow-water strata in the Goz/Corn Creek region despite appropriate facies for potential preservation. The presence of pre-Shuram CIE Ediacara-type fossils occurring only in deep-water facies within a basin that has equivalent well-preserved shallow-water facies provides the first stratigraphic paleobiological support for a deep-water origination of the Ediacara biota. In contrast, new occurrences of Ediacara-type fossils (including juvenile fronds, Beltanelliformis , Aspidella , annulated tubes, and multiple ichnotaxa) are found above the Shuram CIE in both deep- and shallow-water deposits of the Blueflower Formation. Given existing age constraints on the Shuram CIE, it appears that Ediacaran organisms may have originated in the deeper ocean and lived there for up to ~15 million years before migrating into shelfal environments in the terminal Ediacaran. This indicates unique ecophysiological constraints likely shaped the initial habitat preference and later environmental expansion of the Ediacara biota.

Geobiology↗

USGS1945

Stratigraphy and structure of west-central Vermont

The lithologic units recognizable in the fossiliferous succession along southern Lake Champlain are structurally continuous with and traceable eastward into the “marble belt” of west-central Vermont immediately west of the Green Mountain Front. They are also traceable northward through west-central Vermont into a succession in northwestern Vermont bounded on the east and west by major thrusts, where they pass laterally northeastward into fossiliferous shales, the faunal zones of which are correlated with those along southern Lake Champlain and in the Hudson and Mohawk valleys. The Cambrian strata are traced into west-central from northwestern Vermont whereas the Ordovician correlation is with rocks in the Mohawk-Hudson-Champlain region. Certain of the Upper Cambrian strata can be correlated with established formations of this age in both of the outlying areas. The structural pattern reflects movements dependent upon the original distribution of sedimentary facies. Interbedded Carnbro-Ordovician limestones and dolomites grade westward into foreland sandstones and eastward into geosynclinal shales. Cambrian and early Ordovician sandstone tongues extend far to the east. Later Ordovician strata of the shale facies overlie the calcareous and sandy succession and are locally unconformable on it. In the Taconic Range allochthonous Cambrian strata of shale facies are superposed upon the autochthonous Ordovician beds. The rocks of the klippe were derived from a zone at least 50 miles east of the present westernmost exposures, arriving there by movements confined largely to the shale facies. Flexural folding and thrusting, effects of alternating competency and incompetency of the foreland sequence, affected the latter succession and possibly the Taconic Allochthone. Breaking of competent strata in the flexures initiated the thrusts; thrusting continued by the stripping of competent from incompetent beds. This was accompanied by counterclockwise rotation of the thrust slices around pivotal zones bordering on a foreland massif to the southwest. Thus rock cropping out in the north-south thrust slices originally extended northeast-southwest. The thrusts are concentric to the Adirondack crystallines and are cut by the Adirondack normal faults; probably they were warped during normal faulting and uplift of the Adirondacks.

Vermont↗

USGS1962

Hornblendes formed during progressive metamorphism of amphibolites, northwest Adirondack Mountains, New York

Hornblendes in amphibolite interlayers in the paragneiss of the northwest Adirondack Mountains undergo systematic changes in color, composition, and density during progressive metamorphism from almandine- amphibolite to hornblende -granulite facies. In contrast, indices of refraction of the hornblendes remain about constant. In the almandine- amphibolite facies the amphibolite layers have the bulk composition of a saturated basalt and consist of bluish-green hornblende , andesine, and quartz. As these layers are traced into the hornblende -granulite facies, their composition undergoes a progressive change to that of an olivine basalt with brownish-green hornblende , clinopyroxene and orthopyroxene, and calcic andesine as major constituents. Compositional changes in the hornblendes with increasing grade of metamorphism include increases in Ti, Na, K, Cr, V, and Sc. Decreases occur in the amounts of Mn, Zn, OH + F + Cl, and in the ratios Fe 2 O 3 /FeO and Fe/Mg. Density of the hornblendes increases from 3.260 to 3.278 with the increasing grade of metamorphism . These changes in the hornblendes with increasing T and P, although well denned, are less pronounced than those measured in biotites and garnets of the enclosing paragneiss. Large variations in the physical and chemical properties of hornblendes in metamafic rocks reconstituted above the epidote- amphibolite facies appear to be induced principally by critical changes in the bulk composition of the total rock, and not by the regional gradients in T, P, or by changes in kind, or composition, of the coexisting minerals.

New York↗

USGS1975

Chino Valley formation (Cambrian?) in northwestern Arizona

A thin persistent unit (maximum thickness 13 m) of probable Cambrian age in the Chino Valley region of northwestern Arizona consists of three laterally equivalent, mutually exclusive lithofacies. These are, from west to east, lithic sandstone, pebble to boulder conglomerate, and dolomite. The unit, named the Chino Valley Formation, is younger than middle Middle Cambrian and older than Devonian in age. A Late Cambrian age is probable, although no fossil evidence supports this conclusion. The lithic sandstone and conglomerate facies were derived from two apparently active source areas and were probably deposited in shallow marine water. Uplift to the south or southwest of the area almost reversed the regionally westward-dipping Cambrian paleoslope and exposed sedimentary rocks to erosion. Sediment derived from this terrain formed the lithic sandstone facies. Synchronous movement initiated erosion of the Mazatzal Quartzite (Precambrian) from which the conglomerate facies was derived. The dolomite facies is inferred to have accumulated on a mud flat in the supratidal zone.

Arizona↗

USGS1982

Mid-Paleozoic age of the Roberts thrust unsettled by new data from northern Nevada

The Roberts thrust is a major thrust in Nevada on which Ordovician to Devonian siliceous facies rocks were carried more than 80 km eastward over contemporaneous carbonate facies. For more than two decades, a mid-Paleozoic age for this structure has been widely accepted. The bases for dating the thrust are (1) the assumption that the Roberts thrust formed at the time of the Antler orogeny—a reliably dated mid-Paleozoic event, and (2) the concept of an overlapping assemblage consisting of sedimentary strata deposited on both the allochthon and autochthon. The first assumption is unproved, and the second relies on another assumption: that the overlap assemblage was not cut and telescoped by the Roberts thrust. In the years since a mid-Paleozoic age for the Roberts thrust became widely accepted, much new information on the stratigraphy and facies relations of Mississippian rocks in northern Nevada has accumulated. One interpretation of these new data suggests that in the Pinon Range, where a mid-Paleozoic age for the thrust was most convincingly displayed, the overlap assemblage actually may have been cut by a major thrust that juxtaposed contrasting facies of Mississippian rocks. If so, the principal evidence used to date the Roberts thrust is compromised, and the time has come for an agonizing reappraisal of all evidence bearing on the question. A mid-Paleozoic age can no longer be taken for granted, and a post-Paleozoic age cannot be ruled out.

Nevada↗

USGS1993

Exhumation of eclogitized continental basement during Variscan lithospheric delamination and gravitational collapse, Sudety Mountains, Poland

A Variscan,deep-crustal-level (eclogite-facies),continental basement massif in western Poland, the Snieznik complex, was tectonically exhumed. Crustal-penetrating mylonite zones record three main kinematic events: early top-to-the-north-directed thrusting, right-slip transpression-tension, and late top-to-the-south and -east normal faulting. Thrusting resulted in extreme crustal thickening and associated eclogite-facies metamorphism. Right-slip movements produced retrogressive crystal-plastic simple-shear zones. Normal faults flank Carboniferous to Early Permian terrigenous sedimentary basins, documenting tectonic and erosional denudation of the Snieznik complex during lithospheric extension. Sm/Nd isotopic dates previously reported for the in situ eclogite-facies metamorphic mineral assemblages are 341, 337, and 329 Ma (Brueckner et al., 1991). 40 Ar/ 39 Ar isotopic dates for metamorphic hornblende (338, 333, and 332 Ma), muscovite (329 and 329 Ma), and biotite (328 Ma) reflect times of cooling through the ∼500, 350, and 300 °C isotherms, respectively. These nearly concordant mineral dates document rapid cooling from ∼850 °C (eclogue-facies temperatures) to ∼300 °C. Rapid denudation of these deep-crustal rocks (∼19-22 kbar pressures, >70 km depth) is attributed to processes, similar to those of metamorphic-core complexes, that operated during lithospheric delamination and gravitational collapse. The sequence of late Paleozoic (Alleghanian) crustal thickening followed by right-slip transpression-tension followed by normal faulting recognized in the U.S. Appalachians implies that this tectonic pattern may exist throughout the Afleghanian-Variscan belt.

Sudety Mountains↗

USGS2006

U-Pb SHRIMP geochronology and trace-element geochemistry of coesite-bearing zircons, North-East Greenland Caledonides

Obtaining reliable estimates for the timing of eclogite-facies metamorphism is critical to establishing models for the formation and exhumation of high-pressure and ultrahigh-pressure (UHP) metamorphic terranes in collisional orogens. The presence of pressure-dependent phases, such as coesite, included in metamorphic zircon is generally regarded as evidence that zircon growth occurred at UHP conditions and, ifdated, should provide the necessary timing information. We report U-Pb sensitive high-resolution ion microprobe (SHRIMP) ages and trace-element SHRIMP data from coesite-bearing zircon suites formed during UHP metamorphism in the North- East Greenland Caledonides. Kyanite eclogite and quartzofeldspathic host gneiss samples from an island in J??kelbugt (78??00'N, 18??04'W) contained subspherical zircons with well-defined domains in cathodoluminescence (CL) images. The presence of coesite is confirmed by Raman spectroscopy in six zircons from four samples. Additional components of the eclogite-facies inclusion suite include kyanite, omphacite, garnet, and rutile. The trace-element signatures in core domains reflect modification of igneous protolith zircon. Rim signatures show flat heavy rare earth element (HREE) patterns that are characteristic of eclogite-facies zircon. The kyanite eclogites generally lack a Eu anomaly, whereas a negative Eu anomaly persists in all domains of the host gneiss. The 207 Pb- corrected 206 Pb/ 238 U ages range from 330 to 390 Ma for the host gneiss and 330-370 Ma for the kyanite eclogite. Weighted mean 206 Pb/ 238 U ages for coesite-bearing domains vary from 364 ?? 8 Ma for the host gneiss to 350 ?? 4 Ma for kyanite eclogite. The combined U-Pb and REE data interpreted in conjunction with observed CL domains and inclusion suites suggest that (1) Caledonian metamorphic zircon formed by both new zircon growth and recrystallization, (2) UHP metamorphism occurred near the end of the Caledonian collision, and (3) the 30-50m.y. span of ages records long residence times at eclogite-facies conditions for the UHProcks of North-East Greenland. This spread in observed ages is interpreted to be characteristic of metamorphic rocks that have experienced relatively long (longer than 10 m.y.) residence times at UHP conditions. ?? 2006 Geological Society of America.

Special Paper of the Geological Society of America↗

USGS2009

Petrology and geochronology of crustal xenoliths from the Bering Strait region: Linking deep and shallow processes in extending continental crust

Petrologic, geochemical, and metamorphic data on gneissic xenoliths derived from the middle and lower crust in the Neogene Bering Sea basalt province, coupled with U-Pb geochronology of their zircons using sensitive high-resolution ion microprobe-reverse geometry (SHRIMP-RG), yield a detailed comparison between the P-T-t and magmatic history of the lower crust and magmatic, metamorphic, and deformational history of the upper crust. Our results provide unique insights into the nature of lithospheric processes that accompany the extension of continental crust. The gneissic, mostly maficxenoliths (constituting less than two percent of the total xenolith population) from lavas in the Enmelen, RU, St. Lawrence, Nunivak, and Seward Peninsula fields most likely originated through magmatic fractionation processes with continued residence at granulite-facies conditions. Zircon single-grain ages (n ??? 125) are interpreted as both magmatic and metamorphic and are entirely Cretaceous to Paleocene in age (ca. 138-60 Ma). Their age distributions correspond to the main ages of magmatism in two belts of supracrustal volcanic and plutonic rocks in the Bering Sea region. Oscillatory-zoned igneous zircons, Late Cretaceous to Paleocene metamorphic zircons and overgrowths, and lack of any older inheritance in zircons from the xenoliths provide strong evidence for juvenile addition of material to the crust at this time. Surface exposures of Precambrian and Paleozoic rocks locally reached upper amphibolite-facies (sillimanite grade) to granulite-facies conditions within a series of extension-related metamorphic culminations or gneiss domes, which developed within the Cretaceous magmatic belt. Metamorphic gradients and inferred geotherms (??30-50 ??C/km) from both the gneiss domes and xenoliths aretoo high to be explained by crustal thickening alone. Magmatic heat input from the mantle is necessary to explain both the petrology of the magmas and elevated metamorphic temperatures. Deep-crustal seismic-reflection and refraction data reveal a 30-35-km-thick crust, a sharp Moho and refl ective lower and middle crust. Velocities do not support a largely mafic (underplated) lower crust, but together with xenolith data suggest that Late Cretaceous to early Paleocene maficintrusions are likely increasingly important with depth in the crust and that the elevated temperatures during granulite-facies metamorphism led to large-scale flow of crustal rocks to produce gneiss domes and the observed subhorizontal refl ectivity of the crust. This unique combined data set for the Bering Shelf region provides compelling evidence for the complete reconstitution/re-equilibration of continental crust from the bottom up during mantle-driven magmatic events associated with crustal extension. Thus, despite Precambrian and Paleozoic rocks at the surface and Alaska's accretionary tectonic history, it is likely that a significant portion of the Bering Sea region lower crust is much younger and related to post-accretionary tectonic and magmatic events. ?? 2009 The Geological Society of America.

Special Paper of the Geological Society of America↗

USGS2015

Sedimentology of SPICE (Steptoean positive carbon isotope excursion): A high-resolution trace fossil and microfabric analysis of the middle to late Cambrian Alum Shale Formation, southern Sweden

The Cambrian Alum Shale Formation in the Andrarum-3 core from Scania, southern Sweden, consists of black siliciclastic mudstone with minor carbonate intercalations. Four facies comprise three siliciclastic mudstones and one fine-grained carbonate. The facies reflect deposition along a transect from deep ramp to basin on a Cambrian shelf. The three mudstone facies contain abundant clay clasts and laterally variable siltstone laminae. Bed-load transport processes seem to have dominated deposition on this deep shelf. These sedimentary rocks record mainly event deposition, and only relatively few, thin laminae probably resulted from suspension settling. The Alum Shale Formation deep shelf did not show a bioturbation gradient, but fecal strings are common and Planolites burrows are rare in all mudstone facies. Evidence for biotic colonization indicates that this mudstone environment was not persistently anoxic, but rather was most likely intermittently dysoxic. The Alum Shale Formation in the Andrarum-3 core shows an overall decrease of grain size, preserved energy indicators, and carbonate content upsection interpreted to reflect a deepening upward. The succession can also be divided into four small-scale fining-upward cycles that represent deepening, and four overlying coarsening-upward cycles that represent upward shallowing.

Book chapter↗

USGS2023

Construction and modification of debris-flow alluvial fans as captured in the geomorphic and sedimentary record: Examples from the western Sangre de Cristo Mountains, south-central Colorado

Debris-flow alluvial fans are iconic features of dynamic landscapes and are hypothesized to record tectonic and climatic change. Here, we highlight their complex formation and evolution through an exemplary suite of Quaternary debris-flow alluvial fans emanating from the western range front of the Sangre de Cristo Mountains in south-central Colorado, USA. To evaluate the constructive and modifying processes that produce fan form and the associated sedimentary signatures, we applied a combined geomorphologic and sedimentologic approach using sedimentary facies analysis, soils mapping, high-resolution topographic data, and luminescence geochronology to document timing of fan construction and modification. We explored two subsets of fans in the study area: a southern set sourced from the extensively glaciated drainages of the Blanca Peak massif, and a northern set from the unglaciated drainages south of Great Sand Dunes National Park. Both sets of fans have: (1) active and successively abandoned surfaces that show evolving degradation of primary features through modification by secondary processes, (2) associated facies that display distinct characteristics representative of primary depositional and secondary modifying sedimentary processes, and (3) evidence of primary debris flow with subsequent modification by secondary processes. We found that surface geomorphology and facies assemblages in exposed alluvial-fan deposits represent sediment transport processes on both active and abandoned lobes. The link between fan surface morphologies and the sedimentary facies of their deposits provides a basis for an evolutionary process–based interpretation of debris-flow alluvial-fan geomorphology and provides a better understanding of complexities in buried paleosurfaces (intraformational progressive unconformities), surficial deformation, and landform development as recorded in debris-flow fan deposits in the sedimentary record.

Colorado↗

USGS2013

Controls on the deposition and preservation of the Cretaceous Mowry Shale and Frontier Formation and equivalents, Rocky Mountain region, Colorado, Utah, and Wyoming

Regional variations in thickness and facies of clastic sediments are controlled by geographic location within a foreland basin. Preservation of facies is dependent on the original accommodation space available during deposition and ultimately by tectonic modification of the foreland in its postthrusting stages. The preservation of facies within the foreland basin and during the modification stage affects the kinds of hydrocarbon reservoirs that are present. This is the case for the Cretaceous Mowry Shale and Frontier Formation and equivalent strata in the Rocky Mountain region of Colorado, Utah, and Wyoming. Biostratigraphically constrained isopach maps of three intervals within these formations provide a control on eustatic variations in sea level, which allow depositional patterns across dip and along strike to be interpreted in terms of relationship to thrust progression and depositional topography. The most highly subsiding parts of the Rocky Mountain foreland basin, near the fold and thrust belt to the west, typically contain a low number of coarse-grained sandstone channels but limited sandstone reservoirs. However, where subsidence is greater than sediment supply, the foredeep contains stacked deltaic sandstones, coal, and preserved transgressive marine shales in mainly conformable successions. The main exploration play in this area is currently coalbed gas, but the enhanced coal thickness combined with a Mowry marine shale source rock indicates that a low-permeability, basin-centered play may exist somewhere along strike in a deep part of the basin. In the slower subsiding parts of the foreland basin, marginal marine and fluvial sandstones are amalgamated and compartmentalized by unconformities, providing conditions for the development of stratigraphic and combination traps, especially in areas of repeated reactivation. Areas of medium accommodation in the most distal parts of the foreland contain isolated marginal marine shoreface and deltaic sandstones that were deposited at or near sea level lowstand and were reworked landward by ravinement and longshore currents by storms creating stratigraphic or combination traps enclosed with marine shale seals. Paleogeographic reconstructions are used to show exploration fairways of the different play types present in the Laramide-modified, Cretaceous foreland basin. Existing oil and gas fields from these plays show a relatively consistent volume of hydrocarbons, which results from the partitioning of facies within the different parts of the foreland basin.

Colorado;Utah;Wyoming↗

USGS1995

Middle Tertiary extension recorded by lacustrine fan-delta deposits, Plush Ranch Basin, western Transverse Ranges, California

The Plush Ranch Formation (upper Oligocene and lower Miocene) consists of more than 1800 m of nonmarine sedimentary and volcanic rocks that record the history of an extensional basin referred to here as the Plush Ranch basin. Distinctive depositional facies, provenance, and sediment transport directions along each basin margin suggest an asymmetric basin shape that is consistent with a half-graben origin. The northern basin margin consists of sandstone-dominated alluvial-plain deposits (0.1-1.5 m thick, normally graded, lenticular sandstone beds). Small deltaic sequences 1-2 m thick were formed where these alluvial systems flowed southward into a lake. Lenses of massive, boulder-rich granitic breccia that represent rockslide deposits derived from a nearby northern granitic provenance interfinger with the alluvial-plain facies. In contrast to the northern margin, the southern basin margin is represented by coarse-grained fan-delta deposits. Matrix- and clast-supported lenticular conglomerate beds 0.2-5 m thick with interbedded trough-cross-bedded pebbly sandstone represent braided-stream and flood-flow and/or noncohesive debris-flow deposits of alluvial fans that drained a highland area to the south. The alluvial-fan deposits interfinger to the north with several types of subaqueous sediment-gravity-flow facies including turbidite sandstone beds and matrix-supported debris-flow conglomerate. Each of the basin-margin depositional systems grades basinward and to the east into lacustrine deposits that include organic-rich dark shale, evaporite, and limestone. The lacustrine deposits represent the central and eastern parts of the Plush Ranch basin, which received little coarse siliciclastic sediment. Basalt deposits that are at least 50 m thick in the west and thicken eastward are interbedded mainly with the lacustrine facies. The southern margin of the Plush Ranch basin formed along a north-dipping, normal-slip fault along which dip separation increased toward the southwest; the northern margin developed on the tilted hanging-wall block of this fault. This fault was later reactivated in post-middle Miocene time as the present left-lateral strike-slip Big Pine fault. The Plush Ranch is one of several extensional and transtensional basins that formed in southern California and western Arizona about 25-20 Ma as a response to the change from a convergent to a strikeslip tectonic regime along western North America.

California↗

USGS2014

Telescoping metamorphic isograds: Evidence from 40Ar/39A dating in the Orange-Milford belt, southern Connecticut

New 40 Ar/ 39 Ar ages for hornblende and muscovite from the Orange-Milford belt in southern Connecticut reflect cooling from Acadian amphibolite facies metamorphism between ∼380 to 360 Ma followed by retrograde recrystallization of fabric-forming muscovite and chlorite during lower greenschist facies Alleghanian transpression at ∼280 Ma. Reported field temperature and pressure gradients are improbably high for these rocks and a NW metamorphic field gradient climbing from chlorite-grade to staurolite-grade occurs over less than 5 km. Simple tilting cannot account for this compressed isograd spacing given the geothermal gradient of ∼20 °C/km present at the time of regional metamorphism. However, post-metamorphic transpression could effectively telescope the isograds by stretching the belt at an oblique angle to the isograd traces. Textures in the field and in thin section reveal several older prograde schistosities overprinted by lower greenschist facies fabrics. The late cleavages commonly occur at the scale of ∼100 μm and these samples contain multiple age populations of white mica. 40 Ar/ 39 Ar analysis of these poly-metamorphic samples with mixed muscovite populations yield climbing or U-shaped age spectra. The ages of the low temperature steps are late Paleozoic, while the ages of the older steps are late Devonian. These results support our petrologic interpretation that the younger cleavage developed under metamorphic conditions below the closure temperature for Ar diffusion in muscovite, that is, in the lower greenschist facies. The correlation of a younger regionally reproducible age population with a pervasive retrograde muscovite ± chlorite cleavage reveals an Alleghanian (∼280 Ma) overprint on the Acadian metamorphic gradient (∼380 Ma). Outcrop-scale structures including drag folds and imbricate boudins suggest that Alleghanian deformation and cleavage development occurred in response to dextral transpression along a northeast striking boundary. Alleghanian oblique collision of accreting terranes from the northeast would have resulted in northeast-southwest dextral transpression against the New York promontory. This deformation was responsible for crystallization of pervasive retrograde muscovite + chlorite cleavages and associated telescoping of the Acadian metamorphic isograds in southern Connecticut at ∼280 Ma.

Connecticut↗

USGS1987

Alleghanian deformation, metamorphism, and granite emplacement in the central Piedmont of the southern Appalachians

Evidence of late Paleozoic (Alleghanian) penetrative deformation, amphilbolite-facies, metamorphism, and syntectonic granite emplacement has been found in an area of the central Piedmont in the southern Appalachians. The High Shoals Granite batholith in the Kings Mountain belt of south-central North Carolina consists of coarse-grained, megacrystic biotite granite with a strong, nearly vertical, gneissoid, foliation, defined by parallel feldspar megacrysts and biotite. U-Ph isotopic data on zircons from the granite yield a concordant age of 317 Ma (Pennsylvanian). This pervasively deformed late Paleozoic granite in the central Piedmont is far west of those known in the Kiokee and Raleigh belts. Field relationships indicate that the High Shoals Granite was emplaced during the late stages of regional F2 folding, close in time to the thermal peak of amphibolite-facies metamorphism. Mineral assemblages typical of Barrovian metamorphism are found in zones of decreasing grade away from the High Shoals contact. These zones, delimited by a kyanite(out)-sillimanite (in) isograd which completely surrounds the batholith and by part of a regional chloritoid (out)-staurolite (in) isograd west and southwest of it, conform to the shape of the batholith and cut across F2 folds. Furthermore, hornblendes from epidoteamphibolite facies to upper amphibolite facies metamorphic rocks in the Kings Mountain belt of North Carolina and South Carolina yield 40Ar/39Ar plateau ages of 318 to 323 Ma. These dates are analytically the same as the crystallization age of High Shoals Granite as established of zireon using the U-Ph method and demonstrate the synchroneity of intrusion of the High Shoals with the dominant regional metamorphism in this part of the Kings Mountain belt.

Southern Appalachians↗

USGS2003

Brookian stratigraphic plays in the National Petroleum Reserve - Alaska (NPRA)

The Brookian megasequence in the National Petroleum Reserve in Alaska (NPRA) includes bottomset and clinoform seismic facies of the Torok Formation (mostly Albian age) and generally coeval, topset seismic facies of the uppermost Torok Formation and the Nanushuk Group. These strata are part of a composite total petroleum system involving hydrocarbons expelled from three stratigraphic intervals of source rocks, the Lower Cretaceous gamma-ray zone (GRZ), the Lower Jurassic Kingak Shale, and the Triassic Shublik Formation. The potential for undiscovered oil and gas resources in the Brookian megasequence in NPRA was assessed by defining five plays (assessment units), one in the topset seismic facies and four in the bottomset-clinoform seismic facies. The Brookian Topset Play is estimated to contain between 60 (95-percent probability) and 465 (5-percent probability) million barrels of technically recoverable oil, with a mean (expected value) of 239 million barrels. The Brookian Topset Play is estimated to contain between 0 (95-percent probability) and 679 (5-percent probability) billion cubic feet of technically recoverable, nonassociated natural gas, with a mean (expected value) of 192 billion cubic feet. The Brookian Clinoform North Play, which extends across northern NPRA, is estimated to contain between 538 (95-percent probability) and 2,257 (5-percent probability) million barrels of technically recoverable oil, with a mean (expected value) of 1,306 million barrels. The Brookian Clinoform North Play is estimated to contain between 0 (95-percent probability) and 1,969 (5-percent probability) billion cubic feet of technically recoverable, nonassociated natural gas, with a mean (expected value) of 674 billion cubic feet. The Brookian Clinoform Central Play, which extends across central NPRA, is estimated to contain between 299 (95-percent probability) and 1,849 (5-percent probability) million barrels of technically recoverable oil, with a mean (expected value) of 973 million barrels. The Brookian Clinoform Central Play is estimated to contain between 1,806 (95-percent probability) and 10,076 (5-percent probability) billion cubic feet of technically recoverable, nonassociated natural gas, with a mean (expected value) of 5,405 billion cubic feet. The Brookian Clinoform South-Shallow Play is estimated to contain between 0 (95-percent probability) and 1,254 (5-percent probability) million barrels of technically recoverable oil, with a mean (expected value) of 508 million barrels. The Brookian Clinoform South-Shallow Play is estimated to contain between 0 (95-percent probability) and 5,809 (5-percent probability) billion cubic feet of technically recoverable, nonassociated natural gas, with a mean (expected value) of 2,405 billion cubic feet. The Brookian Clinoform South-Deep Play is estimated to contain between 0 (95-percent probability) and 8,796 (5-percent probability) billion cubic feet of technically recoverable, nonassociated natural gas, with a mean (expected value) of 3,788 billion cubic feet. No technically recoverable oil is assessed in the Brookian Clinoform South-Deep Play, as it lies at depths that are entirely in the gas window. Among the Brookian stratigraphic plays in NPRA, the Brookian Clinoform North Play and the Brookian Clinoform Central Play are most likely to be objectives of exploration activity in the near-term future because they are estimated to contain multiple oil accumulations larger than 128 million barrels technically recoverable oil, and because some of those accumulations may occur near existing infrastructure in the eastern parts of the plays. The other Brookian stratigraphic plays are not likely to be the focus of exploration activity because they are estimated to contain maximum accumulation sizes that are smaller, but they may be an objective of satellite exploration if infrastructure is extended into the play areas. The total volumes of natural gas estimated to occur in B

Open-File Report↗