SEARCH · Geology Reports

Results for “The Sedimentary Record”

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 343 records · Page 19Linked to original sources

USGS2000

Report for explosion and earthquake data acquired in the 1999 Seismic Hazards Investigation of Puget Sound (SHIPS), Washington

This report describes the acquisition, processing, and quality of seismic reflection and refraction data obtained in the Seattle basin, central Puget Lowland, western Washington, in September 1999 during the Seismic Hazards Investigation of Puget Sound (SHIPS). As a sequel to the 1998 SHIPS air gun experiment (also known as 'Wet SHIPS'), the 1999 experiment, nicknamed 'Dry SHIPS,' acquired a 112-km-long east-west trending multichannel seismic reflection and refraction line in the Seattle basin. One thousand and eight seismographs were deployed at a nominal spacing of 100 meters, and 29 shot points were detonated at approximately 4 km intervals along the seismic line. The wide-angle seismic profile was designed to (1) determine the E-W geometry of Seattle basin, (2) measure the seismic velocities within the basin, and (3) define the basement structure underlying the Seattle basin. In this report, we describe the acquisition of these data, discuss the processing and merging of the data into common shot gathers, and illustrate the acquired profiles. We also describe the format and content of the archival tapes containing the SEGY-formatted, common-shot gathers. Data quality is variable, but useful data were acquired from all 29 shot points fired along the Dry SHIPS seismic line. The data show pronounced travel time delays associated with the low velocity sedimentary rocks filling the Seattle basin. Thirty-five REFTEK stations, deployed at 4 km intervals along the Dry SHIPS line, recorded 26 regional earthquakes and blasts and 17 teleseismic events, including the main shock and several aftershocks of the Mw=7.6 Chi-Chi (Taiwan) earthquake of 9/20/1999. The teleseismic recordings of the Chi-Chi (Taiwan) mainshock provide useful signals down to 10 second periods. They document a significant (factor between 5 and 10) focusing of compressional- and shear-wave energy by the Seattle basin at periods between 1 and 2 seconds relative to 'bedrock' sites east and west of the basin. Signal durations in the Seattle basin were also substantially increased relative to 'bedrock' sites in the Olympic peninsula and Cascade foothills.

Open-File Report↗

USGS2022

Evidence for the recovery of terrestrial ecosystems ahead of marine primary production following a biotic crisis at the Cretaceous-Tertiary boundary

The fossil record demonstrates that mass extinction across the Cretaceous–Tertiary (K–T) boundary is more severe in the marine than the terrestrial realm. We hypothesize that terrestrial ecosystems were able to recover faster than their marine counterparts. To test this hypothesis, we measured sedimentary δ 13 C as a tracer for global carbon cycle changes and compared it with palaeovegetational changes reconstructed from palynomorphs and cuticles across the K–T boundary at Sugarite, New Mexico, USA. Different patterns of perturbation and timescales of recovery of isotopic and palaeobotanical records indicate that the δ 13 C excursion reflects the longer recovery time of marine versus terrestrial ecosystems.

New Mexico↗

USGS2008

Traces and burrowing behaviors of the Cicada nymph Cicadetta calliope: Neoichnology and paleoecological significance of extant soil-dwelling insects

This study documents the traces and burrowing behaviors of nymphs of the prairie cicada Cicadetta calliope (Hemiptera: Cicadidae), as observed in neoichnological experiments. Cicada nymphs were collected from the C horizons of sandy Fluvents along the Kansas River east of Lawrence, Kansas. The nymphs appeared to be fifth instars, 13-17 mm long and 6-7 mm wide. Nymphs were placed in plastic enclosures containing layers of colored, moist, very fine-grained sand. They burrowed immediately, excavating air-filled, sediment-enclosed cells between 20 mm and 40 mm long and averaging 9 mm wide. Burrowing was completed in three stages: (1) sediment in the forward portion of the cell was excavated and rolled into a ball with the forelimbs; (2) the nymph turned 180?? using a forward roll, and moved to the back of the cell; and (3) the sediment ball was pushed up against the back wall of the cell and kneaded with the forelimbs into a thin layer. Resulting burrow traces are sinuous and distinctly meniscate and demonstrate that insect larvae construct meniscate, backfilled burrows in well-drained terrestrial settings. Cicadetta calliope nymphs and their traces are excellent analogs for meniscate trace fossils commonly found in late Paleozoic-Cenozoic alluvial deposits and paleosols. Such meniscate trace fossils are useful for interpreting the paleoenvironment and paleohydrogeology of the units in which they are found. In addition, such backfilled burrows can be used to supplement the fossil record of cicada-like hemipterans, currently known only from the latest Permian to the Early Triassic. Copyright ?? 2008, SEPM (Society for Sedimentary Geology).

Palaios↗

USGS2011

Source region of a granite batholith: Evidence from lower crustal xenoliths and inherited accessory minerals

Like many granites, the Late Cretaceous intrusives of the eastern Mojave Desert, California, have heretofore provided useful but poorly focused images of their source regions. New studies of lower crustal xenoliths and inherited accessory minerals are sharpening these images. Xenoliths in Tertiary dykes in this region are the residues of an extensive partial melting event. Great diversity in their composition reflects initial heterogeneity (both igneous and sedimentary protoliths) and varying amounts of melt extraction (from <10% to >70%). Mineral assemblages and thermobarometry suggest that the melting event occurred at T ≥ 750°C at a depth of about 40 km. Present-day Sr, Nd, and Pb isotopic ratios indicate a Mojave Proterozoic heritage, but unrealistic model ages demonstrate the late Phanerozoic adjustment of parent/daughter ratios. A link between these xenoliths and the Late Cretaceous granites, though not fully documented, is probable; in any case, they provide invaluable clues concerning a crustal melting event, recording information about nature of source material (heterogeneous, supracrustal-rich), conditions of melting (moderately deep, moderately high T, accompanied by partial dehydration), and melt extraction (highly variable, locally extensive). The Old Woman-Piute granites contain a large fraction of inherited zircon and monazite. A SHRIMP ion probe investigation shows that these zircons record a Proterozoic history similar to that which affected the Mojave region. Zonation patterns in zircons, and to a lesser extent monazites and xenotimes, document multiple phases of igneous, metamorphic, and sedimentary growth and degradation, commonly several in a single grain. Low Y in portions of the cores of inherited zircons and monazites and in monazites and outer portions of zircons from the xenoliths appear to indicate growth in equilibrium with abundant garnet.

California↗

USGS1992

Source region of a granite batholith: evidence from lower crustal xenoliths and inherited accessory minerals

Like many granites, the Late Cretaceous intrusives of the eastern Mojave Desert, California, have heretofore provided useful but poorly focused images of their source regions. New studies of lower crustal xenoliths and inherited accessory minerals are sharpening these images. Xenoliths in Tertiary dykes in this region are the residues of an extensive partial melting event. Great diversity in their composition reflects initial heterogeneity (both igneous and sedimentary protoliths) and varying amounts of melt extraction (from <10% to >70%). Mineral assemblages and thermobarometry suggest that the melting event occurred at T≥750°C at a depth of about 40 km. Present-day Sr, Nd, and Pb isotopic ratios indicate a Mojave Proterozoic heritage, but unrealistic model ages demonstrate the late Phanerozoic adjustment of parent/daughter ratios. A link between these xenoliths and the Late Cretaceous granites, though not fully documented, is probable; in any case, they provide invaluable clues concerning a crustal melting event, recording information about nature of source material (heterogeneous, supracrustal-rich), conditions of melting (moderately deep, moderately high T, accompanied by partial dehydration), and melt extraction (highly variable, locally extensive). The Old Woman-Piute granites contain a large fraction of inherited zircon and monazite. A SHRIMP ion probe investigation shows that these zircons record a Proterozoic history similar to that which affected the Mojave region. Zonation patterns in zircons, and to a lesser extent monazites and xenotimes, document multiple phases of igneous, metamorphic, and sedimentary growth and degradation, commonly several in a single grain. Low Y in portions of the cores of inherited zircons and monazites and in monazites and outer portions of zircons from the xenoliths appear to indicate growth in equilibrium with abundant garnet.

California↗

USGS2012

Turbidite event history—Methods and implications for Holocene paleoseismicity of the Cascadia subduction zone

Turbidite systems along the continental margin of Cascadia Basin from Vancouver Island, Canada, to Cape Mendocino, California, United States, have been investigated with swath bathymetry; newly collected and archive piston, gravity, kasten, and box cores; and accelerator mass spectrometry radiocarbon dates. The purpose of this study is to test the applicability of the Holocene turbidite record as a paleoseismic record for the Cascadia subduction zone. The Cascadia Basin is an ideal place to develop a turbidite paleoseismologic method and to record paleoearthquakes because (1) a single subduction-zone fault underlies the Cascadia submarine-canyon systems; (2) multiple tributary canyons and a variety of turbidite systems and sedimentary sources exist to use in tests of synchronous turbidite triggering; (3) the Cascadia trench is completely sediment filled, allowing channel systems to trend seaward across the abyssal plain, rather than merging in the trench; (4) the continental shelf is wide, favoring disconnection of Holocene river systems from their largely Pleistocene canyons; and (5) excellent stratigraphic datums, including the Mazama ash and distinguishable sedimentological and faunal changes near the Pleistocene-Holocene boundary, are present for correlating events and anchoring the temporal framework. Multiple tributaries to Cascadia Channel with 50- to 150-km spacing, and a wide variety of other turbidite systems with different sedimentary sources contain 13 post-Mazama-ash and 19 Holocene turbidites. Likely correlative sequences are found in Cascadia Channel, Juan de Fuca Channel off Washington, and Hydrate Ridge slope basin and Astoria Fan off northern and central Oregon. A probable correlative sequence of turbidites is also found in cores on Rogue Apron off southern Oregon. The Hydrate Ridge and Rogue Apron cores also include 12-22 interspersed thinner turbidite beds respectively. We use 14 C dates, relative-dating tests at channel confluences, and stratigraphic correlation of turbidites to determine whether turbidites deposited in separate channel systems are correlative - triggered by a common event. In most cases, these tests can separate earthquake-triggered turbidity currents from other possible sources. The 10,000-year turbidite record along the Cascadia margin passes several tests for synchronous triggering and correlates well with the shorter onshore paleoseismic record. The synchroneity of a 10,000-year turbidite-event record for 500 km along the northern half of the Cascadia subduction zone is best explained by paleoseismic triggering by great earthquakes. Similarly, we find a likely synchronous record in southern Cascadia, including correlated additional events along the southern margin. We examine the applicability of other regional triggers, such as storm waves, storm surges, hyperpycnal flows, and teletsunami, specifically for the Cascadia margin. The average age of the oldest turbidite emplacement event in the 10-0-ka series is 9,800±~210 cal yr B.P. and the youngest is 270±~120 cal yr B.P., indistinguishable from the A.D. 1700 (250 cal yr B.P.) Cascadia earthquake. The northern events define a great earthquake recurrence of ~500-530 years. The recurrence times and averages are supported by the thickness of hemipelagic sediment deposited between turbidite beds. The southern Oregon and northern California margins represent at least three segments that include all of the northern ruptures, as well as ~22 thinner turbidites of restricted latitude range that are correlated between multiple sites. At least two northern California sites, Trinidad and Eel Canyon/pools, record additional turbidites, which may be a mix of earthquake and sedimentologically or storm-triggered events, particularly during the early Holocene when a close connection existed between these canyons and associated river systems. The combined stratigraphic correlations, hemipelagic analysis, and 14 C framework suggest that the Cascadia margin has three rupture modes: (1) 19-20 full-length or nearly full length ruptures; (2) three or four ruptures comprising the southern 50-70 percent of the margin; and (3) 18-20 smaller southern-margin ruptures during the past 10 k.y., with the possibility of additional southern-margin events that are presently uncorrelated. The shorter rupture extents and thinner turbidites of the southern margin correspond well with spatial extents interpreted from the limited onshore paleoseismic record, supporting margin segmentation of southern Cascadia. The sequence of 41 events defines an average recurrence period for the southern Cascadia margin of ~240 years during the past 10 k.y. Time-independent probabilities for segmented ruptures range from 7-12 percent in 50 years for full or nearly full margin ruptures to ~21 percent in 50 years for a southern-margin rupture. Time-dependent probabilities are similar for northern margin events at ~7-12 percent and 37-42 percent in 50 years for the southern margin. Failure analysis suggests that by the year 2060, Cascadia will have exceeded ~27 percent of Holocene recurrence intervals for the northern margin and 85 percent of recurrence intervals for the southern margin. The long earthquake record established in Cascadia allows tests of recurrence models rarely possible elsewhere. Turbidite mass per event along the Cascadia margin reveals a consistent record for many of the Cascadia turbidites. We infer that larger turbidites likely represent larger earthquakes. Mass per event and magnitude estimates also correlate modestly with following time intervals for each event, suggesting that Cascadia full or nearly full margin ruptures weakly support a time-predictable model of recurrence. The long paleoseismic record also suggests a pattern of clustered earthquakes that includes four or five cycles of two to five earthquakes during the past 10 k.y., separated by unusually long intervals. We suggest that the pattern of long time intervals and longer ruptures for the northern and central margins may be a function of high sediment supply on the incoming plate, smoothing asperities, and potential barriers. The smaller southern Cascadia segments correspond to thinner incoming sediment sections and potentially greater interaction between lower-plate and upper-plate heterogeneities. The Cascadia Basin turbidite record establishes new paleoseismic techniques utilizing marine turbidite-event stratigraphy during sea-level highstands. These techniques can be applied in other specific settings worldwide, where an extensive fault traverses a continental margin that has several active turbidite systems.

British Columbia, California, Oregon, Washington↗

USGS2012

Active transtensional intracontinental basins: Walker Lane in the western Great Basin

The geometry and dimensions of sedimentary basins within the Walker Lane are a result of Plio-Pleistocene transtensive deformation and partial detachment of the Sierra Nevada crustal block from the North American plate. Distinct morpho-tectonic domains lie within this active transtensive zone. The northeast end of the Walker Lane is partly buried by active volcanism of the southern Cascades, and adjacent basins are filled or poorly developed. To the south, the basin sizes are moderate, 25–45km × 15–10 km, with narrow 8-12km wide mountain ranges mainly oriented N-S to NNE. These basins form subparallel arrays in discrete zones trending about 300° and have documented clockwise rotation. This is succeeded to the south by a releasing stepover domain ∼85-100km wide, where the basins are elongated E-W to ENE, small (∼15-30km long, 5-15km wide), and locally occupied by active volcanic centers. The southernmost part of the Walker Lane is structurally integrated, with high to extreme relief. Adjacent basins are elongate, 50-200km long and ∼5 -20km wide. Variations in transtensive basin orientations in the Walker Lane are largely attributable to variations in strain partitioning. Large basins in the Walker Lane have 2-6km displacement across basin bounding faults with up to 3 km of clastic accumulation based on gravity and drill hole data. The sedimentary deposits of the basins may include interbedded volcanic deposits with bimodal basaltic and rhyolitic associations. The basins may include lacustrine deposits that record a wide range of water chemistry from cold fresh water conditions to saline-evaporative

Cascades;Great Basin;Sierra Nevada;Walker Lane↗

USGS2016

South Park, Colorado: The interplay of tectonics and sedimentation creates one of Colorado’s crown jewels

Recent mapping efforts and hydrocarbon exploration in the South Park Basin have brought to light the magnitude in complexity of a structural basin already recognized for its unique sedimentary and tectonic setting. This fi eld trip to one of Colorado’s scenic gems will examine how Paleozoic, Mesozoic, and Cenozoic strata record the tectonic signatures of at least three orogenic episodes. We will cross the Elkhorn–Williams Range thrust system into the structural block caught between Laramide uplifts, and preserving synorogenic sediments from the Pennsylvanian– Permian ancestral Rocky Mountain tectonic episode in juxtaposition with synorogenic sediments from the subsequent Laramide tectonic episode. Late Cretaceous marine sediments from the Western Interior Seaway caught up in complex fold-fault structures between Laramide uplifts create targets for petroleum exploration. Evidence of evaporitic tectonism originating from Pennsylvanian evaporite deposits and hinting at structural complexity dots the landscape. The trip will also explore a postLaramide surface preserved in a graben developed in the hanging wall of the Elkhorn fault system and view post-Laramide volcanic features. Glacier-carved ranges held up by Precambrian crystalline basement and Paleozoic sediments hardened by contact metamorphism from Paleogene stocks and sills rim the basin. Pleistocene glaciofl uvial deposits fan out from the high ranges to blanket the highly deformed basin, masking many of the primary structural features.

Colorado↗

USGS2013

U-Pb ages of detrital zircons from the Tertiary Mississippi River delta plain in central Louisiana: Insights into sediment provenance

The sources of the tremendous amount of Cenozoic siliciclastic sediment deposited in the Gulf of Mexico region remain debated because of a lack of definitive provenance-identifying characteristics. In an effort to build on prior provenance analysis, we present 101–160 single-grain detrital zircon U-Pb ages for each of 10 outcrop samples from Upper Paleocene to Upper Miocene sandstones from a ∼10,000 km2 swath of central Louisiana corresponding to the ancient Mississippi River Delta, the largest Cenozoic depocenter in the northern Gulf of Mexico region. Sample depositional age control is derived from biostratigraphy and/or regional lithostratigraphic correlation. U-Pb ages in each of the samples range from Cenozoic to Archean, and correspond to the ages of various geologic terranes that underlie the modern Mississippi River drainage basin. However, the prominence of various age distributions changes systematically through the Cenozoic stratigraphy, and pronounced shifts in the abundance of certain age distributions between stratal packages appear to be correlated to shifts in heavy mineral assemblages recorded across the northern Gulf of Mexico coastal plain. Comparison of coastal plain detrital zircon age distributions to age distributions from North American sedimentary cover and the ages of major North American crystalline basement rocks, aided by a sediment mixing model, illuminates the provenance of each of the stratal packages, and suggests that (1) the Mississippi River catchment has resembled its present configuration, at least in the east-west dimension, for much, if not all, of the Cenozoic, and (2) depositional episodes on the Louisiana coastal plain characterized by high sediment supply also corresponded to high proportions of sediment sourcing from the Sevier-Laramide region of the interior western United States. Sediment supply to the Louisiana coastal plain by the paleo–Mississippi River has generally been high during the Cenozoic, except for an anomalous low during the Middle Eocene, when the abundance of sediment derived from the Rocky Mountain region decreased dramatically relative to sediment derived from the Appalachian region.

Louisiana↗

USGS1980

Paleoenvironment of the New Albany Shale Group ( Devonian- Mississippian) of Illinois

The distribution of lithofacies in the New Albany Shale Group of Illinois was determined by wave energy, bottom oxygenation, and bottom topography in a deep water stratified anoxic basin. A transect from the margin to the center of the Illinois Basin reveals a complete transition from high energy, aerobic, shallow-water environments to quiet, anaerobic, deep-water environments. Shallow areas at the margin of the basin are characterized by rapid facies transitions over short distances. High energy, very shallow conditions are recorded by oolitic-skeletal grainstones and packstones with abundant brachiopods, crinoids, trilobites, and other calcified marine invertebrates. Bioturbation did not destroy primary sedimentary structures in these facies. Offshore, less agitated areas are represented by highly bioturbated carbonate wackestones, argillaceous quartz siltstones, and greenish-gray mudstones. Calcified invertebrates are generally rare in these facies, indicating deposition in dysaerobic conditions. Basinward, slope areas are characterized by olive-gray to black, weakly bioturbated shales commonly interbedded with thickly laminated black shales. Trace fossils, including Zoophycos, Chondrites , and Planolites , are abundant along the bases of the olive-gray beds. In areas where the anaerobic/dysaerobic boundary intersected the bottom slope, slight fluctuations of the position of the boundary resulted in thin interbedding of olive-gray and black shales and laterally persistent interfingering of the two lithologies. Anaerobic conditions prevailed during most of New Albany time in the deepest areas of the basin, and finely laminated, undisturbed, pelagic black shales were deposited.

Journal of Sedimentary Petrology↗

USGS2021

Basin and site effects in the U.S. Pacific Northwest estimated from small‐magnitude earthquakes

Earthquake hazards in the U.S. Pacific Northwest (PNW) are increased by the presence of deep sedimentary basins that amplify and prolong ground shaking. To better understand basin and site effects on ground motions, we compile a database of recordings from crustal and intraslab earthquakes. We process 8028 records with magnitudes from 3.5 to 6.8 and hypocentral depths up to 62 km to compute Fourier amplitude spectra of ground acceleration for frequencies of 0–20 Hz. We compute residuals relative to the Bayless and Abrahamson (2019; hereafter, BA18) ground‐motion model and perform a series of linear, crossed, mixed‐effects regressions. In addition to estimating the bias, event, and site terms, we incorporate groupings for broad regionalized site response in three different regions (Seattle basin, Puget Lowland, non‐Puget Lowland), for effects from seismotectonic regime (crustal and intraslab sources), and for interactions between the regions and seismotectonic regimes. We find that the scaling of site response with respect to V S 30 (time‐averaged shear‐wave velocity from the surface to a depth of 30 m) and to basin depth indicators Z 1.0 and Z 2.5 (depths to the 1.0 and 2.5 km/s shear‐wave velocity horizons) is generally consistent with BA18; however, the region terms display strong spatial amplification patterns. For frequencies less than 5 Hz, the Seattle basin amplifies ground motions up to a factor of four, relative to the non‐Puget Lowland, with a maximum amplification around near 0.5 Hz. Sites in the Puget Lowland amplify low frequencies up to a factor of 2.5. At higher frequencies ( ⁠ f > 5 Hz ⁠ ), the Puget Lowland and Seattle basin show regional deamplification of ground motions, with the smallest average amplification factor of 0.65 occurring at 10.0 Hz. Although we observe slight differences in the seismotectonic regime terms, we find that the region terms are significantly more important for modeling earthquake hazard in the PNW.

Oregon, Washington↗

USGS2012

Deep-Sea Turbidites as Guides to Holocene Earthquake History at the Cascadia Subduction Zone—Alternative Views for a Seismic-Hazard Workshop

This report reviews the geological basis for some recent estimates of earthquake hazards in the Cascadia region between southern British Columbia and northern California. The largest earthquakes to which the region is prone are in the range of magnitude 8-9. The source of these great earthquakes is the fault down which the oceanic Juan de Fuca Plate is being subducted or thrust beneath the North American Plate. Geologic evidence for their occurrence includes sedimentary deposits that have been observed in cores from deep-sea channels and fans. Earthquakes can initiate subaqueous slumps or slides that generate turbidity currents and which produce the sedimentary deposits known as turbidites. The hazard estimates reviewed in this report are derived mainly from deep-sea turbidites that have been interpreted as proxy records of great Cascadia earthquakes. The estimates were first published in 2008. Most of the evidence for them is contained in a monograph now in press. We have reviewed a small part of this evidence, chiefly from Cascadia Channel and its tributaries, all of which head offshore the Pacific coast of Washington State. According to the recent estimates, the Cascadia plate boundary ruptured along its full length in 19 or 20 earthquakes of magnitude 9 in the past 10,000 years; its northern third broke during these giant earthquakes only, and southern segments produced at least 20 additional, lesser earthquakes of Holocene age. The turbidite case for full-length ruptures depends on stratigraphic evidence for simultaneous shaking at the heads of multiple submarine canyons. The simultaneity has been inferred primarily from turbidite counts above a stratigraphic datum, sandy beds likened to strong-motion records, and radiocarbon ages adjusted for turbidity-current erosion. In alternatives proposed here, this turbidite evidence for simultaneous shaking is less sensitive to earthquake size and frequency than previously thought. Turbidites far below a channel confluence, instead of representing the merged flows from two tributaries, monitor the dominant tributary only. Sandy beds low in the turbidites, instead of matching from channel to channel, permit divergent stratigraphic correlations; and rather than approximating strong-motion seismograms, the sandy beds more likely record processes internal to the generation and transformation of subaqueous mass movements. The age adjustments, instead of supporting other evidence that all the northern ruptures were long, are uncertain enough to accord with variation in rupture mode, and this variation improves agreement with onshore paleoseismology. Many of the turbidites counted as evidence for frequent earthquakes on the southern Cascadia plate boundary may instead reflect nearness to steep slopes. This report is meant to aid in the updating of national maps of seismic hazards in Canada and the United States. It offers three main conclusions for consideration at a U.S. hazard-map workshop slated for March 21-22, 2012: If giant earthquakes are the norm for the plate boundary offshore southern Washington, the strongest paleoseismic evidence for this rupture mode is the average earthquake-recurrence interval of about 500 years that is evidenced both offshore in lower Cascadia Channel and onshore at estuaries of southern Washington and northernmost Oregon. The plate boundary offshore southern British Columbia and northern Washington may be capable of producing great earthquakes at an average interval as short as 300 years that is evidenced mainly onshore. Review of more of the turbidite evidence now in press may clarify implications for the hazard maps. Further work on the deep-sea turbidites could target sedimentary processes and chronological uncertainties that may affect the turbidites' sensitivity to fault-rupture lengths and recurrence rates.

Washington;Oregon;California↗

USGS1969

A seismic-refraction survey of crustal structure in central Arizona

The U.S. Geological Survey conducted a seismic-refraction study of the earth's crust and upper mantle near the Tonto Forest Seismological Observatory (TFO), located 10miles south of the Mogollon Rim near Payson in central Arizona. Two recording lines 400 km long intersect in the approximate form of a cross at TFO; one line trends southeast and the other northeast. The sedimentary layer at most places southwest of the rim is less than 1 km thick, but north of the rim it is 2 to 3 km thick. The velocity in this uppermost layer ranges from 2.6 to 4.7 km/sec, with the higher limit measured near or north of the rim. Arrivals refracted in the upper crust (P g ) can be attributed to two layers for all the shot points south of the rim. The velocity in the upper layer is about 5.9 km/sec with thickness ranging from 2 to 8 km; beneath the upper layer the velocity is about 6.1 km/sec. The upper layer seems to be absent northeast of the rim, where two shot points generated P g arrivals that show only a velocity of 6.2 km/sec. A Poisson ratio of 0.22 for the upper crustal layers was measured from shear and compressional arrivals. The lower crust could not be identified from the first and later refraction arrivals; however, minimum depths to the intermediate layer were determined. An average crustal velocity of 6.2 km/sec was measured from wide-angle reflection alignments. A thin intermediate layer would explain the seismic measurements. A delay-time method was used to map the configuration of the M-discontinuity. The depth below sea level is about 36 km along the northwest-trending line. The northeast-trending line shows a shallow depth of 21 km near Gila Bend, increasing depth to about 34 km under TFO, and a flat M-discontinuity at 40 km depth under the Mogollon Mesa northeast to Sunrise Springs. There is evidence of an abrupt depth change of about 4 km on the M-discontinuity in the vicinity of TFO. The velocity in the upper mantle is 7.85 km/sec. The relation of topographic elevation to crustal thickness suggests an approach to isostatic equilibrium, which is deduced from a near-zero regional free-air gravity anomaly. However, lateral density change in the upper mantle is required to make the crustal-refraction model fit the observed gravity-anomaly values, provided that velocity and density are linearly related.

Arizona↗

USGS1990

Eocene diatom chert from Adak Island, Alaska

Bedded quartz cherts that contain recognizable diatoms are rare in the geologic record and are described here for the first time. The Eocene Andrew Lake Formation on Adak Island, Alaska consists of about 800 m of sedimentary and volcanogenic rocks. Quartz cherts containing diatoms occur in the upper part of the Andrew Lake Formation and crop out on the northern part of the island. The quartz chert formed at about 70 degrees C as determined by its oxygen isotopic composition. The diatoms were preserved in the chert because early and rapid alteration of ubiquitous volcanic glass in the section released silica and saturated the pore waters with respect to opal-A. Then, temperature rapidly increased with burial and the pore waters became undersaturated with respect to opal-A (biogenic silica), which occurred at a temperature greater than that needed to convert opal-CT to quartz. At this stage, delicate species of diatoms dissolved and quartz precipitated around the remaining more robust diatoms, forming diatom theft. Subsequently, grain-growth occurred and quartz replaced the frustules on a very fine scale.

Alaska↗

USGS1977

Reconnaissance geology and geochronology of the Precambrian of the Granite Mountains, Wyoming

The Precambrian of the western part of the Granite Mountains contains a metamorphic complex of gneisses, schists and amphibolites that were derived through amphibolite-grade metamorphism from a sedimentary-volcanic sequence perhaps similar to that exposed in the southeastern Wind River Mountains. Whole-rock Rb-Sr dating places the time of metamorphism at 2925 ± 81 m.y. A high initial 87 Sr/ 86 Sr ratio of 0.7048 suggests that either the protoliths or the source terrane of the sedimentary component is several hundred million years older than the time of metamorphism. Following an interval of 300 ± 100 m.y. for which the geologic record is lacking or still undeciphered, the metamorphic complex was intruded by a batholith and satellite bodies of medium to coarse-grained, generally massive biotite granite and related pegmatite and aplite. The main body of granite is dated at 2602 ± 60 m.y. by the Rb-Sr method. Limited data suggest that diabase dikes were emplaced and nephrite veins were formed only shortly after intrusion of the granite. Emplacement of the granite at about 2600 m.y. ago appears to be related to a major period of regional granitic plutonism in the Precambrian of southern and western Wyoming. Granites sensu stricto that are dated between 2500 and 2600 m.y. occur in the Teton Range, the Sierra Madre, the Medicine Bow Mountains and the Laramie Range. This episode of granitic plutonism occurred some 50 to 100 m.y. later than the major tonalitic to granitic plutonism in the Superior Province of northern Minnesota and adjacent Ontario--the nearest exposed Precambrian W terrane that is analogous to the Wyoming Province. Initial 87 Sr/ 86 Sr ratios of some of the Wyoming granites are higher than expected if the rocks had been derived from juvenile magmas and it is likely that older crustal rocks were involved to some degree in the generation of these granites. Slightly to highly disturbed Rb-Sr and K-Ar mineral ages are obtained on rocks of the metamorphic complex arid on the granite. These ages range from about 2450 to 1450 m.y. and are part of a regional pattern of lowered mineral ages of Precambrian W rocks of southern Wyoming. A major discontinuity in these mineral ages occurs along a line extending from the northern Laramie Range, through the northern part of the Granite Mountains, to the southeastern Wind River Mountains. North of this line, Rb-Sr and K-Ar biotite ages are 2350 m.y. or greater whereas to the south, the biotite ages decrease drastically over a short distance to a common range of 1400-1600 m.y. We suggest that these lowered ages represent regional cooling below the 300° C isotherm as a consequence of uplift and erosion of the large crustal block occurring south of the age discontinuity. In this interpretation, the westerly-trending age discontinuity would be a zone of major crustal dislocation that resulted from vertical tectonics in late Precambrian X or early Precambrian Y time.

Wyoming↗

USGS2025

A 400-k.y. perspective on arc volcanism: An exceptional explosive eruption record from Central Mexico

Volcanic eruption records provide key information for hazard planning but suffer from a time-dependent loss of resolution, hindering long-term evaluations of volcano behavior. Here, we investigate an exceptional sedimentary sequence from Lake Chalco, on the SE edge of Mexico City, which provides a perspective on volcanic activity over hundred-thousand-year time scales, and we develop a methodological and analytical protocol applicable to core datasets globally. The lava-dominated base of the sequence (~105 m) is followed by ~155 m of lateral-collapse derived deposits and overlain by ~295 m of lacustrine sediments containing at least 450 visible tephra fall deposits (TFDs), spanning 400 k.y. These TFDs include 205 events sourced from large-magnitude (predominantly volcanic explosivity index [VEI] ≥5) silicic explosive eruptions, principally from regional polygenetic sources, and 205 deposits from the nearby Sierra Chichinautzin volcanic field (SCVF). A gradual decline in both frequency and apparent magnitude in the silicic eruption record is consistent with the gradual migration of volcanism south along the adjacent Sierra Nevada volcanic range, toward Popocatépetl. In contrast, the SCVF-derived deposits imply persistent but episodic activity, on time scales of 40–70 k.y., suggesting that the SCVF has been more continuously active than previously recognized. Decoupled trends between the SCVF and regional silicic sources implies that the total magmatic flux is not dictated at the arc scale, or by external (e.g., climatic) drivers, but instead reflects the independent development of individual volcanic systems. The records indicate a minimum long-term frequency of impactful eruptions on Mexico City to be one per 900 years (>1 cm tephra deposited) or 9000 years (≥10 cm tephra).

Central Mexico↗

USGS1990

Flexure and faulting of sedimentary host rocks during growth of igneous domes, Henry Mountains, Utah

A sequence of sedimentary rocks about 4 km thick was bent, stretched and uplifted during the growth of three igneous domes in the southern Henry Mountains. Mount Holmes, Mount Ellsworth and Mount Hillers are all about 12 km in diameter, but the amplitudes of their domes are about 1.2, 1.85 and 3.0 km, respectively. These mountains record successive stages in the inflation of near-surface diorite intrusions that are probably laccolithic in origin. The host rocks deformed along networks of outcrop-scale faults, or deformation bands, marked by crushed grains, consolidation of the porous sandstone and small displacements of sedimentary beds. Zones of deformation bands oriented parallel to the beds and formation contacts subdivided the overburden into thin mechanical layers that slipped over one another during doming.

Journal of Structural Geology↗

USGS2016

Paleomagnetic record determined in cores from deep research wells in the Quaternary Santa Clara basin, California

Paleomagnetic study of cores from six deep wells provides an independent temporal framework for much of the alluvial stratigraphy of the Quaternary basin beneath the Santa Clara Valley. This stratigraphy consists of 8 upward-fining cycles in the upper 300 m of section and an underlying 150 m or more of largely fine-grained sediment. The eight cycles have been correlated with the marine oxygen isotope record, thus providing one means of dating the section. The section has also proved to contain a rich paleomagnetic record despite the intermittent sedimentation characteristic of alluvial environments. Each well was designed to reach a depth of ~300 m, although 2 were terminated at shallower depth where bedrock was encountered and one (GUAD) was deepened to bedrock at 407.2 m. Cores were taken at intermittent intervals in most of the wells, composing ~20%–25% of their depths. In GUAD an attempt was made to core the entire upper 300 m, with core recovery of 201.8 m (67%). The paleomagnetic framework ranges from the 32 ka Mono Lake excursion near the top of the second sedimentary cycle to below the 780 ka Brunhes-Matuyama geomagnetic reversal beneath the eighth cycle. These ages nicely fit those assigned to the section based on correlation with the marine oxygen isotope record. Several episodes of anomalous magnetic inclinations were also found within the cyclic section in some of the wells. Some of the episodes of anomalous magnetic inclinations are only separated by short normal intervals in a pattern similar to that described for some welldocumented excursions. We consider that a geomagnetic excursion was likely only if the anomalous inclinations were found at approximately the same stratigraphic position in more than one drill hole. A deeper time constraint is provided by the upper boundary (990 ka) of the Jaramillo Normal Polarity Subchron recognized at a depth of 302 m in one deeply penetrating well (GUAD). Approximately 100 m of normal Jaramillo section is evident below that in wells GUAD and EVGR. The reversal that we identify as the 780 ka Brunhes-Matuyama boundary, found at depths of 291–303 m in three wells, indicates an average rate of deposition in this upper section of ~37 cm/k.y. In GUAD, the top of the underlying normally polarized section, which we assign to the upper part of the Jaramillo Normal Polarity Subchron, was found between 301.8 and 304.5 m. The resultant 10 m of reversed polarity section above the Jaramillo seems anomalously short for this 210 k.y. part of the Matuyama Chron, during which several times that thickness of section probably should have accumulated. This observation indicates that a significant unconformity should be present in that short section between the Jaramillo Subchron and the Brunhes-Matuyama boundary. Deeper cores in two wells (GUAD and EVGR) all have normal polarity and seem to represent much of the Jaramillo Subchron, although no base for that subchron was found. The resultant minimum rate of sedimentation for this lower section beneath the unconformity is 170 cm/k.y. The Mono Lake (ca. 32 ka), Pringle Falls (ca. 210 ka), and Big Lost (ca. 565 ka) geomagnetic excursions all seem to be represented in the Santa Clara Valley wells. Possible correlations to the Laschamp (ca. 40 ka) and Blake (ca. 110 ka) excursions are also noted. Three additional excursions that have apparently not been previously reported from western North America occur within cycle 6 (between 536 and 433 ka), near the base of cycle 5 (after 433 ka), and near the middle of cycle 2 (before ca. 75 ka).

California↗