SEARCH · USGS Science

Results for “Episodes”

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 523 records · Page 29Linked to original sources

USGS1979

Late Precambrian Sixtymile Formation and orogeny at top of the Grand Canyon Supergroup, northern Arizona

The Sixtymile Formation, a 60-m-thick red-bed unit at the top of the late Precambrian Chuar Group, crops out at three places in the Chuar syncline in the eastern Grand Canyon. Its base is marked by a transition from gray marine shale to red sandstone. The Sixtymile was deposited in the deepening trough of the north-trending Chuar syncline during and as a consequence of regional uplift, tilting, and large-scale block faulting. Folding to form the Chuar syncline occurred in response to faulting on the parallel-trending Butte fault, about 1 km to the east. About 3.2 km of structural relief was developed across the Butte fault, principally during deposition of the lower member of the Sixtymile Formation. Landslide debris shed from the upthrown block on the east constitutes a major part of the material in the lower member. Conglomeratic strata of the upper member of the Sixtymile were deposited after the last increment of subsidence on the Chuar syncline, indicating that deposition of the Sixty mile Formation spanned the time of the structural disturbance. The folding and faulting are part of a regional structural episode that here is called the Grand Canyon orogeny, an event that signaled the end of deposition of strata of the Grand Canyon Supergroup and the beginning of a long interval of erosion. The age of the Grand Canyon orogeny is estimated at about 830 m.y., inferred from an evaluation of maximum and minimum reset K-Ar ages reported for the 1,100-m.y.-old Cardenas Lavas of the Unkar Group. In age and structural style, the Grand Canyon orogeny appears generally correlative with the East Kootenay orogeny of British Columbia, which separates the Purcell (Belt) and Windermere Supergroups. By analogy, strata of the Chuar Group below the Sixtymile Formation are correlated with strata of the upper part of the Belt Supergroup, and conglomeratic strata of the Sixtymile deposited after the structural episode are correlated with conglomeratic strata of the Windermere that unconformably overlie the Belt.

Professional Paper↗

USGS1982

Tertiary volcanic rocks and uranium in the Thomas Range and northern Drum Mountains, Juab County, Utah

The Thomas Range and northern Drum Mountains have a history of volcanism, faulting, and mineralization that began about 42 m.y. (million years) ago. Volcanic activity and mineralization in the area can be divided into three stages according to the time-related occurrence of rock types, trace-element associations, and chemical composition of mineral deposits. Compositions of volcanic rocks changed abruptly from rhyodacite-quartz latite (42-39 m.y. ago) to rhyolite (38-32 m.y. ago) to alkali rhyolite (21 and 6-7 m.y. ago); these stages correspond to periods of chalcophile and siderophile metal mineralization, no mineralization(?), and lithophile metal mineralization, respectively. Angular unconformities record episodes of cauldron collapse and block faulting between the stages of volcanic activity and mineralization. The youngest angular unconformity formed between 21 and 7 m.y. ago during basin-and-range faulting. Early rhyodacite-quartz latite volcanism from composite volcanoes and fissures produced flows, breccias, and ash-flow tuff of the Drum Mountains Rhyodacite and Mt. Laird Tuff. Eruption of the Mt. Laird Tuff about 39 m.y. ago from an area north of Joy townsite was accompanied by collapse of the Thomas caldera. Part of the roof of the magma chamber did not collapse, or the magma was resurgent, as is indicated by porphyry dikes and plugs in the Drum Mountains. Chalcophile and siderophile metal mineralization, resulting in deposits of copper, gold, and manganese, accompanied early volcanism. Te middle stage of volcanic activity was characterized by explosive eruption of rhyolitic ash-flow tuffs and collapse of the Dugway Valley cauldron. Eruption of the Joy Tuff 38 m.y. ago was accompanied by subsidence of this cauldron and was followed by collapse and sliding of Paleozoic rocks from the west wall of the cauldron. Landslides in The Dell were covered by the Dell Tuff, erupted 32 m.y. ago from an unknown source to the east. An ash flow of the Needles Range(?) Formation was erupted 30-31 m.y. ago from an unknown source. Mineralization probably did not occur during the rhyolitic stage of volcanism. The last stage of volcanism was contemporaneous with basin-and-range faulting and was characterized by explosive eruption of ash and pumice, forming stratified tuff, and by quiet eruption of alkali rhyolite as viscous flows and domes. The first episode of alkali rhyolite volcanism deposited the beryllium tuff and porphyritic rhyolite members of the Spor Mountain Formation 21 m.y. ago. After a period of block faulting, the stratified tuff and alkali rhyolite of the Topaz Mountain Rhyolite were erupted 6-7 m.y. ago along faults and fault intersections. Erosion of Spor Mountain, as well as explosive eruptions through dolomite, provided abundant dolomite detritus to the beryllium tuff member. The alkali rhyolite of both formations is fluorine rich, as is evident from abundant topaz, and contains anomalous amounts of lithophile metals. Alkali rhyolite volcanism was accompanied by lithophile metal mineralization which deposited fluorite, beryllium, and uranium. The structure of the area is dominated by the Thomas caldera and the younger Dugway Valley cauldron, which is nested within the Thomas caldera; the Thomas caldera is surrounded by a rim of Paleozoic rocks at Spor Mountain and Paleozoic to Precambrian rocks in the Drum Mountains. The Joy fault and Dell fault system mark the ring-fracture zone of the Thomas caldera. These structural features began to form about 39 m.y. ago during eruption of the Mt. Laird Tuff and caldera subsidence. The Dugway Valley cauldron sank along a series of steplike normal faults southeast of Topaz Mountain in response to collapse of the magma chamber of the Joy Tuff. Caldera structure was modified by block faulting between 21 and 7 m.y. ago, the time of widespread extensional faulting in the Basin and Range Province. Vents erupted alkali rhyolite 6-7 m.y. ago along basin-and-range faults.

Professional Paper↗

USGS1987

Geomorphological interpretations of late Quaternary terraces in western Tennessee and their regional tectonic implications

By use of large-scale topographic maps and traditional techniques of geomorphology. a series of stream terraces was identified and delineated on the Obion, Forked Deer, Hatchie, Loosahatchie, and Wolf Rivers in western Tennessee. From lowest and youngest to highest and oldest, they are designated the Finley, Hatchie, Humboldt, and Henderson terraces. The areal extent and continuity of the relict flood plains are ample to permit reconstruction of former elevations and valley slopes. Downvalley gradients of the terraces and morphology are strongly indicative of the processes and events that influenced terrace formation. Nil gradients for several tens of kilometers upstream from the Mississippi alluvial valley on the Finley and Hatchie terraces and shoreline features on the Finley terrace are interpreted as evidence that backwater flooding from the Mississippi Valley produced extensive seasonal or permanent lakes in the tributary valleys. Therefore, in part. these terraces are relict lacustrine plains. The Humboldt and Henderson terraces do not have any of these characteristics and are interpreted as being entirely of fluvial origin. All terraces are capped by a veneer of loess representing one or more episodes of deposition. Interstream terrace correlations made by using only geomorphic evidence are not definitive, and two interpretations are nearly equally viable. One of these has potentially significant regional tectonic implications because it suggests Quaternary uplift in the vicinity of a known pluton along the flank of the major rift in the New Madrid seismic zone. However, the preferred interpretation suggests that no appreciable regional structural deformation has taken place. Neither interpretation indicates Quaternary deformation in the upland area relative to the Mississippi alluvial valley. Topographic relationships of the terraces to features resulting from the sequence of Pleistocene aggradation and degradation in the Mississippi Valley indicate correlation of the Finley terrace with the early Wisconsinan episode of outwash deposition. Several lines of evidence suggest correlation of the Hatchie terrace with the Prairie terrace of the lower Mississippi Valley, which is Sangamonian in age. By inference, the Humboldt terrace could be of Yarmouthian age.

Tennessee↗

USGS1984

The evolution of the southern California uplift, 1955 through 1976

The southern California uplift culminated in 1974 as a 150- km-wide crustal swell that extended about 600 km eastward and east-southeastward from Point Arguello to the Colorado River and Salton Sea, respectively; it was characterized by remarkably uniform height changes between 1959 and 1974 of 0.30-0.35 m over at least half of its 60,000-70,000 km 2 area. At its zenith, the uplift included virtually the entire Transverse Ranges geologic province and parts of the Coast Ranges, San Joaquin Valley, Sierra Nevada, Basin and Range, Mojave Desert, Peninsular Ranges, and Salton Trough provinces. The alinement of the western part of the uplift closely paralleled the east-trending Transverse Ranges, whereas the southern flank of the eastern lobe roughly coincided with the west-northwest-trending San Andreas fault. The position and configuration of the uplift associate it with a singularly complex section of the boundary between the North American and Pacific plates that has certainly sustained major modification during the past 5 million years and probably during the past 1 million years. Surface deformation can be categorized as tectonic or nontectonic. Nontectonic vertical displacements associated with the activities of man have overwhelmed natural compaction and areally significant soil expansion in the southern California area. Because tectonic displacements are implicitly defined as those that cannot be otherwise explained, those vertical movements that can be reasonably attributed to artificial processes have been subtracted from our reconstructed configurations of the uplift. Hence this reconstruction has necessarily included the assembly and evaluation of an enormous volume of data on oil-field operations, changes in ground-water levels, and measured subsidence (or rebound) associated with changes in the underground fluid regimen. Measured changes in height at various stages in the evolution of the uplift have been based chiefly on first-order levelings carried out between 1953 and 1976. Exceptions to this generalization consist largely of the results of pre-1953 surveys through the western Transverse Ranges and the eastern Mojave Desert. Errors in measured height differences derive from blunders, systematic survey errors, random survey errors, improperly formulated orthometric corrections, and intrasurvey movement; the last of these has created the most serious problems encountered in our reconstruction of the basic data. A variety of independent tests indicate that survey error associated with the utilized levelings was generally small and fell largely within the predicted random-error range. Moreover, the redundancy and coherence displayed by the entire data set provide convincing evidence of survey accuracy and the virtual absence of height- and slope-dependent error in particular. Our reconstructions of the changing configuration of the uplift derive chiefly from comparisons among sequentially developed observed elevations along the same route. Most of the observed elevations from which the vertical displacements were computed have been reconstructed with respect to bench mark Tidal 8, San Pedro, as invariant in height. Because the San Pedro tide station has been characterized by a history of modest relative uplift, vertical displacements referred to this station are biased slightly toward the appearance of subsidence. Where the observed elevations cannot be conveniently tied to Tidal 8, they have been referred to secondary control points whose history with respect to Tidal 8 can be independently established. Each of the lines of observed elevation changes provides, accordingly, a section athwart or along the axis of the uplift from which the changes in the configuration of the uplift can be roughly generalized. Because relatively few surveys were run in 1955, which we choose as a representative temporal datum, we have commonly incorporated the results of earlier or of somewhat later levelings as the equivalents of 1955 surveys. Although this procedure introduces a certain subjectivity, the probable equivalence between the results of these earlier or later surveys with those that would have been obtained had this leveling been carried out in 1955, usually can be independently tested. Wherever the calculated vertical displacements are based on comparisons between the results of levelings over different routes, the observed elevations have been orthometrically corrected to agree with those that would have been produced had each of these surveys been along the same route. The growth of the southern California uplift consisted of two well-defined spasms of positive movement, the second of which was closely followed by partial collapse. Our reconstruction, although it clearly errs in detail, indicates that the uplift, together with marginal and apparently ephemeral tectonic subsidence, nucleated in the west-central Transverse Ranges near Ozena, sometime between the spring of 1959 and the spring of 1960. The uplift expanded rapidly eastward (and probably westward as well), and by the fall of 1961 much of the Transverse Ranges and the Mojave Desert at least as far east as Twentynine Palms had risen by as much as 0.25 m. Between 1962 and 1972 the area included by the initially developed (1959-61) uplift sustained additional but clearly decelerating uplift accompanied locally by oscillatory displacements. Between 1972/ 73 and 1974 a second crustal spasm extended the uplift eastward to the Colorado River and elevated much of the eastern Mojave Desert by values that equaled or exceeded those developed within the western lobe. Between 1974 and 1976, at least the central part of the uplift sustained partial collapse that nowhere amounted to less than 50 percent of the cumulative uplift since 1959. Whether this collapse affected the entire uplift is conjectural, but we now recognize well-defined evidence of major down-to-the-north tilting that must have occurred within the eastern part of the uplift at some time between 1974 and 1976. Accumulating evidence indicates that nearly all the area included with the southern California uplift underwent similar uplift and partial collapse during the early part of the 20th century. Thus we infer that the recent uplift represents but a single event in an ongoing, more or less cyclic deformational process characterized by a period of about 50 years. Even though less than two full cycles are expressed in the geodetic record, the cumulative rate of uplift near the center of the recent uplift probably has averaged about 5 mm/yr, a value that is roughly consistent with the uplift rates that have been deduced for the late Quaternary emergent marine terraces along the south flank of the Transverse Ranges. Although the evolution of the recent uplift is relatively well defined, its correlation with the regional seismicity is poorly defined. A comparison between the occurrence of southern California earthquakes of magnitude ≥4 during the period 1932 to 1976 with the 1974 configuration of the uplift demonstrates the existence of (1) relatively aseismic areas within the western lobe of the uplift (in the western Transverse Ranges), in the central part of the uplift (in the western Mojave Desert), and along an east-trending zone that extends into the eastern Mojave athwart the south flank of the uplift (north of the Salton Sea) and (2) localized concentrations of seismic activity along the flanks of the uplift. Moreover, 9 of the 10 largest earthquakes recorded within or around the area of the southern California uplift during the period 1932 to 1976 (the 1933 Long Beach, the 1941 Santa Barbara, the 1946 Walker Pass, the 1947 Manix, the 1948 Desert Hot Springs, and the four major 1952 Kern County shocks) occurred before the inception of the uplift in 1959 or 1960. The area embraced by the southern California uplift has been identified with geodetically defined horizontal strain, part of which may have accumulated as a major north-south contractional event that roughly coincided with the first spasm of uplift. Nonetheless, continuing contractional strain associated with regionally developed partial collapse argues that the uplift cannot be fully explained simply as the vertical expression of continuing north-south compression. Consideration of the two well-defined historical episodes of uplift and partial collapse indicate that the southern California uplift may be the product of decoupling and viscous flow beneath the seismogenic zone, presumably driven by continuing motion between the irregularly margined plates south of the great bend of the San Andreas fault. Because the magnitude of the maximum uplift associated with each episode was approximately the same, there may be some threshold value above which collapse (viscous flow) may ensue; the absence of total collapse may be a function of precollapse strain hardening within the postulated subseismogenic viscoelastic layer.

California↗

USGS1993

Distribution, facies, ages, and proposed tectonic associations of regionally metamorphosed rocks in east- and south-central Alaska

Most of the exposed bedrock in east- and south-central Alaska has been regionally metamorphosed and deformed during Mesozoic and early Cenozoic time. All the regionally metamorphosed rocks are assigned to metamorphic-facies units on the basis of their temperature and pressure conditions and metamorphic age. North of the McKinley and Denali faults, the crystalline rocks of the Yukon- Tanana upland and central Alaska Range compose a sequence of dynamothermally metamorphosed Paleozoic and older(?) metasedimentary rocks and metamorphosed products of a Devonian and Mississippian continental-margin magmatic arc. This sequence was extensively intruded by postmetamorphic mid-Cretaceous and younger granitoids. Many metamorphic-unit boundaries in the Yukon-Tanana upland are low-angle faults that juxtapose units of differing metamorphic grade, which indicates that metamorphism predated final emplacement of the fault-bounded units. In some places, the relation of metamorphic grade across a fault is best explained by contractional faulting; in other places, it is suggestive of extensional faulting. Near the United States-Canadian border in the central Yukon- Tanana upland, metamorphism, plutonism, and thrusting occurred during a latest Triassic and Early Jurassic event that presumably resulted from the accretion of a terrane that had affinities to the Stikinia terrane onto the continental margin of North America. Elsewhere in the Yukon-Tanana upland, metamorphic rocks give predominantly late Early Cretaceous isotopic ages. These ages are interpreted to date either the timing of a subsequent Early Cretaceous episode of crustal thickening and metamorphism or, assuming that these other areas were also originally heated during the latest Triassic to Early Jurassic and remained buried, the timing of their uplift and cooling. This uplift and cooling may have resulted from extension. South of the McKinley and Denali faults and north of the Border Ranges fault system, medium-grade metamorphism across much of the southern Peninsular and Wrangellia terranes was early to synkinematic with the intrusion of tonalitic and granodioritic plutons of primarily Early and Middle Jurassic age in the Peninsular terrane and Late Jurassic age in the Wrangellia terrane. Areas metamorphosed during the Jurassic episode that crop out near the Border Ranges fault system were subsequently retrograded and deformed in Cretaceous and early Tertiary time during accretion of younger units to the south. North of the Jurassic metamorphic and plutonic complex, low-grade metamorphism affected the rest of the Wrangellia terrane sometime during Jurassic and (or) Cretaceous time. North of the Wrangellia terrane and immediately south of the McKinley and Denali faults, flyschoid rocks, which were deposited within a basin that separated the Wrangellia terrane from the western margin of North America, form a northeastward-tapering wedge. Within the western half of the wedge, flysch and structurally interleaved tectonic fragments were highly deformed and weakly metamorphosed; much of the metamorphism and deformation probably occurred sometime during mid- to Late Cretaceous time. In the eastern half of the wedge, flyschoid rocks form an intermediate-pressure Barrovian sequence (Maclaren metamorphic belt). Metamorphism of the Maclaren metamorphic belt was synkinematic with the Late Cretaceous to earliest Tertiary intrusion of foliated plutons of intermediate composition. Isotopic data suggest metamorphism extended into the early Tertiary and was accompanied by rapid uplift and cooling. Low- to medium-grade metamorphism throughout the wedge was probably associated with the accretion of the outboard Wrangellia terrane, as has been proposed for the Maclaren metamorphic belt. South of the Border Ranges fault system lie variably metamorphosed sequences of oceanic rocks that comprise the successively accreted Chugach, Yakutat, Ghost Rocks, and Prince William terranes. The Chugach terrane consists of three successively accreted sequences of differing metamorphic histories. Metamorphism in all the sequences was associated with north-directed underthrusting beneath either the combined Peninsular-Wrangellia terrane or the older and inner parts of the Chugach terrane. These sequences, from innermost to outermost are: (1) intermediate- to highpressure, transitional greenschist- to blueschist-facies metabasalt and metasedimentary rocks that were metamorphosed during the Early and Middle Jurassic; (2) prehnite-pumpellyite-facies melange that was metamorphosed sometime during the Jurassic and Cretaceous; and (3) low-pressure prehnite-pumpellyite- or greenschist- facies flysch and metavolcanic rocks that were initially metamorphosed during latest Cretaceous to early Tertiary time and, in the eastern Chugach Mountains, were subsequently overprinted by low-pressure amphibolite-facies metamorphism that accompanied widespread intrusion during Eocene time. A similar low-pressure-facies series also developed within melange and flysch of the Yakutat terrane; these rocks are also intruded by Eocene plutons and are correlated with similar rocks of the Chugach terrane. Seaward of the Chugach terrane are the strongly deformed but weakly metamorphosed (prehnite-pumpellyite-facies) deep-sea metasedimentary rocks and oceanic metavolcanic rocks of the Ghost Rocks and Prince William terranes. Metamorphism and deformation occurred during underthrusting of these terranes beneath the Chugach terrane in early Tertiary time and predated, perhaps by very little, intrusion by early Tertiary granitoids.

Alaska↗

USGS2007

Seismic stability of the Duwamish River Delta, Seattle, Washington

The delta front of the Duwamish River valley near Elliott Bay and Harbor Island is founded on young Holocene deposits shaped by sea-level rise, episodic volcanism, and seismicity. These river-mouth deposits are highly susceptible to seismic soil liquefaction and are potentially prone to submarine landsliding and disintegrative flow failure. A highly developed commercial-industrial corridor, extending from the City of Kent to the Elliott Bay/Harbor Island marine terminal facilities, is founded on the young Holocene deposits of the Duwamish River valley. The deposits of this Holocene delta have been shaped not only by relative sea-level rise but also by episodic volcanism and seismicity. Ground-penetrating radar (GPR), cores, in situ testing, and outcrops are being used to examine the delta stratigraphy and to infer how these deposits will respond to future volcanic eruptions and earthquakes in the region. A geotechnical investigation of these river-mouth deposits indicates high initial liquefaction susceptibility during earthquakes, and possibly the potential for unlimited-strain disintegrative flow failure of the delta front.

Washington↗

USGS2005

Paleontology of the upper Eocene to quaternary postimpact section in the USGS-NASA Langley core, Hampton, Virginia

The USGS-NASA Langley corehole was drilled in 2000 in Hampton, Va. The core serves as a benchmark for the study of calcareous nannofossils, dinoflagellates, diatoms and silico flagellates, mollusks, ostracodes, planktonic foraminifera and bolboformids, and vertebrate remains in the upper Eocene, Oligocene, Miocene, and Pliocene sediments in southeastern Virginia. These sediments were deposited after the comet or asteroid impact that produced the Chesapeake Bay impact structure. The Chickahominy Formation was deposited rapidly in outer neritic to upper bathyal marine environments during the last 2 million years of the late Eocene. The Drummonds Corner beds are newly recognized in the Langley core. These beds appear to represent shallower water or more nearshore deposition than the underlying Chickahominy deposits. Paleontology indicates an age in the later part of the early Oligocene, with a sharp floral and faunal break between the Drummonds Corner beds and the underlying Chickahominy Formation. Late Oligocene sedimentation is represented by the Old Church Formation. The record of early and middle Miocene deposition in the Langley core is incomplete. The Calvert Formation records brief episodes of deposition in three members, which are separated by unconformities and are called the informal Newport News beds, the Plum Point Member, and the Calvert Beach Member. A second episode of rapid deposition at the Langley site is preserved as the upper Miocene St. Marys and Eastover Formations. The Eastover is overlain unconformably by the Yorktown Formation, which is both late early and early late Pliocene. The highest unit, the Tabb Formation (Pleistocene), was not studied paleontologically. Continued movement along faults and fractures of the crater and nearby region may have enhanced the contributions of older material into the various units filling the Chesapeake Bay impact crater, as suggested by the conspicuous reworking of microfossils in many of the samples from the upper Eocene and younger units in the USGS-NASA Langley core.

Virginia↗

USGS2008

Overview of the 2004 to 2006, and continuing, eruption of Mount St. Helens, Washington

Rapid onset of unrest at Mount St. Helens on September 23, 2004, initiated an uninterrupted lava-dome-building eruption that continues to the time of writing this overview (spring 2006) for a volume of papers focused on this eruption. About three weeks of intense seismic unrest and localized surface uplift, punctuated by four brief explosions, constituted a ventclearing phase, during which there was a frenzy of media attention and considerable uncertainty regarding the likely course of the eruption. The third week exhibited lessened seismicity and only minor venting of steam and ash, but rapid growth of the uplift, or welt, south of the 1980-86 lava dome proceeded as magma continued to push upward. Crystalrich dacite (~65 weight percent SiO 2 ) lava first appeared at the surface on October 11, 2004, beginning the growth of a complex lava dome of uniform chemical composition accompanied by persistent but low levels of seismicity, rare explosions, low gas emissions, and frequent rockfalls. Petrologic studies suggest that the dome lava is chiefly of 1980s vintage, but with an admixed portion of new dacite. Alternatively, it may derive from a part of the magma chamber not tapped by 1980s eruptions. Regardless, detailed investigations of crystal chemistry, melt inclusions, and isotopes reveal a complex magmatic history. Largely episodic extrusion between 1980 and 1986 produced a relatively symmetrical lava dome composed of stubby lobes. In contrast, continuous extrusion at mean rates of about 5 m 3 /s in autumn 2004 to <1 m 3 /s in early 2006 has produced an east-west ridge of three mounds with total volume about equal to that of the old dome. During much of late 2004 to summer 2005, a succession of spines, two recumbent and one steeply sloping and each mantled by striated gouge, grew to nearly 500 m in length in the southeastern sector of the 1980 crater and later disintegrated into two mounds. Since then, growth has been concentrated in the southwestern sector, producing a relatively symmetrical mound with steep gougecovered slabs on its east flank. Throughout the eruption, the position of the extrusive vent has remained more or less fixed. Lack of geodetic evidence for either volume increase or pressure increase in the deep magmatic system since about 1990 and geodetic modeling that can account for only 20 to 30 percent of the 2004-to-present dome volume puzzles geodesists. Better constraints on parameters such as magma-chamber volume, crustal properties, and magma compressibility are needed to improve the models. Development of the welt and the new dome bisected horseshoe-shaped Crater Glacier, which formerly wrapped around three sides of the 1980s dome, and fractured, compressed, and thickened the glacier’s surviving east and west arms. Doubling of ice thickness resulted in increased flow rate and advance of termini, although rapid infiltration of water into the highly porous glacier bed prevented substantial basal sliding. Overall, dome growth and disintegration has removed surprisingly little ice. The outcome of the ongoing eruption remains uncertain, but Mount St. Helens’ varied eruptive history suggests multiple possibilities. One dynamical model and several petrologic investigations regard the current eruption as an extension of 1980s dome building that may persist continuously or episodically for years to come.

Washington↗

USGS2008

Extrusion rate of the Mount St. Helens lava dome estimated from terrestrial imagery, November 2004-December 2005

Oblique, terrestrial imagery from a single, fixed-position camera was used to estimate linear extrusion rates during sustained exogenous growth of the Mount St. Helens lava dome from November 2004 through December 2005. During that 14-month period, extrusion rates declined logarithmically from about 8-10 m/d to about 2 m/d. The overall ebbing of effusive output was punctuated, however, by episodes of fluctuating extrusion rates that varied on scales of days to weeks. The overall decline of effusive output and finer scale rate fluctuations correlated approximately with trends in seismicity and deformation. Those correlations portray an extrusion that underwent episodic, broad-scale stick-slip behavior superposed on the finer scale, smaller magnitude stick-slip behavior that has been hypothesized by other researchers to correlate with repetitive, nearly periodic shallow earthquakes.

Washington↗

USGS2012

History of surface displacements at the Yellowstone Caldera, Wyoming, from leveling surveys and InSAR observations, 1923-2008

Modern geodetic studies of the Yellowstone caldera, Wyoming, and its extraordinary tectonic, magmatic, and hydrothermal systems date from an initial leveling survey done throughout Yellowstone National Park in 1923 by the U.S. Coast and Geodetic Survey. A repeat park-wide survey by the U.S. Geological Survey (USGS) and the University of Utah during 1975-77 revealed that the central part of the caldera floor had risen more than 700 mm since 1923, at an average rate of 14±1 mm/yr. From 1983 to 2007, the USGS conducted 15 smaller surveys of a single level line that crosses the northeast part of the caldera, including the area where the greatest uplift had occurred from 1923 to 1975-77. The 1983 and 1984 surveys showed that uplift had continued at an average rate of 22±1 mm/yr since 1975-77, but no additional uplift occurred during 1984-85 (-2±5 mm/yr), and during 1985-95 the area subsided at an average rate of 19±1 mm/yr. The change from uplift to subsidence was accompanied by an earthquake swarm, the largest ever recorded in the Yellowstone area (as of March 2012), starting in October 1985 and located near the northwest rim of the caldera. Interferometric synthetic aperture radar (InSAR) images showed that the area of greatest subsidence migrated from the northeast part of the caldera (including the Sour Creek resurgent dome) during 1992-93 to the southwest part (including the Mallard Lake resurgent dome) during 1993-95. Thereafter, uplift resumed in the northeast part of the caldera during 1995-96, while subsidence continued in the southwest part. The onset of uplift migrated southwestward, and by mid-1997, uplift was occurring throughout the entire caldera (essentially rim to rim, including both domes). Consistent with these InSAR observations, leveling surveys indicated 24±3 mm of uplift in the northeast part of the caldera during 1995-98. The beginning of uplift was coincident with or followed shortly after an earthquake swarm near the north caldera rim during June-July 1995 - the strongest swarm since 1985. Rather than a single deformation source as inferred from leveling surveys, the InSAR images revealed two distinct sources - one beneath each resurgent dome on the caldera floor. Subsequently, repeated GPS surveys (sometimes referred to as "campaign" surveys to distinguish them from continuous GPS observations) and InSAR images revealed a third deformation source beneath the north caldera rim. The north-rim source started to inflate in or about 1995, resulting in as much as 80 mm of surface uplift by 2000. Meanwhile, motion of the caldera floor changed from uplift to subsidence during 1997-8. The north rim area rose, while the entire caldera floor (including both domes) subsided until 2002, when both motions paused. Uplift in the northeast part of the caldera resumed in mid-2004 at a historically unprecedented rate of as much as 70 mm/yr, while the north rim area subsided at a lesser rate. Resurveys of the level line across the northeast part of the caldera in 2005 and 2007 indicated the greatest average uplift rate since the initial survey in 1923-53±3 mm/yr. Data from a nearby continuous GPS (CGPS) station showed that the uplift rate slowed to 40-50 mm/yr during 2007-8 and to near zero by September 2009. Following an intense earthquake swarm during January-February 2010, this one near the northwest caldera rim and the strongest since the 1985 swarm in the same general area, CGPS stations recorded the onset of subsidence throughout the entire caldera. Any viable model for the cause(s) of ground deformation at Yellowstone should account for (1) three distinct deformation sources and their association with both resurgent domes and the north caldera rim; (2) interplay among these sources, as suggested by the timing of major changes in deformation mode; (3) migration of the area of greatest subsidence or uplift from the northeast part of the caldera to the southwest part during 1992-95 and 1995-97, respectively; (4) repeated cycles of uplift and subsidence and sudden changes from uplift to subsidence or vice versa; (5) spatial and temporal relationships between changes in deformation mode and strong earthquake swarms; and (6) lateral dimensions of all three deforming areas that indicate source depths in the range of 5 to 15 km. We prefer a conceptual model in which surface displacements at Yellowstone are caused primarily by variations in the flux of basaltic magma into the crust beneath the caldera. Specifically, we envision a magmatic conduit system beneath the northeast part of the caldera that supplies basalt from a mantle source to an accumulation zone at 5-10 km depth, perhaps at a rheological boundary within a crystallizing rhyolite body remnant from past eruptions. Increases in the magma flux favor uplift of the caldera and decreases favor subsidence. A delicate equilibrium exists among the mass and heat flux from basaltic intrusions, heat and volatile loss from the crystallizing rhyolite body, and the overlying hydrothermal system. In the absence of basalt input, steady subsidence occurs mainly as a result of fluid loss from crystallizing rhyolite. At times when a self-sealing zone in the deep hydrothermal system prevents the escape of magmatic fluid, the resulting pressure increase contributes to surface uplift within the caldera; such episodes end when the seal ruptures during an earthquake swarm. To account for the north rim deformation source, we propose that magma or fluid exsolved from magma episodically escapes the caldera system at the three-way structural intersection of (1) the northern caldera boundary, (2) an active seismic belt to the north-northwest that is associated with the Hebgen Lake fault zone, and (3) the Norris - Mammoth corridor - a zone of faults, volcanic vents, and thermal activity that strikes north from the north rim of the caldera near Norris Geyser Basin to Mammoth Hot Springs near the northern boundary of Yellowstone National Park. Increased fluid flux out of the caldera by way of this intersection favors subsidence of the north rim area, and decreased flux favors uplift. This model does not account for poroelastic and thermoelastic effects, nonelastic rheology, or heat and mass transport in the hot and wet subcaldera crust. Such effects almost surely play a role in caldera deformation and are an important topic of ongoing research.

Wyoming↗

USGS2015

Arctic Alaska’s Lower Cretaceous (Hauterivian and Barremian) mudstone succession—Linking lithofacies, texture, and geochemistry to marine processes

We present new images and descriptions of the lithofacies and organic facies of the pebble shale unit and lower part of the Hue Shale (Lower Cretaceous) of Arctic Alaska at a high magnification that illustrates their textural characteristics. Our aims were to describe and determine the distribution of facies in these petroleum source rocks and to identify the processes that formed them. We sampled at high-resolution and applied new petrographic techniques combined with scanning electron microscopy and geochemical analyses to samples collected from three widely spaced sections—located in exposures along the Canning River and continuous core from the Mikkelsen Bay State 1 and Orion 1 wells. Results from these three locations indicate that this succession consists primarily of clay-rich mudstones that are variously silt- or sand-bearing and clay-dominated mudstones that exhibit mainly relict, 2–5 millimeter thick bedding and common but variable microbioturbation, rare macrobioturbation, and common fabrics of pelleted clay and silt. These mudstones contain rare, poorly sorted, silt-rich basal laminae that are often discontinuous and have wavy, sharp bases and crude upward fining. In addition, mud-supported, outsized clasts (dropstones) of fine sand to pebble size are present throughout the succession as isolated clasts or in clusters. We interpret these textures and much of this succession to result from intermittent deposition by suspension settling from melting seasonal sea ice—sometimes sediment-laden—and associated primary productivity. Overall, this mudstone succession fines and deepens upward from the pebble shale unit into the Hue Shale. In the Hue Shale of the Orion well, however, different processes intermittently deposited thin, discrete intervals of coarser sediment that probably represent deposition from density currents. Also in the Hue Shale of the Orion well, several thicker sandstone and silt-dominated mudstone units with discordant, scoured bases and cut and fill structures represent erosion during higher energy events such as major storms. Other lithofacies within the succession are graded tuffs/bentonites and tuffaceous/bentonitic mudstones from episodic volcanic ash falls; these are abundant in the Hue Shale, and very rare in the pebble shale unit of the two wells. Organic-carbon rich strata associated with volcanic ash intervals of the pebble shale unit and Hue Shale in the Mikkelsen 1 well have some of the best petroleum source rock potential determined for this succession. Authigenic pyrite and carbonate-cement-dominated mudstone are also present in both units of all three sections. The carbonate-cemented units indicate breaks in sedimentation and are common in the Hue Shale and in sections of the pebble shale unit interpreted to be more distal, such as along the Canning River. Our results document the variation in facies and textures of the Hauterivian and Barremian Lower Cretaceous mudstone succession of Arctic Alaska. Comparison of these characteristics to the products of modern processes on the North Slope of Alaska, in the Beaufort Sea, and elsewhere suggest that this succession formed primarily from depositional processes related to seasonal sea ice with intermittent fluvial-sourced sediment deposited by density currents and episodic erosion and reworking by storms and other currents.

Alaska↗

USGS1966

Gravity survey and regional geology of the Prince William Sound epicentral region, Alaska

Sedimentary and volcanic rocks of Mesozoic and early Tertiary age form a roughly arcuate pattern in and around Prince William Sound, the epicentral region of the Alaska earthquake of 1964. These rocks include the Valdez Group, a predominantly slate and graywacke sequence of Jurassic and Cretaceous age, and the Orca Group, a younger sequence of early Tertiary age. The Orca consists of a lower unit of dense-average 2.87 g per cm 3 (grams per cubic centimeter) pillow basalt and greenstone intercalated with sedimentary rocks and an upper unit of lithologically variable sandstone interbedded with siltstone or argillite. Densities of the clastic rocks in both the Valdez and Orca Groups average about 2.69 g per cm 3 . Granitic rocks of relatively low density (2.62 g per cm 3 ) cut the Valdez and Orca Groups at several localities. Both the Valdez and the Orca Groups were complexly folded and extensively faulted during at least three major episodes of deformation: an early period of Cretaceous or early Tertiary orogeny, a second orogeny that probably culminated in late Eocene or early Oligocene time and was accompanied or closely followed by emplacement of granitic batholiths, and a third episode of deformation that began in late Cenozoic time and continued intermittently to the present. About 500 gravity stations were established in the Prince William Sound region in conjunction with postearthquake geologic investigations. Simple Bouguer anomaly contours trend approximately parallel to the arcuate geologic structure around the sound. Bouguer anomalies decrease northward from +40 mgal (milligals) at the southwestern end of Montague Island to -70 mgal at College and Harriman Fiords. Most of this change may be interpreted as a regional gradient caused by thickening of the continental crust. Superimposed on the gradient is a prominent gravity high of as much as 65 mgal that extends from Elrington Island on the southwest, across Knight and Glacier Islands to the Ellamar Peninsula and Valdez on the northeast. This high coincides with the wide belt of greenstone and pillow basalt of the Orca Group and largely reflects the high density of these volcanic rocks. A large low in the east-central part of the sound is inferred to have a composite origin, and results from the combined effects of low-density sedimentary and granitic rocks. The Prince William Sound gravity high extends southwest-northeast without major horizontal offset for more than 100 miles. Thus the belt of volcanic rocks causing the high constitutes a major virtually continuous, geologic element of south-central Alaska.

Alaska↗

USGS2006

Rebuilding Mount St. Helens

On May 18, 1980, Mount St. Helens, Washington exploded in a spectacular and devastating eruption that shocked the world. The eruption, one of the most powerful in the history of the United States, removed 2.7 cubic kilometers of rock from the volcano's edifice, the bulk of which had been constructed by nearly 4,000 years of lava-dome-building eruptions. In seconds, the mountain's summit elevation was lowered from 2,950 meters to 2,549 meters, leaving a north-facing, horseshoe-shaped crater over 2 kilometers wide. Following the 1980 eruption, Mount St. Helens remained active. A large lava dome began episodically extruding in the center of the volcano's empty crater. This dome-building eruption lasted until 1986 and added about 80 million cubic meters of rock to the volcano. During the two decades following the May 18, 1980 eruption, Crater Glacier formed tongues of ice around the east and west sides of the lava dome in the deeply shaded niche between the lava dome and the south crater wall. Long the most active volcano in the Cascade Range with a complex 300,000-year history, Mount St. Helens erupted again in the fall of 2004 as a new period of dome building began within the 1980 crater. Between October 2004 and February 2006, about 80 million cubic meters of dacite lava erupted immediately south of the 1980-86 lava dome. The erupting lava separated the glacier into two parts, first squeezing the east arm of the glacier against the east crater wall and then causing equally spectacular crevassing and broad uplift of the glacier's west arm. Vertical aerial photographs document dome growth and glacier deformation. These photographs enabled photogrammetric construction of a series of high-resolution digital elevation models (DEMs) showing changes from October 4, 2004 to February 9, 2006. From the DEMs, Geographic Information Systems (GIS) applications were used to estimate extruded volumes and growth rates of the new lava dome. The DEMs were also used to quantify dome height variations, size of the magma conduit opening, and the mechanics of dome emplacement. Previous lava-dome-building eruptions at the volcano have persisted intermittently for years to decades. Over time, such events constructed much of the cone-shaped mountain seen prior to the May 18, 1980 eruption. Someday, episodic dome growth may eventually rebuild Mount St. Helens to its pre-1980 form.

Scientific Investigations Map↗

USGS2007

Surficial Geologic Map of the West Franklin Quadrangle, Vanderburgh and Posey Counties, Indiana, and Henderson County, Kentucky

The valley of the Ohio River is filled with alluvium and outwash (unit Qal), which total 33-39 m thick under the land surface in the southeast part of the West Franklin quadrangle in Indiana, and 30.5-35 m thick under Diamond Island in the southwest corner of the quadrangle. The deposits are chiefly fine- to medium-grained, lithic quartzose sand, interbedded by lenses of clay, clayey silt, silt, coarse sand, granules, and gravel. Although grain size of the river alluvium varies widely, in general it fines upward-being gravelly sand to sandy gravel in the lower part, mainly sand in the middle part, and silty and clayey in the upper part (Holocene). The middle and lower parts probably accumulated during the Wisconsin Episode (late Pleistocene). The sandy middle part contains interbeds of clay, silt, and minor gravel. At the base is highly consolidated mud (silt and clay), sand, and gravel 2-10 m thick. This unit may be valley train that predates the Wisconsin Episode. Creek alluvium (unit Qa) is silt, clayey silt, and subordinate intercalated fine sand, granules, and pebbles; the coarser grains are generally concentrated in the basal 1-2 m of the deposit. Lenses and beds of clay are present locally. Fossil wood collected from an auger hole in the alluvial deposits of Little Creek, at depths of 10.6 m and 6.4 m, were dated 16,650?50 and 11,120?40 radiocarbon years, respectively. Probable lacustrine terrace silt and clay (Qlt), so-called slackwater-lake or backwater deposits, form deposits 12-22 m thick in the lowest reaches of tributary creeks to the Ohio River. The surfaces of the lacustrine deposits are terraces a few meters higher than the modern creek flood plains. Covering the bedrock upland is loess (Ql) 3-7.5 m thick, deposited about 18,000-12,000 years before present. Most surficial deposits in the quadrangle are probably no older than about 35,000 yrs. Lithologic logs, shear-wave velocities, and other cone penetrometer data are used to interpret depositional environments and geologic history of the alluvium and lacustrine deposits.

Scientific Investigations Map↗

USGS2011

Geologic map of the Bailey 30' x 60' quadrangle, North-Central Colorado

The Bailey, Colo. 1:100,000-scale quadrangle lies within two physiographic and geologic provinces in central Colorado: 1) the Front Range and 2) South Park. Most of the Front Range is composed of Proterozoic rocks ranging in age from 1,790 Ma to 1,074 Ma. Along the eastern flanks and within the Denver Basin, sedimentary rocks ranging from Pennsylvanian to Cretaceous are deformed and steeply tilted to the east. Upper Cretaceous through Paleocene rocks were deposited in the foreland (that is, the Front Range eastern flank) and hinterland (that is, South Park) of this thrust and reverse fault system developed during the Late Cretaceous to Paleocene Laramide orogeny. Within South Park, rocks range in age from Pennsylvanian to Miocene with Quaternary deposits indicating tectonic subsidence of the basin. These rocks record five major geologic episodes: 1) the Paleozoic Anasazi uplift that formed the Ancestral Rockies, 2) the Late Cretaceous to Paleocene Laramide orogeny, 3) widespread Eocene to Oligocene volcanism, 4) Oligocene-Quaternary tectonics, and 5) Quaternary glacial episodes.

Colorado↗

USGS2006

Geology, Water, and Wind in the Lower Helmand Basin, Southern Afghanistan

This report presents an overview of the geology, hydrology, and climate of the lower Helmand Basin, a large, closed, arid basin in southern Afghanistan. The basin is drained by the Helmand River, the only perennial desert stream between the Indus and Tigris-Euphrates Rivers. The Helmand River is the lifeblood of southern Afghanistan and has supported desert civilizations in the Sistan depression for over 6,000 years. The Helmand Basin is a structurally closed basin that began to form during the middle Tertiary as a consequence of the collision of several Gondwanaland fragments. Aeromagnetic studies indicate the basin is 3-5 kilometers deep over basement rocks. Continued subsidence along basin-bounding faults in Iran and Pakistan throughout the Neogene has formed the Sistan depression in the southwest corner of the basin. Lacustrine, eolian, and fluvial deposits are commonly exposed in the basin and were intruded by latest Miocene-middle Quaternary volcanoes, which indicates that depositional environments in the lower Helmand Basin have not substantially changed for nearly 10 million years. Lakes expanded in the Sistan depression during the Quaternary; however, the size and extent of these pluvial lakes are unknown. Climate conditions in the lower Helmand Basin likely mirrored climate changes in the Rajasthan Desert to the east and in Middle Eastern deserts to the west: greater aridity during global episodes of colder temperatures and increased available moisture during episodes of warmer temperatures. Eolian processes are unusually dominant in shaping the landscape in the basin. A strong wind blows for 120 days each summer, scouring dry lakebeds and creating dune fields from annual flood deposits. Nearly one-third of the basin is mantled with active or stabilized dunes. Blowing winds combined with summer temperatures over 50? Celsius and voluminous insect populations hatched from the deltaic wetlands create an environment referred to as the 'most odious place on earth' by 19th century visitors. During dry years, large plumes of dust originating from Sistan are recorded by weather satellites. The Helmand River drains about 40 percent of Afghanistan and receives most of its moisture from melting snow and spring storms. Similar to many desert streams, the Helmand and its main tributary, the Arghandab River, are characterized by large fluctuations in monthly and annual discharges. Water from the Helmand accumulates in several hamuns (shallow lakes) in the Sistan depression. The wetlands surrounding these hamuns are the largest in western Asia and are directly affected by droughts and floods on the Helmand. Average annual discharge on the Helmand is about 6.12 million megaliters (million cubic meters), and the annual discharge varies by a factor of five. In 2005, the region was just beginning to recover from the longest drought (1998-2005) of record back to 1830. Annual peak discharges range from less than 80 cubic meters per second in 1971 to nearly 19,000 cubic meters per second in 1885. Large floods fill each hamun to overflowing to create one large lake that overflows into the normally dry Gaud-i Zirreh basin. The interaction of flooding, active subsidence, and wind erosion causes frequent channel changes on the Helmand delta. A major development effort on the Helmand River was initiated after World War II with substantial aid from the United States. Two dams and several major canals were completed in the 1950s; however, poor drainage conditions on the newly prepared agricultural fields caused extensive waterlogging and salinization. New drains were installed and improved agricultural methods were implemented in the 1970s, and some lands became more productive. Since 1980, Afghanistan has endured almost constant war and civil and political strife. In 2005, the country was on a path to rebuild much of its technical infrastructure. Revitalization of agricultural lands in the lower Helmand Basin and improved managem

Scientific Investigations Report↗

USGS2007

Sedimentation History of Halfway Creek Marsh, Upper Mississippi River National Wildlife and Fish Refuge, Wisconsin, 1846-2006

The history of overbank sedimentation in the vicinity of Halfway Creek Marsh near La Crosse, Wis., was examined during 2005?06 by the U.S. Geological Survey and University of Wisconsin?Madison as part of a broader study of sediment and nutrient loadings to the Upper Mississippi River bottomlands by the U.S. Environmental Protection Agency, U.S. Fish and Wildlife Service, and U.S. Geological Survey. Historical sedimentation patterns and rates were interpreted from field-scale topographic surveys and sediment cores collected from the marsh and upstream flood plains. Historical maps and aerial photographs were used to establish the timing of disturbances and to document changes in channel patterns after Euro-American settlement (post 1846). Episodic overbank sedimentation patterns and rates were linked to watershed agricultural activity, large floods, artificial levee construction, channel alterations, and dam failures over the past 160 years. These forces affected sedimentation on and between levees, the development of alluvial fans and flood-plain splays, and the general pattern of flood-plain sedimentation through the upper and lower marsh. Historical overbank deposits, episodically deposited after about 1860, are as much as 6 feet thick in the upper marsh and as much as 4 feet thick in the lower marsh, representing a total volume of approximately 1.8 million cubic yards. These stratified deposits consist of multiple layers of silt and clay, very fine to fine sand, and some medium to very coarse sand. Coarse-grained deposits are associated with flood-plain splays caused by breaches in artificial levees during large floods. Estimated sedimentation rates were highest from 1919 to 1936 [26,890 cubic yards per year (yd3/yr)] and exceeded by about 30 times the 1846?85 rate of 920 yd3/yr and exceeded by 7 times the 1994?2006 rate of 3,740 yd3/yr. The 1994?2006 sedimentation rate was the lowest since Euro-American settlement, but natural levees along the 1994?2006 channel of Halfway Creek through the lower marsh continued to form and are currently (2006) about 1 foot higher than the surrounding marsh. Natural levee building in the lower marsh from 1994?2006 was accentuated by the lack of overbank sediment storage in the upper marsh. The historical storage of sediment in the upper and lower marsh affects modern streamflow and sediment transport processes of Halfway Creek and Sand Lake Coulee through the marsh, and it also affects marsh vegetation and wildlife habitat. Results from this investigation will help improve the understanding of how past overbank sedimentation patterns continue to influence modern and future water quality, sediment transport, nutrient loads, and water-related resources in riparian habitats common to the Upper Mississippi River National Wildlife and Fish Refuge.

Scientific Investigations Report↗

USGS2007

Sources, dispersal, and fate of fine sediment supplied to coastal California

We have investigated the sources, dispersal, and fate of fine sediment supplied to California coastal waters in a partnership between the U.S. Geological Survey (USGS) and the California Sediment Management Workgroup (CSMW). The purpose of this study was to document the rates and characteristics of these processes so that the State can better manage its coastal resources, including sediment. In this study, we made the following observations: - Rivers dominate the supply of fine sediment to the California coastal waters, with an average annual flux of 34 megatonnes (Mt). - Cliff and bluff erosion in central and southern California is a source of fine sediment, with a delivery rate of approximately 10 percent of river loads. In the southern most part of the State, however, where river-sediment loads are low, cliff and bluff erosion represent approximately 40 percent of the total fine-sediment flux. - Temporal variation in the sources of fine sediment is high. River floods and bluff erosion are episodic and dominated by winter storms, which supply most sediment flux to the coast. The magnitude of winter storms is generally related to the El Niño-Southern Oscillation (ENSO) and Pacific Decadal Oscillation (PDO) climate cycles. - The three rivers that dominate fine-sediment flux to the California coast are the Eel, Salinas, and Santa Clara Rivers. Because the sediment delivery from these and all other California coastal watersheds is episodic, individual rivers discharge most of their annual loads over the course of only a few days per year. - Spatial variation in river-sediment discharge is high and generally related to such watershed characteristics as geology, precipitation, and drainage area. For example, the Transverse Range of southern California represents only 9 percent of the watershed-drainage area but 18 percent of the fine-sediment flux, a function of the young sedimentary bedrock and active tectonics of this region. The urban rivers of southern California were observed to discharge sediment at rates consistent with those of the surrounding Transverse Range rivers, which share the same geologic setting. - Direct observations of fine-sediment dispersal have been limited to the river-mouth settings of the Eel and Santa Clara Rivers, where sediment has been observed to settle quickly from buoyant plumes and be transported along the seabed during periods of storm waves. - After heavy loading of fine sediment onto the continental shelf during river floods, there is increasing evidence that fluid-mud gravity flows occur within a layer 10 to 50 cm above the seabed and efficiently transport fine sediment offshore. - All along the California coast, the timing of river discharge and coastal winds and waves from storm events are strongly coherent; however, of large wave events with the potential for resuspending and transporting fine sediment occur during periods without significant rainfall and therefore no significant river discharge. - Although fine sediment dominates the midshelf mud belts offshore of California river mouths, these mud belts are not the dominant sink of fine sediment, much of which is deposited across the entire continental shelf, including the inner shelf, and offshelf into deeper water depths. - Accumulation rates of fine sediment, which can exceed several millimeters per year, are generally highest near river sources of sediment and along the inner shelf and midshelf. - Sediment-accumulation rates, as summarized from both long-term and recent investigations of continental-shelf geochronology, are generally consistent across California except in southern California, where recently the sediment-accumulation rate has been tenfold greater than the long-term rate, possibly as a result of increased river discharge, wastewater outfall inputs, or other anthropogenic sources. Thus, fine sediment is a natural and dynamic element of the California coastal system because of large, natural sediment sources and dynamic transport processes.

California↗