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At least 91 records · Page 5Linked to original sources
USGS2001

Microbial lime-mud production and its relation to climate change

Microbial calcification has been identified as a significant source of carbonate sediment production in modern marine and lacustrine environments around the globe. This process has been linked to the production of modern whitings and large, micritic carbonate deposits throughout the geologic record. Furthermore, carbonate deposits believed to be the result of cyanobacterial and microalgal calcification suggest that the potential exists for long-term preservation of microbial precipitates and storage of carbon dioxide (CO 2 ). Recent research has advanced our understanding of the microbial-calcification mechanism as a photosynthetically driven process. However, little is known of the effects of this process on inorganic carbon cycling or of the effects of changing climate on microbial-calcification mechanisms. Laboratory experiments on microbial cellular physiology demonstrate that cyanobacteria and green algae can utilize different carbon species for metabolism and calcification. Cyanobacterial calcification relies on bicarbonate (HCO 3 – )utilization while green algae use primarily CO 2 . Therefore, depending on which carbonate species (HCO 3 – or CO 2 ) dominates in the ocean or lacustrine environments (a condition ultimately linked to atmospheric partial pressure PCO 2 ), the origin of lime-mud production by cyanobacteria and/or algae may fluctuate through geologic time. Trends of cyanobacteria versus algal dominance in the rock record corroborate this conclusion. These results suggest that relative species abundances of calcareous cyanobacteria and algae in the Phanerozoic may serve as potential proxies for assessing paleoclimatic conditions, including fluctuations in atmospheric PCO 2 .

AAPG Studies in Geology↗
USGS2013

Sedimentary history of the Tethyan margins of eastern Gondwana during the Mesozoic

A composite Mesozoic geological history for the Gondwana margins to the Eastern Tethys Ocean can be assembled from stratigraphic successions on the Australian and Himalayan margins and from drill sites of Ocean Drilling Program Legs 122 and 123. During the Triassic, this region drifted northwards, entering tropical paleolatitudes during the Late Triassic-Early Jurassic, then returned to mid-latitudes for the Middle Jurassic through Early Cretaceous. Shallow-water carbonates are restricted to the tropical-latitude interval; at other times, the margins are dominated by clastic deposition. Episodes of deltaic sandstone progradation over the shelves are caused by eustatic sealevel fluctuations, by wet climatic conditions within the source regions and by local tectonic activity. A major hiatus between Callovian shallow-water shelf deposits and Oxfordian deep-water sediments is an ubiquitous feature, which may be related to a widespread plate tectonic reorganization and the cascading effects of associated sealevel rise and elevated carbon dioxide levels. Off Northwest Australia, this Callovian/Oxfordian event also coincides with an episode of block faulting. Marginal sediments deposited during the Late Jurassic are mainly marine claystone containing abundant terrigenous organic matter. Shallow depths of carbonate compensation (CCD) during the Late Jurassic through Early Cretaceous prevented the preservation of carbonate over most of the Argo basin off Northwest Australia, and these deep-sea sediments consist mainly of condensed, oxygenated radiolarian-rich claystone. During the late Kimmeridgian-early Tithonian, a downward excursion in the CCD enabled limited preservation of some larger nannofossils and mollusc fragments within the pelagic deposits, a feature also recorded in coeval deposits in the Atlantic. Explosive volcanism accompanied the final stages of rifting between India and Australia during the late Berriasian and Valanginian, producing volcaniclastic debris washing into the deltas and widespread ash deposits. The late Barremian and Aptian sediments indicate a rise in the CCD, accompanied by warming of the region and an increased delivery of organic-rich claystone into the basins.

Book chapter↗
USGS2018

Exposed subsurface ice sheets in the Martian mid-latitudes

Thick deposits cover broad regions of the Martian mid-latitudes with a smooth mantle; erosion in these regions creates scarps that expose the internal structure of the mantle.We investigated eight of these locations and found that they expose deposits of water ice that can be >100 meters thick, extending downward from depths as shallow as 1 to 2 meters below the surface.The scarps are actively retreating because of sublimation of the exposed water ice.The ice deposits likely originated as snowfall during Mars’ high-obliquity periods and have now compacted into massive, fractured, and layered ice.We expect the vertical structure of Martian ice-rich deposits to preserve a record of ice deposition and past climate.

Science↗
USGS2007

Eruption and deposition of the Fisher Tuff (Alaska)--Evidence for the evolution of pyroclastic flows

Recognition that the Fisher Tuff (Unimak Island, Alaska) was deposited on the leeside of an ∼500–700‐m‐high mountain range (Tugamak Range) more than 10 km away from its source played a major role in defining pyroclastic flows as momentum‐driven currents. We reexamined the Fisher Tuff to evaluate whether deposition from expanded turbulent clouds can better explain its depositional features. We studied the tuff at 89 sites and sieved bulk samples from 27 of those sites. We find that the tuff consists of a complex sequence of deposits that record the evolution of the eruption from a buoyant plume (22 km) that deposited ∼0.2 km 3 of dacite magma as a pyroclastic fall layer to erupting ∼10–100 km 3 of andesitic magma as Scoria‐rich pyroclastic falls and flows that were mainly deposited to the north and northwest of the caldera, including those in valleys within the Tugamak Range. The distribution of the flow deposits and their welding, internal stratification, and the occurrence of lithic breccia all suggest that the pyroclastic flows were fed from a fountaining column that vented from an inclined conduit, the first time such a conduit has been recognized during a large‐volume caldera eruption. Pyroclastic flow deposits before and after the mountain range and thin veneer deposits high in the range are best explained by a flow that was stratified into a dense undercurrent and an overriding dilute turbulent cloud, from which deposition before the range was mainly from the undercurrent. When the flow ran into the mountain range, however, the undercurrent was blocked, but the turbulent cloud continued on. As the flow continued north, it restratified, forming another undercurrent. The Fisher Tuff thus records the passing of a flow that was significantly higher (800–1100 m thick) than the mountain range and thus did not require excessive momentum.

Alaska↗
USGS2003

Mobility of authigenic rhenium, silver, and selenium during postdepositional oxidation in marine sediments

Sedimentary records of redox-sensitive trace elements hold significant potential as indicators of paleoceanographic environmental conditions. Records of Re can reveal the intensity of past reducing conditions in sediments at the time of deposition, whereas records of Ag may record the magnitude of past diatom fluxes to the seafloor. Confidence in paleoenvironmental reconstruction from records of either metal, however, requires it to have experienced negligible redistribution since deposition. This study examines diagenetic rearrangements of Re and Ag that occur in response to exposure to bottom-water O 2 in environments of low sedimentation rate, including Madeira Abyssal Plain turbidites and eastern Mediterranean basin sapropels. Authigenic Re was remobilized quantitatively by oxidation but poorly retained by the underlying sediments. All records are consistent with previous work demonstrating that only a limited reimmobilization of Re occurs preferentially in C org -rich, reducing sediments. Silver was also mobilized quantitatively by oxidation, but it was subsequently immobilized more efficiently in all cases as sharp peaks immediately into anoxic conditions below active oxidation fronts, and these peaks remain immobile in anoxic conditions during long-term burial. Comparison of Ag, S, and Se records from various cores suggests that Ag is likely to have been immobilized as a selenide, a mechanism previously proposed for Hg in similar situations (Mercone et al., 1999) . Coexisting narrow peaks of Ag and Hg with Se offer a means of assessing whether oxidative burndown has ever occurred at the top of C org - and sulfide-rich sedimentary units. Although these results suggest that caution must be used when inferring paleoenvironmental information from records of Ag and Re in cores with low sediment accumulation rates (<5 cm ka −1 ), they should not affect the promise that authigenic Ag and Re records hold for paleoenvironmental reconstruction in sediments with higher accumulation rates and where anoxic conditions have been maintained continuously.

Geochimica et Cosmochimica Acta↗
USGS2011

Porosity variability in limestone sequences

Porosity is the state of being porous, as measured by the percentage of bulk volume of a rock or soil that is occupied by space, whether isolated or connected. In hydrocarbon-bearing limestone settings, subsurface porous strata containing the oil or gas usually underlie non-porous caprock through which hydrocarbons cannot pass. In settings, subsurface freshwater aquifers beneath caprock can become contaminated by saltwater intrusion during periods of drought. Islands of the Florida Keys consist of two types of emergent 125-ka limestone, a highly porous fossil coral reef with large voids and a less porous oolite with small grains and interstices. Both limestones are capped by impervious laminated Holocene calcrete whose dimensions differ greatly (Figure 1a and b). Porosity variability in the limestones is thought to be the cause. The less permeable oolite retained rainfall moisture longer, allowing longer periods of calcrete buildup. Reddish and brownish layers in both illustrated calcrete samples represent periods of influx of non-carbonate minerals on African dust. The hiatus or gap in these rock records represents an interval of >115 kyr during which no marine or terrestrial deposition is recorded.

Book chapter↗
USGS2002

Coastal geoindicators of environmental change in the humid tropics

The primary geoindicators appropriate for monitoring environmental changes in the humid tropics are transitory surface water levels, shoreline position, wetlands distribution, coral reefs, landforms, and sediment sequence and composition. Lateral zonations and temporal successions of vegetation also can be used as geoindicators of riverine and shoreline changes. All of these coastal geoindicators are sensitive to regional tectonic processes and anthropogenic alterations and they typically reflect significant changes in coastal conditions such as fluvial processes, coastal energy, water quality, relative sea level, and sediment supply. Where humid tropical coasts coincide with active tectonic margins, indicators of seismic activity are critical to understanding Coastal changes associated with co-seismic subsidence or uplift, tsunamis, and liquefaction of coastal sediments. Coastal landforms and sedimentary deposits that record late Quaternary environmental changes include perched fluvial and marine terraces, delta-plain morphologies, crevassesplay deposits, peats and other paleosols, beach ridges, mud capes, and mud volcanoes. Although these features and deposits typically reflect environmental changes spanning more than 100 years, they are relevant to modern processes, management of coastal lands and resources, and prediction of future conditions. In some regions of the humid tropics, large coastal areas are unaffected by hurricanes or typhoons. Nevertheless, these tropical coasts are vulnerable to other non-storm processes, such as El Nin??o events, tsunamis, and monsoons that increase water levels, and cause widespread flooding and beach erosion. The environmental and political significance of coastal geoindicators increases when they are integrated and applied to issues of human safety and health such as hazards mapping, risk assessment, and dispersion of contaminated sediments. However, to be relevant, those socio-environmental applications demand accurate predictions of future trends and rates of change.

Environmental Geology↗
USGS2023

Paleohydrologic history of pluvial lake San Agustin, New Mexico: Tracking changing effective moisture in southwest North America through the last glacial transition

Paleoclimate records across the Intermountain West region of North America show significant regional variation in timing and magnitude of wet conditions that accompanied the last glacial-interglacial transition. To understand the climate controls on paleohydrologic change, well-dated records are needed across the region. The Plains of San Agustin (New Mexico, USA) is a closed-basin watershed of the American Southwest influenced by both winter westerly and summer North American Monsoon precipitation. The flat valley floors of the Plains contain lake and groundwater discharge deposits that record multiple periods of past wet climate. We present a record of hydroclimate for the past 26,000 years based on radiocarbon, U–Th series, and OSL dating of these deposits and stratigraphic correlation across the three sub-basins of the lake system. We find that two major lake oscillations occurred, coincident with the global Last Glacial Maximum (∼23–18 ka) and with Heinrich Stadial 1 (∼17–14 ka). The LGM lake cycle created a deep lake in the lowermost sub-basin, fed by marsh/lake overflow in the upper sub-basins. The Heinrich Stadial 1 wet interval attained the highest recorded lake level between ∼17.0 and 15.3 ka, merging the lower two sub-basins into one lake. Both lake cycles agree well in timing and magnitude with other lake-based records from the southwestern U.S., supporting previous interpretations that a south-shifted cool season storm track brought a dipole-like pattern of enhanced moisture to the southwest at the expense of drier conditions in the northwest during the LGM and deglaciation. A transition from lake to groundwater discharge conditions followed during the Bølling-Allerød (14.7–12.9 ka) across the study area. Wet meadows prevailed in the lower sub-basin during the Younger Dryas (12.9–11.7 ka), with marsh and open lake conditions in the upper and middle sub-basins, respectively. During the early Holocene, discrete wet intervals are recorded by ages for wet meadow deposits in all sub-basins with centennial-millennial frequency at 9.9, 8.8, 8.2 ka. These events agree well with other Intermountain West records showing wetter-than-present conditions prior to 8 ka. Two additional wet periods, centered at 6.8 and 5.4 ka, occurred during the driest known interval of the middle Holocene, and likely were partly supported in the Plains of San Agustin by summer moisture associated with the peak strength of the North American Monsoon. Our record highlights that both winter and summer moisture support water resources in New Mexico watersheds.

New Mexico↗
USGS1995

Sedimentology, Behavior, and Hazards of Debris Flows at Mount Rainier, Washington

Mount Rainier is potentially the most dangerous volcano in the Cascade Range because of its great height, frequent earthquakes, active hydrothermal system, and extensive glacier mantle. Many debris flows and their distal phases have inundated areas far from the volcano during postglacial time. Two types of debris flows, cohesive and noncohesive, have radically different origins and behavior that relate empirically to clay content. The two types are the major subpopulations of debris flows at Mount Rainier. The behavior of cohesive flows is affected by the cohesion and adhesion of particles; noncohesive flows are dominated by particle collisions to the extent that particle cataclasis becomes common during near-boundary shear. Cohesive debris flows contain more than 3 to 5 percent of clay-size sediment. The composition of these flows changed little as they traveled more than 100 kilometers from Mount Rainier to inundate parts of the now-populated Puget Sound lowland. They originate as deep-seated failures of sectors of the volcanic edifice, and such failures are sufficiently frequent that they are the major destructional process of Mount Rainier's morphologic evolution. In several deposits of large cohesive flows, a lateral, megaclast-bearing facies (with a mounded or hummocky surface) contrasts with a more clay-rich facies in the center of valleys and downstream. Cohesive flows at Mount Rainier do not correlate strongly with volcanic activity and thus can recur without warning, possibly triggered by non-magmatic earthquakes or by changes in the hydrothermal system. Noncohesive debris flows contain less than 3 to 5 percent clay-size sediment. They form most commonly by bulking of sediment in water surges, but some originate directly or indirectly from shallow slope failures that do not penetrate the hydrothermally altered core of the volcano. In contrast with cohesive flows, most noncohesive flows transform both from and to other flow types and are, therefore, the middle segments of flow waves that begin and end as flood surges. Proximally, through the bulking of poorly sorted volcaniclastic debris on the flanks of the volcano, flow waves expand rapidly in volume by transforming from water surges through hyperconcentrated stream flow (20 to 60 percent sediment by volume) to debris flow. Distally, the transformations occur more slowly in reverse order - from debris flow, to hyperconcentrated flow, and finally to normal streamflow with less than 20 percent sediment by volume. During runout of the largest noncohesive flows, hyperconcentrated flow has continued for as much as 40 to 70 kilometers. Lahars (volcanic debris flows and their deposits) have occurred frequently at Mount Rainier over the past several thousand years, and generally they have not clustered within discrete eruptive periods as at Mount St. Helens. An exception is a period of large noncohesive flows during and after construction of the modern summit cone. Deposits from lahar-runout flows, the hyperconcentrated distal phases of lahars, document the frequency and extent of noncohesive lahars. These deposits also record the following transformations of debris flows: (1) the direct, progressive dilution of debris flow to hyperconcentrated flow, (2) deposition of successively finer grained lobes of debris until only the hyperconcentrated tail of the flow remains to continue downstream, and (3) dewatering of coarse debris flow deposits to yield fine-grained debris flow or hyperconcentrated flow. Three planning or design case histories represent different lengths of postglacial time. Case I is representative of large, infrequent (500 to 1,000 years on average) cohesive debris flows. These flows need to be considered in long-term planning in valleys around the volcano. Case II generalizes the noncohesive debris flows of intermediate size and recurrence (100 to 500 years). This case is appropriate for consideration in some structural design. Case III flows are

Professional Paper↗
USGS1988

Neogene stratigraphy, foraminifera, diatoms, and depositional history of Maria Madre Island, Mexico: Evidence of early Neogene marine conditions in the southern Gulf of California

Foraminifera and diatoms have been analyzed from an upper Miocene through Pleistocene(?) sequence of marine sediments exposed on Maria Madre Island, largest of the Trés Marias Islands off the Pacific coast of Mexico. The Neogene stratigraphic sequence exposed on Maria Madre Island includes a mid-Miocene(?) non-marine and/or shallow marine sandstone unconformably overlain by a lower upper Miocene to uppermost Miocene upper to middle bathyal laminated and massive diatomite, mudstone, and siltstone unit. This unit is unconformably overlain by lower Pliocene middle to lower bathyal sandstones and siltstones which, in turn, are unconformably overlain by upper Pliocene through Pleistocene(?) upper bathyal to upper middle bathyal foraminiferal limestones and siltstones. These beds are unconformably capped by Pleistocene terrace deposits. Basement rocks on the island include Cretaceous granite and granodiorite, and Tertiary(?) andesites and rhyolites. The upper Miocene diatomaceous unit contains a low diversity foraminiferal fauna dominated by species of Bolivina indicating low oxygen conditions in the proto-Gulf Maria Madre basin. The diatomaceous unit grades into a mudstone that contains a latest Miocene upper to middle bathyal biofacies characterized by Baggina californica and Uvigerina hootsi along with displaced neritic taxa. An angular unconformity separates the upper Miocene middle bathyal sediments from overlying lower Pliocene siltstones and mudstones that contain a middle to lower bathyal biofacies and abundant planktonic species including Neogloboquadrina acostaensis and Pulleniatina primalis indicating an early Pliocene age. Significantly, this Pliocene unit contains common occurrences of benthic species restricted to Miocene sediments in California including Bulimina uvigerinaformis . Pliocene to Pleistocene(?) foraminiferal limestones and siltstones characterize submarine bank accumulations formed during uplift of the Trés Marias Island area, and include abundant planktonic foraminifera such as Pulleniatina obliquiloculata and Neogloboquadrina duterteri . Common benthic foraminifera in this unit are indicative of upper bathyal water depths. The Neogene depositional history recorded on Maria Madre Island involves an early late Miocene subsidence event marking formation of the Trés Marias Basin with relatively undiluted diatomaceous sediment deposited in a low oxygen setting. Subsidence and deepening of the basin continued into the early Pliocene along with rapid deposition of terrigenous clastics. Uplift of the basinal sequence began in late Pliocene time accompanied by deposition of upper Pliocene-Pleistocene foraminiferal limestones on a rising submarine bank. Continued episodic uplift of the Neogene deposits brought the island above sea level by late Pleistocene time.

Maria Madre Island↗
USGS1985

Geologic history of Goban Spur, Northwest Europe continental margin

Drilling on Leg 80 of the Deep Sea Drilling Project-International Phase of Ocean Drilling was conducted on a transect of four sites (548-551) across the continent-ocean boundary at Goban Spur, a prominent southwest-trending structural and topographic high on the Irish continental slope. Drilling results have been integrated with physiographic, gravimetric, paleomagnetic, and seismostratigraphic data to provide a comprehensive interpretation of the geologic history of this sediment-starved passive margin. The geologic history of Goban Spur and adjacent regions may be divided into three periods: (1) a pre-rift period, beginning at the end of the Hercynian orogeny, was marked by several phases of regional faulting. From the late Paleozoic to the Triassic, a major phase was responsible for the development of northeast-trending grabens and horsts, which were especially active areas of deposition during the early and middle Mesozoic. A second phase created northwesttrending listric, normal fault systems which delineated the pre-Atlantic rift system of the Early Cretaceous. (2) A terminal period of active rifting began approximately at the Jurassic/Cretaceous boundary (late Cimmerian phase), and was marked by a regional emergence. Syn-rift deposition is recorded at Goban Spur by accumulation of a transgressive sequence of hyposaline to pelagic sediments, chiefly Barremian. Interpretation of seismic profiles indicates that (?)Aptian sequences also were deposited in the deeper half-grabens during this phase of rifting. Volcanic rocks appear to be rare or absent in the syn-rift deposits on Goban Spur. (3) The post-rifting period began when ocean crust was accreted in a trough approximately 2000 m deep, beneath what is now the Porcupine Abyssal Plain. This period was marked by the outpouring of pillows and flows of typical oceanic tholeiites, some of which were recovered at Site 550. At the same time, the seaward edge of the continental crust appears to have been deeply intruded by oceanic tholeiites, forming an intermediate crust of transitional geophysical characteristics. Post-rift sedimentation started in the early Albian, and was accompanied by continuous regional subsidence. Before the Campanian, variable depositional environments produced quite diverse sedimentary sequences in the isolated halfgrabens. Carbonaceous shales were recovered in Cenomanian strata of the deepest site (550) and in lower Turonian beds at sites of intermediate depth (549-551). Depositional environments became more uniform across the margin after the late Campanian, and sedimentation was clearly influenced by more regional or global oceanographic events. Sea-level oscillations, climatic variations, changes in bottom circulation, and vertical fluctuations of the carbonate compensation depth (CCD) were responsible for lithologic changes and major hiatuses.

Initial reports of the Deep Sea Drilling Project↗
USGS2008

Hydrodynamic implications of textural trends in sand deposits of the 2004 tsunami in Sri Lanka

Field observations and sediment samples at a coastal-plain setting in southeastern Sri Lanka were used to document the erosional and depositional impacts of the 2004 Indian Ocean tsunami and to interpret the hydrodynamic processes that produced an extensive sand-sheet deposit. Tsunami deposit thicknesses ranged from 6 to 22??cm with thickness being controlled partly by antecedent topography. The deposit was composed of coarse to medium sand organized into plane-parallel laminae and a few laminasets. Vertical textural trends showed an overall but non-systematic upward fining and upward thinning of depositional units with an upward increase in heavy-mineral laminations at some locations. Repeated patterns in the vertical textural trends (upward fining, upward coarsening, uniform) were used to subdivide and correlate the deposit into five hydro-textural stratigraphic units. The depositional units were linked to hydrodynamic processes and upcurrent conditions, such as rates of sediment supply and composition of the sediment sources. Vertical changes in grain-size distributions recorded the depositional phases associated with flow acceleration, initial unsteady pulsating flow, relatively stable and uniform flow, flow deceleration, slack water, and return flow or flow redirection. Study results suggest that vertical textural trends from multiple cross-shore sections can be used to interpret complex tsunami flow histories, but at the location examined, interpretation of the lateral textural trends did not provide a basis for identifying the correct sediment transport pathways because flow near the landward boundary was multidirectional.

Sedimentary Geology↗
USGS2016

Geochemistry, Nd-Pb Isotopes, and Pb-Pb Ages of the Mesoproterozoic Pea Ridge Iron Oxide-Apatite–Rare Earth Element Deposit, Southeast Missouri

Iron oxide-apatite and iron oxide-copper-gold deposits occur within ~1.48 to 1.47 Ga volcanic rocks of the St. Francois Mountains terrane near a regional boundary separating crustal blocks having contrasting depleted-mantle Sm-Nd model ages (T DM ). Major and trace element analyses and Nd and Pb isotope data were obtained to characterize the Pea Ridge deposit, improve identification of exploration targets, and better understand the regional distribution of mineralization with respect to crustal blocks. The Pea Ridge deposit is spatially associated with felsic volcanic rocks and plutons. Mafic to intermediate-composition rocks are volumetrically minor. Data for major element variations are commonly scattered and strongly suggest element mobility. Ratios of relatively immobile elements indicate that the felsic rocks are evolved subalkaline dacite and rhyolite; the mafic rocks are basalt to basaltic andesite. Granites and rhyolites display geochemical features typical of rocks produced by subduction. Rare earth element (REE) variations for the rhyolites are diagnostic of rocks affected by hydrothermal alteration and associated REE mineralization. The magnetite-rich rocks and REE-rich breccias show similar REE and mantle-normalized trace element patterns. Nd isotope compositions (age corrected) show that: (1) host rhyolites have ɛ Nd from 3.44 to 4.25 and T DM from 1.51 to 1.59 Ga; (2) magnetite ore and specular hematite rocks display ɛ Nd from 3.04 to 4.21 and T DM from 1.6 to 1.51 Ga, and ɛ Nd from 2.23 to 2.81, respectively; (3) REE-rich breccias have ɛ Nd from 3.04 to 4.11 and T DM from 1.6 to 1.51 Ga; and (4) mafic to intermediate-composition rocks range in ɛ Nd from 2.35 to 3.66 and in T DM from 1.66 to 1.56. The ɛ Nd values of the magnetite and specular hematite samples show that the REE mineralization is magmatic; no evidence exists for major overprinting by younger, crustal meteoric fluids, or by externally derived Nd. Host rocks, breccias, and magnetite ore shared a common origin from a similar source. Lead isotope ratios are diverse: (1) host rhyolite has 206 Pb/ 204 Pb from 24.261 to 50.091; (2) Pea Ridge and regional galenas have 206 Pb/ 204 Pb from 16.030 to 33.548; (3) REE-rich breccia, magnetite ore, and specular hematite rock are more radiogenic than galena; (4) REE-rich breccias have high 206 Pb/ 204 Pb (38.122–1277.61) compared to host rhyolites; and (5) REE-rich breccias are more radiogenic than magnetite ore and specular-hematite rock, having 206 Pb/ 204 Pb up to 230.65. Radiogenic 207 Pb/ 206 Pb age estimates suggest the following: (1) rhyolitic host rocks have ages of ~1.50 Ga, (2) magnetite ore is ~1.44 Ga, and (3) REE-rich breccias are ~1.48 Ga. These estimates are broadly consistent and genetically link the host rhyolite, REE-rich breccia, and magnetite ore as being contemporaneous. Alteration style and mineralogical or textural distinctions among the magnetite-rich rocks and REE-rich breccias do not correlate with different isotopic sources. In our model, magmatic fluids leached metals from the coeval felsic rocks (rhyolites), which provided the metal source reflected in the compositions of the REE-rich breccias and mineralized rocks. This model allows for the likelihood of contributions from other genetically related felsic and intermediate to more mafic rocks stored deeper in the crust. The deposit thus records an origin as a magmatic-hydrothermal system that was not affected by Nd and Pb remobilization processes, particularly if these processes also triggered mixing with externally sourced metal-bearing fluids. The Pea Ridge deposit was part of a single, widespread, homogeneous mixing system that produced a uniform isotopic composition, thus representing an excellent example of an igneous-dominated system that generated coeval magmatism and REE mineralization. Geochemical features suggest that components in the Pea Ridge deposit originated from sources in an orogenic margin. Basaltic magmatism produced by mantle decompression melting provided heat for extracting melts from the middle or lower crust. Continual addition of mafic magmas to the base of the subcontinental lithosphere, in a back-arc setting, remelted calc-alkaline rocks enriched in metals that were stored in the crust. The St. Francois Mountains terrane is adjacent to the regional T DM line (defined at a value of 1.55 Ga) that separates ~1600 Ma basement to the west, from younger basements to the east. Data for Pea Ridge straddle the T DM values proposed for the line. The Sm-Nd isotope system has been closed since formation of the deposit and the original igneous signatures have not been affected by cycles of alteration or superimposed mineralizing events. No evidence exists for externally derived Nd or Sm. The source region for metals within the Pea Ridge deposit had a moderate compositional variation and the REE-rich breccias and mineralized rocks are generally isotopically homogeneous. The Pea Ridge deposit thus constitutes a distinctive isotopic target for use as a model in identifying other mineralized systems that may share the same metal source in the St. Francois Mountains terrane and elsewhere in the eastern Granite-Rhyolite province.

Missouri↗
USGS2005

Geologic map of northwestern Seattle (part of the Seattle North 7.5' x 15' quadrangle), King County, Washington

This map is the first of four new geologic maps covering the city of Seattle that are based on field exposures and an extensive database of subsurface geologic explorations. The landforms and near-surface deposits here record a relatively brief, recent period in the geologic history of the region that was dominated by the last advance of the continental ice sheet that covered the region about 17,000 years ago. Beneath the deposits of this ice sheet is a complex succession of older sediments that extends far below sea level across most of the map area. These older sediments are now exposed where modern erosion and landslides have sliced through the edge of the upland, most notably in coastal bluffs along Puget Sound.

Washington↗
USGS2015

Geologic map of the Orchard 7.5' quadrangle, Morgan County, Colorado

The Orchard 7.5' quadrangle is located along the South Platte River corridor on the semi-arid plains of eastern Colorado, and contains surficial deposits that record alluvial, eolian, and hillslope processes that have operated through environmental changes from the Pleistocene to the present. The South Platte River, originating high in the Colorado Front Range, has played a major role in shaping the geology of the quadrangle, which is situated downstream of where the last of the major headwater tributaries (St. Vrain, Big Thompson, and Cache la Poudre) join the river. Recurrent glaciation (and deglaciation) of basin headwaters affected river discharge and sediment supply far downstream, influencing alluvium deposition and terrace formation in the Orchard quadrangle. Kiowa and Bijou Creeks, unglaciated tributaries originating east of the Front Range also have played a major role by periodically delivering large volumes of sediment to the river during flood events, which may have temporarily dammed the river. Eolian sand deposits of the Greeley (north of river) and Fort Morgan (south of river) dune fields cover much of the quadrangle and record past episodes of sand mobilization during times of drought. With the onset of irrigation during historic times, the South Platte River has changed from a broad, shallow, and sandy braided river with highly seasonal discharge to a much narrower, deeper river with braided-meandering transition morphology and more uniform discharge. Along this reach, the river has incised into Upper Cretaceous Pierre Shale, which, although buried by alluvial deposits in Orchard quadrangle, is locally exposed downstream along the South Platte River bluff near the Bijou Creek confluence, in some of the larger draws, and along Wildcat Creek.

Colorado↗
USGS2015

Geologic map of the Masters 7.5' quadrangle, Weld and Morgan Counties, Colorado

The Masters 7.5' quadrangle is located along the South Platte River corridor on the semiarid plains of eastern Colorado and contains surficial deposits that record alluvial, eolian, and hillslope processes that have operated in concert with environmental changes from Pleistocene to present time. The South Platte River, originating high in the Colorado Front Range, has played a major role in shaping the surficial geology of the quadrangle, which is situated downstream of where the last of the major headwater tributaries (St. Vrain, Big Thompson, and Cache la Poudre) join the river. Recurrent glaciation (and deglaciation) of basin headwaters affected river discharge and sediment supply far downstream, influencing deposition of alluvium and terrace formation in the Masters quadrangle. Kiowa and Bijou Creeks, unglaciated tributaries originating in the Colorado Piedmont east of the Front Range and joining the South Platte River just downstream of the Masters quadrangle, also have played a major role by periodically delivering large volumes of sediment to the river during flood events, which may have temporarily dammed the river. Eolian sand deposits of the Greeley (north of river) and Fort Morgan (south of river) dune fields cover much of the quadrangle and record past episodes of sand mobilization during times of prolonged drought. With the onset of irrigation and damming during historical times, the South Platte River has changed from a broad, shallow sandy braided river with highly seasonal discharge to a much narrower, deeper river with braided-meandering transition morphology and more uniform discharge. Along the reach of river in the Masters quadrangle, the river has incised into Upper Cretaceous Pierre Shale, which, although buried by alluvial deposits here, is locally exposed downstream along the South Platte River bluff near the Bijou Creek confluence, in some of the larger draws, and along Wildcat Creek.

Colorado↗
USGS2011

Geologic map of the Cochiti Dam quadrangle, Sandoval County, New Mexico

The Cochiti Dam quadrangle is located in the southern part of the Española Basin and contains sedimentary and volcanic deposits that record alluvial, colluvial, eolian, tectonic and volcanic processes over the past seventeen million years. The geology was mapped from 1997 to 1999 and modified in 2004 to 2008. The primary mapping responsibilities were as follows: Dethier mapped the surficial deposits, basin-fill sedimentary deposits, Miocene to Quaternary volcanic deposits of the Jemez volcanic field, and a preliminary version of fault distribution. Thompson and Hudson mapped the Pliocene and Quaternary volcanic deposits of the Cerros del Rio volcanic field. Thompson, Minor, and Hudson mapped surface exposures of faults and Hudson conducted paleomagnetic studies for stratigraphic correlations. Thompson prepared the digital compilation of the geologic map. The mapped distribution of units is based primarily on interpretation of 1:16,000-scale, color aerial photographs taken in 1992, and 1:40,000-scale, black-and-white, aerial photographs taken in 1996. Most of the contacts on the map were transferred from the aerial photographs using a photogrammetric stereo-plotter and subsequently field checked for accuracy and revised based on field determination of allostratigraphic and lithostratigraphic units. Determination of lithostratigraphic units in volcanic deposits was aided by geochemical data, 40 Ar/ 39 Ar geochronology, aeromagnetic and paleomagnetic data. Supplemental revision of mapped contacts was based on interpretation of USGS 1-meter orthoimagery.

New Mexico↗