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At least 73 records · Page 4Linked to original sources
USGS2011

Mineralogy and composition of the oceanic mantle

The mineralogy of the oceanic basalt source region is examined by testing whether a peridotite mineralogy can yield observed whole-rock and olivine compositions from (1) the Hawaiian Islands, our type example of a mantle plume, and (2) the Siqueiros Transform, which provides primitive samples of normal mid-ocean ridge basalt. New olivine compositional data from phase 2 of the Hawaii Scientific Drilling Project (HSDP2) show that higher Ni-in-olivine at the Hawaiian Islands is due to higher temperatures ( T ) of melt generation and processing (by c . 300°C) related to the Hawaiian mantle plume. D Ni is low at high T , so parental Hawaiian basalts are enriched in NiO. When Hawaiian (picritic) parental magmas are transported to shallow depths, olivine precipitation occurs at lower temperatures, where D Ni is high, leading to high Ni-in-olivine. Similarly, variations in Mn and Fe/Mn ratios in olivines are explained by contrasts in the temperatures of magma processing. Using the most mafic rocks to delimit Siqueiros and Hawaiian Co and Ni contents in parental magmas and mantle source compositions also shows that both suites can be derived from natural peridotites, but are inconsistent with partial melting of natural pyroxenites. Whole-rock compositions at Hawaii and Siqueiros are also matched by partial melting experiments conducted on peridotite bulk compositions. Hawaiian whole-rocks have elevated FeO contents compared with Siqueiros, which can be explained if Hawaiian parental magmas are generated from peridotite at 4–5 GPa, in contrast to pressures of slightly greater than 1 GPa for melt generation at Siqueiros; these pressures are consistent with olivine thermometry, as described in an earlier paper. SiO 2 -enriched Koolau compositions are reproduced if high-Fe Hawaiian parental magmas re-equilibrate at 1–1·5 GPa. Peridotite partial melts from experimental studies also reproduce the CaO and Al 2 O 3 contents of Hawaiian (and Siqueiros) whole-rocks. Hawaiian magmas have TiO 2 contents, however, that are enriched compared with melts from natural peridotites and magmas derived from the Siqueiros depleted mantle, and consequently may require an enriched source. TiO 2 is not the only element that is enriched relative to melts of natural peridotites. Moderately incompatible elements, such as Ti, Zr, Hf, Y, and Eu, and compatible elements, such as Yb and Lu, are all enriched at the Hawaiian Islands. Such enrichments can be explained by adding 5–10% mid-ocean ridge basalt (crust) to depleted mantle; when the major element composition of such a mixture is recast into mineral components, the result is a fertile peridotite mineralogy.

Hawai'i↗
USGS1987

New evidence on the state of stress of the San Andreas fault system

Contemporary in situ tectonic stress indicators along the San Andreas fault system in central California show northeast-directed horizontal compression that is nearly perpendicular to the strike of the fault. Such compression explains recent uplift of the Coast Ranges and the numerous active reverse faults and folds that trend nearly parallel to the San Andreas and that are otherwise unexplainable in terms of strike-slip deformation. Fault-normal crustal compression in central California is proposed to result from the extremely low shear strength of the San Andreas and the slightly convergent relative motion between the Pacific and North American plates. Preliminary in situ stress data from the Cajon Pass scientific drill hole (located 3.6 kilometers northeast of the San Andreas in southern California near San Bernardino, California) are also consistent with a weak fault, as they show no right-lateral shear stress at ∼2-kilometer depth on planes parallel to the San Andreas fault.

California↗
USGS2000

Overview: Ancient Lake Creede

Lake Creede was moderately saline closed-basin lake that developed in the 26.9 Ma Creede caldera in the San Juan Mountains in the southwest Colorado. The volcaniclastic sediments deposited within the late Oligocene lake were first described and named as the Creede Formation by Emmons and Larsen (1923). The lake and its sedimentary fill are of interest first as representatives of a caldera-hosted lake in a silicic volcanic terrane, and second because of the likely involvement of lake fluids or related pore waters in the deposition of the 25 Ma silver and base-metal ores of the Creede mining district north of the Creede caldera (Fig. 1), as proposed Bethke and Rye (1979). Much of the material presented in this volume is based on observation of core samples and on downhole geophysical measurements obtained as part of a U.S. Continental Scientific Drilling Program in the moat of the Creede caldera. These core and downhole studies are supplemented by outcrop studies, some initiated in support of the drilling program (Bethke and Lipman, 1987), and by conceptual studies of the evolution of the Creede caldera and its surrounding landscape. Not surprisingly, not all authors agree on all interpretation. Most disagreements are pointed out in this overview chapter, and may present opportunities for future study.

Colorado↗
USGS2000

Hydrologic budget of the late Oligocene Lake Creede and the evolution of the upper Rio Grande drainage system

The filling history, hydrologic budget, and geomorphic development of ancient Lake Creede and its tributary basin are evaluated to determine the factors that controlled its character. The lake filled the Creede caldera that formed in the late Oligocene as a consequence of the eruption of the Snowshoe Mountain Tuff. The caldera's sedimentary fill accumlated to a depth of about 1.26 km and had a volume of about 89 km 3 . The highest lake level was ~3300 m (10,800 ft) present altitude before it drained eastward across a broad volcanic plateau as the ancestral Rio Grande. A tributary canyon several hundred meters deep was cut into hard rhyolite in the north wall of the caldera before the lake was more than half full; its presence demonstrates that ancient Lake Creede filled slowly and thus occupied a long-lived, closed basin. The slow filling rate is incompatible with the present water flux through the Creede caldera basin, because such a flow would fill the basin geologically instantaneously. This mismatch, together with the recognition that the Oligocene climate was similar to that of today, forces the reexamination of the hydrologic and geomorphic history of the caldera. That appraisal shows that the caldera cannot have resurged rapidly immediately after caldera collapse, and that ancient watershed must have been lass than half as large as the present upper Rio Grande basin. The ancient lake had a more or less constant surface area of about 200 km 2 that approximated a steady-state condition between inflow and evaporation. Although the lake level fluctuated with climatic variations, its surface elevation steadily climbed as sediment accumulated, accelerating as resurgance and dome growth usurped spacewithin the basin. It could have had one playa stage early in its development and another after the basin had nearly filled with sediment, but there is no direct evidence for either. At least the lower half of the sedimentary column (the part sampled by the scientific drilling) formed in an euxinic environment. This argues against a persistent early playa, although evaporative accumulation of brine was inevitable. When the rate of resurgance was rapid relative to sedimentary infilling, the lake would have been deep (i.e., bordered by bedrock rather than sedimentary fans). The geomorphic evolution of the Creede caldera and its watershed tracks a two-phase topographic history, the first the Oligocene through Miocene, and the second for Pliocene to the recent. In Oligocene time, the San Juan volcanic field was a hydrologically immature, gently undulating, and outward sloping, constructional volcanic plateau straddling the ancient Continental Divide. West of the Creede caldera, a dendritic drainage discharged northeastward into ancestral Cebolla Creek (a tributary of the ancestral Gunnison River) through an early stage of the Clear Creek graben in the vicinity of Spring Creek Pass. Miocene basalt choked, but did not reconstruct, the drainage. By the end of Miocene time a mature topography of moderate relief developed, exposing some of the higher ores in the Creede district to weathering. In the late Miocene-early Pliocene time the San Juan Mountains were uplifted and titled eastward; the ancestral Rio Grande was revitalized and cut deeply into the older terrain, excavating much of the accessible sediment from the moat of the Creede caldera and exposing successively lowe levels in the Creede district to oxidation. Simultaneously, the southeast end of the Clear Creek graben was reactivated and breached the southwest wall of the Creede caldera. The rejuvenated Rio Grande captured the formerly northeast-directed headwaters of ancestral Cebolla Creek, shifting more than 1000 km 2 from the Pacific-directed drainage to the Atlantic. The water budget for ancient Lake Creede was strictly limited by the early stages of the fist geomorphic cycle; the modern water budget is the product of the second cycle.

Colorado↗
USGS2009

Geologic columns for the ICDP-USGS Eyreville B core, Chesapeake Bay impact structure: Impactites and crystalline rocks, 1766 to 1096 m depth

The International Continental Scientific Drilling Program (ICDP)-U.S. Geological Survey (USGS) Eyreville drill cores from the Chesapeake Bay impact structure provide one of the most complete geologic sections ever obtained from an impact structure. This paper presents a series of geologic columns and descriptive lithologic information for the lower impactite and crystalline-rock sections in the cores. The lowermost cored section (1766-1551 m depth) is a complex assemblage of mica schists that commonly contain graphite and fibrolitic sillimanite, intrusive granite pegmatites that grade into coarse granite, and local zones of mylonitic deformation. This basement-derived section is variably overprinted by brittle cataclastic fabrics and locally cut by dikes of polymict impact breccia, including several suevite dikes. An overlying succession of suevites and lithic impact breccias (1551-1397 m) includes a lower section dominated by polymict lithic impact breccia with blocks (up to 17 m) and boulders of cataclastic gneiss and an upper section (above 1474 m) of suevites and clast-rich impact melt rocks. The uppermost suevite is overlain by 26 m (1397-1371 m) of gravelly quartz sand that contains an amphibolite block and boulders of cataclasite and suevite. Above the sand, a 275-m-thick allochthonous granite slab (1371-1096 m) includes gneissic biotite granite, fine- and medium-to-coarse-grained biotite granites, and red altered granite near the base. The granite slab is overlain by more gravelly sand, and both are attributed to debris-avalanche and/or rockslide deposition that slightly preceded or accompanied seawater-resurge into the collapsing transient crater.

Chesapeake Bay↗
USGS2009

Supplemental materials for the ICDP-USGS Eyreville A, B, and C core holes, Chesapeake Bay impact structure: Core-box photographs, coring-run tables, and depth-conversion files

During 2005-2006, the International Continental Scientific Drilling Program and the U.S. Geological Survey drilled three continuous core holes into the Chesapeake Bay impact structure to a total depth of 1766.3 m. A collection of supplemental materials that presents a record of the core recovery and measurement data for the Eyreville cores is available on CD-ROM at the end of this volume and in the GSA Data Repository. The supplemental materials on the CD-ROM include digital photographs of each core box from the three core holes, tables of the three coring-run logs, as recorded on site, and a set of depth-conversion programs. In this chapter, the contents, purposes, and basic applications of the supplemental materials are briefly described. With this information, users can quickly decide if the materials will apply to their specific research needs.

Chesapeake Bay↗
USGS2009

Paleontological interpretations of crater processes and infilling of synimpact sediments from the Chesapeake Bay impact structure

Biostratigraphic analysis of sedimentary breccias and diamictons in the Chesapeake Bay impact structure provides information regarding the timing and processes of late-stage gravitational crater collapse and ocean resurge. Studies of calcareous nannofossil and palynomorph assemblages in the International Continental Scientific Drilling Program (ICDP)-U.S. Geological Survey (USGS) Eyreville A and B cores show the mixed-age, mixed-preservation microfossil assemblages that are typical of deposits from the upper part of the Chesapeake Bay impact structure. Sparse, poorly preserved, possibly thermally altered pollen is present within a gravelly sand interval below the granite slab at 1392 m in Eyreville core B, an interval that is otherwise barren of calcareous nannofossils and dinocysts. Gravitational collapse of watersaturated sediments from the transient crater wall resulted in the deposition of sediment clasts primarily derived from the nonmarine Cretaceous Potomac Formation. Collapse occurred before the arrival of resurge. Low pollen Thermal Alteration Index (TAI) values suggest that these sediments were not thermally altered by contact with the melt sheet. The arrival of resurge sedimentation is identified based on the presence of diamicton zones and stringers rich in glauconite and marine microfossils at 866.7 m. This horizon can be traced across the crater and can be used to identify gravitational collapse versus ocean-resurge sedimentation. Glauconitic quartz sand diamicton dominates the sediments above 618.2 m. Calcareous nannofossil and dinoflagellate data from this interval suggest that the earliest arriving resurge from the west contained little or no Cretaceous marine input, but later resurge pulses mined Cretaceous sediments east of the Watkins core in the annular trough. Additionally, the increased distance traveled by resurge to the central crater in turbulent flow conditions resulted in the disaggregation of Paleogene unconsolidated sediments. As a result, intact Paleogene clasts in Eyreville cores are rare, but clasts of semilithified Potomac Formation silts and clays are common.

Chesapeake Bay↗
USGS2009

Paleoenvironmental recovery from the Chesapeake Bay bolide impact: The benthic foraminiferal record

The late Eocene Chesapeake Bay bolide impact transformed its offshore target site from an outer neritic, mid-shelf seafloor into a bathyal crater basin. To obtain a depositional record from one of the deepest parts of this basin, the U.S. Geological Survey (USGS) and the International Continental Scientific Drilling Program (ICDP) drilled a 1.76-km-deep core hole near Eyreville, Virginia. The Eyreville core and eight previously cored boreholes contain a rarely obtainable record of marine deposition and microfossil assemblages that characterize the transition from synimpact to postimpact paleoenvironments inside and near a submarine impact crater. I used depositional style and benthic foraminiferal assemblages to recognize a four-step transitional succession, with emphasis on the Eyreville core. Step 1 is represented by small-scale, silt-rich turbidites, devoid of indigenous microfossils, which lie directly above the crater-filling Exmore breccia. Step 2 is represented by very thin, parallel, silt and clay laminae, which accumulated on a relatively tranquil and stagnant seafloor. This stagnation created a dead zone, which excluded seafloor biota, and it lasted ~3-5 ka. Step 3 is an interval of marine clay deposition, accompanied by a burst of microfaunal activity, as a species-rich pioneer community of benthic foraminifera repopulated the impact site. The presence of a diagnostic suite of agglutinated foraminifera during step 3 indicates that paleoenvironmental stress related to the impact lasted from ~9 ka to 400 ka at different locations inside the crater. During step 4, the agglutinated assemblage disappeared, and an equilibrium foraminiferal community developed that contained nearly 100% calcareous species. In contrast to intracrater localities, core sites outside and near the crater rim show neither evidence of the agglutinated assemblage, nor other indications of long-term biotic disruption from the bolide impact.

Chesapeake Bay↗
USGS2009

The Eocene-Oligocene sedimentary record in the Chesapeake Bay impact structure: Implications for climate and sea-level changes on the western Atlantic margin

A multidisciplinary investigation of the Eocene-Oligocene transition in the International Continental Scientific Drilling Program (ICDP)-U.S. Geological Survey (USGS) Eyreville core from the Chesapeake Bay impact basin was conducted in order to document environmental changes and sequence stratigraphic setting. Planktonic foraminifera and calcareous nannofossil biostratigraphy indicate that the Eyreville core includes an expanded upper Eocene (Biozones E15 to E16 and NP19/20 to NP21, respectively) and a condensed Oligocene-Miocene (NP24-NN1) sedimentary sequence. The Eocene-Oligocene contact corresponds to a =3-Ma-long hiatus. Eocene- Oligocene sedimentation is dominated by great diversity and varying amounts of detrital and authigenic minerals. Four sedimentary intervals are identified by lithology and mineral content: (1) A 30-m-thick, smectite- and illite-rich interval directly overlies the Exmore Formation, suggesting long-term reworking of impact debris within the Chesapeake Bay impact structure. (2) Subsequently, an increase in kaolinite content suggests erosion from soils developed during late Eocene warm and humid climate in agreement with data derived from other Atlantic sites. However, the kaolinite increase may also be explained by change to a predominant sediment input from outside the Chesapeake Bay impact structure caused by progradation of more proximal facies belts during the highstand systems tract of the late Eocene sequence E10.Spectral analysis based on gamma-ray and magnetic susceptibility logs suggests infl uence of 1.2 Ma low-amplitude oscillation of the obliquity period during the late Eocene. (3) During the latest Eocene (Biozones NP21 and E16), several lithological contacts (clay to clayey silt) occur concomitant with a prominent change in the mineralogical composition with illite as a major component: This lithological change starts close to the Biozone NP19/20-NP21 boundary and may correspond to sequence boundary E10-E11 as observed in other northwest Atlantic margin sections. It could result from a shift to more distal depositional environments and condensed sedimentation during maximum fl ooding, rather than refl ecting a climatic change in the hinterland. The distinct 1% increase of the oxygen isotopes may correspond to the short-term latest Eocene "precursor isotope event." (4) The abrupt increase of sediment grainsize, carbonate content, and abundance of authigenic minerals (glauconite) across the major unconformity that separates Eocene from Oligocene sediments in the Eyreville core refl ects deposition in shallower settings associated with erosion, winnowing, and reworking. Sediments within the central crater were affected by the rapid eustatic sea-level changes associated with the greenhouse-icehouse transition, as well as by an abrupt major uplift event and possibly enhanced current activity on the northwestern Atlantic margin. ?? 2009 The Geological Society of America.

Chesapeake Bay↗
USGS2013

Modeling volcano growth on the Island of Hawaii: Deep-water perspectives

Recent ocean-bottom geophysical surveys, dredging, and dives, which complement surface data and scientific drilling at the Island of Hawaii, document that evolutionary stages during volcano growth are more diverse than previously described. Based on combining available composition, isotopic age, and geologically constrained volume data for each of the component volcanoes, this overview provides the first integrated models for overall growth of any Hawaiian island. In contrast to prior morphologic models for volcano evolution (preshield, shield, postshield), growth increasingly can be tracked by age and volume (magma supply), defining waxing alkalic, sustained tholeiitic, and waning alkalic stages. Data and estimates for individual volcanoes are used to model changing magma supply during successive compositional stages, to place limits on volcano life spans, and to interpret composite assembly of the island. Volcano volumes vary by an order of magnitude; peak magma supply also varies sizably among edifices but is challenging to quantify because of uncertainty about volcano life spans. Three alternative models are compared: (1) near-constant volcano propagation, (2) near-equal volcano durations, (3) high peak-tholeiite magma supply. These models define inconsistencies with prior geodynamic models, indicate that composite growth at Hawaii peaked ca. 800–400 ka, and demonstrate a lower current rate. Recent age determinations for Kilauea and Kohala define a volcano propagation rate of 8.6 cm/yr that yields plausible inception ages for other volcanoes of the Kea trend. In contrast, a similar propagation rate for the less-constrained Loa trend would require inception of Loihi Seamount in the future and ages that become implausibly large for the older volcanoes. An alternative rate of 10.6 cm/yr for Loa-trend volcanoes is reasonably consistent with ages and volcano spacing, but younger Loa volcanoes are offset from the Kea trend in age-distance plots. Variable magma flux at the Island of Hawaii, and longer-term growth of the Hawaiian chain as discrete islands rather than a continuous ridge, may record pulsed magma flow in the hotspot/plume source.

Hawai'i↗
USGS1991

An analysis of a seismic reflection from the base of a gas hydrate zone, offshore Peru

Seismic reflection data recorded near ODP Site 688, offshore Peru, exhibit a persistent bottom-simulating reflector (BSR) from a depth corresponding to the theoretical base of the gas hydrate stability field. Although gas hydrate has been recovered in cores from scientific drilling in both the Pacific and the Atlantic, drilling in areas exhibiting a BSR has usually been avoided because of the perceived risk of releasing free gas trapped beneath the hydrated sediments. To carry out a quantitative analysis of the BSR, the seismic data were reprocessed using signature deconvolution and true amplitude recovery techniques. Synthetic seismograms that were created using acoustic parameters extracted from the seismic data and physical properties from Site 688 borehole measurement were compared to the observed seismic data to estimate the thickness of the free-gas zone. Results indicate the BSR is discontinuous laterally. Where the BSR is of high amplitude, free gas in a zone 5.5-17 m thick beneath the hydrated sediments provides the observed waveform; where the BSR amplitude is low, the free-gas zone is much thinner than 5.5 m or is entirely absent.

American Association of Petroleum Geologists Bulle↗
USGS2009

Biomineralization associated with microbial reduction of Fe 3+ and oxidation of Fe 2+ in solid minerals

Iron-reducing and oxidizing microorganisms gain energy through reduction or oxidation of iron, and by doing so play an important role in the geochemical cycling of iron. This study was undertaken to investigate mineral transformations associated with microbial reduction of Fe 3+ and oxidation of Fe 2+ in solid minerals. A fluid sample from the 2450 m depth of the Chinese Continental Scientific Drilling project was collected, and Fe 3+ -reducing and Fe 2+ -oxidizing microorganisms were enriched. The enrichment cultures displayed reduction of Fe 3+ in nontronite and ferric citrate, and oxidation of Fe 2+ in vivianite, siderite, and monosulfide (FeS). Additional experiments verified that the iron reduction and oxidation was biological. Oxidation of FeS resulted in the formation of goethite, lepidocrocite, and ferrihydrite as products. Although our molecular microbiological analyses detected Thermoan-aerobacter ethanolicus as a predominant organism in the enrichment culture, Fe 3+ reduction and Fe 2+ oxidation may be accomplished by a consortia of organisms. Our results have important environmental and ecological implications for iron redox cycling in solid minerals in natural environments, where iron mineral transformations may be related to the mobility and solubility of inorganic and organic contaminants.

American Mineralogist↗
USGS2003

Tufts submarine fan: turbidity-current gateway to Escanaba Trough

Turbidity-current overflow from Cascadia Channel near its western exit from the Blanco Fracture Zone has formed the Tufts submarine fan, which extends more than 350 km south on the Pacific Plate to the Mendocino Fracture Zone. For this study, available 3.5-kHz high-resolution and airgun seismic-reflection data, long-range side-scan sonar images, and sediment core data are used to define the growth pattern of the fan. Tufts fan deposits have smoothed and filled in the linear ridge-and-valley relief over an area exceeding 23,000 km2 on the west flank of the Gorda Ridge. The southernmost part of the fan is represented by a thick (as much as 500 m) sequence of turbidite deposits ponded along more than 100 km of the northern flank of the Mendocino Fracture Zone. Growth of the Tufts fan now permits turbidity-current overflow from Cascadia Channel to reach the Escanaba Trough, a deep rift valley along the southern axis of the Gorda Ridge. Scientific drilling during both the Deep Sea Drilling Project (DSDP) and the Ocean Drilling Program (ODP) provided evidence that the 500-m-thick sediment fill of Escanaba Trough is dominantly sandy turbidites. Radiocarbon dating of the sediment at ODP Site 1037 showed that deposition of most of the upper 120 m of fill was coincident with Lake Missoula floods and that the provenance of the fill is from the eastern Columbia River drainage basin. The Lake Missoula flood discharge with its entrained sediment continued flowing downslope upon reaching the ocean as hyperpycnally generated turbidity currents. These huge turbidity currents followed the Cascadia Channel to reach the Pacific Plate, where overbank flow provided a significant volume of sediment on Tufts fan and in Escanaba Trough. Tufts fan and Tufts Abyssal Plain to the west probably received turbidite sediment from the Cascadia margin during much of the Pleistocene.

Escanaba Trough↗
USGS2007

Advances through collaboration: sharing seismic reflection data via the Antarctic Seismic Data Library System for Cooperative Research (SDLS)

The Antarctic Seismic Data Library System for Cooperative Research (SDLS) has served for the past 16 years under the auspices of the Antarctic Treaty (ATCM Recommendation XVI-12) as a role model for collaboration and equitable sharing of Antarctic multichannel seismic reflection (MCS) data for geoscience studies. During this period, collaboration in MCS studies has advanced deciphering the seismic stratigraphy and structure of Antarctica’s continental margin more rapidly than previously. MCS data compilations provided the geologic framework for scientific drilling at several Antarctic locations and for high-resolution seismic and sampling studies to decipher Cenozoic depositional paleoenvironments. The SDLS successes come from cooperation of National Antarctic Programs and individual investigators in “on-time” submissions of their MCS data. Most do, but some do not. The SDLS community has an International Polar Year (IPY) goal of all overdue MCS data being sent to the SDLS by end of IPY. The community science objective is to compile all Antarctic MCS data to derive a unified seismic stratigraphy for the continental margin – a stratigraphy to be used with drilling data to derive Cenozoic circum-Antarctic paleobathymetry maps and local-to-regional scale paleoenvironmental histories.

Open-File Report↗
USGS2018

Geology and assessment of undiscovered oil and gas resources of the Jan Mayen Microcontinent Province, 2008

The Jan Mayen Microcontinent encompasses a rectangular, mostly submarine fragment of continental crust that lies north of Iceland in the middle of the North Atlantic Ocean. These continental rocks were rifted away from the eastern margin of Greenland as a consequence of a westward jump of spreading centers from the now-extinct Aegir Ridge to the currently active Kolbeinsey Ridge in the Oligocene and early Miocene. The microcontinent is composed of the high-standing Jan Mayen Ridge and a series of smaller ridges that diminish southward in elevation and includes several deep basins that are underlain by strongly attenuated continental crust. The geology of this area is known principally from a loose collection of seismic reflection and refraction lines and several deep-sea scientific drill cores. The Jan Mayen Microcontinent petroleum province encompasses the entire area of the microcontinent and was defined as a single assessment unit (AU). Although its geology is poorly known, the microcontinent is thought to consist of late Paleozoic and Mesozoic rift basin stratigraphic sequences similar to those of the highly prospective Norwegian, North Sea, and Greenland continental margins. The prospectivity of the AU may be greatly diminished, however, by pervasive extensional deformation, basaltic magmatism, and exhumation that accompanied two periods of continental rifting and breakup in the Paleogene and early Neogene. The overall probability of at least one petroleum accumulation of >50 million barrels of oil equivalent was judged to be 5.6 percent. As a consequence of the low level of probability, a quantitative assessment of this AU was not conducted.

Professional Paper↗
USGS2008

Application of RHIZON samplers to obtain high-resolution pore-fluid records during geochemical investigations of gas hydrate systems

Obtaining accurate, high-resolution profiles of pore fluid constituents is critical for characterizing the subsurface geochemistry of hydrate-bearing sediments. Tightly-constrained downcore profiles provide clues about fluid sources, fluid flow, and the milieu of chemical and diagenetic reactions, all of which are used to interpret where and why gas and gas hydrate occur in the natural environment. Because a profile’s quality is only as good as the samples from which the data are obtained, a great deal of effort has been exerted to develop extraction systems suited to various sedimentary regimes. Pore water from deeply buried sediment recovered by scientific drilling is typically squeezed with a hydraulic press (Manheim, 1966); whereas pore water in near-surface, less consolidated sediment is more efficiently pushed from the sediment using compressed gas (Reeburgh, 1967) or centrifugation.

Fire in the Ice: NETL Methane Hydrate Newsletter↗
USGS2022

Comparison of sediment composition by smear slides to quantitative shipboard data: A case study on the utility of smear slide percent estimates, IODP Expedition 353, northern Indian Ocean

Smear slide petrography has been a standard technique during scientific ocean drilling expeditions to characterize sediment composition and classify sediment types, but presentation of these percent estimates to track downcore trends in sediment composition has become less frequent over the past 2 decades. We compare semi-quantitative smear slide composition estimates to physical property (natural gamma radiation, NGR) and solid-phase geochemical (calcium carbonate, CaCO3 %) measurements from a range of marine depositional environments in the northern Indian Ocean (Bay of Bengal, Andaman Sea, Ninetyeast Ridge) collected during International Ocean Discovery Program (IODP) Expedition 353. We show that presenting smear slide estimates as percentages, rather than abundance categories, reveals similar downcore variation in composition to the more quantitative core analyses. Overall downcore trends in total calcareous components from smear slides (foraminifers + nannofossils + shell fragments + authigenic carbonate) follow similar downcore trends to samples measured by CaCO3 coulometry. Total lithogenic components (clay + mica + quartz + feldspars + lithic grains + vitric grains + glauconite + heavy minerals + iron oxides) and clay from smear slides track reasonably well with NGR measurements. Comparison of site averages of absolute percentages of total calcium carbonate from coulometry and total calcareous components from smear slide observations reveals an overestimation in carbonate percentages in smear slides (likely due in part to underestimation of the clay fraction), especially in sediments rich in smectite clays. Differences in sediment color between sites and settling of clay particles during slide preparation may contribute to this discrepancy. Although smear slide estimates range in accuracy depending on the training of the operator, we suggest that sedimentologists describing cores obtained during scientific drilling can use the percent estimates of sedimentary components in smear slides to identify trends and cyclicity in marine sediment records.

Scientific Drilling↗
USGS2014

Methane hydrates in nature - Current knowledge and challenges

Recognizing the importance of methane hydrate research and the need for a coordinated effort, the United States Congress enacted the Methane Hydrate Research and Development Act of 2000. At the same time, the Ministry of International Trade and Industry in Japan launched a research program to develop plans for a methane hydrate exploratory drilling project in the Nankai Trough. India, China, the Republic of Korea, and other nations also have established large methane hydrate research and development programs. Government-funded scientific research drilling expeditions and production test studies have provided a wealth of information on the occurrence of methane hydrates in nature. Numerous studies have shown that the amount of gas stored as methane hydrates in the world may exceed the volume of known organic carbon sources. However, methane hydrates represent both a scientific and technical challenge, and much remains to be learned about their characteristics and occurrence in nature. Methane hydrate research in recent years has mostly focused on: (1) documenting the geologic parameters that control the occurrence and stability of methane hydrates in nature, (2) assessing the volume of natural gas stored within various methane hydrate accumulations, (3) analyzing the production response and characteristics of methane hydrates, (4) identifying and predicting natural and induced environmental and climate impacts of natural methane hydrates, (5) analyzing the methane hydrate role as a geohazard, (6) establishing the means to detect and characterize methane hydrate accumulations using geologic and geophysical data, and (7) establishing the thermodynamic phase equilibrium properties of methane hydrates as a function of temperature, pressure, and gas composition. The U.S. Department of Energy (DOE) and the Consortium for Ocean Leadership (COL) combined their efforts in 2012 to assess the contributions that scientific drilling has made and could continue to make to advance our understanding of methane hydrates in nature. COL assembled a Methane Hydrate Project Science Team with members from academia, industry, and government. This Science Team worked with COL and DOE to develop and host the Methane Hydrate Community Workshop, which surveyed a substantial cross section of the methane hydrate research community for input on the most important research developments in our understanding of methane hydrates in nature and their potential role as an energy resource, a geohazard, and/or as an agent of global climate change. Our understanding of how methane hydrates occur in nature is still growing and evolving, and it is known with certainty that field, laboratory, and modeling studies have contributed greatly to our understanding of hydrates in nature and will continue to be a critical source of the information needed to advance our understanding of methane hydrates.

Journal of Chemical and Engineering Data↗