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USGS2016

Geochemistry of host rocks in the Howards Pass district, Yukon-Northwest Territories, Canada: implications for sedimentary environments of Zn-Pb and phosphate mineralization

Detailed lithogeochemical data are reported here on early Paleozoic sedimentary rocks that host the large Howards Pass stratiform Zn-Pb deposits in Yukon-Northwest Territories. Redox-sensitive trace elements (Mo, Re, V, U) and Ce anomalies in members of the Duo Lake Formation record significant environmental changes. During the deposition of lower footwall units (Pyritic siliceous and Calcareous mudstone members), bottom waters were anoxic and sulphidic, respectively; these members formed in a marginal basin that may have become increasingly restricted with time. Relative to lower members, a major environmental change is proposed for deposition of the overlying Lower cherty mudstone member, which contains phosphorite beds up to ∼0.8 m thick in the upper part, near the base of the Zn-Pb deposits. The presence of these beds, together with models for modern phosphorite formation, suggests P input from an upwelling system and phosphorite deposition in an upper slope or outer shelf setting. The overlying Active mudstone member contains stratabound to stratiform Zn-Pb deposits within black mudstone and gray calcareous mudstone. Data for unmineralized black mudstone in this member indicate deposition under diverse redox conditions from suboxic to sulphidic. Especially distinctive in this member are uniformly low ratios of light to heavy rare earth elements that are unique within the Duo Lake Formation, attributed here to the dissolution of sedimentary apatite by downward-percolating acidic metalliferous brines. Strata that overlie the Active member (Upper siliceous mudstone member) consist mainly of black mudstone with thin (0.5–1.5 cm) laminae of fine-grained apatite, recording continued deposition on an upper slope or outer shelf under predominantly suboxic bottom waters. Results of this study suggest that exploration for similar stratiform sediment-hosted Zn-Pb deposits should include the outer parts of ancient continental margins, especially at and near stratigraphic transitions from marginal basin facies to overlying slope or shelf facies.

Mineralium Deposita↗

USGS2013

Melts of garnet lherzolite: experiments, models and comparison to melts of pyroxenite and carbonated lherzolite

Phase equilibrium experiments on a compositionally modified olivine leucitite from the Tibetan plateau have been carried out from 2.2 to 2.8 GPa and 1,380–1,480 °C. The experiments-produced liquids multiply saturated with spinel and garnet lherzolite phase assemblages (olivine, orthopyroxene, clinopyroxene and spinel ± garnet) under nominally anhydrous conditions. These SiO 2 -undersaturated liquids and published experimental data are utilized to develop a predictive model for garnet lherzolite melting of compositionally variable mantle under anhydrous conditions over the pressure range of 1.9–6 GPa. The model estimates the major element compositions of garnet-saturated melts for a range of mantle lherzolite compositions and predicts the conditions of the spinel to garnet lherzolite phase transition for natural peridotite compositions at above-solidus temperatures and pressures. We compare our predicted garnet lherzolite melts to those of pyroxenite and carbonated lherzolite and develop criteria for distinguishing among melts of these different source types. We also use the model in conjunction with a published predictive model for plagioclase and spinel lherzolite to characterize the differences in major element composition for melts in the plagioclase, spinel and garnet facies and develop tests to distinguish between melts of these three lherzolite facies based on major elements. The model is applied to understand the source materials and conditions of melting for high-K lavas erupted in the Tibetan plateau, basanite–nephelinite lavas erupted early in the evolution of Kilauea volcano, Hawaii, as well as younger tholeiitic to alkali lavas from Kilauea.

Contributions to Mineralogy and Petrology↗

USGS2020

Conditions and timing of high-grade metamorphism and ductile deformation of the southern segment of the Central Anatolian Ophiolite

Ophiolitic fragments scattered over a wide area of Central Anatolia exhibit varying degrees of metamorphism, from unmetamorphosed to upper amphibolite facies, although geochemical similarities suggest they are all part of the Central Anatolian Ophiolite (CAO). Magmatic crystallization of oceanic crust in the CAO at ~ 91 Ma coincided with high-grade metamorphism of rocks that underlie the southern, highest grade part of the CAO, raising questions about the tectonic relationship of the ophiolite to underlying metasedimentary and plutonic rocks. New geochronology results show that the 40 Ar/ 39 Ar hornblende age of amphibolite-facies metagabbro in the high-grade metamorphic part of the CAO is ~ 87 Ma, similar to hornblende ages from amphibolite in the underlying Niğde metamorphic/plutonic massif. Biotite in a deformed quartzofeldspathic rock associated with high-grade meta-ophiolitic rocks yielded an 40 Ar– 39 Ar age of ~ 78 Ma, similar to biotite ages from the Niğde Massif. Hornblende in gabbro from unmetamorphosed CAO yielded an older 40 Ar/ 39 Ar age of ~ 90 Ma, similar to the previously determined crystallization age of the ophiolite. These data indicate that the southern part of the ophiolite was incorporated into and therefore metamorphosed and deformed with the orogenic mid-crust now exposed in the Niğde metamorphic–plutonic complex, whereas the northern, unmetamorphosed part of the ophiolite was obducted onto the continent. This distinct difference in different parts of the ophiolite may indicate oblique collision or irregularities in the continental margin, resulting in part of the ophiolite being incorporated into the orogenic crust and subsequently exhumed and cooled with it, and another part being obducted.

Anatolia↗

USGS1993

Monazite paragenesis and U-Pb systematics in rocks of the eastern Mojave Desert, California, U.S.A.: implications for thermochronometry

Studies of the paragenesis and U-Pb systematics of monazite in rocks from the eastern Mojave Desert, California, corroborate its potential usefulness as a prograde thermochronometer and in dating granite inheritance. Unmetamorphosed Latham Shale and its equivalents at grades ranging from greenschist to upper amphibolite facies are virtually identical in composition. Monazite is absent in the shale and low-grade schists, but it is abundant in schists at staurolite and higher grades. Lower-grade schists instead include minute Th- and Ce-oxides and unidentified Ce-poor LREE-phosphates that apparently are lower-temperature precursors to monazite. Thus monazite originates when the pelite passes through lower-amphibolite-facies conditions. Monazites from three Upper Cretaceous granites yield ages that are strongly discordant. Upper intercepts of 1.6-1.7 Ga are similar to those defined by U-Pb data for coexisting zircons and coincide with a period of copious magmatism in the Mojave crust. As the host Upper Cretaceous granitic magmas were all above 700??C, effective closure of the restitic monazites to Pb loss must be well in excess of this temperature. U-Pb compositions of monazite from Proterozoic granitoids and schist also indicate high Pb retentivity. Taken together, these studies support the suggestion that monazite can be an effective prograde thermochronometer. At least in pelites, it is not usually retained as a detrital mineral, but rather forms during moderate-temperature metamorphism. Its U-Pb system should not be reset by subsequent higher-grade metamorphism. ?? 1993.

Chemical Geology↗

USGS1984

Shoreface translation and the Holocene stratigraphic record: Examples from Nova Scotia, the Mississippi Delta and eastern Australia

Classic descriptive models of barrier sedimentation have been developed with data from the Atlantic and Gulf coasts of the United States. These models are dominated by low to moderate rates of relative sea level (RSL) rise and wave energy. Barriers respond by landward recycling of sediment through the mechanism of shoreface retreat. Sedimentation processes on the central coast of New South Wales (N.S.W.), Australia, consist of rapid RSL rise in early Holocene times followed by a stillstand since 6500 B.P. Wave energy is relatively high year-round and sand sources for barrier formation are only found on the inner shelf. Barrier sedimentation on the central coast of N.S.W. exhibits a thick, composite sequence composed of a basal marine transgressive sand overlain by regressive beach and dune facies. The Louisiana coast surrounding the Mississippi delta is underlain by compacting deltaic muds which generate very rapid rates of RSL rise. The Louisiana coast experiences low wave energy punctuated by high-energy tropical and extra-tropical storm events. Barrier sediments accumulate from the erosion of deltaic headlands and undergo a transformation from subaerial barrier island systems to subaqueous shoals located on the inner shelf. Drumlins experience coastal erosion on the Eastern Shore of Nova Scotia and provide a sediment source for compartmented estuary mouth barriers. An ongoing, moderate rise of RSL results from the passage of a glacial forebulge. Wave energy is intermediate between Louisiana and N.S.W. and displays a seasonal pattern dominated by frequent winter storms. Coastal barrier sedimentation is episodic, consisting of a period of beach ridge progradation followed by barrier destruction and re-establishment further landward. The three contrasting sedimentary sequences found in examples from Louisiana, N.S.W. and Nova Scotia indicate that presently available sedimentation models from locations such as the middle Atlantic or Texas coasts of the United States may only represent well-documented regional case studies. A true generalised coastal sedimentation model is required which can identify the parameters controlling vertical and horizontal translation of the depositional surface and provide relationships between these parameters which quantitatively predict the genesis, distribution and geometry of coastal sedimentary facies.

Marine Geology↗

USGS1985

Barrier island arcs along abandoned Mississippi River deltas

Generation of transgressive barrier island arcs along the Mississippi River delta plain and preservation of barrier shoreline facies in their retreat paths on the inner shelf is controlled by: (1) shoreface translation; (2) age of the transgression; and (3) the thickness of the barrier island arc sediment package. Barrier island arcs experience an average relative sea level rise of 0.50–1.00 cm yr −1 and shoreface retreat rates range from 5–15 m yr −1 . Young barrier island arc sediment packages (Isles Dernieres) are thin and have experienced limited landward retreat of the shoreface. Older barrier island arcs (Chandeleur Islands) are thicker and have experienced significant landward movement of the shoreface because of the greater time available for retreat. If the transgressed barrier shoreline sediment package lies above the advancing ravinement surface, the entire sequence is truncated. A thin reworked sand sheet marks the shoreface retreat path. The base of the transgressive sediment package can lie below the ravinement surface in older barrier shorelines. In this setting, the superstructure of the barrier shoreline is truncated, leaving the basal portion of the transgressive sequence preserved on the inner shelf. A variety of transgressive stratigraphic sequences from sand sheets to truncated barrier islands to sand-filled tidal inlet scars have been identified by high resolution seismic profiling across the shoreface retreat paths of Mississippi delta barrier island arcs. One of these examples, the Isles Dernieres, represents a recently detached barrier island arc in the early stages of transgression. An older example, the Chandeleur Islands, represents a barrier island arc experiencing long-term shoreface retreat. This paper describes the stratigraphic character and preserved transgressive facies for the Isles Dernieres and Chandeleur Islands.

Marine Geology↗

USGS1993

Mesozoic stratigraphy and paleoenvironments of the Georges Bank Basin: A correlation of exploratory and cost wells

The Exxon 975-1, Conoco 145-1, and Mobil 312-1 hydrocarbon exploratory wells and the Continental Offshore Stratigraphic Test (COST) G-1 and G-2 wells were drilled in the southeastern part of the Georges Bank Basin. We used drill cuttings and logs from these wells to describe and correlate the dominant lithostratigraphic units and to document lateral changes in the depositional environments. The strata penetrated by the Exxon 975-1 and COST G-1 wells are much more terrestrial than at the seaward (downdip) COST G-2, Conoco 145-1, and Mobil 312-1 wellsites. Oldest rocks penetrated by the exploratory wells represent a Middle Jurassic carbonate-evaporite sequence that correlates to the Iroquois Formation. The Iroquois records nonmarine to marginal-marine sabkha, tidal-flat, and restricted lagoonal paleoenvironments in the Exxon 975-1 and COST G-1 wells, but progressively changes to inner neritic, littoral, and lagoonal paleoenvironments at the Mobil 312-1 wellsite. The nonmarine deltaic siliciclastics of the overlying Mohican Formation, Misaine Shale, and Mic Mac-Mohawk Formations are thicker and the marine carbonates of the Scatarie and Bacarro Limestones are usually thinner in the Jurassic strata of the Exxon 975-1 and COST G-1 wells than at the other wellsites. Similarly, lower delta-plain and delta-front facies of the Early Cretaceous Missisuaga and Logan Canyon Formations at the Exxon 975-1 and COST G-1 wellsites reflect a greater terrestrial influence than the laterally-equivalent shallow marine to delta-front facies present at the downdip wellsites. ?? 1993.

Georges Bank Basin↗

USGS1980

Depositional environments of some Pleistocene coastal terrace deposits, southwestern Oregon - case history of progradational beach and dune sequence.

These deposits comprise a basal gravelly unit and 3 overlying sandy units, each with mud beds, a paleosol, or the modern soil in its uppermost part. The gravelly unit is interpreted as a progradational deposit. The main parts of the sandy units are made up of 1) a crossbedded sand facies, the dominant structure in which is medium-scale crossbedding (interpreted as the product of small eolian dunes), and 2) an irregularly bedded sand facies, which is locally pebbly and is dominated by scour-and-fill structures, interpreted as deposits of interdune ephemeral streams, ephemeral ponds, and wet to dry subaerial flats. The mud beds and paleosols represent times of temporary stabilization of the dune field.- from Author

Sedimentary Geology↗

USGS1984

Fluvial sedimentation on a quivering craton: Influence of slight crustal movements on fluvial processes, upper Jurassic Morrison formation, western Colorado Plateau

One of the most important challenges facing the fluvial sedimentologist is identification of processes outside the stream channel that influence deposition of fluvial sediments. Detailed studies in the lower sequence of the Salt Wash Member (Morrison Formation, Upper Jurassic) demonstrate that crustal deformation at the site of deposition may considerably influence braided-stream processes. Late Jurassic crustal movements in the western part of the Colorado Plateau are interpreted largely from thickness variations and facies distribution, but other features such as vertical repetition of facies, coincidence with at least parts of present-day folds, and the geographic distribution of bedding parameters measured in the fluvial deposits, are also used as corroborating evidence of syndepositional tectonism. These features indicate that several of the large uplifts and basins in the region as well as some of the smaller folds within them were actively moving during deposition of the lower sequence. Tectonic activity altered the stream gradients, which in turn governed sinuosity, flow regime, energy levels, and sediment distribution. Cross-bedding studies indicate that reduced gradients within downwarped areas led to slight increases in sinuosity of the braided-stream channels and of the small sub-channels within them. The lowered gradients apparently resulted in a decrease in the depth of the channels and allowed the streams to flood more readily, producing abundant upper-flow regime horizontal laminations in the channel deposits. In addition, greater quantities of sediment containing higher proportions of sand were deposited in downwarped areas than in positive localities. The inability of the streams to transport bed load through downwarped areas indicates loss of stream energy. However, an increase in the quantity of upper-flow regime horizontal laminations in the same downwarped areas suggests that an increase in flow regime is not necessarily accompanied by an increase in energy levels, at least in regions of slight tectonic activity where the local configuration of the stream channels may change appreciably. Strata presently dip less than 2° throughout most of the region, and this relatively small amount of deformation reflects the combined effects of Late Jurassic, Cretaceous and Tertiary tectonism. This demonstrates that the amount of structural deformation at the site of deposition may appear to be insignificant, yet it can cause appreciable changes in the nature of braided-stream deposits.

Arizona, Utah↗

USGS1988

Synthesis of late Paleozoic and Mesozoic eolian deposits of the Western Interior of the United States

Late Paleozoic and Mesozoic eolian deposits include rock units that were deposited in ergs (eolian sand seas), erg margins and dune fields. They form an important part of Middle Pennsylvanian through Upper Jurassic sedimentary rocks across the Western Interior of the United States. These sedimentary rock units comprise approximately three dozen major eolian-bearing sequences and several smaller ones. Isopach and facies maps and accompanying cross sections indicate that most eolian units display varied geometry and complex facies relations to adjacent non-eolian rocks. Paleozoic erg deposits are widespread from Montana to Arizona and include Pennsylvanian formations (Weber, Tensleep, Casper and Quadrant Sandstones) chiefly in the Northern and Central Rocky Mountains with some deposits (Hermosa and Supai Groups) on the Colorado Plateau. Lower Permian (Wolfcampian) erg deposits (Weber, Tensleep, Casper, Minnelusa, Ingleside, Cedar Mesa, Elephant Canyon, Queantoweap and Esplanade Formations) are more widespread and thicken into the central Colorado Plateau. Middle Permian (Leonardian I) erg deposits (De Chelly and Schnebly Hill Formations) are distributed across the southern Colorado Plateau on the north edge of the Holbrook basin. Leonardian II erg deposits (Coconino and Glorieta Sandstones) are slightly more widespread on the southern Colorado Plateau. Leonardian III erg deposits formed adjacent to the Toroweap-Kaibab sea in Utah and Arizona (Coconino and White Rim Sandstones) and in north-central Colorado (Lyons Sandstone). Recognized Triassic eolian deposits include major erg deposits in the Jelm Formation of central Colorado-Wyoming and smaller eolian deposits in the Rock Point Member of the Wingate Sandstone and upper Dolores Formation, both of the Four Corners region. None of these have as yet received a modern or thorough study. Jurassic deposits of eolian origin extend from the Black Hills to the southern Cordilleran arc terrain. Lower Jurassic intervals include the Jurassic part of the Wingate Sandstone and the Navajo-Aztec-Nugget complex and coeval deposits in the arc terrain to the south and west of the Colorado Plateau. Major Middle Jurassic deposits include the Page Sandstone on the Colorado Plateau and the widespread Entrada Sandstone, Sundance Formation, and coeval deposits. Less extensive eolian deposits occur in the Carmel Formation, Temple Cap Sandstone, Romana Sandstone and Moab Tongue of the Entrada Sandstone, mostly on the central and western Colorado Plateau. Upper Jurassic eolian deposits include the Bluff Sandstone Member and Recapture Member of the Morrison Formation and Junction Creek Sandstone, all of the Four Corners region, and smaller eolian deposits in the Morrison Formation of central Wyoming and apparently coeval Unkpapa Sandstone of the Black Hills. Late Paleozoic and Mesozoic eolian deposits responded to changing climatic, tectonic and eustatic controls that are documented elsewhere in this volume. All of the eolian deposits are intricately interbedded with non-eolian deposits, including units of fluvial, lacustrine and shallow-marine origin, clearly dispelling the myth that eolian sandstones are simple sheet-like bodies. Rather, these units form some of the most complex bodies in the stratigraphic record.

Western Interior↗

USGS1967

Glaucophane schists from California and New Caledonia

In California and New Caledonia, metamorphism of eugeosynclinal rocks has produced blueschist facies in limited areas. The outcrop pattern and structure suggest that the shape of the zone of blueschist metamorphism is elongate parallel to major tectonic trends. Juxtaposition of large ultramafic bodies, subparallel to the blueschist belts, indicates a close tectonic relationship between metamorphism and the tectonic emplacement of the ultramafic masses. Initial emplacement of ultramafics along the depressed axis of the eugeosyncline may have produced deformation related to blueschist metamorphism. Mineral assemblages developed in blueschist facies are characterized by having formed under conditions where pressure is predominant over temperature. That pressure is relatively high requires extremely low thermal gradients combined with a rheology that would allow development of tectonic overpressures.

California↗

USGS1988

Seismic anisotropy in mylonites: An example from the Mannin Thrust Zone, southwest Connemara, Ireland

Mylonites associated with the Mannin Thrust zone of southwesternmost Connemara formed when the high-grade metamorphic rocks typical of most of the Connemara massif were thrust to the southeast over low metamorphic grade (low greenschist facies?) acid volcanics and volcaniclastic sediments, while being metamorphosed in the epidote-amphibolite facies. Triaxial and biaxial ultrasonic velocity measurements of mylonite specimens from a 240 m borehole have established that there is significant seismic anisotropy up to about 11% when comparing velocities perpendicular and parallel to the foliation. This would ultimately lead to a reflection coefficient of about 0.02 when comparing the mean "isotropic" seismic velocity with that perpendicular to the foliation. The finely striped, discontinunous mineral lithons that define mylonitic foliation, but which form no real and continuous surfaces, could interact with seismic energy to produce "reflections" that do not relate to lithological contacts within the rocks but to a tectonically induced, orientated acoustic impedance. However, the results support the work of others in suggesting that on its own the fabric would not produce the high amplitude reflections observed on deep seismic lines and other mechanisms need to be investigated.

southwest Connemara↗

USGS1994

Third-order Middle Miocene-Early Pliocene depositional sequences in the prograding delta complex of the Pannonian Basin

Few studies exist in the geologic literature that show the distribution of seismic facies and depositional sequences within a lacustrine basin. The Pannonian Basin of Central Europe offers a unique opportunity to evaluate the influence of the eustatic signal on lacustrine deposition. Seismic stratigraphie and sedimentological studies indicate that the Middle Miocene-Early Pliocene infill of the transtensional Pannonian Basin was formed by large delta systems. Systematic sequence stratigraphie analysis of 6000 km of reflection seismic data and more than 100 hydrocarbon exploration wells in Hungary allowed the identification of twelve third-order sequence boundaries in the late Neogene sedimentary fill. This number of depositional sequences corresponds to that of the published global eustatic curve for this time period. Furthermore, based on magnetostratigraphic and radiometric data, the ages of these depositional sequences can be tentatively correlated with the global eustatic curve. The Pannonian Basin became isolated from the world sea at the Sarmatian/Pannonian (11.5 Ma) boundary and formed a large lake. The stratal patterns and sedimentary facies of individual systems tracts within the lacustrine sequences display the same characteristics as marine depositional sequences. The relatively low rate of thermal subsidence and the high rate of sediment supply resulted in a good sequence resolution. Within the third-order sequences higher-order sequences can be recognized with an average duration of about 0.1-0.5 Ma.

Tectonophysics↗

USGS1988

Petrographic characteristics of the Wyodak-Anderson coal bed (Paleocene), Powder River Basin, Wyoming, U.S.A.

Six lithofacies of the thick ( > 30 m) Wyodak-Anderson subbituminous coal bed of the Fort Union Formation (Paleocene), Powder River Basin, Wyoming, can be delimited using megascopic and petrographic data. Previous lithofacies analysis of the rock types associated with the Wyodak-Anderson bed suggested that raised peat accumulated in restricted parts of an inland flood plain. The peat bodies were separated by deposits of contemporaneous, possibly anastomosed channels. In this study, megascopic descriptions from four mine highwalls of the Wyodak-Anderson coal bed were found to be similar to facies defined by microscopic data from core and highwall samples. The data indicate that the upper and lower parts of the coal bed are rich in preserved wood remains (for instance, humotelinite), whereas the middle part of the bed contains comparatively larger amounts of material that resulted from degradation and comminution of the peat (e.g. eugelinite). The facies are interpreted to be the result of different chemical and biological environments at the time of peat formation.

Wyoming↗

USGS1982

Application of automated image analysis to coal petrography

The coal petrologist seeks to determine the petrographic characteristics of organic and inorganic coal constituents and their lateral and vertical variations within a single coal bed or different coal beds of a particular coal field. Definitive descriptions of coal characteristics and coal facies provide the basis for interpretation of depositional environments, diagenetic changes, and burial history and determination of the degree of coalification or metamorphism. Numerous coal core or columnar samples must be studied in detail in order to adequately describe and define coal microlithotypes, lithotypes, and lithologic facies and their variations. The large amount of petrographic information required can be obtained rapidly and quantitatively by use of an automated image-analysis system (AIAS). An AIAS can be used to generate quantitative megascopic and microscopic modal analyses for the lithologic units of an entire columnar section of a coal bed. In our scheme for megascopic analysis, distinctive bands 2 mm or more thick are first demarcated by visual inspection. These bands consist of either nearly pure microlithotypes or lithotypes such as vitrite/vitrain or fusite/fusain, or assemblages of microlithotypes. Megascopic analysis with the aid of the AIAS is next performed to determine volume percentages of vitrite, inertite, minerals, and microlithotype mixtures in bands 0.5 to 2 mm thick. The microlithotype mixtures are analyzed microscopically by use of the AIAS to determine their modal composition in terms of maceral and optically observable mineral components. Megascopic and microscopic data are combined to describe the coal unit quantitatively in terms of (V) for vitrite, (E) for liptite, (I) for inertite or fusite, (M) for mineral components other than iron sulfide, (S) for iron sulfide, and (VEIM) for the composition of the mixed phases (X i ) i = 1,2, etc. in terms of the maceral groups vitrinite V, exinite E, inertinite I, and optically observable mineral content M. The volume percentage of each component present is indicated by a subscript. For example, a lithologic unit was determined megascopically to have the composition (V) 13 (I) 1 (S) 1 (X 1 ) 83 (X 2 ) 2 . After microscopic analysis of the mixed phases, this composition was expressed as (V) 13 (I) 1 (S) 1 (V 63 E 19 I 14 M 4 ) 83 (V 67 E 11 I 13 M 9 ) 2 . Finally, these data were combined in a description of the bulk composition as V 67 E 16 I 13 M 3 S 1 . An AIAS can also analyze textural characteristics and can be used for quick and reliable determination of rank (reflectance). Our AIAS is completely software based and incorporates a television (TV) camera that has optimum response characteristics in the range of reflectance less than 5%, making it particularly suitable for coal studies. Analysis of the digitized signal from the TV camera is controlled by a microprocessor having a resolution of 64 gray levels between full illumination and dark current. The processed image is reconverted for display on a TV monitor screen, on which selection of phases or features to be analyzed is readily controlled and edited by the operator through use of a lightpen. We expect that automated image analysis, because it can rapidly provide a large amount of pertinent information, will play a major role in the advancement of coal petrography.

International Journal of Coal Geology↗

USGS1995

The role of palynology in paleoecological analyses of Tertiary coals

Pollen, spores, and other plant microfossils are important constituents of Tertiary coals, both as petrographic components (the maceral sporinite) and as distinctive indicators of the nature of the depositional environment in which the coal-forming peat accumulated. Palynological analyses, which have been conducted on Tertiary coals since the 1930's, play an increasingly important role in modern paleoecological analyses of coals. Among the most important factors controlling the petrographic composition and facies characteristics of coal deposits are the types and relative abundances of plants that composed the peat. At present, wetland environments in which peat accumulates (mires) range from marshes and bogs to forested swamps; each is inhabited by distinctive plant communities. These plant communities are associated with varied conditions of climate, water depth, chemistry, and nutrient supply, and the floras can be indicative of these conditions. In the Tertiary Period, plant communities of mires were also highly diverse and specialized (especially as compared with those of the Carboniferous). The dominant species in some Paleocene mire floras were gymnosperms, but continuing a trend begun in Late Cretaceous time, angiosperms became increasingly prominent through the Tertiary. The diverse: angiosperm-dominated mire floras produced coals that vary significantly in facies and hence in quality. Such contrasts are evident in contemporaneous coals from different paleolatitudes, as well as among, coals of different ages within the Tertiary, as exemplified by selected deposits in the western United. States. The palynofloras of these coals reflect the floristic composition of the plant communities of the ancient mires.

International Journal of Coal Geology↗

USGS1989

Deformation of the Eastern Franciscan Belt, northern California

The late Jurassic and Cretaceous Eastern Franciscan belt of the northern California Coast Range consists of two multiply deformed, blueschist-facies terranes; the Pickett Peak and Yolla Bolly terranes. Four deformations have been recognized in the Pickett Peak terrane, and three in the Yolla Bolly terrane. The earliest recognized penetrative fabric, D 1 , occurs only in the Pickett Peak terrane. The later penetrative fabrics, D 2 and D 3 , occur in both the Yolla Bolly and Pickett Peak terranes. D 1 and D 2 apparently represent fabrics that formed during subduction and accretion of the terranes. Fabrics from both D 1 and D 2 are consistent with SW-NE movement directions with respect to their present geographic positions. D 3 postdates blueschist-facies metamorphism of the terranes and may be related to emplacement of the terranes to higher structural levels. A broad regional warping, D 4 , is evident from the map pattern and folding of large metamorphosed thrust sheets. D 4 folds may be related to deformation associated with oblique convergence along the continental margin in late Cretaceous and (or) early Tertiary time.

Journal of Structural Geology↗

USGS1985

Catastrophic eruptions of the directed-blast type at Mount St. Helens, Bezymianny and Shiveluch volcanoes

This paper describes catastrophic eruptions of Mount St. Helens (1980), Bezymianny (1955–1956), and Shiveluch (1964) volcanoes. A detailed description of eruption stages and their products, as well as the quantitative characteristics of the eruptive process are given. The eruptions under study belong to the directed-blast type. This type is characterized by the catastrophic character of the climatic stage during which a directed blast, accompanied by edifice destruction, the profound ejection of juvenile pyroclastics and the formation of pyroclastic flows, occur. The climatic stage of all three eruptions has similar characteristics, such as duration, kinetic energy of blast (10 17 −10 18 J), the initial velocity of debris ejection, morphology and size of newly-formed craters. But there are also certain differences. At Mount St. Helens the directed blast was preceeded by failure of the edifice and these events produced separable deposits, namely debris avalanche and directed blast deposits which are composed of different materials and have different volumes, thickness and distribution. At Bezymianny, failure did not precede the blast and the whole mass of debris of the old edifice was outburst only by blast. The resulting deposits, represented by the directed blast agglomerate and sand facies, have characteristics of both the debris avalanche and the blast deposit at Mount St. Helens. At Shiveluch directed-blast deposits are represented only by the directed-blast agglomerate; the directed-blast sand facies, or blast proper, seen at Mount St. Helens is absent. During the period of Plinian activity, the total volumes of juvenile material erupted at Mount St. Helens and at Besymianny were roughly comparable and exceeded the volume of juvenile material erupted at Shiveluch, However, the volume of pyroclastic-flow deposits erupted at Mount St. Helens was much less. The heat energy of all three eruptions is comparable: 1.3 × 10 18 , 3.8−4.8 × 10 18 and 1 × 10 17 J for Shiveluch, Bezymianny, and Mount St. Helens, respectively.

Journal of Geodynamics↗