Platform and off-platform carbonates of the Upper Cambrian of western Maryland, U.S.A.

Upper Cambrian carbonates in western Maryland are comprised of platform facies (Conococheague Limestone) west of South Mountain and basin facies (Frederick Limestone) east of South Mountain. Conocheague platform carbonates contain interbedded non-cyclic and cyclic facies. Non-cyclic facies consist of cross-stratified grainstones, thrombolitic bioherms, and graded, thin-bedded dolostones. These were deposited in shallow, subtidal shelf lagoons. Cyclic facies are composed of repeated sequences of cross-stratified grainstone; ribbon-rock; wavy, prism-cracked laminite; and planar laminated dolostone. The cyclic facies are shallowing-upward cycles produced by lateral progradation of tidal flats over shallow, nearshore subtidal environments. Cyclic and non-cyclic facies are interbedded in the Conococheague in a layer cake fashion, but no higher-order cyclicity can be found. The Frederick Limestone is dominated by monotonously thick sequences of graded, thin-bedded limestones, interbedded with massive peloidal grainstones and beds of breccia up to 10 m thick in the lower Frederick. The breccias contain transported megaclasts of Epiphyton-Girvanella boundstones. The basal Frederick was deposited in a slope-to-basinal setting east of a rimmed shelf. An Epiphyton-Girvanella marginal reef along the shelf edge was the source of the blocks in the breccias. The upper Frederick Limestone formed on a carbonate ramp.     

Geochemistry and isotope chemistry of CaNaCl brines in Silurian strata,Michigan Basin,U.S.A.

Formation waters from Silurian-aged reefs in the northern and southern trends of lower Michigan were collected and analyzed for major, minor and isotope compositions. The results were combined with an analysis of an exceptionally concentrated (TDS 640 g/l) Silurian brine reported by Case in 1945 to determine the origin and possible evolutionary pathways for the chemical and isotope components of the brines. The waters are extremely concentrated(TDS> 450g/l) CaNaCl brines. Bromide values support that they originated from seawater concentrated into the MgSO₄ and possibly the KCl salt facies. The brines have, however, evolved considerably from an expected seawater composition and now contain a dominant CaCl composition. Dolomitization appears to have been very important in the brine evolution, but this process cannot explain all the Ca present in these brines. Four scenarios may explain the enrichment in Ca: (1) halite dissolution accompanied by the exchange of Na for Ca; (2) reactions involving aluminosilicate minerals, carbonates and halite; (3) an input of CaCl₂ solutions derived from altered MgCl₂ fluids released during the metamorphism of carnallite into sylvite; and (4) a pre-existing enrichment of CaCl in the Early Paleozoic seawater that filled the basin. All four are possible, but the favored explanation involves the diagenesis of the Salina A-1 potash salts. The isotope composition of the waters is consistent with evaporated seawater, perhaps enriched by exchange with carbonates or by the input of hydration water from evaporite minerals. The isotopic evolution, however, is equivocal but the brine composition does not indicate they have been diluted with meteoric water. This implies the waters have remained isolated from surface-controlled hydrological systems.     

Iron deposition as acidic groundwater encounters carbonates in the alluvium of Pinal Creek,Arizona, U.S.A.

In a column experiment, acidic groundwater from Pinal Creek Arizona, a Cu mining area, was eluted through a composited alluvial sample obtained from a core that had been removed from a well downgradient of the acidic groundwater. The minerals present in typical grains and flakes in the alluvium before and after the elution were determined by X-ray diffraction (XRD), scanning electron microscopy, and energy dispersive multichannel analyses (EDX). The concentrations of Fe, Ti, Mn, Si, Al, Na, Ca, K, Mg and S in these grains and flakes and in their microcrystalline surface coatings were measured by EDX.In addition to magnetite, hematite, and Fe-Ti oxides, Fe was most concentrated in micas (especially biotite-like flakes) and in the microcrystalline coatings. The measured elements in these microcrystalline coatings were primarily K, Fe, Al, and Si. The microcrystalline coatings on the mica flakes also contained Mg. The approximate l:3 Mg:Si atomic ratios (ARs)_of the biotite-life flake both before and after the elution would suggest that the Fe deposited during the elution had not substituted for Mg in these flakes. As a result of the elution, assuming no loss of Si, the averaged recorded Fe:Si AR of the microcrystalline coatings increased from (0.46 to 0.58):3.00.Iron deposition on the typical grains and flakes may relate to the presence of Fe in the particle on which it is deposited or to the presence of Fe in the microcrystalline surface coatings before elution. The data here are not sufficient for a statistical evaluation, but elution caused the following trends: (l) The Fe:Si AR increased in the (K,Fe,Al,Si)-microcrystalline surface coatings; (2) For the mica flakes, there was more than a 2-fold increase in the Fe:Si AR for the microcrystalline surface coatings of the Fe-rich biotite-like flakes but no measurable increase of the Fe:Si AR for the microcrystalline surface coatings of the muscovite-like flakes that contained 3–5 times less Fe; (3) Also for the biotite-like flakes, the increase in Fe:Si AR was greater in the flakes that had a higher Fe:Si AR; (4) The Fe deposition on the Fe-rich microcrystalline surface coatings of the feldspar was much greater than on the Fe-poor, beige quartz and feldspar grains that, prior to elution, had only CaS0₄ microcrystalline coatings; and (5) No Fe was deposited on Fe-poor grains with no microcrystalline surface coating.     

Decisions of the Standing Stratigraphic Commission on Ordovician and Silurian of U.S.S.R.

The Committee on Ordovician and Silurian Stratigraphy of the U. S. S. R. defines a number of time-stratigraphic subdivisions of the Russian Ordovician and Silurian Systems. — C.G. Tillman.     

A strontium isotopic study of formation waters from the Illinois basin, U.S.A.

The isotopic composition of Sr has been measured in 73 formation-water samples from Paleozoic strata in the Illinois basin; ⁸⁷Sr/⁸⁶Sr ratios range from 0.7079 to 0.7108. With the exception of four samples, the waters are more radiogenic than corresponding Paleozoic sea-water values. The relatively narrow range of slightly elevated ⁸⁷Sr/⁸⁶Sr rations is uniformly distributed in waters throughout the stratigraphic column and in Silurian waters across the basin. Isotopic analyses of core samples from reservoir rocks show an absence of water-rock Sr isotopic equilibration. Basin lithology and analyses of detrital rock units indicate that clay minerals in shales and in quartz sandstone matrices represent the only significant source of radiogenic Sr for the waters. Silurian and Devonian water show a two-component mixing relation which suggests that they comprise a single hydrogeological system that evolved when radiogenic water from New Albany shales entered Silurian-Devonian carbonate rocks and mixed with marine interstitial water. Regional migration of the waters and associated petroleum within the Silurian-Devonian strata, proposed in other studies, is consistent with the Sr isotopic data. Under favorable circumstances subsurface waters are capable of retaining a Sr isotopic recor of their evolution.     

Growth history of shallow-water carbonates: control of accommodation on ecological and depositional processes.

We explore the role played on the growth of shallow-water carbonates by changes in accommodation using case studies from Late Quaternary to modern carbonate environments, i.e. coral reefs and carbonate platforms and ramps. Accommodation appears not to influence either primary productivity or community structure (K- versus r-strategists) as these are directly governed by nutrient supply; but it does control submarine depositional profiles and their stratigraphic expression (i.e. volumetric sedimentary partitioning). The growth of communities from incipient to senescent stages is dictated by relative rates of sea-level variations, although ambient temperatures and trophic conditions may modify the community response. A decrease or stabilization of accommodation favours lateral migration, thus generating ecological and sedimentary innovations. Organic carbon production tends to be positively correlated to that of carbonate, both declining as accommodation decreases. Shallow water carbonates develop as a result of the interplay of factors, including the types and rates of biological and physical processes. Applying depositional models of Recent shallow-water carbonates to the interpretation of ancient carbonate platforms first needs to acknowledge the respective role of the two contrasting phases that operated during the post-glacial sea-level rise (an earlier, fast-rising phase versus a later, slowly rising to stabilizing phase); the period of relative sea-level stabilization (i.e. the past 6,000 years) appears a relevant reference for still-stand-related carbonate systems. Furthermore, the oligotrophic nutrient model, a dominant feature of the modern shallow-water tropics, can be used as an analogue in reconstructing carbonate growth histories during greenhouse episodes of Earth history.     

Sedimentary characteristics,formation mechanisms and geological significance of the Shiniulan Formation reefs (lower Silurian) in the Upper Yangtze Platform

The lower Silurian Shiniulan Formation of the Upper Yangtze Platform contains small-scale coral–stromatoporoid reefs, generally from less than 10?km long and from 10 to 30?m in thickness, especially in the middle–upper parts. The reef-building organisms were dominantly tabulate and rugose corals, with fewer stromatoporoids. Reef-inserted organisms include bryozoans, brachiopods, cephalopods, algae, crinoids and bivalves. The Shiniulan Formation is characterised by biohermal limestone, bioclastic limestone, sandstone and mudstone, and is divided into four members according to lithological characteristics. The reefs generally occur in the middle–upper layer, which corresponds to four growth stages: stabilisation, colonisation, diversification and domination. In the reefs, the argillaceous and sandy contents decrease, and the carbonates increase from the base to the top. The growth characteristics, evolution, scale and size of the reefs in the Shiniulan Formation were influenced by three factors: agitation of terrigenous debris, fluctuation of sea level and seawater temperature. The Upper Yangtze region was mainly a shallow marine environment with a warm or torrid tropical and subtropical climate and high salinity in the late Aeronian. However, the Shiniulan Formation has significant differences in growth, evolution and extension scale compared with corresponding reefs in Laurentia and the Siberia and Kazakhstan blocks. Analysis of the geological factors of the Shiniulan Formation and the Menier Formation of Anticosti Island, which has revealed that the two sections have similar reef types, lithology and biology, aims to explain the under-development of large-scale reefs in the Upper Yangtze Platform during the middle–upper Silurian.     

The mineralogy and origin of some Silurian bentonites, Welsh Borderland, U.K.

ABSTRACT Thirty clay-rich horizons from the Wenlock and Ludlow Series of the Welsh Borderland were analysed to investigate their mineralogy and possible origins. Two mineralogical assemblages were determined: an assemblage of illite-smectite and kaolinite of volcanic origin which was associated in many samples with an assemblage of illite and chlorite of detrital origin. Immobile chemical discriminants (Zr/TiO₂ versus Nb/Y) infer an originally rhyolitic ash composition with a possible temporal progression, from the Wenlock to Ludlow, to more differentiated compositions. The thin nature of the bentonites and lack of juxtaposed subduction related calc-alkaline magmatism suggests that the tuffs were derived from distant explosive eruptions from volcanic centres, possibly in NE Europe.     

High-titania alkali-olivine basalts of north-central Oregon,U.S.A.

In north-central Oregon numerous small flows of alkali-olivine basalt occur in the Oligocene to early Miocene John Day Formation. Chemically, these basalts are characterized by relatively low SiO₂ and K₂O and very high TiO₂ and iron. Fifteen analysed specimens (44 to 48 percent SiO₂) have an average of 3.6 percent TiO₂ and 15 percent total iron. The average composition of the Oregon basalts compares closely with the average hawaiite of the Hawaiian Islands, differing only in having slightly higher iron and slightly lower SiO₂ and total alkalis. Closely associated flows of trachyandesite and quartz latite are chemically related to the basalts and probably formed by differentiation of an alkali-olivine basalt magma.Typical basalt specimens have 10 to 15 percent of modal olivine, interstitial alkali feldspar, and abundant clay minerals and chlorophaeite. Textures are subophitic or intersertal and phenocrysts are rare. Plagioclase laths are slightly zoned and range in composition from An₆₈ to An₄₄. Purplish-brown titaniferous augite is the only pyroxene, and ilmenite is the dominant opaque mineral.Distinct differences in composition and age, and the lack of transitional varieties indicate that these basalts are unrelated to the younger Columbia River basalts. They presumably represent a separate parent magma of alkalic affinity that was generated independently within the mantle.     

Ecological succession, palaeoenvironmental change, and depositional sequences of Barremian–Aptian shallow-water carbonates in northern Oman

Abstract Barremian and Aptian shallow‐water carbonate facies (uppermost Lekhwair, Kharaib and Shuaiba Formations) are described from outcrops in northern Oman. Based on facies analysis and bedding pattern, three orders of depositional sequences are defined (third to fifth order) and correlated between sections. Over the course of three third‐order sequences, covering the Barremian to Lower Aptian, a third‐order depositional pattern is documented that consists of a succession of three distinct faunal assemblages: discoidal orbitolinids and calcareous algae were deposited during early transgression; microbialites and microencrusters dominate the late transgressive to early highstand facies; and a rudist‐ and miliolid‐dominated facies is typical of the highstand. This ecological succession was controlled largely by palaeoenvironmental changes, such as trophic level and clay influx, rather than sedimentological factors controlled by variations in accommodation space. Orbitolinid beds and carbonates formed by microbialites and microencrusters seem to be the shallow‐water carbonate response to global changes affecting Late Barremian to Aptian palaeoclimate and palaeoceanography.     

Hydrogeochemical exploration for barite, Ouachita Mountains, U.S.A.

Approximately 100 springs were sampled as part of a general geochemical survey in a 130 by 140 km multi-mineralized area of the Ouachita Mountains. The study area was subdivided into Area I and Area II based on differences in lithology and mineralization. Area I is composed primarily of the Paleozoic Stanley Shale and the Arkansas Novaculite Formation, and has considerable manganese (often present as psilomelane) and barite mineralization. The barite occurs primarily as replacement deposits in shale in Area I. Area II is composed of Paleozoic and Cretaceous sedimentary rocks, and Quaternary alluvium. Mercury, antimony, strontium and barite mineralization is present. The barite mineralization occurs as cement in the Cretaceous sandstone and gravel in this area.The widely prevalent barite cement in Area II is reflected by higher median and upper range values of barium concentration and also higher median and threshold % BaSO₄ saturation values. The presence of barite as cement in the sediments of Area II tends to mask the detection of the barite deposits in this area. Approximately 83% of the sites within 1.7 km of known barite mineralization in Area I were characterized by anomalous barium concentration, and 67% within 3.4 km. Anomalous % BaSO₄ saturation values and Ba/Sr ratios produced success rates generally similar to those of anomalous barium concentrations. Anomalous strontium concentrations did not prove effective in detecting barite mineralization.     

Hydrogeochemical exploration for Mississippi valley-type deposits, Arkansas, U.S.A.

Three areas located in northern Arkansas, U.S.A., representing differing host rock and mineralization, were selected to investigate the usefulness of hydrogeochemical exploration for Mississippi Valley-type Pb-Zn mineralization. Despite the geologic differences among the areas, there were no great differences in groundwater chemistry and threshold values. Anomalous Pb concentrations, and also anomalous Zn concentrations to a lesser extent, are useful in detecting the Pb-Zn mineralized areas; however, specific deposits could not be located. Because of the low threshold values (about 20 μg/1) for Pb and Zn, spring water must be utilized in order to avoid plumbing contamination.     

Tufa formation in Tatar A.S.S.R

Tufa occurs widely in the Tatar A.S.S.R. and adjacent republics. It is associated with springs escaping from aquifers in deposits of Permian age. No tufa deposits were observed in the region where Jurassic and later deposits are developed. Tufa deposits occur because of a breakdown of calcium bicarbonate to CaCO₃, water and CO₂. The presence of organic matter and phosphorous inhibits this reaction, accounting in part for the lack of tufa deposition in the other regions. It appears also that tufa deposition is favored by 1) increase of water surface, hence increased evaporation and CaCO₃ concentration and 2) slopes where argillaceous material is present.—J.J. Finney     

Compaction of wyodak coal,powder river basin,wyoming, U.S.A.

Precise measurements of 54 deformed tree trunks within the subbituminous Wyodak coal seem in northeastern Wyoming were made at eight locations in surface mines along the outcrop north and south of Gillette. Theory of elasticity applied to the individual measurements gives ratios of peat thickness to coal thickness which range from 1.7:1 to 31:1 and average 7.1:1. This result is similar to those previously determined by other methods in other locations. Statistical analysis establishes to a high degree of confidence, however, that the ratios for each mine site vary widely and cannot be considered to be constant or to vary uniformly within the study area.     

Composition of the Precambrian Llano Uplift,central Texas,U.S.A.

The Llano uplift exposes rocks of approximately 1000 m.y. age. The weighted average composition of the exposed crust is: 70.7% SiO₂; 0.35% TiO₂; 13.6% A1₂O₃; 3.4% total Fe as Fe₂O₃; 1.1% MgO; 2.6% CaO; 3.3% Na₂O; and 4.4% K₂O. This composition is similar to, but more potassic, than equivalent estimates for the Canadian shield.     

Geological evolution of the Ishpeming Greenstone Belt,Michigan, U.S.A.

The Ishpeming Greenstone Belt is an Archean belt in the southern part of the Canadian Shield in the Upper Peninsula of Michigan, U.S.A. Two volcanic cycles are preserved in it. The oldest formation, and basal to the first cycle (the Kitchi Schist), consists of mafic metavolcanics, has a major serpentinized ultramafic body near its base, and grades upward to a coarse felsic volcanic breccia at the top of the cycle. This unit in turn is overlain by a sequence of mafic flows that grades upward to interbedded mafic flows and exhalites of the Mona Schist. This sequence has been intruded by the Dead River Pluton.The Ishpeming Greenstone Belt probably represents the keel of a previously much more extensive Greenstone Belt.Gold mineralization occurs associated with mafic basaltic volcanic rocks and serpentinized ultramafics low in the succession, and with carbonate-rich quartz-chlorite-sericite schists and exhalites higher in the sequence. No mineral deposits are now being exploited here.     

Crystallization of the West Farrington Pluton, North Carolina, U.S.A.

The West Farrington pluton in the North Carolina Piedmont isconcentrically zoned from gabbro-diorite near the chilled marginsto leucogranodiorite in the center. A crystallization modelfor the West Farrington pluton has been derived utilizing chemical,petrographic, field, and experimental data. The model involvessimple in situ fractional crystallization from the margins inward,with minimal contamination, crystal settling and floating, ormetasomatism. Rocks of the pluton can be considered as mixturesof early crystallizing minerals (liquidus or near-liquidus phases)and crystallized interstitial liquids. Relative percentage ofentrapped pore liquid increased with increasing degree of crystallization. The original tonalite magma began crystallizing Fe-Ti spinels,plagioclase, and hornblende within a short temperature interval.Crystallization of these minerals controlled fractionation trends.The initial water content in the magma was probably 2–3per cent; under such conditions water saturation would havebeen reached after about 60 per cent of the magma crystallized,assuming crystallization in the lower epizone at about 2000bars total pressure.     

The sillimanite-potash feldspar isograd in Western Maine,U.S.A.

The techniques of electron probe microanalysis and x-ray diffractometry have been utilized in a study of the sillimanite-potassium feldspar isograd in western Maine. The isograd reaction is theoretically a discontinuous one, calling for the nearly instantaneous loss of muscovite and crystallization of sillimanite and orthoclase, with a small contribution of albite from the pre-existing plagioclase. In fact, muscovite coexists with orthoclase, sillimanite, and plagioclase for a distance of at least seven miles from the isograd (marked by the initial coexistence of orthoclase and sillimanite). In this assemblage, muscovite has an extremely narrow range of composition, about an average of Ms_93.5Pg_6.5. A possible explanation for the divariant character of the isograd reaction is that, during dehydration, PH₂O slowly increased from initial values less than P_total + rock strength, under conditions of low permeability, the actual value of PH₂O being controlled by a buffer assemblage and local conditions of P and T. An alternative explanation postulates the flattening of thermal gradients following the onset of fractional melting. The isograd reaction is dependent in only a minor way upon the anorthite content of the plagioclase. Below the isograd, a continuous reaction takes place leading to a diminution in paragonite content of muscovite stable in the presence of quartz. It is possible that this reaction leads to the nearly ubiquitous normal zoning of the plagioclase. Changes in the composition of biotite at the isograd are not conspicuous, and can be satisfactorily explained by the release of Mg, Fe, and Ti impurity from the muscovite, and a continuous reaction between ilmenite, quartz, and muscovite. Garnets are not abundant and are high in Mn, both facts probably due to the low pressure of metamorphism, The presence of garnet probably relates to the Mn content of the rock, and seems to be independent of the Mg/Fe ratio of the biotite. The garnets are zoned with respect to Mn and Mg, but often Mn is enriched and Mg depleted in the marginal zone. The Mg/Fe ratio of the biotite varies twofold depending on the presence or absence of pyrrhotite. The transition: microcline → orthoclase depends upon the amount of dissolved albite; the polymorph is orthoclase in the pelitic schists but microcline in the calc-silicate rocks which are much lower in sodium. The plagioclases are of “low” structural type, although is slightly greater than many other “low” plagioclases. A correlation of d(002) of muscovite and paragonite solid solution for the range 0 to 20 % paragonite is given. An appreciable positive volume of mixing for the binary system muscovite-paragonite is indicated.     

Quaternary lavas from the southern Cascades,western U.S.A.

A series of basaltic and andesitic lavas from three centers in the Cascades (Lassen, Medicine Lake, Mt. Shasta) have been investigated. The lavas are weakly porphyritic, containing phenocrysts of plagioclase, augite, and olivine or orthopyroxene; these phases are also found in the groundmass. Titanomagnetite is a groundmass phase in most lavas but it appears to be absent in some. A sub-calcic augite is found in the groundmass in some of the basic lavas. Orthopyroxenes are present only in the salic lavas and show an increase in calcium with increasing iron. The range in composition shown by both phenocryst and groundmass plagioclase is very similar except that the phenocrysts extend to slightly more calcic compositions. The residual glasses in many of the lavas have a rhyolitic composition. However, only those from the Shasta andesites have normative salic constituents that plot near the ternary minimum in the Ab-Or-Qtz system at 500 bars. Both chemical and mineralogical data allow the lavas of the different centers to be distinguished from one another. The most likely origin for the orogenic lavas of the Cascades is by partial melting of the upper mantle.     

Holocene lake-level trends in the Rocky Mountains, U.S.A.

The availability of water shapes life in the western United States, and much of the water in the region originates in the Rocky Mountains. Few studies, however, have explicitly examined the history of water levels in the Rocky Mountains during the Holocene. Here, we examine the past levels of three lakes near the Continental Divide in Montana and Colorado to reconstruct Holocene moisture trends. Using transects of sediment cores and sub-surface geophysical profiles from each lake, we find that mid-Holocene shorelines in the small lakes (4–110 ha) were as much as 10 m below the modern lake surfaces. Our results are consistent with existing evidence from other lakes and show that a wide range of settings in the region were much drier than today before 3000–2000 years ago. We also discuss evidence for millennial-scale moisture variation, including an abruptly-initiated and -terminated wet period in Colorado from 4400 to 3700 cal yr BP, and find only limited evidence for low-lake stands during the past millennium. The extent of low-water levels during the mid-Holocene, which were most severe and widespread ca 7000–4500 cal yr BP, is consistent with the extent of insolation-induced aridity in previously published regional climate model simulations. Like the simulations, the lake data provide no evidence for enhanced zonal flow during the mid-Holocene, which has been invoked to explain enhanced mid-continent aridity at the time. The data, including widespread evidence for large changes on orbital time scales and for more limited changes during the last millennium, confirm the ability of large boundary-condition changes to push western water supplies beyond the range of recent natural variability.