The Scots Bay formation, Nova Scotia, Canada, a Jurassic carbonate lake with Silica-rich hydrothermal springs

During Early Jurassic time, the Scots Bay Formation accumulated in an aerobic lake on the floor of the subtropical Fundy rift valley. A diverse biota flourished in the lake as evidenced by algal stromatolites, oncolites, charophytes, ostracods, gastropods, conchostracans, fish bones, calcispheres and logs. Carbonate wackestone and packstone were deposited in the littoral zone and lime mudstone accumulated offshore on a proximal slope. Silica-rich hydrothermal springs and seeps around and on the floor of the lake precipitated siliceous tufa and silicified adjacent carbonate strata. The tufa is characterized by the progressive sequence: (1) open-framework, porous substrate of goethite spheres, (2) encrusting chalcedony spherulites where goethite inclusions decrease in size and number outwards towards the surfaces of the spherulites, (3) a fringe of quartz on the chalcedony spherulites and (4) coarsening inwards mosaics of quartz that filled the remaining voids. Carbonate diagenesis included conversion of low-Mg calcite ooze to micrite and microspar-pseudospar, and local dissolution and reprecipitation of calcite cement. At times the lake shrank in size, concentrating silica in increasingly alkaline lake water which then silicified some of the carbonate strata and logs. The latest cements were calcite, zeolites and celadonite.     

POTENTIAL EFFECTS OF CLIMATE CHANGE ON FRESHWATER ECOSYSTEMS OF THE NEW ENGLAND/MID-ATLANTIC REGION

Numerous freshwater ecosystems, dense concentrations of humans along the eastern seaboard, extensive forests and a history of intensive land use distinguish the New England/Mid-Atlantic Region. Human population densities are forecast to increase in portions of the region at the same time that climate is expected to be changing. Consequently, the effects of humans and climatic change are likely to affect freshwater ecosystems within the region interactively. The general climate, at present, is humid continental, and the region receives abundant precipitation. Climatic projections for a 2 × CO₂ atmosphere, however, suggest warmer and drier conditions for much of this region. Annual temperature increases ranging from 3–5°C are projected, with the greatest increases occurring in autumn or winter. According to a water balance model, the projected increase in temperature will result in greater rates of evaporation and evapotranspiration. This could cause a 21 and 31% reduction in annual stream flow in the southern and northern sections of the region, respectively, with greatest reductions occurring in autumn and winter. The amount and duration of snow cover is also projected to decrease across the region, and summer convective thunderstorms are likely to decrease in frequency but increase in intensity. The dual effects of climate change and direct anthropogenic stress will most likely alter hydrological and biogeochemical processes, and, hence, the floral and faunal communities of the region's freshwater ecosystems. For example, the projected increase in evapotranspiration and evaporation could eliminate most bog ecosystems, and increases in water temperature may increase bioaccumulation, and possibly biomagnification, of organic and inorganic contaminants. Not all change may be adverse. For example, a decrease in runoff may reduce the intensity of ongoing estuarine eutrophication, and acidification of aquatic habitats during the spring snowmelt period may be ameliorated. Recommendations for future monitoring efforts include: (1) extending and improving data on the distribution, abundance and effect of anthropogenic stressors (non-point pollution) within the region; and (2) improving scientific knowledge regarding the contemporary distribution and abundance of aquatic species. Research recommendations include: (1) establishing a research centre(s) where field studies designed to understand interactions between freshwater ecosystems and climate change can be conducted; (2) projecting the future distribution, activities and direct effects of humans within the region; (3) developing mathematical analyses, experimental designs and aquatic indicators that distinguish between climatic and anthropogenic effects on aquatic systems; (4) developing and refining projections of climate variability such that the magnitude, frequency and seasonal timing of extreme events can be forecast; and (5) describing quantitatively the flux of materials (sediments, nutrients, metals) from watersheds characterized by a mosaic of land uses. © 1997 John Wiley & Sons, Ltd.     

The Geochemical Evolution of Anorthosite Residual Magmas in the Laramie Anorthosite Complex, Wyoming

Olivine- and pyroxene-bearing Fe-enriched dioritic rocks inthe 1434 Ma Laramie anorthosite complex are interpreted to representvariably fractionated and contaminated magmas residual afterthe crystallization of anorthosite. Geochemical characteristicsof this suite include the following: high contents of TiO₂,, and P₂O₅; high incompatibletrace element contents; rare earth element patterns with a largerange of Eu anomalies; and isotopic compositions that reflectthe geographic location of individual samples, with I_Sr increasingand _Nd decreasing from south to north. After extraction fromanorthosite, fractionation of ferrodioritic residual magmasresulted in secondary residual monzodioritic melts and complementaryoxide-rich ferrodiorite cumulates. Geographic trends in isotopiccomposition reflect an increasing Archean crustal componentfrom south to north. Dioritic dikes and cumulates with isotopiccompositions similar to associated anorthosites were derivedlocally. Large isotopic discrepancies between some dioritesand their hosting anorthosites reflect preferential contaminationof residual magma during ascent and emplacement of mantle-derivedplagioclase-rich diapirs, followed by subsequent extractionand isolation of Fe-enriched interstitial melt. Strong isotopiccontrasts between anorthosite and associated Fe-enriched rocksin anorthosite complexes do not preclude a direct relationshipbetween them and reflect the diversity and complexity of processesduring their petrogenesis. KEY WORDS: anorthosite; ferrodiorite; geochemistry; Laramie anorthosite complex; residual magma ^*Corresponding author: Present address: Ceramic Science and Technology, Mail Stop K762, Los Alamos National Laboratory, Los Alamos, NM 87545, USA. Phone: (505) 665-3934. Fax: (505) 665-3935. E-mail: jeremy{at}lanl.gov.     

Tufa as a record of perennial fresh water in a semi-arid rift basin, Kapthurin Formation, Kenya

Lithological and biological features of a fossiliferous tufa in the Kapthurin Formation, Baringo, Kenya, reveal the presence of a lush wetland in a semi-arid environment during the Middle Pleistocene ( ca 500 ka) in this portion of the East African Rift Valley. Four geological sections, each between 3 m and 8 m in thickness, exposed over a distance of 0·5 km, reveal a 1 to 2 m thick paludal tufa which is composed of three carbonate beds, two dark grey silty claystones and a reddish-brown silty palaeosol. High resolution stratigraphic analysis, carbonate petrography, stable isotope and elemental geochemistry, clay mineralogy and fossil remains (molluscs, ostracods, diatoms and charophytes) reveal a ground water-fed system that fluctuated in depth and periodically disappeared altogether. Oxygen isotope ratios (δ¹⁸O) of tufa matrix range from −4·5‰ to −8·0‰ (Vienna Pee Dee Belemnite) and become more positive up section, indicating the decreasing influence of fault-related fluids and increasing residence time or freshness of wetland water, rather than evaporative enrichment. This spring was situated on a lake margin during low lake levels, thrived during periods of increased ground water input and was ultimately replaced by an alkaline lake. The wetland would appear to have existed during a cool interval within the generally warm Marine Isotope Stage 13 or perhaps during the warm second half of Marine Isotope Stage 13. The ground water source of this wetland arose through a fault system. Thus, the position of the tufa deposit is controlled structurally but the timing and duration of the wetland system may have been influenced by both climatic and tectonic factors.     

Petrogenesis of Mesozoic, Peraluminous Granites in the Lamoille Canyon Area, Ruby Mountains, Nevada, USA

Two groups of closely associated, peraluminous, two-mica graniticgneiss were identified in the area. The older, sparsely distributedunit is equigranular (EG) with initial     

Petrology of the Vandfaldsdalen Macrodike, Skaergaard Region, East Greenland

The Vandfaldsdalen macrodike is a layered and differentiatedgabbroic dike approximately 3?5 km long and from 200 to 500m wide. It appears to cut the eastern margin of the Skaergaardintrusion and may have served as a feeder for the Basistoppensill. The macrodike can be divided into three series of rocks:a marginal series of differentiated gabbros adjacent to thewalls of the dike; a central series of differentiated and subhorizontallylayered gabbros and ferrodiorites in the interior of the dike;and an upper felsic series of granophyric rocks with abundantquartzo-feldspathic xenoliths. The mineral and bulk-rock compositionsthrough both the marginal series and central series show progressiveiron enrichment. The most Ca-rich plagioclase (An₆₉) and mostmagnesian pyroxene (Wo₄₂ En₄₆ Fs₁₂) occur in olivine-bearingrocks of the marginal series about 5 m from the contact withwall rocks. The most Na-rich plagioclase (An₃₉) and Fe-richpyroxene (Wo₃₈ En₂₄ Fs₃₈) are in olivine-free ferrodiorite ofthe central series, about 20 m below the contact with the felsicseries. Evidence from field observations, bulk-rock chemical compositions,and Sr and Nd isotopic data indicate the felsic series formedas a mixture of the initial macrodike magma and granitic countryrock. ⁸⁷Sr/⁸⁶Sr ratios of specimens from the felsic series rangebetween 0?7129 and 0?7294. ¹⁴³Nd/¹⁴⁴Nd ratios vary between 0?51208and 0?51118. Both ratios vary serially with the SiO₂ contentsof the specimens. We suggest that the felsic series evolvedas a separate body of low density liquid which floated on thedenser gabbroic magma of the central series. Heat from crystallizationof the gabbroic magma must have diffused into the felsic layer,enabling extensive assimilation of the granitic xenoliths, butour data indicate there was very little exchange of chemicalcomponents between the two liquids.     

Asymmetry of Zoophycus burrows as a way-up criterion

ABSTRACT Zoophycus burrows from Neogene hemipelagic sediments in the equatorial Atlantic contain an asymmetric spreite fill. The long limb of each lamella is down-section. This is useful as way-up evidence in folded sequences.     

Origin of the Charnockites of the Louis Lake Batholith, Wind River Range, Wyoming

The 2·63 Ga Louis Lake batholith, a calc-alkalic plutonexposed in Wind River Range of western Wyoming, consists ofminor diorite, quartz diorite, granodiorite, and granite. Atshallow structural levels the batholith is pyroxene free, butat deeper levels, all units of the batholith contain pyroxenes.On its northern margin the batholith was emplaced at P = 5–6kbar, T = 775–800°C, fO₂ at FMQ (fayalite–magnetite–quartz)+ 1·5 to FMQ + 1·8, and aH₂O