Geochemistry of carbonate cements in the Sag River and Shublik Formations (Triassic/Jurassic), North Slope, Alaska: implications for the geochemical evolution of formation waters

Carbonate cements (calcite, siderite, dolomite, and ankerite) formed throughout the diagenetic history of the Sag River and Shublik Formations. The trace element and isotopic geochemistry of these cements varies as a function of the timing of precipitation. Earliest calcites, formed prior to significant compaction of the sediment, are relatively enriched in Mg (up to 4·4 mol%), and have ⁸⁷Sr/⁸⁶Sr values (mean = 0·707898) compatible with the original marine pore waters. Later calcites are relatively Fe-rich (up to 5·0 mol%) and are characterized by increasing ⁸⁷Sr/⁸⁶Sr values (up to 0·712823) and Sr content with decreasing age. The Fe content of zoned siderite and dolomite/ankerite rhombs increases towards the outside of the rhombs (i.e. increasing Fe content with decreasing age). These geochemical variations appear principally to result from changes in pore-water chemistry during diagenesis. The increase in ⁸⁷Sr/⁸⁶ Sr and Sr content of the cements is most likely due to interaction between pore waters and ⁸⁷ Sr-rich clay and possibly feldspar in Ellesmerian mudrocks (whole rock ⁸⁷Sr/⁸⁶ Sr signatures for the mudrocks are > 0·716). Pore-water Fe²⁺ concentration was probably controlled by diagenetic alterations involving Fe-bearing minerals (e.g. pyrite precipitation). A reconnaissance examination of carbonate cements in the overlying Kingak Shale indicates that similar alterations occurred in the Kingak. The low δ¹⁸ O value of some calcite cements (-11·96% PDB) suggests that an influx of meteoric water may have occurred in the mid-Neocomian, though the low value could also result from an abnormally high geothermal gradient associated with mid-Neocomian rifting.     

Anaerobic diagenesis within Recent, Pleistocene, and Eocene marine carbonate frameworks

Porewaters from a variety of Recent, Pleistocene, and Eocene lithified marine carbonate frameworks displayed similar chemical characteristics: highly depleted concentrations of dissolved oxygen (>20 μM), elevated levels of dissolved methane (25-5000 nM), and near-seawater sulphate levels. These porewaters also had low pH values (7·5-7·9), and contained elevated concentrations of sulphide (4–10 μM), dissolved inorganic carbon (2·05–2·46 mM), and inorganic nutrients. Hydrocarbon composition data indicate that the methane is biogenic, whereas the methane δ¹³C values (–47·4 ± 2·7%0) suggest that it has been subject to oxidation. The porewater dissolved inorganic carbon δ¹³C values varied from –0·6 to –39%0, suggesting input of carbon dioxide from organic matter oxidation. We conclude that anaerobic diagenesis involving bacterial degradation of organic matter is a common process in lithified marine carbonates and hypothesize that it may be an important factor controlling their carbonate geochemistry.     

Polar fossil forests

In the Arctic and Antarctic there are spectacular fossil forests. They are true 'polar forests' because not only are they found as fossils in high latitudes today-they actually grew in the polar regions. They provide important evidence that the climate of the Earth was once much warmer, and that the trees must have been adapted to the strange polar light-regime of winter darkness and continuous summer sunlight.     

Major Addition of Magma at the Pyroxenite Marker in the Western Bushveld Complex, South Africa

A major, but gradual, reversal in the cryptic variation patternof the plagioclase and pyroxenes, of 13 mol% anorthite and 10mol% Mg/ (Mg + Fe) respectively, is documented in the Main Zoneof the western Bush veld Complex. These changes are accompaniedby a decrease in initial ⁸⁷Sr/⁸⁶Sr ratio from > 0.708 to< 0.707. The Pyroxenite Marker, a distinctive orthopyroxenitelayer, occurs close to the top of this reversed differentiationsequence. This is attributed to addition of less differentiatedmagma. On the basis of a mass balance calculation of the initial⁸⁷Sr/ ⁸⁶Sr ratios, it is estimated that the volume of magmaadded was comparable to that of the resident magma. Increases in the Fe₂O₃, TiO₂, Al₂O₃, and Na₂O contents of thepyroxenes above the level of magma addition indicate that thenew magma had a lower silica activity and higher fO₂ than theresident magma. Quantification of the trace element and REEcontent of the two magmas is hampered by the very low proportionof trapped intercumulus component in these adcumulate rocks.However, semi-quantitative modelling indicates that the traceand REE signatures of the two magmas were similar, with moderateLREE enrichment and flat HREE profiles. The new magma had aslightly higher La/ Sm ratio than the resident magma, consistentwith its more alkaline nature. The new magma was probably added gradually, while 100–150m of cumulates formed. It probably intruded at an intermediatelevel within an existing stratified magma chamber, where itcooled and crystallized, and composite packets of liquid pluscrystals plunged to the base of the chamber. The cores of plagioclasegrains formed during this mixing interval show a wider rangeof compositions than in other sections, and plagioclase primocrystsfrom both magmas may be preserved within single samples. Therefore,although intimate physical mixing of packets of unknown sizeof the two magmas occurred, re-equilibration of the major oxidecomposition of the plagioclase primocrysts was not achieved.However, the data and calculations based on diffusion ratesindicate that partial Sr isotopic resetting of plagioclase mayhave occurred.     

Petrology and Geochemistry of the Rhobell Volcanic Complex: Amphibole-Dominated Fractionation at an Early Ordovician Arc Volcano in North Wales

The Rhobell Volcanic Complex is a remnant of a late Tremadoc,dominantly calc-alkaline, arc volcano. It is the only substantiallypreserved representative in the southern British Caledonidesof an early phase of Ordovician ensialic arc volcanism whichfollowed the onset of southeasterly subduction of Iapetus oceaniclithosphere beneath the northern margin of Gondwanaland. Thecomplex includes extrusive basalts and associated breccias,known as the Rhobell Volcanic Group, which rest unconformablyon folded, uplifted and eroded rift-basin sediments of the earlierpassive margin of the Iapetus Ocean. Amongst the basalts arerelatively primitive pargasite-bearing varieties which containcognate cumulate blocks, dominantly of pargasite but also withcalcic clinopyroxene, Ti-magnetite, and (rarely) plagioclase.Basaltic rocks also occur in an associated feeder-sheet intrusioncomplex, and as numerous minor sills and dykes. In the intrusioncomplex, basaltic sheets are cut by microdiorites and scarcemicrotonalites. The compositional range in the volcanic complex,from low-SiO₂ basalts to microtonalites (SiO₂ 45–66 wt.per cent), is attributable to fractional crystallization, earlystages of which were dominated by removal of pargasite at (water-undersaturated)pressures close to 10 kb, within the mantle. The parental magmawas derived by hydrous partial melting of a supra-subductionzone mantle wedge. Trace-element patterns indicate that themantle was slightly depleted relative to the putative primordialcomposition (Ta/Yb = 0?1), prior to metasomatism by componentsfrom the subduction zone. Textural variations in cumulate blocks,and various phenocryst forms in basalts, are interpreted asindicating that the erupted magmas came from a thermally andcompositionally stratified magma chamber with associated layeredcrystal accumulations, and that materials from initially separatelayers were mixed prior to eruption.     

Shelf construction in a foreland basin: storm beds, shelf sandbodies, and shelf-slope depositional sequences in the Upper Cretaceous Mesaverde Group, Book Cliffs, Utah

The Upper Cretaceous (Campanian) Kenilworth Member of the Blackhawk Formation (Mesaverde Group) is part of a series of strand plain sandstones that intertongue with and overstep the shelfal shales of the western interior basin of North America. Analysis of this section at a combination of small (sedimentological) and large (stratigraphical) scales reveals the dynamics of progradation of a shelf-slope sequence into a subsiding foreland basin. Four major lithofacies are present in the upper Mancos and Kenilworth beds of the Book Cliffs. A lag sandstone and channel-fill shale lithofacies constitutes the thin, basal, transgressive sequence, which rests on a marine erosion surface. It was deposited in an outer shelf environment. Shale, interbedded sandstone and shale, and amalgamated sandstone lithofacies were deposited over the transgressive lag sandstone lithofacies as a wave-dominated delta and its flanking strand plains prograded seaward. Analysis of grain size and primary structures in Kenilworth beds indicates that there are four basic strata types which combine to build the observed lithofacies. The fine- to very fine-grained graded strata of the interbedded facies are tempestites, deposited out of suspension by alongshelf storm flows (geostrophic flows). There is no need to call on cross-shelf turbidity currents (density underflows) to explain their presence. Very fine- to fine-grained hummocky strata are likewise suspension deposits created by waning storm flows, but were deposited under conditions of more intense wave agitation on the middle shoreface. Cross-strata sets in this region are bed-load deposits that accumulated on the upper shore-face, in the surf zone. Lag strata are multi-event, bed-load deposits that are the product of prolonged storm winnowing. They occur on transgressive surfaces. While the graded beds are tempestites in the strict sense, all four classes of strata are storm deposits. The distribution of strata types and their palaeocurrent orientations suggests a model of the Kenilworth transport system driven by downwelling coastal storm flows, and probably by a northeasterly alongshore pressure gradient. The stratification patterns shift systematically from upper shoreface to lower shoreface and inner shelf lithofacies partly because of a reduction in fluid power expenditure with increasing water depth, but also because of progressive sorting, which resulted in a decrease in grain size in the sediment load delivered to successive downstream environments. The Kenilworth Member and an isolated outlier, the Hatch Mesa lentil, constitute a delta-prodelta shelf depositional system. Their rhythmically bedded, lenticular, sandstone and shale successions are a prodelta shelf facies, and may be prodelta plume deposits. Major Upper Cretaceous sandstone tongues in the Book Cliffs are underlain by erosional surfaces like that beneath the Blackhawk Formation, which extend for many tens of kilometres into the Mancos shale. These surfaces are the boundaries of Upper Cretaceous depositional sequences. The sequences are large-scale genetic stratigraphic units. They result from the arranging of facies into depositional systems; the depositional systems are in turn stacked in repeating arrays, which constitute the depositional sequences. The anatomy of these foreland basin sequences differs     

COSMOGENIC 10BE-AGES FROM THE STORE KOLDEWEY ISLAND, NE GREENLAND

Earlier work in northeast Greenland has suggested a limited advance of the Greenland Ice Sheet during the Last Glacial Maximum (LGM). However, this concept has recently been challenged by marine geological studies, indicating grounded ice on the continental shelf at this time. New ¹⁰Be‐ages from the Store Koldewey island, northeast Greenland, suggest that unscoured mountain plateaus at the outer coast were covered at least partly by cold‐based ice during the LGM. It is, however, still inconclusive whether this ice was dynamically connected to the Greenland Ice Sheet or not. Regardless of the LGM ice sheet extent, the ¹⁰Be results from Store Koldewey add to a growing body of evidence suggesting considerable antiquity of crystalline unscoured terrain near present and Pleistocene ice sheet margins.     

U-series Isotope Data on Lau Basin Glasses: the Role of Subduction-related Fluids during Melt Generation in Back-arc Basins

New thermal ionization mass spectrometry U-series disequilibriumdata are presented for 24 basaltic to dacitic glasses from activespreading centres in the back-arc Lau Basin (SW Pacific), togetherwith additional inductively coupled plasma mass spectrometrytrace element analyses and Sr–Nd–Pb isotope data.Valu Fa Ridge samples, adjacent to the arc front, have highU/Th and (²³⁰Th/²³⁸U) <1, implying a recent (<<350ka) addition of a U-rich slab-derived fluid. The Valu Fa datacan be combined with existing ²³⁰Th–²³⁸U data for theCentral Tonga arc to infer a fluid addition event at