Deep channel sedimentation in the Lower Cretaceous McMurray Formation, Athabasca Oil Sands, Alberta

ABSTRACT In the region of the Athabasca Oil Sands, Alberta, the Lower Cretaceous McMurray Formation comprises 50-80 m of uncemented quartz sand and associated shale, saturated throughout by bitumen. The sediments are dominantly of continental origin, except in the uppermost parts of the formation where sedimentation was influenced by the encroaching boreal sea.
In most outcrop and mine face exposures of the McMurray Formation, a sequence of three facies is recognized. In ascending order these are: (1) an erosionally based thick-bedded sand facies, 2-20 m thick, dominated by large-scale trough cross-beds; (2) an epsilon cross-stratified facies with solitary sets up to 25 m in thickness, consisting of decimetre to metre thick couplets of sand/mud, with depositional slopes of 8-12° and palaeocurrent indications parallel to the strike of the epsilon cross-set; and (3) a horizontally bedded argillaceous sand facies up to a few metres thick. The three-fold sequence is interpreted as a single upward-fining cycle of channel sedimentation, the trough cross-bedded sands resulting from channel bottom deposition, the epsilon cross-strata accumulating by lateral accretion of channel point bars, and the upper argillaceous sand representing floodplain sedimentation. Where the McMurray Formation is relatively thin (less than 50 m), virtually the entire formation is commonly composed of a single upward-fining channel deposit.
Details of the size and physiographic setting of the channels are somewhat uncertain, but the present evidence suggests that the epsilon-dominated McMurray Formation sequence in the Athabasca Deposit region represents the coastal plain culmination of a very large fluvial drainage system.     

Mineral Chemistry and Metasomatic Growth of Aluminous Enclaves in Gedrite--Cordierite-Gneiss from Southwestern New Hampshire, USA

The aluminous enclaves occur in gedrite-cordierite-gneissesof the Middle Ordovician Ammonoosuc Volcanics, and are composedof combinations of the aluminous minerals sillimanite (Sill),kyanite, corundum (Cor), staurolite (St), sapphirine (Sa), andspinel (Sp), which are set in a matrix of cordierite (Crd) orplagioclase (Plag). Generally, where plagioclase is present,both it and the aluminous minerals are separated from gedrite(Ged) and rare hornblende (Hbl) by cordierite. The enclavesarc interpreted to have formed near the peak of Acadian (Devonian)metamorphism at sillimanite-staurolite-muscovite grade by reactionsthat were encountered during the pressure decrease which accompaniedthe rise of gneiss domes in the region. The enclaves are divided into two main types: (1) enclaves ofcordierite surrounding aluminous minerals; and (2) enclavesof cordierite and plagioclase surrounding aluminuous minerals.Sapphirine grains contain between 9?2 and 9?3 Al atoms per formulacalculated to 14 cations. Staurolites from the enclaves areMg-rich and have (Fe²⁺+ Mn)/(Fe²⁺+Mn+Mg) ratios of 0-59–0?64. The textures and mineralogy of the enclaves suggest that theserocks originally consisted of Ged+Sill?Qz?Hbl?Sp?Plag. Theseminerals reacted to form Crd+Aluminous Minerals?Plag. The mineralogyof both main types of enclaves can be explained by two analogoussets of continuous Fe-Mg reactions:The structure of the enclavessuggests that the mineral growth by the above reactions wasdiffusion controlled, which would have resulted from oversteppingthe above reactions (i.e. the P change exceeded the reactionrate). Therefore, chemical potential gradients (relative mobilityof diffusing components) between gedrite and sillimanite controlledthe location of mineral growth. The Fe-Mg ratio of the bulkcomposition and the proportions of non-Fe-Mg minerals (quartzand sillimanite) appear to determine which continuous Fe-Mgreactions were encountered. Examples of mineral sequences in the cordierite enclaves are:Sill (core)/St+Crd/Ged (matrix); Cor+Crd (core)/Ged (matrix),and Sill (core)/St+Crd/Sa+Crd/Ged (matrix). Examples of themineral sequences in the cordierite-plagioclase enclaves are:Sill (core)/St+Plag/Plag+Crd/Hbl+Ged (matrix); Cor+Plag (core)/St+Plag/Sa+Plag/Ged+ Hbl (matrix); and St+Plag (core)/Plag+Crd/Ged+Hbl (matrix). P–µ_FeMg–1 diagrams proved to be an importanttool for understanding and illustrating the development of theenclaves. These diagrams allow one to view simultaneously allthe discontinuous and continuous Fe-Mg reactions along a P–µ_H2O(or T) rock path. With this information it is possible to determinequalitatively which reactions and what sequence of reactionsmight be encountered by bulk compositions with variable Fe-Mgratios and modal proportions of phases.     

Paleoecology of the Mollusca of the Tjörnes sequence, Iceland

A long sequence of open-coast tidal flat deposits with a restricted, mainly boreal fauna, comprises the Tapes and Mactra Zones. Characteristic species are Venerupis aurea, Spisula arcuata, Arctic islandica and Lentidium complanatum . This stage is provisionally correlated with the Upper Pliocene of the North Sea Basin. During the time of deposition of the Serripes Zone the basin apparently deepened and the fauna greatly diversified with immigration of arctic elements. It went back finally to beach-deposit conditions. The characteristic species — Serripes groenlandicus, Macoma praetenius , and Beta borealis — are also characteristic of the Early Pleistocene of the North Sea Basin with which the Serripes Zone is provisionally correlated. The climate is interpreted as having been colder than that of the Tapes and Mactra zones. A preliminary list of the Mollusca is given.     

Igneous Petrology of the Synorogenic Fongen-Hyllingen Layered Basic Complex, South-Central Scandinavian Caledonides

The 160 km² Caledonian Fongen-Hyllingen complex is an extremelydifferentiated, layered, basic intrusion, synorogenically emplacedat 5–6 kb in the allochthonous Trondheim nappe complex,situated in the Trondheim region of Norway. A zone of gabbroic rocks without rythmic layering usually occursalong the margin and a supposed feeder to at least part of thecomplex is preserved. A wide variety of magmatic sedimentarystructures are present in the c. 10,000 m thick sequence ofrhythmically layered rocks which vary from olivine-picotitecumulates at the base to quartz-bearing ferrosyenites at thetop. Mineral compositions, fractionation trends, and the compositionof feeder rocks suggest a tholeiitic parent. Mineral compositions cover extreme ranges. Olivine varies fromFo_86·2 to Fo_0·2 with a hiatus between about Fo₇₁and Fo₆₁. Plagioclase ranges from An_79·5 to An_1·5,albite coexisting with orthoclase microperthite in the finaldifferentiates. Cumulus Ca-poor pyroxene (Wo_2.4En_66.8Fs_30.8-Wo_2·0En_17·0Fs_81·0)first shows sporadic inversion from pigeonite at the Fe-richcomposition of Fs₆₇ and the final Ca-poor pyroxenes are replacedby magmatic grunerite which reaches an Mg: Fe ratio of 12:88.Ca-rich pyroxenes (Wo_44·7En_43·8Fs_11·5-Wo_47·0En₀Fs_53·0)are highly calcic and have a slight Ca-minimum in the earlystages, unrelated to the disappearance of Ca-poor pyroxene.Calcic amphibole, a constant intercumulus phase in most of thecomplex, becomes a cumulus phase in the later stages and variesfrom titanian-pargasite to ferro-edenite. Magnetite and ilmenitejoin the cumulate assemblage at Fo₅₅ and ilmenite persists intothe final quartz-bearing ferrosyenite where it shows replacementby sphene. Apatite, biotite, zircon, quartz, K-feldspar andallanite join the final extreme differentiates in the namedsequence. The fractionation trend is, in many respects, transitionalbetween those typical of the tholeiitic and calc-alkaline series,and is interpreted as reflecting crystallization under moderate,increasing P_H2O. Cryptic layering shows several reversals to higher temperatureassemblages with increasing stratigraphic height. Successivereversals are to irregular compositions and measured in termsof olivine composition, can be up to about 30 mole per centFo. The minimum stratigraphic thickness to include the entirefractionation range is reduced to about 2200 m after ‘removal’of the compositional overlaps due to the reversals. Thus roughlythree-quarters of the present cumulate stratigraphic sequencerepresents magma replenishment. A mechanism involving the mixingof fresh magma batches with the residual, differentiated magmafrom the previous influx, is envisaged. The periodic influxof fresh magma took place into a chamber which was probablyclosed to the exit of material.     

Experimental Phase Relations in the System Tonalite-Peridotite-H2O at 15 kb; Implications for Assimilation and Differentiation Processes near the Crust-Mantle Boundary

Experiments at 15 kb in the tonalite-peridotite-H₂O system provideinformation on some of the phase equilibrium factors that mayinfluence reaction and assimilation processes between quartznormativemagmas and ultramafic rocks in the deep crust and upper mantle.Experiments were done with 5 or 10 wt.% H₂O added to powderednatural samples of tonalite, and mixtures of tonalite with 5or 10 wt.% peridotite added (TP5 and TP10, respectively). Theliquidus phase relations of these starting compositions wereinvestigated between 850 and 1100?C at 15 kb, using gold capsulesso that iron loss to the sample containers was not a problemand meaningful glass and mineral analyses could be obtained.Experiments on the tonalite alone show either liquidus garnet,for samples with 5% H₂O added, or liquidus hornblende, for sampleswith 10% H₂O. In contrast, orthopyroxene is the sole liquidusphase, irrespective of water content, in experiments using startingmixtures of 5 or 10 wt.% peridotite added to tonalite. Glassanalyses of partially crystallized tonalite define a crystallizationpath diverging significantly from the calc-alkaline trend towardshigher Ca/(Mg + Fe) in the CaO–(MgO + FeO)–?SiO₂triangle. In contrast, glasses from partially crystallized mixturesof tonalite with 5 or 10 wt.% peridotite added define a liquidtrend close to natural calc-alkaline compositions in terms ofCa/(Mg + Fe). Of more general significance, the proximity ofa field ofliquidus orthopyroxene on the high (Mg + Fe) sideof compositions along the calc-alkaline trend serves to limitthe Mgenrichment of such melts by interaction with ultramaficrocks. Unless heat is added to the system, reaction of tonaliticcomposition melts with ultramafic rocks will produce only slightlyMg-enriched melts: increasing degree of reaction simply resultsin further precipitation of orthopyroxene + garnet ? clinopyroxeneonce melt compositions reach the orthopyroxene field boundary.     

Relationships between cyclicity and stromatolite form in the Late Proterozoic Bitter Springs Formation, Australia

Stromatolite biostromes and bioherms in the lower two units of the Late Proterozoic Loves Creek Member of the Bitter Springs Formation represent shallowing upward and deepening upward sequences. In the central unit stromatolite form is governed by relative position in an asymmetric shallowing upward sequence. Ooid and/or peloid-intraclast grainstones and small, irregular bulbous and columnar stromatolites characterize the basal, transgressive portion of cycles. Domal, columnar and stratiform stromatolites comprise the bulk of the cycle. These forms accreted in a gradually shallowing epeiric sea. Domal stromatolites predominate in the deeper parts of cycles. Here synoptic relief gradually increases upwards. Columnar and stratiform stromatolites predominate in the shallower parts of cycles, where synoptic relief rapidly diminishes upwards. In thin-bedded dolo-mudstones at the tops of cycles the co-occurrence of desiccation cracks, tepee structures, scalloped dissolution surfaces, gypsum moulds and anhydrite nodule pseudomorphs provides evidence for subaerial exposure. In contrast, stromatolites in a unit at the base of the Loves Creek Member accreted during a gradual rise in sealevel. Stratiform, columnar and domal stromatolitic building blocks of the shallowing upward cycle are present in this deepening sequence, but only the lower half of the shallowing upward cycle is represented. Synoptic relief of the stromatolitic laminae gradually increases upward throughout the basal stromatolitic unit. Recognition of a deepening upward stromatolite sequence at the base of the Loves Creek Member, and a disconformity surface between this sequence and the underlying Gillen Member, permits palaeoenvironmental re-interpretation of the Loves Creek Member as a single ‘large scale’ sea-level cycle.     

A Late Pleistocene coarse-grained spit-platform sequence in northern Jylland, Denmark

Several well-preserved Late Pleistocene spit systems occur uplifted in northern Jylland, Denmark. Their present-day morphological expression allows detailed study of spit growth patterns while the internal sedimentological organisation can be examined in a series of pits distributed along the length of the spits. Two characteristic vertical sequences are recognized in the systems. The first (Sequence I) consists of a giant-scale cross-bedded foreset unit, overlain by topset and beach units, while the second (Sequence II) consists of the foreset unit overlain by bar-trough and beach units. The two sequence types pass laterally into each other with a short overlap zone. They can be interpreted in terms of Meistrell's (1966, 1972) model for spit-platform growth based on scaled wave tank experiments. The giant-scale cross-bedded unit corresponds to prograding of a coarse-grained subaqueous spit-platform while the topset, bar-trough and beach units reflect the growth of the subaerial spit. The alternation between sequence I and II reflects the inversely related growth of the spit and platform structures: when the rate of subaqueous platform progradation declines, the subaerial spit grows uniformly, and when the platform progrades uniformly spit growth declines. The model is probably only valid for relatively coarse-grained systems because only these deposits would have a relatively steep front. The water depth in which the spit system progrades and thus bottom topography, determines the thickness of the giant-scale cross-bedded foreset unit because the water depth over the top of the platform is relatively constant. If the water is less than a few metres deep the spit-platform is not developed as seen where the Late Pleistocene spit systems prograded over elevations of the sea bottom. Conversely, the correct recognition of spit-platform sequences allows precise determination of sea-level and water depth at the time of formation. Finally, the model adds one further mode of formation of giant-scale cross-bedding to those already known from fluvial transverse, lateral and point bars, subtidal sand waves and Gilbert deltas.     

Experimental Study of the Composition Join NaAlSiO4-CaCO3-H2O and the Genesis of Alkalic Rock--Carbonatite Complexes

Phase equilibrium studies have been carried out on the compositionjoin NaAlSiO₄-CaCO₃-H₂O with 25 wt per cent H₂O at 1 kb pressurein the temperature range 600–960 °C. Liquid, in equilibriumwith crystalline phases and a sodic, aqueous vapor phase persistsacross the join down to temperatures of about 600 °C. Fractionalcrystallization of a carbonated nepheline-rich liquid in equilibriumwith vapor is capable of generating successively the crystallineassemblages (1) nepheline, (2) melilite+nepheline, (3) hydroxyhaüyne+melilite,(4) cancrinite+melilite, and (5) calcite+cancrinite+melilite.Late-stage liquid fractions are rich in CaCO₃, whereas the vaporphase is enriched in Na. The experimental assemblages are strikinglysimilar to rocks in alkalic rock-carbonatite complexes in generaland in the Oka, Quebec, complex in particular. The successionof assemblages at Oka and at other melilite rock-bearing complexesmay be interpreted as the products of fractionation of a carbonatednephelinite magma by analogy with the experimental results.The sodium-bearing vapor phase of the experiments may be analogousto the fenitizing agent of some carbonatite complexes.