Climatic and tectonic influences on fan deltas and wave- to tide-controlled shoreface deposits: evidence from the Archaean Keskarrah Formation, Slave Province, Canada

The 2.6 Ga Keskarrah Formation, located in the central Slave Province, Northwest Territories, Canada, is a late-orogenic, tectonically controlled sedimentary sequence that developed under unusual climatic and depositional conditions. The formation is adjacent to the crustal-scale, north-trending Beniah Lake Fault and overlies the 3.15 Ga Augustus Granite, the 2.69–2.7 Ga mafic volcanic Peltier Formation and the turbiditic Contwoyto Formation unconformably. Principal lithofacies in the Keskarrah Formation include conglomerate, sandstone and siltstone–sandstone. The conglomerate lithofacies represents coalescing gravelly streamflow-dominated fan deltas adjacent to topographic highs. Up-section quartz-rich arenites and quartz arenites of the sandstone lithofacies are interpreted to be shallow-water shoreface deposits influenced by wave action and tides. The overlying feldspathic litharenites of the siltstone–sandstone lithofacies are consistent with a lower shoreface to proximal offshore environment dominated by wave and tide interaction. Tidal influence in both sandstone-dominated lithofacies is inferred from the presence of mudstone laminae between bedforms and on foresets of cross-beds, as well as from abundant reactivation surfaces with local mudstone drapes. Intense chemical weathering during the Archaean, resulting from elevated atmospheric levels, higher temperatures and moist climatic conditions, played an important role in the development of quartz-rich arenites that appear to be first-cycle deposits. Few lithic fragments and feldspar grains are preserved due to in-situ host rock weathering, chemical weathering during transport and wave and tide action. Hydraulic sorting and abrasion in the shoreface environment contributed to the continued breakdown and transport of labile minerals. Increased proportions of lithic fragments in sandstone beds of the conglomerate lithofacies are the result of shorter transport distances from source areas to the depositional environment. Abundant conglomerate with up to 4-m large granitic boulders derived from the adjacent Augustus Granite and mafic clasts from the Peltier Formation indicate high relief and fault-related uplift and subsidence. The intimate association of fan deltas and wave- and tide-influenced shallow-marine deposits in association with quartz-rich sandstones forming in a high-relief area make the Keskarrah Formation remarkable in the rock record.     

Chemical geothermometry and fluid–mineral equilibria for the Ömer–Gecek thermal waters, Afyon area, Turkey

Thermal waters of the Ömer–Gecek geothermal field, Turkey, with temperatures ranging from 32 to 92°C vary in chemical composition and TDS contents. They are generally enriched in Na–Cl–HCO₃ and suggest deep water circulation. Silica and cation geothermometers applied to the Ömer–Gecek thermal waters yield reservoir temperatures of 75–155°C. The enthalpy–chloride mixing model, which approximates a reservoir temperature of 125°C for the Ömer–Gecek field, accounts for the diversity in the chemical composition and temperature of the waters by a combination of processes including boiling and conductive cooling of deep thermal water and mixing of the deep thermal water with cold water. It is also determined that the solubility of silica in most of the waters is controlled by the chalcedony phase. Equilibrium states of the Ömer–Gecek thermal waters studied by means of the Na–K–Mg triangular diagram, Na–K–Mg–Ca diagram, K–Mg–Ca geoindicator diagram, activity diagrams in the systems composed of Na₂O–CaO–K₂O–Al₂O₃–SiO₂–CO₂–H₂O phases, log SI diagrams, and finally the alteration mineralogy indicate that most of the spring and low-temperature well waters in the area can be classified as shallow or mixed waters which are likely to be equilibrated with calcite, chalcedony and kaolinite at predicted temperature ranges similar to those calculated from the chemical geothermometers. It was also observed that mineral equilibrium in the Ömer–Gecek waters is largely controlled by CO₂ concentrations.     

Archaean quartz arenites in the Canadian Shield: examples from the Superior and Churchill Provinces

Units of remarkably pure Archaean quartz arenite occur in the northwestern part of the Superior Province and in the northern terrane of the Western Churchill Province (Rae Province) of the Canadian Shield. In the Superior Province, silica-cemented quartz arenites of Archaean age are well preserved in several greenstone belts. The example from the Keeyask Lake sedimentary assemblage displays tabular–planar and trough cross-beds, ripple marks, reactivation surfaces and pebble lag deposits. In spite of penetrative deformation and greenschist-grade metamorphism, primary textures are extremely well preserved, showing framework grains to be very well rounded and sorted. The succession of Keeyask Lake quartz-arenite beds is overlain by siltstones containing small-scale stratiform, domal and columnar stromatolites. A shallow-marine environment of deposition is inferred. Detrital heavy minerals include pyrite, magnetite, zircon, tourmaline, apatite, sphene and topaz. In the northern part of the Western Churchill Province (Rae Province), Archaean quartz arenites occur in northeasterly trending belts where intense structural deformation has in most places obscured or obliterated primary textures and structures. This has led to speculation that some of these units are metachert or recrystallized vein quartz, but local preservation of primary textures and structures provides clear evidence of epiclastic origin. In the example described herein, quartz arenites of the Woodburn Lake Group display sparse occurrences of trough and tabular–planar cross-beds, channels, ripple marks and pebble lag deposits. Probable environments of deposition for these quartz arenites include fluvial systems and shallow-marine shelf settings. The occurrence of unequivocal quartz-arenite clasts in beds of intercalated conglomerate provides direct evidence of at least two episodes of accumulation of almost pure quartz sand. Thin sections and polished slabs reveal frameworks of clastic quartz grains with little to no matrix (now mainly muscovite), and rare detrital grains of accessory heavy minerals, predominantly zircon and opaque iron oxides. Pyrite and other sulphides have been introduced along fractures, but some intergranular sulphide grains may be of detrital origin. The principal source for the quartz arenites in both areas must have been quartz-rich granitoid rocks. Conditions of intense chemical weathering are indicated. The widespread occurrence of extremely mature quartz arenites throughout Archaean terranes of the Canadian Shield, and in other shields of the world, are suggestive of crustal stability during early Earth history. The association of quartz arenites and ultramafic rocks, uncharacteristic of younger terranes, is now recognized in many Archaean greenstone belts of the Canadian Shield.     

The importance of air escape processes in the formation of dish-and-pillar and teepee structures within modern and Precambrian fluvial deposits

Attributed to escape of water from unconsolidated deposits, teepee and dish-and-pillar structures are common features in the Late Proterozoic Torridonian red bed succession of NW Scotland. Study of the first annual flood on a modern depositional analogue, in Wadi Baysh, Saudi Arabia, revealed that a layer of mud, initially deposited across the alluvium, restricted the escape of air being displaced through the normally unconfined aquifer by waters entering the aquifer from the river channels. As air bubbled through high level pool-floor muds, it generated dish-and-pillar structures. The formation of teepee structures by similarly combined air and water escapes nearer to the channel later in the annual cycle of flooding is foreseen. It is suggested that the two sediment structures are generated in response to different parts of the aquifer recharge spectrum. The role of air displacement by influent waters is important in the initial phases of structure formation, and combined air and water escape creates greater disturbance to the sediments. The sedimentary structures formed are not, therefore, solely due to water escape, as has been previously suggested. Ephemeral seasonal flooding appears to have characterized the deposition of at least the Stoer Group and Diabaig Formation sediments in the Torridonian.