W.U. Mueller

Precambrian clastic sedimentation systems

Abstract The unique and evolving nature of the Precambrian geological environment in many ways was responsible for significant differences between Precambrian clastic sedimentary deposits and their Phanerozoic-modern equivalents. Some form of plate tectonics, with rapid microplate collisions and concomitant volcanic activity, is inferred to have led to the formation of greenstone belts. Explosive volcanism promoted common gravity-flow deposits within terrestrial greenstone settings, with braided alluvial, wave/storm-related and tidal coastline sediments also being preserved. Late Archaean accretion of greenstone terranes led to emergence of proto-cratons, where cratonic and rift sedimentary assemblages developed, and these became widespread in the Proterozoic as cratonic plates stabilised. Carbonate deposition was restricted by the paucity of stable Archaean terranes. An Early Precambrian atmosphere characterised by greenhouse gases, including CO2, in conjunction with a faster rotation of the Earth and reduced albedo, provide a solution to the faint young Sun paradox. As emergent continental crust developed, volcanic additions of CO2 became balanced by withdrawal due to weathering and a developing Palaeoproterozoic microbial biomass. The reduction in CO2, and the photosynthetic production of O2, led to aerobic conditions probably being achieved by about 2 Ga. Oceanic growth was allied to atmospheric development, with approximately 90% of current ocean volume being reached by about 4 Ga. Warm Archaean and warm, moist Palaeoproterozoic palaeoclimates appear to have become more arid after about 2.3 Ga. The 2.4–2.3 Ga Huronian glaciation event was probably related to continental growth, supercontinent assembly and weathering-related CO2 reduction. Despite many analogous features among both Precambrian and younger sedimentary deposits, there appear to be major differences as well. Two pertinent examples are rare unequivocal aeolian deposits prior to about 1.8 Ga and an apparent scarcity of Precambrian foreshore deposits, particularly those related to barrier island systems. The significance of these differences is hard to evaluate, particularly with the reduced palaeoenvironmental resolution because of the absence of invertebrate and plant fossils within Precambrian successions. The latter factor also poses difficulties for the discrimination of Precambrian lacustrine and shallow marine deposits. The temporal distribution of aeolian deposits probably reflects a number of possible factors, including few exposed late Archaean–Palaeoproterozoic cratonic areas, extensive pre-vegetative fluvial systems, Precambrian supercontinents and a different atmosphere. Alternatively, the scarcity of aeolian deposits prior to 1.8 Ga may merely reflect non-recognition or non-preservation. Precambrian shallow marine environments may have been subjected to more uniform circulation systems than those interpreted from the Phanerozoic-modern rock record, and Precambrian shelves probably were broad with gentle seaward slopes, in contrast to the narrow, steep shelves mostly observed in present settings. Poorly confined Precambrian tidal channels formed sheet sandstones, easily confused with fluvial or offshore sand sheets. Epeiric seas were possibly more prevalent in the Precambrian, but active tectonism as proto-continents emerged and amalgamated to form early supercontinents, in conjunction with a lack of sufficient chronological data in the rock record, make it difficult to resolve the relative importance of eustatic and tectonic influences in forming epeiric embayments and seaways. Other differences in Precambrian palaeoenvironments are more easily reconstructed. Ancient delta plain channels were probably braided, and much thicker preserved delta successions in the Precambrian are compatible with the inferred more active tectonic conditions. Pre-vegetational alluvial channel systems were almost certainly braided as well. Common fluvial quartz arenites are ascribed to differences in weathering processes, which probably changed significantly through the Precambrian, or to sediment recycling. Although Precambrian glacigenic environments were probably the least different from younger equivalents, their genesis appears to reflect a complex interplay of factors unique to the Precambrian Earth. These include emergence and amalgamation of proto-continents, the early CO2-rich atmosphere, the development of stromatolitic carbonate platforms, early weathering, faster rotation of the Earth and the possible role of changes in the inclination of the Earths axis.

Oblique Archean subduction: accretion and exhumation of an oceanic arc during dextral transpression, Southern Volcanic Zone, Abitibi Subprovince Canada

Abstract The Archean Abitibi Subprovince, divided into Northern (NVZ) and Southern (SVZ) Volcanic Zones, is characterized by the crustal-scale Destor-Porcupine-Manneville Fault Zone (DPMFZ), which links the SVZ and NVZ, and the Cadillac-Larder-Lake Fault Zone (CLLFZ), which separates the Abitibi and Pontiac Subprovinces. The DPMFZ and CLLFZ represent major deformation zones that record over 60 million years of Archean deformation. The dextral transpressional phase in the NVZ, which represents incipient SVZ deformation, occurred between 2700 and 2692 Ma and is characterized by Southeast (SE)-trending dextral faults (e.g. Macamic Fault). Synorogenic flysch sedimentation is focussed at the interface between the SVZ–NVZ and Pontiac–Abitibi Subprovinces. The SE-trending Parfouru fault, linking the two major trench fault systems, is interpreted as a reactivated leaky transform fault and is associated with synorogenic flysch sedimentation. The early monzodioritic suite of the Preissac-Lacorne batholith was related to DPMFZ thrusting (ca. 2692–2690 Ma). The change from early thrusting to transcurrent motion is recorded in the Duparquet strike-slip basin, which formed between 2690 and 2680 Ma along the DPMFZ, and the Granada pull-apart basin, which evolved between 2680 and 2670 Ma along the CLLFZ. The timing of these basins indicates a diachronous evolution. Renewed thrusting affected the Granada basin but not the Northern Duparquet strike-slip basin, so that a southward migrating deformation front is inferred. Late exhumation resulted in extension along both fault zones and was responsible for the juxtaposition of medium- and low-grade metamorphic rocks. The monzogranitic suite of the Lamotte pluton, which occurred between 2660 and 2642 Ma is linked to exhumation. Final dextral transpression post-dating exhumation produced shearing and folding. The SVZ of the Abitibi Subprovince shows the salient attributes of modern oblique orogenic collisions, with alternating phases of thrusting and strike-slip movement along crustal-scale faults during dextral transpression.

Caldera-forming processes and the origin of submarine volcanogenic massive sulfide deposits

Certain volcanogenic massive sulfide (VMS) ore deposits form in submarine calderas. This association is well known, but the link between caldera formation and the origins of the deposits remains poorly understood. Here we show that the size and location of a VMS deposit within a submarine caldera may be determined by how and when the caldera formed. These spatial-temporal conditions control development of the hydrothermal system associated with the VMS deposit. We propose that caldera opening along outward-dipping faults promotes magma degassing, seawater influx, and high-temperature leaching, resulting in a metal-rich hydrothermal fluid. These outward-dipping faults are considered to provide critical pathways for ore-forming fluids responsible for some caldera-hosted VMS deposits and may also be fundamentally important for the formation of many other caldera-hosted ore deposit types.

Felsic fire-fountaining beneath Archean seas: pyroclastic deposits of the 2730 Ma Hunter Mine Group, Quebec, Canada

Abstract The lapilli tuff breccias (LTB-1 and LTB-2) of the Archean Hunter Mine Group in the south-central part of the Abitibi greenstone belt are inferred to be the product of subaqueous lava fountaining. Intercalated sub-wave base iron-formations, interstratified turbiditic tuffs, the absence of wave-induced sedimentary structures, and the stratigraphic position of lapilli tuff breccias beneath basaltic komatiites, support this contention. A complete eruptive sequence shows a tripartite division into (a) massive breccia, (b) stratified lapilli tuff, and (c) turbiditic tuff-lapilli tuff division. The massive breccia division is characterized by clusters of isolated and compressed irregular-shaped clasts inferred to be deposited directly from the hot magmatic lava fountain. Abundant vesicular pyroclasts with a vesicle content of up to 60% exhibit locally coalescing vesicles indicating bubble nucleation prior to eruption. The prevalence of irregular to amoeboid clast shapes suggests transport from the vent in a steamy-rich, high-density current to the site under a self-generated steam cupola. Ubiquitous subequant lapilli-size pyroclasts of the stratified lapilli tuff division suggest that significant ingress of water into the fountain changed the prevalent fragmentation process from magmatic to hydrovolcanic. The turbiditic tuff-lapilli tuff division composed of pumice, lithic fragments and vitric ash is envisaged to have formed by gravitational collapse of a subaqueous turbulent eruptive plume. This type of eruptive mechanism constituted a minor but important process of volcanic construction on the ocean floor during the Archean, and possibly during incipient arc and backarc formation in modern day settings.

Late-orogenic basins in the Archaean Superior Province, Canada: characteristics and inferences

Abstract The late-orogenic Archaean Duparquet, Kirkland and Stormy basins of the Canadian Superior Province are characterized by bounding crustal-scale faults and abundant porphyry stock emplacement. Lava flows and pyroclastic deposits are restricted to the Kirkland and Stormy basins, and coarse clastic detritus characterizes the Duparquet basin. Seven distinct lithofacies are identified: (1) mafic volcanic, (2) felsic volcanic, (3) pyroclastic, (4) volcaniclastic, (5) conglomerate-sandstone, (6) sandstone-argillite (± conglomerate), and (7) argillite-sandstone (± tuffaceous sandstone). The mafic and felsic volcanic lithofacies represent effusive lava flows, the pyroclastic lithofacies is formed of subaerial surge and airfall deposits and the volcaniclastic lithofacies is composed of reworked volcanic debris. The conglomerate-sandstone lithofacies is interpreted as alluvial fan, fan delta or proximal braided stream deposits, whereas the sandstone-argillite lithofacies is consistent with sandy-dominated flood- or braidplain deposits. A dominantly shallow-water lacustrine setting is inferred for the argillite-sandstone lithofacies. These different lithofacies record the basin history and can be used to identify basin-forming processes. Lithofacies stacking and rapid lateral changes of lithological units in conjunction with interformational unconformities and basin margin faults suggest tectonically induced sedimentation. Volcanism can also influence basin evolution and the delicate balance between erosion, sedimentation, and prevalent transport processes is affected by volcanic input. Catastrophic influx of pyroclastic material facilitated mass-wasting processes and formation of non-confined hyperconcentrated flood flow deposits account for local congestion of alluvial or fluvial dispersal patterns. Confined stream flow processes govern sedimentation during intravolcanic phases or prominent tectonic uplift. In addition, climate which controls the weathering processes, and vegetation which stabilizes unconsolidated material, affects the transport and depositional process. A CO 2 -rich aggressive weathering, humid Archaean atmosphere favours traction current deposits and an absence of vegetation promotes rapid denudation. Although tectonism is the prevalent long-term controlling factor in restricted basins, the effects of volcanism, climate and lack of vegetation can also be detected.

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 CO2 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.

Volcanism and related slope to shallow-marine volcaniclastic sedimentation: an Archean example near Chibougamau, Quebec, Canada

Abstract Volcaniclastic sedimentary rocks and K-rich lava flows of the Archean Hauy Formation are discontinuously exposed in a 850 m thick sequence in the Waconichi synclinorium northwest of Chibougamau, Quebec. The interstratified sedimentary rocks comprise two major sedimentary facies associations: the Graded Bedded Facies Association (WBFA), indicative of deposition by sediment gravity flows, and the Wavy Bedded Facies Association (WBFA), defined by wave-induced structures. The GBFA is near the base of the stratigraphic section and is interstratified up-section with the WBFA. In the GBFA the abundance of synsedimentary folding, truncated slump folds, load casts, and low-angle erosional surfaces, as well as sediments deposited by turbidity currents and debris flows, support designation as a volcanic slope deposit. In the WBFA trough crossbeds, wavy bedded gravels, and plane-bedded volcaniclastic sandstones are indicative of a nearshore setting. The abundance of angular to subangular lithic volcanic fragments, and broken and euhedral pyroxene crystals in the sediments is suggestive of a pyroclastic origin. Some beds may represent primary pyroclastic material, but unequivocal petrographic recognition of pumice or glass shards is not possible because of metamorphic overprint. Composition suggests an initial magmatic or phreatomagmatic eruption, subsequently reworked along the shoreline and transported downslope. Lava flows comprising massive to lobate andesites and massive to blocky basalts constitute most of the stratigraphic sequence; pillowed flows are strikingly absent from the section. Both compositional types have an increase in vesicularity and change in flow form up-section from massive to lobate or blocky, suggesting progressively shallower water depths. The composite character represented by Hauy lava flows and associated volcaniclastic sediments in characteristic of stratovolcanoes. The studied section is an integral part of an evolved arc with an adjacent backarc basin. The volcano-sedimentary facies indicate the development of a volcanic island in one of these basins.

Age constraints and characteristics of subaqueous volcanic construction, the Archean Hunter Mine Group, Abitibi greenstone belt

The Archean Hunter Mine Group (HMG) is a 6–7 km-thick south-facing volcanic sequence at the southern limit of the Northern Volcanic Zone, Abitibi greenstone belt. The HMG is composed of (1) felsic lava flows and endogenous and exogenous domes with autoclastic breccia deposits; (2) pyroclastic fire-fountain products and high- to low-concentration volcaniclastic sediment gravity flows of autoclastic and pyroclastic origin interstratified with iron-formation; (3) mafic pillowed and brecciated flows; (4) a prominent felsic dyke swarm; and (5) abundant mafic intrusions. The stratigraphy is complex, as inferred by detailed volcano-sedimentary facies mapping. A 5–7 km-wide, N-trending felsic dyke swarm, coupled with abundant synvolcanic and synsedimentary faults, chaotic breccias and mineralization suggest a central volcanic depression, which is consistent with a caldera structure. Voluminous effusive activity and explosive fire-fountaining eruptions contributed to caldera development. Rifting of this arc edifice is supported by the presence of felsic dykes and mafic tholeiitic sills. The lithofacies and overlying komatiites flows are consistent with a deep-water marine setting. U-Pb age determinations help constrain the HMG caldera formation to ≈6 million years. The sampled quartz–feldspar–phyric dykes yielded U-Pb zircon ages of 2728.3+4/−3.4 and 2728.9±0.8 Ma and together with a previously published dyke age of 2729.6±1.4 Ma from the same dyke swarm, a case is made that the swarm evolved over 1 million years (2728.6–2729.6 Ma). A lobate flow and flow breccia revealed similar ages of 2724.6+4.6/−1.7 and 2727.6+4.2/−2.0 Ma, whereas the up to 1 km-thick, gabbro-diorite Roquemaure sill precedes both felsic dykes and flows, as indicated by the 2731.8+2.2/−2.0 Ma age. The 2706.4+5.7/−4.3 Ma feldspar–quartz–phyric dykes occurred late in the history of the HMG and were not associated with subaqueous volcanic construction. The 207Pb/206Pb ages of analyzed zircon fractions have an inherited component within error of the oldest crystallization ages obtained for the HMG complex. Collectively, these age determinations and inheritance, as well as stratigraphy, support a tripartite division into a 2734–2730 Ma lower formational stage, a 2732 Ma middle formational stage and a 2730–2728 Ma upper formational stage. The study shows that age constraints without detailed facies mapping may lead to erroneous conclusions on volcanic construction and that felsic volcanic dykes and flows may inherit an earlier component of their own volcanic edifice.

Volcanic and tectono-plutonic influences on sedimentation in the Archaean Kirkland Basin, Abitibi greenstone belt, Canada

Abstract A 3–5 km thick volcano-sedimentary succession of the Timiskaming Group (2685-2675 Ma) was deposited in a 50 km elongate Kirkland Basin in the southern part of the Abitibi greenstone belt. An unconformity bounding the northern margin, and the Cadillac Larder Lake fault constraining the basin to the south, are geometric characteristics of an interpreted extensional half-graben structure. Facies associations interstratified with alkaline lava flows reveal a complex history for this small molasse basin which represents a late phase of Archaean evolution in the Canadian Superior Province. Syneruption deposits are characterised by a pyroclastic facies association, massive and brecciated alkaline lava flows and a reworked volcaniclastic facies association. Volcanic edifices dominated the landscape during the syneruption phase. Episodic volcanism was sufficiently voluminous to congest local fluvial dispersal systems, resulting in the widespread accumulation of unconfined volcaniclastic aggradation deposits. Sheetflood deposition from hyperconcentrated flows was an important process. As the effects of volcanism diminished, tectono-plutonic activity influenced basin evolution. The conglomerate-sandstone facies association represents stream-dominated alluvial fans and proximal braided stream deposits. The conglomerate-sandstoneargillite facies association represents distal fan, braided stream or braidplain deposits. The argillite-sandstonetuffaceous sandstone facies association is indicative of a floodplain setting containing small ephemeral lakes and ponds. Local provenance associated with high relief is indicated by the dominance of coarse cobble-size extrabasinal debris at the basin margin, and abundance of intrabasinal clasts derived from volcanic edifices eroded down to their plutonic roots. Volcanism and tectono-plutonic processes provided allocyclic controls for the evolution of the Kirkland Basin. The Cadillac Larder Lake fault appears to have initiated basin formation and continued to influence sedimentation by subsequent fault movements, serving as a locus for emplacement of synvolcanic porphyry stocks, as well as providing the major conduit for alkalic flows.

Shallow-water, eruption-fed, mafic pyroclastic deposits along a Paleoproterozoic coastline: Kangerluluk volcano-sedimentary sequence, southeast Greenland

Abstract The 200–300 m thick, volcano-sedimentary sequence at Kangerluluk is part of the psammite zone, one of four major zones, which constitute the 1.8 Ga Ketilidian orogen in south Greenland. Three lithofacies are emphasized in the study: (1) the conglomerate-sandstone; (2) the volcanic; and (3) the pyroclastic lithofacies. The 2–40 m thick conglomerate-sandstone lithofacies represents a subaerial to subaqueous fan-delta deposit. Matrix- and clast-supported conglomerates are interpreted as debris flow and longitudinal gravel bar deposits. Erosive-based conglomerate channel fills attest to stream incision. Trough crossbedded sandstone, interpreted as lunate megaripples, planar-bedded sandstone indicative of upper flow regime bar-top sands, and small-scale trough crossbeds reflecting ripples follow up-section, form collectively with the conglomerate, 0.40–2.50 m thick fining-upward sequences. The sandstone-dominated unit, up-section from the conglomerates and composed of planar and low-angle crossbeds, minor ripples and graded beds as well as mudstone is indicative of a lower shoreface deposit below normal wave base. The clastic sedimentary rocks are suggestive of a fan-delta setting. The 100–200 m thick volcanic lithofacies, composed of pillowed and pillow brecciated lava flows, is consistent with shallow-water deposition. Interstratification of lava flows with both conglomerate-sandstone and pyroclastic lithofacies, intrusion of dykes into volcaniclastic rocks, and peperite formation accentuate contemporaneity between volcanism and sedimentation and is a common feature of island arcs. The 1–50 m thick, pyroclastic lithofacies with sharp depositional contacts to the overlying volcanic and underlying conglomerate-sandstone lithofacies, was emplaced in a subaqueous setting. The lithofacies is divided into a planar- to crossbedded tuff-lapilli tuff and a bedded lapilli tuff breccia, whereby both deposits are inferred to result from shallow-water surtseyan-type eruptions. The 5–15 m thick, bedded lapilli tuff breccia with abundant bomb sag structures and graded beds is considered a result of subaqueous eruptions strong enough to form an insulating steam cupola characterized by ballistically emplaced bombs that rapidly collapsed allowing for transport via mass flow processes. The deposits are considered proximal to the vent. The 2–50 cm thick, planar- to crossbedded tuff-lapilli tuff featuring abundant euhedral and broken crystals of feldspar (≤2cm) and minor pyroxene (≤1 cm), are massive, graded, crossbedded and stratified. The planar but laterally discontinuous beds, characterized by abundant low-angle scours, are interpreted as low- to high-concentration sediment gravity flows produced directly from subaqueous tephra jets that collapsed due to massive water ingestion. Local breccia-size pyroclasts disrupting beds are interpreted as bomb sags. The mafic, eruption-fed, Surtseyan-type deposits, postulated to be a subaqueous counterpart of cold, subaerial base surges, originate from subaqueous tuff cones formed along a rugged volcanic-dominated shoreline featuring high-energy fan-deltas.