Supermature Quartz Arenites Research Articles

Testing Tectonostratigraphic Hypotheses of the Blountian Phase of the Taconic Orogeny in the Southern Appalachians through an Integrated Geochronological and Sedimentological Study of Ordovician K-Bentonites and Quartz Arenites

The tectonic setting of the southernmost part of the eastern margin of Laurentia during the Blountian tectophase (~472–452 Ma) of the Ordovician Taconic Orogeny remains unresolved. Tephras produced by explosive volcanism during this early phase of the orogeny are now K-bentonites, and in many locations, they are interbedded with mature to supermature quartz arenites. We conducted U-Pb analyses of detrital zircons from the sandstones, and of zoned magmatic zircons from the K-bentonites, to constrain the tectonostratigraphic setting with more precision. We also used geochemical fingerprinting of apatite phenocrysts to correlate the K-bentonites in these sandstones along the tectonic front, and we then integrated these results with a depositional systems study of the quartz arenites to further constrain and test competing models of the regional tectonomagmatic setting during that time. The general dearth of detrital zircons that have ages contemporaneous with the volcanic activity, coupled with the predominantly Precambrian ages of the zircons in these Lower Paleozoic quartz arenites that otherwise lack volcaniclastic components—such as detrital VRFs or a muddy matrix derived from an eroding volcanic arc—suggests that magmatic zircons from the tephra layers entered the depositional system only occasionally, and that the volcanic centers were separated geographically from where these quartzose sands were being deposited. Our findings support a tectonostratigraphic and tectonomagmatic model analogous to a combination of select modern settings in the western Pacific and Indonesia, specifically (1) New Guinea, where mature quartz arenites occur in the Cenozoic foreland succession, and (2) Sumatra, where the enormous Toba caldera formed in association with subduction beneath the Cretaceous-aged continental crust of Sumatra.

Lower Cambrian facies architecture and sequence stratigraphy, NW France: framework for evaluation of basin-wide processes of sedimentation

AbstractA correlation and interpretation of the sequence stratigraphy of the lower Cambrian strata of NW France is presented and used to characterize basin-wide controls on processes of sedimentation. Deposition occurred in two basins, as two third-order stratigraphic sequences. A northern basin in the Cotentin was dominated by deposition of marine siliciclastic sediments of Sequence 1 (Cambrian Stages 2–3). The primary sediment input was from the NW at La Hague. A middle Normandy basin, SW of Caen, was characterized by deposition of platform carbonates and fine-grained, offshore siliciclastics in Sequence 1 and fluvio-deltaic and marine siliciclastics in Sequence 2 (Cambrian Stages 3–4). Major sediment input points lay to the west in Brittany and to the NE of Caen (in Sequence 2). Lowstand and transgressive systems tract strata dominate both sequences, with local preservation of highstand deposits in basin-centre locations. Fluvial deposits occur at basin margins and display sandstone-dominated facies typical of prevegetation systems. Two styles of fluvially dominated siliciclastic delta are recognized, which show delta-front strata composed of fluvial channels overlying either wave-formed deposits or hyperpycnites. The former formed on shallow shelves, whereas the latter formed where basin bathymetry was steeper and commonly fault controlled. Fossil microbial life forms are conspicuous in Sequence 1, as are intervals of low-diversity bioturbation. The balance of life forms is consistent with the punctuated replacement of algal matgrounds with mixgrounds during early Cambrian time. Super-mature quartz arenites were formed by high-energy wave processes on slowly aggrading basin flanks following marine transgression.

EVOLUTION OF THE TETHYS HIMALAYA CONTINENTAL SHELF DURING MAASTRICHTIAN TO PALEOCENE (ZANSKAR, INDIA)

The results of several—year stratigraphic researches on the Northwestern Himalaya Late Cretaceous—Early Tertiary succession are here presented. New biostratigraphic data allowed to recognize several benthic foraminiferal assemblages in five measured stratigraphic sections and to reconsider the position of the Cretaceous—Tertiary boundary. Refined chronostratigraphic calibration and assessment of sedimentation rates were obtained through correlation between benthic and planktonic biozones. Petrographical analysis of sandstones allowed to tentatively put forward a sedimentological model, to give rough estimates for temperatures of anchimetamorphic deformation in the different thrust sheets of the Tethys Himalaya zone and to shed new light on the genesis of superstable and supermature quartzarenites. Integrated sedimentological—petrographical studies allowed to distinguish at least three depositional sequences separated by major unconformities in the previously undifferentiated succession, and to put further constraints to the dating of the India—Asia collision.

Cathodoluminescence petrography for provenance studies of the sandstones of Ora Formation (Devonian–Carboniferous), Iraqi Kurdistan Region, northern Iraq

Advanced techniques such as scanning electron microscope hot-cathodoluminescence (SEM-CL) and electron microprobe analysis (EMPA) as well as petrography are applied to investigate the paragenetic sequence and provenance of intertidal–tidal flat sandstones of the Ora Formation which is exposed in northern Iraq. The Formation consists mostly of supermature quartzarenites with subordinate immature subarkoses and sublitharenites. The mean framework composition of the thinly bedded sandstones of Ora Formation is Q95.7F2.8R1.4 where 90% of the quartz grains are monocrystalline and texturally supermature. Meanwhile the mean composition of the thickly bedded sandstones is Q85.4F9.7R4.8 with 84% of the quartz grains are monocrystalline and texturally immature. The provenance of the Ora sandstones is dominantly craton interior and less recycled orogeny.The Ora Formation has undergone intensive and complex episodes of eogenesis, mesogenesis, and telogenesis. Compaction and quartz cementation is more dominant than other diagenetic processes evident from tight grain supported fabric which predated authigenic illite formation; this is also evident from close packing of detrital framework that resulted in reduction of primary porosity. Albitization has postdated eogenesis of K-feldspar cements. Cathodoluminescence study indicated four main distinctive fabrics in quartz grains, (1) healed fractures, (2) mottled textures, (3) low-intensity dark CL streaks and patches, and (4) shocked quartz. The results indicate the dominance of brown to dark blue CL for quartz of low-temperature metamorphic origin; bright blue colors for the felsic plutonic and high-temperature metamorphic quartz; with considerable amounts of detritus that has originated from felsic and mafic volcanic rocks and are characterized by red, violet colors luminescence.

Post-Acadian sediment recycling in the Devonian Old Red Sandstone of Southern Ireland

The Upper Devonian Munster Basin of southern Ireland has traditionally been viewed as a post-orogenic molasse deposit that was sourced from the Caledonides of central Ireland and subsequently deformed by the end Carboniferous Variscan orogenic event. The basin fill is composed of super-mature quartz arenite sandstone that clearly represents a second cycle of deposition. The source of this detritus is now recognized as Lower Devonian Dingle Basin red bed sequences to the north. This genetic link is based on the degree of similarity in the detrital mica chemistry in both of these units; micas plot in identical fields and define the same trends. In addition, the two sequences show increased textural and chemical maturity up-sequence and define indistinguishable 40Ar/39Ar age ranges for the detrital mica grains. Partial resetting of the Ar ages can be attributed to elevated heat flow in the region caused by Munster Basin extension and subsequent Variscan deformation. The combined evidence from southwest Ireland therefore points to a Caledonian or possibly Taconian primary source area that initially shed detritus into the Lower Devonian Dingle Basin which was subsequently recycled into the Upper Devonian Munster Basin following mid-Devonian Acadian basin inversion.

Petrography and geochemical indices of the Lagos lagoon coastal sediments, Dahomey Basin (southwestern Nigeria): Sea level change implications

The Lagos lagoon, part of the barrier-lagoon geomorphic zone of the Nigerian coastal system, is underlain by Quaternary coastal plain sands of the Dahomey Basin. The study involves sedimentological and geochemical study of the three georeferenced wells, to evaluate the lithotopes distribution, provenance, environment of deposition and the resulting marine transgression-regression episodes from previous sea level changes.A total of twenty-nine core samples were used for this study. Eighteen of the samples (sandstones) were subjected to granulometric, petrography analyses, heavy mineral separation and Inductively Coupled Plasma – Mass Spectrometry (ICP–MS) analysis. Geochemical facies criteria were established based on diagnostic indices.The Quaternary lithosome indicates the presence of clay, siltstone, sandstone and conglomerate. The sandstones are fine to coarse-grained, poorly sorted to moderately well sorted, negatively to positively skewed and mesokurtic to leptokurtic. The heavy mineral assemblage is characterized by zircon, rutile, tourmaline, garnet, staurolite, sillimanite, apatite, epidote, hornblende and kyanite. This assemblage suggests sediments derived from both igneous and metamorphic provenance, probably from the Ibadan Craton. The average ZTR index is 27.84%, indicating mineralogically immature sediments. Petrographic analysis shows that the most abundant detrital minerals are quartz (average of 88.7%), feldspar (average of 4.37%) and rock fragments (average of 7.3%). The QFR ternary plots revealed that the sands are super-mature quartz arenite derived from the uplifted continental block under a humid climatic setting, which have undergone moderate transportation and have been subjected to moderate mechanical and chemical weathering. The ratios of K2O/Na2O and SiO2 suggest passive and active continental margin settings. Linear discriminant analysis revealed that the sediments were deposited within fluvial to shallow marine lithosomes. This corroborated the bivariate plots of the statistical parameter. The results of the trace element and ratios considered with regard to their distribution in the diverse lithofacies showing the range of Sr from 50 to 120 ppm; Ba, 180–340 ppm; Ga, 5–14 ppm; B, 25–48 ppm; V, 53–125 ppm; Sr/Ba, 0.20–0.39 and Ba/Ga, 1.86–7.00, all indicating fluvial, shallow and marginal marine lithosomes. The lithofacies are derived from a mixed provenance, have undergone chemical weathering during transportation, and are deposited in the fluvio-marine environment. These indices indicate the occurrence of weak marine transgressions.

Sequence stratigraphy of the Early–Middle Devonian succession (Padeha Formation) in Tabas Block, East-Central Iran: Implication for mixed tidal flat deposits

The facies and sequence stratigraphic analyses of the Padeha Formation (Early–Middle Devonian) in the East-Central Iran are based on two measured stratigraphic sections in the Ozbak-Kuh (type section) and Deranjal (Dahaneh-Kalot section) Mountains in the northern Tabas Block, with a thickness of 492 and 320m respectively. The field observations and laboratory studies were used to identify sixteen lithofacies and four architectural elements. The facies associations were classified into five categories including sandstone (St, Sp, Sr, Sh, Sl), fine grained (siltstone–claystone) or mud rock (Fl), interbedded sandstone–mud rock (Sr(Fl), Sr/Fl, Fl(Sr)), dolomite (Dl and Ds), and evaporate (El, Efl, Efm, Edl). Structural (wave and interference ripples, planar cross-bedding and herringbone, flaser-wavy and lenticular beddings, tepee and stromatolite structures, entrolothic foldings, shrinkage and syneresis cracks, rain drop imprints, and salt casts) and textural (mature–supermature quartzarenite) features, as well as predominant bimodal pattern of paleocurrents in most lithofacies, show that sediments of the Padeha Formation have been deposited in a tidal flat environment. The sandstone, interbedded sandstone–mud rock, and dolomite facies (SB and LA architectural elements) are related to subtidal and intertidal settings, and fine-grained sediments and evaporate facies (consisting of FF and E architectural elements) were deposited in the supratidal and sabkha settings. Detail sequence stratigraphic study led to identification of two 3rd order complete depositional sequences (DS2 and DS3) with SB1 sequence boundaries and two incomplete sequences (DS1 and DS4). Supratidal and sabkha facies were formed during the fall of relative sea level and are related to the lowstand systems tract (LST). The intertidal facies were formed at a rise of relative sea level and are interpreted as transgressive (TST) and highstand (HST) systems tracts.

Basin stratigraphy, sea‐level fluctuations and their global tectonic connections—evidence from the Proterozoic Cuddapah Basin

The Cuddapah Supergroup succession can be divided into four unconformity‐bound sequences, namely, the Papaghni, Chitravati, Srisailam and Kurnool groups and formation representing four major cycles of sedimentation. The oldest sequence, the Papaghni Group is represented by fan‐delta, prodelta and shallow shelf deposits. The rifting stage of the basin evolution is attested by the immature delta succession deposited as a major fault‐controlled basin evolution, and was followed by the early subsidence stage. Basement uplift and a hiatus in deposition follows the first cycle of sedimentation. The Chitravati Group, representing the second cycle of sedimentation, consists of mature sandstones separated by a heterogeneous shale–sandstone–dolomite interval. The third cycle starts with the deposition of widespread coastal fluvial to shallow marine sandstone of the Srisailam Formation, and the fourth cycle is represented by the Kurnool Group consisting of conglomerates, feldspathic sandstones, supermature quartzarenites, minor shale and carbonates. Each cycle represents a rifting phase followed by a stable subsidence stage when the basin evolved into a large epicontinental sea. The supermature Gandikota Quartzite of the Chitravati Group and the Paniam Quartzite of the Kurnool Group represent relative sea‐level fall and forced regression. The siliciclastics in each of the sequences display signatures of macrotidal sedimentation pointing to open ocean connection. The sequences further display signatures of passive margin sedimentation with multiple events of carbonate‐shale rhythmite deposition. Mafic flows and dykes in the Papaghni and Chitravati groups reflect thermal anomalies associated with phased crustal extension; successive extensional phases were punctuated by basin inversion. Extensive and pulsed development of epicontinental seas as recorded in the Cuddapah sequences in the south Indian craton, possibly reflect global sea‐level changes associated with supercontinent (eg. Columbia in the Palaeoproterozoic) break‐up and assembly. Copyright © 2012 John Wiley & Sons, Ltd.

Shallow marine and coastal eolian quartz arenites in the Neoarchean-Palaeoproterozoic Karutola Formation, Dongargarh Volcano-sedimentary succession, central India

The Neoarchean-Palaeoproterozoic Karutola Formation, a >200 m thick supermature quartz arenite, occurs interlayered between two thick units of intermediate to basic volcanic rocks of the Dongargarh Group. Despite metamorphism and deformation, detrital texture and sedimentary structures are well preserved, allowing detailed process-based facies analysis. The facies recognised include: (1) inner shelf, (2) shoreface, (3) delta mouth/lower delta plain, (4) tidal flat, and (5) foreshore/backshore. A coastal sand sheet, dominated by adhesion structures and consisting of alternating eolian and foreshore aqueous deposits, records eolian processes operative in the Karutola coastal area. Textural characteristics hint at a significant eolian contribution to the shallow marine sand. Overall palaeocurrent indicates a NW–SE shoreline trend with hinterland to the west of the present outcrop belt. A surface indicating an abrupt increase in the water depth divides the Formation into two units. The lower part encloses inner shelf to foreshore–backshore deposits and the upper part of the succession records deposition from the shoreface to lower deltaplain environment. A tidal flat sandstone–mudstone unit is developed only in the north-western end of the study area. A granite gneiss source terrain cropping out to the west and a hot, humid climate appears to have supplied the abundant quartz sand to the Karutola shoreline, and the sand population attained textural maturity through eolian and aqueous reworking. It is postulated that the aqueous–eolian interaction inferred for the Karutola sand should be common in sandy depositional environments that lacked land vegetation.

Two Paleoproterozoic (Statherian) siliciclastic metasedimentary sequences in central Wisconsin: The end of the Penokean Orogeny and cratonic stabilization of the southern Lake Superior region

Statherian siliciclastic sedimentary rocks in the southern Lake Superior region are key to understanding the evolution and stabilization of Paleoproterozoic crust in the midcontinent. Deposited during an important period of significant crustal growth, controversy exists regarding the conditions of sandstone deposition and the relations between magmatism and sedimentation. In central Wisconsin, the Hamilton Mounds inlier, although restricted in areal extent, is significant in that it reveals the existence of two separate Statherian units: an 1800–1760 Ma immature meta-arkose and a post-1760 Ma Baraboo Interval supermature quartzite. The meta-arkose is crosscut by 1762 Ma granite and contains abundant Penokean age detrital monazite grains. The younger quartzite contains detrital monazite as young as 1750 Ma and was deposited unconformably upon these older units. All units experienced 1630 Ma low-grade recrystallization and isotopic disturbance associated with the Mazatzal Orogeny. In addition, the supermature quartzite experienced K-metasomatism and brecciation at 1470 Ma, ascribed to regional-scale hydrothermal alteration related to the influence of the nearby 1470 Ma Wolf River batholith. Collectively, these data suggest an early Statherian episode of arkosic deposition and magmatism during Yavapai orogenesis, followed by a longer interval of deposition of supermature quartz arenite associated with crustal stabilization. The existence of two separate Statherian units reconciles apparent conflicts in depositional models and provides a greater understanding of the Paleoproterozoic evolution of the midcontinent.

The uranium mineralization potential of the Paleoproterozoic Sioux Basin and its relationship to other basins in the southern Lake Superior region

The Paleoproterozoic Sioux Basin and the Baraboo, Barron, Flambeau, McCaslin, and Waterloo quartzites in northcentral USA are all supermature quartz arenites that are among the world's oldest redbeds. These quartzites were deposited on a stable craton at ca. 1750 Ma, following exhumation and uplift of Penokean crust and major orogenic activity related to the amalgamation of Archean microcontinents at ca. 2.0–1.8 Ga to form Laurentia. 40Ar/ 39Ar ages of 1615–1543 Ma from diagenetic muscovite in the Sioux Basin provide the first geochronological evidence for Mazatzal-related deformation and low-grade metamorphism of the quartzites during subsequent convergence on the southern margin of Laurentia. One muscovite sample from the Sioux Basin has a 40Ar/ 39Ar age of 1465 Ma that likely corresponds with fluid and thermal resetting due to felsic magmatism in the region and is similar to 40Ar/ 39Ar ages for muscovite from the Baraboo and Waterloo quartzites. Thus, the Sioux Basin and other quartzites in the region have experienced a similar tectonic and fluid history. The Sioux Basin has similar sedimentologic characteristics as other Paleoproterozoic basins in North America, including the uranium-rich Athabasca Basin, Canada, that is host to some of the world's largest and highest-grade unconformity-type uranium deposits. We compared key characteristics, such as the stratigraphy, character of diagenetic fluids, paleoaquifer characteristics, and composition of leachable Pb in the sandstones from the Sioux Basin with the Athabasca Basin in order to evaluate the fluid history of the basin and the potential of the Sioux Basin to host unconformity-type uranium mineralization. Based on the hydrogen and oxygen isotopic composition of muscovite (M1) that formed during diagenesis at 150–200 °C, the pore fluids that were present during peak diagenesis in the Sioux Basin had δ 18O and δD values of −0.5 to +5.2‰ and −32 to −46‰, respectively, similar to those of the fluids associated with uranium mineralization and alteration in the Athabasca Basin. However, fluid flow in the Sioux Basin was likely restricted to relatively thin and discontinous proximal fluvial facies that could have been diagenetic paleoaquifers, in contrast to the extensive paleoaquifers in the Athabasca Basin that promoted large-scale diagenetic reactions and alteration that led to the formation of unconformity-type uranium mineralization. In addition, the non-radiogenic isotopic composition of leachable Pb in the Sioux Basin sandstones indicates that the majority of the Pb is from a normal crustal source, inconsistent with mobilization of radiogenic Pb from a uranium-rich source, such as a uranium deposit. Although the Sioux Basin shares similar general characteristics to the Athabasca Basin, the results of this study suggest that it has a lower potential than the Athabasca Basin to host significant high-grade unconformity-type deposits.

The lower Proterozoic Fern Creek Formation, northern Michigan: mineral and bulk geochemical evidence for its glaciogenic origin

Bulk chemical data indicate the Fern Creek Formation (Early Proterozoic, Marquette Range Supergroup, Lower Chocolay Group) originated as a glacial till, resolving an old argument about the correct interpretation for diamictite-containing units within the formation. There are poor correlations among SiO2, Al2O3, and K2O, and the chemical index of alteration averages 55. These are qualities characteristic of tills and other sediments accumulated in the absence of chemical weathering and transport-induced sorting and inconsistent with an origin as a fluvial deposit (the principal alternate hypothesis). The Archean Carney Lake Gneiss might be the source for at least the lower portions of the Fern Creek Formation. Compositions are similar, and rare earth element trends in the Fern Creek Formation are consistent with derivation from an Archean high-grade terrane. Pegmatites in the gneiss could be the source for monazite, huttonite, and an unnamed fluor-hydroxy-rare earth mineral that occurs in the midsection of the Fern Creek Formation. In addition, mineral and bulk chemical trends are consistent with models involving deep erosion of a complex Archean source, similar to models previously developed for Gowganda tills in the Huronian Supergroup. Samples of the Sturgeon River Quartzite occurring above the Fern Creek Formation were also analyzed. The bulk chemistry is consistent with a well-sorted, supermature quartz arenite.

Provenance analysis of lower Paleozoic cratonic quartz arenites of the North American midcontinent region: U-Pb and Sm-Nd isotope geochemistry

Lower Paleozoic supermature quartz arenites in Wisconsin and Michigan were derived from several Proterozoic and Archean terranes. Single-grain, detrital zircon populations from the Galesville and St. Peter Sandstones in Wisconsin yield very similar, closely concordant age distributions (Cambrian Galesville Sandstone: 1.1 Ga [n = 4], 1.4 Ga [n = 2], 1.8 Ga [n = 1], and 2.7 Ga [n = 2]; Ordovician St. Peter Sandstone: 1.1 Ga [n = 2], 1.8 Ga [n = 1], and 2.7 Ga [n = 6]). In contrast, most of the nine detrital zircons that were analyzed from the St. Peter Sandstone in the Michigan basin have high U contents (450‐2500 ppm) and are strongly discordant (60%‐90%). Six zircons from the Michigan basin that have ca. 1.0 Ga 207 Pb*/ 206 Pb* ages define a regression line that has an upper intercept of about 1100 Ma and a lower intercept of 15 Ma; one zircon has a 2.7 Ga 207 Pb*/ 206 Pb* age. The zircon data indicate that although a number of different terranes contributed detrital material to the Paleozoic quartz arenites in Wisconsin, 1.1 and 2.7 Ga terranes were the dominant sources, and not the local basement, which primarily consists of the ♢2.7 Ga Marshfield terrane, the 1.8 Ga Penokean orogen, and the 1.4 Ga Wolf River batholith. A terrane that has a 1.1 Ga age is probably the main source for the St. Peter Sandstone in the Michigan basin. Quartz separates were also analyzed for PbPb and Sm-Nd isotope variations, and the data do not indicate significant source differences between the heavy mineral fraction (zircons) and the quartz framework grains. Pb-Pb isochrons and Sm-Nd isotope data for quartz separates reflect mixing of the age groups and approximate relative proportions that are identified from the single zircon results. All isotope data on quartz separates and UPb zircon data indicate that the detrital constituents (zircons and quartz framework grains) that compose the lower Paleozoic quartz arenites in Wisconsin were primarily derived from the 2.7 Ga granite-greenstone terrane of the southern Superior Province and a 1.1 Ga terrane. The latter terrane is either the silicic volcanic rocks associated with the Midcontinent rift system, or, more likely, the voluminous granitic rocks that are associated with the Grenville Province on the eastern margin of North America. The Middle Proterozoic Grenville Province was the most important ultimate source of quartz and zircons to the St. Peter Sandstone in the Michigan basin; lesser amounts of material were contributed from the Archean Superior Province.

Turbidite facies, fluid-escape structures and mechanisms of emplacement of the Oligo-Miocene Aljibe Flysch, Gibraltar Arc, Betics, southern Spain

The Aljibe Flysch, the largest tectono-sedimentary unit in the Gibraltar Arc, provides a record of Oligo-Miocene sedimentation in the Betic Orogen, southern Spain. The Aljibe Flysch can be subdivided into two sub-units, the Beneiza Flysch (Late Oligocene) and the Aljibe Arenites (Early Miocene), which have very different depositional histories and mechanisms of emplacement. The Beneiza Flysch is characterised by laterally continuous and thin-bedded lithic arenites, siltstones and mudstones which contain complete and partial Bouma sequences. These are interpreted to have been deposited in a sheet-turbidite system which may have formed part of the North African Flysch Trough. By contrast, the aljibe Arenites comprise of thick-bedded super-mature quartz arenites which are dominated by fluid-escape structures. These sandstones are similar to Facies Group B1-B2 disorganised sands of Pickering et al. (1989), which are interpreted to be the product of deposition from high-density turbidity currents. Fluid-escape structures have been classified according to the scheme of Lowe (1975) and their vertical arrangement indicates that a large volume of sediment was deposited extremely rapidly during frequent remobilisation events. Consideration of the origin of the fluid-escape structures and their vertical arrangement has identified problems in their interpretation, and identified key areas for future research. The boundary between the Beneiza Flysch and Aljibe Arenites is interpreted to be a significant stratigraphic boundary across which there is a major change in sediment source and mechanism of sediment emplacement. Several models can be put forward for the generation of this stratigraphic break, and these serve to set the agenda for future research into the emplacement of other similar flysch sequences and the origin of the Gibraltar Arc Flysch.

Initiation of ∼2.45-2.1 Ga intracratonic basin sedimentation of the Hurwitz Group, Keewatin Hinterland, Northwest territories, Canada

Continental to shallow-marine rocks of the Hurwitz Group, exposed as outliers across the Hearne Province in northern Canada, were deposited between ∼2.45 and 2.11 Ga on a 3.3-2.6 Ga granite-greenstone basement. Initiation of the Hurwitz Basin was characterized by low relief and rates of subsidence that never progressed beyond the intensity of regional sagging. During stage 1, polymictic conglomerate (lower Noomut Formation) was deposited in palaeovalleys cut into Archaean basement, and subarkose to quartz arenite ± quartz pebble conglomerate (upper Noomut Formation) was deposited on a low-relief sand plain. Abundant first-cycle quartz-rich sandstones are attributed to intense humid climate weathering in low-relief source areas and at sites of temporary storage, as well as to repeated fluvial and eolian reworking. Interbedding of pyritic palaeoplacers and continental redbeds denotes fluctuating levels of atmospheric oxygen; disappearance of pyritic palaeoplacers upsection signifies ultimate changeover to an atmosphere with sustained free oxygen. Stage 2 (Padlei Formation) was marked by a climatic shift toward cold (glacial?) conditions as indicated by dropstones, greywacke (till?) pellets and possible ikaite pseudomorphs in lacustrine rhythmites. Stage 3 (Kinga Formation) represents a return to wet/warm conditions with fluvial (± eolian) deposition of subarkose and quartz arenite on a low-relief sand plain (Maguse Member) and deposition of supermature quartz arenite in a vast (100,000 km 2 minimum), shallow-water lake or series of lakes (Whiterock Member). Local chert beds at the top of the Kinga Formation (Hawk Hill Member) formed by spring discharge due to regional, gravity-driven, convective groundwater flow. An intracratonic setting is implied by: (1) a symmetric, basin-centred fill; (2) absence of features indicating a shelf-slope break; (3) absence of rift rocks; (4) a prolonged slow-subsidence history; (5) likely correlation with other continental to shallow-marine deposits scattered across the Keewatin Hinterland; and (6) positioning 1000–2000 km away from closest coeval passive margin sequences. We speculate that Hurwitz Basin formed due to lithospheric stretching related to a large-scale mantle upwelling during the protracted breakup of an earliest Palaeoproterozoic supercontinent (‘Kenorland’).

Ripple Marks in Quartz Arenites of the Hurwitz Group, Northwest Territories, Canada: Evidence for Sedimentation in a Vast, Early Proterozoic, Shallow, Fresh-Water Lake

ABSTRACT Supermature quartz arenites of the Whiterock Member, deposited in the Early Proterozoic, intracratonic Hurwitz Basin, have a maximum thickness of 400 m and a minimum areal extent of 100,000 km2. The unit is unusual in that it is virtually devoid of other rock types and displays sedimentary structures that consist almost entirely of short-spaced ripple marks and parallel stratification, both formed by oscillatory flow. The ripples have spacings that are consistently small (average

Sedimentology and mineral composition of the Kurnub Sandstone in Wadi Qsieb, SW Jordan

The Kurnub Sandstone of Early Cretaceous age crops out in Wadi Qsieb (in the southwest of Jordan) with a thickness of 130 m. Moderately developed fining-upward cycles with irregular erosive bases, together with the presence of plant fragments and the absence of body or trace fossils, point to a fluviatile depositional environment, probably a river of intermediate sinuosity. Results of grain-size analysis confirm this interpretation. The sandstones are compositionally and texturally supermature quartz arenites formed mainly by recycling from Early Palaeozoc sandstones and less commonly by direct supply from the Arabian-Nubian Shield. The absence of feldspars and unstable heavy minerals is attributed to intense chemical weathering, attacking sands that were deposited with a low rate on a craton bordering the Tethys Sea. The thickness recorded here and the absolute absence of hornblende are in contradiction with what has been published by Nasir and Sadeddin (1989).

Tectonic versus chemical‐weathering controls on the composition of fluvial sands in tropical environments

ABSTRACTThe compositions of fluvial sands collected in mountainous regions along the volcanic front in Central America and along the Andean front in Venezuela and Colombia correlate with the tectonic setting of their source terrains. The sands derived from the Central American volcanic arc are compositionally distinct from those derived from the Andean terrains, and minor variations in tectonic style can also be discriminated. Subtle differences in tectonic setting and source rock composition are recognizable only when modification by chemical weathering is slight; they are strongest in sands from low‐order streams draining mountainous areas with modest alluvial‐plain development. Sands that have been exposed to weathering for extended periods during alluvial storage are progressively enriched in chemically stable phases. With increased weathering, sands from continental terrains converge toward a supermature quartz arenite end member, and the imprint left by source rock composition and tectonic setting is obscured. In intraoceanic terrains, a similar but much less pronounced enrichment in stable phases relative to less‐stable phases is observed, similarly overprinting the tectonic signal. Although detailed tectonic information is easily lost, sands derived from intraoceanic terrains are distinguishable from those derived from continental terrains even after long exposure to intense chemical‐weathering conditions.

Clastic limestone dikes from the late Proterozoic Bhima Group, South India

Clastic dikes of the late Proterozoic Bhima Group, South India, intruding the Kasturpalli Glauconitic Member are reported from two localities. These glauconitic sediments of the Rabanpalli Formation form the oldest transgressive sequence of the Bhima Group, which starts with basal conglomerates and grades upward into supermature quartz arenites. These are followed successively by glauconitic fine-grained sediments and shales. This sequence is capped by thick deposits of micritic flaggy limestones of the Adki Formation. The dikes are generally emplaced vertically, but also have small lateral apophyses along the bedding planes of the host rocks. The clastic dikes are chiefly composed of a matrix of micritic calcite along with quartz, clay minerals, etc. In places, crystals of calcite are observed in the form of veins. In one locality the dikes are emplaced in the vicinity of a post-depositional fault. This faulting seems to have triggered the intrusion of the clastic material, which shows a thixotropic behaviour, into the glauconitic sandstones and siltstones.

Sedimentology of Quartzose Sandstones of Lower Mannville and Associated Units, Medicine River Area, Central Alberta: ABSTRACT

ABSTRACT Quartzose sandstones which overlie Lower Jurassic strata in the Medicine River area can be divided into three different units, each separated by an unconformity. The two lower units (J2 and J3) fill a deep valley cut into older strata; the upper unit (Ellerslie) covers the area. Uniform, porous, fine, supermature quartzarenite up to 50 m thick comprises the J2, the oldest of the three quartzose sandstones units. The high degree of mineralogic and textural maturity and associated small-scale cross stratification suggest that the sand was transported to a local watershed by eolian processes, then concentrated in the valley by fluvial action. Sediments of the J3 unit are immature and submature, kaolinitic chertarenites and sublitharenites derived by humid weathering of Paleozoic carbonates. The unit as a whole coarsens upward, indicating valley-fill by progradation into a standing body of water. Ellerslie sediments are principally mature and supermature quartzarenites and interbedded mudstones whose constituents were derived from the Shield to the east. Extensive bioturbation of sands, the presence of limestone beds and, toward the top of the formation, glauconite pellets, suggest that deposition took place in a brackish-to-marine standing body of water. The three quartzose sandstone units are not unique to the Medicine River area and can be compared with similar units elsewhere in western Canada. J2 and J3 sandstones are the respective lithologic counterparts of the Roseray and Success Formations of southwestern Saskatchewan. Ellerslie sediments are the marine equivalents of widespread continental lower Mannville deposits in eastern Alberta and Saskatchewan.