Sandstone Petrography Research Articles

Tectonic setting and paleoenvironment of terranes in the southern New England orogen, eastern Australia as constrained by radiolarian biostratigraphy

Radiolarians are abundant in the Gamilaroi, Djungati and Anaiwan terranes of the New England orogen in eastern Australia. These microfossils present the first age constraints on the timing of development of hitherto undated lithologies within the various juxtaposed terranes of the orogen. They give a biostratigraphic framework for interpreting the history of marine sedimentation in these terranes and have significant implications for published tectonic models. Radiolarians also show that the stratigraphically lowermost rocks of the Gamilaroi terrane are most probably of Devonian age. Better stratigraphic resolution permits interpretation of these rocks as part of an intra-oceanic island arc succession which accreted to the eastern margin of Australia (Gondwana) around the end of the Devonian. This intra-oceanic arc is distinct from a younger, Carboniferous, superposed continental arc sequence which constitutes a successor basin that developed over the Gamilaroi terrane subsequent to its accretion to the margin of Australia. Radiolarians provide the basis for dating siliceous lithologies which dominate the Djungati terrane and permit a more detailed analysis of the history of this terrane. During the middle Silurian through Late Devonian the Djungati terrane was part of an oceanic basin which was isolated from any source of terrigenous sedimentation. The Djungati terrane was subsequently influenced by volcanic island arc activity and was tectonically disrupted during the latest Devonian to Early Carboniferous. Radiolarian age data show that age correlations inferred, on the basis of similarities in detrital sandstone petrography, between lithostratigraphic units in the Gamilaroi and Djungati terranes, are not always appropriate. Djungati terrane has been widely interpreted as a subduction complex related to the Gamilaroi terrane. Radiolarian data now elucidate much of the structural complexity of this terrane. Significant differences between the Djungati terrane and the style of well-documented subduction complexes include the close spacing of zones of radiolarian chert, the dominance of this lithology over others and the lateral extent of many of the chert horizons. The Anaiwan terrane has features which are characteristic of many well-documented subduction complexes. Radiolarians are abundant in this terrane and can be used to show that it developed along the eastern margin of Australia in response to Early Carboniferous subduction of Late Devonian to Early Carboniferous oceanic crust. Packages of chert accreted into the subduction complex are progressively younger towards the northeast indicating that subduction was directed to the southwest.

A combined petrographical—geochemical provenance study of the Newland Formation, Mid-Proterozoic of Montana

AbstractA provenance study was conducted on the Mid-Proterozoic Newland Formation, in which petrographical features of sandstones and geochemical characteristics of shales were integrated to arrive at an internally consistent interpretation.Sandstones of the Newland Formation are typically arkosic sands and arkoses with very-well-rounded quartz and feldspar grains and only minor amounts of extrabasinal rock fragments. The predominant feldspar types are K-spar and microcline, feldspar grains are smaller than quartz grains, and feldspars show little alteration due to weathering. Detrital modes of Newland sandstones (QFL diagrams) indicate that they were derived from a stable cratonic source. These petrographical features imply a source area dominated by granites and granitoid gneisses, semi-arid to arid climate, tectonic quiescence, and overall peneplain conditions.Shales of the Newland Formation are dominated by illite, quartz silt, and fine crystalline dolomite. They have small La/Th rations, relatively large Hf contents, and small contents of Cr, Co, and Ni, all indicative of derivation from crust of granitic composition. Small Tio2/Al2O3ratios also suggest source rocks of granitic composition. The average chemical index of alteration (CIA) for Newland shales is 71.8, which in light of the probable granitoid source indicates modest amounts of chemical weathering. Relatively large SiO2contents and large K2O/Na2O ratios reflect derivation from stable cratonic areas and tectonic quiescence.Thus, in general, the petrography of sandstones and geochemistry of shales provides the same provenance clues for the Newland Formation. One notable discrepancy between the two approaches is that the sandstones indicate an arid to semi-arid climate with very minor chemical weathering, whereas the CIA of the shales indicates at least modest amounts of chemical weathering. This indicates on one hand the need to better calibrate the CIA with a large variety of muds from modern climatic settings, and on the other hand the possibility that this discrepancy is due to transport segregation.

Petrography, diagenesis and pore-water evolution of a shallow marine sandstone (Hasle Formation, Lower Jurassic, Bornholm, Denmark)

Lower Jurassic marine sandstones in the Hasle Formation on Bornholm Island can easily be separated in two different petrographic types. In the lower part of the type locality, type 1 sandstones are friable with layers and lenses of concretionary siderite. They are separated by a major erosional surface from the overlying type 2 well-cemented sandstones.All parts of the sandstones were subjected to chloritization of original grain coatings, neomorphism of chlorite and local precipitation of authigenic pyrite. During early diagenesis, diagenetic pathways differed between the two sandstone types. Type 1 sandstones were subjected to compaction before kaolinitization and some quartz authigenesis occurred. Type 2 sandstones immediately above the major erosional surface were cemented by concretionary siderite at a very early stage, and thus further diagenetic modifications were prevented. Above this zone, sandstones are tightly cemented by various carbonate minerals. Diagenetic modifications mainly involved alternating events of carbonate precipitation and dissolution. The sequence of carbonate precipitation was: (1) zoned stable/metastable siderite; (2) siderite, gradually with significant Ca- and Mg-substitution; (3) ankerite; and (4) calcite, reflecting a sequence of precipitating carbonates containing progressively lower amounts of iron. A period of kaolinitization predated calcite cementation.Six diagenetic stages were recognized: (1) early burial reducing conditions resulting in precipitation of pyrite; (2) precipitation of chlorite and siderite caused from compactional flow from underlying brackish and fresh-water sediments; (3) dissolution of carbonate and partly oxidation of siderite caused by fresh-water flushing related to uplift; (4) precipitation of siderite with gradually increasing substitution by CaCO3 and MgCO3 and eventually precipitation of ankerite caused by renewed burial; (5) fresh-water diagenesis with precipitation of kaolinite and quartz; (6) precipitation of calcite in remnant porosity.

Detrital record of the early Central American magmatic arc: Petrography of intraoceanic forearc sandstones, Nicoya Peninsula, Costa Rica

The framework compositions of sand-stones from the Costa Rican forearc provide a compositional norm for detrital modes from basins associated with intraoceanic arcs, a poorly documented tectonic setting. Nicoya Peninsula sandstones are made up of mainly volcanic lithic fragments, plagioclase, sedimentary lithic fragments, and mafic accessory minerals. Modal point counts of 70 samples average Q 3 F 32 L 65 , Qm 2 F 32 Lt 66 , Qp 1 Lvm 72 Lsm 27 , and Qm 6 P 94 K 0 , defining these sand-stones as less quartzose than sandstones from most other tectonic settings and less lithic than most quartz-poor sandstones from intraplate oceanic islands. Four newly defined modal parameters quantify temporal changes in composition. These parameters are (1) Vi/V′, the ratio of volcanic fragments with microlitic or felsitic textures to those with lathwork, microlitic, or felsitic textures; (2) Mh/M, the ratio of hydrous to total mafic silicate mineral grains; (3) Po/P, the ratio of oscillatory-zoned to total plagioclase grains; and (4) Sc/S, the ratio of reworked carbonate detritus to total sedimentary lithic fragments. All four parameters increase from negligible values in the basement unit to moderate or high values in cover units. These increases are followed by generally low values in the youngest (Miocene) sandstones. Significant temporal changes are not revealed by previously defined modal parameters that are commonly used to analyze sandstone composition. Petrography of these sandstones provides strong evidence for the initial construction of an andesitic arc edifice by Campanian time (Late Cretaceous). Increases in Vi/V′, Mh/M, and Po/P ratios through most of the section record increased detrital input from evolved volcanic sources. A parallel increase in Sc/S ratios reflects the progressive construction and erosion of a carbonate-rich sedimentary are apron. The Miocene reversal of these trends reflects subaerial erosion of up-lifted basement rocks at the forearc structural high. Miocene sandstones also contain minor plutonic detritus that includes potassium feldspar, signaling incipient unroofing of the arc. Sandstones of the Nicoya Peninsula thus preserve a relatively complete record of the evolution and erosion of the Upper Cretaceous through Miocene intraoceanic arc of southern Central America.

Controls on the petrographic evolution of an active margin sedimentary sequence: the Larsen Basin, Antarctica

Abstract The sedimentary fill of the Cretaceous-Tertiary Larsen Basin, located at the northern tip of the Antarctic Peninsula, records the evolution of the Antarctic Peninsula source terrain. Uplift, and the possible renewal of arc volcanism within this area, is documented by the wide petrographic spread in the lower Gustav Group (Barremian-Coniacian). The Hidden Lake Formation (Coniacian-Santonian) records a major pulse in proximal calc-alkaline arc volcanism. The overlying Marambio Group shows a change from lithic-volcanic sandstones to quartz-feldspar-rich sandstones, reflecting a change in source terrain. An abrupt change in sandstone composition in the upper Santa Marta Formation (Campanian) may reflect both a switch in volcanism and plutonism from the east to the west coast of the Antarctic Peninsula (i.e. away from the basin margin), and a change to more mafic volcanism. The Lopez de Bertodano Formation (Campanian-Palaeocene) dominantly reflects a change to a quartzo-feldspathic source terrain (the Trinity Peninsula Group). This is also related to both a decrease in the intensity of arc volcanism and an increase in distance from the location of concurrent volcanism in relation to the site of deposition. Controls on petrography other than simple arc-unroofing (e.g. location and nature of arc volcanism and depositional setting) play an important role in sandstone petrography. These controls on sandstone composition should always be considered in interpreting sequences from active plate margin settings.

The Mississippian Antler Foreland and Continental Margin in Southern Nevada: The Eleana Formation Reinterpreted

Rocks mapped as the Mississippian Eleana Formation at the type locality on the Nevada Test Site appear to comprise two completely different, but coeval, sedimentary units. In the Eleana Range (Western Eleana Formation), the strata are siliciclastic and carbonate turbidites of Mississippian age. From immediately east of the Eleana Range to Syncline Ridge (Eastern Eleana Formation), the strata are Devonian-Mississippian mudstone and quartzite conformably overlying Devonian limestone and underlying Pennsylvanian limestone. Although the contact between the two sedimentary packages is not exposed, small-scale structures document an east-dipping fault contact and reverse motion. Sandstone petrography and stratigraphic considerations support the age data in identifying two separate Mississippian units. Sandstones from the Western Eleana are chert litharenites with significant amount of feldspar and both volcanic and sedimentary lithic grains. These rocks are interpreted to be a submarine fan deposit; southwest-directed paleocurrent indicators suggest that they were deposited in an elongate trough, filled axially from the northeast. The source of the sediments was the antler allochthon and foreland basin. The authors tentatively correlate this section with the Dale Canyon-Chainman-Diamond Peak section near Eureka, Nevada. Sandstones from the Eastern Eleana are quartz arenites with rare chert and detrital heavy minerals. These strata are tentativelymore » interpreted to be a shallow shelf deposit, with sediments derived from the continent to the east. They tentatively correlate this section with the Guilmette-Pilot-Scotty Wash-Chainman section of eastern Nevada. These sedimentary systems are initially separated an unknown distance across the late Paleozoic continental margin.« less

Lower Brioverian formations (Upper Proterozoic) of the Armorican Massif (France): geodynamic evolution of source areas revealed by sandstone petrography and geochemistry

Formations with interbedded cherts constitute an important part of the Lower Brioverian succession (Upper Proterozoic age) in the Armorican Massif (northwest France). These formations are composed of shale-sandstone alternations with interbedded siliceous carbonaceous members. Petrographic and geochemical study of the detrital facies shows that these rocks are compositionally immature. The wackes are rich in lithic fragments (volcanic fragments: 3–20% modal; sedimentary and metamorphic fragments: 0–7% modal) and in feldspar (5–16%). From the geochemical point of view, they are relatively enriched in Fe 2+MgO (about 5.5%) and in alkalis with Na 2O K 2O ratios greater than 1. The CaO contents are low (about 0.3%). Slightly negative Eu anomalies are observed ( Eu Eu ∗ = 0.8 ). Their chemical compositions are in agreement with a dominantly acidic source area with deposition in a continental active margin setting. Compared with other Upper Proterozoic deposits of the Armorican Massif, the interbedded-chert formations appear rather similar to other deposits in North Brittany which accumulated in an intra-arc or back-arc basin environment. The formations with interbedded cherts are interpreted as having been deposited during an early stage of magmatic arc activity (around 640-630 Ma ago) in an immature marginal basin. The clastic supply to these formations is derived in part from early volcanic products (acidic to intermediate) which are linked to subduction beneath the North Armorican Domain. Another component is inherited from the reworking of 2000 Ma old basement relics. The opening of the back-arc domain, with associated basaltic volcanism, would bring about a progressive displacement of the interbedded-chert depositional basin towards the continental margin.

Petrography and provenance of silicified early Archaean volcaniclastic sandstones, eastern Pilbara Block, Western Australia

ABSTRACTIntense post‐depositional alteration has profoundly affected sandstones in the volcanic portions of Early Archaean (3·5–3·3 Ga) greenstone belts. The mineralogy and bulk compositions of most grains have been completely destroyed by pervasive metasomatism, but grain textures are commonly well preserved. Consequently, microtextural information coupled with present alteration compositions as determined petrographically can be used to estimate original framework modes.Silicified Early Archaean volcaniclastic sandstones assigned to the Panorama Formation and Duffer Formation, Warrawoona Group, eastern Pilbara Block, Western Australia, were originally composed of volcanic (VRF) and sedimentary (SRF) rock fragments, volcanic quartz, feldspar, traces of ferromagnesian minerals and pumice. Only volcanic megaquartz remained stable during alteration. All other primary components were replaced by granular microcrystalline quartz (GMC) and sericite. In most areas, the sandstones were composed of dacitic to rhyolitic VRFs, now totally replaced by sericite‐poor GMC and recognized by preserved microporphyritic textures. In a few areas, quartz‐poor dacitic to andesitic(?) VRFs dominated the detrital assemblage. Minor SRFs and mafic VRFs, now replaced by GMC, are recognized on the basis of colour, internal structures, and internal textures, including skeletal, possible spinifex textures. Detrital feldspar is represented by blocky, sericite‐rich grain pseudomorphs.A semi‐quantitative point‐count scheme, developed for the analysis of heavily altered sandstones, indicates the following primary detrital‐mode ranges for Panorama arenites: quartz, 0–28%; feldspar, 0–28%, VRFs, 58–86%, and SRFs 0–25%. In about half the point‐counted samples, feldspar could not be distinguished from rock fragments. In such cases, both were counted as one grain type, Lv', which makes up from 84 to 100% of the framework modes of these rocks.These sands were derived from a terrane composed largely of fresh felsic volcanic rocks and sediments, but locally including minor mafic, ultramafic, and sedimentary rocks. Much, but not all, of the felsic volcaniclastic sand represents reworked pyroclastic debris. There is no evidence for contributions from plutonic or metamorphic sources. The Panorama modal assemblage represents a provenance that is lithologically more restricted than that of Archaean greywackes and other siliciclastic units common in the sedimentary portions of these same Early Archaean greenstone belts and younger greenstone belts (3·0–2·7 Ga).

Lithofacies and petrography of Wajid sandstone (Cambrian-Ordovician) Saudi Arabia

Six lithofacies are recognized in the Wajid sandstone. These litofacies are identified as: silty and argillaceous sandstone; fine-grained sandstones; coarse-grained sandstones; pebbley to very coarse-grained sandstones; conglomerates and massive sandstones. Mineralogic composition suggests that the Wajid sandstone is considered clean sands for it consists 95% of quartz grains 5% heavy minerals, mica, potash feldspar, clay matrix, ferruginous cement and traces of carbonate cement. Quartz grain extinction types and inclusions plus heavy mineral types and shapes suggest nearby igneous and metamorphic sources for these deposits, i.e. the adjacent Arabian Shield rocks, in addition to repeated recycling of these sediments as indicated by the appearance of a small number of rounded to very rounded grains of heavy minerals. Furthermore, perhaps most of the secondary silica present in the Wajid was developed from solution and from silica reprecipitated later during periods of aeolian reworking of these fluvial sediments. Moreover, the ferruginous material in Wajid was very likely derived either from siderite or came in solution from igneous and metamorphic provenance or generally came from both sources. The carbonate material in the sediments might either be derived from post-Wajid carbonate formations or formed from solution of carbonate shells trapped in these sediments. X-ray diffraction analysis shows that kaolinite is the only clay mineral present in the Wajid deposits. On the other hand, spectroscopic analysis reveals the presence of Si, Mg, Al and Ca elements as major constituents in the Wajid sandstone. Moreover, according to facies characteristics, the Wajid sediments were deposited under conditions of fluvial environments.

Regional significance of the Blow Me Down Brook Formation, western Newfoundland: New fossil evidence for an Early Cambrian age

Allochthonous sandstone-shale sequences cropping out directly beneath the ophiolite massifs of the Bay of Islands region of western Newfoundland have been designated the Blow Me Down Brook Formation. The unit has previously been described as an easterly derived flysch sequence deposited in front of the advancing Humber Arm allochthon at the time of its emplacement during the Middle Ordovician Taconic orogeny. On the basis of presumed stratigraphic relations, the Blow Me Down Brook Formation has been interpreted as the youngest formation within the allochthonous Curling Group, although fossil evidence has not been previously reported from the type locality. Recent questions have arisen regarding the provenance and age of the Blow Me Down Brook Formation as the result of recent sandstone petrography studies. We report 20 new localities in the Bay of Islands region where we have found Oldhamia antiqua , Oldhamia smithi , Oldhamia occidens , and Oldhamia flabellata trace fossils that indicate an Early Cambrian age for the Blow Me Down Brook Formation. These sites confirm that the Blow Me Down Brook Formation and correlative units within the Humber Arm allochthon do not represent easterly derived Middle Ordovician flysch deposits, but westerly derived turbiditic sandstone-shale sequences deposited along the eastern North American rifted margin during latest Precambrian to Early Cambrian time. Intact easterly derived flysch units within the allochthon appear to be scarce. These units were apparently prone to severe disruption and incorporation into meelange zones and areas of broken formation.

Upper cretaceous to paleocene depositional sequences and sandstone petrography of southwestern Patagonia (Argentina and Chile)

Upper Cretaceous to Paleocene strata exposed along the Andean margin of Patagonia south of 50°S were deposited in a foreland basin and comprise a complex transition from continental to marine facies. Three unconformity-bounded sequences are observed within the sedimentary succession studied. Sequence 1 (upper Campanian) displays a rapid north-to-south transition from upper delta plain mudstones (Cerro Fortaleza Formation) through lower delta plain and subaqueous delta plain sandstones (La Anita Formation) to delta-slope and basinal turbidites (Alta Vista and Tres Pasos Formations). Sequence 2 (Maastrichtian-Paleocene) was initiated with braided river sediments (La Irene Formation), deposited over a Type II unconformity ( sensu Vail et al., 1984). These rocks are overlain by meandering fluvial sandstones and mudstones (Chorrillo Formation) that interfinger to the south with fossiliferous shallow marine sandstones (upper Cerro Cazador Formation). Sequence 3 (Paleocene) is composed of shallow marine conglomerates and crossbedded sandstones (Calafate and Cerro Dorotea Formations). These rocks are separated by an angular unconformity from subjacent units. Sandstone petrographic analysis indicates the presence of two main petrofacies within the rocks studied. A quartz-rich petrofacies, present to the north of the basin during late Campanian to early Maastrichtian time, was derived from continental crustal block and recycled orogen sources that were possibly exposed to the northwest of the basin. The second petrofacies (volcanic-rich petrofacies) was restricted to the south of the basin during the late Campanian to early Maastrichtian, but covered the entire area during the late Maastrichtian and Paleocene. These sediments were derived from a dissected magmatic arc located to the west of the study area. The presence of abundant tuffaceous intercalations, as well as fresh andesitic fragments, indicates contemporaneous volcanism near the Cretaceous/Tertiary boundary. The marked contrast in observed petrofacies may be a reflection of the major Andean segmentation at 49°S, at the present intersection of the Chile Rise with the South American coast.

Sandstone petrography and geochemistry of late proterozoic sediments of the armorican massif (France) — A key to basin development during the cadomian orogeny

Brioverian (Late Proterozoic) sediments of the Armorican Massif in Northwest France are contained within isolated basins whose development is linked to Cadomian orogenesis. These basins were formed between c. 650 and 530 Ma ago, mainly by gravitational flow processes. Petrographical study and geochemical analysis of major and trace elements show that the sandstone formations of North Brittany are characterized by great immaturity (10–20% feldspar; 10–20% lithic fragments) and high concentrations of Na 2O, Ba, Sr, V and Cr. The formations from Western and Central Brittany, as well as the southern part of Normandy, are compositionally more mature (< 15% feldspar + lithic fragments; K 2O/Na 2O > 1). Petrographical and geochemical data for North Brittany formations yield plots which suggest a mixed source region composed of a continental active margin (island arc) and ancient basement material; sedimentation of the Brioverian Group was influenced by a distensive tectonic regime. Towards the south, sedimentary formations are characterized by continental source areas and were laid down in a passive tectonic environment. However, REE distribution patterns for sediments of this southern domain preserve evidence for the contribution of magmatic arc material; such formations could have been deposited in a marginal basin.

Provenance Evolution and Unroofing History of a Modern Arc-Continent Collision: Evidence from Petrography of Plio-pleistocene Sandstones, Eastern Taiwan

ABSTRACT Plio-Pleistocene sandstones from the Coastal Range of eastern Taiwan provide a record of shifting source areas and unroofing events in adjacent arc and accretionary-wedge terranes during the early stages of are-continent collision. In latest Miocene and early Pliocene time, arc-derived sediments were deposited in a precollisional forearc basin between the Luzon volcanic arc and the Manila Trench. Sandstones of this age are rich in zoned plagioclase, volcanic lithic fragments, and pelagic foraminifera and contain only minor quartz and sedimentary-metasedimentary lithic fragments. In contrast, lower Pliocene to lower Pleistocene sandstones are poor in feldspar and volcanic fragments and contain abundant sedimentary and metasedimentary lithic fragments. This reflects a provenance shift f om the volcanic arc to the uplifted (meta) sedimentary accretionary wedge of proto-Taiwan, which resulted from the early Pliocene onset of arc-continent collision. A refined classification of sedimentary and metasedimentary lithic fragments is introduced here and used to interpret the early evolution of the collisional orogen. Lithic fragments are rich in pelitic minerals and are divided into: 1) sedimentary (Ls) = mudstone, shale, siltstone, chert and argillite; 2) low-grade metamorphic (Lm1) = slate, slatey siltstone, and quartzite; and 3) medium-grade metamorphic (Lm2) = phyllite-schist, phyllitic quartzite and quartz-mica-albite aggregate. A good correlation is observed between these lithic fragment types and bedrock lithologies exposed in the present-day Central Range of Taiwan. Based on this correlation, the change in relative abundance of sedimentary and metasedimentary lithic fragments through time has been used to interpret an unroofing sequence for the collisional fold-and-thrust belt. Lower Pliocene Taiwan-derived sandstones are rich in sedimentary lithic fragments (Ls), which were shed from the accretionary wedge during the earliest stages of arc-continent collision. From early Pliocene to early Pleistocene time, sandstones became progressively depleted in Ls lithic fragments and enriched first in Lm1 and later in Lm2 fragments, as deeper levels in the metamorphic complex were uplifted and exposed to erosion. Regional metamorphism of biotite-grade rocks (= Lm2) occurred at about 4.5 Ma at depths of 12-15 km (Ernst 1983) and were uplifted to the surface by about 1.4 Ma (this study). These ages give an uplift rate of 4 to 5 km/m.y., which is in good agreement with independent estimates based on fission-track studies, raised coral reefs, Quaternary elevation changes, and present-day denudation rates. Thus, it appears that sandstone detrital modes of clastic orogenic sequences can be used to interpret specific aspects of unroofing and tectonic evolution in nearby active mountain belts.

Petrography and Diagenesis of Reservoir and Non-Reservoir Sandstones in Shattuck Member of Queen Formation, Northwest Shelf of Permian Basin: ABSTRACT

The Shattuck Member is a thick (6-20 m) sandstone that defines the top of the Queen Formation (Permian, Guadalupian) and is a major hydrocarbon reservoir on the Northwestern shelf of the Permian basin. The Shattuck was deposited in desert dune and interdune, dry and wet sand sheet, and sandy sabkha environments during a lowstand of sea level. The desert dune, interdune, and dry sand sheet deposits constitute the producing horizons in the Shattuck, whereas the wet sand sheet and sabkha deposits are generally non-productive. The purposes of this study are to examine the petrographic characteristics of the producing and non-producing horizons with petrographic and scanning electron microscopes, and to determine their provenance and diagenetic history.

Early Archean foredeep sedimentation related to crustal shortening: a reinterpretation of the Barberton Sequence, Southern Africa

The upper, dominantly terrigenous sedimentary successions of the Early Archean Barberton Sequence, Kaapvaal Province, record an evolution similar to that of Phanerozoic foredeeps and may represent the oldest known foredeep sequence, rather than a passive continental-margin sequence as previously proposed. Pre-foredeep ocean crust, the 3.5 Ga-old Onverwacht Group, consists of bimodal volcanics with intercalated shallow-water sediments. Gravity modeling and boundary shear zones indicate that deformed Onverwacht crust was obducted northward onto sialic continental crust immediately before, during or after burial by the synorogenic terrigenous clastic fill. Foredeep basin deepening due to crustal loading from the south is recorded by basinal iron-formation, chert, and mudstone of the basal Fig Tree Group which accumulated paraconformably on the Onverwacht. Overlying sediments of the Fig Tree Group consist of ~ 2 km of submarine-fan greywackes and mudstones. Shoaling of the basin is indicated by the transition to the overlying braided-alluvial and shallow-marine sediments of the ca. 3.3 Ga-old Moodies Group (~ 3 km thick). Changing REE patterns and relative abundances of Ni, Cr, Sr, Th and Ur in the sediments of the Fig Tree and Moodies Groups indicate a significant ultramafic-mafic volcanic component diluted upward in the stratigraphic sequence by an increasing granitic contribution. A similar trend is reflected in sandstone petrography, in which the ratio of extrusive to intrusive rock fragments decreases upwards in the Fig Tree Group whereas the Moodies Group sediments are enriched in quartz and potassium feldspar. A persistent chert component in both units suggests that the tectonic setting of the basin is analogous to that of Phanerozoic recycled orogens. The distribution of facies in the Fig Tree and Moodies Groups, and paleocurrent, petrographic and geochemical data, indicate progressive unroofing of a southerly source terrane consisting of the Onverwacht volcanics and intercalated sediments, and the 3.5-3.2 Ga-old Ancient Gneiss Complex (AGC). In addition, older components of the Fig Tree Group were recycled into overlying stratigraphie units. The AGC consists of mantle-derived 3.5 Ga-old bimodal metavolcanics and metasediments, a tonalite-gneiss batholith, metabasite dikes, and meta-anorthosites. Its structures record progressive northwestward uplift, beginning around 3.4 Ga ago. Intense homogeneous ductile strains were overprinted by northwest verging major folds rotated by large-scale northwestward simple shear, followed by imbricate faulting and pseudotachylite formation. By 3.3 Ga ago the Fig Tree and Moodies Groups had also shortened, with similar kinematics to those of the AGC, by (partly syndepositional) folding and volume loss during the formation of transecting cleavage. Further folding and slide formation followed during subsolidus diapiric rise of the initially tabular Kaap Valley tonalite by 3.2 Ga and emplacement of trondhjemite plutons from 3.3 to 3.0 Ga ago.

Comparative Petrology of Arkosic Sandstone Lithofacies of Cowlitz, Spencer, and Yamhill Formations, Northwest Oregon: ABSTRACT

Recent discovery (1979) and commercial production of natural gas in arkosic sandstone (Clark and Wilson sand, informal) of the Cowlitz Formation near Mist, Oregon, has stimulated interest in the sedimentology and stratigraphy of Narizian sandstones of northwestern Oregon and the Willamette lowlands. Petrographic study of arkosic sandstone in the lower part of the Cowlitz Formation in surface outcrops in Clatsop, Tillamook, and Columbia Counties, Oregon, and in the Texaco 1 Clatskanie well reveals that the composition is more lithic than that of arkosic sandstone (Clark and Wilson sand) in the upper part of the Cowlitz Formation. The petrography of highly permeable, arkosic sandstone in the Spencer Formation in the western Tualatin Valley south of the Mist gas field is identical in framework grain composition, texture, matrix composition, and heavy mineral content to that of the gas producing Clark and Wilson sand. The petrography of sandstone in the Yamhill Formation in the western Tualatin Valley is very like that of sandstone in the lower part of the Cowlitz Formation of northern northwest Oregon. Diagenetic alteration is more pronounced in arkosic sandstone in the lower part of the Cowlitz Formation and in the Yamhill Formation compared to diagenesis, which is almost nilmore » in arkosic sandstone in the upper part of the cowlitz and in the Spencer Formations. Natural gas exploration will be aided by a clearer and more certain understanding of these petrologic differences between Eocene sandstones in northwestern Oregon.« less

Petrography and Provenance of the Glauconitic Sandstone, South-Central Alberta, with Comments on the Occurrence of Detrital Dolomite

ABSTRACT The Glauconitic sandstone (Lower Cretaceous, Albian) of central Alberta was deposited in a range of depositional environments associated with the northward regression of the Clearwater Sea. Deposits of a barrier island complex are cut by sandstones believed to be of fluvial origin. Sandstones of the barrier complex are fine to very fine grained, moderately sorted to well-sorted litharenites or sublitharenites in which quartz, chert and sedimentary rock fragments are the dominant framework constituents. Fluvial sandstones are medium-grained, poorly sorted litharenites that contain, in addition, volcanic rock fragments and feldspar. Detrital dolomite, rare or absent in fluvial sandstones, may constitute up to 32% of the framework grains in sandstones of the barrier complex. Cathodoluminescence microscopy reveals several categories of detrital dolomite based on the brightness and uniformity of luminescence, and demonstrates that some grains have been subjected to more than one cycle of transportation and abrasion. The detrital dolomite is believed to have been derived from Mississippian topographic highs that existed in central Alberta during Glauconitic sandstone deposition. Noncarbonate detrital grains in the Glauconitic sandstone were derived primarily from pre-existing sedimentary rocks to the southwest, and were supplied to the northward-regressing Clearwater Sea by a north-flowing fluvial system. PETROGRAPHIE ET PROVENANCE DES GRES DE LA FORMATION GLAUCONITIC, ALBERTA CENTRALE, AVEC COMMENTAIRES SUR L'OCCURENCE DE DOLOMIE DETRITIQUE RESUME Les gres de la formation Glauconitic (Cretace inferieur, Albien) du centre albertain furent deposes dans une gamme d'environnements lies a la regression vers le nord de la mer Clearwater. Des depots d'un complexe de cordon littoral se trouvent coupes par des gres d'origine supposee fluviale. Les gres du complexe de cordon littoral consistent en litharenites ou sublitharenites a grains fins a tres fins, moderement a bien tries, dans lesquels le quartz, le chert et les fragments de roches sedimentaires dominent l'assemblage. D'autre part, les gres fluviaux consistent en litharenites pauvrement tries a grains moyens renfermant en sus des fragments de roches volcaniques et du feldspath. La dolomie detritique, rare ou absente dans les gres fluviaux, peut cependant composer jusqu'a 32% de l'assemblage dans les gres du complexe de cordon littoral. On denote par microscopie cathodoluminescente plusieurs types de dolomie detritique, la luminance et l'uniformite de luminescence en faisant foi, et on demontrent l'effet de plus d'un cycle de transport et d'abrasion sur certains grains. Cette dolomie detritique proviendrait de points hauts topographiques, d'age Mississipien, disperses dans le centre albertain a l'epoque Glauconitic. Les grains detritiques non-calcaires des gres de la Glauconitic proviennent principalement des roches sedimentaires presentes au sud-ouest et sont transportes par un reseau fluvial drainant vers le nord et vers la mer Clearwater qui regresse elle-meme dans cette direction. Traduit par Jean Pelletier

Sedimentary evidence of a Cambro-Ordovician orogenic event in the northwestern Himalaya

The Late Proterozoic-Early Paleozoic sequence of the Tethys Himalaya provides evidence of an orogenic episode close to the Cambrian-Ordovician boundary (Late Pan-African event). Pre-Middle Cambrian sediments were deposited on a terrigenous tidal flat gradually passing upward to a peritidal carbonate platform. The succession shows an overall transgressive trend, which is reproduced at smaller scale by second- and third-order coarsening-upward cycles terminated by the landward shift of carbonate facies during highstands of sea-level. Sandstone petrography suggests a passive continental margin setting for this part of the sequence. In Middle Cambrian times, the carbonate platform underwent gradual drowning and outer shelf/slope shales with intercalated tuffaceous material erupted from a nearby immature volcanic arc were deposited. The shales are overlain by basinal turbidites attesting to the growth of a sandstone lobe enclosed in lobe-fringe siltstones. In the Middle Ordovician, unconformable alluvial fan conglomerates and braid-plain sandstones derived from uplifted sedimentary sequences indicate erosion of a newly formed orogenic belt. Comparison with Early Paleozoic and recent counterparts allows to interpret the Cambro-Ordovician sedimentary evolution in terms of foreland basin development. The thermal subsidence of a passive margin, temporarily reduced by the passage of a “peripheral bulge”, was enhanced strongly by flexural downwarping towards a subduction zone. Collision of a buoyant continental margin at the trench hampered subduction. Rapid uplift ensued, with formation of a foreland fold-thrust belt shedding sedimentary detritus into the adjacent “molasse” basins.

Sedimentology and Sandstone Petrography of the Upper Kirtland Shale and Ojo Alamo Sandstone, Cretaceous-Tertiary Boundary, Western and Southern San Juan Basin, New Mexico

Sedimentology and Sandstone Petrography of the Upper Kirtland Shale and Ojo Alamo Sandstone, Cretaceous-Tertiary Boundary, Western and Southern San Juan Basin, New Mexico

Sedimentology and sandstone petrography of the upper Kirtland Shale and Ojo Alamo Sandstone, Cretaceous-Tertiary boundary, western and southern San Juan Basin, New Mexico

Sedimentology and sandstone petrography of the upper Kirtland Shale and Ojo Alamo Sandstone, Cretaceous-Tertiary boundary, western and southern San Juan Basin, New Mexico