Polycrystalline Quartz Grains Research Articles

Provenance, tectonic setting and palaeoclimate of Proterozoic Jiran Sandstone, Southeastern Rajasthan, India: A petrographic approach

Proterozoic Jiran Sandstone rests unconformably on Binota Shale and Khori-Malan Conglomerate. The Jiran Sandstone is comprised mainly of fine to medium-grained, varicolored, thickly bedded sandstones, showing diverse primary sedimentary structures such as ripple marks, planar, and trough cross-bedding. Petrographically, Jiran Sandstone is of mainly quartzarenite which is composed of varieties of quartz with ultra-scarcity of feldspar, lithic fragments, micas, and heavy minerals. Quartz is more abundant mineral shown by X-ray Diffraction Analysis. The provenance, tectonic setting, and paleoclimatic condition of the Jiran sandstone were evaluated using integrated petrographic studies. Analysis pursuant, monocrystalline and polycrystalline quartz grains and heavy minerals are driven primarily from metamorphic and plutonic Precambrian basement source rocks of a craton interior setting with a minor quartzose recycled sedimentary source material. Intensive chemical weathering in warm and humid paleoclimate is indicated by lack of feldspar and rock fragments.

Genesis of lamellae in sandy soils: A case study in a semi-arid region in NE-Brazil

The occurrence of lamellae in soils and sediments has direct implications for agricultural production, and for stratigraphic and geomorphological studies. Although lamellae have been studied for more than a century, their origin remains controversial. The aim of this study was to identify the origin and formation mechanisms of lamellae in autochthonous soils developed from metamorphic rocks in a semi-arid area of Northeast Brazil. As such, the spatial organization of soil profiles was evaluated in a topolithosequence, through structural analysis of the pedological coverage for subsequent soil sampling. Morphological, chemical, physical, sedimentological, mineralogical, and micromorphological analyses were carried out on the studied slope and in greater detail in four soil profiles. The results indicated that clay particles predominantly smaller than 0.2 µm are eluviated in a sandy soil matrix, infilling voids and coating quartz grains, which are organized in layers of densely packed fine sand, impeding clay translocation and helping form lamellae. Although the micromorphological properties of the lamellae demonstrate their pedogenic origin, the contribution of strips of polycrystalline quartz constitutes a petrogenic component of the illuviation process. Therefore, the genesis of the lamellae is better described based on a petro-pedogenic origin. We suggest a restructuring of the concept of petro-pedogenic origins, including the structural influence of metamorphic rocks, as illuviated clay deposition occurs in accordance with a physical barrier provoked by the increase and organization of fine sand particles, distributed horizontally and in parallel, arising from the disintegration of polycrystalline quartz grains, which indicates geological control in the genesis of lamellae, corresponding to petrogenic evidence. Furthermore, the morphological distinction of the lamellae seems to be more associated with the fine sand content (<0.25 mm), and its packing with larger grains of quartz in the soils.

Diagenesis and Porosity Evolution of the Pachmarhi Sandstones (Early Triassic), Satpura Gondwana Basin, Central India

This study deals with the diagenetic history of the Pachmarhi Sandstones of Satpura Gondwana Basin, Central India. The sandstones are classified as quartz-arenite and sub-arkose type. Monocrystalline quartz grains dominate the detrital mineralogy followed by polycrystalline quartz grains, feldspars (more alkali-feldspar than plagioclase), rock fragments, detrital mica and heavy minerals. The diagenetic signatures observed in the Pachmarhi Sandstones include mechanical compaction, cementation (Fe-oxide, quartz overgrowths, calcite, matrix and clay minerals), replacement and dissolution of feldspar and calcite cement. The most commonly observed type of secondary porosity is dissolution of feldspars. The porosity loss is mainly due to cementation by pore occlusion and by early stage of mechanical compaction. In addition, several clay minerals occurred as pore-filling and pore-lining cements. The nature of various types of grain- to grain contacts suggests early cementation and consequent minor compaction. Among the various cements, calcite is the earliest followed by iron oxide while silica cementation occurred probably at a late stage. The carbonate cement formed during burial by dissolution and re-precipitation represents redistributed calcite which was buried with the sandstone. The iron cement was perhaps derived from weathering and leaching of ferromagnesian minerals of overlying Deccan traps. Silica cement was derived from the corrosion of quartz and feldspar grains. Types of grain contacts, minus-cement porosity and porosity reduction indicate a shallow depth of burial conditions for these sandstones.

Sources of quartz grains influencing quartz cementation and reservoir quality in ultra-deeply buried sandstones in Keshen-2 gas field, north-west China

Quartz cementation is a critical factor in the reservoir quality of ultra-deeply buried sandstones because of the high temperature and high-pressure at great depths. Therefore, determining the main influences retarding the growth of the quartz overgrowths is important for predicting the sweet spots of tight gas sandstones. The vast Keshen-2 gas field in Kuqa Depression is typical of such ultra-deep gas fields, despite the porosity and permeability of the target sandstones in the Lower Cretaceous Bashijiqike Formation being less than 10% and 0.5 md, respectively. The main gas reservoirs had been buried previously to a depth of 7000 m, with the maximum fluid temperature approaching 160 °C, in which authigenic quartz cements are extremely common. The heterogeneity of the physical properties and quartz cementation was investigated using core analysis, log interpretation, thin sections and scanning electron microscopy (SEM). The findings indicated that the Bashijiqike Formation could be stratified into upper porous and lower tight zones. The porosity of the sandstones in the upper zone is generally higher than 5% and abundant polycrystalline quartz grains are present. However, the reservoirs in the lower zone with low overall porosity values below 5%, contain high contents of monocrystalline quartz grains. The polycrystalline quartz grains show brown cathodoluminescence that indicates mainly a metamorphic source from the southern Tianshan provenance which is located to the north of Keshen-2. The monocrystalline quartz grains display primarily blue to violet luminescence colours, suggesting an origin from volcanic or plutonic mother rocks of the Wensu and Kuluketage provenance areas, situated to the southwest and southeast, respectively. The monocrystalline quartz grains are surrounded commonly by quartz overgrowths, whereas the polycrystalline quartz grains are cemented slightly. The presence of polycrystalline quartz grains hampers potential quartz overgrowths—the critical reason preserving the intergranular pores in the ultra-deeply buried sandstones of this gas field. Accordingly, we propose that the metamorphic quartz grain content is the main factor inhibiting quartz cementation, which preserves reservoir quality. This knowledge is conducive to determining the growing mechanism of high-quality reservoirs and predicting the sweet spots of gas production in the ultra-deeply buried sandstones.

A dorozsmai márvány (Tiszai-főegység) kőzettani újravizsgálata: deformáció és fluidum hatása a mikroszerkezet fejlődésére

A márványok petrográfiai vizsgálata napjainkig alárendelt szerepet kapott a hazai földtani szakirodalomban. A márványt felépítő ásványfázisok, valamint azok szemcsemérete és a szemcsehatár jellegének meghatározása (kőzetszövet), illetve a deformációs bélyegek feltárása és értelmezése révén azonban átfogó képet kaphatunk a kőzet metamorf fejlődéséről és deformációtörténetéről, ami független bizonyítékokkal szolgálhat a befoglaló metamorf kőzettest korrelációjához.&#x0D; A Békés–Codrui-egységen belül elhelyezkedő, Dorozsma környéki kristályos aljzatban egy maximum néhányszor tíz méter vastagságú, markáns töréses deformációt szenvedett márványzóna található. Tanulmányunkban az e zónát mintázó Dorozsma‒4, ‒7 és ‒54 fúrások által feltárt márványból készült archív vékonycsiszolatok petrográfiai vizsgálatát végeztük el.&#x0D; A mikroszkópos vizsgálatok alapján a dorozsmai márvány döntően karbonát + kvarc + Mg-klorit ± talk összetételű, finomszemcsés, heteroblasztos szövetű márványból épül fel. A szemcsehatárok jellemzően szutúráltak, illetve beöblösödők, a deformációs ikerlemezek közül dominánsan a II. és a IV. típus van jelen. A mintákban általánosan jellemzők a kisméretű, kvarccal és karbonáttal kitöltött oldódásos üregek, amelyeket mikrokristályos karbonátból álló szegély övez. Erősen átalakult, mikrokristályos kőzetváltozat szintén megfigyelhető, valamint nagyméretű, szigmoid-alakú karbonátklasztok és rezorbeált, polikristályos, alszemcsés szerkezetű kvarcklasztok is megjelennek a mikrokristályos alapanyagban. Helyenként a nagyméretű karbonátszemcsék deformációs ikerlemezeit apró, kisméretű, zárványmentes karbonátszemcsék helyettesítik, amelyek a nagyméretű szemcsék között is megtalálhatók Mg-klorittal együtt.&#x0D; Az archív vékonycsiszolatok petrográfiai vizsgálata alapján kijelenthető, hogy a vizsgált márványt többfázisú deformáció érte. A D1 esemény a szöveti bélyegek alapján nagyobb, 250 ºC-ot meghaladó hőmérsékleten mehetett végbe, amelyet később felülbélyegzett egy kis hőmérsékletű deformációs esemény (D2), amelyhez fluidumhatás társult. E fluidum gyengítő hatása okozhatta a vékonycsiszolati léptékben felismerhető képlékény alakváltozást, valamint a kis hőmérsékletű dinamikus rekrisztallizációra utaló szöveti bélyegeket.

Provenance constraints on the Tremp Formation paleogeography (southern Pyrenees): Ebro Massif VS Pyrenees sources

A detailed petrological study has been performed for the end-Cretaceous clastic deposits of the southern Pyrenees. Provenance results indicate that the Maastrichtian systems from both the Àger and the Vallcebre synclines show compositional features that mainly consist of a high proportion of single and polycrystalline quartz grains, feldspar and plutonic fragments. By contrast, the sandstone systems of the Tremp syncline exhibit minor contributions from igneous source areas and higher amounts of carbonatic components. These results reveal that the Tremp basin had a source area interpreted as situated to the North in the uplifting Pyrenees. The fact that this basin does not show a high plutonic source signal indicates that the Àger and the Vallcebre basins had been fed from a distinct source area located to the South, here interpreted as the Ebro Massif. Thus, the differences mentioned above might imply that the Montsec High acted as a barrier, avoiding a southern influence in the Tremp basin.

Provenance, Tectonic Setting and Geochemistry of Ahwaz Sandstone Member (Asmari Formation, Oligo-Miocene), Marun Oilfield, Zagros Basin, SW Iran

The Asmari Formation deposited in the Zagros foreland basin during the Oligocene-Miocene. Lithologically, the Asmari Formation consists of limestone, dolomitic limestone, dolomite, argillaceous limestone, some anhydrite (Kalhur Member) and sandstones (Ahwaz Member). This study is based on the analysis of core samples from four subsurface sections (wells Mn-68, Mn-281, Mn-292 and Mn-312) in the Marun Oilfield in the Dezful embayment subzone in order to infer their provenance and tectonic setting of the Ahwaz Sandstone Member. Petrographical data reveal that the Ahwaz Sandstone comprises 97.5% quartz, 1.6% feldspar, and 0.9% rock fragments and all samples are classified as quartz arenites. The provenance and tectonic setting of the Ahwaz Sandstone have been assessed using integrated petrographic and geochemical studies. Petrographic analysis reveals that mono- and poly-crystalline quartz grains from metamorphic and igneous rocks of a craton interior setting were the dominant sources. Chemically, major and trace element concentrations in the rocks of the Ahwaz Sandstone indicate deposition in a passive continental margin setting. As indicated by the CIW' index (chemical index of weathering) of the Ahwaz Sandstone (average value of 82) their source area underwent “intense” recycling but “moderate to high” degree of chemical weathering. The petrography and geochemistry results are consistent with a tropical, humid climate and low-relief highlands.

Palaeoenvironment and provenance of the Early Eocene arenaceous sequence of Neyshaboor, Binalud region, Iran

The Early Eocene sequence of Neyshaboor, Binalud region of Iran is predominantly composed of arenaceous deposits. Two stratigraphically important sections from the Damanjan and Taghan areas have been investigated based on field work, petrographic and geochemical analyses. Eight lithofacies were identified and have been grouped in to conglomerate, sandstone, and mudstone facies association. Petrographic and geochemical data show that the Early Eocene sandstones are mainly composed of arkoses and litharenites classes. Provenance analysis indicates that sediments were supplied from a nearby andesitic–granitic source with minor contribution of metamorphic and sedimentary sources. The presence of predominance of monocrystalline over polycrystalline quartz and abundance of K-feldspars; Cu and Pb contents also support this interpretation. However, subordinate representation of polycrystalline quartz grains, chert, volcanic rock fragments, biotite, zircon, as well as higher percentage of MgO and Fe2O3, suggest some contribution from high-grade metamorphic gneissic rocks and, to a lesser degree, from intermediate to basic volcanics. Climate varied from humid in the beginning of the sedimentation to sub-humid and arid during the later phases. Sedimentation was also influenced by prominent tectonic activity in the source when coarser clastic sediments were deposited as multistoried conglomerates. Lithofacies characters of the rock succession suggest sedimentation took place in a piedmont fan environment, adjacent to a rising orogeny in an active foreland basin setting. Clay to sand and gravel-sized sediments were laid down by meandering and braided rivers and by debris flows under changing conditions of climate and tectonics.

Provenance Variability during Damuda Sedimentation in the Talchir Gondwana Basin, India – A Statistical Assessment

The Talchir Gondwana basin houses the Talchir Formation at the base, succeeded by the Damuda Group and the Kamthi Formation in upward progression. The present study is an attempt to determine the provenance of the Damuda Group and its variability in terms of location, climate and tectonism through time from the composition of sandstone grains and detritus of the constituent Karharbari, Barakar and Barren Measures formations. The Damuda sandstones are composed of variable amounts of monocrystalline undulatory, nonundulatory and polycrystalline quartz grains, potash and plagioclase feldspars as well as metamorphic and sedimentary rock fragments in addition to heavy accessories. Palaeocurrent studies suggest that the Eastern Ghats Supergroup lying to the south of the basin served as the source area of the Damuda sediments. Plots of sandstone composition in tectonic setting discrimination diagrams suggest derivation of the detritus from craton interior, continental block and recycled orogen provinces. Statistical analyses indicate significant differences in the detrital modes of the sandstones of the Karharbari, Barakar and Barren Measures formations, which may be attributed to temporal and spatial variation of the provenance coupled with climate change in commensurate with Damuda sedimentation.

Sandstone petrology and mudstone geochemistry of the Peruc–Korycany Formation (Bohemian Cretaceous Basin, Czech Republic)

We have studied the petrography and the bulk-rock geochemistry of arenites and mudstones of the Cenomanian Peruc–Korycany Formation to characterize their provenance and sedimentary history, as well as the influence of weathering, hydraulic sorting, and recycling of the source rocks. The Peruc–Korycany Formation contains sedimentary facies reflecting both meandering- and braided-river systems and shallow-marine systems. Differences in the three depositional settings did not cause distinctly different modifications of the framework compositions of the arenites. The sand from the fluvial systems is very mature (Qm98F0Lt2). These fluvial arenites were subsequently modified by shallow-marine processes; reworking produced very slight decreases in the abundance of lithic fragments and polycrystalline quartz grains. The Cenomanian strata of the Bohemian Cretaceous Basin were derived dominantly from metasedimentary and crystalline rocks of the Palaeozoic Teplà-Barrandian and Cadomian Moldanubian units, respectively. Periods of low tectonic activity resulted in the deposition of arenites with quartzose framework compositions, indicating that climatic and/or transport/depositional-environmental controls overwhelmed factors such as source-rock compositions. Ultrastable dense minerals are useful indicators of sedimentary recycling within the Peruc–Korycanytarenites. Mudstone samples are characterized by abundant kaolinite, illite, chlorite, and quartz but by negligible amounts of goethite and gypsum. Concentrations normalized to the post-Archaean Australian shale (PAAS) show that the sediments are strongly depleted of Na, K, Ca, Sr, and Ba, probably because of the mobility of these elements during weathering. Chemical indices of alteration (CIA, CIW, and PIA) show that the degree of weathering of the source area was high. The data fall closer to the compositional fields of highly weathered minerals such as kaolinite, gibbsite, and chlorite on an A-CN-K diagram. The indices of compositional variability of the studied samples are much less than 1, suggesting that the samples are compositionally mature and were likely dominated by recycling. The elemental ratios critical of provenance (La/Sc, Th/Sc, Th/Co, Th/Cr, and Cr/Th) are similar to fine fractions derived from the weathering of mostly granitoids rather than mafic rocks.

Petrography and geochemistry of the Cambro-Ordovician Wajid Sandstone, southwest Saudi Arabia: Implications for provenance and tectonic setting

The Wajid Sandstone, of probable Cambro-Ordovician age, rests unconformably on peneplained Precambrian crystalline rocks on the southeastern margin of the Arabian Shield. The sandstone is composed of varicolored siliciclastic rocks, mainly coarse-to fine-grained sandstones with granule conglomerate and siltstone interbeds, displaying diverse sedimentary primary structures such as planar- and trough-cross bedding, and flat bedding. They form repeated fining-upward cycles, typical of deposition from braided streams. Current indicators indicate that flow was from SW to NNE. Petrographically, all samples of the Wajid Sandstone are of quartz arenite type, highly enriched in quartz, but poor in heavy minerals, feldspar and lithic fragments. The provenance and tectonic setting of the Wajid Sandstone have been assessed using integrated petrographic and geochemical studies. Petrographic analysis reveals that mono-and poly-crystalline quartz grains and heavy minerals from metamorphic and igneous rocks of a craton interior setting were the dominant sources. Chemically, major and trace element concentrations in the rocks of the Wajid Sandstone indicate deposition in a passive continental margin setting. Petrographic and geochemical data suggest that the sediments were derived from metamorphic and igneous rocks forming the adjacent Precambrian basement rocks of the Arabian Shield, and were deposited on a passive continental margin. Deposition took place after stabilization of the Arabian Shield following the Late Precambrian Pan-African Orogeny. The Wajid Sandstone can be correlated with similar deposits elsewhere in Arabia and North Africa which were deposited on a low-lying landmass forming the stable continental margin of the Arabian-Nubian Shield, flanking the southern margin of the Paleo-Tethys Ocean.

Water distribution in low-grade siliceous metamorphic rocks by micro-FTIR and its relation to grain size: a case from the Kanto Mountain region, Japan

Infrared microspectroscopy reveals that liquid-like water is always present in polycrystalline quartz grains from low-grade metamorphic cherts and shale in the Chichibu Group and the Mikabu Greenrock Complex of the Kanto Mountains area. The water distribution in these rocks is heterogeneous and is related to rock textures. In the microcrystalline quartz, O–H species due possibly to Si–OH are generally present. The liquid-like water is also present at grain boundaries and/or in sub-micron fluid inclusions in the microcrystalline parts. On the other hand, in quartz grains larger than 20 μm in diameter within veins, only the liquid-like water is present, possibly as fluid inclusions trapped during quartz vein formation. In the microcrystalline cherty matrix of the metamorphic rocks, a general decrease in liquid-like water content is observed with increasing metamorphic grade, associated with an increase in grain size. Two representative grain shapes, a cube and a regular tetradecahedron (having 14 planes), are used to estimate the surface area between grains per unit volume with the grain size D (μm): 3/ D for cubic and 2.37/ D for tetradecahedron. Grain boundary volumes were then calculated assuming grain boundary widths from 0.5 to 20 nm and normalized by unit volume of the rock. The measured IR data on low-grade metamorphic cherty rocks fall closely on these curves with grain boundary width of around 10 nm for the both models, assuming saturation of H 2O at grain boundary. The observed decrease in water content with increase in grain size can be rigorously explained by the decrease in grain boundary volume per unit volume. These results suggest that liquid-like water occurs mainly between grain boundaries in the microcrystalline quartz of low-grade metamorphic rocks. Although this grain boundary water model is the first approximation and requires further details of quantitative grain boundary textures, the present model can provide a new approach to understand water distribution in polycrystalline rocks.

Turbidite provenance in the Lower Palaeozoic Manx Group, Isle of Man: implications for the tectonic setting of Eastern Avalonia

The provenance of turbidites from the predominantly Arenig-aged Manx Group has been constrained using petrographical and geochemical techniques. Petrographical analysis of sandstones reveals that mono- and poly-crystalline quartz grains of cratonic origin are dominant. Major, trace and rare earth element concentrations in both sandstones and mudstones complement the petrographical data indicating that the source of the sediments consisted of quartzose basement and mature sedimentary rocks. There is little evidence of a juvenile volcanic input but a minor palaeovolcanic component of active continental margin affinity is present. There are indications of a more immature character in the oldest units but, on the whole, geochemical data from the Manx Group suggest an overall passive margin setting. These results suggest that Eastern Avalonia was attached to Gondwana during deposition of the Manx Group and that Iapetus oceanic crust did not subduct beneath the region before the end of the Arenig.

Provenance of mineral phases in the Cretaceous-Tertiary boundary sediments exposed on the southern peninsula of Haiti

Acid-insoluble mineral residua of tektite-bearing Cretaceous-Tertiary boundary sediments in the Beloc Formation of Haiti contain abundant shocked quartz and lesser amounts of shocked plagioclase. The shocked quartz grains typically have 2 or 3 sets of planar deformation features, although grains with up to 15 sets were observed. The proportion of shocked quartz in the boundary sediments increases with stratigraphic height; at least 70 ± 11% of the quartz grains are shocked in the uppermost stratigraphic interval. The proportion of shocked quartz throughout the boundary sediments indicates that these grains were excavated primarily from crystalline silicate units, which may have been covered with a small amount of porous quartz-bearing sediments. Polyhedral and moderately sutured margins in shocked polycrystalline quartz grains, the size of the crystal units in these grains and the presence of shocked plagioclase, indicate these ejecta components were excavated from a target with continental affinities, containing quartzites or metaquartzites and a sialic metamorphic and/or igneous component. Other evidence suggests the target may also have contained a significant amount of calcium carbonate and/or sulfate. The large size and amount of shocked quartz grains deposited in Haiti indicate the crater from which they were excavated was produced in the proto-Caribbean region.

Composition of Holocene Colorado River Sand: An Example of Mixed-Provenance Sand Derived from Multiple Tectonic Elements of the Cordilleran Continental Margin

ABSTRACT The Colorado River and its tributaries drain a complex provenance that includes multiple tectonic elements of the Proterozoic to Phanerozoic Cordilleran margin. In order to provide (1) a more reliable estimate than that reported in the literature for the composition of Holocene Colorado River sand, and (2) a standard for comparison with ancient sandstones that may contain detritus derived from a similar complex provenance, 25 unconsolidated samples were collected along the lower reaches of the Colorado River for detailed petrological examination. Each specimen was artificially consolidated with epoxy, thin sectioned, and stained for feldspar. Each author, utilizing the Gazzi-Dickinson point-count method, counted 400 points per sample; the results from the two point-count operators wer averaged. The average Colorado River sand in our collection is medium-grained, moderately to well sorted, and composed of 66.2%Q, 18.1%F, 14.7%L, and 1.3% miscellaneous grains. Q, on average, is composed of 89% monocrystalline and 11% polycrystalline quartz grains. The average plagioclase-to-total-feldspar ratio is 0.66, and the average unstable-aphanitic-rock-fragment population (L) consists of 6% metamorphic (Lm), 43% volcanic (Lv), 51% sedimentary (Ls), and 1% alterite fragments. On the QFL provenance-discrimination diagram developed by W. R. Dickinson and associates, our point-count data plot entirely within the recycled-orogen field. W. R. Dickinson and colleagues ascribed such provenances to subduction complexes, collision orogens, and foreland uplifts ass ciated with fold-and-thrust belts. Rocks in the Colorado River drainage basin, however, include (1) remnants of Precambrian crystalline rocks which form a part of cratonal North America; (2) miogeoclinal and platformal sequences that developed along the North American continental margin during Paleozoic time; (3) deformed Paleozoic and Mesozoic sedimentary successions that make up the Sevier-Laramide fold-and-thrust belt; (4) Mesozoic and Cenozoic sedimentary fill of the Sevier foreland basin; (5) Cenozoic basement-cored uplifts that formed during the Laramide orogeny; (6) plutonic and volcanic rocks of the Mesozoic and Cenozoic Cordilleran continental-margin magmatic arc; and (7) Cenozoic sedimentary and volcanic rocks associated with extension and dextral shear along the Cordilleran co tinental margin. Thus, the QFL compositions of Holocene sand derived from the Colorado River drainage basin do not fit neatly into the recycled-orogen model proposed by W. R. Dickinson and colleagues. In contrast, on the QmFLt provenance-discrimination diagram also engendered by W. R. Dickinson and associates, 16 samples plot in the mixed-provenance field, and nine specimens plot in the quartzose-recycled or transitional-continental fields. On the LmLvLs diagram formulated by R. V. Ingersoll and C. A. Suczek, our samples plot close to the Lv-Ls join, approximately midway between the fields of magmatic arcs, rifted continental margins, and suture belts, probably as a result of their mixed heritage. Thus, the QmFL SUB>t and LmLvLs diagrams produce results that reflect more accurately, than do the results derived from the QFL provenance-discrimination diagram, the complex multiple tectonic characteristics of the source rocks in the Colorado River drainage basin. Our samples, therefore, represent excellent examples of mixed-provenance sands. Casual use of only the QFL diagram can lead to erroneous interpretations of provenances composed of more than one major tectonic element.

Detailed characterization of quartz grains in two sandy soils, Western Australia

Properties of quartz grains from yellow and grey sandy soils on the lateritised Old Plateau land surface of southwestern Australia were studied to provide a basis for the discrimination between the two soil materials and to give an indication of provenance. Properties of quartz investigated were: grain surface textures, particle size distribution, grain strength, and percentages of undulose quartz grains and polycrystalline quartz grains. Several surface textures that have been reported in the literature to be diagnostic of diverse depositional environments occurred on the surfaces of quartz grains from both soils. Post-depositional modification of grain surfaces has occurred so that surface texture could not be used to characterize soil parent materials. Particle size distributions included near-normal, uni, bi, and polymodal examples. Bivariate plots of the graphical sorting parameters: mean size (M z); sorting (σ); skewness (SK 1) and kurtosis ( K 1 G) did not show that the two soils were different, although the grey sandy soils showed a wider range in mean size than did the yellow sandy soils. Quartz grain strength was systematically higher for grey sandy soils than for yellow sandy soils but this difference could be due to post-depositional weathering so that grain strength could not be used to distinguish soil parent materials. Percentages of both undulose and polycrystalline quartz grains were not systematically different between yellow sandy soils and grey sandy soils and showed wider ranges for the grey sandy soils than the yellow sandy soils. Quartz grains in yellow and grey sandy soils only differed in properties that could have been affected by weathering therefore they were not diagnostic of different parent materials.

Downstream Changes in Sand Composition, Roundness, and Gravel Size in a Short-headed, High-Gradient Stream, Northwestern Italy

ABSTRACT We assessed changes in grain composition and roundness of the 1-2 and 2-3 size fractions of sand along a 16-km length of the Graveglia River, Liguria, Italy, to its mouth at the Tyrrhenian Sea and then for 1 km along the beach. Rocks exposed in the drainage area include ophiolites, ribbon chert, limestone, shale, sandstone, and Quaternary sands. The erosion of shale and limestone bedrock yields large amounts of sand, but sand grains of these rock types decrease rapidly in abundance downstream, chiefly through dilution rather than abrasion. Carbonate rock fragments that reach the beach survive wave abrasion, but most shale grains do not. Predictably, monocrystalline and polycry talline quartz and chert are the most durable grain types, but serpentine, basalt, and unidentified rock fragments also show significant resistance to both stream and wave abrasion. Polycrystalline quartz, feldspar, basalt, serpentine, sandstone, and shale grains are more abundant in the coarser-sand-size fraction; monocrystalline quartz, carbonate rock fragments, and unidentified rock fragments are more abundant in the finer-sand-size fraction; and chert and plutonic rock fragments have no size preference. A comparison of the outcrop areas of the different rock types with the corresponding percentages of different grain types present in each of three drainage areas indicates that shale, which breaks down readily to silt- and clay-size particles, and chert, which does not readily break down into sand-size fragments, are underrepresented in the sands. Limestone and serpentine, however, are overrepresented in comparison to outcrop area in certain drainage areas. The overrepresentation of limestone reflects, in part, a sampling bias (sand was sampled adjacent to limestone outcrops), but the overrepresentation of serpentine reflects its resistance to weathering (compared to other aluminosilicates) and its durability. Shale, limestone, and serpentine increase in roundness downstream and along the beach to a maximum (limiting?) value of 5 rho units. Medium-grained shale clasts attain high rounding in only a few kilometers of transport. Downstream decrease in gravel size follows an exponential formula that can be expressed in linear form using phi size: size () = 0.1167 distance (km)- 10.29.

Paleogeographic and tectonic implications of quartzose sandstones of Barbados

The provenance of Paleogene sandstones on the island of Barbados is relevant to various models of the tectonic evolution of the southern Caribbean area. Modal point counts of 26 sandstones from Barbados show that the constituent grains most likely were derived from a composite craton interior and orogenic margin. Abundant detrital polycrystalline quartz grains have affinities to a spectrum of low‐ to high‐rank metamorphic/plutonic crystalline source rocks. Microprobe data suggest that feldspars were derived from metamorphic or plutonic source terrances. The complex heavy‐mineral suite of the terrigenous sandstones, including glaucophane, chloritoid, titanite, zircon, and aluminosilicates, supports derivation from orogenic and cratonal terranes. Due to the complex history of the South American‐Caribbean plate boundary and poor age constraints on timing of deposition, widely contrasting models have been proposed concerning the provenance of the sandstones. Convergence between the Aruba‐Orchila arc and continental South America during the Paleogene resulted in the formation of an E‐W trending foreland fold‐thrust belt and foredeep. Dominant sediment dispersal was to the east–northeast along tectonic strike. The present study shows that the terrigenous sandstones on Barbados probably were deposited in deep‐sea fans to the north of present‐day Araya Peninsula during and shortly after the late middle Eocene. Probable source terranes include the Guayana Shield, the Caribbean Mountain System/interior foreland fold‐thrust belt, and possibly (to a lesser extent) the Lesser Antilles magmatic arc or predecessor arc.

Role of Quartz Type and Grain Size in Silica Diagenesis, Nugget Sandstone, South-Central Wyoming

ABSTRACT Silica-cement-dominated intervals within the Nugget Sandstone illustrate several relationships involving pressure solution and initial silica cementation as influenced by clast size and quartz grain type. For medium-sand-size, nonundulose, undulose, and polycrystalline quartz, the portion of grains with well-developed silica overgrowths (cement occupying > 50% of free grain surface) is 26%, 14% and 4%, respectively. Overall, monocrystalline grains are about three times as likely to have well-developed silica overgrowths as polycrystalline clasts. Well-developed silica overgrowths on monocrystalline quartz are about seven times more common in coarse-sand-size grains than fine-sand-size grains. No apparent grain size-initial overgrowth development relation is characteristic of the polycrystalline quartz population. This may be due to a predominant overgrowth-inhibiting influence related to differences in c-axis orientation between adjacent crystal domains within polycrystalline quartz grains. Pressure solution at grain contacts accounts for slightly more silica generated than precipitated as overgrowths for silica-cement-dominated intervals within the Nugget. The volume of silica produced along more extensive, stylolitic contacts is approximately equal to 25 to 30% of the remaining volume of pore space. Thus, the portion of the Nugget studied is an overall exporter of silica due to the pressure solution process.

The sandstone memory of the evolution of a Triassic volcanic arc in the Southern Alps, Italy

ABSTRACTThe erosion of a southern volcanic belt provided the bulk of the fine‐grained and moderately sorted siliciclastic detritus in the whole deltaic to lagoonal Carnian (Upper Triassic) sequence of the Bergamasc Alps. Volcanic input began in the Early Carnian (Calcare Metallifero Bergamasco) and became prominent in Early‐Middle (Val Sabbia Sandstone) and Late Carnian times (S. Giovanni Bianco Fm.), after an intervening period of diminished supply (Gorno Fm.).The mineralogical and chemical composition of the sandstones compares closely with that of siliciclastics derived from modern, Pacific‐type magmatic arcs and it testifies to an evolution of the source rocks from andesites and dacites during the deposition of the Val Sabbia Sandstone, towards rhyodacitic ignimbrites for the S. Giovanni Bianco Fm. The marked reduction in feldspars at the top of the sequence contrasts with the classical trend from Undissected to Dissected Magmatic Arc provenance and is ascribed to the progressive deepening of erosion into Middle Triassic felsic volcanics. An increase in undulatory and polycrystalline quartz grains in the Late Carnian shows that the erosion did reach deeper into the crystalline roots of the arc, although non‐volcanic detritus never exceeded 10%.After a terminal pyroclastic event in Middle‐Late Carnian times, suggested by the sudden appearance of pumiceous vitric clasts at the top of the Val Sabbia Sandstone, the mid‐Triassic orogenic magmatism ended and was followed by the tensional stage which led to the birth of Neotethys.