Lithic Grains Research Articles

Sedimentary Facies, Provenance and Geochronology of the Heshangzhen Group: Implications for the Tectonic Evolution of the Eastern Jiangnan Orogen, South China

AbstractThe Neoproterozoic Jiangnan orogen plays an important role in the study of the Precambrian tectonic evolution of South China. The tectonic nature of the Neoproterozoic sedimentary basins is still controversial, due to poor understanding of the sedimentary sequences and the lack of geochronological data. Here, we present sedimentological, provenance and geochronological data from the Heshangzhen Group in the eastern Jiangnan orogen. Sedimentological analysis shows that the Luojiamen Formation was deposited in a submarine fan, and the overlying Hongchicun Formation was deposited in front of a fan delta. The youngest detrital zircons constrain the lower Luojiamen and Hongchicun formations with ages of 827.3 ± 8.4 Ma and 825 ± 12 Ma, respectively. The sandstones of the Luojiamen Formation are characterized by a large number of intermediate to felsic volcanic grains, suggesting a volcanic arc source. In contrast, quartz and sedimentary lithic grains increase in the Hongchicun Formation, showing a new input from a collisional orogenic source. Detrital zircon from six sandstone samples in the Luojiamen and Hongchicun formations yield similar age spectra of 930–820 Ma with a peak at ca. 845–860 Ma, with one main cluster at 930–820 Ma. Detrital zircons of 930–845 Ma show a positive value of ∊Hf (t) (+2.4 to +11, mean +7.6), which is similar to the volcanic arc of the nearby Shuangxiwu Group. There are a minor group of zircons with U‐Pb ages ranging from 820 Ma to 845 Ma from the middle part of the Luojiamen Formation and Hongchicun Formation, with ∊Hf (t) values between –20 to +2.4, which are consistent with the characteristics of the Shuangqiaoshan Group. within light of the bidirectional paleocurrents in the Luojiamen Formation, it is speculated that the zircons of 820–845 Ma were recycled from the Shuangqiaoshan Group, which is derived from a continental arc to the northwest. Our data suggests that the Luojiamen Formation was formed in an inter‐arc basin, while the Hongchicun Formation was formed in an accretionary wedge‐top basin. When juxtaposed with the conglomeratic characteristics at the bottom of the Luojiamen Formation, it is believed that the unconformity represented by the ‘Shen Gong Movement’ reflects the rapid erosion and accumulation process of island arc volcanic material. The disconformity between the Luojiamen and Hongchicun formations is the imprint of transition from inter‐arc basin to accretionary wedge‐top basin, which represents the collision between the Shuangxiwu arc and the Yangtze Plate.

A mineralogical and geochemical investigation of modern aeolian sands near Tonopah, Nevada: Sources and environmental implications

The mineralogical and geochemical composition of sands across arid to semi-arid landscapes can be used to evaluate the nature of their source lithologies. Once these characteristics are established, these sands have the potential to be implemented as background reference materials in environmentally-focused studies (e.g., mine waste pollution). Tonopah, Nevada is an economically disadvantaged town with a long history of silver mining. Tonopah and surrounding areas with similar histories are also examples of significantly understudied regions with respect to several environmental concerns.Sands sampled in this study are part of a star-shaped dune system ~15 km north of Tonopah. They are medium-grained and are very well sorted: arithmetic mean grain size from 263 to 335 µm. From Quartz – Feldspar – Lithic (QFL) classification the sands are feldspathic litharenites to weakly litharenitic. Powder X-ray diffraction and polarized light microscopy observations identify quartz, ordered calcian albite, and sanidine as the dominant minerals with rarer calcite, magnesian calcite, and amphibole. Scanning electron microscopy indicates grains have a range of textural maturity, with rhyolitic and basaltic lithic grains present. From bulk elemental chemistry, sands are geochemically akin to that of Earth’s bulk upper continental crust, are quartz-rich, formed in arid to semi-arid climates, and were derived from a felsic igneous province (or provinces).The origin of these sands is further investigated by comparing bulk major elemental signatures to the composition of global, regional, and local dune systems, in addition to local and regional bedrock. Regionally, Tonopah sands are compositionally similar to those of the Mojave Desert dune fields (e.g. Cadiz) and distinct from those of the Algodones and Parker dunes of California and Arizona respectively. Consistent with source inferences for the Cadiz, Danby, and Dale lake sands of the Mojave Desert, Tonopah sands are inferred to be derived from chemically evolved provinces. For Tonopah, derivation from the local, Micoene-aged, andesite-dacite-rhyolite suites is inferred.Through comprehensive characterization of the Tonopah sands a geogenic background composition for the region, and an important reference material for future environmental investigations, has been established.

Guidelines for assessing the provenance of Mesozoic and Cenozoic clastic successions sourced by pre-Jurassic basement complexes in southernmost North America

ABSTRACT Mexico is an attractive place for provenance studies focused on reconstructing the tectonic evolution of North America. This is because Mexico hosts a well-preserved clastic record associated with some of the major Mesozoic and Cenozoic tectonic processes that shaped the face of this continent. However, the available information on Mexican pre-Mesozoic source terranes is presently insufficient for provenance analysis. With the aim of drawing the guidelines for provenance determination, we present here detrital modes, geochemical data, and zircon U-Pb ages for detritus derived from pre-Jurassic basement complexes of Mexico. Our data show that the various basement complexes produce distinctive detrital modes and supply diagnostic and compositionally different detrital heavy minerals that represent powerful provenance tracers. The Oaxacan Complex, Ayú Complex, and East Mexico Arc are the main sources of quartzo-feldspathic and feldspatho-quartzose detritus. Quartz with rutile needles, mesoperthitic K-feldspar, orthopyroxene, augitic to diopsidic clinopyroxene, and Mg- to Ca-rich almandine (Alm71–52Grs7–3Prp43–23Sps3–1Alm74–56Grs21–19Prp23–2Sps5–2) are common minerals in detritus from the Oaxacan Complex. The Ayú Complex supplies detritus marked by the occurrence of sagenitic biotite and white mica, as well as Mn-rich almandine (Alm69–66Grs4–3Prp18–11Sps19–10). Detritus from the East Mexico Arc contains any of these mineral phases ubiquitous in the Oaxacan and Ayú complexes. The Acatlán Complex is the main source of detritus dominated by metamorphic lithic grains and quartz, with minor amounts of feldspar. Lithic grains are rank 2–4 metabasitic, metapelitic, and metapsammitic–metafelsitic fragments. Diagnostic mineral phases are schorl–dravitic tourmaline, Na-amphibole, and helycitic garnet varying from a Ca- to Mn-rich almandine (Alm74–55Grs34–15Prp16–3Sps12–1-Alm70–46Grs20–15Prp3–1Sps32–12). Zircon U-Pb geochronology proves to have some virtues but also major limitations because: 1) the zircon U-Pb age signature of many different sources in Mexico is similar and 2) zircon documents a limited number of sources because of variations in zircon fertility.

Erosional and Tectonic Evolution of a Retroarc Orogenic Wedge as Revealed by Sedimentary Provenance: Case of the Oligocene – Miocene Patagonian Andes

Sedimentary provenance techniques have been widely applied in foreland basin settings to understand tectonic and magmatic processes by tracking the exposure and erosion of distinct sediment source areas through time. We present a case example from the Magallanes-Austral retroarc foreland basin of Chile and Argentina (51°30’S), where modal sandstone and conglomerate compositional data, detrital zircon U-Pb geochronology, and sedimentology data from the Oligocene-Miocene Rio Guillermo Formation document a change in source areas during an important stage of orogenic development. In particular, our results from the ~24.3-21.7 Ma Rio Guillermo Formation record an abrupt shift from transitional to undissected arc provenance that indicate rejuvenated magmatism within the contemporary arc. Minor components of lithic grains suggest a subordinate source of recycled sediments that we interpret may have been derived from the intervening external fold-and-thrust belt, rather than directly from sources in the hinterland thrust domain. Detrital zircon U-Pb geochronology data show mostly Neogene (~20-40 Ma) and Cretaceous (~70-110 Ma) age groups, with minor amounts of Jurassic (~145-155 Ma) and Paleozoic (~260-540 Ma) age groups, which are consistent with a syndepositional arc and recycled external fold-and-thrust belt sources. Stratigraphic data suggest a vegetated, channelized braidplain environment developed above an erosional unconformity with the underlying shallow-marine Rio Turbio Formation. Upsection, the Rio Guillermo Formation locally transitions to a low-energy, organic-rich floodplain setting located within the upper reaches of a fluvial-tidal transition zone of the coastal plain, and the uppermost part of the formation is characterized by a coarse-grained sandy channelized braidplain environment along the foreland basin margin. Moderate sediment accumulation rates and coastal plain progradation during this period is consistent with sustained sediment flux from the Patagonian Andes and tectonic subsidence along the basin margin. Taken collectively, we propose that the abrupt provenance shift dominantly records erosion of the rejuvenated mafic volcanic arc, despite coeval changes in orogenic wedge dynamics brought about by increased plate convergence rates that drove uplift of the intervening external-fold-and-thrust belt along reactivated deep-seated high-angle basin structures.

Multimineral fingerprinting of modern sand generated from the Tethys Himalaya (Nianchu River, Tibet)

The Nianchu, as the largest southern tributary of the middle Yarlung Tsangpo (upper Brahmaputra River in southern Tibet), drains mainly sedimentary rocks of the Tethys Himalaya and subordinately metamorphic and igneous rocks of the Greater Himalaya, Kangmar gneiss dome, and Indus-Yarlung ophiolitic suture zone. This study presents the first detailed compositional fingerprinting of detritus released from the Tethys Himalaya in southern Tibet. Sand carried by the Nianchu and by its major tributaries range from quartzo-lithic to litho-quartzose sedimentaclastic with a few metamorphic, volcanic, and ultramafic lithic grains, reflecting dominant erosion of Tethys Himalayan strata. Transparent heavy-mineral assemblages are moderately poor. They include a mixed recycled assemblage with amphibole, tourmaline and rounded zircon, together with first-cycle sillimanite and garnet grains from the Greater Himalaya, young titanite and monazite grains from either the Greater Himalaya or the Kangmar dome, chloritoid from the low-grade metapelites surrounding the Kangmar dome, and clinopyroxene, olivine and enstatite grains shed from forearc ophiolites. Forward mixing models indicate that ~80% of Nianchu sand is recycled from the Tethys Himalaya, which is roughly the same percentage as the corresponding exposure area. Based on previous geochronological and provenance studies, and on available suspended-load data from the Nianchu, Lhasa and Yarlung Tsangpo rivers, we assess the average erosion rate in the Nianchu catchment as ~0.10 mm a−1, about twice as that in the middle Yarlung Tsangpo catchment (0.05 mm a−1) and about five times as that of the Lhasa River catchment (0.02 mm a−1). This marked difference is principally ascribed to the higher erodibility of Tethys Himalayan sedimentary strata. Within the Nianchu basin, low precipitation in the rain shadow of the high Himalayan range may explain a lower erosion rate in the headwaters (0.07 mm a−1) than in the less steep lower reaches (0.14 mm a−1).

Petrographic characterization and diagenetic evaluation of reservoir sandstones from Smørbukk and Heidrun fields, offshore Norway

A detail qualitative and quantitative analysis based on thin section petrographic analyses of sandstone samples from the reservoir horizons of Smørbukk and Heidrun fields have been carried out to characterize the textural and mineralogical properties, and to evaluate the post depositional diagenetic changes. Smørbukk samples are medium-coarse grained, moderately sorted, tightly packed, submature–mature subarkosic and quartz arenites. Heidrun samples, on the other hand, are fine-medium grained, moderate-well sorted, moderate-loosely packed, mature-submature subarkosic and quartz arenites. Despite the detrital components (quartz, feldspar, lithic grains, mica, etc.) of both the fields are similar, silica cementation is more common in Smørbukk samples whereas early carbonate is the predominant cement in Heidrun samples. Smørbukk sediments are comparatively more compacted than those of the Heidrun fields. Overall reservoir quality in Heidrun field (8%–18%) is slightly better than that of Smørbukk field (5%–15%). The findings of this study will be helpful in the development phase of these fields and/or fields from similar geological setting.

Patterns and origins of the porosity in the productive reservoirs of the upper part of the Chubut Group, southern flank of the Golfo de San Jorge Basin, Patagonia Argentina

The Mina del Carmen, Bajo Barreal and Cañadón Seco Formations bear the most important Cretaceous reservoirs in the southern flank of the Golfo de San Jorge Basin, in which the porosity is mainly secondary in origin. Primary porosity is about 4% in average and can be divided in intergranular and intercrystalline porosity, the former is better preserved where chlorite rim cement is abundant, and the second occurs among crystals of the clay cement (main kaolinite). Secondary porosity comprises dissolution of feldspar, ductile fragments, clay cements, lithic grains, and carbonate-zeolite cements.Dissolution of feldspar produces the formation of authigenic kaolinite and secondary porosity; this mechanism dominates in the western area of the southern flank due to sandstones show higher amounts of plagioclase grains, which were very reactive to diagenetic dissolution. Ductile fragments, composed of tuffaceous mudstones (probably of intrabasinal origin), were strongly deformed during the compaction reducing the primary porosity and closing the pore system. Nevertheless, in sandstones containing similar proportions of intrabasinal and extrabasinal clasts, some ductile grains suffered massive dissolution increasing the secondary porosity. Another source of secondary porosity comprises the dissolution of tuff fragments, and locally, carbonate and zeolite cements.The secondary porosity includes the types redistributional (RDS) and pervasive (PVS). This division can be related to previous diagenetic model for the Bajo Barreal Formation. In this way, redistributional secondary porosity is linked to the first and the second events of diagenetic dissolution, while the pervasive secondary porosity to the third dissolution event, which likely had a telogenetic origin.

Compositional and textural study of modern beach sands in the active volcanic area of the Campania region (southern Italy)

The association among volcanism, tectonics and sedimentation may be critical for interpreting the paleogeographic and plate tectonic settings of the past. In modern and ancient sedimentary systems, volcaniclastic fragment classes have been distinguished on the basis of texture and composition. Criteria adopted for their temporal and spatial correlation with source volcanic areas, in terms of type of volcanism and particle-forming processes are still not well constrained and often a topic of controversy and misinterpretation. Here, we present data for volcaniclastic fragments occurring in modern beach sands of the Campanian coast of southern Italy, where there are two active volcanic areas, the Phlegrean Fields and Mt. Vesuvius. Despite their geographical proximity, these volcanoes are characterized by different magma compositions and eruptive mechanisms. We set out to test whether this translates into different textural and compositional characteristics of their associated volcaniclastic detritus in order to better constrain the criteria relating the type of volcanism to the nature of volcaniclastic sand fragments in the sedimentary record. Along a ~100 km coastal stretch, from the Volturno River mouth to Sorrento, 72 sand samples were collected mainly in beach berm environments. Detrital modes were defined for the medium sand fraction of each sample, where monocrystalline and lithic grains are easily identified. The analyzed samples show a mixed sedimentary and volcanic provenance. Samples from the northernmost coastal stretch show a sedimentary petrofacies at the Volturno River mouth. Bacoli marks the transition between the Apennine sedimentary and Phlegrean volcanic petrofacies. Along the Portici-Sorrento coastal stretch, volcanic lithic fragments dominate. Leucite and sanidine crystal components indicate that detritus from the Phlegrean Fields extends south from Licola to Naples Bay, whereas the Vesuvius provenance extends from Naples/Portici south to Castellammare di Stabia. Phlegrean Fields and Mt. Vesuvius volcaniclastic petrofacies have been discriminated through a different distribution of both compositions of the volcanic lithic fragments, leucite and K-feldspars content. Volcanic lithics with lathwork texture and leucite, are Mt. Vesuvius provenance source sensitive detrital grains, whereas volcanic lithics with vitric texture and K-feldspars marks the transition to the Phlegrean Fields petrofacies. The results formalize three different sandy petrofacies which were mapped from north to south as Sedimentary, Phlegrean (volcanic) and Vesuvius (volcanic) petrofacies.

The diagenetic controls of the reservoir heterogeneity in the tight sand gas reservoirs of the Zizhou Area in China's east Ordos Basin: Implications for reservoir quality predictions

Abstract The Shan 23 sandstone formations are important exploratory targets for tight gas resources in the Zizhou Area of the Ordos Basin. In this study, we investigated the effects of diagenesis on the heterogeneity of tight sandstone deposits in similar sedimentary facies and identified the main controlling factors of reservoir quality. A total of 169 samples were collected from 50 wells for a series of analyses. The heterogeneity of the sandstones is mainly controlled by cement and pore-throat characteristics, which showed regional differences in the study area consistent with the regional variations in the detrital components. These results indicate that the diagenesis influenced by the sediment composition is the main controlling factor of the reservoir heterogeneity. The compaction dominated by ductile grains is the major cause of the primary porosity losses in the sandstones, followed by the quartz and clay (kaolinite and illite) cementation. Residual intergranular pores rarely occur in the sandstones with a ductile lithic grain content greater than 2%. Altered kaolinite and recrystallized illite commonly occur in the sandstones with high ductile grain content. Residual intergranular pores, quartz overgrowths, and authigenic kaolinite are the most common phases in the sandstones rich in detrital quartz. The dissolution formed secondary pores and provided a minor contribution to reservoirs permeability. Secondary pores predominantly occur in the quartz arenite due to the preservation of the primary pores. The main controlling factor for the quality of the sandstones is the pore type, which has pronounced effects on both the pore structure and the pore-throat size distribution. The residual intergranular pores preserved in sandstones will lead to larger radii and better sorting characteristics of the pore-throats, which subsequently contribute to the higher permeability. Therefore, the sandstones with the higher content of primary intergranular pores are considered high-quality reservoirs. They are characterized by quartz richness and mainly located in the Zizhou-Mizhi Area. The inter-crystalline micropores occur in the clay mineral, resulting in small-sized and poor-sorted pore-throats. They give subordinate contributions to the reservoir quality and are the key factor of the extremely low permeability in sandstones. The dry layers of the study area are mainly composed of sandstones dominated by inter-crystalline micropores. These sandstones are characterized by the high content of ductile grains and are mainly located in the Qingjian Area.

Petrographical and geophysical investigation of the Ecca Group between Fort Beaufort and Grahamstown, in the Eastern Cape Province, South Africa

Abstract The outcrop of the Ecca Group in the Eastern Cape Province was investigated in order to reveal petrographic and geophysical characteristics of the formations that make up the group which are vital information when considering fracking of the Karoo for shale gas. The petrographic study reveals that the rocks of the Ecca Group are both argillaceous and arenaceous rock with quartz, feldspar, micas and lithics as the framework minerals. The sandstones are graywackes, immature and poorly sorted, thus giving an indication that the source area is near. The observed heavy minerals aswell as the lithic grains signify that the minerals are of granitic, volcanic and metamorphic origin. The porosity result shows that of all the formations that make up the Ecca Group, the Whitehill Formation is the most porous with an average porosity of about 2.1% and also least dense with an average dry density of about 2.5 g/cm3. The least porous unit is the Ripon Formation with porosity of about 0.8% but has the highest dry density of approximately 2.8 g/cm3. The magnetic map shows some ring-like structures which coincide with dolerites that were mapped in the field. As revealed by the depth slices result, dolerite intrusions are pervasive in the northern part of the study area, extending to a depth of about 6000 m below the ground surface. The appearance of dolerite intrusions at the targeted depth (3000 - 5000 m) for gas exploration could pose a serious threat to fracking of the Karoo for shale gas.

Mud deposition and diagenesis within an Early Palaeozoic clinothem: Power Steps Formation, Newfoundland, Canada

AbstractThe early Ordovician (∼385 Ma) Power Steps Formation, Newfoundland, Canada, exposes a well-preserved mudstone-dominated clinothem that serves as an excellent archive for understanding how mud has been produced, transported and converted into mudstone prior to the evolution of globally widespread, deep soil horizons. Sedimentological analysis of four sandstone and five mudstone facies, along the Ochre Cove clinothem, reveal that mud and sand were delivered by unidirectional currents and experienced episodic reworking by storm waves. Petrographic examination and X-ray diffraction from described mudstone facies reveal significant variability in the distribution of illite versus chlorite between the lower and upper part of the Ochre Cove clinothem. This research highlights that in the present-day clay mineral fraction, illite is often detrital whereas chlorite originated via the alteration of silt-sized, highly unstable, mafic (volcanoclastic?) grains. Throughout all sedimentologic facies, albeit in different proportions, these mafic lithic grains were diagenetically altered via in situ weathering before significant compaction occurred, resulting in the precipitation of significant volumes of pore-bridging, silica- and iron-rich chlorite cement. Compositional, diagenetic and textural attributes across the Ochre Cove mud clinothem vary as a function of starting composition, hydrodynamic sorting and grain density. Given that a significant proportion of clay minerals has been generated via in situ transformation of a mafic, non-stable precursor assemblage, we recommend future studies to incorporate detailed petrographic description along with X-ray diffraction analyses when aiming to employ trends in whole-rock clay mineral data as a proxy in provenance and palaeoclimate studies of very old (pre-Devonian) mudstones and sandstones.

Striking Variation in the Provenance of the Lower and Upper Cretaceous Turbidites in the Central Pontides (Northern Turkey) Related to the Opening of the Black Sea

AbstractThe Lower and Upper Cretaceous turbidites cover large areas in the central Pontides (north‐central Turkey). The Lower Cretaceous turbidites are over 2 km thick and are exposed in an area of 400 × 90 km. The Upper Cretaceous (Campanian‐Maastrichtian) forearc turbidites are up to 1,200 m thick and crop out in NW‐SE trending elongate basins about 40 km wide, extending along the central and eastern Pontides. We present new detrital zircon U–Pb ages, petrography, and paleocurrent measurements from the Campanian‐Maastrichtian turbidites and compare them to those from the Lower Cretaceous turbidites. The Campanian‐Maastrichtian sandstones are dominated by carbonate and magmatic lithic grains. The paleocurrents indicate paleoflow directions parallel to the axis of the basin. The sandstones are dominated by Late Cretaceous zircons indicating derivation mainly from the coeval magmatic arc. In contrast, the Lower Cretaceous sandstones are dominated by quartz and feldspar, and paleocurrents indicate southward transport. The detrital zircons in the Lower Cretaceous sandstones are mainly Archean and Paleoproterozoic, indicating that in the Early Cretaceous, the source of turbidites was from the Archean‐Paleoproterozoic Ukrainian Shield to the north. After the opening of the Black Sea in the Late Cretaceous, the connection between the Pontides and the East European Platform was severed and the Campanian‐Maastrichtian turbidites were sourced principally from the magmatic arc in the Pontides. Our results indicate that the Campanian‐Maastrichtian turbidites represent the first turbiditic sequence deposited after the opening of the Black Sea.

Geochemistry and Provenance of Sandstone Unit in Tanjero Formation in Sulimania Area, NE-Iraq.

Sandstone rocks occurring in the Tanjero Formation in the Sulimania and Pera-Magron areas, which crops out within the Imbricated and High Folded Zones in Northeastern Iraq, are lithic arenites with high proportions of sedimentary rock fragments. The presence of quartz, chert, carbonates and igneous and metamorphic lithic grains and fossils in the Tanjero sandstone clastic rocks indicates that the southern Neotethys Ocean was a shallow seaway during their deposition. Geochemical classification of the an Upper Cretaceous Tanjero sandstone clastic rocks are lithic arenites to Fe-Sand and indicates that they were mainly derived from Albian-Cenomanian Gimo–Qandil sequence ophiolite-bearing terrane and Hemipelagic sediments (Parautochthonous Qulqula rocks)..

Features of ice-rafted debris (IRD) at IODP site U1312 and their palaeoenvironmental implications during the last 2.6 Myr

In this paper, we build an age model by tuning the G. ruber δ18O curve to the LR04 stack and U1308 benthic δ18O curve with the aid of the high-resolution lightness parameter L*, which mirrors the marine oxygen isotope stages, and we analyse the major palaeoenvironmental events indicated by different proxies, particularly the >250 μm fraction and the 150–250 μm fraction of ice-rafted debris (IRD) at IODP Site U1312. In general, the input of IRD at Site U1312 can be divided into two stages. In Stage 1, ranging from ~2.6 Ma to ~1 Ma, the number of lithic grains was small, the frequency of ice rafting events was low, and the components were diverse and mainly comprised quartz, feldspar and clasts. In Stage 2, ranging from ~1 Ma to the present, the number and frequency of lithic grains considerably increased and were accompanied by detrital carbonate additions derived from the Hudson Strait after ~650 ka. Regarding the IRD features in the study area, the distinctive layers are as follows. IRD did not occur until 2.52 Ma at Site U1312, implying that icebergs discharged by the Northern Hemisphere glaciers bearing lithic grains were involved in pelagic sedimentation for the first time. Interestingly, a monotonic component of debris, magnetite with some ilmenite and quartz, was observed in a single layer at approximately 1.4 Ma. The source of this component may simply be the erosion of volcanic formations in Iceland during the glacial period. Detrital carbonate played an important role in the IRD component after ~650 ka, and the input and frequency of IRD increased dramatically, revealing glacier enhancements and the instability of the Laurentide ice sheet. The IRD characteristics at Site U1312 during MIS 22, MIS 12 and MIS 6 were different from those at other sites near the IRD belt and may be closely related to the European ice sheets. During MIS 10-8, the input of IRD was high, the dominant warm species G. ruber was too rare for the isotope test to be conducted, and the abundance of planktonic foraminifera severely declined, which may indicate that extreme conditions occurred at Site U1312 during this interval.

Last Glacial Maximum surface water properties and circulation over Laurentian Fan, western North Atlantic

Millennial scale events marked by the contribution of detrital sand are recorded in North Atlantic sediments during the Last Glacial Maximum (LGM), between Heinrich events (HE) 1 and 2, and left their imprint on Laurentian Fan (LF – 43°N) sediments off eastern Canada. The LF counterpart of the well-known detrital events consist of glacial red-brick sediments resulting from subglacial flows separated by olive-grey sediments appears at ∼21.4–19.9 and ∼19.5–18.65 cal kyr BP. High-resolution analyses of diatom assemblages and lithic grains coupled with planktonic oxygen isotopic records reveal that while the red sediment is almost barren of diatoms, foraminifera and lithics (>150 μm), they are abundant in the olive-grey sediment. Diatom assemblages reveal three phases during these events: (1) initial relatively warm/temperate conditions followed by (2) very cold surface water and drifting ice, and (3) a final phase characterized by relatively warmer waters and the appearance of detrital carbonate. Although these events possibly reflect the variability specific to the slope water region, they are likely the response to Atlantic Meridional Overturning Circulation perturbations and ice-sheet instability. Through a chain of mechanisms, meltwater inputs into the North Atlantic led ultimately to an increased volume of tropical waters and part of the heat stored in the subsurface was flushed by a brief convective episode that was not sustained, accounting for the return of cold conditions after the events. The sequence of mechanisms deduced from the paleo data here and elsewhere is consistent with previous modeling results. These data suggest that the detrital events between Heinrich event 1 and 2 may be synchronous across the North Atlantic, and that the LGM was probably not a time of prolonged steady state in the climate system.

Heavy minerals distribution and provenance in modern beach sands of Campania, Italy

This work focuses on the textural characterization of modern sand supplied from a volcanic island in the Aeolian Archipelago, with particular attention given to the factors that control grain rounding. Twelve beach sands were collected from hightide berms spaced along Lipari’s accessible coastal perimeter. Point-count data were collected using a petrographic microscope with an automated stepper stage. Changes in grain roundness were assessed to evaluate grain resistance to transport(currents and wave abrasion). A numerical value was assigned to very angular (1), angular (2), sub-angular (3), sub-rounded (4), rounded (5) and well-rounded (6) categories. Analyzed particles are monocrystalline grains of plagioclase (Plag) and pyroxene (Py) and volcanic lithic fragments of Lvl (volcanic lithic with lathwork texture), Lvmi (volcanic lithic with microlitic texture), Lvv (volcanic lithic with vitric texture) and Lvf (volcanic lithic with felsitic texture). The most common roundness category among all the grains from the sampled coastal stretches of Lipari island is 3 (sub-angular, 36,1%), immediately followed by 2 (angular, 28,5%), 4 (sub-rounded, 20,0%), 1 (very angular, 10,1%), 5 (rounded, 4,2%) and lower occurrence of 6 (well rounded, 1,1%). We have also determined that some grain types tend to round more quickly in beach environments, such as plagioclase among the monocrystalline grains and Lvmi among the lithic grains population. Lipari’s beach sands are all better rounded on the eastern side which is the more protected part of the island. Alternatively, the sand grains on the east side of the island have alonger residence time on the beach.

Cataclasis and silt smear on normal faults in weakly lithified turbidites

Fault-seal analysis in sand-shale multilayers emphasises the role of shale smear without explicitly accounting for cataclasis. These processes produce low-permeability fault rock and are examined here for small displacement (0.001–70 m) normal faults displacing weakly lithified turbidites comprising ∼55–80% lithic grains. Late Miocene Mount Messenger Formation (MMF) turbidites from the North Island of New Zealand provide fault rock data over a range of scales from individual grains (∼0.1–350 μm) to the height of coastal cliffs (∼10–20 m). Fault rock and unfaulted source beds has been analysed using thin sections, SEM images, particle-size distribution (PSD) measurements and outcrops of faults mainly in cross section. Cataclasis associated with particle size and macroscopic porosity reduction of protolith sandstones commences at low fault shear strains (<1) and continues as fault displacement accrues. The relationship between particle-size reduction and displacement is non-linear with initial rapid cataclasis facilitated by disaggregation of weak lithic and altered feldspar grains along pre-existing grain defects (e.g., grain boundaries, fractures and altered cleavage planes). Silt smear, by contrast, is not accompanied by significant particle-size reduction and appears to have been achieved by intergrain slip and micro-faulting. Despite the occurrence of silt smear, cataclasis can produce a significant proportion (>50%) of the total fault-rock in sand-silt multilayers. The resulting fault-rock thickness varies by up to three orders of magnitude for a given fault displacement and at short distances (2–10 m) along individual faults. Variations in fault-rock thickness and associated cataclasis have the potential to modify the hydraulic properties of faults and may need to be accounted for in fault-seal analysis.

A method for automatic grain segmentation of multi-angle cross-polarized microscopic images of sandstone

Automatic grain segmentation of sandstone is to partition mineral grains into separate regions in the thin section, which is the first step for computer aided mineral identification and sandstone classification. The sandstone microscopic images contain a large number of mixed mineral grains where differences among adjacent grains, i.e., quartz, feldspar and lithic grains, are usually ambiguous, which make grain segmentation difficult. In this paper, we take advantage of multi-angle cross-polarized microscopic images and propose a method for grain segmentation with high accuracy. The method consists of two stages, in the first stage, we enhance the SLIC (Simple Linear Iterative Clustering) algorithm, named MSLIC, to make use of multi-angle images and segment the images as boundary adherent superpixels. In the second stage, we propose the region merging technique which combines the coarse merging and fine merging algorithms. The coarse merging merges the adjacent superpixels with less evident boundaries, and the fine merging merges the ambiguous superpixels using the spatial enhanced fuzzy clustering. Experiments are designed on 9 sets of multi-angle cross-polarized images taken from the three major types of sandstones. The results demonstrate both the effectiveness and potential of the proposed method, comparing to the available segmentation methods.

Mesoproterozoic rift sedimentation, fluid events and uranium prospectivity in the Cariewerloo Basin, Gawler Craton, South Australia

ABSTRACTThe Cariewerloo Basin formed in the Mesoproterozoic following assembly of the Gawler Craton, South Australia, and was filled by arenaceous redbeds of the Pandurra Formation. While previous regional-scale work reveals a basin with similar size and sedimentary fill to the Proterozoic Athabasca and Kombolgie basins that host unconformity-related uranium deposits, few details of the Cariewerloo Basin are known. In this study, stratigraphy, petrography, lithogeochemistry, stable isotope geochemistry and 40Ar/39Ar geochronology are integrated to clarify the depositional history of the Pandurra Formation, and to assess fluid events in the basin that could be linked to the formation of uranium deposits. In the study area, the Pandurra Formation was deposited in two eastward-thickening packages that terminate at faulted basement uplifts, interpreted as half-grabens that formed in a continental rift system as the eastern Gawler Craton underwent extension. Deposition occurred between 1575 Ma (latest Hiltaba Suite age) and ca 1490 Ma, the 40Ar/39Ar age of diagenetic illite in the basal Pandurra. Diagenesis involving fluids having δ18O and δ2H values between –2.1 and 3.6‰, and between –66 and –8‰, respectively, occurred at around 150°C. Protracted diagenesis preferentially occurred in the upper Pandurra Formation based on petrography and Pearce Element Ratios that show complete replacement of detrital lithic and feldspathic grains by diagenetic phyllosilicates, and younger 40Ar/39Ar ages between ca 1330 and 1200 Ma that record fluid events later into basin history. Conversely, the basal Pandurra Formation shows better preservation of detrital grains, and older 40Ar/39Ar ages around 1450 Ma that suggest these strata became closed to fluid flow earlier in basin history. Although, based on O-isotope ratios, fluid–rock interaction did not occur in the Cariewerloo Basin to the same extent as that in the Athabasca or Kombolgie basins, it is possible that a uranium deposit formed where the upper Pandurra Formation was in contact with metasedimentary basement units outside the present basin margins.

Mineralogy and spectroscopy of Owen Group sandstones, Australia: implications for the provenance, diagenesis, and origin of coloration

Mineralogical studies were conducted on sandstones of Late Cambrian-Early Ordovician Owen Group, southern West Coast Range, Tasmania, Australia. Several samples were analysed using conventional thin section and point counting techniques, while others were further analysed under a Scanning Electron Microscope with back scattering and Energy Dispersive X-ray Spectroscopy capabilities. The result of petrographic analyses includes provenance, possible sources and controls over coloration in sandstone samples. The sandstones are mainly comprised of quartz, mica, lithic grains (volcanic, metamorphic and sedimentary), clays, zircon, tourmaline, leucoxene, rutile, and chert grains. These were derived from a Proterozoic Tyennan metamorphic and a Middle Cambrian Mount Read Volcanic provenance. Following deposition, sediments underwent severe diagenesis including quartz overgrowth, iron coatings, haematite, chlorite and sericite alterations, linings and cements. Multi-phase haematite-related diagenesis is present and late stage hydrothermal fluid alterations assemblages have severely altered and colored the sediments.