Chert Fragments Research Articles

Sedimentological Properties of the Sand Dunes and Valley Sediments in Al-Muthanna, Southern Iraq

Sedimentological properties of the dunes and valley terrigenous sediments in Al-Muthanna Governorate, southern Iraq were carried out. Ten samples were collected, where five samples from both sand dunes, and valley sediments. Grain size analysis revealed that sand, silt and clay fractions are the constituents of these sediments. Sand fractions predominant in the dunes and the texture is classified as silty sand, whereas clay fractions dominate in the valley sediments, with sandy clay texture. The mineralogy is determined by X-ray diffraction, which revealed that quartz is the main mineral in both study areas, followed by calcite, feldspars in lesser amount and evaporates (gypsum) in minor component of the light minerals. Petrographically, monocrystalline quartz dominates over polycrystalline quartz in both areas. Rock fragments in the valley sediments are higher than in the sand dunes, which are comprised of carbonate, chert, igneous, metamorphic, evaporate, and mudstone rock fragments. Feldspars are approximately similar in the study areas and comprised mainly alkali feldspar (potash feldspar) and plagioclase. Petrogenically, the sand dunes occupy the quartzose-recycled field, while the valley sediments fall in transitional recycled fields represented recycled orogeny.

On the origin and significance of composite particles in mudstones: Examples from the Cenomanian Dunvegan Formation

AbstractDespite the fact that mud‐dominated composite particles have increasingly been recognized as important components of marine mudstones, the characteristics, types and origins of these composite particles remain poorly understood. This incomplete understanding of critical mudstone parameters (for example, depositional grain size, composition at the particle scale and provenance) severely limits the ability to accurately interpret the conditions (for example, depositional environment and climate) under which these rocks were deposited. Herein, detailed petrographic analysis was conducted in core samples from a transect of contemporaneous proximal to distal deposits from the Cenomanian Dunvegan Formation, a fluvial‐dominated delta system in the Western Canada Sedimentary Basin. Within the well‐developed depositional framework, mud‐dominated composite particles can be tracked along the depositional profile of the Dunvegan Formation from the fluvial to the marine realm (from incised valley to distal prodelta). By following these particles from source (hinterland erosion) to sink (offshore deposition), the origin and dispersal of various types of mud‐dominated composite particles in marine mudstones can be unequivocally determined. Mud‐dominated composite particles derived from a wide range of origins are differentiated. Allochthonous mud‐dominated composite particles, supplied by rivers draining the hinterland, comprise volcanic rock fragments (vitric or felsic in composition), chert fragments, shale lithics (clay‐mineral‐rich or quartz‐rich in composition), metamorphic rock fragments and chlorite/siderite clasts. Autochthonous mud‐dominated composite particles include rip‐up clasts (argillaceous, sideritic or siliceous in composition) formed from intra‐basinal erosion and contemporaneous floccules. The recognition of mud‐dominated composite particles shows that the Dunvegan prodelta ‘mudstones’ are much coarser grained and more heterogeneous in terms of their petrographic composition than the grain size of their component minerals would suggest. Results of this study call for a critical reappraisal of the composition and actual grain size of mudstones in general. Recognition of mud‐dominated composite particles can provide valuable insights to unravel their provenance, transport history and depositional setting, and will greatly enhance their utility as palaeoenvironmental archives.

Consumed tectonic plates in Southeast Asia: Markers from the Mesozoic to early Cenozoic stratigraphic units in the northern and central Philippines

Tectonic reconstruction models of Southeast Asia all invoke in the early Cenozoic the collision of Mesozoic oceanic plates, which have been fragmented, consumed along subduction zones or emplaced onto the overriding plate. However, with marked variations in these models, we reinvestigate the tectonic evolutionary landscape of Southeast Asia through the lens of Philippine geology. In particular, we present revisions to the more recent models by adopting the unique approach of integrating data that we have gathered for the past 17 years from the Upper Mesozoic to Lower Cenozoic stratigraphic formations in northern and central Philippines. These formations, which resulted mainly from submarine mass transport processes, evolved in response to early arc-related processes of oblique subduction, frontal wedge deformation, terrane accretion and strike slip faulting. Additional key constraints for the revisions include: (1) the timing of early Cenozoic magmatism in eastern Luzon; (2) the spatial distribution of the Upper Mesozoic to Lower Cenozoic sedimentary formations with respect to other key features (e.g. distribution of Mesozoic ophiolite fragment and continent-derived rocks) in the Philippine arc; (3) the paleolatitudinal position of Luzon and surrounding regions and; (4) the movement of the surrounding plates since the Late Mesozoic.In revising previous models, a subduction zone (proto-East Luzon Trough) separating Benham Plateau and the Philippine arc was placed to explain the spatial distribution of Eocene arc-related formational units and Mesozoic ophiolite materials comprising the accretionary complex east of Luzon at ~40 Ma period. During this time, Luzon was modeled at the southern margin of the East Asia Sea or the proto-Philippine Sea Plate. Mesozoic ophiolitic complexes that line the eastern Philippine arc as well as the ophiolitic and pelagic limestone and chert fragments included in the arc-derived, Eocene formations in Luzon could very well be traces of the now consumed East Asia Sea-proto-Philippine Sea Plate. Within the same period, we modified the Palawan Microcontinental Block (PCB), positioned at the trailing edge of the proto-South China Sea to include the whole Mindoro island and the Romblon Island Group in Central Philippines. Pieces of the consumed Izanagi Plate, the proto-South China Sea and continental-derived sediments from Asia mainland are reflected in the Mesozoic metamorphic rocks and the Eocene sedimentary formation in western Mindoro. Finally, we model Cebu, Bohol and Negros islands in Central Philippines as being at the leading oceanic edge of the Indo-Australian Plate during the early Cenozoic. With the northward movement of the Indo-Australian plate and the trench roll back of the southern margins of the Philippine Sea Plate, the accretion of the Cretaceous arc-related rocks of Cebu, Bohol and Negros onto the Philippine arc by the end of Eocene or early Oligocene becomes a possibility.

Landslide Zoning and Mitigation in Clapar Area, Madukara District, Banjarnegara Regency, Central Java

Landslide disaster is one of the most frequent disasters in Clapar, Banjarnegara. Landslide is a recurring event and there is no suitable method to respond the threat until now, so that preventive stage have not been maximized. The scienstist explains that the movement of land in the area is very active, but the level of activity that can cause landslides can’t be known qualitatively or quantitatively. The method that used to achieving these purposes is geology and landscape morphology mapping in areas that have been and have possibility landslide disaster. Factors that controls the landslide motion are geomorphology, rock type, geological structure and land use, while the triggers are water infiltration and rock fractures. Slope in Clapar is 30 - 70% and the rock type is clay with quarzite and chert fragments from Totogan Formation, volcanic breccia from Tapak Formation, and polimictic breccia from Ligung Formation. Geological structure that found is right slip fault (Rickard, 1972) with the strike and dip of fault is N 359oE/79o. The land usage are pine forest, salak farm, and settlement. Angle of shear strength from direct shear test soil sample in Clapar slope is 27o with safety factor value is 1.045 included in the critical class. According to those existing datas, Clapar included into area that are very vulnerable to landslides. It is because this area was affected by major slip fault and steep angle. The most land usages is salak farms which is the plants has fibrous root that control the properties of soil. It makes the water easily infiltrate into the soil that causes increase the weight of the soil and make the plane become slippery.

Mineralogy of Duné s Sand of Missan Dune Fields Southeastern Iraq

The aim of this study is to define the mineralogical composition of Missan dune fields South Eastern of Iraq, and try to determine the origin or the source of these dunes .
 Three main types of dunes were recognized in the studied area, these are: barchanoid ridge, barchans and nabkha dunes. The direction of these dunes is northwest to southeast.
 The dunes in Missan dune fields were divided into three fields according to the vari- eties in the composition and geographical position in the studied area, these three fields are: Al-Manziliyah, Middle Chailat, and Said Subair dune fields.
 9 samples were collected from these fields, the samples were separated into light and heavy fraction by heavy liquids. The light fraction composed from quartz, feldspar, and rock fragments, the rock fragments composed mainly of carbonate, chert, igneous, and metamorphic rock fragments .
 The heavy minerals are mostly opaques, chlorite, pyroxenes, amphiboles, epidotes, zircon, garnet, muscovite, biotite, kyanite, staurolite, and rutile .
 The percentages of heavy minerals fraction to light minerals fraction was very vari- able in which the percentage of heavy minerals in Sayid Subair field are higher than the percentage in Al-Manziliyah field.The source area of these sediments is the recent sed- iment that deposited in the flood plain and traces of the Tigris river and the outcrops of upper Miocene-Pliocene rocks in the eastern of studied area (Himreen Anticline and Zagros Mountain).

Kovadúsulásos és alumíniumdúsulásos mállási és üledékképződési folyamatok a kora-albai folyamán a Csehbányai-medencében

The tectonically controlled uplift affecting various sectors of the Csehbanya Basin in early Aptian times seems to have been very inhomogeneous. As a result, individual, tectonically controlled blocks show very different denudation histories reflected by the basal layers of the Albian cover sequence (Tes Clay-Marl Formation) postdating the uplift-cumerosion event. At places where the Tes Fm is underlain by Jurasssic to Aptian formations these basal layers developed in the form of the clastic.Kepekő Member rich in chert fragments. At other places where the immediate bedrock of the Tes Fm is Late Triassic limestone or limestone with intercalated dolomitic layers, at the unconformity surface there are often bauxites or Al-rich clays belonging to the Alsopere Bauxite Formation. This suggests that the pre-Tes Fm. relief must have been highly variegated. Therefore both the depositional and the early diagenetic environments of the Alsopere Bauxite Fm and the clastic, cherty Kepekő Member could have been quite different. The differences may reflect different climates and/or different paleoenvironmental conditions. On the other hand since they both belong to one and the same major regional unconformity they have also some characters in common. These are: (1) the mostly local source material,(2) the predominantly oxidized nature of the sediment, (3) the abundance of “slack-water” microfacies, (4) sphericity, roundness and sorting of the coarse detrital material. However, unlike the bauxite, the material of the Kepekő Member is obviously of local origin. It is essentially an in situ dissolution residue without any signs of transport-related concentration/separation of even the chert fragments. Limestone-fragments in the bauxite on the other hand must have been added to the fine-grained sediment in the course of some transport/redeposition, to the distance of which we have no clue. The original source of the prebauxitic material is not known, either. Had it been of local origin it could not have been of the same age as the cherty Kepekő Member since the climatic and/or hydrologic conditions favourable for bauxitisation are substantially different from those favouring the accumulation of the silica-rich sediments, particularly because the latter show even some degree of silica mobilization and reprecipitation.

Petrography and Geochemistry of Archaean Greywackes from Northern Part of the Dharwar-Shimoga Greenstone Belt, Western Dharwar Craton: Implications for Nature of Provenance

ABSTRACT Greywackes (Dharwar greywackes) are the most abundant rock types in the northern part of the Dharwar-Shimoga greenstone belt of the western Dharwar craton. They are distinctly immature rocks with poorly-sorted angular to sub-angular grains, comprising largely quartz, plagioclase feldspar and lithic fragments of volcanics (mafic+felsic), chert and quartzite, with subordinate biotite, K-feldspar and muscovite. They are characterized by almost uniform silica (59.78-67.96 wt%; av. 62.58), alkali (4.62-7.35 wt%; av. 5.41) contents, SiO2/Al2O3 (3.71-5.25) ratios, and compositionally are comparable to the andesite and dacite. As compared to Ranibennur greywackes, located about 100 km south of Dharwad in the Dharwar-Shimoga greenstone belt, the Dharwar greywackes have higher K2O, CaO, Zr, Y, ΣREE, Th/Sc, Zr/Cr, La/Sc and lower Sr, Cr, Ni, Sc, Cr/Th values. The chondrite normalized patterns of Dharwar greywackes are characterized by moderately fractionated REE patterns with moderate to high LREE enrichment, with almost flat HREE patterns and small negative Eu anomalies, suggesting felsic dominated source rocks in the provenance. The frame work grains comprising felsic and mafic volcanics, feldspars and quartz suggest a mixed source in the provenance. The moderate CIA values ranging between 57 and 73, indicate derivation of detritus from fresh basement rocks and from nearby volcanic sources.The mixing calculations suggest that the average REE pattern is closely matching with a provenance having 40% dacite, 30% granite, 20% basalt and 10% tonalite. These greywackes were deposited in a subduction related fore-arc basin than a continental margin basin. Their La/Sc ratios are high (av. 4.07) compared to the Ranibennur greywackes (1.79), suggesting that the greywackes of the northern part of the basin received detritus from a more evolved continental crust than the greywackes of the central part of the Dharwar-Shimoga basin.

The age of fossil StW573 (‘Little Foot’): An alternative interpretation of 26Al/10Be burial data


 
 
 Following the publication (Granger DE et al., Nature 2015;522:85–88) of an 26Al/10Be burial isochron age of 3.67±0.16 Ma for the sediments encasing hominin fossil StW573 (‘Little Foot’), we consider data on chert samples presented in that publication to explore alternative age interpretations. 10Be and 26Al concentrations determined on individual chert fragments within the sediments were calculated back in time, and data from one of these fragments point to a maximum age of 2.8 Ma for the sediment package and therefore also for the fossil. An alternative hypothesis is explored, which involves re-deposition and mixing of sediment that had previously collected over time in an upper chamber, which has since been eroded. We show that it is possible for such a scenario to yield ultimately an isochron indicating an apparent age much older than the depositional age of the sediments around the fossil. A possible scenario for deposition of StW573 in Member 2 would involve the formation of an opening between the Silberberg Grotto and an upper chamber. Not only could such an opening have acted as a death trap, but it could also have disturbed the sedimentological balance in the cave, allowing unconsolidated sediment to be washed into the Silberberg Grotto. This two-staged burial model would thus allow a younger age for the fossil, consistent with the sedimentology of the deposit. This alternative age is also not in contradiction to available faunal and palaeomagnetic data.
 
 
 
 
 Significance: 
 
 
 
 Data on chert samples taken close to StW573 impose a maximum age for the fossil of 2.8 Ma – younger than the 3.67 Ma originally reported. We propose and explore a two-stage burial scenario to resolve the inconsistency and to reopen the discussion on the age of fossil StW573.
 
 
 

Opal‐CT in chert beneath the toe of the Tohoku margin and its influence on the seismic aseismic transition in subduction zones

AbstractThick accumulation of chert is a ubiquitous feature of old oceanic plates at convergent margins. In this study, we investigate chert fragments recovered by the Integrated Ocean Drilling Program expedition 343 at the Japan Trench where the 2011 Tohoku‐Oki earthquake (Mw 9.0) occurred. This sample provides a unique opportunity to investigate in situ chert diagenesis at an active subduction margin and its influence on the kinematics of megathrust faulting. Our mineralogical analyses revealed that the chert is characterized by hydrous opal‐CT and may therefore be highly deformable via pressure solution creep and readily accommodate shear strain between the converging plates at driving stresses of kilopascal order. As chert diagenesis advances, any further deformation requires stresses of >100 MPa, given the increasing transport distances for solutes as represented in cherts on land. The chert diagenesis is thus related to the mechanical transition from a weakly to strongly coupled plate interface at this margin.

Multi-emission luminescence dating of heated chert from the Middle Stone Age sequence at Sodmein Cave (Red Sea Mountains, Egypt)

Sodmein Cave in Egypt is one of the rare archaeological sites in north-eastern Africa conserving human occupation remains of a period most relevant for the ‘Out of Africa II’ hypothesis. This underlines the need for establishing a chronological framework for the more than 4 m of stratified sediments ranging from the Middle Stone Age (MSA) to the Neolithic. The lowest layer J hosts huge fireplaces, from which we report luminescence ages of heated chert fragments unearthed from different depths. The ‘multi-emission’ dating approach – using both the blue and red TL of each specimen as well as the OSL emission of one sample – allowed identifying the most reliable ages. Samples yield ages between <121 ± 15 ka (maximum age) and 87 ± 9 ka. These data evidence human presence at the site during MIS 5. For integrating Sodmein Cave into the actual discussion of the dispersal patterns of modern humans and to identify potential connections with other sites in the Nile Valley and in the Middle East, a sound and reliable chronology is indispensable.

In the footsteps of Pliny: tracing the sources of Garamantian carnelian from Fazzan, south-west Libya

References in the ancient sources indicate that the Libyan desert was a source of ‘carbunculi’: semi-precious red stones and gemstones variously interpreted as ruby, garnet and spinel, amongst others. While gemstones are not attested in the geological strata of Fazzan (south-west Libya), a range of silica-based stones including chert, chalcedony, agate and carnelian are known to originate in this area, linked to an early civilisation known as the Garamantes. It has been long proposed that the geochemical signature and the variations in the relative proportions of quartz:moganite phases can be used to distinguish between groups of stones of different origin. The proposed methodology was tested on a number of archaeological samples from the Garamantian sites of Jarma (ancient Garama) and Saniat Jibril, in Fazzan. Fragments of chert, carnelian and amazonite found at the two sites have been identified as raw materials associated with beadmaking. Trace elemental data obtained by LA-ICP-MS were combined with mineralogical data obtained by X-ray powder diffraction and Raman spectroscopy on the same samples and a group of reference samples. The dataset has been compared with the available literature and data from other localities around the world. To this purpose a preliminary database of silica-based materials was established for provenance work. Based on the scarce data available in the literature, the importation of these stones from Eastern localities such as India may be ruled out. The measured data on archaeological samples and debitage allow us to define a reliable reference group of parameters for materials from Fazzan, which are likely to be derived from a unique geological source. The methodology should be extended and compared with cherts and carnelians from a range of Mediterranean and Sub-Saharan sites. This characterisation work is a tool of high potential utility for a new investigation of ancient contact and trade across the Trans-Saharan zone.

Origin of gypsiferous intrusions in the Hawasina Window, Oman Mountains: Implications from structural and gravity investigations

ABSTRACT Gypsiferous intrusions are exposed in road-cuts in the south-central Hawasina Window in the central Oman Mountains. They are located at lower structural levels in the allochthonous Hawasina Complex and lie along faults that cut Upper Cretaceous structures related to the obduction of the Semail Ophiolite and Hawasina Complex deep-water sediments onto the Arabian Plate. The intrusions form gypsiferous pods that are up to 200 m long, in which the gypsum occurs as a dark, fine-grained matrix that contains a pervasive network of anastomosing veins of gypsum and anhydrite. The intrusions contain abundant sub-angular to sub-rounded litharenites, and less common fragments of chert and fine-grained limestone. Although these clast types are undated, their petrographic characteristics suggest they originate from the local Hawasina (Hamrat Duru Group) country rock. Very well-rounded pebbles and cobbles of feldspathic litharenites, some of which show a well-developed cleavage, and rarer cobbles of well-rounded vein quartz appear to have come from the basement. Gravity investigations indicate salt diapirs are not present beneath the Hawasina Window. Instead, the gypsiferous intrusions are interpreted as having been brought up from depth during compression to form disconnected pods along deep-rooted faults, bringing with them small amounts of the basement country rock. Strontium isotope analysis and regional considerations, in particular the distribution, age and nature of other evaporite units on the eastern Arabian Plate, suggest the gypsum may have its origins in the Neoproterozoic (Ediacaran) to lower Cambrian Ara Group evaporites, perhaps from a previously unknown extension of the Fahud Salt Basin beneath the Hawasina thrust sheets.

Petrography, mineralogy and geochemistry of Cretaceous sediment samples from western Khorat Plateau, Thailand, and considerations on their provenance

At Mo Hin Khao on the western flank of Khorat Plateau, Thailand, the Phra Wihan Formation reveals litharenite and sublitharenite with some subarkose and arkose. A cuesta in the eroded sedimentary sequence exhibits spectacular rock pillars of considerable geotourist potential. The rock sequence is high in silica (SiO2 67–98wt%) and contains quartz, mica, magnetite, chert fragments and accessory minerals such as zircon and tourmaline and amphibole species. These accessory minerals suggest felsic rocks, such as granite, granodiorite and pegmatite, were sources for the sandstones. Geochemical analyses of the sedimentary sequence suggest that source rocks may lie in the passive continental margin, before sediment transport and deposition in the Khorat Basin by rivers flowing across a large flood plain. Many depositional sequences/episodes formed thick beds of cross bedded clastic rocks. A high average maturity index (>5) indicates sedimentary reworking/recycling. Chemical Index of Alteration (CIA) values range from 47 to 98, suggesting variable chemical weathering within the source area rocks, largely representing moderate to high degrees of weathering. The average CIA value of these sediments (78) suggests that relatively extreme alteration factors were involved.

Magmatic occurrences in the Central Arava (southern Israel) based on Geology and Magnetometry

The Eshet Ridge is located in the Central Arava near Wadi Paran. Geological data were collected using ground magnetic surveys and petrophysical measurements (magnetic susceptibility and density). The goal was to reveal the structure and nature of a concealed magmatic body under the ridge. Integrated gravity and magnetic interpretation together with seismic reflection data (including 2¾ modeling) indicated the presence of a deep-seated basic magmatic intrusion. Occurrence of Fe-mineralized rocks along the ridge supports subsurface data. The magmatic body was intruded in the hard Turonian rocks of the Eshet Ridge. Basic magmatic exposures dating from the Early Miocene were evident along Wadi Ashosh (in the eastern Negev) and its margins 18km. to the north of the ridge. A new outcrop of basic magmatic intrusion was found southwest of the Tzukim settlement. Magnetic measurements indicated a similarity between their magnetic pattern and the Ashosh basic magmatics, which were dated to 20.4±0.7Ma. Volcanoclastic tuff pebbles (magnetic susceptibility of around 2–7*10−3SI) surrounded by conglomerate were discovered at two sites in Wadi Demma and Wadi Menuha (streams that drain near the Eshet Ridge and 2km south of it, respectively). The conglomerates contained mostly limestone, chalk and chert fragments; not more than 5% of which were volcanic pebbles. Two pebbles were dated to 24.4±0.7and 21.5±0.5Ma. The magmatic outcrops in Wadi Ashosh, the magmatic dyke near Tzukim and the volcanic purple and black pebbles near Wadi Menuha were all dated to the very Late Oligocene–Early Miocene. The magmatic body identified in this study under the Eshet Ridge was termed the Eshet Intrusion and is connected to, and even the source of, all these phenomena.

Drilling the Waqf as Suwwan impact structure

The about 6-km diameter, near-circular Waqf as Suwwan structure located at E36°48′/N31°03′ in eastern Jordan was only recently recognized as a somewhat eroded, complex impact structure. Surface geological mapping, geophysical interpretation, remote sensing, and petrographic and mineralogical analyses have been carried out to understand the structure. In particular, the complex geology of the remnant of the central uplift has been scrutinized. A recent drilling project afforded an opportunity to expand the investigation of the structure to previously inaccessible strata of the ring syncline in the environs of the central uplift. Three boreholes were drilled, two to 140 and 110 m depth to the north and outside of the central uplift, and a further short hole to 5 m depth into the innermost part of the central uplift. Preliminary assessment of these cores has revealed the presence of around 11 m of fluvial breccias (wadi deposit) that are dominated by chert fragments at the top of the syncline fill. This is underlain by a normal succession of late Maastrichtian to Campanian strata. A variety of microstructures such as fracturing with vertical, as well as inclined at 45° and 30° fractures occurs throughout the cores. Some joints have slickensides along their walls. Limestone and marly limestone constitute the most abundant rocks in the boreholes. Distinct shock deformation effects are entirely lacking in the cores from the syncline. These observations are interpreted as a result of substantial erosion of the impact structure down to a level within the crater floor. The microstructures and the preliminary results of the analyses of sediment ages, textures, and compositions (nanofossils and sediment mineralogy) show that sediments as old as Campanian and as young as late Maastrichtian were affected by the impact. Unfortunately, the drilling did not expose any lithologies such as impact melt breccias that could lend themselves to absolute chronological analysis for a better constraint of the impact age.

Late Triassic, Early and Middle Jurassic Radiolaria from ferromanganese-chert ‘nodules’ (Angelokastron, Argolis, Greece): evidence for prolonged radiolarite sedimentation in the Maliac-Vardar Ocean

In the Argolis, the Basal Sequence, constituting the eastern Pelagonian margin which bordered the Maliac-Vardar oceanic domain, includes shallow-water carbonates of Late Triassic-Early Jurassic, condensed pelagic limestones of Early-Middle Jurassic, radiolarian cherts of late Middle-Late Jurassic age and siliceous mudstones and sandstones rich in ophiolite fragments. Up-section, coarse breccias, including clasts of boninites derived from the ophiolite obducted onto the Pelagonian margin in Late Jurassic times crop out. Near Angelokastron a small quarry exposes pervasively sheared dark reddish-brown, radiolarian-bearing cherty shales with disrupted fragments of chert and chert nodules impregnated by ferro-manganese oxides. These shales occur in the footwall of a thrust bringing them into contact with the Pantokrator Limestone of the Basal Sequence. We collected more than 30 samples of the chert fragments and the shaly matrix. Thirteen nodules and one matrix sample yielded determinable radiolarians. Low to non-detectable concentrations of trace metals such as Co, Cr, Cu, Ni, Zn, and Pb indicate a hydrothermal origin of the ferro-manganese mineralization. The radiolarian taxa found indicate four age groups for the nodules that are embedded in the siliceous shale matrix that yielded a Middle Jurassic age (middle Bathonian). The first group includes a nodule of Late Triassic age (late Norian to Rhaetian); the second group nodules of Early Jurassic age (late early to late Pliensbachian and probably middle-late Toarcian); the third group nodules of early Middle Jurassic age (Aalenian–Bajocian); the last group finally includes nodules of late Middle Jurassic age (Bajocian–Bathonian). The presence of Upper Triassic to Middle Jurassic Mn-impregnated chert nodules in a Middle Jurassic matrix indicates a deep oceanic environment of deposition outside the Pelagonian realm (easternmost Adria Plate), which at that time was a shallow-water carbonate platform with a thin pelagic limestone cover. The chert nodules are with all certainty derived from the oceanic Maliac-Vardar domain and were, together with their host formation, tectonically emplaced onto the Pelagonian margin. We speculate that these nodules, more lithified than their matrix, were exhumed on the slope of an intra-oceanic accretionary wedge and were redeposited in the Middle Jurassic siliceous mudstones on the floor of the subducting Maliac-Vardar Ocean.

Biogenically enhanced permeability: A petrographic analysis of Macaronichnus segregatus in the Lower Cretaceous Bluesky Formation, Alberta, Canada

A detailed petrographic analysis was conducted on several core samples from the Cretaceous-aged Bluesky Formation in the La Glace area in Alberta, Canada. Within the gas-producing zone of these upper shoreface sediments, a fine- to mid-medium-grained chert-rich litharenite is intensely bioturbated with Macaronichnus segregatus. Petrographic analysis showed that the burrow mantle is generally lined with dark-colored, iron-rich (mainly chert, shale clasts, and organic grains) fragments, whereas the burrow fill contained mainly quartz and light-colored chert fragments. The reason for the dark-colored grain segregation of the tracemaker is unclear, but in the Bluesky Formation presented in this study, grain segregation has improved the reservoir quality by effectively resorting compaction- and cement-resistant chert and quartz into the burrow fill. Primary reservoir quality can be preserved in the presence of chert as pore-occluding quartz overgrowths do not form on chert fragments as they do on monocrystalline quartz, thus leaving open, well-connected primary pores and hence elevated permeability. Chert fragments are resistant to the effects of mechanical compaction and are not easily squeezed into adjacent pore spaces as are ductile rock fragments. Further research is needed to test the chemical constituents of the grains that modern tracemakers ingest. This may lead to a better understanding of why sands burrowed with Macaronichnus and similar burrowed sediment can have elevated reservoir quality.

Micro‐scale (1.5 µm) sulphur isotope analysis of contemporary and early Archean pyrite

We present a method for in situ sulphur (S) isotopic analysis of significantly small areas (1.5 microm in diameter) in pyrite using secondary ion mass spectrometry (NanoSIMS) to interpret microbial sulphur metabolism in the early earth. We evaluated the precision and accuracy of S isotopic ratios obtained by this method using hydrothermal pyrite samples with homogeneous S isotopic ratios. The internal precision of the delta(34)S value was 1.5 per thousand at the level of 1 sigma of standard error (named 1SE) for a single spot, while the external reproducibility was estimated to be 1.6 per thousand at the level of 1 sigma of standard deviation (named 1SD, n = 25). For each separate sample, the average delta(34)S value was comparable with that measured by a conventional method, and the accuracy was better than 2.3 per thousand. Consequently, the in situ method is sufficiently accurate and precise to detect the S isotopic variations of small sample of the pyrite (less than 20 microm) that occurs ubiquitously in ancient sedimentary rocks. This method was applied to measure the S isotopic distribution of pyrite within black chert fragments in early Archean sandstone. The pyrite had isotopic zoning with a (34)S-depleted core and (34)S-enriched rim, suggesting isotopic evolution of the source H(2)S from -15 to -5 per thousand. Production of H(2)S by microbial sulphate reduction (MSR) in a closed system provides a possible explanation for both the (34)S-depleted initial H(2)S and the progressive increase in the delta(34)S(H2S) value. Although more extensive data are necessary to strengthen the explanation for the origin of the MSR, the results show that the S isotopic distribution within pyrite crystals may be a key tracer for MSR activity in the early earth.

Stratigraphy of the PB-1 Well, Nopal I Uranium Deposit, Sierra Peña Blanca, Chihuahua, Mexico

The Nopal I site in the Peña Blanca uranium district has a number of geologic and hydrologic similarities to the proposed high-level radioactive waste repository at Yucca Mountain, making it a useful analogue to evaluate process models for radionuclide transport. The PB-1 well was drilled in 2003 at the Nopal I uranium deposit as part of a DOE-sponsored natural analogue study to constrain processes affecting radionuclide transport. The well penetrates through the Tertiary volcanic section down to Cretaceous limestone and intersects the regional aquifer system. The well, drilled along the margin of the Nopal I ore body, was continuously cored to a depth of 250 m, thus providing an opportunity to document the local stratigraphy. Detailed observations of these units were afforded through petrographic description and rockproperty measurements of the core, together with geophysical logs of the well. The uppermost unit encountered in the PB-1 well is the Nopal Formation, a densely welded, crystal-rich, rhyolitic ashflow tuff. This cored section is highly altered and devitrified, with kaolinite, quartz, chlorite, and montmorillonite replacing feldspars and much of the groundmass. Breccia zones within the tuff contain fracture fillings of hematite, limonite, goethite, jarosite, and opal. A zone of intense clay alteration, encountered in the depth interval 17.45-22.30 m, was interpreted to represent the basal vitrophyre of this unit. Underlying the Nopal Formation is the Coloradas Formation, which consists of a welded lithic-rich rhyolitic ash-flow tuff. The cored section of this unit has undergone devitrification and oxidation, and has a similar alteration mineralogy to that observed in the Nopal tuff. A sharp contact between the Coloradas tuff and the underlying Pozos Formation was observed at a depth of 136.38 m. The Pozos Formation consists of poorly sorted conglomerate containing clasts of subangular to subrounded fragments of volcanic rocks, limestone, and chert. Three thin (2-6 m) intervals of intercalated pumiceous tuffs are present within this unit. The contact between the Pozos Formation and the underlying Cretaceous limestone basement was encountered at a depth of 244.40 m. The water table is located at a depth of ~223 m. Several zones with elevated radioactivity in the PB-1 core occur above the current water table. These zones may be associated with changes in redox conditions that could have resulted in the precipitation of uraninite from downward-flowing waters transporting U from the overlying Nopal deposit. All of the intersected units have low (typically submillidarcy) matrix permeability, thus fluid flow in this area is dominated by fracture flow. These stratigraphic and rock-property observations can be used to constrain flow and transport models for the Peña Blanca natural analogue.

Long and continuous or short term and occasional occupation? The human use of Leang Sarru rock shelter in the Talaud Islands, northeastern Indonesia

Leang Sarru is a small rockshelter on the coast of Salebabu Island (Talaud group, northeastern Indonesia). Excavation has revealed a circa 100 cm thick cultural deposit, consisting of at least four observable stratigraphic layers and containing pottery (within the uppermost layers), lithic materials, food shells, and occasional pieces of ochre. C14 dates suggest c. 30,000 years of human occupation here. Although it is possible to establish a general cultural sequence for this site (three cultural phases are proposed), difficulties arise when one attempts to reconstruct more detailed habitation processes, especially in the second phase (21,000-10,000 BP). In this time span, it is unclear whether the cultural deposit is the result of relatively continuous habitation over a long period of time, or whether it resulted from a single but very intensive period of use of the site as a lithic workshop. After considering the available data on lithic technology, ecofactual evidence, and the provenances of the absolute dates, it is possible to suggest a general history of human use of the cave from c. 30,000 years ago up to the ethnographic present. Leang Sarru is a small rockshelter on the coast of Salebabu Island of the Talaud group in northeastern Indonesia (Fig. 1). It is situated in an uplifted coral limestone block about 400 m inland from the beach and about 15 m above the sea level in the middle of a clove plantation. It faces northeast and has a sheltered area of about 5 m by 3 m, with a curving ceiling about 2.5 m high at the dripline. The floor is dry and flat in the sheltered area, but slopes down slightly from the dripline outwards. Fragments of reddish chert were scattered quite abundantly in and around the rockshelter when it was discovered. In 1995, a grid of 1X1 m squares was laid out in the sheltered area and two adjacent squares, denoted B2 and C2, were excavated by 10 cm spits down to 90 and 80 cm respectively below the surface. Seven C14 dates were derived from the site ranging from circa 30,000 to 9,000 years ago (Table 1). CULTURAL PHASES AND DATING At least four stratigraphic layers could be recognized during excavation in Leang Sarru (Fig. 2). However, it turned out to be difficult to follow the layers when digging, since the boundaries between them were sometimes gradual. The uppermost layer is a topsoil about 10-25 cm thick. It is brownish grey in colour and fairly compact. Underneath the topsoil lies the second layer, a dark brown fairly compact deposit. It has an uneven thickness of about 20-45 cm. The third layer consists of a loose pale brown sandy deposit. This layer is about 5-20 cm thick and becomes thinner towards the front of the rockshelter. The basal layer comprises a hard calcareous deposit with relatively small quantities of archaeological materials. Figure 1. The location of Leang Sarru, on the eastern coast of Salebabu Island. Those four stratigraphic layers correspond fairly closely to the distribution of archaeological materials (Table 1) and can be grouped into three cultural phases. TANUDIRJO: HUMAN USE OF LEANG SARRU ROCKSHELTER, TALAUD ISLANDS 16 Table 1. Distribution of cultural materials in Leang Sarru, Square B2 (top) and Square C2 (bottom)