Sedimentary Laminations Research Articles

Early Cretaceous solar cycles recorded in lacustrine laminations in North China

Solar cycles are important moderators of the Earth’s global climate system. Although modern-day solar cycles are well known, they have been less studied over geological time. High-resolution records such as varves have been previously used for reconstructing solar cycles from the Paleoproterozoic through Quaternary. In this paper, very fine (<1 mm) sedimentary laminations of the Early Cretaceous Yixian Formation in Xiushui Basin were studied in Northern Liaoning Province, North China. Two different microfacies of the striped shale in the Third Member of the Yixian Formation were identified. These include the light-gray to gray siltstone (Mf 1) and the gray to black organic-rich mudstone (Mf 2). Laminations of Mf 2 are mainly made of biofilms. Sub-millimeter scaled couplets of biofilm and siliciclastic-rich sublamina record seasonal growth and withering of microbial mats during the warm season (summer) and cold season (winter), respectively. Evolutionary spectral analyses of three binary rank series (the binary boxcar series, triangle series, and midpoint-triangle series), varve couplet thickness and gray scale image data (gray data) show multiple periodicities consistent with solar cycles, including the robust Schwabe sunspot cycle (10.0–10.6 year) and solar Bruckner cycle (31.0–40.6 year), and relatively weaker signals for the solar Hale cycle (21.9 year) and 16.5-year solar cycles that have been linked to solar magnetic activity. Solar cycles recognized in this paper indicate the total solar irradiance (TSI) influenced microbial mat growth in the Early Cretaceous in North China. Further, we extend our new record with a compilation of varve-recorded sunspot cycles throughout geological time to show that the 11-year Schwabe sunspot cycle and the 22-year Hale cycle have persisted since the Paleoproterozoic.

Chondrules from high-velocity collisions: thermal histories and the agglomeration problem

ABSTRACT We assess whether chondrules, once-molten mm-sized spheres filling the oldest meteorites, could have formed from super-km s−1 collisions between planetesimals in the solar nebula. High-velocity collisions release hot and dense clouds of silicate vapour which entrain and heat chondrule precursors. Thermal histories of CB chondrules are reproduced for colliding bodies ∼10–100 km in radius. The slower cooling rates of non-CB, porphyritic chondrules point to colliders with radii ≳ 500 km. How chondrules, collisionally dispersed into the nebula, agglomerated into meteorite parent bodies remains a mystery. The same orbital eccentricities and inclinations that enable energetic collisions prevent planetesimals from re-accreting chondrules efficiently and without damage; thus the sedimentary laminations of the CB/CH chondrite Isheyevo are hard to explain by direct fallback of collisional ejecta. At the same time, planetesimal surfaces may be littered with the shattered remains of chondrules. The micron-sized igneous particles recovered from comet 81P/Wild-2 may have originated from in-situ collisions and subsequent accretion in the proto-Kuiper belt, obviating the need to transport igneous solids across the nebula. Asteroid sample returns from Hayabusa2 and OSIRIS-REx may similarly contain chondrule fragments.

Development of a greenalite-silica shuttle during incursions of hydrothermal vent plumes onto Neoarchean shelf, Hamersley region, Australia

Deep-water cherts are common in early Precambrian successions. Their mode of formation is unclear but proposed mechanisms include seawater silica precipitation, silica adsorption onto iron oxide particles and silica replacement of seafloor sediments. Here we document ≥80 laminated chert beds (0.02–7 m thick) in 2.56–2.49 Ga shelf sequences in drill-hole ABDP9 from the Hamersley Group, Australia. The laminated chert beds, including thin intervals of banded iron formation (BIF), are concentrated in six major intervals that first appear ca. 75 m.y. before deposition of the 2.49 Ga Brockman Iron Formation. The chert beds do not preserve textures indicative of seafloor replacement of precursor grains but of a chemical sediment comprising greenalite nanoparticles enclosed in silica cement with syneresis cracks that truncate sedimentary lamination. The greenalite is interpreted to have precipitated from dissolved Fe(II) and SiO2(aq) transported by hydrothermal plumes. Its ubiquity in laminated chert cement and the capacity of clay minerals to sorb silica, suggest the operation of a greenalite-silica shuttle by which excess dissolved silica in the seawater-plume mixture adsorbed onto settling Fe(II)-silicate nanoparticles. Upon deposition, pore-water silica concentrations surpassed the threshold for cementation due to the drawdown of silica into sediment pore waters and dewatering of underlying siliceous muds. In contrast, the lack of non-ferruginous chert beds in deep-water shelf sediments suggests that silica concentrations were lower in non-plume seawater. The appearance and disappearance of ferruginous cherts in shelf sequences is interpreted to record the waxing and waning of distal vent plumes. Deposition of chert beds starting at 2.56 Ga culminated with the development of thick, laterally extensive BIFs after 2.49 Ga, signalling enhanced ocean ridge volcanism and the transgression of vent plumes across a drowned shelf. The disappearance of basinal cherts and major BIFs after the Great Oxidation Event (2.45–2.3 Ga) may record the disruption or shutdown of the oceanic greenalite-silica shuttle.

Quantifying the anisotropy and tortuosity of permeable pathways in clay-rich mudstones using models based on X-ray tomography

The permeability of shales is important, because it controls where oil and gas resources can migrate to and where in the Earth hydrocarbons are ultimately stored. Shales have a well-known anisotropic directional permeability that is inherited from the depositional layering of sedimentary laminations, where the highest permeability is measured parallel to laminations and the lowest permeability is perpendicular to laminations. We combine state of the art laboratory permeability experiments with high-resolution X-ray computed tomography and for the first time can quantify the three-dimensional interconnected pathways through a rock that define the anisotropic behaviour of shales. Experiments record a physical anisotropy in permeability of one to two orders of magnitude. Two- and three-dimensional analyses of micro- and nano-scale X-ray computed tomography illuminate the interconnected pathways through the porous/permeable phases in shales. The tortuosity factor quantifies the apparent decrease in diffusive transport resulting from convolutions of the flow paths through porous media and predicts that the directional anisotropy is fundamentally controlled by the bulk rock mineral geometry. Understanding the mineral-scale control on permeability will allow for better estimations of the extent of recoverable reserves in shale gas plays globally.

Sedimentary laminations in the Isheyevo (CH/CBb) carbonaceous chondrite formed by gentle impact-plume sweep-up

Prominent macroscopic sedimentary laminations, consisting of mm- to cm-thick alternating well-sorted but poorly mixed silicate and metal-rich layers cut by faults and downward penetrating load structures, are prevalent in the Isheyevo (CH/CBb) carbonaceous chondrite. The load structures give the up direction of this sedimentary rock that accumulated from in-falling metal- and silicate-rich grains under near vacuum conditions onto the surface of an accreting planetesimal. The Isheyevo meteorite is the end result of a combination of events and processes that we suggest was initiated by the glancing blow impact of two planetesimals. The smaller impactor was disrupted forming an impact plume downrange of the impact. The components within the plume were aerodynamically size sorted by the nebular gas and swept up by the impacted planetesimal before turbulent mixing within the plume could blur the effects of the sorting. This plume would have contained a range of materials including elementally zoned Fe-Ni metal grains that condensed in the plume to disrupted unaltered material from the crust of the impactor, such as the hydrated matrix lumps. The juxtaposition of hydrated matrix lumps, some of which have not been heated above 150 °C, together with components that formed above 1000 °C, is compelling evidence that they were swept up together. Sweep-up would have occurred as the rotating impactor moved through the plume producing layers of material: the Isheyevo sample thus represents material accumulated while that part of the rotating planetesimal moved into the plume. Vibrations from subsequent impacts helped to form the load structures and induced weak grading within the layers via kinetic sieving. Following sweep-up, the particles were compacted under low static temperatures as evidenced by the preservation of elementally zoned Fe-Ni metal grains with preserved martensite α2 cores, distinct metal-metal grain boundaries, and metal-deformation microstructures. This meteorite provides evidence of gentle layer-by-layer accretion in the early Solar System, and also extends the terrestrial sedimentary source-to-sink paradigm to a near vacuum environment where neither fluvial nor aeolian processes operate.

Greenalite precipitation linked to the deposition of banded iron formations downslope from a late Archean carbonate platform

Banded iron formations (BIFs) were deposited as deep-water facies distal to the late Archean Campbellrand carbonate platform, Transvaal Supergroup, South Africa. They are traditionally interpreted to have formed from iron oxides/hydroxides and silica that precipitated when upwelling hydrothermal water enriched in Fe2+ and silica mixed with cooler, shallow seawater enriched in dissolved oxygen. The ferric oxides/hydroxides were then converted to hematite or, in the presence of organic matter, reduced and incorporated into siderite. New high-resolution microscopy of BIFs from distal facies of the Campbellrand platform questions that interpretation, revealing the presence of abundant greenalite nanoparticles, interpreted to represent primary precipitates from ferruginous seawater, consistent with recent results from equivalent-aged BIFs of the Hamersley Group, Western Australia. The particles locally define primary sedimentary lamination, and are engulfed in diagenetic chert that preserves shrinkage structures, considered to have formed during dehydration and recrystallization of amorphous silica cement. The replacement of greenalite nanoparticles by coarser grained oxide, silicate and carbonate minerals, with destruction of primary textures and structures, indicates that iron-silicate muds were originally much more widespread. Based on our findings we propose a new model for the deposition of the BIFs, involving iron silicate precipitation from seawater followed by diagenetic silica cementation on the seafloor. We suggest that ferrous iron and silica were transported in hydrothermal plumes sourced from acidic vent fluids. Upon mixing with cooler, more alkaline seawater, the solubility of ferrous iron and silica fell to levels that favored rapid nucleation and the precipitation of iron-silicate nanoparticles over vast areas of the seafloor. The close association between iron and silica during the deposition of BIFs may be explained by the precipitation of iron-silicate nanoparticles and possible sorption of silica onto the surfaces of sinking nanoparticles. Furthermore, on the seafloor, iron-silicate nanoparticles presumably acted as nucleation sites for dissolved silica, promoting early diagenetic silica precipitation and cementation of the iron-rich muds. Our results indicate that iron silicates were important primary precipitates and suggest that changes in alkalinity rather than redox state played a key role in the precipitation of the precursor sediments of BIFs.

Flow regulation manipulates contemporary seasonal sedimentary dynamics in the reservoir fluctuation zone of the Three Gorges Reservoir, China

Since the launch of the Three Gorges Dam on the Yangtze River, a distinctive reservoir fluctuation zone has been created and significantly modified by regular dam operations. Sediment redistribution within this artificial landscape differs substantially from that in natural fluvial riparian zones, due to a specific hydrological regime comprising steps of water impoundment with increasing magnitudes and seasonal water level fluctuation holding a range of sediment fluxes. This study reinterpreted post-dam sedimentary dynamics in the reservoir fluctuation zone by stratigraphy determination of a 345-cm long sediment core, and related it to impact of the hydrological regime. Seasonality in absolute grain-size composition of suspended sediment was applied as a methodological basis for stratigraphic differentiation. Sedimentary laminations with relatively higher proportions of sandy fractions were ascribed to sedimentation during the dry season when proximal subsurface bank erosion dominates source contributions, while stratigraphy with a lower proportion of sandy fractions is possibly contributed by sedimentation during the wet season when distal upstream surface erosion prevails. Chronology determination revealed non-linear and high annual sedimentation rates ranging from 21.7 to 152.1cm/yr. Although channel geomorphology may primarily determine the spatial extent of sedimentation, seasonal sedimentary dynamics was predominantly governed by the frequency, magnitude, and duration of flooding. Summer inundation by natural floods with enhanced sediment loads produced from upstream basins induced higher sedimentation rates than water impoundment during the dry season when distal sediment supply was limited. We thus conclude that flow regulation manipulates contemporary seasonal sedimentary dynamics in the reservoir fluctuation zone, though little impact on total sediment retention rate was detected. Ongoing reductions in flow and sediment supply under human disturbance may have profound implications in affecting sedimentary equilibrium in the reservoir fluctuation zone. The results herein provide insights of how big dams have disrupted the sediment conveyance processes of large scale fluvial systems.

Precipitation of iron silicate nanoparticles in early Precambrian oceans marks Earth’s first iron age

Research Article| April 01, 2015 Precipitation of iron silicate nanoparticles in early Precambrian oceans marks Earth’s first iron age Birger Rasmussen; Birger Rasmussen * 1Department of Applied Geology, Curtin University, Kent Street, Bentley, WA 6102, Australia *E-mail: B.Rasmussen@curtin.edu.au Search for other works by this author on: GSW Google Scholar Bryan Krapež; Bryan Krapež 1Department of Applied Geology, Curtin University, Kent Street, Bentley, WA 6102, Australia Search for other works by this author on: GSW Google Scholar Janet R. Muhling; Janet R. Muhling 1Department of Applied Geology, Curtin University, Kent Street, Bentley, WA 6102, Australia2Centre for Microscopy, Characterisation and Analysis, The University of Western Australia, Stirling Highway, Crawley, WA 6009, Australia Search for other works by this author on: GSW Google Scholar Alexandra Suvorova Alexandra Suvorova 2Centre for Microscopy, Characterisation and Analysis, The University of Western Australia, Stirling Highway, Crawley, WA 6009, Australia Search for other works by this author on: GSW Google Scholar Geology (2015) 43 (4): 303–306. https://doi.org/10.1130/G36309.1 Article history received: 26 Sep 2014 rev-recd: 04 Jan 2015 accepted: 22 Jan 2015 first online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Birger Rasmussen, Bryan Krapež, Janet R. Muhling, Alexandra Suvorova; Precipitation of iron silicate nanoparticles in early Precambrian oceans marks Earth’s first iron age. Geology 2015;; 43 (4): 303–306. doi: https://doi.org/10.1130/G36309.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract The early ocean was characterized by anoxic, iron-rich (ferruginous) conditions before the rise of atmospheric oxygen ∼2.45 b.y. ago. A proxy for ferruginous conditions in the ancient ocean is the deposition of banded iron formations (BIFs), which are iron- and silica-rich chemical sediments whose constituents were largely derived from seawater. Although experiments simulating ancient ocean chemistry support the rapid growth of iron-silicate phases, the main iron precipitates are hypothesized to have been ferric oxyhydroxides. The paradox between the prevailing reducing conditions and the deposition of oxidized iron phases is explained by biologically mediated oxidation in the water column. New high-resolution microscopy of BIFs and shales throughout the 2.63–2.45 b.y. old Hamersley Group, Australia, reveals the presence of vast quantities of nanometer-sized iron-silicate particles in laminated chert. The nanoparticles are finely disseminated in early diagenetic chert and locally define sedimentary lamination, indicating that they represent relicts of the original sediments. By inference from experimental studies simulating the composition of the early Precambrian ocean, we suggest that the nanoparticles precipitated from anoxic seawater enriched in silica and dissolved iron, and were silicified upon deposition. The prevalence of iron-silicate nanoparticles implies that they were pervasive background precipitates in ferruginous, silica-enriched oceans, forming the primary sediments of BIFs during periods of enhanced submarine mafic volcanism. Our results imply that silicate precipitation was a major sink of seawater iron and silica before the Great Oxidation Event and, because of the reactivity of nanoparticle surfaces, may also have influenced the transport and geochemical cycling of trace metals and nutrients. Our hypothesis that the basic building blocks of BIFs were predominantly iron-silicate muds rather than iron oxides and/or hydroxides may lead to new insights into seawater chemistry on the early Earth and the role of biology in the deposition of BIFs. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Microstructures and rheology of a calcite-shale thrust fault

A thin (∼2 cm) layer of extensively sheared fault rock decorates the ∼15 km displacement Copper Creek thrust at an exposure near Knoxville, TN (USA). In these ultrafine-grained (<0.3 μm) fault rocks, interpenetrating calcite grains form an interconnected network around shale clasts. One cm below the fault rock layer, sedimentary laminations in non-penetratively deformed footwall shale are cut by calcite veins, small faults, and stylolites. A 350 μm thick calcite vein separates the fault rocks and footwall shale. The vein is composed of layers of (1) coarse calcite grains (>5 μm) that exhibit a lattice preferred orientation (LPO) with pores at twin–twin and twin-grain boundary intersections, and (2) ultrafine-grained (0.3 μm) calcite that exhibits interpenetrating grain boundaries, four-grain junctions and lacks a LPO. Coarse calcite layers crosscut ultrafine-grained layers indicating intermittent vein formation during shearing.Calcite in the fault rock layer is derived from vein calcite and grain-size reduction of calcite took place by plasticity-induced fracture. The ultrafine-grained calcite deformed primarily by diffusion-accommodated grain boundary sliding and formed an interconnected network around shale clasts within the shear zone. The interconnected network of ultrafine-grained calcite indicates that calcite, not shale, was the weak phase in this fault zone.

Iron silicate microgranules as precursor sediments to 2.5-billion-year-old banded iron formations

Research Article| April 01, 2013 Iron silicate microgranules as precursor sediments to 2.5-billion-year-old banded iron formations Birger Rasmussen; Birger Rasmussen * 1Department of Applied Geology, Curtin University, Kent Street, Bentley, WA 6102, Australia *E-mail: B.Rasmussen@curtin.edu.au. Search for other works by this author on: GSW Google Scholar Daniela B. Meier; Daniela B. Meier 1Department of Applied Geology, Curtin University, Kent Street, Bentley, WA 6102, Australia †Current address: School of Earth and Environment, University of Leeds, Leeds LS2 9JT, UK. Search for other works by this author on: GSW Google Scholar Bryan Krapež; Bryan Krapež 1Department of Applied Geology, Curtin University, Kent Street, Bentley, WA 6102, Australia Search for other works by this author on: GSW Google Scholar Janet R. Muhling Janet R. Muhling 2Centre for Microscopy, Characterisation and Analysis, University of Western Australia, Crawley, WA 6009, Australia Search for other works by this author on: GSW Google Scholar Geology (2013) 41 (4): 435–438. https://doi.org/10.1130/G33828.1 Article history received: 06 Jul 2012 rev-recd: 16 Oct 2012 accepted: 24 Oct 2012 first online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Birger Rasmussen, Daniela B. Meier, Bryan Krapež, Janet R. Muhling; Iron silicate microgranules as precursor sediments to 2.5-billion-year-old banded iron formations. Geology 2013;; 41 (4): 435–438. doi: https://doi.org/10.1130/G33828.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Banded iron formations (BIFs) are chemical sedimentary rocks comprising alternating layers of iron-rich and silica-rich minerals that have been used to infer the composition of the early Precambrian ocean and ancient microbial processes. However, the identity of the original sediments and their formation is a contentious issue due to postdepositional overprinting and the absence of modern analogues. Petrographic examination of the ca. 2.5 Ga Dales Gorge Member of the Brockman Iron Formation (Hamersley Group), Western Australia, reveals the presence of abundant silt-sized microgranules composed of stilpnomelane. The microgranules are most common in the least-altered BIF where they define sedimentary laminations, implying a depositional origin. We suggest that the precursor mineral was an iron-rich silicate that formed either in the water column or on the seafloor. The microgranular texture may have developed due to clumping of amorphous mud, forming silt-sized floccules. The microgranules were resedimented by dilute density currents and deposited in lamina sets comprising a basal microgranular-rich lamina overlain by amorphous mud with dispersed microgranules. The lamina sets collectively define plane-lamination structure, probably of the lower flow regime. The microgranular textures are preserved only where early diagenetic silica prevented the compaction of lamina sets. Episodic resedimentation of iron silicates alternating with periods of nondeposition and seafloor silicification provides an explanation for some of the characteristic banding in BIF. We propose that for most of the early Precambrian, the persistence of ferruginous oceans with elevated silica concentrations favored the widespread growth of iron silicate minerals, which in environments starved of continental sediments formed extensive deposits of the precursor sediment to iron formation. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Sedimentary laminations in the lacustrine Jianshangou Bed of the Yixian Formation at Sihetun, western Liaoning, China

Extremely fine sedimentary laminations of the lacustrine Early Cretaceous Yixian Formation were studied in the excavated Sihetun Fossil Museum section in western Liaoning, China. The section consists mainly of mudstones, shales and layers of volcanic ash. The mudstones and shales are composed of siliciclastic and organic-rich laminae. Most of the laminations are varves that record seasonal climatic changes. Varve thickness measurements show that the sedimentation rate for the majority of the mudstones and shales was 0.2–0.7 mm/year. Quiet, anoxic lacustrine bottom waters were critical for the preservation of the laminations.

Mineralogical Composition of Diabase and Altered Dolostone from the St. Francois Mountains near Annapolis, Missouri, USA

Dolostones of Cambrian and Ordovician age are found in the St. Francois Mountains on the Ozark Plateau in southeastern Missouri and often lie unconformably over Precambrian basement rocks due to the paleo-topography. In some areas, there is distinct evidence of alteration and mineralization within the dolostones. One such location is found near Annapolis, Missouri where nearly all of the original dolomitic material is replaced with quartz. A diabase dike is in close proximity to the silicified dolostone and may have provided the conduit for fluids during alteration. The purpose of this study was to examine samples from the relatively unaltered dolostone, the silicified dolostone, and the diabase in order to establish the mineral changes associated with the alteration. Several samples were collected and analyzed in hand sample, in thin section, and using X-Ray Diffraction. In the diabase, plagioclase was abundant in both hand sample and thin section. The XRD results confirmed the presence of plagioclase as labradorite and supported evidence of magnetite as well. The highly altered dolostone exhibited original sedimentary laminations and varied in color. In thin section, the altered dolostone was very fine grained and the minerals that appeared to be present were quartz and an opaque mineral. After XRD analysis it was concluded that quartz was present and a pyrite structured sulfide mineral was present as well. The other four samples were dolomitic with laminations that varied in color from dark red to cream. XRD results indicated that dolomite, ankerite, and minrecordite were present in the dolostone.

Annual lamination and its sedimentary implications in the Yangtze River delta inferred from High-resolution biogenic silica and sensitive grain-size records

Annual sedimentary layers in core A from the Yangtze River (Changjiang) subaqueous delta were identified based on high-resolution biogenic silica (BSi) and grain-size records. The contents of BSi, the sediment grain-size and the sensitive grain-size fractions of <13.14μm and 13.14–250μm illustrated steady coincident fluctuations with the depth profile of core A. These fluctuations were inferred to reflect seasonal changes in physical and biological processes. For example, variations in the content of BSi likely represent changes in primary production in flood versus dry seasons. It in flood seasons was several times higher than that in dry seasons. The amounts of <13.14 and 13.14–250μm fractions were complimentary to each other and co-varied with the BSi amount. Therefore, the BSi, grain-size and sensitive fractions can be used as proxies for the annual sedimentation. From 1992 to 2005, the estimated sedimentary rate based on the annual layer varied from 1.0 to 3.5 cm/a (mean of 2.07 cm/a), and these values varies with the annual sediment discharge from the Yangtze River to the sea. The significant seasonal variations in the water discharge, sediment load, ocean hydrodynamics and primary production, which are common in the area affected by the East Asian monsoon, are likely responsible for the formation of the annual sedimentary lamination.

A high-resolution micromorphological record of the Late Quaternary paleosols from Ganga–Yamuna interfluve: Stratigraphic and paleoclimatic implications

This study reports the micromorphological record of a continuous set of samples from paleosol-bearing intervals of two subsurface cores of ∼50 m depth representing typical pedosedimentary sequences of the Ganga–Yamuna interfluve in the western Ganga Plains. Micromorphology (microstructures, rhizocretes, Fe–Mn features, pedogenic carbonate, illuvial clay coatings and relict pedofeatures) was used to characterize pedogenic processes in the 13 paleosols in the IITK core and the 10 paleosols in the Bhognipur core. Formation of these paleosols took place through incipient to moderate weathering on relatively stable surfaces of the Ganga–Yamuna interfluve over the last 100 ka. These paleosols range in character from simple ones having weakly developed pedofeatures to mature paleosols with strongly developed pedofeatures and are similar to modern Entisols, Inceptisols, Alfisols and Vertisols of the Ganga Plains. Pedosedimentary evolution in the two interfluve successions differs in terms of stratigraphic stacking of paleosols, types of paleosols, and heterogeneous nature of sediments. In the northern part, the IITK interfluve core is marked by a thick mature paleosol with a strongly developed argillic (Bt) horizon at 41–45 m depth and weakly developed paleosols in the remaining part of the core. The mature paleosol represents a major discontinuity when prolonged pedogenic activity took place on stable surfaces with little or no deposition. On the other hand, thin-simple–thick cumulic paleosols with weakly developed pedogenic features, even with some preservation of sedimentary laminations indicate short stratigraphic breaks in rapidly aggrading sediments. In the southern part, the Bhognipur core is marked by the heterogeneous nature of sediments with a predominance of cratonic sediments in the lower part and the Himalayan sediments in the upper parts. In contrast, the IITK core shows a Himalayan source throughout. The entire interfluve succession in the Bhognipur region is marked by rapid sedimentation and short pedogenic intervals of weakly developed paleosols excepting one mature paleosol with well-developed argillic (Bt) and vertic (Bss) horizons at 10–14 m depth. This mature paleosol possibly formed synchronously with a mature paleosol from the northern part of this interfluve. The mature paleosol in both the cores represents a major stratigraphic break of 8000–10,000 years when prolonged pedogenesis occurred over large areas of the Ganga–Yamuna interfluve following regional degradation and local gullying in response to climate change from sub-humid to semi-arid conditions during the MIS 5–4 transition.

Petrogenesis of the Platinova skarn in the Belmont domain (Composite Arc Belt, SE Ontario, Canada)

The Platinova skarn is hosted by greenschist facies calcitic marble, contiguous to the hypersolvus Deloro pluton, in the Belmont domain, southeastern Ontario, Canada. The skarn is approximately 4 million tones grading 35% wollastonite. The contact between the pluton and the skarn is not exposed. The skarn is divided into wollastonite-rich and poor units. Garnet and pyroxene from the wollastonite-rich unit exhibit the composition Grs67–89Adr11–36Alm0–3Sps<2, and Di50–90Hd10–48Jo0–2, and from the wollastonite-poor unit Grs17–98Adr0–82Alm0–2Sps<1.5 and Di39–95Hd3–59Jo0.3–4, respectively. The skarn development is ascribed to the incursion of magmatogene, silica-rich, CO2-poor (<0.15) fluids at temperatures of approximately 580 °C, and at pressures of 200 MPa. The genetic model proposed for skarn development, based on field and mineralogical evidence indicates addition of many components relative to essentially pure calcitic marble precursor, including Si, Al, Fe, Mg, Mn, B, Na, and loss of CO2. Podiform fabrics of the wollastonite unit are interpreted as products of metasomatism rather than as relict sedimentary laminations or metamorphic structures.

A fan delta succession rich in water escape structures (Upper Turonian, Brandenberg, Austria): Possible record of paleoseismicity

The Upper Turonian of Brandenberg (Austria) is based by a transgressive fan delta succession rich in water escape structures that, at least in part, may have formed in association with earthquakes. The investigated fan delta is among the oldest deposits of the Lower Gosau Subgroup (Upper Turonian to Lower Campanian), a terrestrial to neritic succession that unconformably overlies older carbonate rocks. In its subaerial part, the fan succession consists mainly of conglomerates deposited from mass flows, interlayered with red claystones to siltstones. Along the fringe of marine transgression, beachface/channel mouth conglomerates and bioturbated siltstones to fine sandstones accumulated. The marine part of the fan delta succession consists mainly of cross-laminated and hummocky cross-laminated arenites deposited in a wave/storm-domainated shoreface. Excellent preservation of sedimentary lamination throughout and near absence of bioturbation indicate (intermittently) rapid sediment accumulation. Intercalated shoreface conglomerates are present as compound channel-fills, and as thin sheets alongside and off channels. Offshore transport of gravels to cobbles into the shoreface may have been driven by river floods (in the most proximal positions) and by storm rip currents (farther seaward). Towards the top of the succession, conglomerate sheets disappear, and the arenites become bioturbated. In the succession of shore zone arenites, abundant water escape structures include distorted/convoluted lamination, short fluidization planes, tabular fissures (some associated with offset of beds), pods and lenses of internal breccias, pillow beds up to more than 1 m thick, and hitherto undescribed, cyclindrical structures (“onion structures”) built by concentrically arranged planes interpreted as water escape routes. The tabular fissures, internal breccias and the pillow beds are closely similar to water escape structures documented from historical earthquakes and from inferred paleo-earthquakes. Storm wave loading or wave-induced microseisms are considered less probable triggers of the larger dewatering structures. Water escape structures represent an hitherto unappreciated, although not strictly diagnostic, indicator of syndepositional tectonism in the Upper Cretaceous of the Eastern Alps.

Chert from the Munsungun Lake Formation (Maine) in Palaeoamerican Archaeological Sites in Northeastern North America: Recognition of its Occurrence and Distribution

Numerous fluted point Palaeoamerican sites in the northeast contain significant numbers of chert artefacts whose origin is from the Munsungun Lake Formation in northern Maine. These sites include: Adkins, Bull Brook, Dam, Hedden, Lamareau, Michaud, Morss, Munsungun Lake (three sites), Neponset, Plenge, Point Sebago, Searsmont, Spiller Farm, Vail, Vail Kill (two sites), Wheeler Dam (two sites) and Whipple. Lithic materials in order of decreasing importance at these sites include: (1) chert whose physical characteristics are wholly consistent with chert outcrops and quarries near Munsungun Lake; (2) a variety of cherts whose physical characteristics are inconsistent with outcrops and quarries near Munsungun Lake; (3) a spherulitic, flow-banded rhyolite; (4) miscellaneous fine-grained igneous rocks; (5) clear or milky quartz; (6) miscellaneous very fine-grained or fine-grained metamorphic rocks. Portions of all collections contain artefacts which are completely weathered, and which are not assignable to any specific rock type and cannot be classified as to specific rock type.We developed a hierarchy to classify Munsungun Lake chert, ranging from hand specimen characteristics to polarized light microscopy and X-ray diffraction to chemical analysis. We conclude that hand specimen characteristics combined with polarized light microscopy are viable techniques for assigning Munsungun Lake Formation provenance. Three distinctive hand specimen features are sedimentary lamination, organic burrow structures and colour variation. Using these, chert quarries and outcrops in the geographic vicinity of Munsungun Lake may be assigned to one of the following categories: (A) structureless grayish red, dusky red, blackish red, or dark reddish brown; (B) laminated grayish red, dusky red, blackish red, or dark reddish brown chert with varve-like laminations; (C) mottled or colour laminated red and dark grey, or mottled red and greenish grey chert; (D) colour laminated and burrow mottled chert; (E) greyish to blackish laminated chert; (F) dark grey to black, structureless or weakly and thinly laminated; (G) olive grey to olive black laminated chert which is locally pyrite-rich. The nature of weathered surfaces may be a diagnostic tool for assistance in recognizing Munsungun chert. Chemical weathering tends to produce much lighter colourations and enhances the visibility of sedimentary structures. Munsungun cherts tend to weather to a very light tan, very light grey or greyish white. Radiolarians are uncommon (<10%) in all varieties, and are concentrated in zones parallel to bedding.Chert palaeoquarries are numerous in the Munsungun Lake area. They are of two types. One type occurs on topographically flat areas. Here the quarries are small (1×5m) to large (5×30m) size depressions within and along outcrops. In plan view these range in shape from roughly circular to broad ovals. Along the margins of these depressions are piles of poorly sorted worked and discarded debitage. These “tailing piles” are generally less than 1m in height, but exhibit wide variations in areal extent and shape. Larger tailing piles overlap one another suggesting that they have experienced more than one episode of growth due to repeated quarry activity. The second quarry type occurs on steeper slopes. These are near-vertical quarried faces which range from 1 to nearly 3m in height. Excavation here has occurred both inwards to the slope and downwards. Small cavern or adit-like “mines” have not been recognized.

Characterization of Reservoir Lithologies within Subunconformity Pools: Pekisko Formation, Medicine River Field, Alberta, Canada

The Pekisko Formation in western Canada is a third-order sequence comprised of an open-marine grainstone through restricted carbonate mudstone succession. Truncation of the Pekisko along an unconformity edge prior to the Jurassic and several periods of incision from Early Jurassic-Early Cretaceous have formed an intricately sculptured subcrop belt. In the Medicine River field, oil is trapped in discrete pools close to the unconformity edge. Reservoir facies comprise three lithologies. Medium-crystalline dolostone with intercrystalline and vuggy porosity forms the reservoir in an elongate dolostone body that has replaced grainstone. Fine-crystalline dolostone is facies selective within a lime-mudstone unit in which the common reservoir rock is bioturbated dolomudstone with intercrystalline porosity. Grainstones have secondary porosity formed though leaching of microcrystalline calcite allochems and intergranular cements. Reservoir quality is assessed from consideration of orthogonal permeability values (Kmax = maximum horizontal permeability, K90 =horizontal permeability at 90° to the maximum, and Kv = vertical permeability) obtained from conventional whole-core analyses. Sedimentary lamination primarily affects the Kv, and the presence of fine-grained lamination dictates that Kv 3K90 and Kv > K90. From this relationship, the fracture density in all three lithologies appears to be similar. Low and variable porosity and permeability values in grainstones indicate that the fracture system is poorly connected to the matrix. Despite leaching and fracturing at the unconformity edge, reservoir distribution closely follows patterns of facies that were susceptible to dolomitization. Future exploration potential lies in fine-crystalline dolostone bodies that may form stratigraphic traps downdip from the unconformity edge.

On the Significance of Small-scale and Short-lived Air Escape Structures for the Destruction of Primary Sedimentary Laminations in the Çolakı Beach Deposits, Gulf of Antalya, Turkey (Eastern Mediterranean)

Çolakı beach is a 4km long coastal depositional system on the Mediterranean coast of southern Turkey. It has been accumulating at the foot of Taurus tectonic belt since the late Pleistocene. The beach deposits are composed of fine-grained sands with no significant longshore changes in granulometry and mineralogy. In addition, the sequence of beach deposits is characterized by a homogenous internal appearance due to the lack of preserved primary sedimentary structures. On the foreshore, air-escape structures comprising small tubes, 5–30mm in length and 0·5–3mm in width are extensively formed along the upper swash zone during the backwash phase of wave action. When the next wave swashes across the beach, the former structures are destroyed, at the same time producing conditions for the creation of new ones. The escape structures destroy the original sedimentary lamination. It is thus proposed that the homogenous internal appearance of the older beach deposits has been produced by the same mechanism.

Self-Sedimentation of Fossil Phytoplankton Blooms, Laminated Hemipelagic Sediments and the Oceanic Carbon Cycle: ABSTRACT

The flux of phytoplankton-derived organic carbon from the surface ocean to the deep sea and underlying sediments is a nonuniform process that significantly impacts biogeochemical cycles, atmospheric pCO[sub 2] / O[sub 2] and organic carbon enrichment in marine sediments. Some marine phytoplankton actively drive the sedimentation process by the formation of sticky transparent gels which facilitate aggregation, rapid sinking and efficient export flux. Here we present fossil evidence of unfragmented, low-diversity phytoplankton assemblages preserved as sedimentary laminations and as preserved aggregates that are attributable to a similar phytoplankton-driven sedimentary mechanism, here termed [open quotes]self-sedimentation[close quotes]. Heterogeneities in the texture and/or composition of sediment supply are necessary for the production of laminatedhemipelagic sediments; the absence of hydraulic and biological reworking permits preservation of these sedimentary laminae. Distinctly-laminated core intervals are characterized by large compositional contrasts between adjacent laminae; many such high-bimodality couplets are attributable to self-sedimentation of phytoplankton blooms. Self-sedimentation propels the formation of some conspicuous hemipelagic sedimentary laminations and results in efficient carbon and opal flux to the sediments. These records suggest that phytoplankton-mediated changes in the efficiency of the biological carbon pump may govern many accumulations of organic-rich hydrocarbon source rock as well as many abrupt changes in atmosphericmore » pCO[sub 2] and climate.« less