Siliciclastic Grains Research Articles

Diagenesis of Holocene beachrocks: a comparative study between the Arabian Gulf and the Gulf of Aqaba, Saudi Arabia

Beachrocks occur in present-day intertidal zones of the Arabian Gulf and the Gulf of Aqaba, on the eastern and northwestern coasts, respectively, of Saudi Arabia. The beachrocks occur as linear patches within beach deposits, which have variable grain size and detrital compositions. The Arabian Gulf beachrocks are composed of sand-sized bioclasts and siliciclastic grains, whereas the Gulf of Aqaba beachrocks are composed of sand- to pebble-size grains, which are dominated by igneous rock fragments and small amounts of skeletal carbonate grains. The cement includes micritic high-magnesian calcite and isopachous acicular/bladed aragonite. In addition to cements, intergranular pores are locally filled by a lime–mud matrix. Radiocarbon dating of beachrock samples from the Arabian Gulf yielded ages from ca. 655 to 2185 year bp, whereas the Gulf of Aqaba samples range in age between 2745 and 5075 year bp.

Erratum to: Petrographical and petrophysical characteristics of asynchronous beachrocks along Al-Shoaiba Coast, Red Sea, Saudi Arabia

The present study describes the petrographic and petrophysical characteristics of three stratigraphically asynchronous beachrocks (A, B, and C) along the Red Sea coast at Al-Shoaiba area, Saudi Arabia, to identify the compositional, depositional, and diagenetic controls on beachrock formation and their petrophysical properties. The beachrocks at the three locations consist basically of calcareous skeletal remains of different types and grain size reflecting the composition of adjacent beach sediments and the depositional conditions. They were cemented by aragonite and high-Mg calcite (HMC) in intertidal zone. The cement exhibits three major fabrics: (1) micritic coatings, (2) isopachous to asymmetric aragonite rim, and (3) cryptocrystalline pore-fillings HMC. In addition, some intergranular and intragranular pores were partially filled with infiltrated silt-sized carbonate and siliciclastic sediments. The cementation was accomplished by the combination of inorganic and organic processes, and the former was dominant through evaporation of pore-filling seawater under hot and dry climatic conditions. Cementation in the present beachrocks is selective; carbonate grains are cemented, whereas siliciclastic grains are cement free. The cement nucleation on carbonate grains was easier than on siliciclastic grains which do not provide good seed crystals for carbonate cement growth. The petrophysical measurements are in agreement with petrographic studies. The degree of cementation is the major control on petrophysical characteristics where the lowest and highest porosity and permeability values were obtained from the least cemented beachrocks of location A and the highest cemented beachrocks of location B, respectively. The thickness of cement seems to be controlled by cement precipitation rate, grain size, and size and shape of pore spaces. The coarse sediments of beachrocks at location B were deposited under higher depositional energy than those at locations A and C, and therefore, their primary pore spaces were large enough to be filled with seawaters. Under hot and dry climate, cement precipitation increases leading to partial to complete occlusion of pore spaces.

Sedimentary facies and biofacies of theTorinosuLimestone in theTorinosu area,KochiPrefecture,Japan

AbstractTheTorinosuLimestone represents carbonate platform deposits in a foreland basin, the sedimentary setting of which is highly different from those of well‐knownLateJurassic reefs in the westernTethys that developed on shelf areas of continental margins and intra‐Tethyan platforms. Sedimentological and paleontological analyses were conducted on a 55.5 m‐thickUpperJurassic–LowerCretaceous (Tithonian–Berriasian) carbonate sequence (TorinosuLimestone) at theEasternHitotsubuchiQuarry,KochiPrefecture,SouthwestJapan. The carbonate sequence is composed of two sections that are separated by a subaerial exposure surface. Two and three depositional units have been defined in the lower and upper sections, respectively, based on changes in lithology and the biotic composition of the carbonates; they are numbered from 1 to 5, in ascending order. Calcified demosponges (stromatoporoids and a chaetetidChaetetopsis crinita) are abundant in three units (2, 3, and 5), in which microencrusters (mostlyLithocodium aggregatumandBacinella irregularis) and microbialites are also common to abundant. Although most of them are para‐allochthonous,in‐situbranching stromatoporoids are found on and above the subaerial exposure surface (unit 3). Corals are less common, poorly diverse, and primarily represented by the familyMicrosolenidae. Siliciclastic grains occur in all units, but they are particularly common in units 1 and 4. The co‐occurrence of theLithocodium–Bacinellaassociation, which is typical of oligotrophic or moderately mesotrophic shallow‐water environments, with microsolenids, which are indicative of high nutrient levels and/or low‐light intensity due to high turbidity, suggests repeated changes in nutrient levels associated with terrigenous input. Based on lithology, biotic composition, and succession, we infer that sea‐level changes and related terrigenous input controlled the sedimentary environment of the studied carbonate sequence.

Evolution of marine sedimentation in the Bering Sea since the Pliocene

Sediment of the Bering Sea, derived mainly from biogenic, glaciomarine, and, secondarily, riverine sources, reflects the history of oceanographic changes within the basin and climatic changes on the adjacent continents. Integrated Ocean Drilling Program (IODP) Expedition 323 recovered cores that reveal the evolution of sedimentation in the Bering Sea over the past 5 m.y., a period that includes globally significant events such as the early Pliocene warm period, the onset of extensive Northern Hemisphere glaciation, and the Pleistocene glacial-interglacial and millennial-scale climate cycles. To begin to understand the Bering Sea regional response to and role in these global climate change events, we examined the sedimentary constituents of Expedition 323 sites U1339, U1343, and U1344 on the Bering Slope, and U1340 and U1341 on Bowers Ridge. New particle size and petrographic analyses, combined with shipboard lithostratigraphic and physical property data, are used to characterize sediment types and texture and its distribution through space and time. The sediment comprises mainly two components, opaline diatom valves and siliciclastic grains (mainly clay and fine silt size). Approximately 40% of the variance in particle size can be explained by the abundance and preservation of diatom valves, a rough indicator of biogenic opal productivity. Particle size data indicate that productivity was generally higher during interglacials compared to glacials, and higher during the Pliocene warm period, decreasing as Northern Hemisphere glaciation intensified ∼3 m.y. ago. Although the abundance of diatoms in the sediment varied, diatom ooze and diatom mud are the dominant lithologies at Bowers Ridge, indicating that there was a persistent supply of diatoms to the sediment in the open Bering Sea during the past 5 m.y. This study provides a comprehensive view of sediment types and sedimentation processes; future work should be aimed at validating our interpretations of past changes in productivity and siliciclastic sedimentation mechanisms with multiple additional proxies.

Seagrass-Meadow Sedimentary Facies In A Mixed Siliciclastic-Carbonate Temperate System In the Tyrrhenian Sea (Pontinian Islands, Western Mediterranean)

Abstract The paper presents the characteristics of surface sediments in a seagrass-dominated environment of the inner shelf of the Pontinian Islands (Tyrrhenian Shelf, Central Mediterranean). The aims of this work are to characterize the carbonate production and sedimentary processes of a representative high-latitude, temperate Posidonia oceanica–dominated area, to discuss the role of seagrass in the siliciclastic–carbonate mixing processes, which provide analogs for interpreting the occurrence and importance of these types of deposits in the rock record. The majority of sediments are poorly sorted siliciclastic skeletal sand. Based on grain type and abundance and carbonate content, these deposits include six subfacies. The different subfacies reflect environmental changes within the seagrass environment associated with variations in light penetration and hydrodynamic conditions. Additionally, the seagrass facies range from purely carbonate to mixed carbonate–siliciclastic, depending on the amount of siliciclastic input and on the capacity of seagrasses to trap allochthonous sediments while hosting carbonate-producing biota. The type of substrate and the erosion and transport processes control the siliciclastic input. In these Pontinian seagrass meadows, sedimentary facies contain abundant siliciclastic grains produced by erosion of the volcanic substrate and trapped by seagrass blades and rhizomes. This Pontinian example illustrates that in heterozoan-dominated seagrass settings, mud-producing green algae are not present or are not calcifying, and consequently, seagrass deposits are much more grainy than photozoan counterparts. Ancient heterozoan tropical seagrass deposits likely will be discriminated from their temperate or subtropical counterparts only by careful taxonomic analysis of skeletal grains.

Relative sea-level change, climate, and sequence boundaries: insights from the Kimmeridgian to Berriasian platform carbonates of Mount Salève (E France)

The present study analyses the stratal architecture of the Late Jurassic (Kimmeridgian) to Early Cretaceous (Berriasian) sedimentary succession of Mount Saleve (E France), and four Berriasian stratigraphic intervals containing four sequence-boundary zones reflecting lowering trends of the relative sea-level evolution. Massive Kimmeridgian limestones characterized by the presence of colonial corals appear to be stacked in an aggrading pattern. These non-bedded thick deposits, which are interpreted to have formed in balance between relative sea-level rise and carbonate accumulation, suggest a keep-up transgressive system. Above, well-bedded Tithonian-to-Berriasian peritidal carbonates reflect a general loss of accommodation. These strata are interpreted as a highstand normal-regressive unit. During the early phase of this major normal regression, the vertical repetition of upper intertidal/lower supratidal lithofacies indicates an aggrading depositional system. This is in agreement with an early stage of a highstand phase of relative sea level. The Berriasian sequence-boundary zones investigated (up to 4 m thick) developed under different climatic conditions and correspond to higher-frequency, forced- and normal-regressive stages of relative sea-level changes. According to the classical sequence-stratigraphic principles, these sequence-boundary zones comprise more than one candidate surface for a sequence boundary. Three sequence-boundary zones studied in Early Berriasian rocks lack coarse siliciclastic grains, contain a calcrete crust, as well as marly levels with higher abundances of illite with respect to kaolinite, and exhibit fossilized algal-microbial laminites with desiccation polygons. These sedimentary features are consistent with more arid conditions. A sequence-boundary zone interpreted for the Late Berriasian corresponds to a coal horizon. The strata above and below this coal contain abundant quartz and marly intervals with a higher kaolinite content when compared to the illite content. Accordingly, this Late Berriasian sequence-boundary zone was formed under a more humid climate. The major transgressive–regressive cycle of relative sea level identified and the climate change from more arid to more humid conditions recognized during the Late Berriasian have been reported also from other European basins. Therefore, the Kimmeridgian to Berriasian carbonate succession of Mount Saleve reflects major oceanographic and climatic changes affecting the northern margin of the Alpine Tethys ocean and thus constitutes a reliable comparative example for the analysis of other coeval sedimentary records. In addition, the stratigraphic intervals including sequence-boundary zones characterized in this study constitute potential outcrop analogues for sequence-boundary reflectors mapped on seismic profiles of subsurface peritidal carbonate successions. The detailed sedimentological analyses provided here highlight that on occasions the classical principles of sequence stratigraphy developed on seismic data are difficult to apply in outcrop. A sequence-boundary reflector when seen in outcrop may present successive subaerial exposure surfaces, which formed due to high-frequency sea-level changes that were superimposed on the longer-term trend of relative sea-level fall.

Geochemical trends through time and lateral variability of diatom floras in the Pleistocene Olorgesailie Formation, southern Kenya Rift Valley

The Olorgesailie Formation (1.2–0.49 Ma) consists of fluvial and lacustrine rift sediments that have yielded abundant Acheulean artifacts and a fossil hominin ( Homo cf. erectus). In testing prior understandings of the paleoenvironmental context, we define nine new geochemical zones. A Chemical Index of Alteration suggests increased catchment weathering during deposition of Members 1, 2, 7, 11, and 13. Biophile elements (Br, S) peak in M8–9 and lower M13 possibly reflecting increased input from soil erosion. REE data show that the Magadi Trachytes supplied most siliciclastic grains. Sixteen diatom stages indicate conductivities of 200–16,000 μS cm − 1 and pH of 7.5–9.5 for five deep-water lakes, ten shallow lakes and sixteen wetlands. These results are compared with diatom data from other sections in the basin and show aquatic spatial variability over km-scale distances. Similar floras are traceable over several kilometers for M2, M3 and M9, indicating broadly homogeneous lacustrine conditions during these times, but diatoms in other members imply variable conditions, some related to local tectonic controls. This lateral and temporal variability emphasizes the importance of carrying out stratigraphic sampling at multiple sites within a basin in efforts to define the environmental context relevant to human evolution.

First-Cycle Grain Weathering Processes: Compositions and Textures of Sea Glass from Port Allen, Kauai, Hawaii

Abstract Chemical and mechanical weathering textures on siliciclastic grains have provided important information concerning depositional processes and environments, yet understanding is limited by the multicycle origin of most sedimentary deposits. Sea glass grains sampled from a beach in Port Allen, Kauai, Hawaii, were analyzed for mechanical and chemical weathering features using scanning electron microscopy (SEM), energy dispersive X-ray (EDX) analysis, and X-ray photoelectron spectroscopy (XPS). As a result of a proximal dumping-ground source, the sea glass offers a unique opportunity to study first-cycle grain weathering processes in a moderate- to high-energy shoreline environment. Grain-size analysis indicates that the largest grains are located behind natural barriers, such as outcrops and boulders. The best-sorted samples are from the lower to middle foreshore environment, whereas moderately sorted deposits are located closer to the backshore where storm waves predominate. Although there is no preferred distribution of different-colored sea glass grains on the beach, nonfrosted grains predominate over frosted varieties, suggesting constant input of fresh material. Grain surface textures can be divided into mechanically dominated and chemically dominated weathering groups. Evidence of mechanical weathering is provided by conchoidal fractures, crescentic gouges, and straight grooves, whereas chemical weathering is indicated by c-shaped cracks, halite (salt) and silica precipitation, and solution pits. Combinations of these features demonstrate that both weathering processes work together to degrade the grains. XPS depth profiling of individual grains indicates that chemical weathering occurs at different grain depths, with leaching of sodium from the glass surfaces. This study provides a rare opportunity to relate grain compositions and textures with depositional and weathering processes, due to the first-cycle origin of the sea glass. The results show that using XPS and SEM techniques is a broadly applicable approach toward unraveling chemical and mechanical breakdown of sediment in beach environments.

Stable isotope and trace element stratigraphy across the Permian–Triassic transition: A redefinition of the boundary in the Velebit Mountain, Croatia

Stable isotopes of carbonates (δ 13C carb, δ 18O carb), organic matter (δ 13C org, δ 15N org) and major, trace and rare earth element (REE) compositions of marine carbonate rocks of Late Permian to Early Triassic age were used to establish the position of the Permian–Triassic boundary (PTB) at two continuous sections in the Velebit Mountain, Croatia. The chosen sections — Rizvanuša and Brezimenjača — are composed of two lithostratigraphic units, the Upper Permian Transitional Dolomite and the overlying Sandy Dolomite. The contact between these units, characterized by the erosional features and sudden occurrence of ooids and siliciclastic grains, was previously considered as the chronostratigraphic PTB. The Sandy Dolomite is characterized by high content of non-carbonate material (up to ~ 30 wt.% insoluble residue), originated from erosion of the uplifted hinterland. A relatively rich assemblage of Permian fossils (including Geinitzina, Globivalvulina, Hemigordius, bioclasts of gastropods, ostracods and brachiopods) was found for the first time in Sandy Dolomite, 5 m above the lithologic boundary in the Rizvanuša section. A rather abrupt negative δ 13C carb excursion in both sections appears in rocks showing no recognizable facies change within the Sandy Dolomite, −2‰ at Rizvanuša and −1.2‰ at Brezimenjača, 11 m and 0.2 m above the lithologic contact, respectively. This level within the lower part of the Sandy Dolomite is proposed as the chemostratigraphic PTB. In the Rizvanuša section, the δ 13C org values decline gradually from ~ −25‰ in the Upper Permian to ~ −29‰ in the Lower Triassic. The first negative δ 13C org excursion occurs above the lithologic contact, within the uppermost Permian deposits, and appears to be related to the input of terrigenous material. The release of isotopically light microbial soil-biomass into the shallow-marine water may explain this sudden decrease of δ 13C org values below the PTB. This would support the hypothesis that in the western Tethyan realm the land extinction, triggering a sudden drop of woody vegetation and related land erosion, preceded the marine extinction. The relatively low δ 15N org values at the Permian–Triassic (P–Tr) transition level, close to ≈ 0‰, and a secondary negative δ 13C org excursion of −0.5‰ point to significant terrestrial input and primary contribution of cyanobacteria. The profiles of the concentrations of redox-sensitive elements (Ce, Mn, Fe, V), biogenic or biogenic-scavenged elements (P, Ba, Zn, V), Ce/Ce* values, and normalized trace elements, including Ba/Al, Ba/Fe, Ti/Al, Al/(Al + Fe + Mn) and Mn/Ti show clear excursions at the Transitional Dolomite–Sandy Dolomite lithologic boundary and the chemostratigraphic P–Tr boundary. The stratigraphic variations indicate a major regression phase marking the lithologic boundary, transgressive phases in the latest Permian and a gradual change into shallow/stagnant anoxic marine environment towards the P–Tr boundary level and during the earliest Triassic.

Marine biota response to clastic sediment influx: A quantitative approach

There have been few quantitative studies of the effects of siliciclastic and volcaniclastic sediment influx on carbonate producing organisms from the geological record. This study presents the results of quantitative field, petrographic and acid digestion analyses of samples from three Cenozoic shallow-water mixed carbonate-volcani/siliciclastic depositional environments—the Miocene Wonosari Formation of south Java, the Miocene Batu Putih Limestone of east Borneo, and the Eocene Collsuspina Limestone of northeast Spain. Carbonate production in these systems is dominated by larger benthic foraminifera, coralline algae and corals. Analyses of the quantity, grainsize and type of volcani/siliciclastic sediment within these mixed successions have allowed assessment of the influence of these factors on shallow-water carbonate producers. Larger benthic foraminifera and coralline algae exhibit the greatest tolerance to siliciclastic sediment influx (abundant up to 55 and 38 wt.%, respectively) at a range of grain sizes (to coarse sand). Platy corals are also present within sediments containing a large proportion of insoluble material (< 80%); however, these are limited to clay-dominated horizons with a maximum siliciclastic size of fine sand grade. Branching and massive corals exhibit the least tolerance to large quantities of insoluble material (usually < 40% up to medium sand, and usually < 25% up to very fine sand, respectively). These variations probably reflect organism: 1) mobility, 2) ability to self clean, 3) morphology and 4) feeding mechanisms relative to: a) sediment settling, b) turbidity, c) abrasion, d) energy, e) water depth, f) light levels, and g) nutrients. The influx of volcaniclastic and siliciclastic grains of comparable size does not appear to have any differential effect on carbonate producers; although corals may be more susceptible to damage by angular volcanic shards. Geologically averaged rates of clastic accumulation and their potential influences on carbonate producers inferred herein may be difficult to compare directly with the daily and seasonal fluctuations experienced by modern clastic influenced systems. However, this study may aid our understanding of the likely effects of increased clastic influx (today commonly anthropogenically related) to marine carbonate environments over longer time periods.

Zeroing tests of luminescence sediment dating in the Arctic Ocean: Review and new results from Alaska-margin core tops and central-ocean dirty sea ice

Additional numeric dating techniques are needed for sediments in the Arctic Ocean that span at least the last half million years. Almost all presently suitable geochronometers are relative-dating methods, requiring significant assumptions about sedimentation rates (such as continuous deposition and preservation). An alternative ‘absolute’ geochronometer, luminescence sediment dating, assumes daylight exposure of siliciclastic grains before burial. Early tests and applications of thermoluminescence (TL) techniques in the western Arctic Ocean showed some promise, but trans-ocean application to core tops of more sensitive photon-stimulated-luminescence (PSL) multi-aliquot procedures revealed large regional variations in accuracy. Recent applications of single-aliquot PSL procedures to core tops and dirty sea ice refine our understanding of the limitations and capabilities of PSL dating in this ocean. Dirty sea ice is the source of most of the bottom sediment. Particular procedures for multi-aliquot and single-aliquot PSL dating produce near-zero ages for silt grains from 4 available dirty sea-ice samples. This suggests that TL and PSL age overestimates obtained previously from core-top samples were due mainly to bottom-current reworking (mixing or stripping) of silt grains in abyssal darkness, rather than due to sedimentation of relict-age grains from sea ice. Single-aliquot PSL dating of silt grains from core tops at two different-depth sites at the Alaskan margin reveals large variations in accuracy. This re-emphasizes the critical sensitivity of fine-silt PSL dating accuracy to the effects of bottom-current processes. These recent applications are beginning to provide a more precise knowledge of just where and how PSL methods are likely to be accurate in the Arctic Ocean.

Petrography and temporal changes in petrofacies of deep‐marine Ainsa–Jaca basin sandstone systems, Early and Middle Eocene, Spanish Pyrenees

AbstractA petrographic study of 157 samples from the Early to Middle Eocene deep‐marine sandy systems, Ainsa–Jaca basin, Spanish Pyrenees, shows that each system has a characteristic petrofacies. Three main petrofacies are recognized. Petrofacies 1 sandstones comprise mainly siliciclastic grains (≥80%), subordinate terrigenous carbonate grains and negligible intrabasinal grains. Petrofacies 2 hybrid arenites are characterized by significant amounts (≥10%) of intrabasinal carbonate grains. Petrofacies 3 calcilithites contain relatively abundant (≥10%) extrabasinal carbonate grains. On the basis of these petrofacies, a revised correlation of the sandy systems is proposed between the more proximal Ainsa basin, and the more distal Jaca basin sediments, now separated by the Boltaña anticline, across which it is impossible to actually trace out individual beds or sandstone packages between both basins. The arenite composition in the Ainsa and Jaca basins is interpreted as being controlled mainly by synsedimentary tectonic processes that led to changes in sediment sources during basin evolution.

Sand Composition and Transport History on a Fringing Coral Reef, Molokai, Hawaii

Composition of sand grains from the beaches, reef flat, and fore reef of south Molokai, Hawaii, provides key information about the origin and transport history of sediment on the reef and adjacent beach. The most common grain types include coralline algae, coral, chemically altered carbonate, and siliciclastic grains. Minor components include calcareous algal plates (Halimeda), mollusk fragments, and foraminifera; in selected areas, these components are abundant. Similarities in composition indicate that sand grains are freely transported between the beach and the reef-flat environments, whereas the fore reef has limited exchange of sand with either the beach or the reef flat. Overall, the calcium carbonate fraction of the sand, silt, and clay increases with distance offshore from 400 to 650 m, where the percentages plateau and remain relatively stable across the fore reef. The calcium carbonate content of bottom sediment indicates that sand transport on the reef flat is generally shore parallel with little shore-normal movement. This study has implications for management of beaches and coastal resources along fringing coral reefs in that it documents the effectiveness of the wide reef flat and reef crest in blocking sand transport with the fore reef. It also indicates that sediment introduced to the inner reef flat is not quickly dispersed seaward and therefore has a relatively high residence time in that setting.

Paragenesis of the Morgan Creek Limestone, Late Cambrian, central Texas: Constraints on the formation of glauconite

Abstract Deposition of the Morgan Creek Limestone, a member of the Cambrian Wilberns Formation, began a few meters above a disconformable surface and displays abundant indicators of accumulation in shallow marine to tidal flat environments. These indicators include: intercalation of a lithologic variety of thin beds (e.g. biosparites, biomicrites, oosparites, intrasparites), which display rapid vertical and lateral lithologic changes, numerous stromatolitic horizons, channels filled with graded oosparites and intrasparites that cut through micrite accumulations, and finely laminated (non-burrowed) siltstones. Glauconite is a ubiquitous minor constituent throughout in the form of pellets, replaced skeletal material and mica books, and, most informatively, as an authigenic precipitate in the form of fibroradiating rims on carbonate allochems and siliciclastic grains. Fibroradiating glauconite rims were disrupted and pushed away from the pelmatozoan nuclei on which they precipitated. Combined sedimentological and paragenetic constraints indicate that the glauconite was the earliest diagenetic event to affect these sediments and occurred essentially at the sediment–water interface within these relatively high-energy, shallow marine deposits. Precipitation of glauconite was closely followed by precipitation of carbonate cement as well as dissolution of aragonitic constituents. Later diagenetic processes included selective dolomitization and silicification. In modern seas glauconite is reported to form at and below mid-shelf water depths. Thus, the constraints pertaining to the origin of modern glauconite are not valid for Late Cambrian deposits and probably are also not applicable for Late Cretaceous through Early Paleogene glauconites.

Siliciclastic grain size composition of the Upper Ordovician rocks of the Blidene Formation in western Latvia

New data on the siliciclastic grain size composition of the Upper Ordovician Blidene Formation in its stratotype region in western Latvia are presented. The formation represents a marly interval at the transition from Keila to Oandu stages, where the succession of deposits has been influenced by essential changes in the depositional regime in the Baltic palaeobasin. The Blidene Formation overlies the organodetrital limestones of the Adze Formation and is succeeded by the organic-rich black shales of the Mossen Formation. It is characterized by a high content (50-85%) of siliciclastic material, increasing upwards. The siliciclastic material consists mainly of clay fraction ( 63 µm), forming less than 5%. The data obtained on the Blidene Formation in western Latvia are compared with earlier data from southern Estonia. The increase in the siliciclastic component of the Ordovician carbonate sequence in Keila time is considered to be related with climatic and sea level changes, and with the changes in the sea bottom topography due to tectonic movements.

Carbonate cements in contemporaneous beachrocks, Jaguaribe beach, Itamaracá island, northeastern Brazil: petrographic, geochemical and isotopic aspects

Holocene beachrocks of the Jaguaribe beach, State of Pernambuco, northeastern Brazil, consist of horizontal, cemented layers approximately 40 cm thick. The cement shows three textural varieties: (a) calciferous, surrounding siliciclastic grains, (b) micritic, with an acicular fringe; and (c) cryptocrystalline calcite in pores. Early cementation took place at the water table below beach ridges, where geochemical, hydrodynamic and, perhaps, also microbiological conditions favored rapid precipitation of aragonite and/or high-Mg calcite. delta13C values range from -1.8 to +1.5‰ for dissolved carbonate in interstitial water and from +0.2 to +2.1‰ for bioclastic components. delta18O values range from -2.8 to +0.5‰ for seawater, freshwater and interstitial water. delta13C values and diagenetic features suggest that cementation occurred in meteoric-vadose and/or marine-phreatic water by loss of CO2 during evaporation of the interstitial water. Locally, superimposed low-Mg calcite cements point to subsequent freshwater influence. Total-rock cement composition of vertically stacked beachrock beds at the Jaguaribe beach shows that the highest beachrock bed is older than the one (of same petrographic composition) situated at the current groundwater level. This implies a downward progression of cementation, which probably followed the sea-level fall after a local high stand.

Late Quaternary climatic control on erosion and weathering in the eastern Tibetan Plateau and the Mekong Basin

High-resolution siliciclastic grain size and bulk mineralogy combined with clay mineralogy, rubidium, strontium, and neodymium isotopes of Core MD01-2393 collected off the Mekong River estuary in the southwestern South China Sea reveals a monsoon-controlled chemical weathering and physical erosion history during the last 190,000 yr in the eastern Tibetan Plateau and the Mekong Basin. The ranges of isotopic composition are limited throughout sedimentary records: 87Sr/ 86Sr = 0.7206–0.7240 and ε Nd(0) = −11.1 to −12.1. These values match well to those of Mekong River sediments and they are considered to reflect this source region. Smectites/(illite + chlorite) and smectites/kaolinite ratios are used as indices of chemical weathering rates, whereas the bulk kaolinite/quartz ratio is used as an index of physical erosion rates in the eastern Tibetan Plateau and the Mekong Basin. Furthermore, the 2.5–6.5 μm/15–55 μm siliciclastic grain size population ratio represents the intensity of sediment discharge of the Mekong River and, in turn, the East Asian summer monsoon intensity. Strengthened chemical weathering corresponds to increased sediment discharge and weakened physical erosion during interglacial periods. In contrast, weakened chemical weathering associated with reduced sediment discharge and intensified physical erosion during glacial periods. Such strong glacial–interglacial correlations between chemical weathering/erosion and sediment discharge imply the monsoon-controlled weathering and erosion.

Sediment dynamics in a submarine canyon: a case of river–sea interaction

This paper presents findings from a 1-month field experiment conducted during the flood season of 2000 on the southern coast of Taiwan to examine the delivery of lithogenic (siliciclastic) sediment to Kao-ping Submarine Canyon from the nearby Kao-ping River and shelf sources and the transport of suspended lithogenic and nonlithogenic particles near the canyon floor. Collected time series data included simultaneous measurements of coastal winds, salinity, temperature, flow field, suspended sediment concentration (SSC) and size composition of suspended particles at the river mouth and inside the submarine canyon, as well as results from seawater and sediment trap data. Other collected data included river discharge and sediment load and hydrographic measurements along and across the axis of the canyon. During the experiment, two significant episodic river sediment discharge events occurred relating to a typhoon and extreme precipitation conditions.The findings show that theoretically, the time difference between sand and clay siliciclastic grains to settle from the sea surface to the canyon floor is on the order of hours to months, analogous to a sorting process. River plume dynamics and the coastal wind field largely control the delivery of terrigenous fine-grained sediment to the canyon. Inside the canyon, the ‘behavior’ of suspended lithogenic and nonlithogenic particles of different sizes can be differentiated into a coarse-grained group (sand), which forms the ‘drop-in’ population; and a fine-grained group (mud), which forms the ‘standing’ population. Each group has distinct spectral characteristics. The coarse-grained group is delivered directly to the canyon by the river effluent, and by wave resuspension of the shelf substrate. During the study period, the net transport of suspended particles near the canyon floor was landward mostly by tidal currents, whose direction was also modulated by spring/neap tide. Both lithogenic and nonlithogenic particles contribute to high mass fluxes (exceeding 700 g m−2 day−1) in the lower part of the canyon.This river–sea system represents a type of systems that consist of three geographically separated dynamic entities: the river plume, the shelf, and the canyon. Under fair-weather conditions, the river plume and shelf processes are coupled through the tide and wind field. The river and canyon form a direct source-to-sink link via coarse-grained siliciclastic sediment during the flood season in both continuous and pulsed fashion. In the typhoon season, the shelf becomes a pulsed source region for coarse-grained sediment.

Upper Ordovician sequences of western Estonia

The Upper Ordovician (uppermost Caradoc-Ashgill) section of western Estonia consists of a series of seven open-shelf carbonate sequences. Depositional facies grade laterally through a series of shelf-to-basin facies belts: grain-supported facies (shallow shelf), mixed facies (middle shelf), mud-supported facies (deep shelf and slope) and black shale facies (basin). Locally, a stromatactis mud mound occurs in a middle-to-deep shelf position. Shallow-to-deep shelf facies occur widely across the Estonian Shelf and grade laterally through a transitional (slope) belt into the basinal deposits of the Livonian Basin. Each sequence consists of a shallowing-upward, prograding facies succession. Sequences 1 (Upper Nabala Stage) and 2 (Vormsi Stage) record step-wise drowning of underlying shelf units (lower Nabala) that culminated in the deposition of the most basinal facies (Fjäcka Shale) in the Livonian Basin. Sequences 3–6 comprise the overlying Pirgu Stage and record the gradual expansion of shallow and middle-shelf facies across the Estonian Shelf. The Porkuni Stage (sequence 7) is bracketed by erosional surfaces and contains the shallowest-water facies of the preserved strata. The uppermost part of the section ( Normalograptus persculptus biozone) is restricted to the Livonian Basin, and includes redeposited carbonate and siliciclastic grains; it is the lowstand systems tract of the lowest Silurian sequence 8. Sequence 7 and the overlying basinal redeposited material (i.e., the lowstand of sequence 8) correspond to the latest Ordovician (Hirnantian) glacial interval, and the bracketing unconformities are interpreted as the widely recognized early and late Hirnantian glacial maximums. The sequences appear correlative to Upper Ordovician sequences in Laurentia. Graptolite biozones indicated that the Estonian sequences are equivalent to carbonate ramp sequences in the western United States (Great Basin) and mixed carbonate-siliciclastic sequences in the eastern United States (Appalachian Basin–Cincinnati Arch region). These correlations indicate a strong eustatic control over sequence development despite the contrasting tectonic settings of these basins.

Sediment dynamics in a submarine canyon: a case of river?sea interaction

This paper presents findings from a 1-month field experiment conducted during the flood season of 2000 on the southern coast of Taiwan to examine the delivery of lithogenic (siliciclastic) sediment to Kao-ping Submarine Canyon from the nearby Kao-ping River and shelf sources and the transport of suspended lithogenic and nonlithogenic particles near the canyon floor. Collected time series data included simultaneous measurements of coastal winds, salinity, temperature, flow field, suspended sediment concentration (SSC) and size composition of suspended particles at the river mouth and inside the submarine canyon, as well as results from seawater and sediment trap data. Other collected data included river discharge and sediment load and hydrographic measurements along and across the axis of the canyon. During the experiment, two significant episodic river sediment discharge events occurred relating to a typhoon and extreme precipitation conditions. The findings show that theoretically, the time difference between sand and clay siliciclastic grains to settle from the sea surface to the canyon floor is on the order of hours to months, analogous to a sorting process. River plume dynamics and the coastal wind field largely control the delivery of terrigenous fine-grained sediment to the canyon. Inside the canyon, the ‘behavior’ of suspended lithogenic and nonlithogenic particles of different sizes can be differentiated into a coarse-grained group (sand), which forms the ‘drop-in’ population; and a fine-grained group (mud), which forms the ‘standing’ population. Each group has distinct spectral characteristics. The coarse-grained group is delivered directly to the canyon by the river effluent, and by wave resuspension of the shelf substrate. During the study period, the net transport of suspended particles near the canyon floor was landward mostly by tidal currents, whose direction was also modulated by spring/neap tide. Both lithogenic and nonlithogenic particles contribute to high mass fluxes (exceeding 700 g m−2 day−1) in the lower part of the canyon. This river–sea system represents a type of systems that consist of three geographically separated dynamic entities: the river plume, the shelf, and the canyon. Under fair-weather conditions, the river plume and shelf processes are coupled through the tide and wind field. The river and canyon form a direct source-to-sink link via coarse-grained siliciclastic sediment during the flood season in both continuous and pulsed fashion. In the typhoon season, the shelf becomes a pulsed source region for coarse-grained sediment.