Quaternary Carbonate Deposits Research Articles

Quaternary fluvial carbonate deposits of the Almonda River Valley, Central Portugal

This paper discusses the formation and preservation of a fluvial tufa system influenced by Atlantic climate based on stratigraphical, chronological (amino-acid racemization, AAR), sedimentological and stable-isotope analyses. On the southwestern Iberian Peninsula, the tufas and associated deposits of the Almonda River valley occur as isolated terraced bodies and reach 25 m thick. AAR dated most deposits to within the warm Marine Isotope Stage 5 (MIS-5). Two Holocene ages were reset within MIS-5 based on diverse criteria. Widely varied carbonate and minor allochthonous coarse detrital facies occur arranged in four simple vertical associations. The deposit geometry and facies association distribution correspond to a low- to moderate-sloped fluvial valley consisting of several short knickpoints and extensive flat areas between them. The latter are occupied by slow-flowing water facies (carbonate sand, lime mud, phytoclast and oncoid rudstones, and up-growing stem boundstones). Facies that formed in moderate- to high-slope substrates were stromatolite, moss and down-growing stem boundstones. The homogeneous Miocene bedrock lithology and gentle structural deformation propitiated this depositional architecture. Calcite δ13C and δ18O values suggest that the aquifer water provided the outflowing Almonda water with (1) 18O-enriched water, compared with present precipitation and groundwater δ18O values, and (2) 13C-depleted CO2 from bituminous rocks and vegetation cover in the catchment. The proximity to the Atlantic coast favoured the Mesozoic-rock aquifer recharge with 18O-enriched water precipitation, assuring water availability during the formation of the studied tufas. No evidence of frequent intense erosion phases might indicate stable precipitation regimes, which would have allowed the preservation of loose fine-grained and palustrine deposits.

Quaternary lacustrine carbonate deposits of the Great Basin, USA: Impact of climate, tectonics and substrate

ABSTRACTThe Great Basin included several lacustrine systems that accommodated extensive carbonate buildups: the Lahontan on the western side (dating back to 48 kyr cal bp), and the Bonneville (from 30 to 11.5 kyr cal bp) and Great Salt Lake (starting 11.5 kyr cal bp) on the eastern side of the basin. The eastern lakes show a transition from freshwater to hypersaline conditions. In contrast, the western lakes do not show any significant change from the originally prevailing freshwater conditions. Mapping of the carbonate buildups in the different lakes settings enables a comparison of a biotic versus an abiotic composition of the carbonate buildups, their morphologies and their specific spatial distributions. The morphology, size and distribution of the carbonate deposits are predominantly governed by seasonal to long‐term water level fluctuations, particular geomorphological heritage, fault‐induced processes, groundwater seepage and substrates. All of the lakes show a palaeoshoreline distribution with some buildups containing crusts, hemispheroid domes, and parts of complex domes resulting from climate‐induced lake level variations. However, the presence of columns and complex domes made up of mixed biotic/abiotic carbonates in the western Great Basin is related to the influx of groundwater. Winnemucca Lake and Pyramid Lake contain bigger buildups than the ones observed in the eastern Great Basin lakes. The presence of these large size buildups and thinolites in the Lahontan lacustrine system are a consequence of local hydrological processes associated with influenced groundwater flows through specific sedimentary structures (for example, from springs and delta fronts) and faults. This contrast in the distribution, composition and size of the buildups between the two sides of the Great Basin suggests local changes in water chemistry (for example, [Ca2+]) and groundwater influxes. This work provides a novel conceptual model for the formation of abiotic and/or biotic carbonate buildups in lacustrine settings.

Boron Isotope Geochemistry of the Lakkor Co Salt Lake (Tibet) and Its Geological Significance

The origin of boron in modern salt lakes has long been a controversial subject. This study focuses on the boron geochemistry of Lakkor Co salt lake, located on the Shiquanhe-Lakkor Co-Yongzhu-Jiali Ophiolite Mélange Zone (SYMZ). Lakkor Co is one of the main boron-rich salt lakes in the Tibetan plateau where the boron concentration ranges from 401 to 849 mg L-1. The boron isotope value ( δ 11 B ) of the carbonate deposit within the Lakkor Co region is -28.08‰. The boron isotope value of the Somme river decreases from -9.97‰ to -11.15‰ after it flows through the carbonate deposit region, indicating that the carbonate deposit in this region releases boron, which has a more negative isotope value. However, the boron isotope value of Bai Kok river increases from -15.63‰ to -13.76‰, when it flows through the Quaternary clay region in Ganga Co, indicating that the carbonate deposit undergoes a process of boron adsorption. It can be concluded that the Quaternary carbonate deposits may be a source of boron and also serve as a temporary boron reservoir, thus playing an important role in the formation of modern salt lake boron deposits in Tibet.

Late‐Quaternary miliolite (biogenic carbonate) deposits and their implications for sea‐level fluctuations and climatic variability

AbstractThe western coast of India (Kachchh) has ubiquitous preservations of Quaternary carbonate deposits popularly known as “miliolites”. Field‐based detail documentation of the nature and distribution of miliolites supported by sedimentology and scanning electron microscopy (SEM) microtextural studies indicates that the miliolites were primarily deposited by wind as aeolinites and are termed “primary miliolites (PM)”. These were subsequently fluvially eroded and deposited and such deposits are termed “secondary miliolites (SM)”. The PM are dated between 28 ka to 16 ka largely clustering around the Last Glacial Maxima (LGM). In comparison, the fluvially reworked SM occurs as valley‐fill deposits and were deposited episodically during 23 ka to 10 ka before present. Climatically, the deposition of PMs suggests enhanced aridity whereas SMs represent episodic strengthening of the Indian Summer Monsoon (ISM). The bedding attitudes of the PMs indicate that they were deposited predominantly by the northeasterly winds whereas the presence of the deflation lag deposits suggests downwind migration of sands during the equilibrium condition (reduced sediment supply). The aeolian parameters (based on the grain size and wind velocities) for PMs indicate high shear velocity (~23 to 48 cm−1) and high transport rate (< 0.1 to 0.2 g/cm s). The short average flying distance implies that the biogenic sands were transported in multiple pulses (from coast to inland) at rates varying from ~200 m/h to ~1 km/h. Comparing our data with related occurrences along the biologically productive coasts in the mid‐latitudinal belt indicates that the regional/global aridity during the lowered sea level was responsible for inland occurrences of aeolinites, particularly during the LGM. The strengthened northeast monsoon winds during the LGM was conducive to upwelling and production of biogenic carbonates along the coasts. The subsequent reworking of the aeolinites were mainly the results of local variability in precipitation.

Cathodoluminescence in Quaternary carbonate deposits

The cathodoluminescent oscillatory and sectoral growth zones common in crystals formed in ancient limestone successions in a variety of putative environments appear to be rare or absent from Recent and Pleistocene marine carbonate sequences. The factors controlling cathodoluminescence and reasons for this disparity are examined. The cathodoluminescent zones in the cements of ancient rocks have been interpreted as responses to variations in the redox potential of formative pore waters during crystal growth; although similar cathodoluminescent behaviour is recorded from some deposits, including travertines and Quaternary speleothems, formed in what are thought to have been strongly oxidizing environments. The apparent absence of cathodoluminescence in the most Recent and Pleistocene marine deposits, that presumably reflect deposition and diagenesis in environments that are also characteristically oxidized, therefore seems anomalous. The controlling influences on cathodoluminescence are reviewed, together with evidence relating to observations of Pleistocene marine deposits and likely conditions of formation but, where it is present, the mechanism(s) for its development remain elusive.

Quaternary carbonate deposition and climate variation (Tyrrhenian coast, Calabria, Southern Italy)

A mid-latitude coralgal bioconstruction (Tyrrhenian coast of Calabria, Southern Italy) and associated sedimentary facies record climate and sea-level changes of this coastal area during the Pleistocene. The coralgal bioconstruction, characterized by Cladocora caespitosa bafflestone and red algae bindstone, associated with Spondylus gaederopus sp.-rich boundstone, formed as a result of submergence during Marine Isotope Stage 9 in a moderate shallow-marine environment at a depth of 6 - 20 m. The coast was affected by moderate to high-energy currents and occasional storms; the annual temperature was 12° - 18°C. The lower part of the coralgal body is dominated by Cladocora and the upper part by Lithophylloid algae. The change was likely the result of the occasional influx of coarse clastic material partially buried the C. caespitosa colonies, but provided a suitable substrate for the pioneer encrusting organisms. During a subsequent glacial stage, sea-level fall combined with tectonic uplift induced the partial emergence of the coralgal body, and deposition of an overlying subaerially-deposited breccia. A later resubmergence of the area, likely during the last interglacial period (MIS 5), is recorded as testified by the presence of endolithic microbialites in cavities within the coralgal bioconstruction and the breccia deposit. Microbialite formation took place in a marine environment with cooler water, with respect to the previous submergence phase, possibly during a time of early deglaciation. The low temperature, possibly combined with very shallow depths and an excess of nutrients, could have played a role in preventing further growth of C. caespitosa while favouring the precipitation of microbial carbonate.

OSL dating of Brazilian fluvial carbonates (tufas) using detrital quartz grains

Robust chronologies for Quaternary fluvial carbonate sediments (tufas) from Brazil are still challenging. We obtain Optically Stimulated Luminescence (OSL) ages of Brazilian tufa deposits using the Single Aliquot Regeneration (SAR) dose protocol applied to single grains and multigrain aliquots of quartz extracted from tufa matrix. Additionally, radiocarbon (14C) ages were obtained for the carbonate matrix and organic matter from tufas. The methodology was applied in samples from tufa deposits in southwestern and southeastern regions with Quaternary carbonate deposition. Equivalent doses distributions have overdispersion between 19% and 81%, indicating that the studied samples comprise well bleached as well as poorly bleached sediments. The quartz grains extracted from the studied tufas show an OSL signal dominated by the fast component and reliable dose response curves, with saturation doses (2D0) ranging from 35 to 210 Gy. Most of the samples has low radiation dose rates (0.40 ± 0.02 to 0.78 ± 0.05 Gy/ka), with variation in the water content due to compaction and matrix recrystallization, being the major factors that could induce dose rate changes through time. Hard tufa samples with intense carbonate cementation show quartz OSL ages from 51.9 ± 4.8 to 150.3 ± 35.9 years. These ages are much older than the radiocarbon ages of the carbonate matrix and organic matter. The OSL ages of soft tufa samples related with the active fluvial system ranged from 0.8 ± 0.1 to 5.5 ± 0.4 years and are in the same interval of radiocarbon ages presented in the previous works. We interpret that carbonate recrystallization or uptake of organic matter after tufa formation are responsible for the younger radiocarbon ages compared to quartz OSL ages in the older tufa samples. For these samples, age differences vary up to tens of thousands years between deposition of quartz grains and the last episode of carbonate recrystallization or organic matter uptake. Consequently we believe that the obtained OSL ages are the best estimates for the starting deposition time of the studied tufa samples.

Late Quaternary carbonate deposition at the bottom of the world

Carbonate sediments on polar shelves hold great potential for improving understanding of climate and oceanography in regions of the globe that are particularly sensitive to global change. Such deposits have, however, not received much attention from sedimentologists and thus remain poorly understood. This study investigates the distribution, composition, diagenesis, and stratigraphic context of Late Quaternary calcareous sediments recovered in 15 piston cores from the Ross Sea shelf, Antarctica. Results are used to develop a depositional model for carbonate deposition on glaciated, polar shelves. The utility of the deposits as analogs for the ancient record is explored. In the Ross Sea, carbonate-rich lithofacies, consisting of poorly sorted skeletal sand and gravel, are concentrated in the west and along the outer reaches of the continental shelf and upper slope. Analysis of fossil assemblages shows that deposits were produced by numerous low-diversity benthic communities dominated locally by stylasterine hydrocorals, barnacles, or bryozoans. Radiocarbon dating indicates that carbonate sedimentation was episodic, corresponding to times of reduced siliciclastic deposition. Most accumulation occurred during a time of glacial expansion in the lead-up to the Last Glacial Maximum. A more recent interval of carbonate accumulation postdates the early Holocene sea level rise and the establishment of the modern grounding line for the Ross Ice Shelf. When carbonate factories were inactive, fossil debris was subjected to infestation by bioeroders, dissolution, fragmentation, and physical reworking. This study reveals the episodic nature of carbonate deposition in polar settings and a reciprocal relationship with processes that deliver and redistribute siliciclastic debris. Carbonate production is most active during colder periods of the glacial–interglacial cycle, a potential new sedimentological paradigm for polar carbonate systems. Low accumulation rates and long residence times on the seafloor leave sediments vulnerable to significant post-depositional modification, processes that profoundly affect the appearance of deposits as they enter the rock record. Comparison with other examples of polar carbonates highlights the utility of these Late Quaternary deposits as a well-constrained analog that can aid in the recognition and interpretation of similar deposits from the ancient record.

Joints as fingerprints of stress in the quaternary carbonate deposits along coastal Saurashtra, Western India

Coastal cliffs and shore platforms are important geomorphic features of coastal areas of Saurashtra. These features are composed of medium to coarse grained carbonate sand and are designated as “Miliolitic limestones” that range in age from Middle to Late Pleistocene. Significant jointing has been observed in the Middle Pleistocene Miliolite Formation as well as in the younger shell limestone that comprises Chaya Formation of Late Pleistocene. Along with NE-SW trend which is the direction of maximum horizontal compressive stress [SHmax] for Indian sub-continent, other trends recorded are NNE-SSW, N-S, NW-SE and E-W. When compared with other regional studies, neotectonic episode in Saurashtra peninsula appears to be younger than at least 125ky. The present study on joint sets also indicates that they are important to understand stresses associated with anticlockwise rotation of the Indian plate.

Carbonate eolianites, quartz sands, and Quaternary sea-level cycles, Western Australia: A chronostratigraphic approach

Marine and eolian carbonate deposits, grouped under the name “Tamala Limestone”, have been investigated along thousands of kilometers of coastal Western Australia (WA). Relative-age diagenetic features of carbonate sand dunes or “eolianites” indicate that coastal ridges decrease in age seaward, reflecting coastal accretion during successive sea-level stands. Yellow- to red-stained quartz sands are associated with eolianites as pits, lenses, extensive beds, and even 40-m-high dunes.A regional survey using whole-rock and land snail amino acid epimerization geochronology confirms the eolianite succession and provides a means of correlating widespread deposits along a steep climatic gradient and 16° of latitude. AMS 14C and uranium–thorium (U/Th) ages provide independent radiometric calibration of the amino acid ratios, and eolianite ages can be estimated using a parabolic kinetic model.Over 90% of the sampled eolianite deposits comprise Aminozones A, C, E (125ka), F?, G, and H, and correlate with interglacials from <10ka (Holocene) to ca. 500ka. In contrast, at the type locality of the Tamala Limestone along the Zuytdorp Cliffs, the upper eolianite-paleosol units are characterized by advanced stages of cavernous weathering, pedogenesis, and recrystallization. In the same units, sediment and snail samples generally yield very low or non-detectable levels of amino acids. These factors along with the stratigraphic complexity of the deposits reflect an age much greater than a large majority of sites along the WA coastline. These findings encourage a revision of the existing classification and nomenclature of Quaternary carbonate deposits in WA, as well as a reexamination of the underlying mechanisms related to the formation and emplacement of both carbonate and quartz dunes.

Dating and geochemical tracing of paleoseismic events

Dating of shallow faults is crucial to understanding of the mechanism of earthquake generation and future seismic risk. However, radiometric dating of low temperature authigenic minerals in shallow fault zones has been a major challenge because of the contamination of fault rocks by older mineral phases and the overprinting of the syn-tectonic isotopic signature by post-tectonic re-crystallisation events. We present a technique for a reliable dating of fault movements using a combined application of low temperature geochronology, mineralogy and isotope-trace element geochemistry. Such combined studies further allow constraining the origin and migration of seismically mobilised fluids along active fault zones. We investigated illitic clay minerals from fault gouges and late Quaternary carbonate deposits in co-seismic fissures along currently active fault zones in Turkey. Our results indicate that the North Anatolian Fault Zone has been active since the Late Paleocene ? Early Eocene that followed immediately the continental collision related to the closure of the Neotethys Ocean. Fault movement is considered to have driven deeply sourced fluids, with metamorphic fluids being supplied by the compression along the Neotethyan orogenic suture zone. Precise dating of the late Quaternary carbonate deposits by U-series geochronology provide important constraints on the late Quaternary fissure generation related to the neotectonic processes and active faulting. Isotopic and trace element data indicate that the fissure carbonate deposits precipitated from deeply hydrothermal fluids. Mobilisation of deep fluids and their surface effusion is attributed to tectonic processes such as seismic pumping. K-Ar and U-series dating of clay and carbonate minerals respectively in seismically active areas is very promising for further studies for dating of major earthquake events and their recurrent intervals, with significant implications for the Australian active fault systems.

The Belize margin revisited. 2. Origin of Holocene antecedent topography

The importance of antecedent topography in dictating Holocene facies patterns has been generally recognized. There is, however, disagreement as to origin or lithology of the antecedent topography, particularly with respect to the siliciclastic or carbonate nature of the underlying topography and structural patterns. To help resolve these problems, published and unpublished information have been compiled to produce a structural fabric map of onshore and offshore Belize that includes a new geologic map of the country. The map, along with illustrated seismic lines, demonstrates the occurrence of a number of NNE-trending transpressional faults in which landward directed thrusting is consistently displayed along with tectonic inversion. Offshore wells in conjunction with the seismic lines document the inversion as post-Eocene, suggesting a similar age for transpressional fault movement. Presumably, the landward-directed thrusting reflects the opposing force of eastward-directed subduction along the western margin of Central America relative to the westward seafloor spreading of the Caribbean Cayman Ridge. The Belize faults show little current seismicity, but, nevertheless, the resulting structures have affected Quaternary carbonate deposition as evidenced on an illustrated seismic line by both seaward and landward (bi-directional) progradation of the reef margin from an underlying structural high. The structural influence on the development of Holocene antecedent topography is further suggested by the occurrence of a Bouguer gravity plateau in the same shelf area that marks the occurrence of the Belize lagoon rhomboid shelf atolls. The youngest documented lithology of 12 illustrated offshore exploration wells is Miocene carbonate. In some wells, the carbonate is interpreted as extending into the Pliocene and Holocene although no age diagnostic criteria are in evidence. In other wells, siliciclastics of unknown age and thickness are identified as overlying Neogene carbonates. The regional distribution and age of onshore limestones suggests that unroofing of the Cretaceous carbonate cap of the Maya Mountains siliciclastic sediment source did not occur until late in Neogene time, perhaps no earlier than late Pliocene. Consequently, the Maya Mountains could not have been a major offshore source of siliciclastics until the Quaternary. Information on the lithology of the immediately underlying pre-Holocene is provided by limited penetration core data and shallow resolution seismic lines. These show that antecedent Pleistocene limestones beneath the Holocene reefs were deposited around 130,000 b.p. (isotope stage 5e). The thickness of the overlying Holocene, shelf margin, reef-capped carbonates increases along depositional strike from a few meters in the north to more than 25 m in the south. In contrast, piston cores and seismic data from the southern shelf lagoon collectively document the occurrence of antecedent siliciclastic topography. The southward dip of both carbonate and siliciclastic antecedent surfaces is presumably a reflection of increasing southerly subsidence. Increasing antecedent erosional relief in the same direction reflects the pronounced southerly increase in paleo-rainfall that presumably paralleled that of present-day rainfall.

Significance of relic carbonate deposits along the central and southwestern margin of India for late Quaternary environmental and sea level changes

Environmental and sea level indicators were investigated using dredge samples from late Quaternary carbonate deposits along the shelf break between Goa and Cape Comorin, India. Geomorphic features in the area were identified from sonar profiles and included isolated patch reefs with a relief of up to 10 m, and linear reefs with reliefs between 2 and 15 m. The main clast types recovered from these features include fragmented corals and carbonate nodules dominated by either encrusted foraminifera or coralline algae. Some of these clast types are clearly of shallow-water origin. Fragments of reef-forming Poritid corals, for example, were collected off Mangalore at depths of 110–105 m and dated between 11,520 and 12,610 14C years BP (13.42–14.77 ka). Nodules of similar age dominated by Lithothamnium and capped by foraminiferal veneers were also collected at −90 m off Cape Comorin. Their altered algal tissues are consistent with formation in shallow water, high-energy conditions. In contrast, nodules recovered off Kochi and Mangalore–Goa are of deeper water origin, younger in age (10,980–7350 14C years BP), and are dominated by Gypsina encrustations with volumetrically less algal encrustation. They show cyclic succession of foraminiferal–algal, or foraminiferal–algal–coral laminations in which the algal species are typical of deeper waters. The age and elevation of corals and shallow-water nodules are both consistent with published glacio-eustatic sea-level curves. In addition, the alternate micro-encrustations of foraminifera, algae and encrusting corals could indicate changing conditions from nutrient-rich and turbid to nutrient-poor and clear water that may be attributable to seasonal variations in sediment flux caused by monsoons.

Uranium and thorium series disequilibrium in quaternary carbonate deposits from the Serra da Bodoquena and Pantanal do Miranda, Mato Grosso do Sul State, central Brazil

Activities of gamma-ray emitting members of the uranium ( 238U) and thorium ( 232Th) series were measured in a quaternary limestone deposit that outcrops in the southeastern Pantanal Matogrossense Basin and in quaternary tufas deposited at the drainage of the Serra da Bodoquena. It is a first step in a study of the mobilization of uranium and thorium series and its relation to surface hydrology, in a region where carbonate deposits are being continuously dissolved and reprecipitated. The obtained results show that all these deposits are characterized by very low concentrations of uranium and thorium. The 238U/ 226Ra and 228Th/ 228Ra activity ratios are significantly different than 1.0, indicating that both series are in radioactive disequilibrium. Although the Serra da Bodoquena deposits seem to be very recent, their very fine granulation and high porosity suggest that they behave as open systems for geochemical exchanges of uranium and thorium series members. The Pantanal do Miranda limestone has a radiocarbon age of 3900 yr BP. Since the thorium series is in disequilibrium it is also concluded that this deposit behaves as an open system for geochemical exchanges.

Composition and Morphology of Late Quaternary Carbonate Deposits, Outer Continental Shelf, Northeast Gulf of Mexico: ABSTRACT

Composition and Morphology of Late Quaternary Carbonate Deposits, Outer Continental Shelf, Northeast Gulf of Mexico: ABSTRACT

Surface electromagnetic geophysical exploration of the ground-water resources of Isla de Mona, Puerto Rico, a Caribbean carbonate Island

Two electromagnetic surface geophysical techniques were used to explore the ground-water resources of Isla de Mona, Puerto Rico—a 55-square-kilometer island located between Puerto Rico and Hispaniola, Isla de Mona is a tectonically uplifted carbonate plateau of Neogene age that has an average elevation of about 50 meters above mean sea level. This plateau is bounded by vertical cliffs except on the southwest where there is a narrow, 3-square-kilometer coastal plain. The coastal plain is composed of Quaternary carbonate deposits, and has a maximum elevation of less than 10 meters above mean sea level. No large-scale surface-water features are found on the plateau or on the coastal plain. To better understand the aquifer characteristics of the island, terrain conductivity and transient electromagnetic data were collected on the coastal plain and on the plateau. Computer programs were used to analyze quantitatively the electromagnetic data. Geoelectric models were produced to approximate the depth below land surface of the saline-freshwater interface underlying both the coastal plain and the plateau. Because the geophysical methods could not discern the water-table, it was assumed that it was near sea level. The thickness of the freshwater lens was estimated by subtracting the elevation of the land surface above mean sea level from the depth to the saline-freshwater interface as determined from the geophysical data. Results from the geophysical methods and water-level observations indicate that a freshwater lens with a maximum thickness of about 10 meters exists under the coastal plain. This lens thins towards the ocean, and it also thins away from the ocean toward the plateau. The model produced from the transient electromagnetic data indicates that the freshwater lens under the plateau has a maximum thickness of about 14 meters, which is a much thinner lens than previously estimated. A freshwater lens thickness of 14 meters is similar to direct measurements by divers in Cueva de Agua at Punta los Ingleses (located on the southeast coast), and observations by divers of freshwater seeps into the ocean at depths of 8 to 10 meters along the north coast cliffs after a major rainstorm.

230Th/ 234U and 14C dating of Quaternary carbonate deposits of Saurashtra, India — Comments

Commentaire sur l'article de Baskaran M. et al., 1989 . Chem. Geol. (Isot. Geosci. Sect.), 79: 65-82 et reponse de l'auteur.

230Th/ 234U and 14C dating of Quaternary carbonate deposits of Saurashtra, India — reply

230Th/ 234U and 14C dating of Quaternary carbonate deposits of Saurashtra, India — reply

Seismic expression of Quarternary climatic cycles in the peri-platform carbonate ooze of the northern Bahamas

Stratigraphic, physical, and acoustic properties of peri-platform carbonate sediments in an 18-m-long piston core (core 2218, 655 m) from Northwest Providence Channel, Bahamas, have been investigated. The sediments range in age from recent to about 1 m.y. old. There is a close correlation between changes of compressional wave velocity and Quaternary climate as indicated by δ 18 O. Velocity correlates negatively to aragonite content, water content, and porosity, whereas it correlates positively to grain size, saturated bulk density, and observed degree of cementation. Changes in the degree of cementation exert a particularly strong influence on velocity variations. All parameters correlate to Quaternary climate/sea-level fluctuations. During glacial/low-sea-level stages, the aragonite content, water content, water content, and porosity decrease, but grain size, saturated bulk density, and the degree of cementation increase; all of these increase velocity. In general, water content, porosity, and the aragonite content decrease slightly with core depth, but grain size, saturated bulk density, degree of cementation, and velocity increase slightly. Two processes may contribute to these trends: (1) shallow subsurface diagenesis upon burial and (2) variations of both the interglacial production of bank-top sediments and the intensity of glacial sea-floor diagenesis. Major changes in velocity are associated with transitions from glacial to interglacial stages. Acoustic reflectors correlate one-to-one with the velocity changes occurring at isotopic stage boundaries 7-8, 9-10, and 11-12. At the site of core 2218, deeper reflectors have an interference origin because velocity changes are more closely spaced. The reflectors that correlate with isotopic stage boundaries can be used to study the detailed history of Quaternary peri-platform carbonate deposition and place it into the context of global climate/sea-level cycles.

Formula omitted] and 14C dating of the Quaternary carbonate deposits of Saurashtra, India

Seventy-eight samples from Quaternary carbonate deposits of the Saurashtra peninsula in Gujarat State, India, were dated by three variants of the 230Th/ 234U technique: (I) using a 5% acetic acid leaching method and applying a correction for detrital Th; (II) using 1 M HC1 leaching with corrections for detrital U and Th; and (III) the isochron method. Twenty-one samples were also used for 14C age determination. About forty reliable 230Th/ 234U ages were obtained ranging from 45.9 to 204 ka. Of the 21 samples used for 14C dating, twelve yielded ages > 30 ka whereas the rest ranged from 12.1 to 21.5 ka. Based on the 230Th/ 234U ages it is inferred that miliolite formation took place mainly in three episodes: 50–70, 75–115 and 140–210 ka. Considering the available evidence in favour of tectonic instability of Saurashtra, uplift rates of the miliolite locations are calculated to range from 0.23 to 2.2 mm a −1, which are in good agreement with rates obtained for other regions based on similar studies.