Phreatic Environment Research Articles

Unusual speleothem formation in the Thabazimbi mine cave, Limpopo Province, South Africa, and its chronology

An unusual cave is located in the hills southeast of the town of Thabazimbi in the Limpopo Province. This cave, a V-shaped shallow chamber measuring 110 × 40 m, was discovered serendipitously when an adit was driven into the hill in connection with iron ore mining. It has no natural entry point and no externally derived sediment is found in it. It is situated within the upper part of the Malmani Subgroup and although it was formed as a solution cavity, requiring a phreatic environment, dripwater flow must subsequently have been low: no large speleothems (stalactites, stalagmites or flowstones) were ever formed in the cave. Instead, evaporation produced crystallictitic frostwork bushes consisting of aragonite (metastably grown due to enhanced Mg-content) which were in part redissolved followed by the secondary formation of calcite corals and encrustations. This sequence of events can be understood in the context of an enclosed cave where the CO2 content of the air is high. U/Th disequilibrium dating of two aragonite frostwork samples from the cave yielded two very different ages (155.1 ± 7.4 and 67.2 ± 3.4 ka), thus showing that the formation of this type of frostwork occurred (probably episodically) over a long period of time. The two dates broadly coincide with the waning stages of periods of low total inorganic carbon (TIC) as well as high clastic input in the Tshwaing crater sediment record, both indicating enhanced rainfall at that locality. The younger age coincides with a brief negative excursion in chlorine content and a positive potassium anomaly at Tshwaing, both also indicating high rainfall. However, both ages are also coeval with glacial maxima, thus underlining earlier observations from the Tshwaing record that in southern Africa, wet periods are not necessarily coeval with northern hemisphere interglacials.

From axial parallel to orthogonal groundwater flow during fold amplification: insights from carbonate concretion development during the growth of the Quattro Castella Anticline, Northern Apennines, Italy

This paper is based on a multidisciplinary field and laboratory study of carbonate concretions developed in poorly lithified Quaternary, syn-kinematic sediments along the Quattro Castella Anticline, Northern Apennines, Italy. The studied concretions consist of both tabular (parallel to bedding) and elongate single to coalescent concretionary bodies oriented at different angles to the bedding throughout the exposed stratigraphic succession. The dimensions of the concretions range from a few centimetres for single elongate concretions up to several metres for tabular to coalescent concretions. Field observations and petrophysical data indicate that the concretions developed preferentially in sediments characterized by mean grain sizes of 90–290 μm and a permeability ranging from 7 × 10 2 to 7 × 10 4 mD. Carbon and oxygen stable isotope analyses in conjunction with the petrographic investigations indicate that the precipitation of concretionary calcite occurred in a meteoric vadose realm during early eogenesis and subsequently in a meteoric phreatic environment. Diagenetic data and concretion patterns in syn-tectonic sediments suggest they formed during the lateral propagation of the anticline, which, in turn, promoted a change in the local topographic–hydraulic gradient from fold axial parallel to fold orthogonal. The integrated analysis of carbonate concretions provides a useful tool with which to unravel the palaeo-fluid flow history and therefore to predict fluid circulation patterns in folded siliciclastic rocks. Supplementary material: Complete isotopic data from both hand samples and thin sections, together with statistical analysis, scanning electron microscope cement images and grain size distributions with the standard operative procedure tests performed, and detailed permeability measurements are available at https://doi.org/10.6084/m9.figshare.c.4087019

Miocene weathering environments in Western Australia—Inferences from the abundance and 13C/12C of Fe(CO3)OH in CID goethite

The channel iron deposits (CID) of the Hamersley Province in Western Australia are dominated by pedogenic goethite/hematite-rich ooids and pisoids that were transported to, and deposited in, the meandering channels of Miocene rivers. Information about the Miocene weathering environments that produced the Fe(III) oxides is archived in the mole fraction (X) and δ13C of the Fe(CO3)OH component in solid solution in oolitic CID goethite (α-FeOOH).Values of X and δ13C were measured for 12 oolitic goethite samples from different depths in two cores drilled in CID of the Robe Formation of Mesa J. The weighted-average plateau values of X ranged from 0.0098 to 0.0334, which suggest ambient CO2 concentrations that ranged from ∼50,000 ppm V to perhaps as much as ∼200,000 ppm V at the time of goethite crystallization. In a vadose zone characterized by in situ production of CO2 with steady-state Fickian diffusive transport of the gas, such concentrations would correspond to modeled soil respiration rates (Q) ranging from about 10 to 30 mmol/m2/h. Values for Q of about 10 mmol/m2/h are reported for soils in modern tropical forests with MAP ≥ ∼2000 mm. However, model-derived values of Q that exceed 15 mmol/m2/h are larger than observed in modern systems. This could indicate that some of the CID goethites crystallized in conditions that were phreatic or near phreatic rather than vadose.The δ13C values of the Fe(CO3)OH component in these 12 CID samples ranged from −24.0‰ to −22.3‰, which are among the most negative measured to date. If they reflect steady-state diffusive transport of CO2 in vadose environments, the soil CO2 would have been derived from a source with δ13C values that ranged from ∼−31‰ to −29‰. If, on the other hand, the goethites crystallized in a nearly phreatic environment that was moderately acidic, the inferred δ13C of the ancient CO2 source would have been about −27.6‰ to −25.8‰. In either case, the δ13C values point to in situ oxidation of C3 organic matter as the predominant source of the ambient CO2.The Fe(III) oxides in the CID ooids suggest crystallization in aerobic environments. However, even in aerobic environments, many microbial species can reduce the Fe3+ in oxides to relatively soluble Fe2+ and may have facilitated progressive Fe enrichment during multiple cycles of Fe(III) oxide dissolution and recrystallization. At the same time, microbially mediated oxidation of organic matter could have produced the high concentrations of soil CO2 with the very negative δ13C values recorded in the Fe(CO3)OH component in oolitic goethite.More frequent summer storms in the Miocene, may have been a significant factor in forming and eroding these soil systems and in concentrating large volumes of oolitic Fe(III) oxides in the local river systems to form channel iron deposits. However, published (U-Th)/He ages indicate that the oolitic CID goethites of Mesa J became closed systems after ∼7 Ma, which suggests a change in local climate and/or conditions of burial at about that time in the Miocene.

The complex diagenetic history of discontinuities in shallow‐marine carbonate rocks: New insights from high‐resolution ion microprobe investigation of δ18O and δ13C of early cements

AbstractSedimentary gaps are a major obstacle in the reconstruction of a carbonate platform's history. In order to improve the understanding of the early diagenesis and the succession of events occurring during the formation of discontinuity surfaces in limestones, secondary ion mass spectrometry was used for the first time to measure the δ18O and δ13C signatures of 11 early cement and fabric stages in several discontinuity surfaces from the Jurassic carbonate platform of the Paris Basin, France. Pendant cements show a high variability in δ18O, which was impossible to detect by the less precise microdrilling method. The morphology of a given cement can be produced in various environments, and dogtooth cements especially can precipitate in marine phreatic and meteoric phreatic to vadose environments. Marine dogtooth cements and micritic microbially induced fabrics precipitated directly as low‐magnesium calcite in marine waters, as attested to by the preservation of their initial δ18O and δ13C signals. Five discontinuity types are recognized based on high‐resolution geochemical analyses, and their palaeoenvironmental history can be reconstructed. Two exposure surfaces with non‐ferroan pendant or meniscus cements formed in the oxidizing vadose zone. A hardground displays marine fibrous cements and non‐ferroan dogtooth cements that formed in a subtidal environment in oxidizing water. Two composite surfaces have undergone both marine and subaerial lithification. Composite surface 1 displays non‐luminescent ferroan dogtooth cements that precipitated in reduced conditions in seawater, followed by brown‐luminescent dogtooth cements characteristic of a meteoric phreatic environment. Composite surface 2 exhibits microbially induced fabrics that formed in marine water with abundant organic matter. The latter discontinuity, initially formed in a subtidal environment, was subsequently exposed to meteoric conditions, as evidenced by ferroan geopetal cements. A high‐resolution ion microprobe study is essential to precisely document the successive diagenetic environments that have affected carbonate rocks and discontinuities with a polygenic and intricate history.

Geology, mineralogy, and geochemistry of late Miocene paleosol and calcrete in the western part of the Central Anatolian Volcanic Province (CAVP), Turkey

The late Miocene was characterized by high global temperatures and partly to nearly complete desiccation of the Mediterranean Sea throughout the latter part of the Messinian age (the Messinian salinity crisis). The non-marine paleoenvironmental and paleoclimatic records from the Mediterranean area are highly limited in their spatial range. Late Miocene river flood-plain sediments in the Central Anatolian Volcanic Province (CAVP) of central Turkey consist of siliciclastic and pyroclastic sediments with paleosols and calcrete. Five lithofacies were identified as (A) massive conglomerate, (B) trough-cross-bedded sandstone, (C) massive mudstone, (D) disrupted mud, (E) and disrupted matrix-supported conglomerate. The paleosols were composed predominantly of smectite and illite with smaller amounts of feldspar, quartz, opal-A, pyroxene and amphibole, and the calcretes were composed mainly of calcite, with smaller amounts of feldspar, quartz and opal-A, and minor amounts of smectite, chlorite and illite. Authigenic clay minerals and micritic calcite coating on the detrital grains and soil peds and pendant cement in the calcrete and paleosol horizons were evidence of a vadose zone. In contrast, the large equant spar drusy mosaics were formed in phreatic environments. The weathering of ignimbrite and marble led to the depletion of SiO2, CaO, Al2O3, Fe2O3, TiO2, and K2O through the precipitation of smectite and illite in the paleosols, and CaO in the form of micrite and calcite in the calcrete horizons. Field observations, mineralogy, geochemistry and the results of δ18O and δ13C isotopic analysis reveal that alternating wet and dry periods resulted in an upward increase in precipitation of authigenic smectite and illite within the late Miocene sections, under the influence of geochemical and pH fluctuations in phreatic or vadose water within the river flood-plain paleoenvironment.

Control of cave levels in Kanaan, Kassarat and Jeita karst systems (Central Mount Lebanon, Lebanon)

Mount Lebanon is a mountainous Mediterranean karst ridge, consisting of high plateaus reaching 3,088 m altitude and entrenched by deep valleys. Fluvial drainage systems run on a steep gradient and reach the Mediterranean Sea coast in less than 30 km distance. In Central Mount Lebanon, the downstream parts of Antelias and Kalb valleys host the Kanaan cave (160 m), the Kassarat cave (4.6 km) and the Jeita cave (10.05 km), whose subterranean rivers are connected to the karst springs located close to the contact between the Cretaceous aqui- clude and the Jurassic karst aquifer. The area thus comprises a dammed Jurassic aquifer in which all the studied cave levels developed at certain altitudes. A geomorphological study performed by speleological and geomorphological surveys and completed by analyses of cave sediments allowed the formulation of a first evolutionary framework of the karst system. Several features suggest a long-term and multi-stage evolution of the karst in both Antelias and Kalb valleys. The horizontal caves develop between 60 and 160 m a. s. l. and constitute water table caves in a steeply dipping structure (Mount Lebanon's western fold). These cave levels are part of an ancient system that developed near the water table controlled by the position of the aquiclude and correspond to the base level of a past fluvial network. An important phase of base level cave development is documented in the relict passages above 90 m a. s. l. Speleogenetic features between 90 and 160 m developed in a phreatic environment where paragenesis or per ascensum erosion processes occurred during base level rise. Epiphreatic features were identified at 60–70 m in Kassarat and Jeita caves. Typical morphological features such as horizontal solution passages and solution notches suggest a phase of stillstand of the base-level. The evolution of the water table level from a paragenetic rise above 90 m to a continuous lowering below 90 m and with a temporary stillstand phase at 70–60 m, is related to the fluvial base-level evolution. Preliminar chronological constraints for the evolution of the karst system are discussed based on U/Th dating of speleothems and cave levels (90–160 and 60–70 m) and their relationship with the altitude of the low-gradient surfaces. The cave levels between 90 and 160 m are related to a water table rise that caused an alluviation process at the river's mouth probably during the Pliocene transgression phase of the Mediterranean Sea (PHS). Abandoning of the upper cave levels (90–160 and 60–70 m) is the result of a continuous lowering of the water table related to fluvial incision. The lowering of the water table level below 90 m is mainly related to the fluvial incision of the acquiclude in a high geodynamic context (Mount Lebanon uplift) during the Pleistocene.

Cave morphology and controls on speleogenesis in quartzite: The example of the Itambé do Mato Dentro area in southeastern Brazil

An area of 417 km2 in southeastern Brazil was mapped in detail regarding karst features in quartzite. Landforms typically assigned to carbonate karst terrains are largely absent, except for discrete occurrences of towers and collapse dolines. Caves are the most conspicuous landform, although occurring at a lower density than in most Brazilian carbonate areas. A total of 11 caves were mapped and studied in detail. Caves tend to develop at shallow depths parallel to the slope surface, controlled by lithological facies and the hydraulic gradient. Petrographical analyses suggest that mica (sericite) lenses within the quartzite and both sericite and iron around quartz grains may favour grain disaggregation and later erosional removal in a shallow phreatic environment. Quartzite caves represent a common feature in southeastern Brazil but tend to be isolated features rather than an integrated and areally extensive hydrological system typical of many carbonate settings.

Lithofacies of transgressive–regressive sequence on a carbonate ramp in Vindhyan basin (Proterozoic): a case of tidal-flat origin from central India

Palaeoenvironment of late Proterozoic carbonate deposits of Vindhyan basin of central India has been studied using lithofacies analysis, petrographical character and sediment dispersal patterns. Based on field parameters, 12 lithofacies were identified from five sections. For environmental interpretation, these lithofacies have been grouped into two major lithofacies associations namely a lower association A comprising reefal lithofacies, algal lithofacies, oolitic-intraclastic lithofacies, stromatolitic limestone lithofacies and an upper lithofacies association B, which includes nodular limestone lithofacies, large-scale planar cross-bedded lithofacies, parallel and ripple cross-laminated lithofacies, lenticular and flaser bedded lithofacies, shaly limestone lithofacies, synaeresis cracked lithofacies, deformed limestone lithofacies and finally, channelized and sandy limestone lithofacies. Reefal lithofacies and oolitic-intraclastic lithofacies were deposited under subtidal conditions. Synaeresis cracked lithofacies, lenticular and flaser bedded lithofacies, channelized and sandy limestone lithofacies were deposited under intertidal conditions whereas stromatolitic limestone lithofacies deposition ranged between subtidal to intertidal domains. Lithofacies character of the rock succession as well as the microfacies associations recognized petrographically indicate that the depositional environment belongs to platform interior with restricted circulation to marginal sand shoals in a carbonate ramp setting. Due to calcium-carbonate-saturated water, the diagenesis took place in stagnant marine phreatic environment where little compaction occurred, with abundant micrite as primary groundmass, and rapid cementation by calcite took place. With palaeocurrent current direction towards south and azimuthal dispersion of 270°, a pattern akin to a marine system is corroborated.

Hypogenic origin of Provalata Cave, Republic of Macedonia: a distinct case of successive thermal carbonic and sulfuric acid speleogenesis

Provalata Cave (Republic of Macedonia) is a small but remarkable hypogenic cave, developed in Cambrian marbles by successive thermal carbonic and sulfuric acid speleogenesis. The cave has a thick partly corroded calcite crust, abundant gypsum deposits, with cupolas, ceiling and wall channels, feeders and replacement pockets as some of the most characteristic morphological features. Distribution of morphology and deposits suggest a hypogenic origin in two distinct speleogenetic phases: the first by thermal CO2 rich waters, the second by sulfuric acid dissolution, which were separated by complete infilling of cave passages with pyroclastic-derived clays. In the first phase of speleogenesis, cave passages were formed by dissolution along fractures due to cooling of rising carbonated thermal waters. These phreatic morphologies were later covered with a thick calcite crust deposited in a shallow phreatic environment. In Early Pleistocene the cave was completely filled with clays due to deposition of pyroclastic rocks in a lacustrine environment in the nearby Mariovo Basin. Mariovo Lake sediments were later incised by the Buturica River, which cut down into Cambrian marbles, creating its superimposed valley. Incision lowered the water table and allowed removal of the clay deposits in Provalata Cave. The second phase of speleogenesis started after introduction of H2S associated with rising thermal waters. Oxidation produced sulfuric acid, which rapidly dissolved first calcite crust, then marble host rock. Condensation-corrosion by sulfuric vapors replaced carbonate rock with gypsum producing replacement pockets as well as second generation of pockets and cupolas. The contact of sulfuric acid with the clay deposits formed alunite, jarosite, and natroalunite. 40Ar/39Ar dating gave maximum ages of 1.6 Ma (alunite) and 1.46 Ma (jarosite) for this last stage of speleogenesis, thus making it the second 40Ar/39Ar dating of a sulfuric cave in Europe (after Kraushöhle in Austria), and the first dated cave in the Republic of Macedonia.

Petrography and geochemistry of Paleocene–Eocene limestones in the Ching-dar syncline, eastern Iran

The Ching-dar syncline is located to the west of the city of Birjand, in the east of Iran. The ca. 500 m thick studied section at the eastern flank of the syncline contains a sequence of almost continuous shallow-marine limestones that exhibit no major sedimentary breaks or evidence for volcanic activity. Skeletal grains consist of large benthic foraminifera and green algae whereas non-skeletal grains are mostly peloids and intraclasts. They were deposited on a shallow-marine carbonate ramp. The limestones have undergone extensive diagenetic processes with varying intensities, the most important of which are micritization, cementation, compaction (chemical and mechanical), internal filling and stylolitization. Chemical analysis of the limestone samples revealed high calcium and low magnesium content. Major and minor element values were used to determine the original carbonate mineralogy of these limestones. Petrographic evidence and elemental values indicate that calcite was the original carbonate mineral in the limestones of the Ching-dar syncline. The elemental composition of the Ching-dar carbonates also demonstrates that they have stabilized in a meteoric phreatic environment. Variation of Sr/Ca vs. Mn values suggests that diagenetic alteration occurred in an open geochemical system.

New insights on the red alga Archaeolithophyllum and its preservation from the Pennsylvanian of the Cantabrian Zone (NW Spain)

The new species Archaeolithophyllum asymmetricum nov. sp., from the Bachende Formation (Pennsylvanian, Cantabrian Zone, NW Spain), is described herein using cathodoluminescence microscopy. Under plane-polarized light, A. asymmetricum occurs as elongate and arcuate sheets preserved as calcitic mosaics of tiny anhedral to subhedral crystals. Cathodoluminescence has revealed that skeletal walls are composed of dull-bright (locally bright) luminescent calcite that contrasts sharply with the nonluminescent cements filling the intraskeletal pores. Skeletal walls are currently composed of low-Mg calcite (0.5–2 mol % MgCO3) with low Sr content (average 415 ppm). A. asymmetricum shows a strong asymmetry of the thallus organization. The internal tissue is well differentiated into a thick medullar hypothallus and a thin upper cortical perithallus, the latter being composed of nearly rectangular cells arranged in rows perpendicular to the external surface. Cell fusions commonly occur in the perithallial tissue whereas conceptacles exhibit a highly arched geometry lacking any preserved aperture. A. asymmetricum accumulations display a growth pattern similar to that reported from Late Paleozoic “phylloid algae”, and also resemble Miocene frameworks of the corallinacean Mesophyllum. These accumulations of A. asymmetricum formed micrite-rich bioherms with abundant shelter porosities, which are filled up with radiaxial-fibrous calcite (originally high-Mg calcite) and subsequent blocky spar. They constructed a rigid framework that was basically a combination of the foliaceous growth form, crust fusion and division, and synsedimentary marine cementation. Paleontological and sedimentological evidence suggests that A. asymmetricum thrived in an outer platform environment with relative quiet conditions. The exceptional preservation of these algae was favored by a rapid cementation of the intraskeletal pores under oxidizing conditions in a marine phreatic environment, protecting skeletons from early dissolution and recrystallization. Although the resulting neomorphic microsparite fabric suggests an aragonite precursor, the morphological similarities (especially reproductive organs) between Archaeolithophyllum and Recent calcitic corallinaceans, and the similar trace element composition of the algal thalli and the surrounding high-Mg radiaxial-fibrous cements, suggest that originally, Archaeolithophyllum was probably composed of high-Mg calcite. Based on the morphologic features, framework strategies (crust fusion and division) and growth modes, it is suggested that Archaeolithophyllum might be phylogenetically related to the modern coralline algae.

Diagenetic Pattern in the Citarate Carbonate Rocks, Cilograng Area, Lebak Regency, Banten Province

DOI: 10.17014/ijog.v7i3.142The carbonate sequence overlies conformably the tuffaceous sandstone unit, and in turn is conformably underlain by the tuff-sandstone unit, both of which are members of the Citarate Formation. The Citarate carbonate rocks were deposited in an open platform back reef environment, which was temporarily drowned by local sea level rise. Regional Middle Miocene deformation formed NNE-WSW trend faults and E-W trend folds in the researched area. This paper discusses the nature of diagenetic alteration of the Citarate carbonate rocks based on petrographic analyses of twenty surface samples. Carbonate rocks from bottom to top comprise algae packstone, packstone-grainstone, coral-algae packstone, and foraminifer wackestone-packstone. Fragments of coral, coralline red algae, and large foraminifera are the dominant bioclasts in most of the observed samples, whereas echinoids and bivalves are less abundant; they are set in a recrystallized micrite matrix. Planktonic foraminifera are abundant only in few samples. Fragments of plagioclase, igneous volcanic rocks, pyroclastic rocks (tuff), and much less abundant quartz are commonly present in all the studied samples. A generalized diagenesis includes early marine cementation by fibrous aragonite, compaction, aragonite dissolution and/or neomorphism, precipitation of equant-grained calcite cement in a phreatic environment, dissolution to form moldic porosities, dolomitization, the formation of stylolites and fractures, and precipitation of late ferroan calcite during burial. Multiple carbonate cements occur as pore-filling phases, with ferroan calcite cementation taking place during later-stage burial. Secondary porosities were formed during different stages in diagenetic processes, such as dissolution, dolomitization, and stylolite and fracture formations. Although precipitation of nonferroan and ferroan calcite cement occluded porosities, porosity enhancement during early selective dolomitization might still be significant. Current observations also revealed the presence of intraparticle, micro-vuggy, and fracture porosities in different samples.

Structural, petrographic and mineralogical characteristics and diagenetic aspects of Kandira stone from Northwestern Turkey

Kandira stone is a unique Late Cretaceous (Campanian-Maastrichtian) carbonate rock quarried by small enterprises in Northwestern Turkey. The rock has been used as a construction material in a variety of applications such as cladding and paving, flooring and stairs, roofing and fireplaces. Recently, the demand for this stone in the production of intricately sculpted shapes has raised questions about its workability and durability. Such questions require detailed characterization of structural, petrographic and mineralogical properties of the stone. This paper provides a database on the earlier mentioned properties collected from Kandira stone sampled from quarries. The investigation comprised field measurement and sampling of the Kandira stone, microscopic examination of thin sections, XRD, XRF and SEM studies. The Kandira stone is biomicritic limestone composed mainly of microcrystalline calcite crystals (micrite matrix), pelagic fossils, shell fragments and sponge spicules. Calcite is the sole carbonate mineral and is accompanied by minor quartz and authigenic clay minerals. Diagenetic alteration involved micritization, limited cementation and neomorphism. These processes were variably active in the marine phreatic and limited meteoric phreatic environment. The compact and closed form textural composition results in a stronger, durable stone suitable for masonry and decorative purposes. Key words: Kandira stone, natural stone, petrographic and mineralogical characteristics, biomicritic limestone, pelagic fossils, diagenesis, Late Cretaceous.

Role of sediment in speleogenesis; sedimentation and paragenesis

Although the effects of sedimentation in caves have been recognised for many years, its role in speleogenesis is frequently overlooked. Influxes of sediment into a cave system fundamentally alter the way cave passages develop, either by alluviation in a vadose environment, forcing lateral corrosion and the development of notches, or by upwards dissolution in a phreatic environment through a process known as paragenesis. Sediment influxes affect the hydrological functioning of a karst aquifer by changing the way conduits behave and subsequently develop both in plan and long section.Here we give an overview of the mechanisms of cave sedimentation and describe how the process of alluviation and paragenesis affect speleogenesis. A characteristic suite of meso- and micro-scale dissolutional features can be used to recognise paragenetic development, which is reviewed here. In a vadose environment these include alluvial notches, whilst in a phreatic environment, half tubes, anastomoses and pendants, bedrock fins and paragenetic dissolution ramps result. Using these to identify phases of sedimentation and paragenesis is crucial for reconstructing denudation chronologies from cave deposits. We suggest that sedimentation and paragenesis are most likely to occur in certain geomorphological situations, such as ice marginal and periglacial environments, beneath thick residual soils and where rivers can transport fluvial sediment into a cave, either via stream sinks or back-flooding.

Correlation of Aptian-Albian Carbon Isotope Excursions in Continental Strata of the Cretaceous Foreland Basin, Eastern Utah, U.S.A.

Abstract Nodular carbonates (“calcretes”) in continental foreland-basin strata of the Early Cretaceous Cedar Mountain Formation (CMF) in eastern Utah yield δ13C and δ18O records of changes in the exogenic carbon cycle related to oceanic anoxic events (OAEs), and terrestrial paleoclimate. Chemostratigraphic profiles of both forebulge and foredeep sections show two prominent positive δ13C excursions, each with a peak value of −3%0 VPDB, and having background δ13C values of about −6%0 VPDB. These excursions correlate with the global early Aptian (Ap7) and late Aptian–early Albian (Ap12-Al1) carbon isotope excursions. Aptian–Albian positive δ13C excursions in the CMF also correspond to 3–4 per mil increases in carbonate δ18O. These phenomena record local aridification events. The chemostratigraphic profile on the thinner forebulge section of the CMF is calibrated, for the first time, by a radiogenic U-Pb date of 119.4 ± 2.6 Ma on a carbonate bed, and by detrital zircon U-Pb dates on two bounding sandstone units (maximum depositional ages of 146 Ma and 112 Ma). Petrographic observations and diagenetic analyses of micritic to microsparitic carbonates from nodules indicate palustrine origins and demonstrate that they crystallized in shallow early meteoric phreatic environments. Meteoric calcite lines derived from CMF carbonates have δ18O values ranging between −8.1 to −7.5%0 VPDB, supporting an estimate of zonal mean groundwater δ18O of −6%0 VSMOW for an Aptian–Albian paleolatitude of 34° N. Furthermore, our two chemostratigraphic profiles exhibit a generally proportionate thinning of correlative strata from the foredeep on to the forebulge, suggesting that there were consistently lower rates of accumulation on the forebulge during the Aptian–Albian. Identification of the global Aptian–Albian δ13C excursions in purely continental strata, as demonstrated in this paper, opens a new avenue of research by identifying specific stratigraphic intervals that record the terrestrial paleoclimatic impacts of perturbations of the global carbon cycle.

Mineralogical and geochemical investigations of the Dajia Salento-type bauxite deposits, western Guangxi, China

The Dajia Salento-type bauxite deposit in western Guangxi is hosted within the Quaternary ferrallitic soil profile, and it formed via breaking up, weathering and oxidizing of Permian bauxite orebodies occurring as a semi-continuous layer in the upper Permian. Mineralogical analyses reveal that diaspore, hematite and kaolinite are the major minerals in bauxite ores with small amounts of anatase, chamosite, gibbsite, goethite, illite, zircon, quartz and pyrite. The ore texture and mineral assemblage reveal that the depositional/diagenetic environment of the Dajia bauxite was much close to phreatic environment. Both the ore texture and the morphology of zircon grains also indicate that most of the bauxitic soils were transported a short distance. Diaspore is suggested to be non-metamorphic in origin and mainly formed in a reducing condition of diagenetic environment, while kaolinite is the product of the in situ epigenetic replacement of alumina in diaspore by dissolved silica. Geochemical analyses indicate that Al 2O 3, Fe 2O 3, SiO 2 and TiO 2 are the main components of the bauxite ores and trace elements such as Zr, Hf, Nb, Ta, Th and U were enriched during the bauxitization process. Simultaneously, Zr vs. Hf and Nb vs. Ta show a high correlation. Geochemical indices such as Zr/Hf, Nb/Ta and Eu/Eu* (among others) denote that the magmatic rocks related to the Emeishan plume in western Guangxi and the carbonates in the underlying Maokou Formation provided the main sources of material for the bauxite ores.

A sustained +21 m sea-level highstand during MIS 11 (400 ka): direct fossil and sedimentary evidence from Bermuda

A small, protected karstic feature exposed in a limestone quarry in Bermuda preserved abundant sedimentary and biogenic materials documenting a transgressive phase, still-stand, and regressive phase of a sea-level in excess of 21.3 m above present during Marine Isotope Stage (MIS) 11 (400 ka) as determined by U/Th dating and amino acid racemization. Cobbles and marine sediments deposited during the high-energy transgressive phase exhibit rim cements indicating a subsequent phreatic environment. This was succeeded stratigraphically by a still-stand deposition of fine calcareous lagoonal sediments containing bioclasts of red algae and benthic and planktonic foraminifera that was intensely burrowed by marine invertebrates, probably upogebiid shrimp, that could not be produced under any condition other than sustained marine submergence. Overlying this were pure carbonate beach sands of a low-energy regressive phase containing abundant remains of terrestrial and marine vertebrates and invertebrates. The considerable diversity of this fauna along with taphonomic evidence from seabird remains indicates deposition by high run-up waves over a minimum duration of months, if not years. The maximum duration has yet to be determined but probably did not exceed one or two thousand years. The most abundant snails in this fauna are two species indicative of brackish water and high-tide line showing that a Ghyben-Herzberg lens must have existed at > + 20 m. The nature of these sediments and fossil accumulation is incompatible with tsunami deposition and, given the absence of evidence for tectonic uplift of the Bermuda pedestal or platform, provide proof that sea-level during MIS 11 exceeded +20 m, a fact that has widespread ramifications for geologists, biogeographers, and human demographics along the world's coastlines.

Microfacies and geochemistry of the Ilam Formation in the Tang-E Rashid area, Izeh, S.W. Iran

The Ilam Formation (Santonian–Campanian in age), part of the Bangestan Group, is disconformably overlain by the Sarvak Formation and underlain by the Gurpi Formation in the Tang-E Rashid, Peyon area, Izeh (Zagros), southwest of Iran. Facies analyses indicate that the Ilam carbonates formed in four microfacies belts: tidal flat, lagoon, shoal and open marine, in a platform ramp environment. Major and minor elements and carbon and oxygen isotope values were used to determine the original carbonate mineralogy of the Ilam Formation. Petrographic evidence and elemental and oxygen and carbon isotope values indicate that aragonite was the original carbonate mineralogy in the Ilam Formation. The elemental and isotopic compositions of the Ilam carbonates also illustrate that they have stabilized in the marine phreatic environment. Variations of Sr/Ca and δ 18O values versus Mn suggest that diagenetic alteration occurred in a closed system. Temperature calculation based on the oxygen isotope value of the least-altered sample indicates that the very early shallow burial fluid temperature was around 28 °C. Recognition of the exact boundary between the Ilam and Sarvak Formations is difficult, due to similar lithologies and the absence of the Surgah Formation in the study area. However, elemental and oxygen and carbon isotope analysis were used to determine the boundary between these formations. The δ 18O and δ 13C values, along with elemental results, clearly indicate a subaerial exposure surface, below which meteoric diagenesis affected the sediments.

Dolinas em arenitos da Bacia do Paraná: evidências de carste subjacente em Jardim (MS) e Ponta Grossa (PR)

In the Parana Basin, dolines in sandstones occur in the Aquidauana Formation in Jardim (MS) and in the Furnas Formation in Ponta Grossa (PR). In the Ponta Grossa region, the retreat of arenitic rocks is marked by the “Furnas Formation Escarpment”, whereas in Jardim there is no associated escarpment. The dolines of the Jardim are generally extensive and shallow, and those of the Ponta Grossa deep and scarped. In Ponta Grossa, the dolines have had their origin associated, in most studies, with the subsurface erosion of sandstones, due to the presence of ruiniform relief features, the proximity of escarpments and the mistaken absence of carbonatic rocks in the basement of the basin. The Jardim and Ponta Grossa dolines are collapse phenomena generated by a piping process, brought about by the development of a subjacent karst system, at depth, in a phreatic environment, in the carbonatic basement rocks. The role of subjacent karst in doline formation in sandstones is demonstrated by: 1) rather than being distributed along all the arenitic units of the Parana Basin, dolines are restricted to only those locations with carbonatic rocks in the basement structures of the basin; 2) ruiniform relief features are surface structures originating from erosive or weathering processes in sandstones and can be attributed to pseudokarst, having no direct relationship with the dolines, which even when present at the surface, represent points of injection of meteoric water, connected at depth to the karst system; 3) besides being evidence of structural control in the sandstones, the alignment of surface features shows the direction of karst conduits at depth. The “Vila Velha Depression”, described for the first time in this study, which includes the dolines in the Ponta Grossa region, is a reflection of the piping process brought about by the subjacent karst. In the exposed karst area typical features are not very frequent or evident, due to dolomitic composition, and the dolines are more evident in sandstones than the actual carbonate rocks. However, in Jardim, the gradual geomorphological transition between sandstones and carbonate rocks makes the subjacent karst more visible, which exposed the karst features, in a different manner to the Ponta Grossa region, where the karst is encountered at a distance from the dolines and at greater depth.

Dolinas em arenitos da Bacia do Paraná: evidências de carste subjacente em Jardim (MS) e Ponta Grossa (PR)

In the Paraná Basin, dolines in sandstones occur in the Aquidauana Formation in Jardim (MS) and in the Furnas Formation in Ponta Grossa (PR). In the Ponta Grossa region, the retreat of arenitic rocks is marked by the “Furnas Formation Escarpment”, whereas in Jardim there is no associated escarpment. The dolines of the Jardim are generally extensive and shallow, and those of the Ponta Grossa deep and scarped. In Ponta Grossa, the dolines have had their origin associated, in most studies, with the subsurface erosion of sandstones, due to the presence of ruiniform relief features, the proximity of escarpments and the mistaken absence of carbonatic rocks in the basement of the basin. The Jardim and Ponta Grossa dolines are collapse phenomena generated by a piping process, brought about by the development of a subjacent karst system, at depth, in a phreatic environment, in the carbonatic basement rocks. The role of subjacent karst in doline formation in sandstones is demonstrated by: 1) rather than being distributed along all the arenitic units of the Paraná Basin, dolines are restricted to only those locations with carbonatic rocks in the basement structures of the basin; 2) ruiniform relief features are surface structures originating from erosive or weathering processes in sandstones and can be attributed to pseudokarst, having no direct relationship with the dolines, which even when present at the surface, represent points of injection of meteoric water, connected at depth to the karst system; 3) besides being evidence of structural control in the sandstones, the alignment of surface features shows the direction of karst conduits at depth. The “Vila Velha Depression”, described for the first time in this study, which includes the dolines in the Ponta Grossa region, is a reflection of the piping process brought about by the subjacent karst. In the exposed karst area typical features are not very frequent or evident, due to dolomitic composition, and the dolines are more evident in sandstones than the actual carbonate rocks. However, in Jardim, the gradual geomorphological transition between sandstones and carbonate rocks makes the subjacent karst more visible, which exposed the karst features, in a different manner to the Ponta Grossa region, where the karst is encountered at a distance from the dolines and at greater depth.