Carbonate Infilling Research Articles

U–Pb ages via LA-ICP-MS of carbonate from brittle structures of Jandaíra formation, Potiguar basin, NE-Brazil

U–Pb carbonate dating by LA-ICP-MS is becoming a popular tool to investigate the timing of brittle deformation. In northeast Brazil, the absolute dating of brittle deformation in a series of rift-related intracontinental basins associated with the South Atlantic Rift has been little explored. Here we add on that, by investigating carbonate infilling and dating of fractures/veins from the Jandaíra limestone, part of the post-rift sequence of the Potiguar Basin. Until now, these features' ages (Lower Cretaceous to Middle Miocene) were inferred by field relationship and stratigraphic studies. Carbonates that were previously considered impossible to date, have been dated using a recent and challenging technique of U–Pb dating via LA-ICP-MS. Our data, the first ones with this method on this basin, indicate that post-rift brittle deformation can be constrained between 45 and 30 Ma. This interval is coincident with a major magmatic event (Macau-Messejana) suggesting a causal relationship between magmatism and brittle deformation, but the origin of the stress associated with this tectono-magmatic remains debatable. We validate here the use of U–Pb carbonate dating and its application for dating brittle deformation in northeast Brazil rift-related basins.

Meso to Neoproterozoic polyphase rifting and tectonic inversion: The example of the São João del Rei region at the southern border of the São Francisco Craton, Brazil

The up to 3000 m thick Mesoproterozoic and Neoproterozoic sedimentary succession exposed at the southern border of the São Francisco Craton, São João del Rei region (southern Minas Gerais State, Brazil), records the stacking of five sedimentary basins. These basins evolved due to multiple rifting episodes related to the southernmost Espinhaço rift system and were deformed around 600 Ma during the evolution of the Ribeira Orogen. These basins are the 1- São João del Rei Basin with quartzite units with maximum depositional age of ca. 1.5 Ga preserved in a half graben associated to the younger rift phases, 2- Carandaí Basin with rift related fault bounded debris flow deposits with maximum depositional age of ca. 1.38 Ga, and a sag carbonate infill recording the post-rift thermal relaxation, 3- Prados Basin, an NE-SW aulacogen linked to the passive margin Andrelândia Basin (<1.04 Ga) and 4- a foreland basin of the Brasiliano Orogeny with early syn-collisional plagioclase wacke deposits with maximum depositional age of ca. 650 Ma. The present paper discusses the role of the faults at the São João del Rei region in the evolution of these basins and the subsequent orogenic deformation. Five phases of fault activity were recognized. The first one is recorded by a few E-W faults and a single N–S fault and associated growth strata within the São João del Rei Basin. The second phase is recorded by activity of NE-SW, E-W and minor NW-SE faults during the Carandaí rifting (1.53–1.38 Ga). The main faults are the NW-SE Lenheiro and São João del Rei faults and the E-W São José Fault. Minor NW-SE faults are interpreted as release faults associated with the last two faults. NE-SW mafic dikes occurring along the faults suggest precursor/coeval magmatism. The third phase is characterized mainly by the reactivation of the NW-SE Lenheiro Fault leading to the development of the NW-SE Prados aulocogen. The fourth phase is inferred to be a reverse reactivation of NW-SE faults related to the NW tectonic transport direction of the Ribeira Orogen. The NW verging D1 folds and steeply SE dipping cleavage contribute to this interpretation. However, the reverse reactivation was insufficient to overcome the cumulative previous normal throw. During the fifth phase NE-SW faults were reactivated as dextral shear zones. Subhorizontal stretching lineation and mylonitic foliation occur along the Lenheiro Fault for at least 60 km. These structures are related to the E-W compression that generated other NE-SW regional dextral shear zones, such as the Três Corações and Caxambu shear zones during the late stages of the Brasiliano Orogeny. Structural data also reveals that during the Carandaí and/or Prados rifting, prior to the orogenic contraction, rotational oblique tilting in what was to become the northern limb of the Lenheiro Anticline generated the difference between the trends of the Beta axis and L1 in the northern limb of the anticline in relation to the other structural domains. This also explains the ENE-WSW trend of the Lenheiro Anticline contrasting with the NE-SW trend of the Águas Santas Syncline and of the other D1 regional folds south of the studied area. A forward model based on a cross section through the Águas Santas Syncline reveals that the NE-SW Lenheiro Fault acted as a major listric fault responsible for the development of a rollover anticline and the half-graben in which the São João del Rei basin deposits are preserved. Rotation and partial erosion of the hanging wall resulted in the deposition of the Carandaí diamictites in angular unconformity on the São João del Rei basin deposits. The model indicates 41% of total extension with a cumulative displacement of 4.2 km produced by the Carandaí and Prados rifting phases. The minimum shortening of the orogenic compressive phase is estimated to be about 11% with 1.57 km of displacement. These values may be underestimated since the model does not take into account parasitic folds, crystalplastic deformation and volume loss produced by layer parallel shortening and metamorphic reactions.

Determination of carbonate vein chemistry using portable X-ray fluorescence and its application to mineral exploration

The composition of carbonate minerals infilling hydrothermal veins can vary as a function of factors including fluid chemistry, temperature and pressure. If effectively quantified, carbonate vein chemistry can represent an important tool in mineral exploration. We present a method for determining carbonate vein chemistry using portable X-ray fluorescence analysis (pXRF) utilising matrix matched certified reference materials (CRMs) to assess pXRF analyser performance, before establishing strict QA/QC procedures, along with linear calibration equations for Mg, S, Mn, Fe, Zn, Sr and Pb. However, low concentrations of mineralization-related elements in commercially available CRMs meant raw S, Zn and Pb concentrations determined for case study samples could not be reliably corrected. Our new workflow was then applied to carbonate vein samples collected from Mount Isa, Northwest Queensland, Australia. Though complexity of this system limited the interpretation of results, the case study showed that pXRF can also be used to identify veins with multiple generations of carbonate infill. Portable X-ray fluorescence analyses, used in conjunction with robust QA/QC processes is a powerful tool that can quickly and cost-effectively inform decisions during exploration programs. Supplementary material: Results of pXRF analyser performance for six reference materials (S1) and results from analysis of carbonate veins collected from Mount Isa, Australia (S2). All tables are available at https://doi.org/10.6084/m9.figshare.c.3831385

Multi‐scale constraints of sediment source to sink systems in frontier basins: a forward stratigraphic modelling case study of the Levant region

AbstractRecent scientific work has highlighted the presence of an up to 12 km thick Cenozoic siliclastic and carbonate infill in the Levant Basin. Since the Late Eocene, several regional geodynamic events affecting Afro‐Arabia and Eurasia (collision and strike slip deformation) induced marginal uplifts. The initiation of local and long‐lived regional drainage systems in the Oligo‐Miocene period (e.g., Lebanon, Arabia and Nile) provoked a change in the depositional pattern along the Levant region from carbonate‐dominated to mixed clastic‐rich systems. Herein, we explore the importance of multi‐scale constraints (i.e., seismic, well and field data) in the quantification of subsidence history, sediment transport and deposition of a Middle to Upper Miocene “multi‐source” to sink system along the northern Levant frontier region. Through a comprehensive 4D forward stratigraphic modelling workflow, we suggest that the contribution to basin infill is split between proximal and more distal clastic sources as well asin situcarbonate and hemipelagic deposition. The results show that single‐source scenarios could not reasonably satisfy the basin‐scale constraints. The worldwide application of such new multi‐disciplinary workflows in frontier regions highlights the additional data constraints that are needed to de‐risk highly uncertain geological models in the hydrocarbon exploration phase.

Variations in petrophysical properties along a mixed siliciclastic carbonate ramp (Upper Jurassic, Ricla, NE Spain)

A multi-scale petrophysical study was performed on an Upper Jurassic siliciclastic – carbonate ramp system near the village of Ricla (Spain). A combination of textural (field observations, thin section and grain-size analysis), chemical – mineralogical (thermo gravimetric analysis) and physical (porosity, gamma ray and velocity measurements) properties was used to capture the heterogeneity of this mixed sedimentary system. Subsequently, synthetic seismic sections with varying resolution were created to show the characteristics of the internal ramp-structures. The translation from the measured petrophysical properties to a synthetic seismic section helps to better understand subsurface seismic. Although upscaling problems are a recurring issue, the homogeneity of the samples and the continuity of the outcrop and facies reduce the number of uncertainties.Acoustic properties are generally controlled by the amount and type of pore spaces. Due to the low porosities (ϕ < 7.2%) of this mixed ramp system the influence on the acoustic properties is limited; therefore other controlling parameters come into play. One of the primary controlling factors is the mineral composition, thus the presence of clay and the ratio between siliciclastic and carbonate components. A dominant presence of carbonate relates to relative high P- and lower S-wave velocities, a high percentage of quartz relates to relative lower P- and higher S-wave velocities, the presence of clay reduces the overall velocities even more. Carbonate infill of intergranular space (micrite or cement) slightly influences the acoustic properties. In general, the infill of microporous micrite shows lower velocities compared to the cemented samples. Finally microcracks may significantly reduce the velocity, although these cannot be quantified, their presence can be estimated using elastic property models.The ramp geometry is characterized by an overall lateral homogeneity and vertical heterogeneity. Textural, sedimentological and stratigraphic transitions in the outcrop are related to the defined facies, each has its own unique mineralogical signature. Amplitude variations in the synthetic seismic profile are primarily caused by changes in mineral composition thus facies transitions.

Genesis of the central zone of the Nolans Bore rare earth element deposit, Northern Territory, Australia

The Nolans Bore rare earth element (REE) deposit consists of a network of fluorapatite-bearing veins and breccias hosted within Proterozoic granulites of the Reynolds Range, Central Australia. Mineralisation is divided into three zones (north, central, and south-east), with the north and south-east zones consisting of massive REE-bearing fluorapatite veins, with minor brecciation and carbonate infill. The central zone is distinctively different in mineralogy and structure; it features extensive brecciation, a high allanite content, and a large, epidote-rich enveloping alteration zone. The central zone is a reworking of the original solid apatite veins that formed during the Chewings Orogeny at ca. 1525 Ma. These original apatite veins are thought to derive from phosphate-rich magmatic–hydrothermal fluid exsolved from as-yet unrecognised alkaline magmatic bodies at depth. We define four ore breccia types (BX1–4) in the central zone on the basis of detailed petrological and geochemical analysis of drillcore and thin sections. BX1 ore comprises fluorapatite with minor crackle brecciation with carbonate infill and resembles ore of the north and south-east zones. Breccia types BX2, BX3, and BX4 represent progressive stages of ore brecciation and development of calc-silicate mineral (amphibole, epidote, allanite, calcite) infill. Comparison of bulk ore sample geochemistry between breccia types indicates that REEs were not mobilised more than a few centimetres during hydrothermal alteration and brecciation. Instead, most of the REEs were partitioned from the original REE fluorapatite into newly formed allanite, REE-poor fluorapatite and minor REE carbonate in the breccias. Negative europium (Eu) anomalies in the breccia minerals are accounted for by a large positive Eu anomaly in epidote from the alteration zones surrounding the ore breccias. This observation provides a direct link between ore recrystallisation and brecciation, and the formation of the alteration halo in the surrounding host rocks. Where allanite and fluorapatite are texturally related, the fluorapatite is relatively depleted in the light rare earth elements (LREEs), whereas allanite is relatively LREE enriched, suggesting co-crystallisation. We tentatively date the BX1 ore stage to 1440 ± 80 Ma based on U–Pb dating of thorianite. Sm–Nd isotope isochrons derived from in situ isotope analysis of cognate apatite and allanite date the BX2 and BX3 events to ca. 400 Ma, while U–Pb dating of late-stage monazite from the BX4 ore stage returned an age of ca. 350 Ma. Therefore, formation of the central zone at Nolans Bore involved multiple alteration/brecciation events that collectively span over 1 billion years in duration. We suggest that the BX1-type veins and breccias were formed from REE-rich, saline (F- and Cl-bearing) fluids that infiltrated the granulite-grade host rocks in association with either shear activation events of the Redbank Shear Zone (1500–1400 Ma) or intrusion of late-stage pegmatites of the Mt Boothby area. BX2, BX3, and BX4 events record deformation and hydrothermal alteration associated with the Alice Springs Orogeny (400–350 Ma). These hydrothermal events occurred at temperatures of 450 to ~600 °C, due to inflow of highly acidic hydrous fluids derived from a magmatic source, or from mixing of meteoric and metamorphic fluids. Our data testify to the long and complex geological history of not only the Nolans Bore REE deposit, but also of the rocks of the eastern Reynolds Range, and demonstrate the great utility of using hydrothermally derived REE minerals to trace the timing of crustal deformation events and source of associated hydrothermal fluids.

Microstructure variability in freshwater microbialites, Pavilion Lake, Canada

Calcite microbialites in Pavilion Lake, British Columbia, exhibit a diverse range in macro-morphology, biomass abundance, porosity, and mineral content. To evaluate the role of microorganisms in their formation, samples collected from a range of depths were examined by scanning electron microscopy (SEM) and synchrotron radiation-based micro-X-ray fluorescence (μ-XRF) spectroscopy to characterize both their outer surfaces as well as internal structures. Observed trends in both surface colonization as well as microbialite framework with increasing lake depth include decreasing microbial abundance on outer surfaces as well as increasing ratios of carbonate:biomass in the microbialites. Microscopic investigations of the interiors show bacteria and algae entrapped within calcite, with this calcite exhibiting micropores and casts similar in size and shape to microorganisms. Based on these observations, it is hypothesized that microbialite development in Pavilion Lake initiates calcite precipitation in phototrophic microbial mats, i.e., combined phototrophy and heterotrophy, followed by heterotrophic oxidation of organic matter leading to eventual carbonate infilling of the microbial–mineral matrix. In addition, an observed shift from cyanobacteria to algae with increasing lake depth suggests variability in contemporary conditions controlling microbialite growth and diagenesis. High photosynthetic growth rates at shallower depths result in significant porosity and friability due to biomass accumulation outpacing carbonate precipitation. At intermediate depths, lower light levels and slower growth rates of phototrophs lead to a greater proportion of the microbialite matrix being in-filled by carbonate. Carbonates precipitate initially within the bacteria-EPS matrix, with abundant uncalcified algae maintaining microbialite porosity. In the deepest waters, the presence of only sparse algal colonization as well as fine-grained, laminated metal-rich sediments covering microbialites suggests that present-day insolation levels are too low to support the development of photosynthetic microbial mats. As a consequence, heterotrophic carbonate precipitation has progressively in-filled these microbialite interiors to create lithified calcite fabrics that exhibit minimal porosity but preserve the casts of microorganisms as biosignatures. While the origin of microbialites in Pavilion Lake remains unknown, current observations provide valuable information in evaluating how environmental conditions influence microbialite growth in a freshwater, lacustrine environment.

COVARIANCE OF MICROFOSSIL ASSEMBLAGES AND MICROBIALITE TEXTURES ACROSS AN UPPER MESOPROTEROZOIC CARBONATE PLATFORM

Early diagenetic chert nodules and beds in the upper Mesoproterozoic Angmaat (formerly Society Cliffs) Formation, Baffin and Bylot islands, preserve microfossils and primary petrofabrics that record microbial mat deposition and lithification across a range of peritidal carbonate environments. Five distinct microfossil assemblages document the distribution of mat-building and mat-dwelling populations across a gradient from restricted, frequently exposed flats to more persistently subaqueous environments. Mats built primarily by thin filamentous or coccoidal cyanobacteria give way to a series of more robust forms that show increasing assemblage diversity with decreasing evidence of subaerial exposure. Distinct fabric elements are associated with each microbial assemblage, and aspects of these petrofabrics are recognizably preserved within unsilicified carbonate in the same beds. These include some features that are distinctly geologic in nature (e.g., seafloor cements) and others that reflect microbial growth and decomposition (e.g., tufted microbialites). A particularly distinctive, micronodular fabric is here interpreted as carbonate infilling of primary voids within microbial mat structures. Such structures mark the co-occurrence of cyanobacterial photosynthesis that produced oxygen gas, filamentous mat builders that imparted the coherence necessary to trap gas bubbles, elevated carbonate saturation required to preserve void fabrics via penecontemporaneous cementation, and a relative paucity of detrital sediment that would have inhibited mat growth. Petrofabrics preserved in Angmaat samples are widespread in upper Paleoproterozoic and Mesoproterozoic carbonate successions but are rare thereafter, perhaps recording, at least in part, the declining carbonate saturation state of seawater. Covariation of microfossil assemblages with petrofabrics in both silicified and unsilicified portions of carbonate beds supports hypotheses that link stromatolite microstructure to the composition and diversity of mat communities.

Ornithogenic weathering of an ultramafic plutonic rock: St. Peter and St. Paul Archipelago, Central Atlantic

A phosphatic alteration rind is described comprising a leucophosphite crust above a phosphatised substrate of serpentinized peridotite from an exceptionally isolated location experiencing high rainfall conditions. The formation of the rind is intimately linked to the deposition and subsequent leaching of guano associated with the high concentration of seabirds that nest on the islands and its transformation into a ‘weathering rind’. The favoured pathway into the substrate appears to be along a pre-existing microfracture network, via the alteration of its carbonate infill into soluble apatite. This apatite is only retained within the protected environment of the microfracture network. Surrounding the network, there is evidence of the continued inward migration of the leachate into the surrounding peridotite resulting in its gradual phosphatisation in the form of a ‘halo’ around individual fractures. On the exposed rock surface, the consequence of the reaction between the guano leachate and the underlying substrate is its transformation into a ‘weathering rind’ composed principally of leucophosphite. It appears that the widespread formation of a phosphate crust is constrained by continual surface loss in a high energy, salt-rich maritime environment exposed to extensive overwash, wave attack and ultimately erosion.

Using coralline algae to understand historic marine cloud cover

Free-living coralline algae lay down growth bands formed by areas of more and less calcified cells which, in certain species, are associated with winter and summer growth respectively . Band width and cell calcification of Lithothamnion glaciale from Scotland were compared to annual and seasonal cloud cover and sea surface temperature (SST). There was a negative relationship between summer calcification (the degree of cellular carbonate infilling) and winter cloud cover. Annual and summer SST were also negatively related to summer calcification. This indicates that summer calcification may be affected by the previous winter's cloud cover and that summer's/year's SST. No relationships between band width and cloud cover were observed. A cloud cover hindcast using summer calcification and SST indicated a modest rise in cloud cover trends from 1910 to 2006 and a 12 (mean) year cyclicity in cloud cover, however, the latter may be associated with other site-specific factors. This study demonstrates the utility of densitometric algochronology in understanding marine temperature and cloud cover histories.

Chemosynthetic microbialites in the Devonian carbonate mounds of Hamar Laghdad (Anti-Atlas, Morocco)

Veins cutting across the Devonian conical mounds of the Hamar Laghdad Ridge (eastern Anti-Atlas, Morocco) were the carriers of geofluids. Although not completely determined, the composition of these fluids seems related to hydrothermal and seepage fluxes that were the primary contributors to the establishment of sub-seafloor (cryptic) environments linking deeper fluid sources to the seafloor. The optical and scanning electron microscope investigation of the laminated carbonate infill of these veins has revealed that they consist of stromatolite- and oncolite-like fabrics. In the Early and Middle Devonian mounds, the laminated carbonates contain morphologies that are attributable to microbially mediated processes that in turn suggest a direct biological contribution to the deposition of the veins infill. Microbial evidence include rod-shaped and cocci clusters embedded in amorphous membranes, iron-rich filaments organized as mat-like and biodictyon filamentous aggregates, or isolated filaments, microbial clotted textures and peloids. Moreover, the oncoids from the veins of the Middle Devonian (Hollard) mound have been interpreted as the mineral replacement of crypto-microorganisms as Frutexites. Other microbial evidence are biominerals and include carbonate with spheroids and pyrite framboids, in which typifying hydrocarbon seep environments. The Hollard Mound, which is the only one with a well-documented origin by hydrocarbon seepage, shares a number of geomicrobiological evidence with other Early Devonian mounds. These evidence, which are supplemented by δ 13C-depleted data, suggest that the infill of the veins system in the Hamar Laghdad mounds would be the product of chemotrophic/chemosynthetic microbial communities that are adapted to crypto-habitats sustained by hydrocarbon (and other components) fluid fluxes.

Pottery manufacture in early Roman Galilee: a micromorphological study

The characteristics of, and the sources of the soil materials for, pottery groups manufactured in three ancient settlements of early Roman (late first century BC–first century AD) Galilee were studied by micromorphological techniques. The site-specific manufacturing proveniences of the three respective pottery groups have been previously demonstrated by chemical analysis. One of the sites, Kefar Hananya, is located near the geographical boundary between the Upper and Lower Galilee while the other two, Shikhin and Yodefat, are in the Lower Galilee. Each of the settlements made pottery from different soil types. In one case (Yodefat) temper was added to the main soil material. The Kefar Hananya pottery was made from noncalcareous Red Mediterranean soils (Terra-Rossa soil type) derived from hard limestone. Although most of the soils in the Galilee derive from Upper Cretaceous geological formations, the Kefar Hananya pottery was prepared from soils, found near that settlement, that were derived from Lower Cretaceous material rich in the kaolinite clay mineral. Micromorphological analysis of 41 randomly selected samples of the most common early Roman storage jar type thought to have been produced at Shikhin showed that all of the vessels were made from one of three soil types, Colluvial–Alluvial soils, Brown Grumusols, or Pale Rendzina. Study of the lithology and soils in the area of Shikhin showed that all three soil types derive from soil material eroded from the hilly area, on which the ancient settlement of Shikhin was built, into the adjacent valleys below the site. Samples of the Shikhin pottery group collected on the surface, in contrast with excavated samples, exhibit postdepositional carbonate infilling by dissolution and reprecipitation of the carbonates due to repeated exposure to water. No added temper was used for the preparation of either the Kefar Hananya or the Shikhin pottery groups. The Yodefat pottery, made from Rendzinic material rich in calcareous material, is of particular interest. This soil material contains about 70–80% calcareous material homogeneously mixed with the clayey plasma. In order to diminish the effect of the large amount of calcareous material and improve the properties of this raw material for pottery manufacture, leached clayey soil material of Terra-Rossa soil type was added as temper. The micromorphological comparison of the pottery groups and soil materials shows that two main factors influenced the raw-material procurement strategies of the potters of these settlements: the proximity of the soil material to the potters' settlement, and the suitability of the soil material for pottery manufacture.

Timing and genesis of early marine caymanites in the hydrothermal palaeokarst system of Buda Hills, Hungary

Sedimentological, biostratigraphic and magnetostratigraphic studies were carried out on five Late Eocene–Early Oligocene profiles in the Buda Hills, Hungary. The Szépvölgy Limestone Formation accumulated on the pre-Eocene basement. Basal strata of the limestone consist of a thin conglomerate followed by a coralgal limestone. The overlying limestone contains abundant Nummulites, Discocyclina . The contact between these two members is sharp. The Szépvölgy Limestone body, covering 60–65 km 2 , is considered a carbonate bank. Above the limestone, the siliciclastic slope deposits of the pelagic and turbiditic Buda Marl, and the euxinic Tard Clay accumulated. A huge multiphase hydrothermal cave system developed in the Szépvölgy Limestone resulting in a long-term composite palaeokarstic evolution lasting from the Late Eocene to the Quaternary. The first palaeokarst phase during the Late Eocene is represented by two generations of early marine cavity filling sediments of caymanite-type, deposited at sea-level. The earlier, carbonate infilling is conformable while the younger, siliciclastic one is disconformable with the bedding of the host rock. Bio- and magnetostratigraphic studies indicate that deposition of the caymanites-bearing palaeokarst host sequence started in the Late Eocene, during Chron C15r (35.3 Ma) and terminated in the Early Oligocene during Chron C13n (33 Ma). Two marker horizons are present, the first is between the Szépvölgy Limestone and Buda Marl at ∼34.6 Ma, and the second horizon is between the Buda Marl and the Tard Clay at 33.5 Ma. The Szépvölgy Limestone, deposited on a mobile shelf, represents a deepening upward sequence, interrupted by two lowstand events. They can be correlated with the PHd event of Keller et al., 1987 (Global distribution of late Palaeogene hiatuses. Geology 15, 199–203) and resulted in marine palaeokarstification. The carbonate shelf with the infillings drowned at 34.6 Ma. The caymanites accumulated at about 35.2–35.0 Ma and represent short-term palaeokarstic events. Their deposition record was estimated in several thousand years.