Subaerial Exposure Surfaces Research Articles

Facies and Stratigraphic Anatomy of a Temperate Carbonate Sequence (Capo Colonna Terrace, Late Pleistocene, Southern Italy)

Abstract Accurate interpretation of ancient non-tropical carbonate systems requires the development of appropriate facies and sequence stratigraphic models. These models should account for the marked differences in sedimentation style among non-tropical carbonates formed under different energy settings. This paper presents data and a conceptual model of the sequence architecture of non-tropical carbonates of moderate energy environments based on the late Pleistocene deposits of Capo Colonna marine terrace (Calabria, southern Italy). These deposits have the advantage of being continuously exposed along a proximal–distal transect across the paleo-shelf and their geometry has not been disturbed by later tectonic activity. They consist of a transgressive–regressive (T–R) cycle whose genesis can confidently be related to one of the interstadial glacio-eustatic fluctuations of marine isotope stage (MIS) 5. Eight different mixed carbonate–siliciclastic and carbonate facies are present in the studied sequence. Of these, biogenic-dominated facies are invariably made up of skeletal remains of light-independent organisms plus or minus red calcareous algae (the heterozoan association). The relatively young age of the deposits allows calibration of the paleoecological significance of observed facies by direct comparison with modern analogues from the Mediterranean Sea. Four laterally persistent discontinuity surfaces were identified along the transect, and interpreted, from bottom to top, as (1) a basal ravinement surface, (2) a submarine hardground locally acting as a substrate for algal boundstones, (3) a regressive surface of marine erosion, and (4) a surface of subaerial exposure. Sequence stratigraphic analysis indicates that the preserved T–R cycle is asymmetric, with a thicker regressive portion. Highstand deposits can be subdivided into a shore-connected sandy prism and more distal autochthonous and parautochthonous skeletal accumulations and algal boundstones. During forced regression, a thin, sharp-based, strongly progradational unit of calcarenites was deposited above the regressive surface of marine erosion. The deposits of Capo Colonna terrace document the response of non-tropical, moderate-energy carbonate ramps to sea-level fluctuations during icehouse conditions. Their architecture can facilitate the interpretation of other successions from similar settings and provide an end member to be compared and contrasted with high-wave-energy cool-water carbonates.

Anatomy of the Holocene succession of the southern Venice lagoon revealed by very high-resolution seismic data

The southern portion of the Venice lagoon contains a relatively thick (up to 20 m) Holocene sedimentary body that represents a detailed record of the formation and evolution of the lagoon. New very high-resolution (VHR) seismic profiles provided a detailed investigation on depositional geometries, internal bounding surfaces and stratal relationships. These informations, combined with core analysis, allowed the identification of large- to medium-scale sedimentary structures (e.g. dunes, point bars), the corresponding sedimentary environment, and of retrogradational and progradational trends. In addition, the availability of dense seismic network produced a 3D reconstruction of the southern lagoon and the recognition of the along-strike and dip variability of the stratal architecture. Three main seismic units (H1–H3), separated by key stratal surfaces (S1-S3), form the Holocene succession in the southern Venice lagoon. This succession is bounded at the base by the Pleistocene/Holocene boundary (the surface S1), which consists of a surface of subaerial exposure locally subjected to river incision. The lower part of the Holocene succession (up to 13 m thick) consists of incised valley fills passing upward into lagoon and then shallow-marine sediments (Unit H1), and therefore shows a deepening-upward trend and a retrogradational stacking pattern. A prograding delta and adjacent shorelines, showing internal clinoforms downlapping onto the top of Unit H1 (the surface S2), form the middle part of the Holocene succession (Unit H2, up to 7.5 m thick). Unit H2 is interpreted as a result of a regressive phase started about 6 kyr BP and continued until recent time. The upper part of the Holocene succession (Unit H3) consists of lagoonal deposits, including tidal channel and tidal and subtidal flat sediments, that abruptly overlie Unit H2. Unit H3 is thought to represent a drowning of the area primarily due to human interventions that created rivers diversion and consequent delta abandonment during historical time.

Polyphased dolomitization of a shoal-rimmed carbonate platform: example from the middle Turonian Bireno dolomites of central Tunisia

The middle Turonian Bireno carbonate platform of central Tunisia consists of a shoal-rimmed platform including at least three carbonate units deposited in inner to outer shelf depositional settings. The Bireno middle unit is dominantly composed of dolomites capped by a subaerial exposure surface correlative with the global SB 90.5 Ma. Dolomitization occurred during a major sea-level fall which resulted in a progressive modification of the marine groundwater by meteoric water or by evaporation The multidisciplinary study, including petrographic and geochemical analyses, shows that the dolomites include three dolomite phases which consecutively occurred in quite different platform environments, according to different mechanisms: (1) the first dolomite phase resulted from a replacive dolomitization that affected the fine-grained limestone intercalations associated with the layered anhydrites in the evaporitic supratidal zone; (2) the second dolomite phase resulted from a replacive dolomitization that simultaneously affected the rudist lithosomes on the platform border as well as the alternation of the peritidal laminated limestone and desiccation breccias of the intertidal zone, the resulting dolomicrite was more or less recrystallized during shallow burial into turbid dolomicrosparite and dolosparite; (3) the third dolomite phase resulted from the precipitation of limpid dolomite cement in the tectonic fractures affecting especially the rudist shoal carbonates on the border of the platform. Through this study it was established that dolomitization occured in close relation with the major lowring of the sea level in different sub-environments belonging to the same sedimentary system, but following different mechanisms that can result in different dolomite phases displaying quite different petrographic and geochemical signatures. In addition to the role of the sea-level fall, the local tectonic framework played a great role in controlling dolomitizing fluids circulation and distribution of the dolomitized bodies within the platform.

炭素・酸素同位体組成に基づく地表露出面の認定～宮古島南部ボーリングコアでの検討～

炭素・酸素同位体組成に基づく地表露出面の認定～宮古島南部ボーリングコアでの検討～

Facies stacking patterns in high-frequency sequences influenced by long-term sea-level change on a Permian Panthalassan oceanic atoll: An example from the Akiyoshi Limestone, SW Japan

Detailed core examination of Middle Permian (Guadalupian) lagoonal successions of mid-Panthalassan atoll carbonates (Akiyoshi Limestone) revealed distinct stratigraphic variation of facies and sequence stacking patterns. The long-term sea-level change reconstructed from the atoll stratigraphy of the Akiyoshi includes a Bolorian to Midian gentle sea-level fall, punctuated by a short-lived Murgabian sea-level rise. These sea levels can be regarded as eustasy, judging from the monotonous subsidence of the Akiyoshi atoll and its good correspondence with the sea-level curves of other areas. Two sequence types are defined. Sequence type A consisting of shoal to lagoon grainstone occurred in low accommodation during the sea-level highstand-to-falling stages of the late Murgabian to Midian. This sequence type is bounded by two successive subaerial exposure surfaces and exhibits the uneven but generally low δ 13C values throughout the section. In contrast, sequence type B occurred in increasing accommodation during the sea-level rise of the early Murgabian. This sequence type represents a composite sequence comprising parasequences and consists of various types of facies such as peritidal lime-mudstone, lagoonal packstone and boundstone, and shoal grainstone. Sequence type B is characterized by consistently high δ 13C values except for the negative shift immediately below the sequence-top surface. These lines of evidence demonstrated that long-term eustatic sea-level change obviously controls the variations of sequence types in atoll carbonates.

Carbon isotope lateral variability in a Middle Frasnian carbonate platform (Belgium): Significance of facies, diagenesis and sea-level history

Carbon isotopic variations of Frasnian shallow-water carbonates from Belgium are related to facies and major sea-level trends. The influence of the diagenetic overprint was assessed in order to determine the primary signal of the Frasnian carbonates. Shallow-water microfacies are characterized by biostromes with stromatoporoids and lagoonal deposits dominated by carbonate mud and calcareous algae with subaerial exposure surfaces. The diagenetic history was controlled by three main events: early meteoric diagenesis (short-term subaerial exposure during deposition), late meteoric diagenesis (major Famennian regression) and burial diagenesis. The oxygen isotopic values are almost constant with respect to facies, original material (carbonate mud and cement) and sedimentological units (no differences before or after the main regression). This homogeneity is related to resetting during late meteoric diagenesis. The carbon isotopic values are related to facies (with the more negative values for the shallowest facies) and to major sea-level variations (most negative values after the main regression). This pattern is interpreted as being related to primary signals. This trend was enhanced by early meteoric diagenesis and the influence of more negative values from paleosols. The carbon isotope patterns reflect the influence of sea-level and water circulation on shallow-water deposits and this influence implies that shallow-water carbonates are not necessarily good material for assessing the primary isotopic values of the ocean because of the influence of long residence time (“aging”) of the platform-top water and because of early diagenesis.

Facies analysis of Lofer cycles (Upper Triassic), in the Argolis Peninsula (Greece)

The Upper Triassic carbonate sediments of Argolis Peninsula are part of the Upper Triassic–Lower Jurassic extensive and thick neritic carbonate formations (Pantokrator facies) that formed at the passive Pelagonian margin and are considered as Dachstein-type platform carbonates. Facies analysis of the Upper Triassic “Lofer-type” lagoonal–peritidal cycles in the Dhidimi area, proved that cycles, although mostly incomplete, were regressive shallowing-upward. The ideal elementary cyclothems are meter-scale in thickness and begin with a subtidal bed (Member C), represented by a peloidal dolostone with megalodonts (wackestone or packstone), being followed by a stromatolitic intertidal dolomitic mudstone and/or fenestral intertidal dolomitic mudstone (Member B) that is overlain by dolocrete (terrestrial stromatolites or pisoidic dolomite) or a supratidal “soil conglomerate” in red micritic matrix (Member A). Lofer-cycle boundaries are defined at the erosional surfaces and accordingly the Lofer cyclothems are unconformity-bounded units. Due to common post-depositional truncation of the subtidal and intertidal facies, the supratidal members prevail, being developed, in places, directly upon subaerial exposure surfaces (erosionally reduced cyclothems). Peritidal layers are characterized by a well-expressed lamination, sheet cracks, tepee structures, fenestral pores and karst dissolution cavities. The studied lagoonal–peritidal cycles are considered to have been deposited in a tidal-flat setting (inner platform), repeatedly exposed under subaerial conditions, in the context of a broader tropical rimmed platform. Although the studied area was tectonically active due to rift-activity and the autocyclic processes should also be taken in consideration, the great lateral correlatability of cycles, the facies shifting and the widespread erosion that resulted in superposition of supratidal-pedogenic facies directly upon subtidal members (subaerial erosional unconformity), indicating a sea-level drop, reflect allocyclic control via high-frequency eustatic sea-level oscillation (orbital forcing). Sediment deposition occurred during low-stand system tract (LST), that probably continued also in the transgressive system tract (TST) and reflects an overall sea-level fall. Under these conditions dissolution and cement precipitation episodes, as well development of paleosols and karsts, were triggered, during a relatively less arid interval.

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.

Heterogeneity in Geochemical Expression of Subaerial Exposure in Limestones, and Its Implications for Sampling to Detect Exposure Surfaces

Abstract Replicate sampling along stratigraphic horizons in one outcrop of Ordovician limestones of the Nashville Dome reveals considerable variation in δ13C, δ18O, and Sr concentration among seemingly identical samples. Ten vertical transects were sampled, each consisting of samples taken at 10 cm intervals from 2 m below to 1 m above the M4–M5 sequence boundary of Holland and Patzkowsky (1997, 1998). The 10 transects were each 2 m apart. The resulting 10 replicate and thus seemingly interchangeable samples from each horizon have ranges of δ13C as great as 2.4‰ and ranges of δ18O as great as 2.8‰, and Sr concentrations that vary by as much as a factor of four. Variance is great both just below and just above surfaces of subaerial exposure. As a result, maxima in variance in δ13C, as well as or (in some cases) instead of minima in mean δ13C, can be used to test such surfaces for exposure. Maxima in variance and minima in mean δ13C may be found either below a surface of subaerial exposure in the meteorically altered sediment, or above that surface in the transgressive lag where reworked pre-exposure sediment is mixed with unaltered post-exposure sediment. The large variances and ranges found in this study, and the dissimilarity of geochemical profiles from the 10 vertical transects, indicate that single samples of stratigraphic horizons, and thus single vertical transects, can yield misleading characterizations of stratigraphic variation of geochemical parameters. Instead, replicate sampling along horizons better characterizes isotopic and elemental variation through stratigraphic sections than does existing single-sample strategies, and it will be more useful in future testing of surfaces of subaerial exposure. For example, the replicate geochemical data used in this project confirm the presence of three surfaces of subaerial exposure, whereas only two had been recognized as exposure surfaces before. Data from replicate samples also make unnecessary the use of arbitrary criteria for inferring exposure, such as the predetermined numerical cutoffs used by Railsback et al. (2003).

The diversification and extinction of Doushantuo‐Pertatataka acritarchs in South China: causes and biostratigraphic significance

AbstractThe Ediacaran Period immediately follows the last Cryogenian glaciation—the ∼635 Ma Marinoan or Nantuo glaciation, and it is also punctuated by another brief glaciation—the ∼582 Ma Gaskiers glaciation. It is possible that these glaciations may have had significant impact on Ediacaran biological evolution (e.g. the appearance or disappearance of Doushantuo‐Pertatataka acritarchs). Alternative hypotheses propose that the diversification of Doushantuo‐Pertatataka acritarchs was caused by the Acraman bolide impact or by emerging eumetazoans. To test these hypotheses, high‐resolution geochronological and biostratigraphic data are required. The Doushantuo Formation in South China, radiometrically constrained between ∼635 and ∼551 Ma, has the potential to clarify the global picture of early‐middle Ediacaran evolution. Here we present preliminary biostratigraphic data from the Doushantuo Formation in the East Yangtze Gorges area and new δ13C chemostratigraphic data from the Doushantuo Formation at Weng'an. These and previously published palaeontological data, aided by the tool of δ13C chemostratigraphy, indicate that the biostratigraphic record of the Doushantuo Formation is locally sensitive to the availability of specific taphonomic windows. In the East Yangtze Gorges area, Doushantuo‐Pertatataka acritarchs first appeared shortly after the termination of the Nantuo glaciation and gradually diversified throughout the Doushantuo Formation. At Weng'an, however, such acritarchs first appear—abruptly and in much greater diversity—in phosphorite of the upper Doushantuo Formation, immediately above a subaerial exposure surface. Thus, the biostratigraphic pattern in the East Yangtze Gorges area permits, whereas that at Weng'an apparently disallows, a causal relationship between the Nantuo glaciation and the diversification of Doushantuo‐Pertatataka acritarchs. We conclude that the biostratigraphic record is incomplete at Weng'an, where the early Ediacaran evolutionary history is not preserved. The South China data indicate that special attention has to be paid to taphonomic and palaeoenvironmental analysis before extrapolating local and regional biostratigraphic ranges to make global interpretations.It is less clear when Doushantuo‐Pertatataka acritarchs disappeared. Previous investigators have variously suggested that they disappeared before, at, or after, the Gaskiers glaciation. These hypotheses are difficult to test because of the lack of sedimentary evidence for the Gaskiers glaciation in South China and other regions (e.g. South Australia) where Doushantuo‐Pertatataka acritarchs are abundant. In Australia, Doushantuo‐Pertatataka acritarchs are thought to have experienced a sharp decline after the Egan glaciation, which may well be equivalent to the Gaskiers glaciation. If true, then Doushantuo‐Pertatataka acritarchs are largely restricted to the interval between the Nantuo and Gaskiers glaciations. This conclusion allows us to place constraints on the possible causes of the diversification and extinction of Doushantuo‐Pertatataka acritarchs and has important implications for the biostratigraphic significance of Doushantuo‐Pertatataka acritarchs. Copyright © 2006 John Wiley & Sons, Ltd.

Sequence stratigraphy of a carbonate‐evaporite succession (Zechstein 1, Hessian Basin, Germany)

AbstractThe evaporitic Hessian Zechstein Basin is a sub‐basin of the Southern Zechstein Basin, situated at its southern margin. Twelve facies groups were identified in the Zechstein Limestone and Lower Werra Anhydrite in order to better understand the sequence‐stratigraphic evolution of this sub‐basin, which contains economically important potassium salts. Four different paleogeographic depositional areas were recognized based on the regional distribution of facies. Siliciclastic‐carbonate, carbonate, carbonate‐evaporite and evaporite shallowing‐upward successions are developed. These allow the establishment of parasequences and sequences, as well as correlation throughout the Hessian Basin and into the Southern Zechstein Basin. Two depositional sequences are distinguished, Zechstein sequence 1 and Zechstein sequence 2. The former comprises the succession from the Variscan basement up to the lowermost part of the Werra Anhydrite, including the Kupferschiefer as part of the transgressive systems tract. The highstand systems tract is defined by the Zechstein Limestone, in which two parasequences are developed. In large parts of the Hessian Basin, Zechstein sequence 1 is capped by a karstic, subaerial exposure surface, interpreted as recording a type‐1 sequence boundary that formed during a distinct brine level fall. Low‐lying central areas (Central Hessian Sub‐basin, Werra Sub‐basin), however, were not exposed and show a correlative conformity. Topography was minimal at the end of sequence 1. Widely developed perilittoral, sabkha and salina shallowing‐upward successions indicate a renewed rise of brine level (interpreted as a transgressive systems tract), because of inflow of preconcentrated brines from the Southern Zechstein Basin to the north. This marks the initiation of Zechstein sequence 2, which comprises most of the Lower Werra Anhydrite. In the Central Hessian Sub‐basin, situated proximal to the brine inflow and on the ridges within the Hessian Basin, physico‐chemical conditions were well suited for sulphate precipitation to form a thick cyclic succession. It consists of four parasequences that completely filled the increased accommodation space. In contrast, only minor sulphate accumulation occurred in the Werra Sub‐basin, situated further southwards and distal to the inflow. As a result of substantially different sulphate precipitation rates during increased accommodation, water depth in the region became more variable. The Werra Sub‐basin, characterized by very low sedimentation rates, became increasingly deeper through time, trapping dense halite brines and precipitating rock salt deposits (Werra Halite). This ‘self‐organization’ model for an evaporitic basin, in which depositional relief evolves with sedimentation and relief is filled by evaporite thereafter, contradicts earlier interpretations, that call upon the existence of a tectonic depression in the Werra area, which controlled sedimentation from the beginning of the Zechstein.

Sequence stratigraphy of the Taebaek Group (Cambrian–Ordovician), mideast Korea

This study focuses on the sequence stratigraphic implications of the Taebaek Group (Cambrian–Ordovician), a mixed carbonate–siliciclastic sequence (ca. 1200 m thick), in the mideastern part of the Korean peninsula. The group contains 11 lithologic units (formations) that can be partitioned into four packages (supersequences I–IV). Each package comprises alternation of siliciclastic and carbonate successions, corresponding to the second-order sequence (supersequence) formed in a period of 10–100 m.y. Supersequence I (Jangsan/Myeonsan, Myobong, and Daegi fms) represents the initial inundation and subsequent drowning during the late Early to middle Middle Cambrian. It experienced the drowning between the middle and late Middle Cambrian, forming type-3 sequence boundary. Supersequence II (Sesong and Hwajeol fms) was formed by prolonged marine flooding during the late Middle and Late Cambrian. It was terminated by a large supply of siliciclastics due to the rejuvenation of the drainage basin, and the subsequent shallowing at the Cambrian–Ordovician boundary (type-2 sequence boundary). Supersequence III (Dongjeom, Dumugol, and Makgol fms) comprises the lowstand coarse-grained siliciclastics, transgressive shale by marine flooding, and shallow platform carbonates by highstand progradation in the Early Ordovician. Following the large-scale eustatic fall of sea level in the earliest Middle Ordovician, supersequence IV (Jigunsan and Duwibong fms) formed on the subaerial exposure surface (type-1 sequence boundary) by resumed marine flooding in the Middle Ordovician. This study provides basis for basin-scale correlation with the coeval sequences in northeastern China.

Carbon Isotope Excursions and Crinoid Dissolution at Exposure Surfaces in Carbonates, West Texas, U.S.A.

Abstract Carbonates from the Horseshoe Atoll, Reinecke Field, West Texas, U.S.A., show four subaerial exposure events by the presence of rhizoliths, alveolar septal fabric, and thin calcretes that subdivide the Pennsylvanian succession into five sequences. Vertical profiles of bulk rock δ13C analyses show values that are generally ~ +1‰ (PDB), but negative shifts of up to 2‰ occur beneath exposure surfaces. The bulk-rock δ13C values ~ +1‰ could be explained as a mixture of 63% marine carbonate (δ13C = +4‰) and 37% pedogenic carbonate (δ13C = −4‰) while a negative bulk-rock excursion of δ13C = ~ −1‰ can be explained as a mixture of 37% marine and 63% pedogenic calcite. Distinctive negative shifts in the δ13C profiles do not occur under some exposure surfaces that contain soil-generated features, and one negative shift occurs that apparently is unrelated to an exposure surface. Bulk-rock δ13C values are difficult to interpret because the volume of their various components is unknown. Furthermore, δ13C profiles may miss subaerial exposure surfaces. Certain styles of crinoid alteration may indicate paleo-subaerial exposure surfaces. Fossil crinoids can be preserved in a variety of ways, but crinoids with dissolution holes that subsequently either become filled with calcite cement or were crushed during burial are restricted to 2 m below exposure surfaces. Dissolution of Mg calcite crinoids is due to their relatively high Mg composition, 12 mole % MgCO3, combined with their location beneath soil-capped surfaces from which meteoric waters greatly undersaturated with respect to calcite emerged. Late-stage dissolution removed finely crystalline calcite, but crinoids by this time had stabilized to coarsely crystalline calcite and dolomite and thus were unaffected. Evaluation of other stratigraphic systems is needed to determine whether the use of holey crinoids to identify subaerial exposure surfaces has broad application.

Jurassic and Cretaceous clays of the northern and central North Sea hydrocarbon reservoirs reviewed

AbstractThe principal clays of the northern and central North Sea are illite (sometimes with interlayered smectite) and kaolin. Chlorite is only locally important. Although it has been proposed that kaolin within North Sea sandstones is detrital in origin, the majority of workers have concluded that it is authigenic, largely the product of feldspar alteration. Kaolin is found within a wide range of sedimentary settings (and within shales) apparently defying the notion that kaolin is an indicator of meteoric water deposition. Within sandstones, the earliest authigenic kaolin has a vermiform morphology, the distribution of which is controlled by the availability of detrital mica to act as a nucleus, and the composition of the post-depositional porewaters. This vermiform kaolin formed in meteoric water, the presence of which is easily accounted for below sub-aerial exposure surfaces in non-marine formations, and below unconformities over marine units. In fully marine sands, and even marine shale units, kaolin still occurs. It has therefore been suggested that even these locations have been flushed with meteoric water.Early vermiform kaolin recrystallizes to a more blocky morphology as burial proceeds, at least in the Brent Group. Blocky kaolin has been reported as growing before, synchronously with, and after the formation of quartz overgrowths, though oxygen isotope studies support low-temperature growth, pre-quartz. Blocky kaolin may form during meteoric flushing associated with lower Cretaceous uplift and erosion, though it is found in fault blocks that are thought to have remained below sea level. Here, the kaolin may form in stagnant meteoric water, relics of the post-depositional porewater. It has also been proposed that the blocky kaolin grew in ascending basinal waters charged with carboxylic acids and CO2, though this hypothesis is not supported by stable oxygen isotope data. Some of the blocky kaolin is dickite, the stable polymorph above ∼100°C.Fibrous illite occurs almost ubiquitously within the clastic sediments of the North Sea. An early pore-lining phase has been interpreted as both infiltrated clastic clay, and as an early diagenetic phase. Early clays may have been quite smectite-rich illites, or even discrete smectites. Later, fibrous illite is undoubtedly neoformed, and can degrade reservoir quality significantly. Both within sandstones and shales, there is an apparent increase in the K content deeper than 4 km of burial, which could be due to dilution of the early smectite-rich phase by new growth illite, or to the progressive illitization of existing I-S. Much of the ‘illite’ that has been dated by the K-Ar method may therefore actually be I-S.The factors that control the formation of fibrous illite are only poorly known, though temperature must play a role. Illite growth has been proposed for almost the entire range of diagenetic temperatures (e.g. 15–20°C, Brent Group; 35–40°C, Oxfordian Sand, Inner Moray Firth; 50–90°C, Brae formation; 100–110°C, Brent Group; 130–140°C, Haltenbanken). It seems unlikely that there is a threshold temperature below which illite growth is impossible (or too slow to be significant), though this is a recurring hypothesis in the literature. Instead, illite growth seems to be an event, commonly triggered by oil emplacement or another change in the physiochemical conditions within the sandstone, such as an episode of overpressure release. Hence fibrous illite can grow at any temperature encountered during diagenesis.Although there is an extensive dataset of K-Ar ages of authigenic illites from the Jurassic of the North Sea, there is no consensus as to whether the data are meaningful, or whether the purified illite samples prepared for analysis are so contaminated with detrital phases as to render the age data meaningless. At present it is unclear about how to resolve this problem, though there is some indication that chemical micro-analysis could help. It is a common belief that illite ages record the timing of oil charge, and so can be used to calibrate basin models.Grain-coating Fe-rich chlorite cements can preserve exceptional porosity during burial. They are found in marginal marine sandstones, formed during diagenesis from precursor Fe-rich clays such as berthierine or verdine.

Dry climate near the Western Pacific Warm Pool: Pleistocene caliches of the Nansha Islands, South China Sea

The Nansha (Spratly) Islands are located in the middle of the South China Sea (at about 10°N) near the northwestern margin of the Western Pacific Warm Pool (WPWP). A borehole was drilled at an atoll at Nansha, and cores were taken. The upper 165.4 m of the borehole consists entirely of limestone of reef facies and lagoon facies, which had been influenced by extensive meteoric diagenesis. Petrographic and geochemical studies have identified at least four subaerial exposure surfaces (SES) in the Pleistocene carbonate sequence. These SES are thought to have resulted from global sea-level changes and characterized by caliche formation. When developed in limestone, caliches typically form in areas where annual precipitation ranges from 500 mm up to about 1200 mm, compared with 1800–2200 mm of annual rainfall of the present-day Nansha Islands. The Nansha caliche therefore indicates the existence of several dry climate episodes during Pleistocene sea-level lowstands. Lower rainfall and higher evaporation during such dry conditions may explain the higher sea-surface salinities reported elsewhere in the South China Sea. The Nansha caliche may also indicate reduced extent of the WPWP and southerly shift of the Intertropical Convergence Zone during major sea-level lowstands.

Factors controlling carbonate dissolution rates quantified in a field test in the Austrian alps

Carbonate dissolution rates were investigated by measuring the mass difference of carbonate tablets exposed to natural dissolution for 1 year. 70 tablets were distributed over 13 test sites on the north slope of the Hochschwab Massif in the Austrian Alps. The influences of altitude, subsoil vs. sub-aerial exposure, vegetation, karst morphology, soil humidity, sample lithology, and sample surface morphology were investigated. The observed dissolution rates varied between 13 and 40 μm/a for subcutaneous samples and about 11 μm/a for sub-aerial exposure. Outstandingly high rates of 48 μm/a were observed in a doline and nearly zero rates were measured at a site influenced by seeping spring water. A mass balance, using high-resolution hydrological data, was calculated for the Kläffer Spring, which has an average outflow of 4.8 m 3/s. It indicated a loss of 21×10 6 kg of carbonate rock per year which gives an average dissolution rate of 95 μm/a for the catchment area of 83 km 2. The dissolution rates of ∼10 μm/a from the sub-aerially exposed samples are comparable to values from limestone pedestals, which were protected from dissolution by glacial erratics for the past 15 ka.

Stratigraphy of upper Viséan carbonate platform rocks in the Carlow area, southeast Ireland

AbstractThe stratigraphy of the upper Viséan (Asbian to Brigantian) carbonate succession in southeast Ireland is revised on the basis of seven quarry and two borehole sections. Six lithological units have been distinguished, two units (units 1 and 2) in the upper Asbian Ballyadams Formation, and four units (units 4 to 6) in the Brigantian Clogrenan Formation (both formations are dated precisely using foraminiferans, calcareous algae and rugose corals). The boundary between the Ballyadams and Clogrenan formations is redefined 19 m below the horizon proposed by the Geological Survey of Ireland, and thus, lithological characteristics of both formations are redescribed. The upper part of the Ballyadams Formation is characterized by well‐developed large‐scale cyclicity, with common subaerial exposure surfaces. Fine‐ to medium‐grained thin‐bedded limestones with thin shales occur in the lower part of cycles, passing up into medium‐grained pale grey massive limestones in the upper part. The Clogrenan Formation is composed mainly of medium‐ to coarse‐grained thick limestone beds with variable presence of shales; but no large‐scale cyclicity. There is a decrease in the number of subaerial exposure surfaces towards the top of the formation and common chert nodules; macrofauna occurs mostly concentrated in bands. The six units recognized in the Carlow area are comparable with other units described for the same time interval (Asbian–Brigantian) from south and southwest Ireland, demonstrating the existence of a stable platform for most parts of southern Ireland, controlled principally by glacioeustatics. Copyright © 2004 John Wiley & Sons, Ltd.

‘A 3D outcrop analogue model for Ypresian nummulitic carbonate reservoirs: Jebel Ousselat, northern Tunisia’ by E. Vennin et al .

Simon J. Beavington-Penney and Andrew J. Barnett write: Vennin et al . (2003) have provided an interesting study of the Ypresian (Eocene) El Garia Formation of Tunisia. This unit has been the subject of numerous studies, largely because it reservoirs hydrocarbons within the Gulf of Gabes (e.g. Loucks et al . 1998; Racey et al . 2001; Jorry et al . 2003). Because of the economic significance of this unit, its correct interpretation, especially in terms of the continuity and correlation of depositional sequences (and, therefore, reservoir layering) is crucial. Using the well-studied outcrops to the west of Kairouan in north-central Tunisia, Vennin et al . (2003) present a sequence stratigraphic model of the El Garia Fm. Their high-resolution model recognizes three orders of depositional sequences (third to fifth), with their ‘small-scale’ cycles occurring as individual decimetre- to metre-scale beds (which, they suggest, fall within the range of the duration of Milankovitch cycles) and groups of these beds which form metre- to decametre-scale, fifth-order packages. Vennin et al . (2003) claim that this hierarchy of cycles was produced by a nested hierarchy of eustatic sea-level change. We would like to suggest that these cycles (and the invoked causal sea-level change) are debatable, not least because they contain no unambiguous evidence of relative sea-level change (i.e. subtidal deposits capped directly by subaerial exposure surfaces). The El Garia Fm. was deposited over a period of approximately 2.8 Ma (Racey et al . 2001), during which time several third-order sea-level changes might be expected to have occurred. However, earlier studies from both outcrop and subsurface (offshore Tunisia borehole core) have not noted widespread surfaces that can be correlated within the El Garia Fm. (such as the Fe-rich hardgrounds which Vennin et al . (2003) record as bounding their fifth-order cycles) and have …

Subaerial exposure surfaces in a Palaeoproterozoic [formula omitted]-rich dolostone sequence from the Pechenga Greenstone Belt: palaeoenvironmental and isotopic implications for the 2330–2060 Ma global isotope excursion of [formula omitted

This article reports the discovery of numerous subaerial exposure surfaces which occur in the ca. 2200 Ma Kuetsjärvi Sedimentary Formation (KSF) from the Pechenga Greenstone Belt, NE Fennoscandian Shield, Russia. The formation was accumulated within an intraplate rift and is composed of fluviatile-deltaic siliciclastic deposits and lacustrine dolostones. The dolostones are enriched in 13 C ( δ 13 C=+6.6 to +7.6‰ V-PDB, δ 18 O=16.7 to 21.5‰ V-SMOW), which is, in general, assigned to a global perturbation of the carbon cycle. The lacustrine dolostone unit contains desiccated and in situ brecciated carbonate beds, subaerial erosion and dissolution surfaces, epikarst, surficial silicification (silcretes) and hot-water spring travertines. The dissolution surfaces—marked by formation of a few centimetre thick, ‘vuggy’, non-laminated, dolomicrite capped by silicified nodular zone—resemble modern calcretes which formed by capillary rise of surface waters (per-ascensum calcrete) following saturation of the regolith zone under arid or semi-arid conditions. Abundant red, iron-stained, superficial dolocrete implies the existence of an O 2-rich atmosphere. Rezolith, ‘black pebbles’, alveolar septal structures, ‘ Microcodium’ and other fabrics associated with plant roots and input of terrestrial organic matter, which are typical for post-Cretaceous pedogenic calcretes, have not been identified. Alfa fabrics can be applied to describe dolocretes though rhombic calcite crystals are lacking. Neither 13C-depletion nor 13C-enrichment has been documented in the dolocrete, which suggests no contribution of CO 2 derived from soil or from any other C org-rich sources enriched in 12C, and apparent lack of biological uptake of 12C during formation of the dolocrete. Both petrographic and carbon isotope data do not support earlier assertions that Palaeoproterozoic subaerial surface was colonised by microbial mats. A few samples have high δ 18 O values (up to 30‰) despite diagenetic and metamorphic resetting of the oxygen isotope system, suggesting evaporation during formation of the superficial dolocrete. Such documented episodes of subaerial exposure, formation of hot-water travertines, superficial dolocrete and silcrete suggest that there must have been frequent decoupling between the Kuetsjärvi depositional system and the bordering sea. This imposes an additional restriction on the use of the isotopic composition of the KSF dolostones for reconstruction of the δ 13 C carb global secular curve.

Reef‐capping laminites in the Upper Silurian carbonate‐ to‐evaporite transition, Michigan Basin, south‐western Ontario

AbstractSilurian pinnacle reefs in the subsurface of the south‐western Ontario portion of the Michigan Basin display a variety of laminated carbonates (laminites) within predominantly muddy reef‐capping facies in the upper part of the Guelph Formation and the overlying A‐1 Carbonate of the Salina Group. Laminites, which are limestone, dolomite or partially dolomitized limestones, have a range of morphologies, from simple planar to a variety of wavy and serrated forms. Individual laminae are composed mainly of micrite, microspar or replacive dolomite, and vary internally from isopachous and continuous over the diameter of the core to non‐isopachous and often discontinuous. Clotted and peloidal micrite, sometimes defining small knobs and chambers, is interpreted as being microbial in origin and occurs within all types of laminites. Fibrous cement locally comprises laminite clasts in breccias or coats clasts in breccias, and also occurs as spherulites in the interparticle spaces in breccias. Although similar laminites have been described from elsewhere in the Michigan Basin and interpreted as caliche, travertine and abiotic subtidal stromatolites, the laminites in south‐western Ontario are most realistically regarded as microbial. The causes for the variations in morphology and characteristics of the constituent laminae are uncertain, although fluctuations in local microenvironmental conditions would have been important, set against a backdrop of an increasingly restricted overall setting. Caliche or travertine origins for these laminites are unlikely in general, except perhaps locally at the subaerial exposure surface at the tops of pinnacle reefs.