Skeletal Wackestone Research Articles

Facies, Facies Association, and Depositional Environment of The Klapanunggal Formation, Klapanunggal District, West Java

The research focuses on the Klapanunggal Formation, composed of limestone in the West Java Basin, specifically in the Klapanunggal area, Klapanunggal District, Bogor Regency, West Java Province. A massive limestone outcrop was found in this location. The method used is field data collection, stratigraphic cross-section measurement, and petrographic analysis. It aims to determine the facies characteristics, depositional environment, and association of the limestone facies of the Klapanunggal Formation. The results obtained from macroscopic and microscopic descriptions resulted in six (6) facies: Skeletal Wackestone, Skeletal Packstone, Benthic Foram Grainstone, Skeletal Rudstone, Coral Rudstone, and Bindstone. Then, the facies are classified into facies associations, resulting in three (3) facies associations: Platform Interior (Open Marine), Platform Margin Reef, and Slope.

Sedimentology, sequence stratigraphy, palynology and diagenetic evaluation of the Triassic Jilh Formation. New insights from Saudi Arabia

The Jilh Formation (late Anisian to Carnian) was deposited on the southwestern margin of the Neotethys Ocean recording the flooding of a broad shallow marine platform, and is characterized by restricted clastic-prone mudrocks and sabkha anhydrite with interbedded shallow marine dolomitized carbonates. Within the Lower Jilh Formation, deposits encompass tidal flat to low-energy inner ramp thrombolytic microbial deposits with local anhydrite nodules and fenestrae structures interbedded with ft-scale intraclast-rich floatstones with desiccation surfaces. Within the interbedded mudrocks, land-derived miospores (including common Lunatisporites spp. and other taeniate bisaccate pollen) and clastic material are common. During the Carnian, the higher sea level resulted in a change to tidal flat bioturbated and microbially induced mudstones, along with inner ramp subtidal peloidal sand banks and skeletal wackestones with benthic foraminifera within the Upper Jilh Formation. Terrestrially derived miospores are still common (assemblage including common Camerosporites secatus, Conipollenites arabicus, Duplicisporites spp., Samaropollenites speciosus), but an increasing marine influence is also noted during the Carnian from the presence of marine palynomorphs such as acritarchs and prasinophytes (including acanthomorphic acritarchs, Veryhachium spp. and Dictyotidium spp.). The backstepping of the platform is marked by the progressive flooding of the tidal flats and the deposition of open marine facies culminating in a regional 2nd order maximum flooding surface. Petrographic and geochemical analyses show the development of fabric-replacive planar-s to planar-e dolomite fabrics associated with negative δ18O values and positive δ13C values, suggesting precipitation of the dolomite on top of 3rd and 2nd order sequences from possible transgressive reflux syn or post deposition of the Jilh Formation. Zoned dolomite rhombs lining macropores might be related to burial diagenesis and pore-filling anhydrite and calcite are locally common. The local presence of native sulfur replacing primary anhydrite nodules, along with the minor calcitization of the anhydrite also suggest the onset of thermochemical sulfate reduction during the burial history at possibly high temperature.

Construction mode of a Middle Permian sponge reef in theChangning–MenglianBelt, western Yunnan, China

The Changning–Menglian Belt in western Yunnan, China has long been acknowledged as a major suture zone of the Palaeo‐Tethys in south‐east Asia. Although bearing a wealth of environmental information for this vanished ocean, the Permo‐Carboniferous limestones of seamount resemblance in this belt have only been sketchily studied heretofore in terms of their sedimentary features. This study presents a detailed investigation on a Middle PermianArchaeolithoporella‐microbial‐sponge reef in the Damingshan Formation of these limestones. This reef could be ascendingly subdivided into five parts according to its petrographic and sedimentolotgic properties. The first, third, and fifth parts are characterized by the mixture ofArchaeolithoporella‐sponge boundstone, sponge cementstone, microbial boundstone, and skeletal wackestone. They are interpreted to be constructed in a moderate‐energy environment through the following processes: sponges in clusters were partly encrusted byArchaeolithoporella, and mostly toppled over after death; whereafter, spaces within frameworks formed by sponges and/orArchaeolithoporellawere occluded by syndepositional cements; during the processes, sponge clusters were bound together by microbial domes, and both acted as bafflers that trapped sediments. In contrast, the construction of the second and fourth parts occurred in a high‐energy environment and is almost devoid of microbial contribution: sponges that were encrusted byArchaeolithoporellatoppled over posthumously, and then spaces among them were filled by syndepositional cements, ultimately formingArchaeolithoporella‐sponge boundstone. The construction mode of the studied reef, especially sponge toppling and microbial domal frameworks in‐between sponge clusters, differs from that of most coeval sponge reefs, in which the framework is characterized by erect sponges and microbial encrustations onArchaeolithoporella. Our work not only enriches the sedimentary data of limestones in the Changning–Menglian Belt, but also would hopefully deepen the understanding of the growth model of Permian sponge reefs.

Bed‐scale spatial patterns in dolomite abundance: Part II. Effect of varied fluid chemistry, flow rate, precursor mineralogy, temperature, textural heterogeneity, nucleation density and bed geometry

AbstractReaction‐transport modelling shows that a lateral, metre‐scale pattern in dolomite abundance can form during replacive dolomitization of calcite and aragonite precursors by Mississippian, Triassic and modern seawaters advecting at >20 cm year−1 in low temperature (≤60°C) hydrogeological systems. The modelled pattern develops best in beds >1 m thick with a relatively uniform nucleate density and a low variance in reactive surface area (for example, grainstones). These conditions suggest that a lateral pattern in dolomite abundance should be expected in dolomites formed by any shallow hydrodynamic system that advects dolomitizing fluid horizontally, including the down‐gradient portion of a reflux system, the oceanward reaches of geothermal (Kohout) convection systems, and the seaward reaches of a seawater entrainment zone below a freshwater aquifer. However, even in those systems, a pattern will not form in all dolomites. Pattern will be muted in thinner (≤ ca 50 cm thick) beds and non‐emergent where the precursor had a high variance in reactive surface area (for example, a skeletal wackestone) and/or large variation in nucleate density. Pattern also did not form at temperatures above ca 60°C, which implies that pattern should not be expected in dolomites formed at intermediate to deep burial depths. As the patterns are horizontal, they also should not be expected where dolomitizing fluids moved vertically (for example, reflux immediately below a brine source). Pattern metrics (short‐range correlation length, and wavelength and amplitude of the longer cyclic component) vary with flow rate, fluid chemistry, bed thickness, porosity, grain size, temperature and the flow complexity (one‐dimensional, two‐dimensional or three‐dimensional) within the bed. However, no modelled pattern produced metrics larger than those documented on dolomite outcrops. The results thus constrain the length scales of porosity and permeability variance to include in petrophysical models of dolomite reservoirs, as well as the geological scenarios in which to consider metre‐scale lateral petrophysical variability.

Exploring the end‐Ordovician extinctions in Hirnantian near‐shore carbonate rocks of northern Guizhou, SW China: A refined stratigraphy and regional correlation

Richly fossiliferous Hirnantian shelly strata of near‐shore facies in northern Guizhou, South China, known as the Kuanyinchiao Formation, superbly record glacioeustatic sea‐level fluctuations and benthic faunal turnover. Recent studies of the temporal and spatial distribution of these carbonates and shelly fossils permit a critical stratigraphic revision and establishment of a robust regional stratigraphic correlation. The formation is revised to include three informal subdivisions, that is, units A, B, and C, in ascending order. Unit B of the formation typically is dominated by peloidal or oolitic grainstones, and unit C is composed of skeletal wackestone and calcareous mudstone, both units sharing distinctive coral and brachiopod faunas. This contrasts sharply with unit A of the formation, consisting of mudstone, silty mudstone, or calcareous mudstone, that yields the cool‐water Hirnantia fauna and associated coral fauna. In view of the presence of carbonate ooids and peloids, rugose corals, and a distinctive brachiopod assemblage, all indicative of warm‐water conditions, unit B, as well as unit C yielding the same shelly fauna, is interpreted as representing postglacial sedimentation immediately following the major Hirnantian glaciation, thus marking a significant climatic shift. Similar warm‐water carbonate rocks have been recognized in a number of regions along the margin of the Qianzhong Oldland, including Bijie, Renhuai, Tongzi, and Fenggang of northern Guizhou. Such a vast distribution area of these rocks indicates that postglacial carbonates are more widespread on the Yangtze Platform of South China than previously thought, providing a rare window into rocks and fossils of the survival interval immediately following the extinction event associated with the Hirnantian glacial episodes.

Depositional environments and sequence stratigraphy of carbonate mudrocks using conventional geologic observations, multiscale electrofacies visualization, and geochemical analysis: The case of the Tuwaiq Mountain and Hanifa Formations in a basinal setting, Saudi Arabia

ABSTRACT Depositional interpretation and sequence stratigraphic analysis of carbonate mudrocks requires numerical analysis and data integration to achieve quantitative, predictive stratigraphic and geochemical models. A depositional and sequence stratigraphic analysis is built for a basinal interval of the Tuwaiq Mountain and Hanifa Formations, Saudi Arabia. Conventional geologic interpretation, automated electrofacies analysis, and geochemical interpretation are integrated using quantitative means. Cluster analysis of well logs using self-organizing maps and hierarchical clustering allowed for a multiscale electrofacies analysis. This is useful for identifying major lithological surfaces, which commonly correspond to sequence stratigraphic surfaces. Geochemical data were used for depositional environment interpretation, such as sediment provenance, redox, and paleoproductivity conditions. Factor analysis is used to group element data. Redox and paleoproductivity indices were calculated using electrofacies clustering of different elemental groups. Electrofacies analysis shows good correlation with the lithofacies; that is, lithofacies can be identified from logs. Five major lithofacies have been identified in the studied interval: (1) wispy laminated skeletal wackestones to mudstones; (2) differentially cemented skeletal wackestones, packstones, and grainstones; (3) laminated peloidal mudstones to wackestones; (4) bioturbated packstones and wackestones; and (5) palmate anhydrite. The majority of the interval is interpreted to be deposited by gravity processes. Four major sequences have been identified in the Tuwaiq Mountain and Hanifa Formations made of transgressive and highstand systems tracts. The uppermost bioturbated packstones of the Hanifa Formation are interpreted to be a lowstand systems tract with subsequent restriction leading to the deposition of gypsum. In the studied interval, total organic carbon (TOC) content correlates well with suboxic to anoxic intervals that have high paleoproductivity. Complete anoxia is not a prerequisite for organic matter preservation. These high TOC intervals are mainly in transgressive systems tracts.

Lateral variability of shallow-water facies and high-frequency cycles in foreland basin carbonate platforms (Pennsylvanian, NW Spain)

The kilometer-sized and 100-meter-thick carbonate platforms of the Escalada Fm. I and II (Middle Pennsylvanian) accumulated in the foredeep of a marine foreland basin during the transgressive phases of 3rd-order sequences and were buried by prograding siliciclastic deltaic systems in the course of the subsequent highstand. The carbonate successions show a general upward trend from grain- to mud-supported carbonates, interfingering landwards with siliciclastic deposits of a mixed siliciclastic-carbonate shelf (Fito Fm.) adjacent to deltaic systems. The spatial variability of the carbonate facies and the high-frequency (4th–5th order) cycles, from the platform margin-outer platform to the deltaic systems, has been interpreted from basin reconstruction. Carbonate facies include skeletal grainstone to packstone, ooidal grainstone, burrowed skeletal wackestone, microbial and algal boundstone to wackestone forming mounds, various algal bafflestone and coral biostromes in areas with siliciclastic input. These high-frequency transgressive–regressive cycles are interpreted to record allocyclic forcing of high-amplitude glacioeustasy because they show characteristic features of icehouse cycles: thickness >5 m, absence of peritidal facies, and in some cases, subaerial exposure surfaces capping the cycles. In the mixed cycles, siliciclastics are interpreted as late highstand to lowstand regressive deposits, whereas carbonates as transgressive-early highstand deposition. The lateral and vertical variability of the facies in the glacioeustatic cycles was a response to deposition in a rapidly subsiding, active foreland basin subjected to siliciclastic input, conditions that might be detrimental to the growth of high-relief carbonate systems.

Sequence Stratigraphic Context of Extensive Basin-Margin Evaporites: Middle Jurassic Gypsum Spring Formation, Wyoming, U.S.A.

Abstract Ancient marine evaporites are commonly far more extensive than any known modern evaporite system. This is true not only for basin-central saline giants, but also for evaporites that occur in basin-margin settings, and the origin of these basin-margin systems is unclear. Extensive basin-margin evaporites of the Middle Jurassic Gypsum Spring Formation are well exposed along the eastern flank of the Bighorn Basin in Wyoming, USA, and a facies and sequence stratigraphic analysis was undertaken to understand the origin of these evaporites and associated deposits. The Gypsum Spring Formation was deposited on a northwestward-dipping mixed evaporate–carbonate–siliciclastic ramp, with facies and depositional environments that include distal ooid shoal grainstone, open-marine shallow-subtidal peloidal–skeletal wackestone to packstone, restricted shallow subtidal laminated lime mudstone, peritidal dolostone and microbialite, as well as red mudstone and gypsum deposited on mud flats, sabkhas, and salinas. Three depositional sequences are present, with the J-1, J-1a, and J-1b sequence boundaries characterized locally by chert-pebble lags, silicified horizons, and iron-rich paleosols. Evaporites accumulated in a basin-margin position, with 1–6 m beds of gypsum forming in salinas and sabkhas, and nodular anhydrite and gypsum forming in red mudstones deposited on sabkhas and mud flats. The limited lateral extent of salina-type gypsum beds and the variable number of these beds among the exposures indicates substantial autocyclic control on the formation of salinas. Salinas are well developed in the TST of the J-1 sequence, and sabkhas and mud flats are well developed in the J-1 HST, the J-1a HST, and the J-1b TST. The great lateral extent of the Gypsum Spring evaporites reflects the original expanse of the depositional environment in which they formed, and is not the result of diachroneity of a narrow evaporite-precipitating belt, such as the modern-day coastal sabkhas of the Trucial coast. The great lateral extent of Gypsum Spring evaporites suggests that they were precipitated mainly from continental brines on an expansive desert mudflat.

Lithofacies of the Devonian Marcellus Shale In the Eastern Appalachian Basin, U.S.A.

The Marcellus Shale is an important hydrocarbon source rock and an unconventional reservoir whose origin has engendered much recent debate among geologists. Our analysis of the formation in West Virginia and adjacent States aims to identify and describe the several lithofacies in the eastern Appalachian basin and to interpret their depositional setting. The study is based on field, hand-sample, and microscopic observations, supplemented by X-ray diffraction, thermogravimetric, and gamma-ray-scintillometer data. We identify five major lithofacies: (1) calcitic coarse mudsto ne, quartz-silty and sparsely fossiliferous; (2) skeletal wackestone–packstone limestone , exhibiting a low to moderate faunal diversity; (3) calcitic carbonaceous medium mudstone , containing a high organic-carbon content and low-density, low-diversity fossil community; (4) siliceous carbonaceous fine mudstone , exceedingly radioactive with a high organic-carbon content and usually barren of benthic fossils; and (5) argillaceous coarse mudstone , quartz-silty, micaceous, and kaolinitic. Most layers of Facies 4 mudstone—the quintessential black shale of the Marcellus—formed in a stagnant, anoxic environment beneath a stratified water column and starved of terrigenous sediment. Yet, some layers of this same facies plus the other closely associated mudstones and limestone record markedly different conditions. Rhythmic organic-rich and -poor laminae indicate periods of a stratified water column followed by mixing of the water body. Rip-up clasts, graded bedding, and small-scale cross-bedding resulted from moderate storm action down to the sea floor. Alternating styliolinid-rich and -poor laminae point to a repeated disruption of the thermocline and the recycling of bottom nutrients up to the surface water. The communities of benthic organisms and reduced organic-carbon contents reflect times of oxic–dysoxic bottom water. Widely varying thorium/uranium geochemical ratios suggest that the redox potential in the depositional environment shifted frequently between anoxic (Th/U 4.0). Finally quartz-silty, micaceous, and kaolinitic mudstones demonstrate a considerable influx of terrigenous coarse mud. Our research argues for a relatively shallow marine setting in which the sea floor everywhere in the study area remained above storm-wave base, the thermocline was intermittent being tied to a seasonal stratification–mixing cycle, the redox state of the benthic zone varied considerably, and the supply of detrital silt was at times quite high.

Linking carbonate cyclicity in platforms to depositional and diagenetic overprints: an example from the Lower Eocene Drunka Formation, west of Assiut–Minia stretch, Western Desert, Egypt

Early diagenetic modifications in carbonate sequences can be traced to fluctuations in relative sea level and, hence, sequence stratigraphy. These variations in carbonate cyclicity can have an impact on the spatial and temporal distribution of diagenetic alterations. This study aims to investigate the depositional and diagenetic attributes of early Eocene Drunka carbonate sequences in the Western Desert of Egypt as a function of changes in cyclicity and relative sea-level variations. Drunka Formation is a thick carbonate platform consisting of two members; the lower member (34 m thick) consists of four depositional shallowing-upward or emergence carbonate cycles that range in thickness from 4 to 10.75 m. Each cycle commences with a thin lime mudstone and/or algal skeletal wackestone and capped with thick oolitic–peloidal grainstone. These cycles represent nongradual cycles that signify no regular balance between subsidence and sedimentation rates. They indicate high-frequency sea-level fluctuations and/or short-time sea-level oscillations, accompanied with high production of carbonates. The upper member (99.75 m thick) comprises of emergence carbonate cycles that begin with a thick lime mudstone, followed by algal wackestone/packstone and capped by a thin bed of nummulitic algal packstone or peloidal grainstone (0.5 to 5 m thick). These cycles represent gradual cycles that denote regular relative vertical increase in sea level concurrent with increase in sedimentation rate. They resemble low-frequency sea-level fluctuations with high rate of subsidence outpacing the increase in sea-level rise. The drowning of carbonate platform is evidenced by increase in thickness of emergent carbonate cycles upward especially in the upper member, the increase of thickness in the lower parts of cycles (lime mudstone and wackestone) at the expense of cycle cap (packstone and grainstone), and the decrease of skeletal particles. The carbonate components of the grainstone lithofacies (topmost part of cycles) are characterized by luminescence characteristics and depleted oxygen isotopic values (−7.57 to −9.07 ‰ Vienna Pee Dee Belemnite (VPDB); av. = −8.32 ‰ VPDB) and carbon isotopic values (−4.90 to −6.45 ‰ VPDB; av. = −5.675 ‰ VPDB). These values reflect formation/stabilization under meteoric phreatic conditions. The coated grains, which show aggrading neomorphism, display oxygen isotopic values that range between −6.48 and −9.48 ‰ VPDB (av. = −7.98 ‰ VPDB). The close and relatively depleted δ18 O values of the base of cycles (lime mudstone, wackestone, and packstone) have probably suffered neomorphic stabilization in freshwater-dominated environment on the spatial and temporal distribution of diagenetic alterations.

Arabian carbonate reservoirs: A depositional model of the Arab-D reservoir in Khurais field, Saudi Arabia

The Upper Jurassic Arab Formation in the Arabian Peninsula, the most prolific oil-bearing interval of the world, is a succession of interbedded thick carbonates and evaporites that are defined stratigraphically upsection as the Arab-D, Arab-C, Arab-B, and Arab-A. The Arab-D reservoir is the main reservoir in Khurais field, one of the largest onshore oil fields of the Kingdom of Saudi Arabia. In Khurais field, the Arab-D reservoir is composed of the overlying evaporitic Arab-D Member of the Arab Formation and the underlying upper part of the Jubaila Formation. It contains 11 lithofacies, listed from deepest to shallowest: (1) hardground-capped skeletal wackestone and lime mudstone; (2) intraclast floatstone and rudstone; (3) pelletal wackestone and packstone; (4) stromatoporoid wackestone, packstone, and floatstone; (5) Cladocoropsis wackestone, packstone, and floatstone; (6) Clypeina and Thaumatoporella wackestone and packstone; (7) peloidal packstone and grainstone; (8) ooid grainstone; (9) crypt-microbial laminites; (10) evaporites; and (11) stratigraphically reoccurring dolomite. The Arab-D reservoir lithofacies succession represents shallowing-upward deposition, which, from deepest to shallowest, reflects the following depositional environments: offshore submarine turbidity fans (lithofacies 1 and 2); lower shoreface settings (lithofacies 3); stromatoporoid reef (lithofacies 4); lagoon (lithofacies 5 and 6); shallow subtidal settings (lithofacies 7 and 8); peritidal settings (lithofacies 9); and sabkhas and salinas (lithofacies 10). The depositional succession of the reservoir represents a prograding, shallow-marine, reef-rimmed carbonate shelf that was subjected to common storm abrasion, which triggered turbidites.

Siphodinarella costata n. gen., n. sp., a new benthic foraminifer from the Coniacian of the Adriatic Carbonate Platform (Slovenia, Croatia)

A new benthic foraminifer is described as Siphodinarella costata n. gen., n. sp. from Coniacian shallow-water platform-interior carbonates of Slovenia and Croatia. The new foraminifer is found in skeletal wackestone in association with small benthic foraminifera, thaumatoporellaceans, and calcimicrobes (Decastronema, Girvanella-type tubes). The existence of an internal siphon in Siphodinarella n. gen. is interpreted as an entosolenian tube and discussed in terms of its generic and suprageneric importance.

Regional Sequence Stratigraphy, Biostratigraphy, and Facies of the Upper Cretaceous Austin Chalk in South and Central Texas

This regional study constructs a sequence stratigraphic framework for the Upper Cretaceous Austin Chalk in South and Central Texas, based on new nannofossil biostratigraphy, core descriptions, correlations of wireline logs, and synthesis of outcrop, core, and paleontological descriptions from literature. The study area extends along strike from the Rio Grande to Austin, and along dip from the outcrop belt to the limit of downdip well control at the Lower Cretaceous shelf edges. New biostratigraphic control enables correlations across the region and ties the stratigraphic nomenclature from the outcrop to the major producing petroleum fields on either side of the San Marcos Arch. The upper Turonian–lower Campanian Austin Chalk consists of three unconformity-bounded sequences, with a basal unconformity, two internal unconformities, and an overlying unconformity. Sequence boundaries are recognized by nannofossil biostratigraphic hiatuses, which coincide with hardgrounds, Glossifungites ichnofacies, and coarse-grained glauconitic and phosphatic skeletal lags in cores and outcrops. Glauconite-rich beds have regionally extensive log character that overlie angularly truncated section. Facies within sequences can be distinguished as inner ramp (light colored skeletal wackestone, with diverse burrowers and macrofossils), middle ramp (medium gray horizontally burrowed skeletal wackestone), and outer ramp (interbedded dark horizontally burrowed and laminated skeletal wackestone), based on a depositional model of increasing oxygen content in shallower water. Regional cross sections show that the three sequences (AC–I to AC–III) in the Austin Chalk have an extremely asymmetric distribution across the San Marcos Arch. The upper Turonian–lower Coniacian section (AC–I) is very thick west of the San Marcos Arch but is almost completely eroded in the east. The upper Coniacian–Santonian section (AC–II) is very thick east of the San Marcos Arch but is almost completely eroded over the San Marcos arch, and is thin on the west side. The upper Santonian–lower Campanian sequence (AC–III) is more balanced, but similar to the middle sequence. Downdip, the Austin Chalk thins dramatically toward the relict Lower Cretaceous Edwards margin. The Austin Chalk was deposited as a downdip thinning wedge, due to greater carbonate productivity updip and non-deposition or erosion downdip. This interpretation is based on the geometry of the overlying Anacacho Formation, which thickens as the Austin Chalk thins, and internal facies relationships and markers within the Austin Chalk. Petroleum reservoirs in the Austin Chalk occur across a range of inner to outer ramp depositional environments but generally are older on the west side of the San Marcos Arch compared to the east side.

Depth transect of an Upper Triassic (Rhaetian) reef from Gosau, Austria: Microfacies and community ecology

In the Late Triassic (~235–201.3Ma) scleractinian corals and hypercalcified sponges built large, diverse reef ecosystems, the most famous of which are the Dachstein reefs. This study presents a depth transect along an Upper Triassic (lower Rhaetian) Dachstein reef from the Gosausee margin of the Dachsteingebirge (Gosau, Austria). The Gosausee microbial-sponge-coral reef is a complete barrier reef with an almost continuous fore reef to lagoon transect preserved (a very rare occurrence for the Triassic), and thus provides a window into depth zonation of Dachstein-type reef facies and biotic succession. The Gosausee reef facies exhibit strong depth control and five classic reef facies or zones are identified: the fore reef, reef front, reef crest, back reef, and lagoon facies. Thin, rare microbial fabrics and a high abundance of fine-grained, mud-rich skeletal wackestones (transported reef debris) characterize the deepest fore reef. As the reef shallows, muddy sediments decrease in abundance and are replaced by microbial fabrics, corals, and cements. Abundant sponges, microbial crusts, and thick, marine cements typify the reef crest, whereas microbialite-coated phaceloid corals are dominant in the back reef facies. Heavily cemented oncoids or microbial-sponge bindstones are characteristic of the lagoon. Based on their compositional and biotic similarities, the Gosausee reef was likely part of the same barrier reef systems as the source reef for the Gosaukamm reef breccia (one of the classic Norian–Rhaetian Dachstein reefs). The reef zones of the Gosausee margin can be used to interpret the depth or reef zone of less well preserved reef fragments, can inform models of community ecology and niche utilization in the Late Triassic, and highlight the need for additional research into the environmental factors that controlled biotic distribution in Upper Triassic reefs.

Sequence development on a sediment-starved, low accommodation epeiric carbonate ramp: Silurian Wabash Platform, USA mid-continent during icehouse to greenhouse transition

Abstract Twelve cored wells through the Silurian section of the Wabash Platform, Indiana, U.S.A., and 2 non-cored basinal wells with cuttings, were studied to define the sequence stratigraphy of a sediment-starved ramp within a low-latitude epeiric sea during the latest Ordovician icehouse to Silurian greenhouse transition. The Wabash Platform (approximately 200,000 km2 area) was bounded to the north by the Michigan Basin, to the east by the Appalachian foreland basin, and passed to the southwest into the Vincennes Basin, which was open to the ocean. Facies developed include: platform-fringing buildups (stromatactis wackestone core, stromatoporoid skeletal wackestone caps and crinoidal rudstone/packstone flank facies); and non-buildup crinoid grainstone–packstone sheets (shoals), skeletal wackestone (variably cherty; between fair weather and storm wave base; deep lagoon and deep ramp) and carbonate mudstone (clean to argillaceous; sub-storm wave base, deep lagoon and deep ramp/basin). The eleven sequences, 2 m to 30 m thick and 1 to 4 m.y. durations (average ∼ 2.5 m.y.), generally can be correlated to global sea-level cycles. The lower three sequences are disconformity-bounded (reflecting moderate ice sheets) but the others are relatively conformable. The most easily mapped regional surfaces are the transgressive surfaces, because the correlative conformities are cryptic. The wide well spacing prevented mapping of lowstand system tracts. The transgressive systems tracts are upward-deepening, upward-fining carbonate units, that become more argillaceous and silty upward. The highstand systems tracts are upward-coarsening carbonate mudstone to wackestone–packstone and rare grainstone. Barrier bank complexes and isolated buildups aggraded during deposition of the upper 4 sequences to form a discontinuous raised rim (∼ 40 m relief) to the ramp while the interior of the platform remained sediment starved. Subsidence rates in the basin were very low (0.6 to 1.2 cm/k.y.), but the low platform accumulation rates (0.2 cm/k.y. to 0.8 cm/k.y.) prevented the seafloor from building to sea level in all but the lower three sequences and later over barrier banks (accumulation rates 2 cm/k.y.). Parasequence development in high-accommodation settings elsewhere in the Silurian in North America records transition from moderate ice-sheets to an ice-free world. This is weakly expressed on the Wabash Platform due to the dominantly deeper subtidal setting. The remarkably low sedimentation rates resulted from the epeiric sea being isolated from the open ocean, and they likely are typical of the large epeiric seas of the Paleozoic.

A biodetrital coral mound complex: Key to early diagenetic processes in the Mississippian Bangor Limestone

The Bangor Limestone is a Mississippian (Chesterian) shallow marine carbonate formation exposed over a large portion of the Interior Low Plateaus province of northern Alabama. It is dominated by oolitic grainstone and skeletal wackestone and packstone, but in one outcrop near Moulton, Alabama, the Bangor contains a five m thick, 25 m wide, oolite-biodetrital mound-tidal flat succession. This sequence is interpreted as a 4th order sea level cycle.

Endemic foraminifera of the LateMaastrichtian from the northern branch of the Neotethys, NE Turkey

The study of benthic foraminifera from the upper Maastrichtian intraplatform open shelf limestones in northeastern (Eastern Pontides) and central (Anatolides parts of Turkey reveals the presence of some benthic foraminiferal taxa,which appear to characterize the northern branch of the Neo-Tethys. The limestones are mainly composed of two microfacies: skeletal wackestone and bioclastic wackestone. These facies include Sirelina orduensis (Meric and Inan), Laffitteina turcica (Inan), Cideina soezerii (Sirel), Selimina spinalis (Inan), Sivasella goekceni (Meric and Inan) and Postomphalocyclus merici (Inan). The geographic distribution of these species seems to be limited to the southern continental margin of the northern branch of the Neo-Tethys Ocean and the southern margin of the Rhodope-Pontide Fragment. These endemic taxa are associated with Orbitoides medius, Orbitoides apiculatus, Omphalocyclus macroporus, Pseudomphalocyclus blumenthali, Simplorbites papyraceus, Hellenocyclina beotica, Siderolites calcitrapoides, Loftusia minor, Smoutina cruysi, Sirtina orbitoidiformis, Laffitteina marsicana, Cuneolina ketini, Moncharmontia sp. and Dargenionella sp. which indicate a Late Maastrichtian age.

MAPPING AND CLASSIFYING FLOW UNITS IN THE UPPER PART OF THE MID‐CRETACEOUS SARVAK FORMATION (WESTERN DEZFUL EMBAYMENT, SW IRAN) BASED ON A DETEMINATION OF RESERVOIR ROCK TYPES

The mid‐Cretaceous Sarvak Formation, the second‐most important reservoir unit in Iran, is composed mainly of grain‐supported carbonates. For the purposes of this study, flow units in the upper part of the formation were identified, mapped and classified as part of an integrated reservoir characterization study at a giant oilfield in SW Iran. Pore types and geometries, pore‐scale diagenetic history and core‐scale depositional attributes were logged using conventional petrographic and lithological methods. The resulting data were combined with core descriptions, mercury‐injection capillary pressure data, and wireline log and geophysical data to identify five reservoir rock types: (i) highly oil‐stained, grain‐supported carbonates, including patch reef and barrier complex deposits with high porosities and permeabilities; (ii) leeward and seaward shoal deposits including grain‐supported packstones and skeletal wackestones with high porosities and permeabilities; (iii) dominantly mud‐supported lagoonal and open‐marine facies with fair porosity and permeability; (iv) grain‐supported but highly cemented facies which had poor reservoir characteristics; and (v) calcareous shales and shaly limestones with no reservoir quality.Based on the reservoir rock types, eight flow units were recognised. Subsequently, four reservoir zones were defined based on these flow units at a field scale.This study has contributed to our understanding of flow units in this complex carbonate reservoir, and has improved our ability to characterize and model the architecture of the reservoir from pore to core to field scale.

The mid-Carboniferous Arctic Lake Formation, northwestern Stikine terrane, British Columbia

The Lower Devonian to Upper Permian Stikine Assemblage is well exposed in northwestern British Columbia, near the west-central margin of the Stikine terrane. The stratigraphic base of the assemblage is not exposed and the Upper Triassic Stuhini Group disconformably overlies the assemblage locally. The assemblage consists mainly of various types of volcanic rocks, and there is a large, polyphase, Devono-Mississippian sub-volcanic plutonic complex. The assemblage includes a distinctive, but laterally discontinuous mid-Carboniferous limestone named the Arctic Lake Formation. Facies are mainly locally re-worked, well-bedded skeletal wackestone, but whole fossil floatstone and intraformational olistostromal deposits are present. Stratigraphic thickness varies from 32 to 80 metres. Stratigraphic contacts with bounding Lower and Upper Carboniferous successions are conformable and intergradational, but paraconformities may also be present. The formation records weak development of disjointed carbonate ramps on volcanic highlands. Neritic faunal assemblages have warm water, open marine affinity. The formation spans the Serpukhovian-Bashkirian boundary and encompasses microfacies assigned to Global Foraminiferal zones 17, 18 and 20. Integrated foraminiferal-coralconodont biostratigraphy and uranium-lead geochronology support a circa 350 Ma age for the Tournaisian-Visean boundary, and a circa 320 Ma age for the Serpukhovian-Bashkirian (Mississippian-Pennsylvanian) boundary.

Cherty carbonate facies of the Montoya Group, southern New Mexico and western Texas and its regional correlatives: a record of Late Ordovician paleoceanography on southern Laurentia

The Upper Ordovician Montoya Group in southern New Mexico and westernmost Texas records predominantly subtidal deposition on a gently dipping carbonate ramp that was subsequently nearly entirely dolomitized. The medial unit of the Montoya Group, the Aleman Formation is unique because it contains abundant chert (10–70% by volume). The chert occurs as: (1) thin continuous beds of sponge spicules within mudstone or calcisiltite; (2) discontinuous, lenses or nodules within skeletal wackestones and packstones; or (3) as a replacement of skeletal grains and burrows. Coeval skeletal grainstones and muddy peritidal facies contain little chert. Phosphate (up to 5 wt.%) occurs within the underlying Upham Formation and the Aleman Formation as replacement of fossils and peloids. The abundance of chert and phosphate in these subtidal facies indicates they formed within a region of strong upwelling. Regional correlation with Upper Ordovician cherty units along the periphery of southern Laurentia and other low latitude continents suggests that upwelling was widespread and long-lived during the Late Ordovician. The upwelling is interpreted to record vigorous oceanic circulation produced by the onset of glaciation on Gondwana during this period. Late Ordovician relative sea-level curves around the periphery of Laurentia indicate correlative third-order (1–3 my duration) fluctuations that may provide a means for high-resolution global correlations. However, the mechanism(s) that produced these long-term fluctuations are unclear.