Extensive Carbonate Platform Research Articles

Insights into a million-year-scale Rhenohercynian carbonate platform evolution through a multi-disciplinary approach: example of a Givetian carbonate record from Belgium

AbstractIn this paper we formulate answers to three important questions related to Givetian carbonate records and their use for reconstructing million-year-scale past palaeoenvironmental changes. First, we provide detailed illustrations of the fascinating diversity that shaped a significant shallow reefal platform during early to late Givetian time in the Rhenohercynian Ocean; secondly we improve the sedimentological model of the extensive Givetian carbonate platform in the Dinant Basin; and thirdly we evaluate the application of magnetic susceptibility as a tool for long-term trend correlations and palaeoenvironmental reconstructions. These goals are reached by making a sedimentological, geophysical and geochemical study of the La Thure section. Through the early–late Givetian interval we discerned 18 microfacies ranging from a homoclinal ramp to a discontinuously rimmed shelf and then a drowning shelf. The comparison of these sedimentological results with those published for the south of the Dinant Syncline allowed us to provide an up to date model of the vertical and lateral environmental development of one of the largest Givetian carbonate platforms in Europe. This comparison also increased the knowledge on the distribution of facies belts in the Dinant Basin and allowed us to highlight the Taghanic Event. Palaeoredox proxies reveal a substantial change in the oxygenation level, from oxygen-depleted to more oxic conditions, between middle and late Givetian time. We demonstrated the relationship between variation in magnetic susceptibility values and proxies for siliciclastic input (such as Si, Al). The La Thure section is considered a key section for the understanding of internal shelf settings bordering Laurussia's southeastern margin.

High frequency peritidal cycles of the upper Araras Group: Implications for disappearance of the neoproterozoic carbonate platform in southern Amazon Craton

The Araras Group is an extensive carbonate platform developed at the southeastern margin of the Amazon Craton during the Neoproterozoic. The Nobres Formation corresponds to the upper unit of the Neoproterozoic Araras Group. It is exposed in road cuts and quarries in the Northern Paraguay Belt, and is characterized by meter-scale shallowing upward cycles. Forty-four fourth-to fifth-order parasequence cycles are enclosed into three third order sequences/megacycles, unconformably overlain by siliciclastic deposits of the Alto Paraguay Group. The cycles are generally of peritidal type, limited by exposure surfaces composed of asymmetrical tidal flat/sabkha lithofacies in the basal Nobres Formation. They consist of fine dolostone, intraclastic dolostones with megaripples, stromatolites biostrome, sandy dolostone with enterolithic structures and silicified evaporite molds. Upsection, the cycles progressively become symmetrical, comprising arid tidal flat deposits with abundant stromatolite biostrome, fine-grained sandstone and rare evaporitic molds. The stacking patterns for hundreds of meters indicate continuous and recurrent generation of accommodation space, probably triggered by subsidence concomitant with relative sea-level changes. Palynomorphs found in the upper part of Nobres Formation comprehend spheroidal forms, such as Leiospharidia, rare filamentous and acanthomorphous acritarchs, mostly Tanarium correlated to the Ediacaran Complex Acantomorph Palynoflora of ∼580–570 Ma. Previous data of carbon isotopes and paleogeographic reconstructions, and also the presence of evaporites and storm-influenced deposits in the Araras Group, suggest a wet to tropical setting for Amazonia during the Mid-Ediacaran, which is incompatible with previous claims for Gaskiers-related glacial sedimentation in the region. During the final stages of evolution of the Araras carbonate platform, a progressive input of terrigenous has occurred in the peritidal setting likely due tectonic activity in the vicinity of the basin and the rapid uplift of source areas to the west and to the east of the basin preceding the progradation of the siliciclastic fluvial-coastal environments related to the Alto Paraguay sedimentation. This event resulted in erosion of shallow carbonate basins and, potentially, the demise and disintegration of the Neoproterozoic carbonate platforms in southern Amazon Craton.

Tectonic setting and stratigraphic architecture of an Early Cretaceous lacustrine carbonate platform, Sugar Loaf High, Santos Basin, Brazil

Abstract The Sugar Loaf High is an extensive basement structure within the Santos Basin, offshore Brazil, in which all of the elements of the pre-salt hydrocarbon play are present. Following a clastic-dominated early synrift phase, the later synrift and sag phases are characterized by an extensive non-marine carbonate platform over the basement high. The study area within this new 3D seismic dataset contains carbonates stratigraphically equivalent to the reservoirs within the Lula and Sapinhoá fields. Analysis of the 3D seismic data shows that the pre-salt carbonates onlap the basement of the Sugar Loaf High and form aggradational geometries on a platform that has a proximal to distal width of about 100 km and an interpreted relief of some 900 m. The carbonate platform has an aggradational and mounded basinward-facing gullied margin with slope deposits. Progradational units are imaged on the platform top that may be either carbonate or clastic in composition. The platform is preserved beneath evaporites that form the regional seal within the Santos Basin. Platforms of this size are previously unknown from lacustrine settings outside the pre-salt stratigraphy of the Santos and Campos Basins, Brazil.

A NEW MODEL FOR THE FORMATION OF DOLOMITE IN THE TRIASSIC DOLOMITES, NORTHERN ITALY

The Pale di San Martino and Pale di San Lucano (referred to together as the “Pale”) are remnants of an originally more extensive carbonate platform in the Dolomite Mountains of northern Italy. The platforms are composed of Middle Triassic dolomites and limestones up to 1.6km thick. Limestones comprise 2–3% of the platform carbonates and are restricted to narrow corridors (tens to a few hundred metres wide, hundreds of metres long and high) within the dolomite. The mainly sucrosic dolomites of the Pale are interpreted as the result of recrystallization of a depositional, nearly stoichiometric Mg calcite under burial temperatures of ca. 40–70°C. The principal arguments are: The quantitative composition indicates that all platform carbonates are composed mainly of micritic crusts (45%; boundstone fabric prevails) and early cement (35%; microcrystalline, fibrous). The platform carbonates were probably mainly bacterial precipitates and tight at the sediment-water interface (porosities <5%, permeabilities in the micro-Darcy range). The limestone-dolomite transitions (centimetres to decimetres wide) lack dolomite gradients. The lack of evidence for flowing fluids causing dolomitization suggests stagnant pore waters. The δ13C of average dolomite is 1.3‰ heavier than that of coeval limestone (666 analyses). The difference corresponds to a primary difference of 50mol% MgCO3 and is interpreted as the result of fractionation. It suggests a dolomite precursor of very high Mg calcite, whereas present-day limestone of the Pale was probably deposited as a basically Mg-free polymorph (aragonite and/or calcite). The dolomite δ18O (+1 to −11‰ VPDB) values show a scatter over the platform thickness and preserve randomly distributed values around 0‰. The scatter is probably due to selective re-setting of δ18O near pore spaces and is mainly a sampling effect. The observation that 87Sr/86Sr ratios (77 analyses) of limestone and dolomite are either slightly higher or lower than Middle Triassic seawater, but almost never “normal marine”, suggests that the platform carbonates of the Pale were deposited from seawater contaminated with artesian freshwater. The limestone corridors are probably caused by artesian springs of somewhat higher than ambient depositional temperature, with low Mg calcite and/or aragonite deposited in or near fracture zones. The volumetrically subordinate cycle-cap dolomite is possibly a primary precipitate.

Spatial and temporal patterns of Cenozoic dynamic topography around Australia

Despite its importance, the spatial and temporal pattern of dynamic topography generated by mantle convective circulation is poorly known. We present accurate estimates of dynamic topography from oceanic basins and continental margins surrounding Australia. Our starting point is measurement of residual depth anomalies on the oldest oceanic floor adjacent to the continental shelf. These anomalies were determined from a combined dataset of ~200 seismic reflection and wide‐angle images of well‐sedimented oceanic crust. They have amplitudes of between −1 km and +0.5 km, and their spatial variation is broadly consistent with long‐wavelength free‐air gravity and shallow seismic tomographic anomalies. Along the Northwest Shelf, a regional depth anomaly of −300 to −700 m intersects the adjacent continental shelf. The temporal evolution of this anomaly was determined by analyzing the stratigraphic architecture of an extensive carbonate platform, which fringes the shelf and records a dramatic switch from progradation to aggradation during Neogene times. Three‐dimensional seismic mapping calibrated by boreholes was used to calculate water‐loaded subsidence histories at rollover points of clinoforms along the shelf. At 9 ± 3 Ma, the rate of subsidence increases from 5 to up 75 m Myr−1, generating a subsidence anomaly of −300 to −700 m. The amplitude of this anomaly varies along the shelf and cannot be generated by glacio‐eustatic sea‐level variation. Instead, we propose that a regional subsidence episode, which affects both the proximal shelf and the distal oceanic basin, was generated by convective drawdown. By combining our results with other published estimates of uplift and subsidence, a map of Australia, which shows the spatial and temporal pattern of dynamic topography is presented. Most, but not all, of Australia's epeirogeny can be attributed to rapid northward motion of the Australian plate over a pre‐existing pattern of convective circulation.

CSEM data interpretation constrained by seismic and gravity data: an application in a complex geological setting

We describe a novel approach to the interpretation of marine controlled source electromagnetic (CSEM) data based on electromagnetic attributes in combination with gravity and seismic data. This integrated approach involves a new electromagnetic attribute of resistor probability, and has been applied in a complex exploration area in the Barents Sea adjacent to an extensive carbonate platform. We used the data recorded by a total of 172 CSEM receivers from two different surveys. Gravity data were also used to highlight large-scale geological features. Integrating seismic, electromagnetic, and gravity information helped to distinguish resistivity anomalies caused by geological variations from those caused by hydrocarbons. Finally, the hydrocarbon distributions in two stacked reservoirs were accurately mapped. Our integrated approach significantly improved the appraisal of the field, reducing the exploration risk in the surrounding area and facilitating the placement of future wells.

아나톨리아 반도의 지질구조: 대륙 충돌에 따른 구조적 성장

아나톨리아 반도는 북쪽의 폰타이드 지판, 남쪽의 아나톨리드-토우리드 지판 및 남동쪽의 아라비안 석회암 대지를 포함하는 몇 개의 대륙 조각들로 구성되어 있다. 이러한 대륙 조각들은 제 3기 말에 하나의 대륙으로 병합되었으나 현생 이언 동안에는 고테티스해와 신테티스해와 같은 고해양에 의해 분리되어 있었다. 북쪽에 위치하는 폰타이드 지판은 고생대 동안에 바리스칸 조산운동과 중생대 동안의 키메리안 조산운동을 받은 증거들을 보존하고 있으며 비교적 약한 알파인 조산운동의 영향을 받는 등 라우라시아 대륙과 비슷한 지체구조 발달사를 보인다. 폰타이드 지판은 스트랜드자대, 이스탄불대, 사카라야대로 세분되며 이들은 백악기 중기에 하나의 지판으로 병합되었다. 남쪽에 위치하는 아나톨리드-토우리드 지판은 삼첩기에 곤드와나 대륙으로부터 분리되어 중생대 동안에 광역적인 탄산염 대지로 존재하는 등 곤드와나 대륙과 비슷한 지체구조 발달사를 보인다. 아나톨리드-토우리드 지판은 알파인 조산운동 시기에 의해 심하게 변형되었고 부분적으로 변성작용을 받아 변형과 변성의 시기와 형태를 기준으로 세분될 수 있다. 남동쪽에 위치하는 아라비안 석회암 대지는 아라비아 판의 북쪽 연장으로 고생대 동안에는 탄산염암이, 중생대 동안에는 쇄설성 퇴적암이 우세한 아나톨리드-토우리드 지판과 비슷한 층서를 보인다. 이러한 지판들이 마지막으로 합병된 이후 올리고세와 마이오세 동안에는 새로운 구조운동의 시기가 있었다. 이러한 신기구조운동은 주향이동성 단층작용과 확장, 육성 퇴적작용, 광역적인 칼크-알카리 계열의 화성작용 등으로 특징지어 지며 이러한 신기 구조운동은 아나톨리아 반도의 오늘날의 지형을 형성하는데 매우 중요한 역할을 하였다. The Anatolia peninsula consists of several continental fragments that include the Pontide Block in north and the Anatolide-Touride Block in south as well as the Arabian Platform in southeast. These continental blocks were joined together into a single landmass in the late Tertiary. During most of the Phanerozoic these continental blocks were separated by paleo-oceans, such as Paleo-Tethys and Neo-Tethys. The Pontide Block in north show Laurasian affinities, and was only slightly affected by the Alpide orogeny; they preserve evidence for the Variscan and Cimmeride orogenies. The Pontic Block is composed of the Strandja, Istanbul and Sakarya zones that were amalgamated into a single terrane by the mid Cretaceous times. The Anatolide-Tauride Block in south shows Gondwana affinities but was separated from Gondwana in the Triassic and formed an extensive carbonate platform during the Mesozoic. The Anatolide-Tauride Block was intensely deformed and partly metamorphosed during the Alpide orogeny; this leads to the subdivision of the Anatolide-Tauride Block into several zones on the basis of the type and age of metamorphism and deformation. The Arabian Platform in southeast forms the northernmost extension of the Arabian Plate that shows a stratigraphy similar to the Anatolide-Tauride Block with a clastic-carbonate dominated Palaeozoic and a carbonate dominated Mesozoic succession. A new tectonic era started in Anatolia Peninsula in the Oligocene-Miocene after the final amalgamation of these continental blocks and plate. This neotectonic phase is characterized by extension, and strike-slip faulting, continental sedimentation, and widespread calcalkaline magmatism, which played a very important role in producing beautiful landscapes of the Anatolia Peninsula today.

Growth fold controls on carbonate distribution in mixed foreland basins: insights from the Amiran foreland basin (NW Zagros, Iran) and stratigraphic numerical modelling

AbstractThe evolution from Late Cretaceous to early Eocene of the well dated Amiran foreland basin in the NW Iranian Zagros Mountains is studied based on the reconstruction of successive thickness, palaeobathymetry and subsidence maps. These maps show the progressive forelandwards migration of the mixed carbonate‐siliciclastic system associated with a decrease in creation of accommodation. Carbonate facies variations across the basin suggest a structural control on the carbonate distribution in the Amiran foreland basin, which has been used as initial constraint to study the control exerted by syndepositional folding in basin architecture and evolution by means of stratigraphic numerical modelling. Modelled results show that shallow bathymetries on top of growing folds enhance carbonate production and basin compartmentalization. As a consequence, coarse clastics become restricted to the internal parts of the basin and only the fine sediments can by‐pass the bathymetric highs generated by folding. Additionally, the development of extensive carbonate platforms on top of the anticlines favours the basinwards migration of the depositional system, which progrades farther with higher fold uplift rates. In this scenario, build‐ups on top of anticlines record its growth and can be used as a dating method. Extrapolation of presented modelling results into the Amiran foreland basin is in agreement with an early folding stage in the SE Lurestan area, between the Khorramabad and Kabir Kuh anticlines. This folding stage would enhance the development of carbonate platforms on top of the anticlines, the south‐westward migration of the system and eventually, the complete filling of the basin north of the Chenareh anticline at the end of the Cuisian. Incremental thickness maps are consistent with a thin (0.4–2 km) ophiolite complex in the source area of the Amiran basin.

Palaeoproterozoic sedimentation in the Cuddapah Basin, south India and regional tectonics: a review

Abstract The Proterozoic Cuddapah Basin of south India hosts a number of unconformity-bound sequences deposited over continental crust. The Palaeoproterozoic Papaghani Group with basal polymict conglomerate unconformably overlies the Archaean Peninsular Gneiss and set the stage for Palaeoproterozoic sedimentation following a long hiatus. Two cycles of fluvial–shallow-marine psammite–carbonate–pelite sedimentation in the western part of the basin (Papaghni sub-basin) is punctuated at different levels by mafic flows, sills and dykes, and less common acidic tuffs. Each of the sedimentation cycles fosters extensive carbonate platforms with dolomitic stromatolites and algal laminites. Emplacement of shallow crustal intrusives at the end of the first cycle was possibly linked to thermal anomalies in the mantle and associated widespread crustal extension in south India. Brittle fault-slip analysis from the western Cuddapah indicates repetitive extensional to strike-slip regimes. This paper proposes a likely late Palaeoproterozoic shallow-marine sedimentary sequence of the Nallamalai Fold Belt as allochthonous, thrusted over the successions of the Papaghni sub-basin and the Kurnool Group in the western part of the basin. Inversion of the Papaghni sub-basin and the development of regional erosional unconformities is discussed in the context of Palaeoproterozoic and early Mesoproterozoic orogenic events at the SE margin of India.

Rugose coral biostromes in the late Viséan (Mississippian) of NW Ireland: Bioevents on an extensive carbonate platform

The extensive upper Viséan (Asbian) platform carbonates in NW Ireland (Bricklieve Limestone Fm, Glencar Limestone Fm and Dartry Limestone Fm) contain distinctive rugose coral biostromes, which are dominated by different species of the genus Siphonodendron. These are in stratigraphic sequence: pauciradiale biostrome (oldest), martini biostrome and several junceum biostromes (youngest). They represent bioevents caused by special short-lasting ecological conditions and can be used as approximately synchronous horizons to correlate within the region. The pauciradiale biostrome is the thickest, laterally most persistent and most variable in facies and biotic composition of all biostromes. It formed on a tectonically influenced platform with a landward–seaward zonation from northwest to southeast, mainly above storm wave-base and below fair-weather wave-base. The northwestern Streedagh facies is characterized by the presence of clusters of large sheet-like colonies of S. pauciradiale. The intermediate O'Donnell's Rock facies is unique for the predominance of the fasciculate genus Solenodendron. The southeastern Bricklieve facies represents the amalgamation of autochthonous and allochthonous coral debris and bioclastic debris with localized small patches of coral boundstone. Mass occurrences of fasciculate rugose corals re-appear in the martini biostrome. This biostrome developed in a shallower water setting, just above fair-weather wave-base on a levelled carbonate platform. The junceum biostromes are thinner, decimetre to some metres thick, and less persistent. They formed in deeper water mostly below storm wave-base, except for the composite 2nd junceum biostrome of the Bricklieve Mountains, which records a period of shallowing. According to facies and coral morphologies, which were compared with modern scleractinian growth forms, sea-level variation exerted one of the strong controls on the rise and decline of the biostromes. The pauciradiale biostrome formed during an extended shallowing-upward cycle in a depth interval leading to the climax of Siphonodendron pauciradiale. The cycle ended with the development of the martini biostrome in shallow water. Its demise is probably caused by drowning during the late Asbian sea-level rise. During that time slice the junceum biostromes flourished in deeper water on the platform. Coral growth ceased almost completely after formation of the junceum biostromes during the falling sea level of the latest Asbian. Siliciclastic input and resulting turbidity, as well as turbulence formed a complex cascade of ecological constraints. In addition to the local tectonic influences, they combined to result in the “depth” factor which controlled the distribution of predominating coral populations and the succession of the different biostromes in the Asbian of NW Ireland.

Palynology of the Toarcian–Aalenian Global Boundary Stratotype Section and Point (GSSP) at Fuentelsaz (Lower–Middle Jurassic, Iberian Range, Spain)

The Fuentelsaz Section is located in the Castilian Branch of the Iberian Range (Guadalajara, Spain). Its exceptional Lower–Middle Jurassic transition outcrops led to its designation as the Global Boundary Stratotype Section and Point for the base of the Aalenian. The sediments of the Toarcian–Aalenian transition at Fuentelsaz are composed of marls with interbedded limestones in rhythmic alternation. Marls are dominant in the Toarcian and the Opalinum Zone up to the middle part of the Comptum Subzone. In the remaining Comptum Subzone, marly materials are scarce and limestones predominate. Sedimentation at Fuentelsaz took place over an extensive marine epeiric carbonate platform that was well connected with the open sea. In the Fuentelsaz Section, a total of 43 palynomorph taxa were recorded: 23 spore taxa, 13 pollen taxa, 4 acritarchs, 2 prasinophytes and 1 dinoflagellate cyst. The studied sediment samples were always dominated by terrestrial allochthonous miospores. In general, miospore assemblages are biased due to the transport of pollen from land or islands to the continental platform. Spheripollenites, Classopollis or indeed both pollens numerically dominate the assemblages. Other miospores appear in low numbers. Aquatic palynomorphs are also scarce; Micrhystridium lymensis is the most common. Five palynological assemblages (PA) were distinguished: PA1, PA2 and PA3 are Toarcian in age, PA4 is located at the boundary between the Toarcian and the Aalenian and PA5. This latter palynological assemblage which is lower Aalenian in age shows a strong reduction in palynomorph diversity and preservation in the Comptum Subzone. Palaeoecological analysis of the palynomorphs indicated the presence of calm, oligotrophic sea water. The palaeofloral communities of gymnosperms and vascular cryptogams—which grew in subtropical arid conditions—were poorly diversified.

Carbonate facies and biostromal distribution in a tectonically controlled platform in northwest Ireland during the late Viséan (Mississippian)

SUMMARY The North–West Carboniferous Basin (NWCB) contains a c. 3 km-thick succession of Mississippian (Courceyan to Arnsbergian) limestone, mudstone and sandstone, mostly of marine origin. The litho- and biostratigraphy of some sections in the basin are revised and new logged sections described in upper Viséan, limestone-dominated platform successions. In the O’Donnell’s Rock area, a precise correlation of the Glencar Limestone Formation with the lower part of the Bricklieve Limestone Formation, and the Dartry Limestone Formation with the upper part of the Bricklieve Limestone Formation is confirmed. Consequently, in late Viséan (Asbian) times the NWCB developed as an extensive (&gt;12,000 km 2 ) carbonate platform, in which sediments of the facies typical of outer, middle and inner parts of the platform were deposited. The Asbian rocks of this platform contain abundant solitary and fasciculate rugose corals, with laterally persistent biostromes dominated by different species of Siphonodendron . Evidence of regional uplift and subaerial exposure occurs at the top of the Dartry Limestone/Bricklieve Limestone formations, especially in the northern half of the platform. The base of the succeeding Meenymore Formation is defined by a change from marine limestone below an erosional disconformity surface, to sediments deposited on a sabkha and in supra-tidal–peritidal facies. Evolution of the platform was controlled, in part, by syn-sedimentary faults, which allowed different thicknesses of sediment to accumulate. Between the margins of the NWCB there existed a turbid carbonate platform with relatively shallow to moderately deep water characterized by impoverished microfaunal assemblages. The platform margin extended SE into deeper water facies of the Dublin Basin.

Carbon isotope chemostratigraphy and precise dating of middle Frasnian (lower Upper Devonian) Alamo Breccia, Nevada, USA

At Hancock Summit West, Nevada, western USA, uppermost Givetian (upper Middle Devonian) and lower and middle Frasnian (lower Upper Devonian) rocks of the lower Guilmette Formation include, in stratigraphic sequence, carbonate-platform facies of the conodont falsiovalis, transitans, and punctata Zones; the type Alamo Breccia Member of the middle punctata Zone; and slope facies of the punctata and hassi Zones. The catastrophically deposited Alamo Breccia and related phenomena record the ~ 382 Ma Alamo event, produced by a km-scale bolide impact into a marine setting seaward of an extensive carbonate platform fringing western North America. Re-evaluation of conodonts from the lower Guilmette Formation and Alamo Breccia Member, together with regional sedimentologic and conodont biofacies comparisons, now firmly locates the onset of the Johnson et al. (1985) transgressive–regressive (T–R) cycle IIc, which occurred after the start of the punctata Zone, within a parautochthonous megablock low in the Alamo Breccia. Whole-rock carbon isotope analyses through the lower Guilmette Formation and Alamo Breccia Member reveal two positive δ 13C carb excursions: (1) a small, 3‰ excursion, which is possibly correlative with the falsiovalis Event previously identified from sections in Western Europe and Australia, occurs below the breccia in the Upper falsiovalis Zone to early part of the transitans Zone; and (2) a large, multi-part excursion, dominated by a 6‰ positive shift, begins above the start of the punctata Zone and onset of T–R cycle IIc and continues above the Alamo Breccia, ending near the punctata– hassi zonal boundary. This large excursion correlates with the punctata Event, a major positive δ 13C excursion previously recognized in eastern Laurussia and northern Gondwana. Consistent with previous studies, at Hancock Summit West the punctata Event is apparently not associated with any regional extinctions or ecosystem reorganizations. In the study area, onset of the main punctata Event began after the start of both the punctata Zone and T–R cycle IIc, and preceded the Alamo impact by less than 650 k.y., as inferred from conodont biochronologic and regional rock-accumulation rate estimates. Although complicated by the heterolithic, high-energy deposits of the Alamo Breccia, the carbon isotope record of the breccia and post-breccia beds does not indicate a major impact-correlative perturbation to the carbon cycle. This study extends recognition of the punctata Event to western Laurussia, further reinforcing the potential global scale of the event and its potential importance to understanding early to middle Frasnian marine geochemistry and palaeoenvironments. Based on previous models and our observations, increased tectonic activity, increased nutrient flux to oceans, increased marine bioproductivity, widespread anoxia, and increased organic carbon burial were all likely key factors in driving the punctata Event excursion. Furthermore, periodic eustatic and regional relative sea-level rises may have played an important role in promoting organic carbon burial and in maintaining a link between the primary open-marine geochemical signal and that recorded on the shallow-marine, lower Guilmette carbonate platform.

Comparison of the Late Triassic carbonate platform evolution and Lofer cyclicity in the Transdanubian Range, Hungary and Pelagonian Zone, Greece

For comparative studies of Upper Triassic cyclic platform carbonates, the Transdanubian Range (Hungary) and the Pelagonian Zone (Greece) were chosen. Paleogeographically they represent two distant segments of the passive margin of the Neotethys Ocean. During the Late Triassic, on this wide margin a very extensive tropical carbonate platform domain was developed, referred to as the Dachstein-type carbonate platform system. The Transdanubian Range (TR) represents a segment of a continent-encroaching platform system, whereas the Pelagonian-Subpelagonian Zone (PG) may have been a large isolated platform, surrounded by deep-water basins. The discussed Upper Triassic thick platform carbonates (Fődolomit/Hauptdolomit Formation and Dachstein Limestone in the TR, and Pantokrator Formation in the PG) are made up of cyclically arranged facies deposited under similar environmental conditions in the interior zones of carbonate platforms. Three characteristic major facies types can be distinguished: shallow subtidal-lagoonal, intertidal and supratidal-pedogenic, which correspond to the three typical lithofacies (members C, B and A) of Fischer's (1964) Lofer-cycle. The cycles are usually bounded by discontinuity surfaces related to subaerial exposure and pedogenic alteration. The meter-scale (Lofer) cyclicity is predominant throughout the successions. However, various stacking patterns including symmetric complete, truncated, incomplete, and condensed cycles or even alternating peritidal and subtidal facies without any disconformity are recognized in both areas studied. Pervasive or partial early diagenetic dolomitization affected some parts of the cyclic successions in both areas. However, age-dependence of the early dolomitization was clearly demonstrated only in the TR, where the older part of the platform carbonate succession was subject to pervasive dolomitization, whereas the younger part is non-dolomitized and there is a transitional unit between them. This trend is attributed to the climate changing from semiarid to more humid. The Upper Triassic platform carbonates of the TR and PG show strikingly similar features concerning the litho- and biofacies, the stacking pattern and the thickness of the elementary cycles, despite their distant and different paleogeographic setting within the western Neotethys realm. This suggests a eustatic signal, i.e. the cyclic deposition was essentially controlled by orbitally-forced eustatic sea-level changes, although the contribution of autocyclic mechanisms cannot be excluded either. Definite signatures of subaerial exposure (karstic features and vadose meteoric diagenesis) at and below the cycle boundaries also support allocyclic control. In the northeastern part of the TR the carbonate platform was drowned at the Triassic-Jurassic boundary, whereas platform conditions persisted until the end of the Hettangian in the southwestern part. However, the Hettangian part of the succession is characterized by non-cyclic subtidal limestone, implying an upward-deepening trend. In contrast, in the PG the platform conditions continued until early to middle Liassic, and the Liassic succession is typified by well-developed pedogenic features, suggesting long-lasting subaerial exposure intervals, i.e. an upward-shallowing trend.

Sequence stratigraphy and architecture of a late Early–Middle Aptian carbonate platform succession from the western Maestrat Basin (Iberian Chain, Spain)

The attributes of a ‘four-systems-tract’ sequence are at times difficult to identify in outcrop-scale carbonate successions. Poor exposure conditions, variable rates of sediment production, erosion and/or superposition of surfaces that are intrinsic to the nature of carbonate systems frequently conceal or remove its physical features. The late Early–Middle Aptian platform carbonates of the western Maestrat Basin (Iberian Chain, Spain) display facies heterogeneity enabling platform, platform-margin and slope geometries to be identified, and provide a case study that shows all the characteristics of a quintessential four systems tract-based sequence. Five differentiated systems tracts belonging to two distinct depositional sequences can be recognized: the Highstand Systems Tract (HST) and Forced Regressive Wedge Systems Tract (FRWST) of Depositional Sequence A; and the Lowstand Prograding Wedge Systems Tract (LPWST), Transgressive Systems Tract (TST) and subsequent return to a highstand stage of sea-level (HST) of Depositional Sequence B. An extensive carbonate platform of rudists and corals stacked in a prograding pattern marks the first HST. The FRWST is constituted by a detached, slightly cross-bedded calcarenite situated at the toe of the slope in a basinal position. The LPWST is characterized by a small carbonate platform of rudists and corals downlapping over the FRWST and onlapping landwards. The TST exhibits platform backstepping and marly sedimentation. Resumed carbonate production in shelf and slope settings characterizes the second HST. A basal surface of forced regression, a subaerial unconformity, a correlative conformity, a transgressive surface and a maximum flooding surface bound these systems tracts, and are well documented and widely mappable across the platform-to-basin transition area analyzed. Moreover, the sedimentary succession studied is made up of four types of parasequence that constitute stratigraphic units deposited within a higher-frequency sea-level cyclicity. Ten lithofacies associations form these basic accretional units. Each facies assemblage can be ascribed to an inferred depositional environment in terms of bathymetry, hydrodynamic conditions and trophic level. The architecture of the carbonate platform systems reflects a flat-topped non-rimmed depositional profile. Furthermore, these carbonate shelves are interpreted as having been formed in low hydrodynamic conditions. The long-term relative fall in sea-level occurred during the uppermost Early Aptian, which subaerially exposed the carbonate platform established during the first HST and resulted in the deposition of the FRWST, is interpreted as one of global significance. Moreover, a possible relationship between this widespread sea-level drop and glacio-eustasy seems plausible, and could be linked to the cooling event proposed in the literature for the late Early Aptian. Because of the important implications in sequence stratigraphy of this study, the sedimentary succession analyzed herein could serve as an analogue for the application of the four-systems-tract sequence stratigraphic methodology to carbonate systems.

Termination of the Arabian shelf sea: Stacked cyclic sedimentary patterns and timing (Oligocene/Miocene, Oman)

In the Janabah region of southeastern Oman, Oligocene/Miocene limestones of the Shuwayr, Warak and Ghubbarrah formations are widely exposed. They were deposited on an extensive shallow carbonate platform that was part of the Arabian Shelf and located on the Gulf of Aden's northeastern rift shoulder, which emerged during the Early Miocene. The uppermost part of the studied sedimentary succession developed immediately before the permanently subaerial exposure of the carbonate platform. Cyclic changes of intertidal and subtidal facies document a fluctuating relative sea level at different frequencies and a continuous decline of accommodation. Single erosive surfaces with palaeokarst cavities and caliche crusts separate larger depositional cycles. These disconformities imply relatively long episodes of subaerial exposure and are interpreted to have been formed during lowstands of third-order sea level cycles that denuded the platform. Taxonomic studies of the accompanying mollusc faunas and certain benthic foraminifers allow a correlation of the recognised subaerial disconformities with the Ru4/Ch1 to Ch4/Aq1 sequence boundaries of Hardenbol et al. [Hardenbol, J., Thierry, J., Farley, M.B., Jacquin, T., Graciansky, P.-C., Vail, P.R., 1998. Mesozoic and Cenozoic Sequence Chronostratigraphic Framework of European Basins. In: Graciansky, C.-P., Hardenbol, J., Jacquin, T., Vail, P.R. (Eds.), Mesozoic and Cenozoic Sequence Stratigraphy of European Basins. Society for Sedimentary Geology, Special Publication 60, 13–603.]. This demonstrates that the termination of the Arabian shelf sea must be back-dated from the middle Burdigalian to the early Aquitanian.

Integrated 3D seismic and petrophysical data of the Sarvak formation in the Persian Gulf

During the Middle Cretaceous, extensive carbonate platforms covered the eastern part of the Arabian plate, including, the eastern Persian Gulf. Many of the major hydrocarbon accumulations are found in the Middle Cretaceous reservoir rocks which developed as the carbonate platforms evolved.

Unlocking paleo-environmental information from Early Cretaceous shelf sediments in the Helvetic Alps: stratigraphy is the key!

The northern alpine Helvetic thrust-and-fold belt includes an Early Cretaceous shallow-water carbonate succession, which was part of an extensive carbonate platform rimming the northern Tethyan margin. The structural architecture of the Helvetic zone allows for the palinspastic reconstruction of proximal-distal transects across the former platform into the outer-shelf realm for distances surpassing 80 km. The Early Cretaceous platform sediments preserved therein provide, therefore, excellent insight into the spatial and temporal evolution of this platform. Furthermore, the presence of ammonites in marker horizons within the Helvetic succession is key to unprecedented time control. During the life span of the Helvetic platform, carbonate build up and build out occurred along two distinct pathways: we discern between a mode including oligotrophic photozoan communities (latest Tithonian – Late Berriasian; Late Barremian; Early Aptian) and a mode dominated by mesotrophic heterozoan communities (Valanginian – Early Barremian; earliest Aptian; late Early Aptian – Late Aptian). The heterozoan mode was frequently interrupted by incipient platform drowning episodes, which materialized either in an important erosive hiatus, or in the deposition of highly condensed, glauconiteand phosphate-rich intervals (Early Valanginian – Early Hauterivian; late Early – early Late Hauterivian; latest Hauterivian – latest Early Barremian; middle Late Barremian; late Early Aptian – early Late Aptian; and latest Aptian – Early Albian). The photozoan mode is interpreted as essentially oligotrophic, whereas the heterozoan and drowning phases were associated with the input of coarser-grained detrital sediments and a correspondingly increased nutrient load, which were both the consequence of intensified chemical weathering on the continent due to warmer and more humid climate conditions. Their onset is signaled by increases in oceanic phosphorus burial rates and major positive excursions in the stable carbon isotope record. Oceanic anoxic episodes occurred during these latter phases. The northern Tethyan platform was not only controlled by climatic, environmental and paleoceanographic change, but changes in platform morphology and the composition of carbonate-producing benthic communities also influenced the quality and quantity of dissolved and particulate material exported into adjacent basins.

Progressive drowning of carbonate platform in the Moravo-Silesian Basin (Czech Republic) before the Frasnian/Famennian event: facies, compositional variations and gamma-ray spectrometry

The Moravo-Silesian Basin (MSB; eastern Czech Republic and southern Poland) hosted an extensive shallow-water carbonate platform in the Middle Devonian to Frasnian interval. The platform drowned in a stepwise fashion from the Palmatolepis hassi to the Pa. linguiformis zone. Three types of drowning successions were revealed from conodont biostratigraphy, facies, microfacies and gamma-ray spectrometry data: (A) drowning to periplatform turbidite setting; (B) drowning to (hemi)pelagic seamount setting and (C) drowning associated with the stratigraphical gap. In the lower Pa. hassi zone, rapid subsidence caused the platform to drown locally along the N–S to NW–SE trending faults (type A drowning). In the upper Pa. rhenana to the Pa. linguiformis zone, the drowning accelerated in the western part of the MSB due to locally higher subsidence rates combined with the Late Frasnian biotic crisis (type B). In the southern part of the basin, the platform emerged shortly before the Frasnian/Famennian (F/F) boundary and drowned in the Early to Late Famennian (type C). The primary cause of drowning was differential subsidence at the Laurussian passive margin. Eustatic sea-level fluctuations, if any, contributed only to a minor extent to the Late Frasnian drowning, but were effective in type C drowning during the Famennian. The drowning boundaries are associated with increased contents of K and Th, reflecting the deceleration of carbonate production. Uranium contents display isolated peaks that roughly correlate with the drowning boundaries or the stratigraphic gaps associated with the F/F boundary. The uranium contents are considered to reflect local depositional conditions and are not suitable for stratigraphic correlation. On the other hand, from the K and Th contents, we can infer Late Frasnian sea-level fluctuations with duration on the order of 1 Myr. These cyclic variations in K and Th contents proved to be useful in platform-to-basin stratigraphic correlation.

Using multi-attribute neural networks classification for seismic carbonate fades mapping: a workflow example from mid-Cretaceous Persian Gulf deposits

Abstract Seismic facies mapping in large seismic surveys can be time consuming, even if only a basic overview of the facies distribution is needed. Therefore this study outlines an approach for the use of volume-based seismic attributes from 2D surveys for automated seismic facies mapping within carbonate settings. The study area is located in the central Persian Gulf, offshore Iran. The interval of interest is the mid-Cretaceous Sarvak Formation, which is part of the extensive Cretaceous shallow-water carbonate platform of the eastern Arabian Plate. A set of nine volume-based seismic attributes, calculated from time, amplitude and frequency information of post-stacked 2D seismic data, was chosen to characterize geological information within the interval of interest. The volume-based attributes were supplemented by two grid-based attributes to highlight structural elements. The geological significance of each attribute was evaluated by comparing it with results of seismic sedimentological/geomorphological studies. Furthermore, statistical methods were applied to highlight direct relationships amongst the attributes. The results of these tests were then used to choose a limited set of attributes for neural network-based multi-attribute classifications. The results show that seismic attributes derived from 2D surveys can be used to map basic seismic facies types in carbonate settings and that the outlined, general approach might be applied in other studies.