Observations on the diagenesis of the Lower Triassic Sudair Formation,Abu Dhabi,United Arab Emirates

Summary The Lower Triassic Sudair Formation in the United Arab Emirates (U.A.E.) ranges in thickness from 178–297 m and comprises three units consisting of interbedded limestone, argillaceous limestone, dolomite and anhydrite. The Lower Unit contains variable energy shallow marine, slightly argillaceous mudstones and subordinate ooliticpeloidal packstones and grainstones with minor dolomite and anhdrite. The Middle Unit consists of argillaceous and ferroan dolomite deposited in a lagoonal to supratidal setting. The Upper Unit comprises argillaceous mudstones and dolomites at the base grading upward into argillaceous anhydrite deposited in a restricted shallow marine to sabkha setting. These units represent the transition from a carbonate/evaporite shelf with significant terrestrial input to an evaporitic platform defined by an overall shallowing-upward sequence. Diagenesis in the Sudair includes extensive leaching of grain-supported carbonates, partial to complete dolomitization, evaportie formation, clay nucleation, fracturing/pressure solution, late cementation by coarse calcite spar and saddle dolomite, and hematite formation. These processes have had the cumulative effect of reducing the secondary porosity. Dolomitization occurred in two stages: an earlier progression of rhombic-sucrosic-aphanocrystalline dolomite, and a later coarse crystalline and saddle dolomite fracture fill.     

Sedimentation on Rasdhoo and Ari Atolls, Maldives, Indian Ocean

A first systematic study of composition, texture, and distribution of modern sediments in two Maldivian atolls reveals the predominance of skeletal carbonates. Fragments of corals, calcareous algae, mollusks, benthic foraminifera, and echinoderms are identified in the grain-size fraction >125 μm. Non-skeletal grains such as cemented fecal pellets and aggregate grains only occur in small percentages. Fragments of skeletal grains, aragonite needles, and nanograins (<1 μm) are found in the grain-size fraction <125 μm. Needles and nanograins are interpreted to be largely of skeletal origin. Five sedimentary facies are distinguished (1–5), for which the Dunham-classification is applied. Fore reef, reef, back reef, as well as lagoonal patch reef and faro areas in both atolls are characterized by the occurrence of coral grainstones (1), which also contain fragments of red coralline algae, the codiacean alga Halimeda, and mollusks. On reef islands, coral-rich sediment is cemented to form intertidal beachrock and supratidal cayrock. Skeletal grains in atoll-interior lagoons are mainly mollusks and foraminifera. The lagoon of Rasdhoo Atoll is covered in the west by mudstones (2), in the center by mollusk packstones (3) and mollusk wackestones (4), and by hard bottoms with corals in the east adjacent to channels through the atoll reef margin. The interior lagoon of Ari Atoll contains mollusk wackestones (4) in the center and mollusk-foraminifer packstones (5). Marginal lagoon areas are characterized by hard bottoms with corals. Facies distribution appears to be an expression of depositional energy, which decreases from the atoll margin towards the center in Ari Atoll, and towards the west in Rasdhoo Atoll. Predominant sediment mineralogies include aragonite and high-magnesium calcite. Mean aragonite content decreases from 90% in coral grainstone to 70–80% in mollusk packstone, mollusk wackestone, and mudstone, and to 50% in mollusk-foraminifer packstone. Stable isotopes of oxygen and carbon in bulk samples range from −3 to −1.5 (δ¹⁸O) and from +0.4 to +3.2 (δ¹³C). It is not possible to delineate facies based on O- and C-isotopes.     

Lofer cyclothems revisited (Late triassic, Northern Alps, Austria)

Summary Fischer's classic study (1964) of the Upper Triassic “Lofer” cyclothems in the Dachstein Limestone of the Northern Calcareous Alps was seminal to many studies of the Dachstein and to carbonate cycles globally.Fischer's idealized cycle is deepening upward, ABC in his terminology, where member A is a surface or interval of subaerial exposure, B is tidal deposits, and C is shallow subtidal. Studies of the Dachstein from the elsewhere in the northern Alps have substantiatedFischer's upward-deepening ABC cycle, butGoldhammer et al. (1990) andSatterley (1996a) reinterpreted the type Lofer cycles as shoaling upward. We measured 139 m of Dachstein Limestone incorporating 25 cycles at Steinernes Meer, Austria, nearFischer's most extensive section. In this section we identified no A members. The section is punctuated by slightly reddish horizons (‘pink partings’) that in some cases may reflect brief subaerial exposure, but generally appear to be pressure-solution zones that have concentrated iron oxides but lack a distinctive isotopic signature. B members are readily distinguished by fenestral porosity, stromatolitic lamination, partial dolomitization, intraclasts, or desciccation cracks. They are relatively thin (5 to 155 cm; median thickness is 38 cm.) and in some cases laterally variable or discontinuous. C members are characterized by molluscan wackestones and packstones with diverse biota. C intervals are 25 cm to 26 m thick (median 4.1 m) and comprise 91% of the interval measured. Pervasive bright-red internal sediment, which appears commonly within the B and C members, does not derive from any interval observed within the measured section, but from sources, possibly paleosols, much higher in the section. It is spatially associated with near-vertical, ENE-trending (62^o) fractures filled with the sediment, brachiopods, and cement. Such fractures cut stratigraphic intervals as thick as 70 m without an exposed top or base. If “pink partings” were accepted as indicative of subaerial exposure, three cyclothems (12%) would correspond toFischer's ideal upward-deepening cyclothem, seven cyclothems (29%) are shoaling-upward, four (17%) are symmetrical and the remaining 10 (42%) are incomplete with both deepening and shoaling components. If subaerial disconformities are absent, the intervals are better described as BCBC rhythms than as true cycles. Our study is intended to stimulate new discussion of patterns and origin of the Lofer cycles.     

Facies analysis and sequence stratigraphy of an Upper Jurassic carbonate ramp in the Eastern Alborz range and Binalud Mountains, NE Iran

Upper Jurassic (Oxfordian-Kimmeridgian-Tithonian?) strata of NE Iran (Lar Formation) are composed of medium- to thick-bedded, mostly grainy limestones with various skeletal (bivalves, foraminifera, algae, corals, echinoderms, brachiopods, and radiolaria) and nonskeletal (peloids, ooids, intraclasts, and oncoids) components. Facies analysis documents low- to high-energy environments, including tidal-flat, lagoonal, barrier, and open-marine facies. Because of the wide lateral distribution of facies and the apparent absence of distinct paleobathymetric changes, the depositional system likely represents a westward-deepening homoclinal ramp. Four third-order depositional sequences can be distinguished in each of five stratigraphic measured sections. Transgressive system tracts (TST) show deepening-upward trends, in which shallow-water (tidal flat and lagoonal) facies are overlain by deeper-water (barrier and open-marine) facies. Highstand systems tracts (HST) show shallowing-upward trends in which deep-water facies are overlain by shallow-water facies. All sequence boundaries in the study area (except at the top of the stratigraphic column) are of the nonerosional (SB2) type. Correlation of depositional sequences in the studied sections show that relatively shallow marine (tidal-flat, lagoonal, barrier, and shallow open-marine) conditions dominated in the area. These alternated with deep-water open-marine wackestone and mudstones representing zones of maximum flooding (MFZ).     

Sedimentary facies analysis of the subsurface triassic and hydrocarbon potential in the United Arab Emirates

Summary The Triassic sediments in the subsurface of the United Arab Emirates has been divided into three formations (from bottom to top): Sudair, Jilh (Gulailah) and Minjur. The Sudair Formation consists of four lithofacies units composed mainly of limestones and minor dolomites interbedded with terrigenous shaley mudstones and anhydritic dolomitic limestones. These were deposited in shallow marine supratidal to subtidal settings. The Jilh (Gulailah) Formation has five lithofacies units dominated by anhydritic dolomitic limestone, fine terrigenoclastic sediments and bioclastic and intraclastic limestones. The formation was laid down under lagoonal to supratidal sabkha conditions with little normal marine influence. The Minjur Formation is composed of three lithofacies units characterized by argillaceous quartzitic sandstones, shales, mudstones, dolomitic and ferruginous limestones with thin coal seams. These facies represent deposition in prograding delta lobes, reflecting humid continental to marginal-marine conditions. Diagenesis plays a major role in the reservoir development in the Triassic sediments, the pores are occluded by dolomite and anhydrite. The grains are compacted, leached or cemented by marine cements. Porosity generally ranges from fair to poor with values from 6% to 9% in the carbonates and from 6% to 15% in the clastics. Interparticle and vuggy porosities are the main pore types. The porosity was controlled by diagenesis, depth of burial and lithology. No oil has been discovered so far in the Triassic sediments of the United Arab Emirates but pronounced gas shows have been reported from offshore fields. Western offshore United Arab Emirates is a promising area for potential hydrocarbon accumulations. The Triassic sediments have low to moderate source rock potential; the organic matter is mainly sapropelic kerogen, and the degree of thermal alteration ranges between mature to highly mature stages.     

A sequence stratigraphic approach to a Middle Triassic shelf-slope complex of the Ligurian Alps (Ligurian Briançonnais,Monte Carmo-Rialto unit,Italy)

The Mesozoic sedimentary cover belonging to the Monte Carmo-Rialto unit of the Ligurian Briançonnais domain is composed of Scythian clastics and Anisian to Carnian carbonate rocks over 300 m thick. This paper focuses on the stratigraphy of this carbonate complex, its environmental significance, and its evolution in light of dynamic stratigraphy. Our facies analysis of limestones and dolomites of the Triassic complex allowed us to reconstruct an environmental model. Data support a distally steepened carbonate ramp of Anisian age evolving to a more diversified Ladinian platform with an oolitic sand-bar belt separating the lagoon from the slope. The Monte Carmo-Rialto slope facies are the only witnesses of deep sedimentation in the Triassic terrains of the Ligurian Briançonnais domain, otherwise represented by shallow-water carbonate deposits. On the basis of facies succession, we have identified nine medium-scale cycles (third-order sequences) in the study area, comparable to those evidenced in the Briançonnais s.s. domain by the French authors. Small-scale cycles analysis evidenced mainly shallowing-upward trends in the examined sequences; although a few evidences of transgression-related deposits (deepening upward cycles) have been found at the base three sequences, they have been mostly obliterated by dolomitization and masked by local tectonics. For this reason, we can undoubtedly distinguish only the part of each sequence belonging to HST, while the TST, though present, still remains a partition that cannot be precisely characterized. In the same way, LSTs are not present in the Monte Carmo-Rialto unit, due to the original relative landward position of the examined area. Sequence stratigraphy analysis indicates different long-term dynamics for the two evolutionary stages of the Triassic Ligurian platform: a general landward backstepping to moderate progradation during the Early Anisian and true progradation during the latest Anisian and Ladinian. In addition, a good fit with the sequences proposed by the SEPM chart has been found, indicating a correspondence for the third-order sequences of the Middle Triassic.     

Facies variability in Lower Liassic carbonate successions of the Western Dinarides (Croatia)

Summary In the Western Dinarides the Lower Liassic carbonates are underlain by Upper Triassic “Hauptdolomit”, whereas the first appearance of the foraminiferOrbitopsella praecursor (Gümbel) marks the beginning of the Middle Liassic. Their composition, observed at several localities in Western Croatia, shows a correlation of sedimentation events, which took place during Early Liassic on the Adriatic-Dinaridic carbonate platform. Facies variability is interpreted as result of autocyclic sedimentary processes on which the carbonate platform reacted by periodical oscillations of sea-bottom near the fair-weather wavebase. As a consequence, the Lower Liassic carbonate successions in the Dinarides is characterized by stacking of two main types of coarsening-upward parasequences: (1) the basal part of the Lower Liassic succession is represented by parasequences composed of mudstones or pelletal-bioclastic wackestones as their lower members, and peloidal-bioclastic wackestone/packstones to grain-stones as their upper members; and (2) the upper part of the Lower Liassic succession with parasequences consisting of mudstones or pelletal-bioclastic wackestones overlain by ooid grainstones. Judging from the composition of parasequences and thickness relations of their members, the first type is interpreted to comprise late transgressive system tract (ITST) and/or early highstand system tract (eHST), while the second type corresponds to a late highstand system tract (1HST) and/or early lowstand system tract (eLST) of a third-order sequence.     

Anthracoporella mounds in the Late Carboniferous Auernig Group,Carnic Alps (Austria)

Summary A heretofore undocumented example of skeletal mounds formed by the dasycladacean algaAnthracoporella spectabilis is described from mixed carbonate-clastic cycles (Auernig cyclothems) of the Late Carboniferous (Gzhelian) Auernig Group of the central Carnic Alps in southern Austria. The massive mound facies forms biostromal reef mounds that are up to several m thick and extend laterally over more than 100 m. The mound facies is developed in the middle of bedded limestones, which are up to 16 m thick. These limestones formed during relative sea-level highstands when clastic influx was near zero. The mound facies is characterized by well developed baffler and binder guilds and does not show any horizontal or vertical zonation. Within the massive mound faciesAnthracoporella is frequently found in growth position forming bafflestones and wackestones composed of abundantAnthracoporella skeletons which toppled in situ or drifted slightly.Anthracoporella grew in such profusion that it dominated the available sea bottom living space, forming ‘algal meadows’ which acted as efficient sediment producers and bafflers. BecauseAnthracoporella could not provide a substantial reef framework, and could not withstand high water turbulence, the biostromal skeletal mounds accumulated in shallow, quiet water below the active wave base in water depths less than 30 m. The massive mound facies is under- and overlain by, and laterally grades into bedded, fossiliferous limestones of the intermound facies, composed mainly of different types of wackestones and packstones. Individual beds containAnthracoporella andArchaeolithophyllum missouriense in growth position, forming “micromounds’. Two stages of mound formation are recognized: (1) the stabilization stage when bioclastic wackestones accumulated, and (2) the skeletal mound stage when the sea-bottom was colonized byAnthracoporella and other members of the baffler and binder guilds, formingAnthracoporella bafflestones and wackestones of the mound facies. A slight drop in sea-level led to the termination of the mound growth and accumulation of organic debris, particularly calcareous algae, fusulinids, crinoids and bryozoans, forming well bedded limestones, which overlie the mound facies     

Continental, brackish and marine carbonates from the Lower Cretaceous of Kolone–Barbariga (Istria, Croatia): stratigraphy, sedimentology and geochemistry

During the Early Cretaceous, wide areas of the Dinaric–Adriatic Carbonate Platform emerged for long periods. The Hauterivian–Barremian carbonates from Kolone–Barbariga show a few typical examples of lacustrine facies with dinosaur bones and brackish/palustrine facies. The sequence of the platform is made for the most part by subtidal and intertidal limestones. The bone levels are located in a large depression few meters deep in the uppermost Hauterivian marine limestones. The filling facies of this depression are made by oncolitic rudstones and algal boundstones, which represent marginal lacustrine facies, and by laminated limestones, thin stromatolitic levels and distal fringes of rudstones which represent relatively open lacustrine facies. The fossil content is characterized by rare charophyte stems, ostracods, gastropods and plant remains, while typical marine fauna is absent. At the Hauterivian–Barremian boundary a major emersion event has been observed, then a slow transgressive phase occurred. The transgressive facies are primarily made by mudstones with ostracods, charophytes and Spirillina (brackish and probably freshwater facies), wackestones with Ophtalmidiidae and rare dasyclad algae, storm layers with gastropods and miliolids and breccia-like dinoturbated beds. Wackstones, packstones and very rich in dasyclad grainstones outcrop at the top of the section, representing the maximum of the transgression. Trace elements content, carbon and oxygen stable isotope analyses have been performed to aid the palaeoenvironmental interpretation. In this geological setting, Barium seems to discriminate between brackish and freshwater facies. The isotopic values of the marine carbonates appear to depend on early diagenetic processes, meanwhile lacustrine facies seem to show a weak signal of the depositional environment.     

Sequence stratigraphy and carbonate platform organization of the Devonian Santa Lucia Formation, Cantabrian Mountains, NW-Spain

In this paper, the sedimentology and the stratigraphic architecture of the Devonian Santa Lucia Formation in the Cantabrian Mountains of NW-Spain are described. The Santa Lucia Formation consists of 11 different facies that can be attributed to peritidal/lagoonal, intertidal and subtidal facies associations. These facies associations are arranged in small-scale sedimentary cycles. Three different settings of small-scale sedimentary cycles are recognized: intertidal/supratidal, shallow subtidal/intertidal and subtidal cycles. These cycles reflect spatial differences in the reaction of the depositional system to small-scale relative sea-level changes. Small-scale stratigraphic cycles are stacked into seven medium-scale cycles that in turn are integral parts of three larger-scale cycles. Most of the Santa Lucia Formation (sequences 2–6) forms one major large-scale cycle, whereas sequences 1 and 7 are part of an underlying and an overlying cycle, respectively. Eustatic sea-level changes exerted major control on the formation of these large-scale sequences, whereas the medium-scale cycles seem to be co-controlled by regional tectonism and eustasy. Small-scale cycles seem to be the product of high frequency, eustatic sea-level changes. During the deposition of the Santa Lucia Formation, the morphology of the carbonate platform changed from a gently south-dipping ramp to a rimmed shelf and back to a gently dipping ramp.     

Facies arrangement and cyclostratigraphic architecture of a shallow-marine,warm-water carbonate platform: the Late Carboniferous Ny Friesland Platform in eastern Spitsbergen (Pyefjellet Beds,Wordiekammen Formation,Gipsdalen Group)

During the Late Carboniferous, a spacious warm-water carbonate platform developed across the eastern part of the present Arctic archipelago of Svalbard. The platform initiated in the Moscovian on an uplifted fault block (Ny Friesland High) and progradated during the Late Moscovian to Early Kasimovian into the adjacent Campbellryggen Basin (central Spitsbergen). The fossiliferous platform strata are characterized by a pronounced cyclicity formed by stacked parasequences, which consist of defined, subtidal to supratidal facies-set successions reflecting a general shallowing of the depositional area. Up to 17 of these shallowing-upward cycles, bounded by distinct discontinuity (marine flooding) surfaces due to the recurrent emersion and subsequent flooding of the platform surface, have been recognized within the platform strata. The stacked cycles are the result of global, glacio-eustatic, high-frequent and high-amplitudinal sea-level fluctuations with eccentricity periodicities caused by ice volume changes during the Gondwana Land glaciation. Based on systematic changes of the cycles (thickness and internal facies composition), the upper part of the platform strata is interpreted as a progradational parasequence set of a late highstand system tract. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Sedimentology,magnetic susceptibility and isotopes of a Middle Frasnian carbonate platform: Tailfer section,Belgium

Summary  The Tailfer section (Belgium, northern border of the Dinant Synclinorium) exposes Middle Frasnian shallow-water limestones. This paper proposes a sedimentological analysis based on classical petrology, which leads to the identification of 19 fourth order sequences, forming three third order system tracts. This sedimentological analyses is complemented by carbon isotopic and magnetic susceptibility (MS) data (based on the relation between MS and lithogenic input). Each system tracts is characterised by a particular evolution of the isotope and MS curve:

Cenomanian rudist-dominated shelf-margin limestones from the panormide carbonate platform (Sicily, Italy): Facies analysis and sequence stratigraphy

Summary Sedimentological, paleontological and sequence analyses of Cenomanian limestones in Sicily reveal the facies architecture and dynamics of a Mid Cretaceous rudistdominated platform margin from Western Tethys. The studied deposits outcrop near Palermo, as part of a large structural unit of the Sicilian Maghrebids. They belong to the Panormide carbonate platform, a Mesocenozoic paleogeographic domain of the African margin. The lateral continuity of the beds along three nearly parallel E-W outcrop sections allowed the recording of cm/dm thick lithological and faunal variations. Nine main lithofacies associations have been recognised along about 200 m of subvertical strata. Their vertical and lateral organisation points to a transition from highenergy shelf-margin rudist patches and shoals to more internal lagoonal-tidal environments over a short distance. The lithofacies evolution and stacking pattern along the three sections made it possible to define elementary cycles, composite cycles and larger-scale sequences with a dominant shallowing-upward trend. Their hierarchical organisation implies that sea-level fluctuations were an important factor in their formation. The cycles are characterised by a great variation in facies as a result of transgressive-regressive events in different sectors of the inferred Cenomanian shelf. Subtidal cycles typical of the shelf margin (4–10 m-thick) are particularly well identifiable. They are made of large Caprinidae and Sauvagesiac rudstone-to-floatstone (about 2/3 of the total thickness), capped by rudist-conglomerates, often organised into 3–5 fining-upward amalgamated beds and showing, in places, effects of surface-related diagenesis. In more internal shelf areas the cycles consist of Caprinidae-Radiolitidae floastone grading up into amalgamated beds of angular bioclastic rudstone/grainstone. Alternations of foraminifer/ostracod mudstone/wackestone and bioclastic grainstone/fine-rudstone, capped by loferites and/or by other emersion-related overprintings, characterise the cycles formed in the peritidal zones. these cycles are stacked into three incomplete depositional sequences. The sequence boundaries have been identified by the abrupt interposition of peritidal cycles in subtidal rudist-rich cycles, with evidence of brief subaerial exposure.     

An integrated analysis (microfacies and ichnology) of a shallow carbonate-platform succession: upper Aptian,Lower Cretaceous,Betic Cordillera

Four lithofacies and 12 microfacies types recognized in an upper Aptian section in the Sierra de Bedmar-Jódar (Prebetic of Jaén) represent shallow lagoonal environments (marl and marly limestone) and sand bars that delimited the lagoon. The lagoonal facies reflect subtidal restricted water circulation with low energy. The sand bar facies (intertidal environment) have upper surfaces that show the effects of supratidal and subaerial conditions. The presence of early fractures in particular lithofacies shows the importance of local synsedimentary tectonics during sedimentation. Thalassinoides, ?Arenicolites, Diplocraterion, Circolites, Gastrochaenolites and Trypanites are recorded in different beds of this section, reflecting various states of substrate consistency, in the form of firmground, hardground, and rockground. Whereas firmground conditions were dominant in the lower part of the section, hardgrounds and rockgrounds are mainly present in the upper part of the section. Four types of shallowing-upward elementary sequence are recognized. All the sequences show at the base mudstone or wackestone microfacies representing a lagoonal environment, overlain by sand-bar grain-pack-stone facies corresponding to a bar bounding the lagoon. The factors that controlled their development were carbonate production and tectonic movements.     

Facies,depositional sequences,and biostratigraphy of the Oligo-Miocene Asmari Formation in Marun oilfield,North Dezful Embayment,Zagros Basin,SW Iran

The Asmari Formation in Marun oilfield (south-west Iran), is about 440 m-thick marine carbonate succession with subordinate siliciclastic rocks, characterized by abundant benthic foraminifera (perforate and imperforate). Foraminiferal biostratigraphy indicates that this unit is Oligocene–Miocene in age. The distribution of benthic foraminifera and other components have led to the recognition of three siliciclastic and ten carbonate facies that were deposited in inner ramp (shoreline, tidal flat, restricted and open lagoon and shoal), middle and outer ramp sub-environments. Based on vertical facies trends, three third-order sequences in the Oligocene and three third-order sequences in the Miocene sediments have been identified. These depositional sequences are bounded by both type 1 and type 2 sequence boundaries. The transgressive systems tracts (TST) of sequences show deepening-upward facies trend with a gradual upward increase in perforate foraminifera, whereas the highstand systems tracts (HST) have a shallowing-upward facies trend and contain predominantly imperforate foraminifera. Deposition of these depositional sequences (DS) were controlled by both eustasy and tectonic subsidence.     

Tide-dominated Middle Devonian sequence from the northern part of the Holy Cross Mountains (Central Poland)

Summary The dolomitic Wojciechowice Formation distinctly differs from the remaining, mainly shaly Middle Devonian succession in northern part of the Holy Cross Mountains (Central Poland). The upper Member of the Formation (Crystalline Dolostone Mb.), in greater part dolomitized but also containing limestone beds, is composed of shallowing-upward cyclothems well exposed in Skaly quarry in the Bodzentyn syncline. The lower parts of the cyclothems, interpreted as subtidal facies, contain fossils characteristic for restricted environments. They are grouped into two assemblages. The first, with brachiopods (largeBornhardtina andEmanuella), massive stromatoporoids, and subordinate gastropods and amphiporoids is related to a deeper subtidal environment, while the second (mainly amphiporoids, gastropods, ostracodes and calcareous algae) is shallower subtidal. Towards the top of succession the fossil content radically decreases. The upper parts of cyclothems are composed mainly of different types of laminites. In these parts of the section, interpreted as intertidal/supratidal units, stromatolites, desiccation polygons, intraformational breccias, and common bioturbations are present. The whole succession was deposited in a low-energy environment, only intermittently affected by high-energy events. For their most spectacular example of this, aBornhardtina-coquinite, a tempestitic origin is proposed. The interval with cyclic sedimentation studied correlates with the dolomitized lower “Unit I” of the Stromatoporoid-Coral Kowala Formation from the southern part of the Holy Cross Mountains, which exhibits sabkha-type cyclicity. The differences in development of cyclothems in both regions resemble outer and inner part of an extensive platform, and correspond well with basic trends of the Lower-Middle Devonian transgression in the Holy Cross Mountains. The general succession of formations deposited during this process coincides with transgressive events on Johnson's eustatic curve for the Devonian.     

Palynofacies and sedimentology-based high-resolution sequence stratigraphy of the lignite-bearing muddy coastal deposits (early Eocene) in the Vastan Lignite Mine, Gulf of Cambay, India

Geological records of early Paleogene warming are rare in low latitudinal regions. The Indian subcontinent preserves records of this global event on western and eastern margins. We attempt to decipher paleoenvironmental setup and facies architecture of the paleo-equatorial early Eocene succession at the Vastan Lignite Mine, Gulf of Cambay, western India. The Vastan lignite succession was deposited in a low-energy coastal marsh-bay complex receiving only fine-grained muddy sediments from the weathered Deccan Traps. The lower part of the Vastan lignite deposit, designated as “Vastan Succession A”, comprises four depositional facies representing distinct environments (open bay, restricted bay, creek and channel, and coastal marsh) and one diagenetic facies. Palynofacies analysis, backed by precise sedimentological framework, records changes in terrestrial supply and fluctuating marine characters of bay and marshes. Eleven Palyno-Units are identified in distinct lithofacies sequences stacked in shallowing-upward cycles representing five parasequences that constitute a Transgressive Systems Tract (TST) deposit. Each parasequence starts with a transgressive sheet deposit, followed by shallowing-upward bay fill-marsh deposits. In the vertical succession, each parasequence acquires increasing marine character, culminating in a maximum flooding surface (shell carbonate) that represents large-scale coastal onlap during early Ypresian time. The TST is followed by a Highstand Systems Tract deposit, which shows an erosional surface at the top of the upper lignite indicating Lowstand Systems Tract and a sequence boundary at ~52 Ma. The Vastan Succession A represents TST (3rd-order cycle) deposits with parasequences and hemicycles representing 4th- and 5th-order cycles. The study demonstrates sea level rise along the Indian western coastal margin in response to early Eocene warming between ~55 and ~52 Ma with maximum transgression at 53.7 Ma.     

Platform environments, microfacies and systems tracts of the upper Cenomanian-lower Santonian of Sinai, Egypt

Summary Factors controlling grain composition and depositional environments of upper Cenomanian—Santonian limestones of Sinai are discussed. The mainly shallow-water, inner-platform setting investigated is subdivided into five major facies belts, each represented by several microfacies types (MFTs). Their lateral distribution patterns and their composition underline aclear relation between depositional environment and platform position. The facies belts include sandstones and quartzose packstones of siliciclastic shorefaces, mudstones and bioclastic wackestones of restricted lagoons, shallow-subtidal packstones with diverse benthic foraminifera and calcareous algae, bioclastic and/or oolitic grainstones of inner-platform shoals, and wackestones of deep open-marine environments. The microfacies distribution patterns of the Cenomanian-Santonian strata are evaluated with respect to local and regional large-scale environmental changes. While protected shallow-subtidal environments with only subordinate ooids and oncoids prevail during the late Cenomanian, high-energy oolithic shoals and carbonate sands occur locally during the middle and late Turonian. They were probably related to a change of the platform morphology and a reorganisation of the platform after a late Cenomanian drowning. In the Coniacian-Santonian, the lack of ooids, oncoids, and the decrease of calcareous algae versus an increase in siliciclastics indicate a shift to lower water temperature and to a more humid climate. Especially in the Turonian, the interplay between sea-level changes, accommodation, hydrodynamics, and siliciclastic input is reflected by lithofacies and biofacies interrelation-ships that are elaborated within individual systems tracts. In particular, increasing accommodation intensified circulation and wave-agitation and controlled the distribution of high-energy environments of the middle and upper Turonian trans-gressive systems tracts. During highstands protected innerplatform environments prevailed.     

Nature and distribution of porosity and permeability in jurassic carbonate reservoirs of the Arabian Gulf basin

Summary The Middle-Upper Jurassic section in the Arabian Gulf basin forms one of the most prolific sequences in the world, in which an excellent combination of source, reservoir and seal rocks was developed within a major sedimentary cycle. The sequence consists of a) relatively quiet deep-water mudstone, wackestone and shale (source facies), b) shallow-water high enery grainstone and packstone (reservoir facies), and c) very shallow supratidal anhydrite (seal facies). The principal factors, which controlled the sedimentation of this sequence, are considered to have been eustatic sea-level change and epeirogenic movement of carbonate shelves. The Jurassic reservoirs of the major oil fields in this region show exceptionally high porosity up to 30% for their relatively old geologic age (some 150 million years old) and depths of burial in the range between 1,200 and more than 2,700 m. Porosity occurs most commonly as intergranular/remnant primary pore spaces, but its distribution is quite uneven and very complicated. To account for the existence of such high porosity (and permeability) in the Jurassic reservoirs, probable geological, physical and chemical factors for preserving and enhancing porosity (and permeability), such as acidic formation fluids, reduced fluid mobility, tectonic forces, ductility of intercalated beds (e.g. anhydrite), and dolomitization were examined. It has been observed in various fields in the region that oilsaturated portions of the Jurassic reservoirs tend to retain higher porosity than the surrounding water-saturated zones. Porosity preservation by hydrocarbons is possible primarily because of excess hydrocarbon pressure and of reduced mobility of water in such oil-saturated zones. To continue sediment diagenesis, a steady supply of minerals by formation water and the mobility of the water may have been essential. Because the entrapment of oil in the Jurassic reservoirs in the region is considered to have been as late as early Tertiary, some other (pre-migration) mechanisms which may have worked in the earlier geologic stages for preserving and creating porosity (and permeability) seem to be necessary.