Mixed Siliciclastic Carbonate Research Articles

Dissolution and its impacts on reservoir formation in moderately to deeply buried strata of mixed siliciclastic–carbonate sediments, northwestern Qaidam Basin, northwest China

Petroleum exploration is increasingly extending from shallowly to moderately to deeply buried strata, with reservoir quality being a key. The formation mechanism of conventional clastic and carbonate reservoirs under such conditions has been widely investigated. However, the mechanism of mixed siliciclastic–carbonate reservoirs has not been well studied. In this paper, we address this issue using a case study in the northwestern Qaidam Basin, northwest China (N1–E1+2 age, 2500–4500 m). Dissolution pores and fractures dominate the reservoirs. However, only the fractures were the focus of previous studies. Thus, here we investigate dissolution and its impacts on reservoir formation, providing a complementary understanding of the reservoir formation in the basin. Using core observations and examining thin section, it was discovered that dissolution was both random and followed bedding. The dissolved components primarily included calcareous and gypsum minerals, with fingerprints that are characteristic of burial dissolution. Further electron probe analysis on authigenic minerals revealed that the dissolution fluid might have originated from acidic formation fluids associated with hydrocarbon generation. The fluids passed through faults and fractures. Dissolution pores were an important component of the reservoir, providing approximately 60% of the porosity. In addition, segments of high porosity generally above 5% are associated with dissolution. Based on these observations, a schematic model was established to explain the impacts of dissolution on reservoir formation. Specifically, organic acidic formation fluids enter tectonic fractures, resulting in dissolution as they pass through them. This dissolution enhances the size of the pore space and the reservoir properties of the rocks, eventually developing a fracture-dissolution reservoir.

Paleogeographic and tectonic evolution of the Pranhita-Godavari Valley, Central India: a Stratigraphic perspective

The Pranhita-Godavari Valley of central India preserves records of repeated opening and closing of Proterozoic and Gondwana rifts along the zone of NW-SE trending Neoarchean suture between the Dharwar and Bastar cratonic nuclei. The Proterozoic succession in the Valley comprises several major unconformity-bound sequences, each with a distinctive set of litho-assemblage, deposited under diverse tectonic environments with highly variable modes and tempos of sedimentation. Analysis of sedimentary attributes of unconformity-bound sequences indicates that Purana sedimentation in the Valley started with the deposition of stable platformal assemblage of carbonate-quartzarenite in an extensional sag basin. The basin opened at c. 1700 Ma. The stability of the craton was disrupted when the first major cratonic rift nucleated at c. 1620 Ma, and shortly evolved into an oceanic basin along the northeastern margin of the Valley with consanguineous deposition of a thick wedge of conglomerates, tidal sandstones and shales in the Mulug shelf, and a succession of graywacke, carbonate, siltstone-mudstone-black shale and tuff turbidites of the Somanpalli flysch in two parallel adjoining belts. The Somanpalli flysch is bounded by two shallow marine successions, analogous to several geosynclinal deposits preserved in orogenic belts, and was deposited in a deep oceanic basin along a rifted continental margin. Paleocurrent analysis reveals that siliciclastics in the flysch succession were derived primarily from a continental provenance to the southwest of the Mulug shelf, and were transported to the flysch basin across the margin. Coarse sands were deposited in large submarine fan complexes of graywackes and mixed carbonate-siliciclastics at several points. The voluminous carbonate turbidites in the flysch were also derived from the adjoining Mulug shelf. High quartz content of graywackes, their close association with tidally deposited arenites, and ocean-ward transport of sediments from cratonic hinterlands collectively attest to the deposition of the flysch in a trailing margin basin. Opening of the oceanic basin and deposition of the flysch were accompanied by extensive granulite facies metamorphism at deep crustal level indicating a genetic linkage between them, and the litho-tectonic elements were deformed into an orogenic belt during re-assembly of Dharwar and Bastar blocks. The orogenic belt girdled the northeastern margin of the PG Valley, wrapped around its southeastern corner making a syntaxial bend, and continued into the Ongole-NKSB orogenic belt which also had its peak metamorphism at c. 1600 Ma. The age of the orogeny is not well constrained, but was followed by a Neoproterozoic episode of extension, and formation of cratonic rift basin with the deposition of several thousand meters of quartz-rich sandstones of the Sullavai, Albaka and Usur sequences.

Trace fossil assemblages in mixed siliciclastic-carbonate sediments of the Kaladongar Formation (Middle Jurassic), Patcham Island, Kachchh, Western India

The Middle Jurassic rocks of the Kaladongar Formation well exposed in the Kaladongar Hill range of the Patcham Island and Kuar Bet of the Northern Kachchh comprises of ∼450 m thick sequence of mixed siliciclasticcarbonate sediments intercalated with shales. These Mixed siliciclastic-carbonate sediments show wide variation in textural and mineralogical composition and represent genetically related six rock types: micritic sandstone, allochemic sandstone, sandy allochemic limestone, micrtic mudrock, sandy micrite and muddy micrite; which are highly bioturbated and show behaviourally diverse groups of trace fossils. Total 34 ichnogenera are identified, which includes, Arenicolites, Asterosoma, Beaconites, Bergaueria, Chondrites, Cochlichnus, Dactylophycus, Daedalus, Didymaulichnus, Diplocraterion, Gordia, Gyrochorte, Gyrolithes, Ichnocumulus, Laevicyclus, Lockeia, Margaritichnus, Monocraterion, Nereites, Ophiomorpha, Palaeophycus, Phoebichnus, Phycodes, Pilichnus, Planolites, Plug Shaped Form, Protovirgularia, Rhizocorallium, Scolicia, Skolithos, Taenidium, Teichichnus, Thalassinoides and Walcottia. These trace fossils are classified into six morphological groups namely, circular and elliptical structures; simple structures; branched structures; rosette structures; spreiten structures; and winding and meandering structures. These trace fossils are further group into eight assemblages which occurred together into mixed siliciclastic-carbonate sediments, include, Asterosoma assemblage, Gyrochorte assemblage, Rhizocorallium assemblage, Thalassinoides assemblage, Planolites-Palaeophycus assemblage, Phycodes assemblage, Ophiomorpha assemblage and Skolithos assemblage. The recurring pattern of these assemblages through the sequence displays the development of Skolithos and Cruziana ichnofacies and at places the mixed Skolithos-Cruziana ichnofacies which suggest a low wave and current energy conditions with intervening period of high wave and current energy conditions and an intermediate period of stressful environments, respectively. Sedimentological and ichnological data suggest that the deposition of the mixed siliciclastic-carbonate sediments of the Kaladongar Formation took place in the foreshore to offshore environment under fluctuating wave and current energy condition.

Seagrass-Meadow Sedimentary Facies In A Mixed Siliciclastic-Carbonate Temperate System In the Tyrrhenian Sea (Pontinian Islands, Western Mediterranean)

Abstract The paper presents the characteristics of surface sediments in a seagrass-dominated environment of the inner shelf of the Pontinian Islands (Tyrrhenian Shelf, Central Mediterranean). The aims of this work are to characterize the carbonate production and sedimentary processes of a representative high-latitude, temperate Posidonia oceanica–dominated area, to discuss the role of seagrass in the siliciclastic–carbonate mixing processes, which provide analogs for interpreting the occurrence and importance of these types of deposits in the rock record. The majority of sediments are poorly sorted siliciclastic skeletal sand. Based on grain type and abundance and carbonate content, these deposits include six subfacies. The different subfacies reflect environmental changes within the seagrass environment associated with variations in light penetration and hydrodynamic conditions. Additionally, the seagrass facies range from purely carbonate to mixed carbonate–siliciclastic, depending on the amount of siliciclastic input and on the capacity of seagrasses to trap allochthonous sediments while hosting carbonate-producing biota. The type of substrate and the erosion and transport processes control the siliciclastic input. In these Pontinian seagrass meadows, sedimentary facies contain abundant siliciclastic grains produced by erosion of the volcanic substrate and trapped by seagrass blades and rhizomes. This Pontinian example illustrates that in heterozoan-dominated seagrass settings, mud-producing green algae are not present or are not calcifying, and consequently, seagrass deposits are much more grainy than photozoan counterparts. Ancient heterozoan tropical seagrass deposits likely will be discriminated from their temperate or subtropical counterparts only by careful taxonomic analysis of skeletal grains.

Climatic controls on sedimentary environments in the Triassic of the Transdanubian Range (Western Hungary)

During the Triassic the Transdanubian Range was a segment of the western Neotethys margin where the climate was mostly determined by a strong global monsoonal system. As a consequence of the global climatic conditions and its geographic setting, warm to hot temperature prevailed in this area. However, there were significant changes in the precipitation pattern from dry to wet conditions in connection with the intensity of the monsoon system and the actual setting of this area within the monsoon system. The aim of our research was to interpret the palaeoclimatic conditions and their changes for the Triassic succession of the Transdanubian Range on the base of geochemical, mineralogical, sedimentological and palaeontological data, with special regard to the climate indicator facies (e.g. different types of dolomites; clay mineral assemblages). Nine evolutionary stages were distinguished. During the Early Triassic mixed siliciclastic–carbonate ramp sedimentation took place under arid climate that was interrupted by a humid pulse (Campil Pluvial Event). In the early Anisian a shallow carbonate ramp was developed under arid to semi-arid climate. The predominatly dry conditions prolonged during the middle Anisian,. The latest Anisian and the Ladinian carbonate platforms were developed under mostly semi-arid climate. The semi-humid climate of the earliest Carnian was followed by a definitely wet interval (Carnian Pluvial Event) that was characterised by an intense terrigenous input into the Julian intraplatform basins. Arid and humid conditions alternated during the final stage of basin upfilling from the latest Julian to early Tuvalian. Then a marked climatic change took place in the latest Carnian when the humid climate was changed to semi-arid conditions. From the middle to late Norian a gradual increase of humidity may have taken place that resulted in a decreasing intensity of the early dolomitisation of the carbonate sediments of the broad internal platform. At the very end of the Norian there was a marked climate change leading to enhanced humidity but with a definite seasonality. Humid climatic conditions prevailed during the Rhaetian (Kössen Event).Evaluation of the similarities and differences of the palaeoclimatic conditions for the Transdanubian Range and some selected areas in the western Neotethys (Dolomites, Northern Calcareous Alps, Mecsek), and the Peri-Tethyan Germanic Basin is also presented.

Mixed Siliciclastic-Carbonate Upward-Deepening Cycles of the Upper Cambrian Inner Detrital Belt of Laurentia

Abstract High-resolution sedimentological and biostratigraphic data recently recovered from Upper Cambrian strata in the northern Rocky Mountain and central Appalachian regions reveal that meter-scale cycles of very different character developed synchronously during deposition of the basal subzones of the Ibexian Series in both the carbonate facies belt and the distal part of the inner detrital, mixed siliciclastic–carbonate facies belt of the Laurentian paleocontinent. A typical mixed siliciclastic–carbonate cycle in the Snowy Range Formation of Wyoming and Montana consists of the following lithofacies in ascending order: shale, shale with very thin grainstone interbeds, grainstone with subordinate thin shale interbeds, flat-pebble conglomerate, and (in some cycles) thrombolitic bioherms. Internal transitions between the constituent lithofacies are mostly gradational, but cycle boundaries are sharp with shale directly overlying flat-pebble conglomerate and/or thrombolitic boundstone. We postulate that these mixed siliciclastic–carbonate cycles developed in response to different depositional dynamics than meter-scale cycles developed in carbonate-belt facies. Specifically, cycles in the carbonate belt were produced largely by temporal variation in accommodation space, and associated filling of that space through progradation of the carbonate system and aggradation to sea level. In contrast, cyclic deposition within the inner detrital belt was controlled by variations in terrigenous sediment input and resulting effects on carbonate productivity. In this case, enhanced delivery of siliciclastic mud by rivers was triggered by regression and/or a shift to more humid climatic conditions. An upward increase of carbonate in the main part of the cycles reflects enhanced carbonate production in response to the reduction of terrigenous clay supply during shoreline retreat. Widespread cementation of fine grainstone substrates led to deposition of extensive beds of flat-pebble conglomerate, possibly in association with elevated sea-surface temperatures and enhanced storm intensity. Thrombolites formed at times of maximum transgression as microbial communities thrived in the absence of turbidity, colonizing the irregular surfaces of flat-pebble conglomerate beds. We thus interpret these wholly subtidal cycles to record full changes in paleobathymetry with the bulk of the cycles recording upward deepening, rather than shoaling associated with regression. Comparison with age-equivalent shoaling cycles of the Conococheague Formation of the carbonate belt of the Appalachian Mountains highlights the different depositional dynamics in the mixed siliciclastic–carbonate systems, and underscores the peril in extrapolation of depositional models derived from carbonate systems to mixed systems, regardless of age or paleogeographic setting.

Miocene scleractinian corals from a mixed siliciclastic–carbonate system: Bakhtiari succession, Zagros Basin (central-western Iran)

Colonial scleractinian corals were sampled from three levels within a Miocene marine unit of the Bakhtiari succession, Zagros Basin, central-western Iran. The first two coral-bearing intervals, A and B, contain small-scale scattered colonies and show a poor coral diversity, whereas the third, consisting of a strongly lithified limestone package, reflects a well-developed biostromal framework with higher coral skeletal volume within the Bakhtiari succession. The Bakhtiari succession coral assemblages are characterized by Porites sp. cf. P. maigensis, Porites sp. cf. P. mancietensis, Porites sp. cf. P. collegniana, Tarbellastraea reussiana, Favia sp., Montastrea sp. cf. P. tchihatcheffi, Favites sp. cf. P. neugeboreni, Favites sp. cf. P. neuvillei, Agathiphyllia sp. and Acropora sp. Sedimentological and palaeontological data indicate that the depositional environment is consistent with a mixed carbonate–siliciclastic ramp that was gently deepening basinwards from the shoreline. The hemispherical and massive growth forms of colonies and sparse branching forms dominated the well-illuminated euphotic zone. Abundant domestone and dense pillarstone coral growth fabrics interdigitating with coarse-grained terrigenous sediments developed in the shallow inner ramp environment. Branching forms and meandroid branching colonies together with some massive forms mostly inhabited the low-energy conditions of the lower euphotic to oligophotic zones of the middle ramp. In the middle parts of the mixed carbonate–siliciclastic ramp, sparse pillarstone together with domestone comprises a mixstone coral growth fabric. Fluctuations in nutrient and clastic sediment input, salinity and the growth of red algae likely terminated coral growth.

The Madotz Urgonian platform (Aralar, northern Spain): Paleoecological changes in response to Early Aptian global environmental events

Sudden addition of carbon dioxide to the atmosphere can reduce the CaCO3 saturation and weaken the biocalcification potential of marine organisms in shallow water and in open marine settings. In this study, the response of an Aptian neritic carbonate environment to sudden addition of carbon dioxide at the beginning of Oceanic Anoxic Event 1a is investigated. The beginning of the OAE1a was coupled with a major perturbation on the carbon cycle as indicated by a negative carbon isotope excursion in the sedimentary record. This isotope anomaly is regarded as a proxy for massive addition of volcanic or methane-derived CO2 to the atmosphere within only a few 104years. The impact of a rapid change in atmospheric pCO2 on biocalcifiers in low latitude shallow-water settings can be studied in a well preserved Aptian carbonate shelf succession cropping out today in the Aralar mountains (NE Spain). The Madotz section (N Spain) preserves a continuous shallow water record that was deposited on a mid-latitude, Atlantic-oriented mixed siliciclastic–carbonate ramp. Lower Aptian sediments consist of two neritic limestone successions separated by orbitolinid-rich marlstone enriched in organic matter. The lower neritic limestone succession ends with a submarine hardground and the transition from the lower neritic limestone to the orbitolinid marlstone coincides with a negative spike in the organic carbon isotope record. This negative spike can be correlated with the negative carbon isotope anomaly marking the base of OAE1a. The paleoecological change coinciding with the base of OAE1a occurred at a time of sea level rise and it coincided with a demise of heavily calcified nannoconids in the Tethys and Pacific Oceans. The paleoecological change observed in the Madotz section corresponds to a comparable change seen in the more distal and more expanded carbonate ramp section (Igaratza) at the Aralar Mountains. Ocean acidification caused by sudden increase in pCO2 may explain reduced calcification potential of some shallow water calcifiers. Calcification crisis was amplified by rising sea level, increasing temperatures and increased flux of detrital material and nutrients from continents into coastal seas.

Palaeoenvironmental and palaeoecological change on the northern Tethyan carbonate platform during the Late Barremian to earliest Aptian

AbstractA major shift from Urgonian oligotrophic carbonate accumulation to orbitolinid‐rich mixed siliciclastic–carbonate deposition is observed near the Barremian–Aptian boundary in many sections both within and outside the shallow‐marine Tethyan Realm. This important facies change in the Swiss Helvetic Alps is documented here and interpreted in the context of general palaeoenvironmental change. To achieve this, a detailed micropalaeontological, sedimentological, mineralogical and geochemical study has been carried out on six sections across the upper part of the lower Schrattenkalk Member (Late Barremian), the Rawil Member (formerly ‘Lower Orbitolina Beds’, earliest Aptian) and the lowermost part of the upper Schrattenkalk Member (Early Aptian). The sediments of the Rawil Member exhibit inner‐platform facies with rudists, miliolids, orbitolinids and dasycladals to outer‐platform facies characterized by small benthic foraminifera, orbitolinids, crinoids and bryozoans. Stratigraphic trends in microfacies environments and the composition of microfossil assemblages, indicate that the Rawil Member includes a transgressive systems tract and the base of a highstand systems tract which are composed of an increasing number of parasequences in distal directions (five to nine in the sections studied here). The sea‐level rise discerned in the Rawil Member is coeval with increased detrital input and phosphorus burial, with maximum values up to 80 times and 21 times the background values in the subjacent part of the lower Schrattenkalk Member, respectively. Furthermore, the Rawil Member records the appearance of kaolinite, indicating a change towards tropical and more humid climate conditions. This change may have led to an increase in continental weathering rates and an associated increase in detrital and nutrient fluxes towards the ocean. The phase of climate change observed near the Barremian–Aptian boundary may have been triggered by a phase of intensified volcanic activity linked with the onset of the Ontong Java large igneous province and the Rawil Member may be the expression of a precursor episode to Oceanic Anoxic Event 1a in the shallow‐marine environment.

Petrography and geochemistry of the magnesites and dolostones of the Ediacaran Ibor Group (635 to 542 Ma), Western Spain: Evidences of their hydrothermal origin

The Ediacaran deposits (between 635 and 542 Ma) of the Central Zone of the Iberian Massif consist of alternating siliciclastic and carbonate beds. These carbonates are dolostones and magnesites which are interpreted to have been formed by the replacement of primary peritidal limestones. Through petrographic and geochemical analyses, we recognize different types of dolomites (D1 to D4) and magnesites (M1 and M2). Despite distinct petrographic features of the four types of dolomite, their oxygen and carbon isotopes overlap with δ 18O values ranging from + 15.45 to + 17.51‰ (SMOW) and δ 13C from − 0.13 to 3.21‰ PDB. Sr isotope values for D1 and D2 range from 0.7028 to 0.7091. Magnesites (M1 and M2) show oxygen values higher than + 17.87.0‰, and δ 13C values show the same variability as for the dolomites. D3 and D4 oxygen isotope values are between + 18.91 and 19.61, and the carbon isotope values range are similar to the other diagenetic phases. Sr isotope values for the magnesites and late dolomites (D3 and D4) are 0.7095 to 0.7104, being higher than those of the D1 and D2 dolomites. D1 is a relatively early dolomite phase formed by the replacement of fine grained peritidal limestones. The coarser crystal size of D2, which shows similar geochemical features as D1, suggests formation by dolomitization of coarser grained limestones. The replacement of D1 and D2 by M1 and M2 advanced along stylolites, fractures and bedding planes. This replacement is interpreted to have occurred by hydrothermal fluids, which is suggested by the presence of talc and forsterite. D3, a coarse dolomite, completely destroyed any previous texture and D4 (dolomite cement) post-dates magnesite formation. Interactions of hydrothermal fluids with the prior carbonates reset the oxygen isotopes of the earlier dolomite. The study of these magnesites and related dolostones may offer new insights into the model of formation of sparry magnesites hosted by mixed siliciclastic–carbonate platform deposits. The establishment of the factors and mechanism that control the diagenetic evolution of these carbonates has a great importance in order to understand and predicts porosity and permeability variations of rocks formed under similar geological conditions.

Chapter 62 The Port Askaig Formation, Dalradian Supergroup, Scotland

Abstract The Port Askaig Formation (Fm.) is a thick glaciogenic succession within the Dalradian Supergroup that consists of over 700 m of variably dolomitic diamictite, conglomerate, sandstone mudstone and minor dolomite, and is bounded by mixed siliciclastic–carbonate successions of the Islay (Lossit) and Bonahaven formations. These strata are exposed in the metamorphic Caledonides of Scotland, although excellent preservation of sedimentary structures can be found at several sites. An extensional setting for this succession has been proposed based on stratigraphic and structural arguments. The available chemostratigraphic data include the Chemical Index of Alteration, δ 13 C and Sr-isotope values. Palaeomagnetic analyses have been shown to be subject to post-depositional Caledonian overprinting. There is also continued debate over the regional palaeogeographical reconstructions of the Scottish promontory for this time period. The succession is chronologically poorly constrained with U–Pb analyses of stratigraphically much higher or lower deposits. The thick succession is thought to record glacially influenced marine sedimentation and reworking of unstable sediments in a tectonically active setting with evidence of ice-margin fluctuations. Alternative palaeoenvironmental interpretations that focus on glacial terrestrial processes and emphasize climatic influence instead of tectonic activity have also been proposed. The overlying carbonate is a lithologically diverse coastal complex and so does not fit the Neoproterozoic norm. Research has to date focused on the stratigraphic and sedimentological aspects of this succession, as well as some of the broader palaeogeographical and structural features of the Dalradian basin. Future efforts should focus on the chronological, structural and palaeogeographical constraints of this succession.

Organic matter and palaeoenvironmental signals during the Early Triassic biotic recovery: The Salt Range and Surghar Range records

Latest Permian to the Middle Triassic mixed siliciclastic–carbonate shelf deposits of the northern Gondwana margin have been studied in four sections (Nammal, Chhidru, Chitta–Landu, and Narmia) in the Salt Range and Surghar Range of Pakistan. Sedimentological and palynofacies patterns combined with a high resolution ammonoid based age control have been used to assess environmental changes such as sea-level change, distance from the shore, and oxygenation conditions of the sections in the aftermath of the end-Permian mass extinction. The base and the top of the Early Triassic are marked by second order sequence boundaries (SRT1, SRT8). Within the Early Triassic two third order sequence boundaries could be delineated by means of palynofacies analysis and sedimentology, one near the Dienerian–Smithian (SRT2) and the second one near the Smithian–Spathian boundary (SRT5). The extinction event at the Smithian–Spathian boundary seems to be closely associated to the latter globally recorded sea-level low stand. Five additional sequences of undetermined order (SRT3, SRT 4, SRT5/1, SRT6, and SRT7) are reflected in the sedimentological record of the studied sections. The observed changes in the composition of the particulate organic matter (POM) indicate a general shallowing upward trend, which is modulated by smaller transgressive–regressive cycles supporting the sedimentologically defined sequences. The POM is mostly dominated by terrestrial phytoclasts and sporomorphs. The strongest marine signal is reflected by increased abundance of amorphous organic matter (AOM) in the lower part of the Ceratite Marls at Nammal (late Dienerian) and Chhidru (earliest Smithian) and the Lower Ceratite Limestone at Chitta–Landu (late Dienerian). AOM of marine origin is characteristic for deeper, distal basinal settings and is preferentially preserved under dysoxic and anoxic conditions, indicating reduced oxygen conditions during these intervals. Up-section transgressive events are reflected by increased numbers of acritarchs, reaching up to 50% of the POM. Well oxygenated conditions and low total organic carbon contents (TOC) continue up to the top of the Early Triassic (Mianwali Formation). The most pronounced terrestrial influx is expressed in the Middle Triassic. Organic carbon isotope data parallel the carbonate carbon isotope records from the Tethyan realm; therefore, they reflect real global changes in the carbon cycle independent of the OM composition. The biomarker study of the apolar hydrocarbons of three samples from the Nammal section indicates an enhanced bacterial productivity, especially in the Smithian and Spathian, reflected in high relative abundances of hopanes. POM, TOC data and redox sensitive biomarkers together with high resolution biostratigraphy demonstrate that well-oxygenated environmental conditions prevailed in the Early Triassic with the exception of the Dienerian to earliest Smithian interval. The POM assemblages of Late Permian to late Griesbachian age indicate well oxygenated conditions during this time interval. There is no evidence in support of an anoxic event in the late Griesbachian in these sections.

Tectonically driven carbonate ramp evolution at the southern Tethyan shelf: the Lower Eocene succession of the Galala Mountains, Egypt

The succession of the Galala Mountains at the southern Tethyan margin (Eastern Desert, Egypt) provides new data for the evolution of an isolated carbonate platform in the Early Eocene. Since the Late Cretaceous emergence of the Galala platform, its evolution has been controlled strongly by eustatic sea-level fluctuations and the tectonic activity along the Syrian Arc-Fold-Belt. Previous studies introduced five platform stages to describe platform evolution from the Maastrichtian (stage A) to the latest Paleocene shift from a platform to ramp morphology (stage E). A first Early Eocene stage F was tentatively introduced but not described in detail. In this study, we continue the work at the Galala platform, focussing on Early Eocene platform evolution, microfacies analysis and the distribution of larger benthic foraminifera on a south-dipping inner ramp to basin transect. We redefine the tentative platform stage F and introduce two new platform stages (stage G and H) by means of the distribution of 13 facies types and syn-depositional tectonism. In the earliest Eocene (stage F, NP 9b–NP 11), facies patterns indicate mainly aggradation of the ramp system. The first occurrence of isolated sandstone beds at the mid ramp reflects a post-Paleocene-Eocene thermal maximum (PETM) reactivation of a Cretaceous fault system, yielding to the tectonic uplift of Mesozoic and Palaeozoic siliciclastics. As a consequence, the Paleocene ramp with pure carbonate deposition shifted to a mixed carbonate-siliciclastic system during stage F. The subsequent platform stage G (NP 11–NP 14a) is characterised by a deepening trend at the mid ramp, resulting in the retrogradation of the platform. The increasing deposition of quartz-rich sandstones at the mid ramp reflects the enhanced erosion of Mesozoic and Palaeozoic deposits. In contrast to the deepening trend at the mid ramp, the deposition of cyclic tidalites reflects a coeval shallowing and the temporarily subaerial exposure of inner ramp environments. This oppositional trend is related to the continuing uplift along the Syrian Arc-Fold-Belt in stage G. Platform stage H (NP 14a–?) demonstrates the termination of Syrian Arc uplift and the recovery from a mixed siliciclastic carbonate platform to pure carbonate deposition.

Petrographical and geochemical evidences for paragenetic sequence interpretation of diagenesis in mixed siliciclastic–carbonate sediments: Mozduran Formation (Upper Jurassic), south of Agh-Darband, NE Iran

The Upper Jurassic Mozduran Formation with a thickness of 420 m at the type locality is the most important gas-bearing reservoir in NE Iran. It is mainly composed of limestone, dolostone with shale and gypsum interbeds that grade into coarser siliciclastics in the easternmost part of the basin. Eight stratigraphic sections were studied in detail in south of the Agh-Darband area. These analyses suggest that four carbonate facies associations and three siliciclastic lithofacies were deposited in shallow marine to shoreline environments, respectively. Cementation, compaction, dissolution, micritization, neomorphism, hematitization, dolomitization and fracturing are diagenetic processes that affected these sediments. Stable isotope variations of δ18O and δ13C in carbonate rocks show two different trends. High depletion of δ18O and low variation of δ13C probably reflect increasing temperatures during burial diagenesis, while the higher depletion in carbon isotope values with low variations in oxygen isotopes are related to fresh water flushing during meteoric diagenesis. Negative values of carbon isotopes may have also resulted from organic matter alteration during penetration of meteoric water. Fe and Mn enrichment with depletion of δ18O also supports the contention that alteration associated with higher depletion in carbon isotope values with low variations in oxygen isotopes took place during meteoric diagenesis. The presence of bright luminescence indicates redox conditions during precipitation of calcite cement.

Middle-Miocene (Serravallian) temperate carbonates in a seaway connecting the Atlantic Ocean and the Mediterranean Sea (North Betic Strait, S Spain)

Miocene deposits in the contact area between the Prebetic and Subbetic domains (Betic Cordillera, S Spain) record the last stages of marine deposition on the southern Iberian margin and the final closure of the North Betic Strait in the early Tortonian (Late Miocene). Serravallian (Middle Miocene) shallow-water carbonates and mixed carbonate–siliciclastics formed on a platform at the southern margin of the North Betic Strait. These shallow-water deposits, informally termed the La Escalera Formation, onlap an erosion surface carved into underlying Miocene and older rocks. Carbonate components are mainly bryozoans, coralline red algae, molluscs, and benthic and planktonic foraminifers, typical of heterozoan lithofacies. Several facies types can be distinguished based on different grain sizes and sedimentary structures. Large-scale geometries and stratigraphic facies relationships indicate that the La Escalera Formation was deposited on a homoclinal ramp attached to a deeply eroded, emergent relief to the south. A beach system developed at the southern margin, with backshore lagoons and local incursions of coarse terrigenous sediments. Seawards of the beach system was a submarine-dune belt that extended over several square kilometres at the northeastern end of the La Escalera Formation. Longshore currents shifted submarine dunes to the northeast. Maximum production of carbonate skeletons took place seawards of the dune field. From this factory area, skeletal debris was moved landwards and incorporated into the dunes and beaches, whereas other skeletal particles were transported downslope by storms and fed a facies belt with hummocky and swaley cross-stratified sediments. The finest-grained particles accumulated below a storm-wave base, along with significant planktic debris. Carbonate deposits change laterally westwards to terrigenous sediments formed in similar facies belts. Terrigenous influx limited carbonate production on the western part of the ramp by disturbing the growth of coralline algae and bryozoans. Vertical variations of facies within the formation depict a major transgressive–regressive cycle, modulated by up to 10 higher-frequency cycles. Despite its location in an Atlantic-linked basin, the sedimentary model of the La Escalera Formation shows similarities to the models proposed for Neogene Mediterranean temperate carbonates, which imply a relatively low-energy regime on the ramp in comparison with the open-ocean platforms. This relatively low-energy regime can be explained by the position of the North Betic Strait at the eastern end of the Guadalquivir Basin. However, hummocky and swaley cross-stratified deposits are significantly more developed in La Escalera deposits than in known examples of Neogene Mediterranean carbonates.

Integrated approach to geomodelling and dynamic simulation in a complex mixed siliciclastic–carbonate reservoir, N'Kossa field, Offshore Congo

AbstractThe Lower Sendji Carbonate (Albian age) of the N'Kossa field is a mixed siliciclastic–carbonate reservoir exhibiting very heterogeneous reservoir properties and the development of extensive vertical flow permeability barriers. The reservoir is a succession of interstratified dolomite, limestone, sandstone and shale lithologies of sabkha, tidal flat and lagoonal origin. Early, synsedimentary dolomite cements are extensively developed, particularly over local palaeohighs.Challenges to hydrocarbon production include: (1) the vertical variability of reservoir properties; (2) the presence of many, laterally extensive vertical flow barriers; and (3) variable connectivity between fault-bounded compartments. This complexity was underestimated at the appraisal stage, the initial development plan calling for a relatively simple scheme of pressure support to the critical reservoir fluid via injection into the gas cap and the water leg. This decision was supported by the identification of a single hydrocarbon column (>400 m thick) over the entire structure which suggested a lack of vertical compartmentalization.Recent studies, which include a geological synthesis of more than forty wells and a dynamic data synthesis to determine the production mechanism and identify key heterogeneities, show a lack of pressure support to the oil leg from both water and gas injector wells. Today, integration of static and dynamic data together with an improved geological understanding of the stratigraphic control on vertical flow barriers and mapping of high permeability sandstone layers is the key to identifying unswept zones of the field and future infill drilling targets.

Tectonic and eustatic control on a mixed siliciclastic–carbonate platform during the Late Oxfordian–Kimmeridgian (La Rochelle platform, western France)

Boreal and Tethyan realms of Western Europe present significant sedimentological, paleontological, and stratigraphic differences. The purpose of this study is to constrain regional versus global controls on the dynamics of a sedimentary system located at the interface of these two realms in order to better understand the origin of their differences. Detailed sedimentological, palynofacies and calcareous nannofossil analyses were performed on two sections from the La Rochelle platform (western France). The Pas section includes part of the Late Oxfordian and Early Kimmeridgian, and the Rocher d'Yves section is assigned to the Late Kimmeridgian. They correspond to monotonous marl–argillaceous limestone alternations. Limestones are essentially mudstones with echinoderms, bivalves and foraminifera that suggest low-energy, open-marine conditions. Highly bioclastic and/or peloidal deposits occur commonly, and show wackestones to wacke-pack-grainstones textures. These deposits indicate frequent high-energy events, and are interpreted as storm deposits. Marls dominate in the most proximal depositional environments, while calcareous deposits are more important in more distal environments. The Rocher d'Yves section is globally more marly than the Pas section, suggesting a more proximal setting. Palynofacies are dominated by woody particles, suggesting shallow-water, proximal depositional environments. Calcareous nannofossils are ascidian spicules, coccoliths, and schizospheres. Watznaueria britannica dominate calcareous nannofossil assemblages in the Pas section. The Rocher d'Yves assemblages are quasi-exclusively composed of Cyclagelosphaera margerelii, and indicate more proximal paleoenvironments than those of the Pas section. Different orders of depositional sequences are defined, with sequence boundaries corresponding to the most rapid relative sea-level falls. They are hierarchically stacked, and correlate, on the basis of ammonite zones, with the sequences of contemporaneous sections from Tethyan and boreal realms. The stacking pattern of these sequences suggests an orbital control on sedimentation. Small-, medium- and large-scale sequences correspond to precession (20 ky) cycles and to 100 ky and 400 ky eccentricity cycles, respectively. The elementary sequences have durations shorter than 20 ky. The Kimmeridgian was a period of global sea-level rise that ended in the Late Kimmeridgian. More proximal depositional environments in the Rocher d'Yves section (Late Kimmeridgian) than in the Pas section (Early Kimmeridgian) imply a progradation of the La Rochelle platform during the Kimmeridgian. This progradation resulted from a slowdown of the subsidence in the Aquitaine Basin during the Kimmeridgian, corresponding to the first steps of Atlantic Ocean opening. High-frequency cycles on the La Rochelle platform formed in sync with Milankovitch orbital cycles, while tectonics controlled the formation of the low-frequency cycles.

The western edge of the Mediterranean Pelagian Platform: A Messinian mixed siliciclastic–carbonate ramp in northern Tunisia

The marine Messinian deposits of Tunisia cover a narrow littoral strip some 300 km long between the northern Bizerte and Cap Bon areas and the central–eastern Sahel region. Litho- and biofacies analysis of six stratigraphic sections reveals the distinctive features of these deposits. The lower Messinian deposits are characterized by ubiquitous siliciclastics and abundant oolitic/bioclastic limestones organized in an eastward facing ramp. Westward (landward), this ramp changes into coastal lagoons, sometimes containing evaporites. Eastward, the ramp passes to the reefal Pelagian Platform extending as far as Lampedusa. Two main sedimentary cycles are distinguished: 1) an early Messinian siliciclastic retrogradational then oolitic/bioclastic progradational cycle (Beni Khiar Formation and lower Oued bel Khedim Formation); 2) a late Messinian brackish to continental cycle that probably accumulated in rapidly subsiding lagoons (Oued el Bir Formation and upper Oued bel Khedim Formation). The Tunisian early Messinian cycle is partly eustatically controlled, but the late Messinian cycle cannot be confidently correlated to other well-known Messinian series because of tectonic movements. The lower Messinian deposits of Tunisia are also characterized by abundant suspension-feeding organisms (molluscs and bryozoans) and rare corals, calcareous algae, echinoids, and larger benthic foraminifers. The proposed palaeoenvironmental model shows that the lower Messinian ramp of Tunisia was located on a current-protected margin and subjected to continent-derived sediment and nutrient supply. Eastward, nutrient influx diminished and a shallow-water isolated carbonate platform with coralgal facies developed between the western and the eastern Mediterranean basins. The main hydrological connection between these two basins occurred through a narrow seaway situated to the northeast of the Pelagian Platform, south of Sicily and Malta.

High-Frequency Rhythmicity in a Mixed Siliciclastic-Carbonate Shelf (Late Miocene, Guadix Basin, Spain): A Model of Interplay Between Climatic Oscillations, Subsidence, and Sediment Dispersal

Abstract Rhythmites in pelagic sediments can include information on paleoclimate and with orbital cycles. This paper documents and interprets an unusual rhythmic succession from a cool-water shallow-marine shelf that includes features of Mediterranean-type ramps and open-ocean-type ramps. In the subsiding southwestern margin of the Guadix Basin (southern Spain, Betic Cordillera), during the late Tortonian, a 640-m-thick succession was deposited over a short period (7.8–7.2 Ma). This succession contains 30 sedimentary packages or rhythmites, each defined by two facies associations: a mixed cross-stratified siliciclastic–skeletal sandstone and a homogeneous marl rich in planktonic foraminifera. Sandstones ranging in thickness from 3 m to 20 m contain coralline algae, bivalves, bryozoans, benthic foraminifera, echinoids, and brachiopods (typical temperate-water shelf biofacies). These units exhibit cross-strata sets up to four meters high, interpreted to have been produced by strong unidirectional currents. The marly deposits range in thickness from 2 to 17 m and contain an association of cold-water planktonic foraminifera as well as thin sandstone layers that exhibit normal grading corresponding to typical incomplete Bouma sequences or a lower division with planar lamination and an upper division of very fine- to fine-grained sandstone with hummocky and swaly cross-stratification. On the basis of the sedimentary structures, the sandstone layers are interpreted as sporadic turbidity flows and storm ebb–surge deposits. On the basis of the sedimentary structures, the hydrodynamic conditions during deposition of the mixed siliciclastic–carbonate interval were high-energy. Based on sedimentary and biogenic features of the rhythmites and their temporal durations (ca. 20 ka), rhythmicity is interpreted to reflect alternating cool-wet and cold-dry climate episodes driven by precession orbital cycles. This occurrence of alternating mixed carbonate–siliciclastic sedimentation and pelagic sediments represents a unique occurrence in the geologic record.

Facies and fauna of the Pennsylvanian Buckhorn Asphalt Quarry deposit: a review and new data on an important Palaeozoic fossil Lagerstätte with aragonite preservation

The Pennsylvanian Buckhorn Asphalt Quarry contains the best-preserved Palaeozoic mollusc fauna in the world. Early impregnation of mixed siliciclastic–carbonate rocks (mudstones, pack to grainstones, shell beds, and conglomerates) with hydrocarbons prevented aragonite destruction (“Impregnation Fossil Lagerstatte”). The exceptional preservation comprises shell microstructures, microornaments and early ontogenetic shells. Most gastropods had planktotrophic larval development indicating a high primary production although the remains of phytoplankton are very rare in this and other Late Palaeozoic deposits. Deposition occurred close to a shallow-water coastal area. Mass flow processes (density currents) triggered by storms were involved in the transport mechanisms of some units. Shells of benthic molluscs yield the most diverse known Palaeozoic microboring assemblage, indicating at least partly euphotic conditions. The invertebrate fauna comprises about 160 species and is dominated by molluscs, which is unusual for a Palaeozoic deposit, suggesting that aragonite dissolution produces a major bias in the fossil record. However, most mollusc genera in the Buckhorn deposit are also known from other Pennsylvanian occurrences as recrystallised shells. This shows that preservation bias via preferential aragonite dissolution may be overestimated.