Carbonate Depositional Systems Research Articles

FACIES PARTITIONING AT REGIONAL AND FIELD SCALES IN THE BARREMIAN KHARAIB‐2 CARBONATES, UAE

Carbonates in the Lower Cretaceous (Barremian to early Aptian) Kharaib Formation are reservoir rocks at giant oil fields in the UAE and Qatar. The Barremian Kharaib‐2 member (K60), the focus of this study, is in general composed of a regionally continuous succession of high‐energy, shallow‐water limestones bounded above and below by “dense” low‐energy mud‐rich strata. Despite several decades of research, conventional carbonate facies classification schemes and resulting facies groupings for the Kharaib‐2 member have failed to show a statistically acceptable correlation with core‐ and log‐derived petrophysical data. Moreover, sedimentary bodies potentially responsible for dynamic reservoir heterogeneities have not clearly been identified. This paper proposes a standardized facies classification scheme for the Kharaib‐2 carbonates based on vertical facies proportion curves (VPCs) and variogram analyses of core data to construct stratigraphic correlations at both field and regional scales. Data came from 295 cored wells penetrating the Kharaib‐2 member at ten fields in the on‐ and offshore UAE. Thin, dense intervals separating reservoir units were adopted as fourth‐order transgressive units and were used for stratigraphic correlation. Field‐scale probability maps were used to identify sedimentary bodies such as shallow‐water rudistid shoals.Regional stratigraphic correlations of the Kharaib‐2 member carbonates based on the VPCs identified variations in depositional environments, especially for the lower part of the reservoir unit; depositional facies at fields in the SE of the UAE were interpreted to be more distal compared to those at offshore fields to the NW. At a field scale, the VPCs failed to identify significant lateral variations in the carbonates. However, variogram analyses of cored wells showed spatial concentrations of specific facies in the inner ramp domain which could be correlated with high‐energy depositional bodies such as shoals dominated by rudist debris. The bodies were sinusoidal in plan view with lengths of up to 8 km and widths of ca. 1 km. Although similar‐shaped bodies with these dimensions have been reported from other carbonate depositional systems, they have not previously been reported in the Kharaib Formation. At a regional (inter‐field) scale, the stratigraphic correlation of standardized sedimentary facies remains problematic; however, mapping of facies associations and their relative proportions relative to their environments of deposition demonstrated new patterns for the stratigraphic architecture of the Kharaib‐2 member in the UAE.

Seismic attributes and quantitative reverse reservoir simulations of shallow-marine eocene sedimentary fairways, extensional rifting Indus onshore petroleum system, southern Pakistan: Implications for hydrocarbon exploration

Quantitative reservoir characterization plays a vital role in exploring carbonate depositional systems. The subsurface vertical and lateral changes in the tectonics and sea level adversely impact the signatures of band-limited seismic data. The seismic amplitudes are very sensitive to these vertical and lateral tilts of the stratigraphy and sedimentology of the subsurface stratigraphic layers. The dynamical tectonic-based fluctuating sea creates vital seismic expressions of high amplitude. These seismic expressions can inversely infer the dense-fractured depositional parasequences, thickness, porosity, and lithological change for the accommodation space and the sedimentary influxes for the development of petroleum fairways. Hence, it becomes very challenging to predict whether the producing zone for hydrocarbons sourced from the wet-gas-bearing zones is a potential zone for hydrocarbon exploration or a light oil-bearing zone (WGZ or LOZ). Therefore, the development of reverse quantitative simulations of parasequences within the extensional regimes is always a very complex research problem during stratigraphic-based fuel exploration of thin-bedded carbonate systems. This research utilizes the spectral-based amplitudes along with the instantaneous waveform-based temperature-porosity stratigraphic simulations (WTPS) on middle-Eocene carbonates (MCARB), Pakistan. 18-Hz amplitudes detect the gas-bearing stratigraphic prospect of 230 km2 on the northern flanks. The 18-Hz-based WTPS has predicted the parasequences within the complete petroleum systems. 24 m thick progradational and laterally continuous and fractured reservoirs are deposited within the wet-gas-bearing zones (WGZ) throughout a rapidly falling sea and succeeding growth at 175 °F and 28% porosity pseudo-wells. Deposition of 15–20 m thick deep-marine transgressive shale has been completed throughout a rising sea level at a temperature of 125 °F and 12% poor porosity. There is a lateral change in the dense-fractured then low-velocity from 2.1 to 1.8 g/cc and 2670 to 1690 ms/s predicted by WTPS from the NE to SW zones of MCARB, which impacts the presence of 1.5 km of the regionally developed pure stratigraphic trap. The WGZ has a vertically resolved thickness of 24 m, which is appreciably above the resolvable magnitude threshold of 23 m. Hence, it implies that the extensive fall of sea level followed by the regional development of the accommodation space for the accumulation of WGZ carbonate stratigraphic units is at its accurate location within the sub-surface reservoir system. This workflow has robust consequences towards exploring petroleum systems, Onshore Pakistan. Hence, this workflow may serve as an exemplary move towards regional analogues aimed at world carbonate in extensional regimes.

The interplay between siliciclastic and carbonate depositional systems: Maastrichtian to Danian basin‐floor sediments of the mid‐Norwegian Møre Basin

AbstractSource‐to‐sink sedimentary systems associated with volcanic rifted margins serve as important archives for basin development by recording lithospheric changes affecting the depositional systems. Distinguishing between sediment transport processes and their sediment source(s) can guide the interpretation of a basin's history, and thereby inform regional paleogeographic reconstructions. In this contribution, we integrate and utilize wireline geophysical logs, detailed petrographic observations from side‐wall cores, and seismic analysis to describe and decipher a Maastrichtian to Danian‐aged basin‐floor depositional system in the deep outer Møre Basin, mid‐Norwegian margin. Well 6302/6‐1 (Tulipan) is a spatially isolated borehole drilled in 2001 that penetrates Maastrichtian and younger strata. A succession of hitherto undescribed carbonates and sandstones in the outer Møre Basin was discovered. It is investigated for sediment transport, provenance, and depositional processes on the basin floor surrounded by structural highs and ridges. The strata from the lower parts form a basin‐floor apron consisting of redeposited carbonate sourced from a westerly sub‐aerial high. The apron transitions vertically from mixed siliciclastic and carbonate into a purely siliciclastic fan with intercalated sandstone and mudstone, providing a rare high‐resolution record of how depositional environments experience a complete shift in dominant processes. The development coincides with similar latest Cretaceous‐earliest Palaeocene sequences recorded south of this region (e.g., well 219/20‐1) and may have been influenced by regional uplift associated with the onset of magmatism in the Northeast Atlantic. This study improves our understanding of a late, pre‐breakup source‐to‐sink sedimentary system developed near the breakup axis of an infant ocean, and documents what is possibly the northernmost chalk deposit in the Chalk Group.

Geomorphic and environmental controls on microbial mat fabrics on Little Ambergris Cay, Turks and Caicos Islands

AbstractTo interpret microbially influenced paleoenvironments in the sedimentary record, it is crucial to understand what processes control the development of microbial mats in modern environments. This article reports results from a multiyear study of Little Ambergris Cay, Turks and Caicos Islands, an uninhabited island floored by broad tracts of well‐developed microbial mats on the wind‐dominated and wave‐dominated Caicos Platform. Uncrewed aerial vehicle‐based imaging, differential global positioning system topographic surveys, radiocarbon data, and in situ sedimentological and microbial ecological observations were integrated to identify and quantify the environmental factors that influence the distribution and morphologies of Little Ambergris Cay microbial mats, including their response to large storm events. Based on these data, this study proposes that Little Ambergris Cay initially developed from the accretion and rapid lithification of carbonate sediment delivered by converging wave fronts in the lee of adjacent Big Ambergris Cay. Broad tracts of microbial mats developed during late Holocene time as the interior became restricted due to beach ridge development. Minor elevation differences regulate subaerial exposure time and lead to three categories of microbial mats, differentiated by surface texture and morphology: smooth mats, polygonal mats and blister mats. The surface texture and morphology of the mats is controlled by subaerial exposure time. In addition to elevation, the spatial distribution of mats is largely controlled by hydrodynamics and sediment transport during large storm events. This detailed assessment of the controls on mat formation and preservation at Little Ambergris Cay provides a framework within which to identify and understand the interactions between microbial communities and sediment transport processes in ancient high‐energy carbonate depositional systems.

Carbonate sedimentation and reservoirs associated with a volcanic mound in an open-marine, deep-water, drowned platform setting, Elaine field area, Upper Cretaceous Anacacho Formation, South Texas U.S.A.

In the Upper Cretaceous Balcones Igneous Province in South and Central Texas, numerous small volcanoes developed during Austin Chalk and Anacacho deposition that are similar to the recent Surtsey volcano offshore of Iceland. The volcanoes formed topographic highs on the deep-water (500–900 ft [150–275 m]), earlier-drowned Comanche Platform. The Elaine field in South Texas is associated with one of these relict volcanic highs and integration of a 3D-seismic dataset and cores permitted a detailed analysis of the deeper-water carbonate depositional systems of the field and associated carbonate reservoir development. This reservoir description extends the understanding of petroleum systems associated with relict, open-marine, deep-water, volcanic highs. In the shallow water at the top of the volcanic topographic highs, a carbonate factory developed that produced coarser-grained carbonate sediments that sourced the deeper-water flanks and toe-of-slope of the mound. Finer-grained carbonate chalk was deposited in the open-marine, deep-water environment adjacent to the volcanic mound. The coarser-grained carbonate sediments sourced from the mound top were transported downslope by gravity-flow processes that formed debrites and hyperconcentrated density-flow deposits. The gravity-flow deposits are composed of a mixture of shallow- and open-marine, deep-water biota indicating they are not shallow-water deposits, but are deeper-water mound-flank and toe-of-slope deposits.In the Anacacho Elaine field, the coarser-grained carbonates (i.e., skeletal packstones and grainstones) associated with the volcanic mound are the oil-producing reservoirs. The Elaine volcanic mound itself is tight and not productive. The Anacacho chalk immediately above the mound are also tight and form the top seal for the Elaine field.Associated with the relict volcanic mounds in the Balcones Igneous Province are a spectrum of carbonate plays. The relict volcano can be a reservoir, but not all the relict volcanic mounds are porous. The coarser-grain carbonates on the top, flanks, and at the base of the volcanic mounds are reservoirs in the area of the Balcones Igneous Province. The chalks deposited penecontemporaneously on the open-marine, deep-water platform can also be tight carbonate reservoirs.

Middle platform carbonate depositional systems and lithofacies patterns in the Lower Ordovician Ellenburger Group, Tobosa Basin in West Texas, U.S.A., and subsequent Sauk-Tippecanoe megasequence boundary meteoric karsting

A long, continuous core (1155 ft; 352 m) from the Ellenburger Group in Upton County, Texas allowed for a detailed reservoir description of depositional setting, lithofacies, pore networks, and impact of subsequent karst processes on the reservoir geology. This reservoir description furthers the understanding of the origin and characteristics of pore types relevant for waste-water disposal, carbon dioxide storage, and hydrocarbon reservoir quality in the Ellenburger Group. The Lower Ordovician Ellenburger Group in West Texas is a thick dolomite sequence that was deposited on a broad shelf over a 16 m.y. Time period in the Tobosa Basin. The shallow-water restricted depositional setting produced carbonate sediments that are dominantly composed of middle-shelf microbial thrombolite boundstones and inner-shelf laminated (stromatolitic) mudstones. Recognizable skeletal grains are rare. The lower two thirds of the cored section in the area of investigation in Upton County, Texas is dominated by microbial thrombolite complexes with associated stromatactis features. Interbeds of oolites and evaporites are associated with the complexes and may represent small-scale fluctuations in relative sea level. Up-section, lithofacies transition into a peritidal complex composed of stromatolites and laminated mudstones, some displaying evaporite nodules. The lithofacies stacking pattern is similar to the regional established stacking pattern where the most open-marine lithofacies are at the base of the stratigraphic section and the most restricted lithofacies are at the top of the stratigraphic section. This consistent stacking pattern suggests a stable structural setting for millions of years where constant environmental conditions existed. Following deposition and subsequent lithification and dolomitization of the Ellenburger Group, the section underwent intense meteoric karsting (epigenic) during Late Middle Ordovician Sauk-Tippecanoe megasequence long-term exposure. The meteoric karsting affected the upper 700 ft (215 m) of the Ellenburger Group. The pore network in the studied core is composed of vuggy (small cavities) pores in the thrombolite lithofacies and crackle and mosaic breccia fracture pores and chaotic breccias interbreccia pores in karst-affected intervals. These pore systems support hydrocarbon production, waste-water disposal, and carbon-dioxide storage.

Controls on large-scale architecture and facies distribution of an oolitic-microbial carbonate shelf margin, Nanpanjiang Basin, south China

Carbonate factory types and facies architecture of carbonate depositional systems result from complex interactions among several space- and time-dependent variables, including paleobathymetry, tectonic setting, water chemistry and oceanographic setting, clastic sediment influx, and accommodation. Whereas tropical factories that dominate modern settings have been documented and modelled extensively, microbial-oolitic systems, which are very common throughout the Paleozoic and Mesozoic, have received less attention.Continuous exposures along a platform-to-basin transect of an Early Triassic (Induan) microbial-oolitic carbonate ramp to shelf system in the Nanpanjiang Basin, South China, serves as a viable analogue to similar systems, including the Jurassic Smackover Formation of the Gulf of Mexico, and provide a natural laboratory to test these variables. Carbonate deposition following end-Permian regional drowning and retreat of the Yangtze Platform resulted in low-angle margin-slope systems during the Early Triassic. Lower Triassic carbonate systems display shifts in carbonate factory types, including skeletal, microbial, and abiotic factories, and lateral and vertical architectural variability.We applied stratigraphic forward modelling using DIONISOSFLOW, calibrated with detailed stratigraphic sections and facies mapping, to assess the impact of carbonate factory type, antecedent topography, and variations in accommodation on facies distribution and large-scale geometries of the carbonate shelf margin. Results from model simulations suggest that in the absence of skeletal carbonates in the Lower Triassic, the architecture of the carbonate margin is mainly controlled by rate of microbial growth along the slope.

Facies character and skeletal composition of heterozoan carbonates in a high-energy confined embayment (Miocene, Finale Ligure Limestone, NW Italy)

During the early and middle Miocene, diverse photozoan to heterozoan carbonate and mixed carbonate-siliciclastic depositional systems characterized the Mediterranean region at palaeo-latitudes of 30–40°N, during a time of warm climate. These systems ranged from carbonate ramp to rimmed platforms to current-swept flooded incised palaeo-valleys, seaways and straits. This study focuses on the facies character, skeletal biota composition, diagenesis and stable carbon and oxygen isotope signature of a mixed carbonate-siliciclastic succession (lower-middle Miocene Finale Ligure Limestone and underlying Oligocene-lower Miocene siliciclastic deposits) accumulated in a high-energy coastal setting (Finale Ligure, NW Italy) to improve the knowledge on the variety and controlling factors of Miocene heterozoan skeletal carbonates.The 3D reconstruction of the geometry of the early-middle Miocene Finale Ligure basin identifies an embayment, about 35 km2 wide, limited by uplifted and eroded Alpine tectonic units and connected to the open sea through a strait. The Oligocene-Miocene sedimentary succession unconformably overlying the Alpine deformed substrate consists of 14 lithofacies (L1-L14). Besides pockets of karstic breccias (L1) preceding the marine transgression, discontinuous outcrops of glaucony-bearing litharenite, siltstone, conglomerate and wackestone/packstone with planktonic foraminifera (L2-L4) represent the preserved remnants of an eroded marine shelf affected by tectonic uplift and subsequent erosion, approximately in the Aquitanian-Burdigalian, overlain through an angular unconformity by transgressive conglomerates and bioturbated litharenites (L5-L7). The onset of carbonate skeletal production characterizes the overlying compositionally mixed carbonate-siliciclastic Langhian-Serravallian Finale Ligure Limestone succession (up to 100–150 m thick, L8-L14). These cross-bedded skeletal packstone and grainstone/rudstone are composed of barnacles, echinoderms, bryozoans, bivalves, scleractinian and stylasterid corals, and lack halimedacean algae as previously published. They accumulated in a high-energy setting with strong bottom currents, driven by the amplification of tidal and/or storm currents in a confined embayment, promoting the formation of seaward prograding metre-scale subaqueous dunes.Unlike other lower-middle Miocene carbonate systems, the Finale Ligure heterozoan carbonates contain rare coralline red algae and larger benthic foraminifera but are enriched in barnacles, scleractinian and stylasterid corals. The coastal palaeo-environmental conditions, with hard rocky substrates, strong bottom currents, high nutrients and water turbidity, influenced the composition of the skeletal carbonate producers. These distinctive lithofacies character and composition resulted from the confined embayment morphology inherited from the marine flooded Alpine bedrock that was subjected to land runoff driving nutrient input.

Contradictory coeval vertical facies changes in upper Miocene heterozoan carbonate–terrigenous deposits (Sierra de Gádor, Almería, SE Spain)

ABSTRACT While spatial facies patterns can be observed in modern systems, only vertical facies successions can usually be examined in ancient deposits. Lateral facies relationships (depositional models) and relative sea-level changes throughout time are traditionally deduced from correlation of vertical facies successions along transects perpendicular to inferred paleo-shorelines. Establishing vertical facies successions and their corresponding time-equivalent proximal to distal facies patterns are, therefore, paramount in reconstructing ancient carbonate depositional systems and their response to sea-level change. In the present study of well-exposed panoramics of late Tortonian (8.1 Ma) mixed heterozoan carbonate and terrigenous deposits in La Chanata area in Sierra de Gádor, Almería, SE Spain, we show that frequent changes in facies width make it difficult to predict how variation in sea level impacts the facies distribution. The following facies are recognized: shoreline conglomerates directly overlie an erosion surface on the basement; terrigenous coralline-algal packstones extend basinwards from the conglomerates and are interpreted as shallow-water deposits stabilized by seagrass. There exist three types of facies consisting of relatively well-preserved, parautochthonous bioclasts, which occur generally seawards of the packstones: a) branching-coralline rudstones that formed from rhodolith (maërl) beds, both shorewards and basinwards from seagrass meadows where b) foliose-coralline rudstones to floatstones accumulated, and c) lenses of Heterostegina rudstones to floatstones changing laterally to any of the coralline algal facies. The factory facies of a–c show a patchy distribution with no definite arrangement in shoreline-parallel belts. The evolution of the depositional system is as follows: after filling paleovalleys in the erosion surface, deposition took place on a homoclinal ramp. The hybrid heterozoan carbonate–terrigenous deposits show a general retrogradation altered by one episode of proximal facies progradation. The width of facies across the ramp changes markedly in different episodes of relative sea-level rise: in several episodes of transgression, terrigenous coralline-algal packstones spread across the ramp locally overlying more distal facies, such as branching-coralline rudstones, thus generating regressive vertical patterns during relative sea-level rise. In other transgressive episodes packstones disappear, and the a–b factory facies pass laterally into conglomerate. The stratigraphic changes in facies width might be due to changes in general energy caused by climate variations or, alternatively, to the amount of relative sea-level rise. Large sea-level rise would result in relatively deep and calm conditions, thus favoring little fragmentation of large bioclasts during accumulation of the factory facies. Conversely, little change in accommodation would have resulted in higher-energy environments with concomitant increased physical erosion of the coralline-algae factories, thus resulting in mostly sand-size bioclasts in the packstones. In this environmental context, vertical change from distal to proximal facies can result from relative sea-level rise with increased mobilization and lateral expansion of proximal facies across the ramp. This outcome challenges the adequacy of using vertical lithofacies successions to reconstruct relative sea-level change in carbonate depositional systems.

Ecosystem design as an avenue for improving services provided by carbonate producing marine ecosystems.

Ecosystem Design (ED) is an approach for constructing habitats that places human needs for ecosystem services at the center of intervention, with the overarching goal of establishing self-sustaining habitats which require limited management. This concept was originally developed for use in mangrove ecosystems, and is understandably controversial, as it markedly diverges from other protection approaches that assign human use a minor priority or exclude it. However, the advantage of ED lies within the considered implementation of these designed ecosystems, thus preserving human benefits from potential later disturbances. Here, we outline the concept of ED in tropical carbonate depositional systems and discuss potential applications to aid ecosystem services such as beach nourishment and protection of coastlines and reef islands at risk from environmental and climate change, CO2 sequestration, food production, and tourism. Biological carbonate sediment production is a crucial source of stability of reef islands and reef-rimmed coastlines. Careful implementation of designed carbonate depositional ecosystems could help counterbalance sea-level rise and manage documented erosion effects of coastal constructions. Importantly, adhering to the core ethos of ED, careful dynamic assessments which provide a balanced approach to maximizing ecosystem services (e.g., carbonate production), should identify and avoid any potential damages to existing functioning ecosystems.

Middle Platform Carbonate Depositional Systems and Lithofacies Patterns in the Lower Ordovician Ellenburger Group, Tobosa Basin in West Texas, U.S.A, and Subsequent Sauk-Tippecanoe Megasequence Boundary Meteoric Karsting

Middle Platform Carbonate Depositional Systems and Lithofacies Patterns in the Lower Ordovician Ellenburger Group, Tobosa Basin in West Texas, U.S.A, and Subsequent Sauk-Tippecanoe Megasequence Boundary Meteoric Karsting

Understanding carbonate factories through palaeoecological and sedimentological signals – Tribute to Luis Pomar

AbstractThe carbonate factories, their controlling factors and their palaeoecological and sedimentological signals recorded in sedimentary successions are key elements for understanding the evolution of carbonate platform systems. Luis Pomar has dedicated most of his academic life to the study of carbonate rocks and carbonate factories. The idea for this special issue to celebrate Pomar’s career arose during a session at the 34th International Association of Sedimentologists meeting held in Rome, entitled ‘Understanding carbonate factories through palaeoecological and geochemical signals’. The proposal encountered great response among participants, and additional contributions followed an email invitation to other specialists. This issue contains a variety of papers on carbonate sedimentology and carbonate factories. Here, an introduction that contextualizes the papers and key concepts discussed in this thematic issue is provided. It reviews Luis Pomar’s major achievements in carbonate sedimentology and discusses the evolution of the concept of the carbonate factory and the series of palaeoecological and sedimentological signals used to characterize the wide spectrum of carbonate depositional systems found in the natural world.

From Siliciclastic to Bioclastic Deposits in the Gulf of Naples: New Highlights from Offshore Ischia and Procida–Pozzuoli Based on Sedimentological and Seismo-Stratigraphic Data

This study discusses the siliciclastic to bioclastic deposits (in particular, the rhodolith deposits) in the Gulf of Naples based on sedimentological and seismo-stratigraphic data. The selected areas are offshore Ischia Island (offshore Casamicciola, Ischia Channel), where a dense network of sea-bottom samples has been collected, coupled with Sparker Multi-tip seismic lines, and offshore Procida–Pozzuoli (Procida Channel), where sea-bottom samples are available, in addition to Sparker seismic profiles. The basic methods applied in this research include sedimentological analysis, processing sedimentological data, and assessing seismo-stratigraphic criteria and techniques. In the Gulf of Naples, and particularly offshore Ischia, bioclastic sedimentation has been controlled by seafloor topography coupled with the oceanographic setting. Wide seismo-stratigraphic units include the bioclastic deposits in their uppermost part. Offshore Procida–Pozzuoli, siliciclastic deposits appear to prevail, coupled with pyroclastic units, and no significant bioclastic or rhodolith deposits have been outlined based on sedimentological and seismo-stratigraphic data. The occurrence of mixed siliciclastic–carbonate depositional systems is highlighted in this section of the Gulf of Naples based on the obtained results, which can be compared with similar systems recognized in the central Tyrrhenian Sea (Pontine Islands).

Lacustrine carbonate towers of Lake Abhe, Djibouti: Interplay of hydrologic and microbial processes

Lake Abhe, located at the triple junction between the Red Sea, Gulf of Aden, and Main Ethiopian rift trends, is a hypersaline and alkaline closed lake that is known for its exposures of massive carbonate chimneys on the flat-lying sediments of the lake's eastern margin. This study describes the morphological, textural, and petrographic characteristics of these chimneys at the basin- to microscopic-scale in order to constrain the carbonate depositional system in the context of basin hydroclimate history. Chimneys occur in three main fields and are assessed according to 1) large-scale (>5 m) morphological variations, 2) meso-scale (cm to <5 m) textures, and 3) micro-scale (<1 cm) fabrics. The dominant chimney fabric is a porous crystalline framework of trigonal prismatic and dendritic calcite crystals that locally contain spherulites, sickle-cell calcite fabrics, and entombed microbial cocci. In addition to the crystalline chimneys, other carbonate sediments include stromatolitic crusts composed of microdigitate laminated columns dominated by crystalline fan microfabrics, as well as carbonate-rich mudstones and relatively rare, localized, carbonate-rich diatomites. Chimneys are interpreted to form primarily as products of mixing between hydrothermal sublacustrine springs and lake waters during lake highstand intervals, while stromatolitic crusts are interpreted to form during lowstand lake levels. Our interpretation of mixing processes is supported by the δ18O composition of chimney calcite, which is representative of lake water and area hot spring endmembers. The observation of stromatolitic crusts with more positive values of δ13C and δ18O than crystalline chimneys indicates that crusts formed during periods of high evaporation and low lake level. Crusts also contain Mg-silicate minerals, which are not present in crystalline chimneys, and further support the interpretation of lowstand depositional conditions. Although the age of chimney formation is not well constrained, evidence from seismic reflection data suggests a pattern of chimney formation during lake level rise and highstand times, followed by lake level fall, subaerial exposure and weathering that occurred at several times throughout the Late Pleistocene and Holocene.

Timescale dependent sedimentary record during the past 130 kyr from a tropical mixed siliciclastic–carbonate shelf edge and slope: Ashmore Trough (southern Gulf of Papua)

AbstractIn tropical and sub‐tropical mixed siliciclastic–carbonate depositional systems, fluvial input and in situ neritic carbonate interact over space and time. Despite being the subject of many studies, controls on partitioning of mixed sediments remains controversial. Mixed sedimentary records, from Ashmore Trough shelf edge and slopes (southern Gulf of Papua), are coupled with global sea‐level curves and anchored to Marine Isotope Stage stratigraphy to constrain models of sediment accumulation at two different timescales for the past 130 kyr: (i) 100 kyr scale for last glacial cycle; and (ii) millennial scale for last deglaciation. During the last glacial cycle, carbonate production and accumulation were primarily controlled by sea‐level fluctuations. Export of neritic carbonate to the slopes was initiated during re‐flooding of previously exposed reefs and continued during Marine Isotope Stage 5e and 1 interglacial sea‐level highs. Siliciclastic fluxes to the slope were controlled by interplay of sea level, shelf physiography and oceanic currents. Heterogeneous accumulation of siliciclastic mud on the slope, took place during Marine Isotope Stage 5d to Marine Isotope Stage 3 sea‐level fall. Siliciclastics reached adjacent depocentres during Marine Isotope Stage 2. Coralgal reef and oolitic–skeletal sand resumed at the shelf edge during the subsequent stepwise sea‐level rise of the last deglaciation. Contemporaneous, abrupt siliciclastic input from increased precipitation and fluvial discharge illustrates that climate controlled deglacial sedimentation. Siliciclastic input persisted until ca 8.5 ka. Carbonate accumulation waned at the shelf edge after ca 14 ka, whereas it increased on the slopes since ca 11.5 ka, when previously exposed reef and bank tops were re‐flooded. When comparing the last sea‐level cycle sedimentation patterns of the southern Gulf of Papua with other coeval mixed systems, sea level and shelf physiography emerge as primary controls on deposition at the 100 kyr scale. At the millennial scale, siliciclastic input was also controlled by climate change during the unstable atmospheric and oceanic conditions of the last deglaciation.

Introduction to special section: Mapping mixed clastic and carbonate depositional systems in lacustrine basins

Introduction to special section: Mapping mixed clastic and carbonate depositional systems in lacustrine basins

A quantitative criterion with which to distinguish lithofacies belts from mosaics in carbonate deposystems

The planform pattern of carbonate lithofacies in shallow tropical systems have been described as both relatively simple (i.e. the lithofacies belt model) and highly complex (i.e., the lithofacies mosaic model). The lateral continuity of the lithofacies elements differs between these two descriptions with the former containing continuous strike-oriented elements while the latter contains numerous disjointed elements with no specified orientation under two conditions. First, we consider the lithofacies elements solely by their sediment texture, and second, we do not consider ecological constraints on the benthic communities. Using variography, we develop the spatial correlation index (SCI) for distinguishing between the two types of lithofacies arrangements and deploy the index to visualize the spatial distribution of belt-like and mosaic-like patterning for 7 lithofacies in 36 carbonate platforms. The visualizations suggest that the belt versus mosaic dichotomy does not adequately capture the complexity of lithofacies arrangements in modern carbonate systems. This limitation exists because carbonates primarily consist of biogenic sediments whose distribution is controlled by physical and ecological controls. Thus, taxonomy must be considered when examining these spatial patterns. Furthermore, there appears to be a co-occurrence of arrangements within each of the systems. Four motifs in the statistical distribution of the SCI values observed within the systems are identified. The motifs appear to depend on the biotic and abiotic factors of the sedimentary system under study. Belt-like arrangements for a framestone class appeared to be strongly associated with the higher energy environments, while mosaic-like arrangement for this class often occurred in sheltered, low-energy areas. For grainstone and wackestone classes, belt-like arrangements were located along the transition from platform rim to lagoon. Finally, the results suggest that a greater variety of sediment-texture arrangements may be observed in Modern shallow-water carbonate depositional systems than may be visible in Ancient systems observed within the rock record.

A wet climatic terrestrial carbonate record in the middle-upper Pleistocene, north Tunisia

The middle-upper Pleistocene Borj Edouane unit records a singular spectrum of terrestrial carbonates in a shallow depression on Cretaceous rocks in north Tunisia. This study aims to decipher the depositional system and reveal the climatic and paleohydrological conditions of the carbonate facies deposition. They are grouped into two facies associations: (1) a calcrete–palustrine and (2) a composite microbialite-travertine, surrounded by a marginal alluvial association along the western and northern borders of the basin. The calcrete–palustrine association extends along the westernmost south–north strip of the basin, reflecting calcrete development on distal alluvial fan deposits and pedogenic modification (palustrine facies) of shallow fresh-water carbonates. The composite microbialite-travertine, extending along north–south strips in the middle and eastern areas of the basin, consists of microbialite, clastic microbialite–travertine, and travertine associations. Microbialites were developed in fluvio-lacustrine environments within the central strip and pass eastward into the travertines through the clastic microbialite–travertine association. The latter was deposited from currents and in shallow ponds along a bench next to the travertine-depositing springs, linked to a fault system in the eastern margin of the basin. The carbonate depositional system shows an asymmetrical distribution of facies that parallels the paleohydrological asymmetry along the basin. Water supply was restricted in the western part of the basin and abundant in its eastern areas linked to spring waters that kept stable lake levels in the basin center, favoring the genesis of microbialites. The presence of lacustrine carbonates in the studied area indicates higher water availability during the Middle-Late Pleistocene and that climate in North Africa was wetter than today.

Controls on carbonate platform architecture and reef recovery across the Palaeozoic to Mesozoic transition: A high‐resolution analysis of the Great Bank of Guizhou

AbstractCarbonate platforms spanning intervals of global change provide an opportunity to identify causal links between the evolution of marine environment and depositional architecture. This study investigates the controls on platform geometry across the Palaeozoic to Mesozoic transition and yields new stratigraphic and palaeoenvironmental constraints on the Great Bank of Guizhou, a latest Permian to earliest Late Triassic isolated carbonate platform in the Nanpanjiang Basin of south China. Reconstruction of platform architecture was achieved by integrating field mapping, petrography, biostratigraphy, satellite imagery analysis and δ13C chemostratigraphy. In contrast to previous interpretations, this study indicates that: (i) the Great Bank of Guizhou transitioned during Early Triassic time from a low‐relief bank to a platform with high relief above the basin floor (up to 600 m) and steep slope angles (preserved up to 50°); and (ii) the oldest‐known platform‐margin reef of the Mesozoic Era grew along steep, prograding clinoforms in an outer‐margin to lower‐slope environment. Increasing platform relief during Early Triassic time was caused by limited sediment delivery to the basin margin and a high rate of accommodation creation driven by Indosinian convergence. The steep upper Olenekian (upper Lower Triassic) slope is dominated by well‐cemented grainstone, suggesting that high carbonate saturation states led to syndepositional or rapid post‐depositional sediment stabilization. Latest Spathian reef initiation coincided with global cooling following Early Triassic global warmth. The first Triassic framework‐building metazoans on the Great Bank of Guizhou were small calcareous sponges restricted to deeper water settings, but early Mesozoic reef builders were volumetrically dominated by Tubiphytes, a fossil genus of uncertain taxonomic affinity. In aggregate, the stratigraphic architecture of the Great Bank of Guizhou records sedimentary response to long‐term environmental and biological recovery from the end‐Permian mass extinction, highlighting the close connections among marine chemistry, marine ecosystems and carbonate depositional systems.

Upper Jurassic carbonate buildups in the Miechów Trough, southern Poland – insights from seismic data interpretations

Abstract. The geometry and internal architecture of the Upper Jurassic carbonate depositional system in the epicontinental basin of central and western Europe and within the northern margin of the Tethyan shelf are hitherto only partly recognized, especially in areas with thick Cretaceous and younger cover such as the Miechów Trough. In such areas, seismic data are indispensable for the analysis of a carbonate depositional system, in particular for the identification of the carbonate buildups and the enveloping strata. The study area is located in the central part of the Miechów Trough that in the Late Jurassic was situated within the transition zone between the Polish part of the central and western European epicontinental basin and the Tethys Ocean. This paper presents the results of the interpretation of 2D seismic data calibrated by deep wells that document the presence of large Upper Jurassic carbonate buildups. The lateral extent of particular structures is in the range of 400–1000 m, and their heights are in the range of 150–250 m. The interpretation of seismic data revealed that the depositional architecture of the subsurface Upper Jurassic succession in the Miechów Trough is characterized by the presence of large carbonate buildups surrounded by basinal (bedded) limestone and marly-limestone deposits. These observations are compatible with depositional characteristics of well-recognized Upper Jurassic carbonate sediments that crop out in the adjacent Kraków–Częstochowa Upland. The presented study provides new information about carbonate open-shelf sedimentation within the transition zone in the Late Jurassic, which proves the existence of a much more extensive system of organic buildups which flourished in this part of the basin. The results obtained, due to the high quality of available seismic data, also provide an excellent generic reference point for seismic studies of carbonate buildups in other basins and of different ages.