This paper describes and interprets diver-collected, offshore samples to establish the origin of enigmatic large (100-200 m across) circular, dome-shaped features imaged using Multibeam Echo-Sounding (MBES) on the sea-floor of Weymouth Bay, Dorset, U.K. The structures occur within the Durlston Formation of the Purbeck Limestone Group (Late Jurassic-Early Cretaceous) that accumulated in a lagoon of variable salinity. A previous morphological study using MBES images alone led to four possible hypotheses for their origin; as isolated erosional remnants, as evaporite-related diapiric structures, as periclinal folds, or as eroded carbonate mounds that grew within the Purbeck lagoon. The petrographic study of seafloor samples taken from the centre of these structures results in their classification into nine sedimentary facies; eight limestones and one chert. The most abundant of these facies are similar to the well-known Purbeck limestones outcropping in nearby cliff sections, however four out of the nine facies have previously unrecorded microbialite components (intraclasts of travertine, stromatolites, laminated filamentous mudstones, and post-depositional, cavity-lining endostromatolites). This petrographic analysis suggests a microbial carbonate mound origin for these structures that is also supported by their morphology, their restricted occurrence palaeogeographically and stratigraphically to within the Purbeck Limestone, and the occurrence of microbialites at this level in onshore outcrops. Carbonate mounds of this size, in a lagoonal setting, are previously unknown from the Wessex Basin but show some similarities with Early Cretaceous lacustrine build-ups in South Atlantic offshore basins. The work demonstrates how the interpretation of even an extremely well-known stratigraphy such as that of the Purbeck Group can be limited when only part of the marginal environment is exposed for study. (c) 2025 The Authors. Published by Elsevier Ltd on behalf of The Geologists' Association. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).

In this study, we propose a multiple hypotheses approach to improve interpretations of limited remotely sensed datasets, such as sparsely exposed outcrops, subsurface datasets, or planetary objects using semi‐quantitative scoring and ranking of observable features. This method is demonstrated using an outcrop example from the Broken Beds of the Upper Jurassic–Lower Cretaceous Purbeck Limestone Group exposed along Britain's Jurassic Coast. Four published hypotheses regarding their origin are refined, represented in matrix form, scored and ranked based on carefully selected outcrop features. Semi‐quantitative scoring utilises knowledge of likely processes governing the occurrence of a range of features, some of which might be ignored or down‐played to favour a single hypothesis. Furthermore, by integrating expertise from different sub‐disciplines (e.g. basin analysis, sedimentology, diagenesis), we also consider the combined evidence of multiple features. This new method results in an interpretation that favours a multi‐process origin for the Broken Beds due to evaporite dissolution, overpressure release and tectonic folding, with identified uncertainty, all useful to guide further data collection.

Understanding how carbonate factories influence platform evolution is either based on qualitative conceptual models or quantitative numerical stratigraphic forward models. This study establishes new production depth profiles for four Cenozoic carbonate factories and uses two‐dimensional stratigraphic forward models to explore how interactions between sediment production and transport within carbonate systems influence carbonate platform development. Newly established production/depth profiles are used to model photozoan and heterozoan carbonate grain associations, and the associated carbonate producing factories, and results are compared with well‐studied outcrop successions. Sediment production from photozoan and heterozoan grain associations is also equalized, so that the total sediment production is the same but the depth/production profiles retain their distinctly different form. Thus, the effect of the different production profiles can be assessed. Ramps form when sediment diffusional transport rates are high relative to production rates and flat‐top steep‐margin platforms form when sediment diffusional transport rates are low relative to production rates, whether they are photozoan or heterozoan grain associations. The control exerted by sediment production and transport is expressed as a sediment transport–production ratio where transport ratio is a diffusional sediment transport in two‐dimensions and production ratio is the total sediment production rate which is the product of a production profile that varies in depth and laterally. The transport–production ratio is a key control on the evolution and geometry of carbonate platforms. This is the case with different production profiles (both euphotic and oligophotic) and in mixed grain‐size and mixed transport‐rate systems. Carbonate producing factories significantly influence the rate of sediment production, the depth distribution of sediment production (production profiles), as well as the type of grain sizes produced (influencing resistance to erosion). Thus, different types of carbonate grain associations, and the associated carbonate producing factories, can produce the critical differences between carbonate platform geometries.

We present enigmatic toroidal carbonate concretions retrieved from 700 m water depth from two sites in the upper plateau of the Rio Grande Rise. The concretions have a diameter of similar to 15 cm and a central hole of similar to 5 cm, and were observed on top of loose bioclastic sand over an area of similar to 30 m(2) at 0.5-1.5 m from one another. They consist of brown, porous, bioclastic grainstone, lacking internal structures. Grains consist of sand (< 3% coarse, 30% medium, 35% fine, 25% very fine), composed mainly by planktonic foraminiferal tests, and < 10% lime mud. The observed foraminiferal species indicate initial deposition of the sand in an open ocean setting. Biostratigraphy suggests an age no older than Pleistocene. Petrographic thin sections and SEM reveal that the fossiliferous grainstone contains intraclastic micritic cement and isopachous rim cement made of bladed magnesian calcite. delta O-18 values range from +1.5 to +3.3 parts per thousand (V-PDB) and increase with the degree of cementation, while delta C-13 ranges from +0.5 to +2.3 parts per thousand irrespective of cementation. The cementation of the grainstones is likely to have taken place in the marine phreatic environment. Carbonate precipitation induced by methane oxidation or (subaerial) meteoric diagenesis are ruled out based on both cement fabric and isotopic composition. Plausible causes for the toroidal shape of these structures could be: 1) sediment excavation by organisms, or 2) cementation within biofilms around burrows, followed by selective seafloor erosion. However, unveiling the actual formation mechanisms warrants further investigation.

This paper examines how non‐marine thrombolites are formed through a complex, multiphase process of microbial framework construction, erosion, cementation, recrystallization and episodes of internal sedimentation. Recognition of such phases of thrombolite construction provides a framework for the interpretation of the fluctuating environmental conditions leading to their formation. Microbialite frameworks are examined in detail from the Purbeck Limestone Group and their affinities and palaeo‐environmental significance assessed. Three types of thrombolite, one stromatolite and a leolite are described and interpreted. The thrombolite frameworks include: a peloidal mesoclotted type, a thrombolite constructed by the filamentous alga Cladophorites and a type with concentrically laminated micritic mesoclots. Physical and chemical erosion led to extensive early cavity formation within the frameworks. Early calcite rim cements with associated spherulites then developed over the microbial frameworks and these were reworked into cavities. Frameworks were also replaced by chalcedonic quartz and calcite spherulites. Internal sediments comprise peloids, intraclasts and brackish‐water molluscs and ostracods, together with their debris. The thrombolites grew in moderate‐energy to high‐energy shallow, lacustrine, microbial mounds whereas stromatolites occurred in deeper‐water settings. A brackish‐water, lacustrine setting is indicated by the preserved macro‐biota, microbes, absence of charophytes and syndepositional evaporites, and negative stable carbon and oxygen isotope ratios. Strontium isotopes suggest that the carbonate‐rich waters were fed from erosion of Mid–Lower Jurassic limestones on the western basin margin with possible mixing with waters from nearby uplifted Upper Jurassic limestones and with Late Jurassic seawater. The research indicates that non‐marine thrombolites have a complex, multiphase origin resulting in a diverse succession of textures and structures relating to microbially induced and influenced construction, dissolution, cementation, recrystallization and mineral replacement which have not been previously recorded and indicate the major differences between marine and non‐marine thrombolites.

This article is a review of the calcium carbonate-rich sediments that accumulate on present-day shelf seas of the world. These sediments form the majority of limestones and dolomites in the geological record. They are produced by three different pathways of precipitation of calcium carbonate out of sea water; biotically induced, abiotic and biotically influenced precipitation. These processes occur in four main sites of carbonate sediment production, known as carbonate factories; warm shallow tropical, cold-water, microbial and pelagic. Shelf morphology has a strong influence on sediment type and distribution and flat-topped, steep sloped platforms and gently sloping ramp platforms or shelves are both investigated. Four examples of carbonate shelves are presented so as to represent the range of different shelf carbonate sediments that occur today and in the geological past: semihumid rimmed carbonate shelves of south Florida and the Bahamas, the arid carbonate ramp of the Arabian Gulf, and the cool-water, distally steepened ramp of the north-east Atlantic. Finally, the affects of anthropogenically-driven changes in climate, ocean chemistry and sea-level are discussed. These generally have a negative impact on the production and accumulation of shelf sea carbonate sediment.

Lacustrine carbonate facies distribution is controlled by multiple environmental parameters including climate, hydrology, and tectonic setting, but few published models address this complexity. In this study, seismic and borehole data, integrated with outcrop logging, correlations, and facies models, are used to create a new tectono‐sedimentary model demonstrating how extensional faults, linked by a relay ramp, control distribution of lacustrine carbonate facies in the Upper Jurassic to Lower Cretaceous Purbeck Limestone Group (Wessex Basin, UK). Accumulation occurred in half‐graben sub‐basins south of two extensional east‐west faults, with widespread subaerial emergence of footwall blocks to the north. The lacustrine limestones of the lowest unit of this Group are characterised by in‐situ microbial mounds within bedded inter‐mound packstones‐grainstones. Mounds occur in three depositional intervals separated by paleosols. The distribution of facies indicates more brackish‐water conditions shoreward to the west, and more hypersaline conditions basinward to the east. The relay ramp hosts extensive microbial carbonate buildups formed in response to carbonate‐rich waters sourced from the northern limestone footwall blocks that fed into extensive shallow‐water areas on the low‐angle relay ramp slope.

Sedimentary facies and stratigraphic architecture of non-marine carbonates are controlled by a range of environmental parameters, such as climate, hydrology and tectonic setting, but the few published facies models do not account for this variability. Outcrop and petrographic observations from the Mupe Member of the Purbeck Limestone Group (Upper Jurassic–Lower Cretaceous) in Dorset, southern England, are the basis for new depositional models of non-marine microbialites and associated carbonates in an extensional basin. Ten facies are defined, described and grouped into five facies associations. The Mupe Member is characterised by accumulation of in situ microbial mounds developed around tree remains preserved as moulds and silicified wood. Mounds occur within three stratigraphic units, separated by three palaeosoils, characterised by less-porous, bedded, inter-mound packstone–grainstone that commonly onlap mound margins. Mounds are developed mainly in the shallowest areas of the lake, as indicated by their shapes, facies relationships and association with palaeosoils. These microbial mounds are compared to modern (Laguna Bacalar, Mexico and Great Salt Lake, Utah, USA) and ancient (Eocene Green River Formation, Uinta Basin, Utah, USA) analogues to assess their value as palaeoenvironmental indicators. Facies transitions indicate an earlier, brackish-water lake and a later hypersaline lake for the Mupe Member, both within a semi-arid climate setting in an extensional basin. The fact that the microbialites are covered by evaporitic strata, together with sedimentological, palaeontological and stable isotope data, suggest that there was a sharp change from through-flowing brackish-water, to a closed hypersaline, lacustrine system.

DAN BOSENCE1*, KATHRYN GIBBONS2, DANIEL P. LE HERON1, WILLIAM A. MORGAN3, TIM PRITCHARD4 & BERNARD A. VINING1,5 Department Earth Sciences, Royal Holloway University of London, Egham, Surrey, TW20 0EX, UK Nexen Petroleum UK Ltd., Prospect House, 97 Oxford Road, Uxbridge, Middlesex, UB8 1LU, UK Morgan Geoscience Consulting LLC, 132 W. Ellendale Estates Drive, Houma, LA 70360, USA bg Group, 100 Thames Valley Park Drive, Reading, Berkshire, RG6 1PT, UK Baker Hughes, Bentley Hall, Alton, Hampshire, GU34 4PU, UK

The obduction of an ophiolite sheet onto the eastern Pelagonian carbonate platform complex of the Hellenides began during the Late Bathonian and ended with the final emplacement of the ophiolite during Valanginian time. The early stages of obduction caused subaerial exposure of the platform, recorded by an unconformity of Callovian age, which is marked by laterites overlying folded and faulted, karstic substrates. The laterites have distinct ophiolitic geochemical signatures, indicating that emergent ophiolite had been undergoing lateritic weathering. This unconformity coincides with widespread western Tethyan, Callovian gaps, indicating that the obduction in the Hellenides was probably related to far-reaching plate tectonic processes. Resumed gravitational pull and rollback of the subducted, oceanic leading edge of the temporarily exposed ophiolite. Platform drowning continued into Tithonian–Valanginian time, documented initially by reefal carbonates and then by below-CCD, carbonate-free radiolarian cherts and shales. Subsequently, siliciclastic turbidites, which apparently originated from uplifted Variscan basement, were deposited together with and over the radiolarite as the ophiolite nappe sheet advanced. The nappe substrate underwent tectonic deformations of varying intensity, while polymictic mélange and syntectonic sedimentary debris accreted beneath the ophiolite and at the nappe front. The provenience of the ophiolite nappe complexes of northern Evvoia most probably has to be looked for in the Vardar ocean.

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The outcrops of Late Jurassic microbialites on the Dorset coast (southern England, Fig. 1) have been known to geologists for over 175 years. They form buildups, commonly around the stumps of ancestral conifer trees, and associated with both replaced evaporites and freshwater limestones. Despite the fact that they are visited by numerous university and industrial groups each year the environment of formation of the microbialites from the Purbeck Limestone is still not clear and previous authors have described them as being deposited in either freshwater, hypersaline, lagoonal or lacustrine environments. The limestones have been considered to have been diagenetically altered by either hypersaline or freshwater fluids. The only consensus is that these are non-marine microbial limestones but they have been variously referred to as tufas, algal limestones, stromatolites or by the local quarryman’s term “Caps”. This presentation reviews their occurrence, facies, early diagenetic fabrics and pore systems and discusses the likely depositional and early diagenetic environments that have resulted in the formation of some remarkably porous rocks. The Wessex Basin in Late Jurassic times was undergoing extension-related subsidence with maximum accumulation of clastics and carbonates in the hangingwall sub-basins adjacent to east-west trending normal faults (Fig. 1). The palaeoenvironment of the lower Purbeck Limestones is non-marine and succeeds the shallowmarine tropical grainstone shoals of the underlying Portland Limestone.

The lateral continuity and facies heterogeneities of metre-scale cycles in a greenhouse Lower Jurassic (Sinemurian) carbonate ramp from the northern Iberian Basin (Spain) was evaluated from extensive field analysis carried out on a well-exposed 12 km long outcrop. Eleven high-frequency continuous cycles and their bounding surfaces are traceable laterally through the entire outcrop. However, three of these cycles are found to split laterally into discontinuous cycles of more limited distribution (up to 3 to 5 km of lateral extent). The continuous and discontinuous cycles have a similar field expression in one-dimensional logs. As a consequence, the number of cycles that can be differentiated is variable along the logged sections (i.e. from 11 to 16). Cycles have variable facies heterogeneities and sedimentary trends depending on the environment of formation: shallowing-upward and symmetrical cycles occur in protected lagoon-tidal flat areas and in the open-marine, high-energy domain. These cycles show significant facies heterogeneities, which were controlled mainly by lateral migration of a mosaic of facies over an irregular topography. Deepening-upward and aggradational cycles are generated in low-energy, sub wave-base, open-marine areas. Facies are laterally homogeneous, reflecting low potential for carbonate accumulation and inability to fill the created accommodation space in this low-relief and relatively deep area. Cycle boundaries are generated by stages of rapid accommodation gain, involving the flooding of the carbonate ramp; they are more likely to originate from regional tectonic pulses (related to the extensional tectonics operating in the northern Iberian Basin) rather than greenhouse low-amplitude eustacy. Discontinuous cycles tend to occur in thickened areas and are interpreted as originating from the infill of wedge-shaped accommodation space resulting from differential subsidence (i.e. local tectonic pulses). In conclusion, where thickness variations occur in extensional settings lateral continuity of cycles should not be expected. In less well-exposed, or in one-dimensional sections and in wells, it would not be possible to distinguish continuous from discontinuous cycles, or to understand such two-dimensional heterogeneities. Identification of unique cycle-forming mechanisms or attempting cyclostratigraphic long-distance correlation of cycles is unrealistic without a detailed analysis of the architecture of cycles in laterally continuous outcrops.

The Late Triassic and Jurassic platform and the oceanic complexes in Evvoia, Greece, share a complementary plate-tectonic evolution. Shallow marine carbonate deposition responded to changing rates of subsidence and uplift, whilst the adjacent ocean underwent spreading, and then convergence, collision and finally obduction over the platform complex. Late Triassic ocean spreading correlated with platform subsidence and the formation of a long-persisting peritidal passive-margin platform. Incipient drowning occurred from the Sinemurian to the late Middle Jurassic. This subsidence correlated with intra-oceanic subduction and plate convergence that led to supra-subduction calc-alkaline magmatism and the formation of a primitive volcanic arc. During the Middle Jurassic, plate collision caused arc uplift above the carbonate compensation depth (CCD) in the oceanic realm, and related thrust-faulting, on the platform, led to sub-aerial exposures. Patch-reefs developed there during the Late Oxfordian to Kimmeridgian. Advanced oceanic nappe-loading caused platform drowning below the CCD during the Tithonian, which is documented by intercalations of reefal turbidites with non-carbonate radiolarites. Radiolarites and bypass-turbidites, consisting of siliciclastic greywacke, terminate the platform succession beneath the emplaced oceanic nappe during late Tithonian to Valanginian time.

Meter-scale, peritidal carbonate cycles are a common feature of the geological record but debate continues about what processes lead to their formation. Three conceptual models, or a combination thereof, are commonly invoked to explain cycle formation; eustasy, tectonics, or autocyclicity. These three models are tested with a large new dataset from different Early Jurassic plate margins from western Tethys. Study of seven logged sections from Spain, Italy, Greece, Tunisia, Morocco, and Gibraltar enables an analysis of the possible roles of local versus regional patterns and controls on cyclicity within a Sinemurian time slice. Cycle types are diverse and include shallowing-upward cycles (parasequences) but also deepening-upward and diagenetic cycles (high-frequency sequences) and subtidal cycles. Numbers of cycles per section and cycle stacking patterns within this time slice vary from section to section. Statistical tests (runs tests, time series, and bundling) all indicate random stacking of cycles within sections and an absence of any bundling of thicknesses or of facies trends. Assessment of cycle types by their occurrence and stacking patterns indicates little support for either eustasy or autocyclicity being the dominant cycle-forming mechanism. However, the variability in numbers of cycles per section, thickness variations of the sections, cycle type variability, and randomness of stacking patterns all favor a pulsed, tectonic control for the creation and filling of accommodation space. This conclusion is further supported by evidence that has largely arisen during the course of this studs of syndepositional extensional tectonics in the Sinemurian on these rifted Tethyan margins. Although tectonics appears to be the dominant control, superimposed eustasy and/or autocyclic processes cannot be discounted.

Stratigraphic forward modelling (SFM) has been used to answer important questions in stratigraphic analysis. This has been demonstrated through generic modelling of synthetic data and numerous two-dimensional (2D) real data case studies, but the method has rarely been used to further our understanding of specific, real three-dimensional (3D) carbonate data sets by quantitative comparison. This paper applies CARBONATE-3D, a SFM program, to a hydrocarbon reservoir and outcropping carbonate platforms at both exploration and production scales. We show the value of SFM in the exploration and production of hydrocarbons through predicting facies distributions and stratigraphic geometries between wells, and by analysing and answering fundamental questions about the formation of specific carbonate strata. First, the model is tested using the 3D outcrops of a Miocene attached carbonate platform at Níjar, SE-Spain where 78% of mapped facies are matched in the simulations. Second, SFM of an 80,000km2 Jurassic ramp in NE Spain (exploration scale), predicts new facies in areas without data, leads to an improved understanding of ramp evolution, modification of the interpreted sea-level curve and re-interpretation of the main sediment transport direction. Application to the isolated platform reservoir Judy Creek (Devonian, Canada) exemplifies the advantages of applying SFM at reservoir scale. The simulations yield information on platform geometries and facies distributions, an improved understanding of the factors that control platform evolution, and propose a revised sea-level history. Wider applications of SFM in exploration include assessment of the possibilities and probabilities of facies distributions and evaluation and ranking of different interpretations of platform evolution. At the reservoir scale facies probability distributions and layer-geometries derived from SFM can serve as input into static reservoir models to guide geostatistical simulations.