Complex Facies Research Articles

Eocene stratigraphy of the Wadi Al-Hitan World Heritage Site and adjacent areas (Fayum, Egypt)

The middle and late Eocene stratigraphy and depositional environments of the Wadi Al-Hitan area, Egypt, are described in detail for the first time. Complex facies relationships have been resolved by detailed logging of key sections over a 300 sq. km area and by biostratigraphic analysis. Mid and outer neritic marls and argillaceous limestones of the lower Gehannam Formation are overlain proximally by a major coastal sand body complex, the Birket Qarun Formation, which (as preserved) was deposited almost entirely on the lower shoreface. The sands of the Birket Qarun Formation prograded progressively northwards in four phases, with individual sand bodies separated by transgressive wedges of inner to mid-neritic silts and clays (the upper Gehannam Formation and the newly named Garet El-Naqb Formation). The final phase of progradation was terminated by the transgressive lower Qasr El-Sagha Formation. Calcareous nannofossil Zone NP17 (late Bartonian) is represented in the lowest Gehannam Formation. The Dictyococcites erbae nannofossil acme, previously recorded from Italy and ODP sites, has been identified at a higher level within the lower Gehannam Formation. Its start (in early NP18) is suggested as an excellent criterion to define the base of the Priabonian. Zone NP18 is highly condensed; sedimentation rates increased dramatically in early NP19/20 with the shift to predominantly clastic sedimentation. The Birket Qarun Formation is entirely within Zone NP19/20 (Priabonian). Seven depositional sequences are tentatively differentiated. Previous sedimentologic and paleogeographic interpretations are discussed; there is no evidence for ‘bay’ or otherwise restricted environments within this area, and previously proposed episodes of emersion are also discounted. Levels with relatively abundant marine mammals mainly reflect episodes of reduced clastic sedimentation, but are partly biased by differing surface exposure of successive intervals. There is no evidence that these levels represent whale ‘calving grounds’, or that marine mammals were unusually concentrated in this area in life.

Channel arrangements and depositional styles in the São Lourenço fluvial megafan, Brazilian Pantanal wetland

The Brazilian Pantanal is an extensive lowland tropical basin characterized by the presence of fluvial megafans and seasonally-inundated savanna floodplain wetlands. With an area of about 16,000km2, the São Lourenço is the second largest megafan in the Pantanal. Three distinct fluvial channel styles that formed at different times during the late Quaternary are found here. A geomorphological and sedimentary assessment of these depositional patterns provides valuable insight on the environmental context of their evolution. New optically stimulated luminescence data indicate that the upper five meters of sediment in the São Lourenço megafan has been accumulating since the late Pleistocene. Ancient fan lobes, located in upper and intermediate fan settings, consist of medium- and coarse-grained fluvial sands and exhibit well-preserved distributary braided paleochannels on their surfaces. As the megafan evolved through time, Pleistocene lobes were incised by a prominent valley filled with Holocene-aged meander belt deposits, which consist of silts interbedded with very fine sands and clays. Currently, the incised valley is a zone of sediment bypass. Modern deposition occurs along the distal toe of the megafan system, where lobes characterized by distributary channel-levee ridges are widespread. These features formed by progradation of avulsion belts into a broad swampy floodbasin, which caused the lower portion of the meander belt to be abandoned. The significant differences observed in intra-fan morphology appear to be linked to the variability in effective precipitation. Fan lobes deposited with braided distributary channels occurred under relatively dry conditions in the late Pleistocene. By contrast, aggradational meander belt deposits and lobes with distributary channel-levee ridges formed during fluctuating precipitation conditions of the Holocene, when the Pantanal emerged from deglacial aridity. Modern lobes form under heavy seasonal flooding and deposition occurs in response to very rapid and common avulsion events. These results have implications for interpreting the complexity of megafan facies in similar continental basins.

Castleton, Derbyshire, UK

The area around Castleton in the English county of Derbyshire has extensive outcrops of Carboniferous Limestone, showing complex sedimentary facies changes as well as interleaved extrusive basaltic volcanic rocks. Overlying these are the lower parts of the Millstone Grit, ranging from marine shales through a variety of deltaic sandstones. The limestones are hosts to hydrothermal mineral deposits, with veins containing galena, fluorspar, baryte and calcite. A unique variety of fluorspar is Blue John, mined for ornamental purposes since the mid‐eighteenth century. Landscape features include a massive landslip, dry valleys, loess, dolines, scree slopes and various karstic landforms. A complex hydrological system encompasses numerous caves, of which four are open to visitors.

Vertebrate tracksites in the Obernkirchen Sandstone (late Berriasian, Early Cretaceous) of northwest Germany— their stratigraphical, palaeogeographical, palaeoecological, and historical context

The northern German Lower Cretaceous Buckeberg Formation yields numerous dinosaur tracksites, some of which have produced material of impressive quality. Stratigraphically, the localities are concentrated in the Obernkirchen Sandstone, a thin subunit within this formation. The Obernkirchen Sandstone represents mainly a sandy barrier to back-barrier and lagoonal setting within a limnic deltaic facies complex, which was deposited during the late Berriasian (Cypridea alta formosa ostracod subzone) in the southeast of the Lower Saxony Basin, northwest Germany. A few tracksites occur more proximally in coeval fluvial deposits. Dinosaur footprint assemblages were left by ornithopods, theropods, sauropods, ankylosaurs, and small, bipedal ornithischians. Other vertebrate tracks are those of turtles and, possibly, crocodilians. Due to the decrease in sandstone quarrying in recent decades, many old tracksites are inaccessible today. Additionally, historical descriptions of the tracks were of highly variable quality and often published in remote and today nearly unobtainable sources. Here we provide a catalogue of 13 tracksites compiled from the literature and some new observations. Of these 13 tracksites, only five are still accessible and currently under study. Descriptions of each locality are provided, with a comprehensive compilation of existing data on lithofacies, stratigraphy, palaeogeography and palaeoecology of the Obernkirchen Sandstone and equivalent strata. A short review of the track-bearing lithofacies assemblage indicates that the outcrop areas have distinctly different facies and environments, and, therefore, track-bearing horizons can only be correlated stratigraphically between adjacent outcrops. For this reason, the identification of a megatracksite in the Obernkirchen Sandstone is currently regarded as premature and uncertain.

Sequence architecture and depositional evolution of theOrdovician carbonate platform margins in theTarimBasin and its response to tectonism and sea‐level change

AbstractThe sequence architecture and depositional evolution of theOrdovician carbonate platform margins in theTarimBasin,China, were formed in response to the interplay of tectonism and sea‐level change, their history being documented by the integrated analysis of many seismic lines, drilling and outcrop data. TheOrdovician carbonate system in the basin is divided into four composite sequences defined by major unconformities. Each sequence consists of a regional depositional cycle from transgression with an onlapping transgressive systems tract (TST) to regression with a prograding highstand systems tract (HST), and can be further subdivided into 10 third‐order sequences based on subordinate discontinuous boundaries at the carbonate platform marginal zones. Constrained by the marginal slope of the early‐riftedManjiaer aulacogen, the carbonate platform margins of the Lower and MiddleOrdovician that prograded eastward in an arcuate belt extending generally north‐south across the northern part of the basin. The development of theTazhong uplift due to compression resulted in an extensive paleokarst hiatus between the Middle and the UpperOrdovician in the south‐central basin, and subsequently constrained the formation of a peninsula‐shaped carbonate platform whose margins were controlled by marginal thrust‐fault belts of the paleo‐uplift during the LateOrdovician. In the northern basin, the LateOrdovician carbonate platform margin developed around the marginal slope of theTabei paleouplift. The transgressive–regressive cycles of the carbonate system are comparable and seem to have occurred simultaneously across the entire basin, suggesting that the cyclic sequence architecture was fundamentally controlled by eustatic fluctuations. Stacking patterns of the composite sequences varied due to the interplay between the accommodation produced by tectonism and sea‐level change, and the carbonate production rate. The reef–shoal facies complexes that developed along the platform margins, with paleokarst development at unconformities, constitute the major reservoir of large petroleum reserves in the basin.

CARBONATE GEOBODIES: HIERARCHICAL CLASSIFICATION AND DATABASE – A NEW WORKFLOW FOR 3D RESERVOIR MODELLING

Quantitative data on geobodies are crucial for reservoir modelling. Although abundant quantitative data are available in the literature for siliciclastic depositional systems, equivalent data for carbonate systems are scarce. In this paper we introduce a new approach to the management of quantitative data on carbonate geobodies which is based on a hierarchical classification scheme. The classes to which a carbonate geobody are assigned are: (1) depo‐time (i.e. geological age); (2) depo‐system (i.e. type of carbonate platform); (3) depo‐zone (i.e. facies belt or zone); (4) depo‐shape (i.e. geometry of the geological body); (5) depo‐element (i.e. architectural elements present); and (6) depo‐facies (litho‐ and biofacies). This hierarchical classification is complemented by a set of rules for modifying depo‐shapes which refer to their spatial distribution and patterns of interaction.Based on this classification, an extensive database has been developed which can be used for 3D reservoir modelling. The database holds more than 600 case studies from outcrop analogues and the subsurface and also from satellite images of modern carbonate settings. The database can be used as the basis for a new workflow for reservoir modelling which uses multiple‐point statistics (MPS). MPS makes use of training images to capture and reproduce facies patterns and geometries during stochastic simulations.The application of this new approach is demonstrated by modelling a Cretaceous outcrop reservoir analogue from southern France. The use of MPS allows the generation of geologically realistic and complex facies distributions in the model based on the simplified training images.

Allogenic and autogenic processes combined in the formation of shallow-water carbonate sequences (Middle Berriasian, Swiss and French Jura Mountains)

Sediment production and accumulation on shallow carbonate platforms are controlled by allogenic, externally controlled processes (such as sea level, climate, and/or platform-wide subsidence patterns) as well as by autogenic factors that are inherent to the sedimentary system (such as lateral migration of sediment bodies). The challenge is to determine how and in which proportion these processes interacted to create the observed sedimentary record. Here, a case study of Middle Berriasian, shallow-marine carbonates of the Swiss and French Jura Mountains is presented. Based on vertical facies evolution and bedding surfaces, different orders of depositional sequences (elementary, small-scale, medium-scale) have been identified in the studied sections. The hierarchical stacking pattern of these sequences and the time span represented by the investigated interval imply that eustatic sea-level fluctuations in the Milankovitch frequency band were an important controlling factor. The small-scale and medium-scale sequences relate to the 100 and 400-kyr orbital eccentricity cycles, respectively. The elementary sequences are attributed to the 20-kyr precession cycle. Differential subsidence additionally produced accommodation changes. The present study focuses on one specific small-scale sequence situated at the base of the transgressive systems tract of large-scale sequence Be4, which is identified also in other European basins. This small-scale sequence has been logged in detail at eight different outcrops in the Jura Mountains. Detailed facies analysis reveals that different depositional environments (tidal flats, internal lagoons, open lagoons, carbonate sand shoals) were juxtaposed and evolved through time, often shifting position on the platform. The boundaries of the small-scale (100-kyr) sequence can be followed over the entire study area and thus must have formed through predominantly allogenic processes (eustatic sea-level fall, the effect of which was locally modified by differential subsidence). In two sections, five well-developed elementary sequences constitute the small-scale sequence. In the other sections, the identification of elementary sequences often is difficult because sedimentation was dominated by autogenic processes that overruled the influence of sea-level fluctuations. In low-energy, tidal-flat and internal-lagoonal settings, orbitally induced sea-level changes were recorded more faithfully, while high-energy shoals were mainly submitted to autogenic processes and the allogenic signal is masked. Consequently, the studied Jura platform experienced a combination of auto- and allogenic processes, which created a complex facies mosaic and a complex stacking of depositional sequences. Nevertheless, the 100-kyr orbital signal was strong enough to create correlatable sequence boundaries. Within a 100-kyr sequence, however, the unambiguous definition of sequences related to the 20-kyr orbital cycle is often difficult and the prediction of their lateral or vertical facies evolution impossible.

High-frequency palaeoenvironmental changes on a shallow carbonate platform during a marine transgression (Late Oxfordian, Swiss Jura Mountains)

Based on a well-established bio- and sequence-stratigraphic framework, a narrow time window in the Bimammatum ammonite zone (Late Oxfordian) is investigated in six Swiss Jura sections representing a shallow-water carbonate platform. From the detailed facies and microfacies analysis of oncoid-rich (Hauptmumienbank Member) and ooid-rich (Steinebach Member) limestones, a microfacies classification is established, depositional environments are interpreted, and a depositional model for the Swiss Jura platform is proposed. The sequence- and cyclostratigraphic interpretation has been performed for the transgressive part of the medium-scale sequence Ox6+, independently for each section, with a very high time resolution at the scale of elementary depositional sequences. The good correlation of the elementary and small-scale sequences between the six studied sections and the similar number of elementary sequences in all sections strongly suggest that allocyclic processes were involved in their formation. The hierarchically stacked depositional sequences (small-scale and elementary sequences) result from orbitally controlled sea-level changes with periodicities of 100 and 20 kyr, respectively. Thickness variations in the correlated small-scale and elementary sequences imply variable sedimentation rates, probably resulting from differential subsidence due to the activity of tectonic blocks. The tectonically controlled platform morphology contributed significantly to the general pattern of depositional environments and, combined with high-frequency sea-level fluctuations, created a complex facies distribution in time and space on the Swiss Jura carbonate platform.

Glaciovolcanic landsystems and large-scale glaciotectonic deformation along the Brekknafjöll–Jarlhettur, Iceland

This paper documents the glaciovolcanic landsystem of the Brekknafjöll–Jarlhettur ridge in Central Iceland. Glaciolacustrine diamict is found beneath, and in association with, a complex assemblage of pillow lava, lava breccias and hyaloclastites. Three depositional environments are identified: glaciolacustrine fan, pillow lava dome, and hyaloclastite fan. These subaqueous environments occurred both simultaneously and at different times along the volcanic fissures which underlie the ridge and have given rise to a complex facies architecture. This facies architecture provides evidence that the ridge evolved in a time transgressive fashion during several episodes of volcanism, some of which may have been punctuated by periods of ice erosion. Associated with the ridge are large-diapiric folds in diamict and gravel which form by the loading and lateral displacement of saturated diamict beneath the developing volcanic pile. A depositional model is presented which emphasises the glaciolacustrine component and the time transgressive nature of the glaciovolcanic landsystem. Much of the eruption occurred in subglacial to englacial lakes or vaults, which were probably linked by water and sediment exchange. The initial subglacial vaults appear to have extended beyond the fissure limits and were infilled by glaciolacustrine diamicts, subaqueous outwash and the eruption of pillow lava. This was followed by the eruption of hyaloclastite sand and breccia forming an elongated fan.

The role of the Mediterranean region in the development of sedimentary geology: a historical overview

AbstractThe Mediterranean region, source of so much knowledge in the world, is the site of major advances in sedimentary geology. In addition to its economic and cultural richness, the geological and geographic diversity of the region, plus its active geological processes, have long stimulated indigenous scholars, along with attracting talented outsiders such as Steno, Lyell, Walther, Kuenen and Bagnold. Since classical Hellenic times, debates about the origin of fossils and the changing positions of sea‐level served as catalysts for studies of sediments, sedimentary rocks and ancient life. The presence of geologically young and easily interpreted marine shell beds in many Mediterranean coastal areas adjacent to their modern analogues was a particular stimulus for progress in sedimentary geology, for example, the very advanced stratigraphic ideas of Leonardo da Vinci, expressed solely in his unpublished notebooks. Impeding progress was the geological complexity of facies, faunas and structure in the circum‐Mediterranean Alpine belts. Once the secrets of these were unlocked, however, the Mediterranean region became the source of major discoveries about syn‐sedimentary tectonics, carbonate platforms, pelagic and anoxic sediments, turbidites, evaporites, aeolian processes, cyclostratigraphy, magnetic stratigraphy and impact events. In many of these Mediterranean discoveries, the critical element is the occurrence of extensive Mesozoic‐Cenozoic pelagic successions whose precise age dating was made possible by pioneering biostratgraphic studies using microfossils.

Turbidites and turbidity currents from Alpine ‘flysch’ to the exploration of continental margins

Turbidites and turbidity currents from Alpine ‘flysch’ to the exploration of continental margins

Lithology and stratigraphy of the Ordovician facies complexes of Gornyi Altai

In the Ordovician time, the transform margin of the Gornyi Altai region consisted of two bathymetric stages: (1) shelf and upper parts of continental slope; (2) foothills and lower parts of continental slope. The first stage includes the shallow-water facies complexes (terrigenous and terrigenous-carbonate schlieren and variegated flyschoid), while the second stage is composed of deep-water (black shale terrigenous) and subflysch gray (carbonate terrigenous) complexes. Model series of the facies complexes established in our work should be taken into account during the geodynamic analysis of fold zones.

Facies architecture of a net transgressive sandstone reservoir analog: The Cretaceous Hosta Tongue, New Mexico

Net transgressive sandstones form a significant component of many shallow-marine reservoirs, but their shale-poor character commonly masks complex facies architecture and stratigraphy associated with significant permeability variations that impact reservoir drainage patterns and ultimate recovery. In this article, the controls on net transgressive sandstone reservoir architecture are investigated through a detailed analysis of the Cretaceous Hosta Tongue of the Point Lookout Sandstone (informally termed Hosta sandstone in this article) outcrop in New Mexico. Mapping of facies architecture within a series of adjacent canyons has enabled a quantitative three-dimensional reconstruction of key stratigraphic surfaces and sand body distributions from an updip pinch-out to a downdip pinch-out of the net transgressive sandstone complex.The Hosta sandstone contains a complex arrangement of wave- and tide-dominated facies associations arranged in an overall transgressive pattern. Tidal channel-fill sandstones, tidal sheet-form sandstones, and heterolithic tidal-flat and lagoonal deposits comprise the stratigraphy in the updip part of the system. These deposits pass abruptly downdip into wave-dominated shoreface sandstones. The facies composition indicates that the Hosta sandstone represents a wave-dominated barrier shoreline and a tide-dominated back-barrier lagoon. Facies associations are partitioned both vertically and laterally by a hierarchy of transgressive erosion (ravinement) surfaces cut by wave and tidal processes. Reconstructing the geomorphology and spatial organization of these surfaces is critical to understanding sand body distribution and facies architecture at high-resolution (intrareservoir) scale. The exceptional quality of the Hosta Sandstone outcrops has enabled (1) improved understanding of patterns and controls of facies architecture in net transgressive sandstone reservoirs, (2) construction of predictive templates of facies architecture in interwell volumes, and (3) quantification of geobody dimensions and spatial distribution patterns. In combination, these data provide appropriate qualitative and quantitative conditioning for reservoir models.

The role of microbes in reef-building communities of the Cannindah limestone (Mississippian), Monto region, Queensland, Australia

Reefs in the Cannindah Limestone at Old Cannindah Homestead, Monto region, Queensland, are exceptional in Eastern Australian Mississippian (Carboniferous) build-ups because of their largest dimension and differentiated microbial fabrics. Calcimicrobes and microbial carbonates, which represent a marine reefal environment occupied by both corals and sponges, are particularly abundant in the reef framework fabrics compared to other Mississippian build-ups in the world. They contributed significantly to the rigidity of the reefs on a crinoidal bank setting. Metazoans and calcimicrobes coexisted and played different roles in reef construction. Reef-building and cavity-dwelling microbes include Renalcis, Palaeomicrocodium, Girvanella, problematic Aphralysia, Ortonella, Shamovella-like, Rothpletzella-like, Wetheredella-like, and some problematic calcimicrobes, which occur in inter-corallite infillings of fasciculate rugose corals, in thrombolitic textures, in or within deposits between microdigitate stromatolite and laminated microbialites, and in reef cavities. Some reef intervals are entirely formed by Renalcis, Palaeomicrocodium, problematic calcimicrobes, and cement. Girvanella, as an encrusting calcimicrobe, generally bound bioclasts and micrite, or together with cement, formed boundstone. Microbial carbonates, including thrombolites, microencrusters, microdigitate stromatolite, laminated and tabular microbialite, irregular layers of self-encrusting vesicles, and microbial micrite, occur commonly in reef framestone and boundstone. The role of microbes and relevant microbial carbonates in the Cannindah reef limestone highlighted a significant account of microbial facies complexes associated with the Mississippian reefs.

TRIASSIC-LOWER JURASSIC FORAMINIFERAL INDICES FOR BAHAMAN-TYPE CARBONATE-BANK LIMESTONES, CABLAC MOUNTAIN, EAST TIMOR

Peloidal and oolitic limestones on Cablac Mountain in East Timor contain small calcite-cemented agglutinated and porcelaneous foraminifera that place these limestones in the Triassic or Lower Jurassic, in contrast to the Lower Miocene as previously mapped. An Early Jurassic (Sinemurian-Pliensbachian) age is indicated for some of the limestone by the presence of Meandrovoluta asiagoensis Fugagnoli and Rettori, Everticyclammina praevirguliana Fugagnoli and a palynomorph assemblage. The age of other limestones on the mountain is identified broadly as Late Triassic to Early Jurassic, based on the occurrence of Duotaxis metula Kristan. In basinal facies of the nearby Wai Luli Valley, Gsollbergella spiroloculiformis (Oraveczne Scheffer), palynomorphs, a brachiopod and halobiid bivalves indicate a Late Triassic (Carnian) age for a transported foraminiferal assemblage associated with peloids, ooids and Duotaxis and Siphovalvulina characteristic of carbonate-bank deposits. This occurrence suggests that carbonate banks were developed locally on submerged topographic highs in seas that flooded interior-rift basins in this part of Gondwana and that a complex facies array of deep-water muds, deltaic sands, and carbonate shoals were present in the basins. Taxonomic assessment of Triassic and Early Jurassic species previously placed in Tetrataxis suggests that these are better accommodated in Duotaxis , and some species placed in Trochammina are transferred to Siphovalvulina . The distinction between Duotaxis and Siphovalvulina , as now understood, rests on the shape of adult chambers.

Basic Building Blocks and Process Variability of a Cretaceous Delta: Internal Facies Architecture Reveals a More Dynamic Interaction of River, Wave, and Tidal Processes Than Is Indicated by External Shape

Abstract Although delta-building processes and the resultant facies and facies associations have been studied for over a century, there remains a lack of well-documented facies architectural studies of ancient deltas. In this study, architectural-element analysis is applied to determine the basic building blocks of a Cretaceous delta at the top of the upper Turonian Wall Creek Member exposed along the western flanks of the Powder River Basin, Wyoming. We document the bed-scale basic building blocks of this ancient delta, and show them in the context of their facies and stratal variability, and their hierarchical spatial arrangement within a single deltaic parasequence. We also test the idea that external sand-body shape may not reflect the internal facies complexity, as has recently been suggested in studies of several modern deltas, including the Burdekin in Australia, the Brazos in the Gulf of Mexico, and the Baram–Trusan in Borneo. Five orders of bounding surfaces separate six facies architectural elements in the prodelta and delta front deposits. These elements are prodelta fines (PF), frontal splay (FS), channel (CH), storm sheet (SS), tidally modulated deposit (TM), and bar accretion (BA). Seasonal to decadal river floods are thought to represent the main building phases of the delta, producing channels, bars, and frontal splay elements. During intervening periods, the delta was reworked by waves, storms, and tides, producing tidally modulated and storm sheet elements. Due to the complex interactions of river effluents with waves, storms and tides, the delta is interpreted as mixed-influenced. Regional sandstone isolith, facies, and paleocurrent maps help to reconstruct the paleogeography and show how the various architectural elements varied across the delta front. The channels, bars, frontal splays, and tidally modulated elements are found only near the distributary mouth, whereas the storm sheets occur extensively away from the distributary mouth. The plan-view morphology of the studied delta shows a smooth-fronted, arcuate to cuspate shape, which should indicate wave dominance. In contrast, our analysis of the internal facies architecture shows that the delta was constructed rapidly during major river floods with significant tidal reworking. Additional outcrop mapping shows that storm-wave-reworked sands are attached to the flanks of the system. Previously published delta models predict incorrectly that the tidal reworking of a river-flood dominated system should result in a more bird-foot shape versus the lobate geometry that is actually mapped. Our analysis of internal facies versus external shape matches similar observations on several modern deltas and suggests that external shape is a poor indicator of internal facies complexity.

Depositional environments and an apparent age for the Geci meta-limestones: Constraints on the geological history of northern Mozambique

Strongly contrasting rocks were juxtaposed during the long tectonometamorphic history of the Mozambique Orogenic Belt in northern Mozambique. The latest depositional event was expressed by accumulation of marine sediments, which were subsequently mildly metamorphosed and tectonically juxtaposed with granulite facies complexes. The low-grade metasedimentary rocks comprise the Geci group which was mapped, and studied petrographically, geochemically and isotopically in order to provide constraints on the depositional environments, age and latest history of the Mozambique Orogenic Belt. The group occurs as several large, tectonically dissected, intensively sheared, folded and mylonitised, SW–NE trending lenses within Unango Complex granulite rocks. In places, primary depositional features are well preserved. The dominant rocks are calcarenites, dolarenites, calcite matrix-supported and dolostone clast-supported carbonate breccias forming beds with erosional bases, normal and reverse graded bedding, and well-developed Bouma sequences. Dolomicritic, microbial and oolitic dolostone clasts were apparently derived from the margin of a shallow-water carbonate platform and redeposited by turbidity currents on a continental slope with calcareous sedimentation. The Geci meta-carbonate rocks have low SiO 2 and Al 2O 3 contents. MgO/CaO ratios fluctuate between 0.05 and 0.70 averaging 0.15 ± 0.18 (1 σ, n = 111). Acid-soluble constituents have moderate concentrations of Fe (777 ± 310 ppm), Mn (131 ± 85 ppm) and Sr (566 ± 145). Mn/Sr ratios are relatively low (0.26 ± 0.19), δ 13C and δ 18O values are invariably high, with only moderate scatter: +4.0 ± 0.6% V-PDB and 25.8 ± 0.5% V-SMOW, respectively. The least altered 87Sr/ 86Sr ratio of calcite is 0.70708, whereas dolomite is enriched in 87Sr with the lowest ratio of 0.70730. The least-altered 87Sr/ 86Sr and δ 13C ratios suggest an apparent depositional age of either 595–585 or 630–625 Ma. This provides a lower age limit for juxtaposition of the low-grade Geci group rocks and granulite facies rocks of the Unango Complex.

The reefal margin and slope of a Middle Triassic carbonate platform: the Latemar (Dolomites, Italy)

The Latemar is a mainly aggrading platform, but shows repeated backstepping during its entire development. The behaviour of the slope does not reflect accommodation changes and lateral consistencies of the lagoonal interior; the Latemar contemporaneously reveals different, even contrasting depositional characteristics. The slope of the late stage platform evolution corresponds at least partially to the “base-of-slope apron” model. Controlling factors on slope evolution are of tectonic (proximity of the Stava Line) and autocyclic (repeated oversteepening) nature. Other factors are insignificant and/or overprinted. The reef-facies at Latemar reveals a complex facies pattern; it varies along and across the margin and is rich in encrusting sponges, corals, biogenic crusts and “Microproblematica”. Some biota or fossil assemblages—e.g. foraminifers (Abriolina mediterranea, Turriglomina scandonei) or “Tubiphytes” multisiphonatus thrombolites—have not been described in the Dolomites before. Biostratigraphic evidence from the uppermost reef-facies confirms a mainly Anisian age of the outcropping platform interior.

A multidisciplinary study of middle Miocene seep-carbonates from the northern Apennine foredeep (Italy)

Several pelitic intervals are intercalated at various levels within the marly-arenaceous turbiditic successions of the middle Miocene northern Apennine foredeep. They range in thickness from 30 to 200 m, and represent sedimentation on top of ephemeral structural highs related to blind faults. Sediments are made up of hemipelagites and fine-grained turbidites, and include 13C-depleted carbonates, related to the rising of methane-rich fluids (hydrocarbon seep-carbonates). Large portions of pelitic intervals are involved in chaotic masses by soft sediment deformation (slumps, slides, intraformational breccias), revealing an intense sediment instability during middle Miocene. A stratigraphic, mineralogic and geochemical study was conducted on two of these pelitic intervals (Castagno d'Andrea, Mt Citerna) in order to reconstruct carbonate development, the composition of fluids, and to document the connections between fluid seepage and syndepositional tectonics. This multidisciplinary approach has allowed us to discriminate between the two examined pelitic intervals in terms of age, depositional rates and conditions, organic carbon and post-depositional processes. Seep-carbonates are characterized by chemosymbiotic fossil communities, autoclastic fractures and brecciation; carbonate bodies show complex facies relationships, as indicative of different stages in chemoherm growth. The compositional study evidences the peculiar chemistry of chemoherm carbonates (calcite low in Mg and Sr) compared with carbonates in associated enclosing pelites and with modern chemoherms, in general. The non-carbonate components within the chemoherms are enriched in detrital minerals and depleted in phyllosilicates with respect to the enclosing pelites. The mineralogical changes in the clay component within the brecciated unit of the Castagno d'Andrea chemoherms suggest authigenic precipitation of the Mg-rich phases. Isotopic analyses show the distinct carbon signature in the chemoherms from the two examined intervals (Castagno d'Andrea chemoherms more depleted, from −15.8‰ up to −41.3‰ PDB, than Mt Citerna, from −5.2‰ up to −16.7‰), and the transitional 13C-depletion trend observed moving from chemoherms to the enclosing pelites (moderately depleted) and Te (Bouma sequence) turbidites (in the range of marine carbonates). A slight but significant enrichment in δ 18O (up to +1.4% PDB) is observed for all chemoherms when compared to values of carbonate phases present in enclosing pelites. Geochemical data indicate that the brecciated facies of seep-carbonates are related to an explosive release of gaseous fluids probably associated with the rise of deep hypersaline fluids.

Sedimentary architecture and genesis of Holocene shallow‐water mud‐mounds, northern Belize

Abstract Cangrejo and Bulkhead Shoals are areally extensive, Holocene biodetrital mud‐mounds in northern Belize. They encompass areas of 20 km2 and 35 km2 in distal and proximal positions, respectively, on a wide and shallow‐water, microtidal carbonate shelf where storms are the major process affecting sediment dynamics. Sediments at each mound are primarily biodetrital and comprise part of a eustatically forced, dominantly subtidal cycle with a recognizable deepening‐upward transgressive systems tract, condensed section and shallowing‐upward highstand systems tract. Antecedent topographic relief on Pleistocene limestone bedrock also provided marine accommodation space for deposition of sediments that are a maximum of 7·6 m thick at Cangrejo and 4·5 m thick at Bulkhead. Despite differences in energy levels and location, facies and internal sedimentological architectures of the mud‐mounds are similar. On top of Pleistocene limestone or buried soil developed on it are mangrove peat and overlying to laterally correlative shelly gravels. Deposition of these basal transgressive, premound facies tracked the rapid rate of sea‐level rise from about 6400–6500 years BP to 4500 years BP, and the thin basal sedimentation unit of the overlying mound‐core appears to be a condensed section. Following this, the thick and complex facies mosaic comprising mound‐cores represents highstand systems tract sediments deposited in the last≈ 4500 years during slow and decelerating sea‐level rise. Within these sections, there is an early phase of progradationally offlapping catch‐up deposition and a later (and current) phase of aggradational keep‐up deposition. The mound‐cores comprise stacked storm‐deposited autogenic sedimentation units, the upper bounding surfaces of which are mostly eroded former sediment–water interfaces below which depositional textures have largely been overprinted by biogenic processes associated with Thalassia‐colonized surfaces. Vertical stacking of these units imparts a quasi‐cyclic architecture to the section that superficially mimics metre‐scale parasequences in ancient rocks. The locations of the mud‐mounds and the tidal channels transecting them have apparently been stable over the last 50 years. Characteristics that might distinguish these mud‐mounds and those mudbanks deposited in more restricted settings such as Florida Bay are their broad areal extent, high proportion of sand‐size sediment fractions and relatively abundant biotic particles derived from adjoining open shelf areas.