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In the Kurovice Quarry (southern Moravia), the paleoenvironmental conditions around the Jurassic–Cretaceous boundary were deciphered using geochemical, magnetic, macrofaunal, microfaunal, and ichnologic evidence. In addition to weakly bioturbated horizons, four discernible ichnologic assemblages (IA1–IA4) were distinguished. Geochemical changes were recorded both between individual ichnologic assemblages and within them. IA1 is dominated by Chondrites, Thalassinoides, and Nereites, accompanied by Bergaueria and Phycodes, and occurring in pelagic sediments. IA1 also occurs in an impoverished form, in deposits characterized by a low ichnofabric index (i.e. impoverished IA1). IA2 consists of Trypanites in layers with microbially induced sedimentary structures; it is present in beds 20–21. IA3 is represented by subvertical shafts filled with pyrite and Trypanites isp., found in beds 30 and 63–72. IA4 consists of common Balanoglossites and rare Trypanites, Thalassinoides, and Planolites; it occurs in beds 102–104, 126–127, and 137–140. Four intervals with prominent Balanoglossites (IA4) and occasional Trypanites (IA2) mark probable hardgrounds and/or condensed deposits. These deposits are also characterized by slightly increasing magnetic susceptibility and enhanced biogenic reworking. Enhanced bioturbation/bioerosion commonly concords with higher susceptibility intervals. Geochemically, the beds hosting impoverished IA1 show a slight decrease in the δ13Ccarb followed by an increase in the δ13Ccarb values; this may reflect a general rise in productivity accompanied by organic matter burial. Similar trends in the δ13Ccarb values are also associated with the IA2 and IA3 characterized by the presence of vertical pyrite-filled tubes. Increases in δ18Ocarb values within IA2 and IA3 are interpreted to express cooling episodes suggesting a rise in oceanic productivity. Therefore, this primary isotopic signal reflects nutrient changes within an open ocean, which is not (in this case) influenced by bioturbation activity. IA4 (Balanoglossites) is characterized by an apparent increase in δ18Ocarb values (the trend is similar to that in IA2 and IA3) but δ13Ccarb values are stable. [ABSTRACT FROM AUTHOR]

Trypanites is a common boring in Ordovician hardgrounds of Estonia (Baltica). The depth of the sedimentary basin and sedimentation rates controlled the distribution of Trypanites. The trace-makersâ community was diverse and changing over time. Three ichnospecies of Trypanites can be distinguished: T. sozialis, T. weisei and Trypanites isp. All three morphotypes can be recognized in the same hardground. It is impossible to distinguish between the different ichnospecies based only on the size of the boring aperture. The depth of early lithification of the seafloor determines the morphological variability seen in T. sozialis. The occurrence of elongated borings, such as T. weisei and Trypanites isp., is related to tropical environments, and their trace-makers strongly preferred substrates with a homogeneous and dense texture. The texture and available volume of hard substrate controls the ichnodiversity of Trypanites ichnospecies.

The Upper Cretaceous of central India is represented by a thin transgressive-regressive succession of tropical marine sediments deposited on a Precambrian basement and covered by massive Deccan Trap basalt flows. At the height of the transgression, a few meters thick succession of very thinly and rhythmically bedded, laterally continuous, muddy carbonates of the Turonian–Coniacian Bagh Group was deposited. The planar to wavy beds consist of nodular to massive limestones intercalating with marls. Eroded, bored, and encrusted hardgrounds occur at regular intervals in succession. Field appearance, results of thin-section petrography, and stable isotope signature of micro-sample carbonates suggest that the hardgrounds formed in a supra- to very shallow sub-tidal, restricted marine environment, which was characterised by repeated emergence and soil genesis. Knowing moments of zero-sea level in a regular rhythmically bedded succession with accurate chronostratigraphy allows for better reconstruction of Indian intracratonic basin dynamics during the Upper Cretaceous and its correlation to global events. [ABSTRACT FROM AUTHOR]

Trypanites is a common boring in Ordovician hardgrounds of Estonia (Baltica). The depth of the sedimentary basin and sedimentation rates controlled the distribution of Trypanites. The trace-makers’ community was diverse and changing over time. Three ichnospecies of Trypanites can be distinguished: T. sozialis, T. weisei and Trypanites isp. All three morphotypes can be recognized in the same hardground. It is impossible to distinguish between the different ichnospecies based only on the size of the boring aperture. The depth of early lithification of the seafloor determines the morphological variability seen in T. sozialis. The occurrence of elongated borings, such as T. weisei and Trypanites isp., is related to tropical environments, and their trace-makers strongly preferred substrates with a homogeneous and dense texture. The texture and available volume of hard substrate controls the ichnodiversity of Trypanites ichnospecies. [ABSTRACT FROM AUTHOR]

The term hardground refers to a surface of synsedimentary lithification associated with non-deposition or slow sedimentation. This paper introduces a marine hardground with characteristic microscopic properties in the Pennsylvanian Atoka bank carbonates, which forms a stratigraphic trap for natural gas. The algal bank complex includes several elongated biostromes up to 21 m thick, oriented normally to the paleo-shelf margin. Several lithofacies have been identified in the bank carbonates and surrounding sediments: crinoidal limestone (grainstone/packstone), nodular shaly limestone (wackestone/mudstone), bank margin (grainstone/boundstone), algal bank (wackestone), brecciated limestone, basal bioclastic micritic pile (wackestone), limy shale, and black shale. Phylloid alga, Archaelithopyllum, is the main fossil component in the bank complex and indicates deposition in a shallow marine environment at depths less than 30 m. The other fossils found with phylloid algae such as bryozoans, brachiopods, and echinoderms (sessile benthic suspension feeders) and small benthic foraminifers point out an open shelf environment away from clastic input. The hardground in the carbonates is characterized by an irregularly eroded surface capping well-lithified peloidal grainstone including scattered phylloidal algal plates and overlain by phylloid algal plates and large brachiopod fragments which are extending subparallel to the bedding surface. When the bank complex during the growth approaches to wave base, that increases agitation in the environment and erosion at the upper surface of the banks. Furthermore, agitation leads to the lithification of the peloidal sediments which decreases from the hardground surface into the sediments. Micro-borings penetrate downward into the lithified sediments. The poorly lithified bioclastic sediments with marine internal lime silts in the sheltered pores just above the hardground surface gradually pass upward into algal wackestone and reflect deepening conditions in the environment. [ABSTRACT FROM AUTHOR]

Upper Ordovician hardgrounds display a spectrum of complexity reflecting a range of local to global-scale processes. Hardgrounds are cemented seafloor surfaces typically marked by the presence of encrusting taxa and borings. Many hardgrounds show evidence for successive episodes of colonization by hard substrate specialists and are associated with localized evidence of seafloor erosion such as overhangs and reworked concretions. They commonly show trace amounts of pyrite and dolomite cements indicating an association with sulfate reduction. The most widespread hardgrounds are highly complex and unravelling their history provides insights into global biogeochemical events. The Curdsville and Kirkfield hardgrounds in the Appalachian Basin (Kentucky and Ontario) represent relatively simple end members of the hardground spectrum. They covered 10s to 100s km2 and formed relatively quickly during the early Katian. They display both planar to subplanar and hummocky to topographically complex surfaces (cm-scale) and contain highly diverse encrusting echinoderm faunas. Study of these surfaces yields important insights into the evolutionary history of encrusting communities. By contrast, the slightly younger hardground at the top of the Galena Group (Ka1) is a surface that is present throughout most of the Midcontinent Basin (>7.5 à 105km2). It is an example of a highly complex surface that was repeatedly modified by erosion and mineralization. Near the eastern margin of the basin in Indiana, the capping Galena hardground is pinnacled with cavity-filling sharpstone clasts, phosphate grains and bored crusts, iron ooids, and pyritic impregnated surfaces. It is onlapped by graptolitic shales of the Kope Formation (Fm) (Ka1) indicating an unconformity of approximately 1 m.y. To the west, in Illinois, the Kope Fm is erosionally truncated and the hardground is directly overlain by graptolitic shales of the Waynesville Fm (Ka3), where the unconformity expands to nearly 4 m.y. Toward Iowa, the hardground is onlapped by meters of phosphorite. Taken together, these observations reveal that the capping Galena Group hardground reflects a complicated history of repeated subaerial exposure, karsting, and marine flooding by a dysoxic to anoxic water mass with fluctuating redox conditions, similar to the age equivalent hardground at the base of the Fjäcka Shale in the Baltic Basin. Thus, hardground studies provide important insights for resolving the temporal continuity of the Upper Ordovician rock record and unravelling processes that controlled carbonate precipitation and dissolution and the evolution of sea floor communities. Some simple hardgrounds may have formed through random exhumation of cemented sediments on the sea floor through the effects of storm scour. However, their clustering into certain portions of the Upper Ordovician suggests that processes that affected sea water chemistry may also be involved. The most complex surfaces reflect major environmental perturbations with large amplitude sea level oscillations and redox changes that in some cases generated rare-earth enriched phosphorites.

Thrombolites and ichnofossils are well‐preserved in a carbonate succession of the Cambrian (Miaolingian Series) Zhangxia Formation, south North China Block. This provides a chance to investigate the factors controlling the distributions of thrombolites and metazoans. Three types of thrombolites are identified, on the basis of clot shape and their array styles, namely, bushy, columnar, and blocky thrombolites. Bushy thrombolites occur in the lower part of the study unit, columnar thrombolites in the middle, and blocky thrombolites in the upper. Thrombolitic intervals alternate with bioturbated limestones, with the latter including two ichnogenera: Balanoglossites triadicus Mägdefrau, 1932 and Sulcolithos variabilis Knaust, 2019. Both thrombolites and bioturbated limestones are covered by oolitic limestones. The facies analysis indicates depositional environments from deep to shallow subtidal, then to oolitic shoals upwards. Such a sequence represents an overall shallowing‐upward trend. Thrombolites generally flourished during transgressions, whereas bioturbated and oolitic limestones represent regressive systems. Thus, relative sea level was a main factor controlling the alternations of thrombolites and ichnofossils in the Zhangxia Formation. Although there is no clear evidence that bioturbation can directly destroy thrombolites, exploration of new habitats by benthic animals and the increasing mixing degree of substrates could have destroyed the previous firmground substrate required for thrombolite development. The alternations of thrombolites and bioturbated limestones demonstrate that ecological conditions frequently fluctuated in the neritic environment during the Middle Cambrian. [ABSTRACT FROM AUTHOR]

Abundant Trypanites isp. associated with Gastrochaenolites isp. and Petroxestes pera are evidenced in the first found hardground of the late Albian marine transgressive series, overlying the Continental Intercalaire in the Ouled Nail Range, eastern Saharan Atlas. This hardground is encrusted by oyster shells belonging to Ostrea falco Coquand. The studied hardground developed in warm environment and displays a lateral extension of at least 6 km. It is interpreted as an erosional transgressive surface. The ichnogenus Petroxestes is reported for the first time from Africa. This ichnoassemblage is attributed to the Trypanites ichnofacies. [ABSTRACT FROM AUTHOR]

Distinctive erosional and omission surfaces occur at several stratigraphic levels in the tidal carbonates of the 3rd-order Coniacian sequence (Iberian Basin). They are ancient analogs of omission surfaces developed on lithified carbonates in subaerial and coastal settings. Omission surfaces consist of (i) firmground Glossifungites ichnofacies (Balanoglossites - Thalassinoides); and (ii) hardground Trypanites ichnofacies (scalloped and planar surfaces and Gastrochaenolites - Entobia surfaces). Firmgrounds are also related to erosion or ferruginous crusts. Hardground surfaces are related to bioerosion, dissolution and physical erosion. Grain size and textural features in Balanoglossites and Thalassinoides firmground surfaces are essentially the same, suggesting that even bathymetry could be similar. Several stages in hardgrounds consist of different, scalloped or planar surfaces related to bioerosion, dissolution and physical erosion. Gastrochaenolites - Entobia borers represent a major change in the trace fossil associations and imply different processes in their origin, being originated at slightly different depths with Gastrochaenolites representing shallower environments. The studied field sections display a cyclicity on the scale of meters that tentatively reflects the presence of 4th-order parasequence sets. Two kinds of sedimentary discontinuities have been used for correlation: omission surfaces and ferruginous crusts representing regional sea level falls and rises. Part of the described surfaces does not appear to have been previously recognized in older carbonate deposits. Their common presence of similar surfaces along modern coasts and in karst terrains, as well as their abundance in the Coniacian sequence, suggests that they might also be abundant in the geologic record in other sedimentary basins for defining palaeoshorelines. • Coniacian erosional and omission surfaces in tidal carbonates of the Iberian Ranges • Firmground Glossifungites ichnofacies (Balanoglossites-Thalassinoides) • Hardground Trypanites ichnofacies, scalloped surfaces, Gastrochaenolites-Entobia • Hardground related to bioerosion, dissolution and physical erosion • Omission and ferruginous surfaces as parasequence boundaries [ABSTRACT FROM AUTHOR]

Hardground discontinuities within carbonate platforms form important stratigraphic surfaces which can be used at basin scale to correlate sequence boundaries. Although these surfaces are commonly used in sequence strati‐graphy, the timing and duration of hardground lithification and the crystallization of early cements remain unexplored. Here, early calcite cements were dated by U‐Pb geochronology for five Jurassic hardgrounds, interpreted as third‐order sequence boundaries, situated within a well‐constrained petrographic, sedimentological and stratigraphic framework. The consistency or the slight deviation between the age of the cements and the stratigraphic age of deposition of the formations illustrate that cementation occurred early in the diagenetic history. The ages obtained on dogtooth cements, replacing aragonite in gastropod shells and pendant cements in intergranular spaces, match those of the standard Jurassic biostratigraphic ammonite Zones, making calcite U‐Pb geochronology a promising method for dating third‐order sequence boundaries of depositional sequences and refining the Jurassic time scale in the future. [ABSTRACT FROM AUTHOR]

Based on examination of the collection of stemmed echinoderm holdfasts from the Ordovician of the Leningrad Region (Russia), all holdfasts are divided in two large groups: "unbranched" and "branched". In total, 11 morphotypes of unbranched holdfasts are identified and described in detail. [ABSTRACT FROM AUTHOR]

Lithofacies, sedimentologic and x-ray analytical techniques were employed to reconstitute paleodepositional environments, bounding surfaces and basin-fill model of Mundeck Formation during Lower Cretaceous in Yabassi, NE Douala sub-basin. The basin-fill comprises fissile gravel, sand and mud (GSF) facies from which eighteen sub-facies were identified and grouped under five facies associations (FA1-FA5). These correspond to alluvial settings (FA1), braided channels (FA2), cross-bedded tidal channels (FA3), ephemeral fluvio-lacustrine lake with short-lived marine incursion (FA4), fluvial channels with deeply incised erosional surfaces (FA5). Four hierarchical lithostratigraphic bed bounding surfaces were defined; incipient or intraformational hardgrounds, erosional surfaces abutted by ripped-up mud clasts, an obvious transgressive surface marked by the occurrence of ammonitic in lamellibranch and gastropod-rich silty-mudstones and shales. Humid and warm climates during Lower Cretaceous could be inferred from secondary mineral suites: clays (kaolinite, chlorite, illite and smectite); oxides (hematite and hydralsite); and oxy-hydroxides (goethite). Hematite-rich incipient hardgrounds surfaces depict warm and dry climates, while paleosols suggest channel abandonment, subaerial exposure and pedogenetic processes. Sediment transport regimes have evolved from low velocity cohesive flow regime in braided channels and overbank to high velocity and cohesion-less flow regime in multi-storey cross-bedded sand bars. Granulometric results reveal angular to sub-rounded, coarse-grained to gravelly clast sizes with unimodal, bimodal- and trimodal distributions, whereby all analyzed samples are positively skewed towards fine-grained sands. Morphoscopic analysis reveals 65–90% of very angular to angular and shinning quartz (VAAS), compared to 10–35% sub-angular and sub-rounded (SASR) and 0% rounded (R) shapes which indicate short transport distance from source areas. Sorting index (So) and deviation quartile (dQ) values of analyzed sands vary from 1.52 to 2.89, and 0.38–0.88 respectively and are indicative of moderate to very poor sorting. • Five lithofacies associations were identified in Yabassi, NE of the Douala sub-basin and (FA1-FA5) that range from alluvial, braided channel, tidal fluvial channel and ephemeral lake setting subjected to short-lived marine incursion respectively. • The basin-fill model is hierarchically bounded by hardgrounds, paleosols and erosional surfaces butted by either ripped-up mudstone clasts, or reworked hardground, and a transgressive surface marked by ammonites. • Granulometric analytical results reveal coarse-grained to gravel clast sizes with uni, bi- and tri-modal distributions, whereby all analyzed samples are skewed towards fine-grained fraction. • 65–90% of very angular to angular and shinning quartz (VAAS), compared to 10–35% sub-angular and sub-rounded (SASR) and 0% rounded (R) shapes in analyzed granulometric fractions are indicative of short transport distance from source areas. • Sorting index (So) and deviation quartile (dQ) values of analyzed sands vary from 1.52 -2.89, and 0.38–0.88 respectively, indicative of moderate to very poor sorting. [ABSTRACT FROM AUTHOR]

High-permeability zones (HPZ) are common occurrences in carbonate reservoirs of the Middle East and act to improve productivity in primary depletion but lead to water breakthrough in secondary depletion. Bioturbation aggravated the reservoir heterogeneity of the Khasib Formation carbonates in AD oilfield, Iraq, resulting in the development of HPZ. The observations of cores and thin sections, measurements of porosity, permeability, capillary pressure and probe permeability were conducted to investigate the characteristics and genesis of the HPZ. Well log responses of the HPZ in cored wells were analysed to identify the HPZ in uncored wells and predict its distribution. Results demonstrate that the HPZ is demarcated by Glossifungites ichnofacies and Thalassinoides ichnofabric. The burrow systems filled passively with overlying sediments that were apparently different from the unbioturbated matrix, in terms of the lithologic associations, pores types, and pore connectivity. The burrow systems, which contribute to the high permeability of the HPZ, were merely composed of grainstone with well-connected interparticle pores and vugs. The matrix comprises packstone and hardground-associated strongly cemented grainstone, which developed some fractures and exhibited poorly connected pores, including intraparticle, moldic, and residual interparticle pores. The arithmetic mean permeability of the HPZ is 241.0 mD, which is much higher than that of the non-HPZ (17.3 mD). The entry pressure and pore throat radius of the burrows in HPZ is 0.02–0.05 MPa and 5–20 μm, respectively, which reveal the high-quality pore structure of the burrow systems. There is strong heterogeneity within the HPZ. The permeability of burrow systems could reach up to 10133.5 mD with an arithmetic mean permeability of 712.2 mD, while the permeability of the matrix is only 0.05 to 69.29 mD, with an arithmetic mean permeability of 8.7 mD. Interconnected burrow networks in the HPZ were formed by bioturbation. The efficiency of connectivity of the HPZ was further enhanced by the early dissolution occurred within burrow architecture and matrix in the meteoric freshwater environment. These resulted in the formation of the high-permeable network composing of interparticle pores, vugs, and the dissolution-enlarged fractures. The high-permeable burrow systems and low-permeable matrix jointly contributed to the distinct well log responses, which are markedly different from the conventional shoal HPZ. The bioturbated HPZ can be identified by medium natural gamma (GR), high bulk density value (RHOB), low acoustic transit-time value (DT), low compensated neutron (NPHI), high deep induction value (RILD), and high microspherically focused resistivity value (MSFL). A well log-model based three cutoffs, namely RHOB nor /DT nor , RILD*MSFL and GR, was built and shows good prediction of the HPZ. • Bioturbation-associated high-permeability zones are systematically characterised in all aspects. • The development of the HPZ is jointly controlled by bioturbation and early dissolution. • Well log responses of the bioturbated HPZ is synthesised responses of burrow and matrix. [ABSTRACT FROM AUTHOR]

Abstract: The cold-water Volkhov and warm-water Keila hardgrounds of the Ordovician Baltic Basin represent a specialized product of cyanobacterial communities of the seafloor, another member in a group containing stromatolites and microbially induced sedimentary structures. The leading role of cyanobacterial communities in the development of hardgrounds is suggested by their macrofeatures, structure, and elemental composition revealed by SEM equipped with a microprobe microanalyzer. During the formation of the hardgrounds, the cyanobacterial films were of considerable thickness (up to 4-5 mm), but lacked vertical zonation of bacterial communities, thereby being at an intermediate stage between the mature biofilms and true mats. The soft substrate underlying hardgrounds was inhabited by an abundant infauna. Thanks to bioturbation, the cyanobacterial film was in places destroyed, which prevented the development of hardgrounds. In the warm Late Ordovician basin of northern Estonia, cyanobacterial films formed hardgrounds on the surface of ripple marks. Abundant peloids and warm pore waters facilitated further cementation and hardening of the substrate underneath the hardground before the burial of the ripple marks and the hardground. Cyanobacterial films appeared simultaneously over large areas of the seafloor due to settling of planktonic bacteria. The extracellular polymeric substances (EPSs) released by these bacteria were rapidly mineralized within a maximum of one season, apparently at a depth of 15-25 m, below the fair weather wave base. The formation of hardgrounds in bioherms occurred in a shallower environment and was patchy due to the alternation of living and dead cyanobacterial mats, mineralizing mats and resulting hardgrounds. Inhabitants of biohermal hardgrounds were producers of carbonate detritus, including of micritic size, over vast areas of the seafloor surrounding the bioherm. [ABSTRACT FROM AUTHOR]

Jurassic-Lower Cretaceous rocks in the Kösrelik and Kösrelikızık areas (North of Ankara, Central Turkey) which were deposited in a tectonically controlled basin have outcrops in a very restricted area but they carry very important evidences for elucidating the Late Jurassic paleogeography of the Ankara region subjected to heavy tectonic activities during Hettangian-Oxfordian, especially during Sinemurian. Detailed facies analysis of sequences in the study area indicates that the deposition took place in two different tectonically controlled depositional settings, 3 km apart from each other. At the east of the study area, the succession is represented by Hettangian-Sinemurian aged basin sediments lying on the basement rocks with angular unconformity, but succession at the west comprises sediments of Oxfordian pelagic carbonate platform (PCP) overlying the basement rocks with angular unconformity. The PCP sediments were overlapped by radiolaria bearing pelagic limestone, Oxfordian-early Cretaceous in age, under relatively calm environmental conditions. Anomalies in C and O isotopes, trace elements and rare earth element values coincide with the periods of fast changes in the environmental conditions. The sedimentological and geochemical data indicated that the area was deepened and shallowed several times until early Cretaceous. [ABSTRACT FROM AUTHOR]

El presente trabajo estudia los diferentes tipos de sílex que aparecen en la Formación Calizas de Oriñón (Aptiense superior-Albiense inferior), en un zona concreta de la costa oriental de Cantabria (Liendo-Castro Urdiales), debido a su gran abundancia y espectacularidad. Las rocas que los incluyen son principalmente biocalcarenitas (wackestones/packstones de pellets, fragmentos de equinodermos, ostreidos, foraminíferos y espículas calcáreas o silíceas calcificadas), siendo el ambiente de depósito marino de plataforma abierta, y existiendo un hardground regional, donde las silicificaciones son diferentes. La fuente de la sílice proviene de la calcitización o disolución de las espículas de esponjas silíceas. Se determina que la gran mayoría de sílex (mosaicos de cuarzo micro-microcriptalino y calcedonita principalmente), se forman por la silicificación selectiva de estructuras biogénicas, siendo las galerías de Thalassinoidespreferentemente silicificadas, debido a que en su relleno existe mayor proporción de materia orgánica y mayor porosidad y permeabilidad. En las calizas que soportan el hardground además de las silicificaciones selectivas sobre fósiles (rostros de belemnites, ostreidos y equínidos), la sílice se acumula de forma importante en huellas de alimentación y rellenos de perforaciones, dando lugar a su vez a costras sobre bioturbaciones y perforaciones que a pesar de su importancia no han podido ser clasificadas. La silicificación de todas las estructuras biogénicas, originó inicialmente fases opalinas y tuvo lugar durante la diagénesis temprana cuando la oxidación de la materia orgánica estaba todavía activa, conservándose en los Thalassinoides silicificados formas (filamentos y cocoides) que podrían ser consideradas microbianas. La transformación ópalo-cuarzo fue temprana, preservándose dichas formas. En las galerías silicificadas de Thalassinoides los procesos diagenéticos son diferentes a los de los sedimentos carbonáticos marinos que las incluyen, y al resto de estructuras biogénicas silicificadas, existiendo en estas galerías neoformación de dolomita y calcita, en un microambiente cerrado donde las condiciones cambian de oxidantes a reductoras.

A attachment structure, attributable to the sphaeronitid diploporitan Finitiporus boardmani Frest and Strimple, is documented herein from a hardground surface within the well-known middle Silurian (Wenlock) Massie Formation at the Napoleon quarry of southeastern Indiana, USA. This attachment structure is composed of multiple elongate plates lacking macroscopic pores, terminating in a slightly expanded disc. No portion of the theca is articulated to the specimen, though the morphology of the attachment structure is sufficiently distinctive to permit confident identification. The holdfast was dislodged from its attachment substrate but is present in the area immediately surrounding a bryozoan-micrite microbioherm, suggesting that F. boardmani occupied such elevated, hard environments. As the densely encrusted hardground containing this specimen has been partially destroyed since the initial report on pelmatozoan holdfasts, documentation of this occurrence is important for accurately recording of the total diversity and environmental distribution of echinoderms at an important locality. [ABSTRACT FROM AUTHOR]

The sedimentary succession in central Poland records significant changes in facies at the turn of the Planula and Platynota zones in the Upper Jurassic, expressed by the drowning of the ramp-type platform and development of an extensive isochronous marl horizon. The topmost level of the marl horizon is a regional hardground, which is interpreted as the third-order sequence boundary Kim 1. In some areas, the hardground was eroded and is only preserved as bored and encrusted clasts. The composition of the borings may indicate that colonization and recolonization of the clasts took place in an extremely shallow water environment. The overall low level diversity of clast-encrusting organisms and their occurrence on both sides of clasts indicates frequent overturning and high current activity. However, other extrinsic factors, such as salinity fluctuations, may have been involved. The final redeposition and burial of the clasts were related to subsidence through widespread reactivation of Paleozoic faults. Comparison with Middle Oxfordian-Lower Kimmeridgian sequences of central and southern Poland indicates that the carbonate ramp morphology and paleoenvironmental conditions were periodically subjected to significant modification during phases of extensional tectonics, with the development of fault-controlled intra-platform ridges and basins. These features may have been the NE continuation of parallel swells and basins developed in the southern part of the carbonate platform adjacent to the Tethys. [ABSTRACT FROM AUTHOR]

Large plug- or slightly amphora-shaped borings have been found in the hardground marking the boundary between Early and Middle Ordovician rocks in northeastern Estonia. These borings cut large bioclasts of the trilobite Megistaspis and cannot be assigned with certainty to any known ichnotaxon. They indicate that the diversity of early borings may have been greater than was recognized previously.

The Tortonian sedimentary succession of Menorca Island (Balears, Spain) includes two stratigraphic sequences: the lower Tortonian distally-steepened carbonate ramp and the coral reef complex of late Tortonian-early Messinian age. The boundary between the two sequences corresponds to Heterostegina -rich sediments in the western part of Menorca, and to a phosphatic hardground in the eastern part of the island. In the field, the hardground is represented by a dark-brown layer and a rough surface on top of the Tortonian Ramp deposits. Dark phosphate precipitates infill cracks and borings in the underlying limestone to a depth of up to 50 cm. The phosphatic hardground is overlain by planktonic rich wackestone to packstone, which penetrates downward into the fissures, borings, and vugs. This deposit is characterized also by grains of reworked phosphate, glauconite grains, small benthic foraminifers and is followed by tabular beds of bioclastic packstone to wackestone with abundant thin crusts of coralline algae. Based on the Amphistegina tests shapes and red algal assemblage of sediments below and above the hardground, and the lateral correlation with Heterostegina deposits, the bathymetry for the development of the phosphatization is estimated less than 100 m. The limited localization of the phosphatic hardground at the eastern side of the ramp indicates that a dynamic upwelling of deeper and nutrient-enriched waters favored development of a phosphatic hardground. This upwelling episode is representative of the paleoceanographic settings induced by the climate conditions of the Mediterranean area during the Tortonian. [ABSTRACT FROM AUTHOR]

Carbonate hardgrounds are lithified seafloors formed by synsedimentary cementation of carbonate sediments, which dominantly occur during the period of calcite seas. The earliest typical Phanerozoic hardground known until now was reported from the Furongian of USA, which was suggested to indicate onset of the early Paleozoic calcite sea period. In this study, we report hardgrounds from the early and middle parts of the Cambrian Series 3 (Stage 5 and Drumian) of the North China Platform, which predate previously reported hardgrounds. The hardground surfaces developed on oolitic grainstone, oncolitic wackestone, and microbialite (thrombolite and dendrolite), which sharply truncate the underlying deposits. The radial fibrous calcite cements between the carbonate grains below the hardground surfaces indicate that the cements formed by early marine cementation. EPMA analysis reveals that the fibrous cements typically consist of low-Mg calcite. The hardgrounds are sometimes encrusted by microbialites and coated by hematite, suggesting long exposure to the open seawater after formation of the surface. In addition, detailed review on the sedimentological studies of Cambrian Series 3 to Furongian deposits throughout the world reveals that there may be several other hardgrounds during these times, which could have been overlooked. The abundant occurrence of hardgrounds in Cambrian Series 3 deposits suggests that the general paleoseawater chemistry was suitable to induce synsedimentary cementation of low-Mg calcite, implying that seawater chemistry would have changed from the aragonite to calcite seas during the Cambrian Series 3 or even earlier period. Metazoan encrustors and macroborers possibly could not have adapted to the newly appeared substrate condition yet, until the latest Cambrian Series 3. [ABSTRACT FROM AUTHOR]

During the Middle-Late Berriasian, a long-term climatic and eustatic change occurred, documented in the literature. However, data from the northern Gondwana paleomargin are scarce. This research analyzes the Lamoricière Clay Formation at the Ouled Mimoun section, focusing on fossil assemblages and using taphonomic and ichnological aspects to interpret a transgressive-regressive cycle. The section starts with mudstones and oolitic grainstones representing shallow-water environments in the top part of the Ouled Mimoun Marly Limestones Formation (Upper Tithonian p.p. to lowermost Berriasian). The base of the Lamoricière Clay Formation is characterized by a high clay content but was still deposited in shallow water, as indicated by the record of the ostracod Asciocythere, dasyclad green algae, and the sponge Cladocoropsis. The subsequent record of fossil-rich calcareous beds at the beginning of the Upper Berriasian (Boissieri Zone) with ammonoids and calpionellids is congruent with an increase in water depth. The sedimentation rate in the Late Berriasian was reduced, as indicated by the increment of fossil remains and trace fossils. Ammonoid moulds show taphonomic features pointing to long-lasting exposure on the sea floor prior to burial with corrasion and encrustation by sessile organisms such as serpulids, thecideidinids, and bryozoans. During calm periods, crustaceans and worms intensely burrowed the sea floor. The record of Thalassinoides and Rhizocorallium indicates bottom conditions ranging from soft to firm. The low sedimentation rate and sediment by-passing probably favored early lithification. The increasing carbonate content as well as decreasing sedimentation rate is compatible with the maximum distance to emerged areas during maximum flooding. High-energy events, probably related to storms, favored the exhumation and extreme corrasion of ammonite moulds and trace fossils. In the resulting substrate, limonitic films developed and encrusting organisms proliferated (serpulids, bryozoans, and thecideidinids), colonizing both the bottom surface (hardground) and exhumed moulds of ammonoids and Thalassinoides. The uppermost 0.7 m of the section represents the return to shallow conditions, with increasing sedimentation rate and terrigenous detrital content, along with the disappearance of hemipelagic forms (ammonoids); hence it is interpreted as having developed at the beginning of a regressive context. [ABSTRACT FROM AUTHOR]

Calcareous nannofossil biostratigraphy has been performed on tree sections cropping out in Eastern Sardinia (Orosei Gulf, Mt. Albo). Calcareous nannofossils are rare to few and poorly to moderately preserved. Nevertheless thirteen bioevents have been recognized (S’Adde valley section) and a Late Bathonian-Early Tithonian age is derived for the S’Adde Limestone (Lms.). The inferred age constraints, integrated with data from the literature, allow the revision of the S’Adde Lms. chronostratigraphy, and the formalization of the S’Adde Limestone (Dieni & Massari 1985) as a lithostratigraphic unit. Qualitative evaluations of carbonate production/sedimentation rates for the north Mt. Albo area are proposed: the Late Bathonian-Callovian and Oxfordian were times of pronounced reduction of carbonate production/exportation, in agreement with the European passive margin evolution, also affected by starvation fenomena and condensations. A Middle-Late Jurassic basin-and-swell setting related to regional tensional tectonic activity is reconstructed for the north Mt. Albo area. The comparison of new and literature data allows framing the local and Eastern Sardinia passive margin evolution in the broader geodynamic and paleogeographic context of the southern European margin.

Abstract: Lagoonal carbonates of Kimmeridgian age in NW Switzerland formed in a tropical epeiric sea and exhibit indicators of subaerial emergence such as tidal biolaminites, desiccation cracks, flat pebble conglomerates, and fenestral structures. Additionally, 30 dinosaur tracksites from at least six stratigraphic intervals indicate repeated formation of land bridges between the platform and adjacent massifs. Populations of herbivorous and carnivorous dinosaurs necessitate substantial vegetation and consequently a soil cover, but no striking evidence of paleosols have been found within the track bearing sequences. However, two stratigraphic levels exposed in nine outcrops exhibit distinctive hardgrounds that experienced early meteoric diagenesis during times of emergence. Initial induration began with the precipitation of thin micritic meniscus cements. Contemporaneously meteoric waters completely dissolved all aragonite leaving large voids. Iron-hydroxides were precipitated in the uppermost 7–10cm where they impregnated the porous micrite and constituted a separate cement generation, which caused a penetrative dark red staining. These aspects are suggestive of the formation of Mediterranean-type red soils on the emerged platform. During transgression, the soil was eroded and wave-cut hardgrounds were superimposed on the hardgrounds formed during regression. Erosion is evidenced by micro-karst, truncated Gastrochaenolites isp. borings and the physical removal of steinkerns. The eroded material including floral remains provided surplus nutrients leading to eutrophication. This food-rich environment supported the mass-occurrence of gastropods in the nerineoid limestones and oysters (Nanogyra sp.) in the Virgula Marls. [Copyright &y& Elsevier]

Detailed microstratigraphy and geochemistry of a single-layered phosphatic pebble from the Southern Rocky Plateau at the Head of the Cape Canyon on the western margin of South Africa are utilized to reveal a complex interplay between deposition, erosion and authigenesis in the Miocene. The phosphatic pebble has three micrite and carbonate fluorapatite cemented layers separated by Fe-rich hardground surfaces. The layers contain a diverse grain assemblage of reworked glauconite, phosphorite, detrital quartz and biogenic carbonate. Overlapping Sr-isotope derived ages of point-drilled samples from the three layers range from 19·1 to 17·6 Ma, although a single point near the outer edge of the uppermost layer has a Sr age of 12·8 to 10·4 Ma, and a reworked bivalve shell fragment from the middle layer has a Sr age of 26·3 to 24·6 Ma. The microstratigraphy of the phosphatic pebble provides evidence of multiple Baturin cycles, where periods of deposition and carbonate fluorapatite authigenesis are followed by reworking, erosion and precipitation of iron oxide hardground surfaces. The condensed microstratigraphy of the pebble is consistent with the regional depositional history of the area and indicates significant fluctuations in the depositional environment related to changes in sea-level, sediment supply and upwelling along the western margin of southern Africa during the early to middle Miocene. [ABSTRACT FROM AUTHOR]

The Middle Bathonian to Middle Oxfordian interval in the Eastern External Subbetic (Betic Cordillera, SE Spain) is characterized by Ammonitico Rosso facies including various stratigraphic breaks. Five hardground-bounded units are recognized in relation to hiatuses in the ammonite record at the following stratigraphic boundaries: Hg1 (Lower-Middle Bathonian), Hg2 (Middle-Upper Bathonian), Hg3 (Lower-Middle Callovian), Hg4 (Middle-Upper Callovian), and Hg5 (Callovian-Oxfordian). Interesting features of these hardgrounds include their microfacies, ferruginous crusts and macro-oncoids, taphonomy of macroinvertebrates, trace fossils, neptunian dykes, and the hiatuses associated with each of them. The main hardgrounds (Hg1, Hg2, and Hg5) contain trace fossils of the Cruziana and Trypanites ichnofacies as well as abundant fossil macroinvertebrates with taphonomic features evidencing corrasion, early diagenesis, and reworking, indicating substrate evolution from softground to hardground. Neptunian dykes affected the trace fossils and ammonoid moulds, and their walls and the hardground surfaces were colonized by ferruginous microbial crusts. These features are characteristic of the External Subbetic pelagic swells, where the absence of sedimentation, sediment bypassing and erosion, and early diagenesis during relative sea-level falls produced hardgrounds. The neptunian dykes are indicative of tectonic activity in the areas of pelagic swells. Ferruginous crusts and macro-oncoids developed only on hardground surfaces and neptunian dykes walls prior to deposition of condensed bioclastic beds, which are interpreted as the first deposits after hardground development and are related to the onset of transgression. The varying ranges of the gaps as well as lateral facies changes are related to different local paleobathymetry controlled by the activity of listric faults. [ABSTRACT FROM AUTHOR]

Abstract: Taphonomic analysis of cephalopod assemblages, together with analyses of ichnologic and sedimentologic features, are useful tools in the interpretation of the history of unconformities in Jurassic pelagic swells. The External Subbetic presents a complex evolution during the Middle Bathonian–Middle Oxfordian interval that includes hardgrounds and condensed levels rich in ammonoids. Bathonian and Callovian hardgrounds, separated by a condensed calcarenitic bed, can be differentiated. In the Bathonian hardground, ammonoids are preserved with unfilled phragmocone chambers, as a consequence of initial fast burial. Neomorphic calcite in the ammonoids is related to early diagenesis, indicating a stop in sedimentation. The ichnological features testify to an early diagenesis and progressive cementation of the sea-bottom, which is corroborated by the record of neptunian dykes. Subsequently, sediment by-passing and erosion produced reorientation of the ammonoids and belemnite remains and exhumation of moulds, during which faceting and reelaboration took place. Finally, the sedimentation began resulting in the deposition of a condensed calcarenite bed with Fe–Mn macro-oncoids. The Callovian hardground developed at the top of the calcarenite bed, where the cephalopods show signs of long exposure on the sea-bottom such as colonization by chemosynthetic microbial communities and sessile foraminifera. The study proves that the Middle Bathonian–Middle Oxfordian represents a time interval of strongly reduced and highly discontinuous sedimentation, interrupted by periods of non-deposition and erosion. [Copyright &y& Elsevier]

Abstract: A condensed hemipelagic limestone unit with glauconite and phosphate separates a drowned Late Burdigalian carbonate platform from overlying Langhian pelagic mudstones, marls, and calcarenitic tempestites within the Neogene Manavgat Basin of southwestern Anatolia, Turkey. The unit consists of coeval lenticular limestone bodies, between 1 cm and 15 m in thickness, ranging from 10 m to 3 km in lateral extent. The P2O5 content of this limestone ranges from 0.2 to 10% by weight and the iron-oxide, clay, and other metal-oxide contents of this unit substantially exceed those of the underlying and overlying rocks. This condensed stratigraphic unit was deposited on the southwesterly outer shelf portions of drowned horsts that directly faced upwelling currents. The unit displays three main facies: 1) glauconitic phosphate crusts associated with hardgrounds; 2) bedded glauconitic-phosphatic limestones; and 3) glauconitic-phosphatic limestones interbedded with resedimented limestones. Facies (1) forms the thinnest units whereas facies (3) is the thickest, since the thickness and spatial distribution of this facies are related to environmental contrasts caused by block faulting of the underlying carbonate platform. This lithostratigraphic unit of hemipelagic glauconite–phosphate deposition represents an interval of maximum flooding in tectonically active situations and depositional hiatus on submarine highs that separate the carbonate transgressive systems tract from the overlying terigenic tempestite highstand systems tract. This appears to be the sole condensed unit within the Miocene Manavgat Basin; and is here ascribed to a third-order (2.3) eustatic rise in sea level comprising the maximum flooding of the first-order cycle. This is also the time for early–Mid Miocene major changes in Mediterranean climate from tropical to temperate. [Copyright &y& Elsevier]

Abstract: Negative excursions in δ 13C profiles from platform carbonates that coincide with pyritised hardgrounds are commonly linked to subaerial exposure events, but we show here that they can also result from subsurface bacterially-mediated early cementation in addition to the precipitation of syndepositional marine cements. A non-soil-derived origin for δ 13C-depleted micrites offers an alternative origin and bathymetric interpretation for these surfaces. The Lower Cretaceous Lekhwair Formation platform carbonate successions from offshore Abu Dhabi contain abundant hardgrounds that are important for both regional correlation and control of subsurface flow. The micrite from these hardgrounds have average δ 13C values of +0.7‰; 1.5‰ lower than non-hardground micrites that are similar to contemporary open ocean values. Hardground δ 13C values are due to the addition of 13C-depleted carbonate, generated as a by-product of sulphate reduction, to the ‘normal’ marine calcite that caused hardground lithification. Pyritisation of the hardgrounds occurred before, during and after ‘normal’ calcite precipitation. The persistence of sulphate reduction after hardground lithification is shown by the presence of pyrite and low δ 13C micrite in sediment up to a few mm above the hardgrounds. [Copyright &y& Elsevier]

A hardground surface is well exposed in the Kilop area of Kale (Gu¨mu¨shane, NE Turkey) which forms part of the Eastern Pontides. Here, the hardground is underlain by shallow water Lower Cretaceous limestones, and overlain by Upper Cretaceous red limestones/marls which contains a planktonic microfauna including Globotruncanidae. In the field, the recognition of the hardground is based on the presence of extensive burrows (especially vertical burrows), the encrusting rudistid bivalve Requienia, neptunian-dykes with infills of pelagic sediments and synsedimentary faults. Skolithos and Thalassinoides-type burrows are present. Some burrow walls show iron hydroxide-staining. The extensive burrowing occurred prior to lithification. On the other hand, the neptunian-dykes and synsedimentary faults, which cut the hard ground, occurred after the lithification. These features indicate the progressive hardening of the substrate.The burrowed limestone consists of an intrabioclastic peloidal grainstone which was deposited in an intertidal to shallow, subtidal, moderate to relatively high energy environment. The peloidal limestone shows little or no evidence of submarine cementation, characterized by only scarce relics of isopachous cement rims of bladed calcite spar. The grainstone cement is composed predominantly of blocky calcite and overgrowth calcite cements on the echinoid-fragments. The origin of this cement is controversial.Biostratigraphic analysis of the limestones demonstrates that there is a marked stratigraphic gap (hiatus), spanning the Aptian to the Santonian, in the Cretaceous of the Kilop area.The formation of the Kilop Hardground is related to the break-up and subsidence of the Eastern Pontides carbonate platform during the formation of the Black Sea backarc basin. Hardground development was initiated in a shallow marine environment of slow sedimentation and with moderate to high energy indicating slow subsidence. Later, the hardground subsided abruptly, as shown by the deposition of pelagic sediments on the hardground surface. During drowning, the Kilop area was converted to a bypass-margin where currents were effective. The formation of the hardground may also have been associated with an eustatic rise in sea-level. [Copyright &y& Elsevier]

Modern ooids from Joulters Cay, when baked at 500 °C, turn various shades of black depending upon the organic content. Mucus-rich laminae occur at quasi-regular intervals of a few micrometres within the cortex. When mucus is still present, it turns black; when it is absent, there is a gap. The cortex consists of two types of aragonite: (1) ‘batons’ of circular cross-section capped by a single 0·1-μm (100-nm) ball, which can be interpreted as a single nannobacteria cell that precipitated the baton; (2) elongate crystals made of multiple rows of minute balls of about 0·03 μm (30 nm), which may or may not have been small organisms in the size range of viruses. There are also some crystals that show no evidence of organic precipitation. Hardground cementation begins with the formation of a terminal mucus-rich ring on the ooid that bakes black and is crowded with 0·1-μm (100-nm) balls. Some ooids are then joined by a meniscus also made of mucus with aragonite crystals. The final, most abundant hardground cement forms a fur of inorganic aragonite crystals often shaped like plywood sheets, although some ‘organic’, elongate crystals composed of ≈0·03 μm (≈30 nm) balls are also found in the later cement. For a century, ooids have been known to be closely associated with organic matter; this paper goes further and proposes that the bulk of the ooid may be precipitated by nannobacterial processes. Hardground formation, in the beginning, may also be a microbiological process, but most cementation is accomplished inorganically. [ABSTRACT FROM AUTHOR]

Some of the world's oldest macroborings occur in hardgrounds in lower Ordovician (Arenig) limestones exposed on the island of Öland, southern Sweden. The trace fossils, which are described here as Gastrochaenolites oelandicus isp. nov., appear to be dwelling structures excavated in the indurated substrate by invertebrates of unknown taxonomic affinity. They are the oldest examples of this ichnogenus. The appearance of a macroboring life habit at this early time represents a revolutionary new adaptive strategy for inhabiting carbonate hardgrounds. However, this innovative strategy apparently was not successful for the long term, because this particular macroboring taxon seems to have disappeared shortly after its early Ordovician appearance. [ABSTRACT FROM AUTHOR]

Sedimentology and paleoecology of omission or discontinuity surfaces in carbonate strata is well documented, but the geochemistry of such surfaces is poorly recognized. This work describes morphology, fabrics, geochemistry, and biota succession in omission levels from Middle Triassic (Muschelkalk) platformal carbonates in Upper Silesia, southern Poland. Three types of omission levels are distinguished: firmgrounds, hardgrounds, and intraformational conglomerates. (1) Firmgrounds are characterized by burrows of Balanoglossites triadicus. Some burrows are enclosed by a dark and/or (minor) bright diagenetic halo. The prevailing dark color, lower δ13C and δ18O ratios, and increased concentration of redox-sensitive elements (U, As, Mo, Cu, Zn, Pb, Ni) in the halos compared to the host micrite indicate a significant enrichment in organic matter, which most likely resulted from impregnation of the burrow walls with organic mucus from the burrowing organism, a process observed in modern burrows. The burrow walls were in contact with stagnant saline water (recorded by elevated 87Sr/86Sr), but were locally bleached by oxygenated fresh seawater pumped by the burrow producer. (2) Hardgrounds show encrustations of the bivalve Placunopsis ostracina and borings Trypanites weisei. In many hardgrounds, the uppermost 5-mm-thick layer has a different color and higher δ18O values than the underlying micrite. This heavier δ18O signal reflects a higher content of 18O-enriched early-marine microcrystalline cement precipitated from colder bottom seawater. (3) Intraformational conglomerates consist of flat pebbles derived from reworked firmgrounds and hardgrounds. Many pebbles contain the same encrustations and borings as observed in the hardgrounds, which occur on both sides of the clasts and mimic their shape, indicating that they were developed after pebble formation. All studied types of omission level are laterally discontinuous at some scale and are thus unsuitable for regional correlation. • Firmground burrows are surrounded by dark and/or minor bright early diagenetic haloes. • Dark haloes are enriched in redox-sensitive elements and 12C and 87Sr stable isotopes. • These proxies suggest organic matter in burrow walls and stagnant water in burrows. • Hardground top surfaces show discoloration, bedding-parallel halo, and elevated δ18O. • This elevated δ18O implies early cementation by cold bottom seawater. [ABSTRACT FROM AUTHOR]

Abrupt facies changes from the underlying terrigenous mudstone of deep-water facies upward into reefal limestones were observed on Pleistocene scleractinian reefs in southwestern Taiwan. To reveal the initial mechanisms of reef development, we examined the lithologies and vertical facies changes of 7 outcrops and 37 borehole cores from the Takangshan Reef and performed petrographic and isotopic studies. Various occurrences of dolomitic mudstone were observed from 6 outcrops and in 11 borehole cores, containing massive dolomitic mudstones, carbonate pipes, dolomitic cobbles, and dolomitic pebbles. The δ13C values of 27 samples ranged from -53.7‰ to -10.4‰, indicating that the carbonate cements of these mudstones were all cold-seep carbonates in origin. The majority of the cold-seep carbonates and a funnel-shaped structure packed with dolomitic cobbles were precipitated and formed within fine-grained siliciclastic mudstones. The wide occurrence of seep carbonates in the study area suggests hydrocarbon seepage having occurred extensively. The compact nature and associated large lucinid bivalves in massive cold-seep carbonates further indicate a pronounced, long-lasting seepage of methane occurring antecedently to the development of Takangshan Reef. A schematic model was proposed to illustrate the occurrence of various associations of lithologies and lithofacies. The erosional surfaces on siliciclastic mudstones and the funnel-shaped structure, as well as the exhuming of massive cold-seep carbonates may have occurred concurrently during a tectonically unstable time in SW Taiwan. The deposition of fossiliferous mudstone interfingered with the conglomerate lithofacies represents a rapid facies transition from a siliciclastic (non-carbonate) to a carbonate environment. The root of this rapid facies change is presumed to be tectonic movement, probably related to the westward thrust migration in the Pleistocene foreland basin. The exposed massive seep carbonates provided a substrate for the encrustations of corals as well as coralline algae and might have played a crucial role in the initial development of coral reefs in a siliciclastic paleoenvironment. To our best knowledge, this is the first case in the world of cold-seep carbonates acting as an initial colonization hardground for hermatypic corals and corallines.

The upper Cretaceous Austin Chalk Group in south-central Texas, USA, displays vertical and lateral variations in thickness and facies resulting from an irregular paleotopography of the seafloor and variable preservation of 3rd-order sea-level cycles. During the Coniacian to early Campanian, the tectonic evolution of the San Marcos Arch, a prominent basement feature in the study area, impacted facies and sequence architecture within the Austin Chalk Group. Outcropping sections of the Austin Chalk Group along the margin of the Edwards Plateau in south-central Texas stratigraphically transition between inner and outer ramp environments. The cyclic stacking pattern of facies is recorded in vertical stratigraphic succession as 3rd-order cycles separated by discontinuity surfaces and condensed deposits. Inner ramp facies consist of Thalassinoides -burrowed bivalve wackestone to packstone with minor occurrences of skeletal grainstone. Outer ramp facies comprise Planolites -burrowed foraminiferal chalk and marl with fine skeletal debris. Condensed facies associated with transgressive re-working are characterized by glauconitic packstone with high concentrations of coarse skeletal debris and lithoclasts. Gamma-ray profiles of outcrops investigated in the study area are correlated to subsurface wireline logs of wells drilled along the outcrop and subsurface trend, in order to construct a stratigraphic framework across the San Marcos Arch. This stratigraphic framework is biostratigraphically well constrained and correlated to the type section of the Austin Chalk Group farther north in the vicinity of Austin. The integration of sedimentological and stratigraphic information enables the identification of four regional 3rd-order depositional sequences within the Austin Chalk Group. Thickness maps of each depositional sequence illustrate the tectonic evolution of the San Marcos Arch and its influence on the deposition of the Austin Chalk Group, which is judged as negligible during most of the lower Austin Chalk Group (Coniacian), and started to trigger lateral thickness variations and pinch-outs in the Santonian and early Campanian. Syndepositional tectonic movement played an active role in the geometry of Austin Chalk depositional sequences and should be accounted for in sequence stratigraphic models developed in chalk depositional settings to explain the occurrence of coarser and cleaner facies. Unlabelled Image • Hardground surfaces in the Austin Chalk mark sea level rises. • Grain size variation relates to sea level changes and paleogeography. • Movements of basement features control regional thickness variations. [ABSTRACT FROM AUTHOR]