Urgonian Carbonate Platform Research Articles

Record of latest Barremian-Cenomanian environmental change in tectonically controlled depressions from the Jura-Burgundy threshold (Jura Mountains, eastern France and western Switzerland)

The area of the western Jura Mountains constitutes the former Jura-Burgundy threshold between the Tethys Ocean and the epicontinental Paris Basin Sea. During the Barremian, the area was covered by a shallow-water Urgonian carbonate platform. Tectonic processes influenced the architecture of the Urgonian platform and were notably responsible for the formation of fault-related depressions on top of the Urgonian series, which were subsequently transformed into incised valleys and then to marine depocenters. Their sedimentary infills are mostly represented by the Perte-du-Rhône Formation and record stepwise environmental change on the innermost platform, which was strongly influenced by the nearby landmass. From the latest Barremian onward, sedimentation progressively changed from: i) predominantly oncoidal and detrital charophyte-rich marly limestones; to ii) heterozoan marly limestones and marls; followed by iii) detrital sediments interspersed with numerous phosphatic conglomerates. These changes are linked with the onset of the latest Barremian Taxy Episode and with the following oceanic anoxic episodes (OAE's) 1a to d in the neighbouring basins. The sedimentary changes are associated with sedimentary sequences which formed during 2nd-order transgressions, and which express the effect of sea level fall followed by drowning events. The presence of a rich ammonite-fauna allowed a precise biostratigraphical dating of these events during the latest Barremian to Cenomanian time interval.

Platform-to-basin anatomy of a Barremian–Aptian Tethyan carbonate system: New insights into the regional to global factors controlling the stratigraphic architecture of the Urgonian Provence platform (southeast France)

The recognition of regional ammonite bio-events spanning the late Barremian–early Aptian interval in Provence invalidates previous chronostratigraphic models of the Urgonian carbonate platform. Based on high-resolution correlations across the Provence platform and the Vocontian Basin, this study provides a comprehensive revision of the temporal and spatial evolution of this sedimentary system through the definition of seven depositional sequences. Close to the early–late Barremian transition, the South Provence domain was marked by the inception of rudistid, Agriopleura-dominated, carbonates, while further to the North, the expansion of the latter biota was delayed, presumably due to greater subsidence rates driven by extensive E–W-trending basement faults. Northward installation of a rudist-rich succession in the North Provence region – including, from oldest to youngest, the Agriopleura, requieniid–monopleurid and caprinid episodes – strictly spans the late Barremian. Intermittent shedding of platform-derived material influenced the basinal series in the pre-Vocontian margin at that time. Structural inheritance and active contribution of syn-sedimentary tectonics into the stratigraphic architecture are suggested through similar orientation of depositional environment belts along with important thickness and facies variations within the North Provence area. Large-scale deformation due to the progressive rotation of the Iberian plate is likely to have direct implications into the Provence domain. Concomitant rudistid production crises, replaced by mass occurrence of Palorbitolina deposits, are documented across the peri-Vocontian Urgonian platforms. The final demise of the Urgonian-type rudistid regime, previously linked to the onset of the Oceanic Anoxic Event 1a, rather correlates to the intermittently anoxic Taxy episode. These results call for a reconsideration of the chronology of events controlling the development of Urgonian shallow-water carbonate systems through the Barremian.

Assessing the factors controlling high sedimentation rates from the latest Barremian–earliest Aptian in the hemipelagic setting of the restricted Organyà Basin, NE Spain

The Organyà Basin, south–central Spanish Pyrenees, developed as a marginal depocenter during a rapid extensional phase of anticlockwise rotation of the Iberian plate. As a result of increased subsidence, an important change in sedimentation occurred from the late Barremian to the Aptian leading to unusually high sediment accumulation rates. Approximately 1000 m of hemipelagic marls and limestones accumulated during this time interval.Here we studied the basal 85 m of the hemipelagic facies of the El Pui section, Organyà Basin, that are characterized by alternating 15 cm – ∼3 m thick beds of limestone and marls. Geochemical analyses indicate high total inorganic carbon (TIC) values (average 70%) suggesting enhanced CaCO3 production and deposition. SEM analyses of the samples indicate high abundance of calcareous nannofossils, which together with the absence of shallow water taxa characteristic of the Urgonian Carbonate platform of Organyà, and the lack of sedimentary facies attributable to carbonate platform components point to nannofossils as the main source for the elevated TIC. Organic-rich levels (total organic carbon (TOC) up to 1.74%) concurrent with positive excursions up to 2‰ in δ13Corg, imply enhanced preservation of organic matter (OM) in the basin. In addition, pronounced peaks of δ13Corg higher than the global average suggest superimposed local factors related to intensified 12C removal due to primary productivity. Biomarker analyses and the δ13Corg profile suggest an autochthonous origin of the OM from phytoplankton and possible additional contributions from microbial communities.X-ray diffraction (XRD) results attest for sustained terrestrial fluxes as the source of nutrients to the basin because of a 30% average non-carbonate bulk mineral content in the sediment. The non-carbonate fraction is dominated by quartz (average, 14%) whereas the clay mineral assemblages are characterized by high illite content (>73 relative%) with minor concentrations of kaolinite (<5%), illite /smectite mixed layers (<17%) and chlorite (<15%), consistent with a provenance from the Paleozoic metamorphic terranes adjacent to the Organyà Basin.The integrated results suggest a high sediment accumulation rate (5 cm/ky–7.5 cm/ky) and enhanced carbon burial during the latest Barremian–earliest Aptian in the hemipelagic setting of the El Pui section.

Dissocladella hauteriviana MASSE in MASSE et al., 1999 (non MASSE, 1976), another lower Urgonian Dasycladalean alga revisited

First ascribed to the Triploporellacean genus Dissocladella (Pia in Rao & Pia, 1936), the species D. hauteriviana Masse in Masse et al., 1999, was supposedly characterized by a thallus bearing whorls of laterals each consisting of a stumpy primary with a tuft of four slim secondaries at its top. A restudy of the laterals proves that they split, not only once, but several times, and stepwisely decrease in diameter. The species is re-ascribed to the Family Thyrsoporellaceae in a new combination to the genus Deloffrella Granier & Michaud, 1987. Its known stratigraphic range is rather short (Late Valanginian-earliest Barremian). In addition, it disappears earlier than its companion Polyphysacean alga, i.e., Clypeina paucicalcarea (Conrad, 1970), and its geographical distribution is broader, which makes it a good index fossil for lower Urgonian carbonate platform series.

Carbon-isotope stratigraphy of Early Cretaceous (Urgonian) shoal-water deposits: Diachronous changes in carbonate-platform production in the north-western Tethys

Carbonate platforms are highly sensitive ecological systems that typically show rapid and characteristic response modes to environmental and climatic changes acting both on a regional scale and global scale. A widely accepted hypothesis proposes that the Late Barremian stepwise establishment of the Urgonian carbonate platform on the Northern Tethyan shelf was related to a gradual change from predominantly humid towards more arid greenhouse conditions. This climate change resulted in the reorganisation of the carbonate platform ecosystem from heterozoan towards photozoan-dominated assemblages and a decrease of organic-matter burial in Tethyan and Boreal basins. In order to decipher the palaeoenvironmental and climatic boundary conditions of these major changes in neritic and pelagic settings, a precise chronostratigraphy of Urgonian carbonate platform evolution is needed. Here, we provide an integrated stratigraphic framework of Lower Barremian to Lower Aptian Urgonian carbonate platform sections (Cluses, Forclaz) located at the northern rim of the Tethys (Subalpine Chains, ESE France), with special focus on sedimentological analyses and high-resolution carbon-isotope stratigraphy. A characteristic Barremian–Aptian carbon-isotope pattern permits precise platform-to-basin correlation with cyclostratigraphic and ammonite-dated pelagic and hemipelagic Tethyan sections in the Vocontian Trough (Angles/Combe-Lambert/Glaise) and Umbria Marche Basin (Gorgo a Cerbara). Similar to Helvetic shoal-water settings, the carbonate platform in the Subalpine Chains experienced a gradual transition from heterozoan- to photozoan-dominated ecosystems. This biogenic pattern points to a gradual change of the carbonate platform after the so-called “Early Barremian crisis” towards an oligotrophic rimmed platform system (Urgonian Limestone Formation). According to the chronostratigraphy of Urgonian carbonate platform evolution in the Subalpine Chains established here, the installation of a photozoan-dominated rudist-rich platform ecosystem, referred to as Urgonian limestones sensu stricto, occurred at the Early–Late Barremian transition. The onset of changes in the carbonate production mode (“heterozoan” versus “mixed heterozoan–photozoan” versus “photozoan”) clearly predates the onset of similar lithological changes in the Helvetic realm by about 1.0 to 1.2Myr. The established chronostratigraphic framework points to a link between the change towards photozoan-dominated limestones and fading palaeoenvironmental perturbations related to the mid-Barremian event, which is associated with major faunal turnovers and the onset of cyclic black-shale deposition in the central Tethyan realm. Judging from palaeoenvironmental proxies including kaolinite, phosphorus and black marl-limestone ratios, oscillating predominantly humid and relatively arid greenhouse conditions modulated the observed pattern in Urgonian carbonate platform production mode and the varying frequencies of black-shale deposits in the Tethyan and Boreal realms. Transient blooms of the orbitolinid foraminifera Palorbitolina lenticularis at the onset of a marked Upper Barremian positive carbon-isotope shift are interpreted to reflect increasing nutrient influx, most probably related to accelerated hydrological cycling and/or sea-level rise. These strata do not represent an over-regional correlatable lithostratigraphic unit, but likely an expression of progressive Northern Tethyan environmental and oceanographic change possibly related to Ontong Java large igneous province volcanism in the prelude of oceanic anoxic event 1a (OAE).

Heteroporella ? paucicalcarea (CONRAD, 1970), an Urgonian Dasycladalean alga revisited

When the species Heteroporella ? paucicalcarea (Conrad, 1970) was erected it was left in open nomenclature. Later on new combinations were introduced but did not meet a general agreement among paleophycologists. Considering the current acception of the Dasycladalean families, we ascribe it to the Family Polyphysaceae and subsequently to the genus Clypeina (Michelin, 1845). Its known stratigraphic range is rather brief (Late Hauterivian-Early Barremian), which makes it a good index fossil in Urgonian carbonate platform series. Its geographical distribution is apparently restricted to Western Europe (France, Spain and Switzerland).

Comparisons between the Urgonian platform carbonates from eastern Serbia (Carpatho-Balkanides) and northeast Iran (Kopet-Dagh Basin): Depositional facies, microfacies, biostratigraphy, palaeoenvironments and palaeoecology

In the Getic of the Carpatho-Balcanides (eastern Serbia) and the Tirgan Formation of the Kopet-Dagh Basin (northeast Iran), platform carbonates were deposited during the Barremian/Early Aptian in environments in the domain of the northern Alpine Tethys and deformed during the Alpine orogeny. In this study, Urgonian carbonate platform deposits are discussed in detail with regard to depositional facies, microfacies, biostratigraphy, palaeoenvironments and palaeoecology. Detailed sedimentological and palaeontological investigations have been carried out on five sections in eastern Serbia and three sections in northeast Iran supported by an analysis of 392 thin-sections. Petrographic analysis of thin-sections led to the recognition of eight microfacies types grouped into four facies zones. A supratidal–intertidal (restricted)–intertidal (open-lagoon)–platform-margin sand-shoal transition was recorded in both areas. Supratidal facies are characterized by bioclastic mudstones and fenestral and peloidal wackestones and packstones; intertidal (restricted) facies are represented by bioclastic wackestones, whereas intertidal (open-lagoon) facies are indicated by bioclastic packstones/grainstones and oncoid grainstones. High-energy sand-shoal facies are dominated by ooid grainstones/rudstones followed by orbitolinid packstones. Benthic foraminifera are especially abundant and along with calcareous algae are the most important fossils used for age determination of shallow-marine carbonate deposits. Thirty-two benthic foraminiferal genera were identified from eastern Serbia with an additional 38 genera from northeast Iran dominated by agglutinated forms. Identified calcareous algae provide significant data for depositional environments and palaeoecology. The microfossil associations in the two regions are very similar and share a number of common characteristics, but also some differences and show a strong affinity to those of the northern margins of Tethys. In both study areas shallow-marine environments of the Barremian/Early Aptian were replaced by deep-marine conditions during the Late Cretaceous.

Bridging the Faraoni and Selli oceanic anoxic events: late Hauterivian to early Aptian dysaerobic to anaerobic phases in the Tethys

Abstract. A detailed geochemical analysis was performed on the upper part of the Maiolica Formation in the Breggia (southern Switzerland) and Capriolo sections (northern Italy). The analysed sediments consist of well-bedded, partly siliceous, pelagic carbonate, which lodges numerous thin, dark and organic-rich layers. Stable-isotope, phosphorus, organic-carbon and a suite of redox-sensitive trace-element contents (RSTE: Mo, U, Co, V and As) were measured. The RSTE pattern and Corg:Ptot ratios indicate that most organic-rich layers were deposited under dysaerobic rather than anaerobic conditions and that latter conditions were likely restricted to short intervals in the latest Hauterivian, the early Barremian and the pre-Selli early Aptian. Correlations are both possible with organic-rich intervals in central Italy (the Gorgo a Cerbara section) and the Boreal Lower Saxony Basin, as well as with the facies and drowning pattern in the Helvetic segment of the northern Tethyan carbonate platform. Our data and correlations suggest that the latest Hauterivian witnessed the progressive installation of dysaerobic conditions in the Tethys, which went along with the onset in sediment condensation, phosphogenesis and platform drowning on the northern Tethyan margin, and which culminated in the Faraoni anoxic episode. This episode is followed by further episodes of dysaerobic conditions in the Tethys and the Lower Saxony Basin, which became more frequent and progressively stronger in the late early Barremian. Platform drowning persisted and did not halt before the latest early Barremian. The late Barremian witnessed diminishing frequencies and intensities in dysaerobic conditions, which went along with the progressive installation of the Urgonian carbonate platform. Near the Barremian-Aptian boundary, the increasing density in dysaerobic episodes in the Tethyan and Lower Saxony Basins is paralleled by a change towards heterozoan carbonate production on the northern Tethyan shelf. The following return to more oxygenated conditions is correlated with the second phase of Urgonian platform growth and the period immediately preceding and corresponding to the Selli anoxic episode is characterised by renewed platform drowning and the change to heterozoan carbonate production. Changes towards more humid climate conditions were the likely cause for the repetitive installation of dys- to anaerobic conditions in the Tethyan and Boreal basins and the accompanying changes in the evolution of the carbonate platform towards heterozoan carbonate-producing ecosystems and platform drowning.

Strontium and carbon-isotope chronostratigraphy of Barremian–Aptian shoal-water carbonates: Northern Tethyan platform drowning predates OAE 1a

A widely accepted hypothesis proposes that the Early Aptian demise of carbonate platforms in the northern Tethyan realm reflects the impact of environmental changes that eventually led to the deposition of organic-rich basinal sediments during oceanic anoxic event (OAE) 1a. In fact, the temporal correlation of oceanic anoxia and platform drowning has not been rigorously tested and evidence indicating causality is debated. The present paper provides a high-resolution carbon- and strontium-isotope chronostratigraphy combined with detailed sedimentological analysis applied to the Barremian–Aptian Urgonian carbonate platform development at the northern rim of the Tethys (Subalpine Chains, Haute-Savoie, ESE France). A characteristic Barremian–Aptian carbon- and strontium-isotope pattern below the onset of OAE 1a interval permits precise platform-to-basin correlation with the Barremian stratotype locality of Angles (SE France). This chemostratigraphic pattern equally allows for a correlation with well-studied shoal-water and pelagic records of the northern and central Tethys. Strontium-isotope stratigraphy provides a high-resolution numerical age-data set, indicating an earliest Aptian age (< 124.5 ± 0.4 Ma) of platform demise in the Subalpine Chains. The detailed platform-to-basin correlation obtained in France clearly illustrates that shoal-water carbonate production in the Urgonian platform ceased about 300 kyr before the most negative values of the characteristic negative carbon-isotope anomaly, which marks the beginning of OAE 1a black-shale deposition. The stratigraphic results confirm (i) the theory of an Early Aptian northern Tethyan platform drowning episode predating OAE 1a and (ii) document the temporal response of shoal-water carbonate platforms to the environmental perturbations prior to the OAE acme.

Stratigraphic, sedimentological and palaeoenvironmental constraints on the rise of the Urgonian platform in the western Swiss Jura

Urgonian-type carbonates are a characteristic feature of many late Early Cretaceous shallow-marine, tropical and subtropical environments. The presence of typical photozoan carbonate-producing communities including corals and rudists indicates the prevalence of warm, transparent and presumably oligotrophic conditions in a period otherwise characterized by the high density of globally occurring anoxic episodes. Of particular interest, therefore, is the exploration of relationships between Urgonian platform growth and palaeoceanographic change. In the French and Swiss Jura Mountains, the onset and evolution of the Urgonian platform have been controversially dated, and a correlation with other, better dated, successions is correspondingly difficult. It is for this reason that the stratigraphy and sedimentology of a series of recently exposed sections (Eclepens, Vaumarcus and Neuchâtel) and, in addition, the section of the Gorges de l’Areuse were analysed. Calcareous nannofossil biostratigraphy, the evolution of phosphorus contents of bulk rock, a sequence-stratigraphic interpretation and a correlation of drowning unconformities with better dated sections in the Helvetic Alps were used to constrain the age of the Urgonian platform. The sum of the data and field observations suggests the following evolution: during the Hauterivian, important outward and upward growth of a bioclastic and oolitic carbonate platform is documented in two sequences, separated by a phase of platform drowning during the late Early Hauterivian. Following these two phases of platform growth, a second drowning phase occurred during the latest Hauterivian and Early Barremian, which was accompanied by significant platform erosion and sediment reworking. The Late Barremian witnessed the renewed installation of a carbonate platform, which initiated with a phase of oolite production, and which progressively evolved into a typical Urgonian carbonate platform colonized by corals and rudists. This phase terminated at the latest in the middle Early Aptian, due to a further drowning event. The evolution of this particular platform segment is compatible with that of more distal and well-dated segments of the same northern Tethyan platform preserved in the Helvetic zone of the Alps and in the northern subalpine chains (Chartreuse and Vercors).

Demise of the northern Tethyan Urgonian carbonate platform and subsequent transition towards pelagic conditions: The sedimentary record of the Col de la Plaine Morte area, central Switzerland

The sedimentary succession of the Col de la Plaine Morte area (Helvetic Alps, central Switzerland) documents the disappearance of the northern Tethyan Urgonian platform in unprecedented detail and suggests stepwise platform demise, with each drowning phase documented by erosion and phosphogenesis. The first identified drowning phase terminated Urgonian carbonate production in a predominantly photozoan mode. Using a correlation of the whole-rock δ 13C record with the well-dated record from SE France, its age is inferred to as Middle Early Aptian (near the boundary between the weissi and deshayesi zones). A subsequent drowning phase is dated by ammonites and by a correlation of the whole-rock δ 13C record as Late Early Aptian (late deshayesi to early furcata zone). A third drowning phase provides an ammonite-based age of Early Late Aptian ( subnodosocostatum and melchioris zones) and is part of a widely recognized phase of sediment condensation and phosphogenesis, which is dated as latest Early to Middle Late Aptian (late furcata zone to near the boundary of the melchioris and nolani zones). The fourth and final drowning phase started in the latest Aptian ( jacobi zone) as is also indicated by ammonite findings at the Col de la Plaine Morte. The phases of renewed platform-carbonate production intervening between the drowning phases were all in a heterozoan mode. During the ultimate drowning phase, phosphogenesis continued until the Early Middle Albian, whereas condensation processes lasted until the Middle Turonian. Coverage of the external margin of the drowned Urgonian platform by a drape of pelagic carbonates started only in the Late Turonian. During the Santonian, the external part of the drowned platform underwent normal faulting and saw the re-exposure of already lithified Urgonian carbonates at the seafloor. Based on the here-inferred ages, the first drowning phase just precedes oceanic anoxic episode 1a (OAE 1a or “selli event”) in time, and the second drowning phase partly overlaps with OAE 1a. The onset of the third drowning event slightly predates two further periods of increased organic-matter accumulation in the Vocontian Basin (Noir and Fallot levels), and the onset of the fourth and final drowning phase may coincide with two further periods of increased organic-matter accumulation in the Vocontian Basin (Jacob and Kilian levels, part of OAE 1b). These correlations indicate a relationship between the so-called anoxic episodes and the stepwise demise of the Urgonian platform, even if the onset of environmental change is registered earlier on the platform than in basinal sediments.

The Early Aptian Grünten Member: Description of a new lithostratigraphic unit of the helvetic Garschella Formation

Early Aptian sediments mostly referred to as “upper Orbitolina beds” are known from a relatively small number of outcrop areas throughout the distal part of the Helvetic Zone of Switzerland, Austria and Germany. These sediments are here formally defined as a new, basal member of the Garschella Formation; the Grunten Member. Equivalent and correlatable sediments also exist in the Vercors region of the French Dauphinee Zone. The historical type section (holostratotype) of the Grunten Member is situated on the Grunten Mountain in southern Germany. A new type section (lectostratotype) for the Grunten Member is chosen in the better suited Bauen-Brisen area of Central Switzerland and an additional reference section (hypostratotype) is defined near the Rawil Pass in the Bernese Oberland of Switzerland. In relatively proximal settings, the Grunten Member overlies Early Aptian limestones of the Urgonian Carbonate Platform (Schrattenkalk Formation), documenting its demise and early “drowning”. In relatively distal settings, it overlies the contemporary hemipelagic sediments of the Drusberg and Mittagspitz Formations. In complete successions, the Grunten Member is in turn overlain by the basal, phosphoritic Luitere Bed of the Brisi Member (Garschella Formation) documenting the continuing “drowning” of the Urgonian Carbonate Platform in the Late Aptian. The Grunten Member essentially consists of a single stratigraphic sequence, beginning with a marly base and gradually passing to crinoidal limestones at its top. Rare ammonite finds as well as sequence stratigraphic correlations suggest a late Early Aptian age (parts of the Deshayesi and Furcata Ammonite Zones). In the relatively proximal reference section of Rawil, the Grunten Member contains two phosphoritic horizons. Phosphateenriched horizons are also known from other proximal sections of the Grunten Member.

Geometry and petrophysical parameters of a calcarenitic turbidite lobe (Barremian-Aptian, Pas-de-la-Cluse, France)

Several bioclastic resedimentation systems, late Barremian to early Aptian in age, outcrop in southeastern France. The upslope portion of a calcarenitic turbidite lobe outcrops as a cliff, 2.3 km wide and up to 45 m high, oriented roughly perpendicular to the direction of palaeoflow. It has been studied through 31 sections. The studied body is point-sourced and corresponds to a channel/lobe transition located at the toe-of-slope of a Urgonian carbonate platform on the western edge of the Vocontian Trough (French Subalpine Chains). The internal geometry of the lobe is the result of repeated erosion and filling of channels, creating well-defined stacked units. Filling phases result in lobe-shaped morphology. Units evolve from base to top in an ordered way. First, the sediments exported from the platform and deposited in the basin are represented by decimetre-thick, sheet-like and fine-grained Bouma a-e and ab-e sequences. In this first unit, erosion is limited and represented by shallow and symmetrical channels. This initial phase of deposition corresponds to the formation of a lobe with a positive morphology. The next phase of system development is more erosional and this is dominated by migrating, sinuous channels several metres deep filled with coarse-grained, poorly graded turbidites. These channels are first filled through lateral accretion and then sediment spills over to create a sub-lobe. The mixture of channel and lobe features could be considered as a carbonate equivalent of the channel–levee system of siliciclastic deposits. Finally, the system ends with the deposition of a flat-based unit. This unit extends for more than 2 km and is composed of the southwards lateral accretion of hundreds of metres wide sequences each composed of a debris-flow deposit capped by a coarse-grained calcarenite bed. These sigmoidal bodies could be the ultimate forms of a spreading out phenomenon of the lobe. Overall, the Pas-de-la-Cluse system is a coarsening-upward body. As the grain-size is related to the nature of the material, this coarsening-upward implies either an evolution in the nature of the carbonate material produced on the platform (from grainy to bioconstructed), a displacement of the source sediment (from inner platform to margin) or a displacement of the channel head through time (from distal to proximal). The upward evolution from classic turbidites to debris-flow deposits can be explained either by the way the platform facies evolved with time in a depositional sequence or through retrogressive erosion by successive slope failures that cut deeper and deeper into the platform slope. Although its present-day porosity is low, the size of the system and the fact that it is encased in marls make it an interesting outcrop analogue for hydrocarbon exploration.

La migration de la plate-forme urgonienne entre le Jura plissé et les Chaînes subalpines septentrionales (France, Suisse)

The stratigraphic study of the Urgonian carbonate platform presented here was carried out on a directed transversal NW-SE and passing by Geneva. This transversal extends on 100 km length; it lies between the face of the “folded Jura” (Champagnole, the Jura) and the septentrional subalpine chains (Aravis, Haute-Savoie). Essentially based on the biostratigraphy (ammonites, orbitolinids, echinids) and on sequence stratigraphy, this study illustrates the progradation of the Urgonian platform from the “folded Jura” (top of the Lower Hauterivian) to the Aravais chain (lowermost Barremian).

Carbonate platform sequence stratigraphy in a rapidly subsiding area: the Late Barremian–Early Aptian of the Organyà basin, Spanish Pyrenees

A detailed sedimentological and stratigraphical study of the Late Barremian–Early Aptian Urgonian Carbonate platform of the Organyà basin, Spanish Pyrenees, has identified four depositional sequences. The first two sequences consist of transgressive and highstand systems tracts and the last two sequences of lowstand, transgressive and highstand systems tracts. Sequence boundaries at the top of sequences OR-1 and OR-4 are type 1 boundaries with evidence of exposure and erosion. Sequence boundaries at the top of sequences OR-2 and OR-3 are type 2 with no evidence of exposure or erosion. Sand-shoal (high-energy) facies characterise the first sequence, OR-1. Sequences OR-2 and OR-3 are dominated by lagoonal facies, with an important accumulation of lacustrine facies at the base of OR-3. A succession of lacustrine, deep-platform and sand-shoal facies characterises sequence OR-4. The last sequence also contains the record of an anoxic event (OAE 1a). All these characteristics have been largely influenced by the high subsidence rate of the Organyà basin (1100 m of compacted limestone deposited in about 5 Ma). However, the main global eustatic changes registered in other contemporaneous basins were also identified.

Modeling Carbonate Microfacies in the Context of High-Frequency Dynamic Relative Sea-Level and Environmental Changes: Reply

Sequence stratigraphic models are used to interpret stratal architecture and key stratal bounding surfaces in ancient carbonate platforms within the context of changing accommodation space during third-order relative sea-level cycles. However, individual systems tracts are still described using standard microfacies that give a snapshot of limestone composition, but they do not take into account gradual changes in the marine environment resulting from variations in water depth during the cycle of relative sea-level change. Water depth is the single most significant collective control on a wide range of environ- mental gradients affecting carbonate sediment composition. During dy- namic relative sea-level fluctuations stratigraphic changes in carbonate sediment composition are characterized by systematic shifts in the rel- ative importance of different limestone component grain types, forming temporal continuums or relays. Relays are detected using computer- optimized Jaccard's similarity coefficient matrices to analyze presence/ absence compositional data. Individual relays between grain types may link together samples that are generically unrelated to one another but are nevertheless genetically related to deposition during the same uni- directional dynamic environmental gradient. A stratigraphic relay identified within the basal beds of the mid- Cretaceous Urgonian carbonate platform succession of SE France re- cords unidirectional environmental gradients linked to changing water depth, characteristic of a transgressive systems tract. Periods of static water depth, such as the keep-up phase of a late highstand systems tract at the top of the Urgonian carbonate platform succession, are characterized by fixed compositional assemblages. Stratigraphic breaks between individual compositional relays and assemblages occur at in- flections in changing water depth, marking the boundaries between individual systems tracts and sequences, currently identified using stra- tal geometries and key stratal surfaces. The use of relays to model microfacies and identify individual system tracts and sequence boundaries has several advantages over existing methods. This approach can be used where key stratal surfaces are difficult to distinguish, such as in planar and concordantly bedded, inner platform settings and in arid depositional environments where physical evidence of subaerial platform exposure such as karstification is poorly developed. The technique can also be used to determine the genetic significance of unconformities preserved in outcrop or core within a sequence stratigraphic context by differentiating unconfor- mities formed at the extremes of relative sea-level cycles from those formed by other abrupt acyclical environmental changes.

Modeling carbonate microfacies in the context of high-frequency dynamic relative sea-level and environmental changes

Sequence stratigraphic models are used to interpret stratal architecture and key stratal bounding surfaces in ancient carbonate platforms within the context of changing accommodation space during third-order relative sea-level cycles. However, individual systems tracts are still described using standard microfacies that give a snapshot of limestone composition, but they do not take into account gradual changes in the marine environment resulting from variations in water depth during the cycle of relative sea-level change. Water depth is the single most significant collective control on a wide range of environ- mental gradients affecting carbonate sediment composition. During dy- namic relative sea-level fluctuations stratigraphic changes in carbonate sediment composition are characterized by systematic shifts in the rel- ative importance of different limestone component grain types, forming temporal continuums or relays. Relays are detected using computer- optimized Jaccard's similarity coefficient matrices to analyze presence/ absence compositional data. Individual relays between grain types may link together samples that are generically unrelated to one another but are nevertheless genetically related to deposition during the same uni- directional dynamic environmental gradient. A stratigraphic relay identified within the basal beds of the mid- Cretaceous Urgonian carbonate platform succession of SE France re- cords unidirectional environmental gradients linked to changing water depth, characteristic of a transgressive systems tract. Periods of static water depth, such as the keep-up phase of a late highstand systems tract at the top of the Urgonian carbonate platform succession, are characterized by fixed compositional assemblages. Stratigraphic breaks between individual compositional relays and assemblages occur at in- flections in changing water depth, marking the boundaries between individual systems tracts and sequences, currently identified using stra- tal geometries and key stratal surfaces. The use of relays to model microfacies and identify individual system tracts and sequence boundaries has several advantages over existing methods. This approach can be used where key stratal surfaces are difficult to distinguish, such as in planar and concordantly bedded, inner platform settings and in arid depositional environments where physical evidence of subaerial platform exposure such as karstification is poorly developed. The technique can also be used to determine the genetic significance of unconformities preserved in outcrop or core within a sequence stratigraphic context by differentiating unconfor- mities formed at the extremes of relative sea-level cycles from those formed by other abrupt acyclical environmental changes.