Marl Limestone Research Articles

Magnetic resonance sounding and radiomagnetotelluric measurements used to characterize a limestone aquifer in Gotland, Sweden

► We have tested MRS, RMT, VES and GPR to characterize a limestone aquifer. ► RMT locates the limestone–marl interface and helps to constrain the MRS inversion. ► The T 1MRS of MRS suggests the aquifer to be hosted by small fractures and vugs. ► No real correlation between water content and resistivity exist. ► MRS is hence the only method that measure water and hence the extension of the aquifer. Almost all drinking water in Gotland is groundwater and is mainly found in karst limestone. However, the unpredictable location and geometry of the karst cracks and caverns makes it very difficult to estimate groundwater storage and movement, as well as contaminant transport. The aim of this study was to test the performance of different geophysical techniques like Magnetic Resonance Sounding (MRS), Radiomagnetotelluric (RMT), Vertical Electrical Sounding (VES) and Ground Penetrating Radar (GPR) to characterize aquifers in Gotland, in respect to geometry and storage as well as connectivity over a wider area. The investigated area is located on the south-eastern part of Gotland. The geology here is dominated by 50–60 m thick successions of limestone that gradually turn into marlstone. The use of multiple techniques has shown to give a more coherent interpretation. However, the shallow penetration depth of GPR and the lack of soil cover in some places of the investigated area limit the use of geoelectrical methods and GPR. With MRS, water are found down to 60 m in depth, with a maximum water content at depths of 20–30 m. This coincides with the most resistive sections of the limestone. The water content varies between 0% and 3%, with a relaxation time ( T 1MRS ) less than 400 ms suggesting that the aquifer is hosted in small fractures, molds and vugs rather than larger karst fractures and caverns. Two potential aquifers were identified with MRS, possibly separated by marlstone. From modeling it can be seen that such boundary separating two aquifer apart can be more easily discriminated in the N/S-, than in the E/W direction. In summary, MRS is therefore the only method in this survey that can detect and determine the vertical and lateral distribution of water within the aquifer together with the total volume of free water. The RMT method has shown to be effective in characterizing the limestone/marl interface, but also to locate anomalous low resistive zones, possibly associated with salt water. RMT also helps to constrain the final MRS model by choosing a suitable regularization for the MRS 3D inversion. All together, the combination of MRS and RMT seems most efficient of the tested methods and therefore most promising for future groundwater explorations in geological environments like in eastern Gotland.

Geochemical and palynological analyses of the Serikagni Formation, Sinjar, Iraq

Serikagni Formation (lower–middle Miocene) was studied at four outcrop sections within Sinjar Anticline, Para (the western plunge), Jaddala, Sinjar, (Southern limb), and Noaniaa (Northern limb) of Sinjar Mountain, which is part of foothill zone in northwest Iraq. This formation is composed of Globigerina limestone, marly limestone intercalated with marl, deposited in quiet, deep marine environment. Palynological analyses and the percentages of palynological matter were determined. Palynofacies analysis was recorded for paleoenvironmental interpretation. Mineralogical investigations and petrographical analyses were done. Geochemical analyses for the determination of the major constituents (CaO, MgO, and I.R.),the secondary constituents (SiO2, Al2O3, Fe2O3, Na2O, K2O, and trace elements (Sr, Mn, Cu, Zn, Co, Cr, Pb) in the rock samples were accomplished. The calcite is the predominant mineral which is mostly of micrite types as well as spary calcite. The insoluble residue (I.R.) has negative correlation with the carbonate percentages in the rock formation as observed by the XRD analyses. Clay minerals show low occurrence represented by palygorskite, montmorillonite, and sepiolite. Geochemical differences can be noticed between the limy and marly facies constituting the same formation. Differences are also observed between the geochemistry and mineralogy of the four studied localities such as the relatively more pure limestone in Jaddala and Para sections than Sinjar and Noaniaa sections which indicate the higher percentages of marly limestone. This is believed to be due to the geographic location of these sections within the main depositional environment, as well as the consequences of sea level fluctuation. Two palynofacies (PF-1) and (PF-2) were identified. Palynomorphs are more numerous with high percentages (>80%) of amorphous organic matter (AOM) is present in the PF-1. The lithological association of this palynofacies is limestone and marly limestone. This palynofacies 1, indicate the deep marine depositional environment. The second palynofacies (PF-2) reflect low quantities (less than 40%) of AOM, with increase of phytoclasts (translucent-type) to more than 30% and the miospores (small spores and pollen grains) to more than 20%. The lithological association of this palynofacies is marl and marly limestone. This palynofacies 2, indicate the nearshore and shallow marine depositional environment.

Characterization of groundwater in the Souss upstream basin: Hydrochemical and environmental isotopes approaches

The hydrochemistry of major ions and environmental isotope compositions ( 18 O, 2 H) of water samples have been used to identify the chemical characteristics and the origin of groundwater in the Souss upstream basin. The total dissolved solids (TDS) did not exceed 1000 mg.L -1 with an average of 635 mg.L -1 . Two chemical water types were observed: A calcium and magnesium bicarbonate type (Ca 2+ -Mg 2+ -HCO 3- ), forming the dominant water type generally observed in the plain along the Souss river and towards the piedmont of the Anti-Atlas Mountains; and a calcium sulphate type (Ca 2+ -SO 4 2- ), from the northwest of the study area, linked to the presence of phosphatic gypseous marls and limestone marls (Upper Cretaceous). The calculations of saturation indexes versus the main mineral phases have been carried out, using the PHREEQC program. The groundwater is saturated and slightly oversaturated with respect to carbonate minerals and under saturated with respect to evaporite minerals; surface waters show an oversaturation with respect to carbonate mineral phases, mainly dolomite. The groundwater composition is largely controlled by the dissolution of carbonate rocks known in this part of the basin. Stable isotope contents of groundwaters ranged from -7.96 to -6.26‰ for δ 18 O and from -49.47 to -39.28‰ for δD. The hydrogen (δD) and oxygen (δ 18 O) isotope signatures indicate a low evaporation of precipitations during infiltration and that the aquifer is highly influenced by the contribution of recharge water recharge from the High Atlas Mountains. Key words: Groundwater, hydrochemistry, stable isotopes, arid climate, Souss, Morocco.

Reply to the comment on "Orbitally forced climate and sea-level changes in the Paleoceanic Tethyan domain (marl–limestone alternations, Lower Kimmeridgian, SE France)" by S. Boulila, M. de Rafélis, L. A. Hinnov, S. Gardin, B. Galbrun, P.-Y. Collin [ Palaeogeography Palaeoclimatology Palaeoecology 292 (2010) 57–70

In their comment on our paper (Boulila S. et al. 2010 — Orbitally forced climate and sea-level changes in the Paleoceanic Tethyan domain, marl–limestone alternations, Lower Kimmeridgian, SE France, Palaeogeog. Palaeoclim. Palaeoecol., 292, 57–70), Mattioli and co-authors (E. Mattioli et al., Comment on "Orbitally forced climate and sea-level changes in the Paleoceanic Tethyan domain (marl–limestone alternations, Lower Kimmeridgian, SE France) " by S. Boulila, M. de Rafélis, L. A. Hinnov, S. Gardin, B. Galbrun, P.-Y. Collin [Palaeogeography Palaeoclimatology Palaeoecology 292 (2010) 57–70], Palaeogeography Palaeoclimatology Palaeoecology, this issue) criticize our depositional model of Lower Kimmeridgian marl-limestone alternations in the Vocontian Basin (SE France) and contest the nannofossil contribution to the build-up of micritic limestones. The model that we proposed links maxima of orbitally forced insolation with high sea-level, weaker continental erosion, and reduced detrital input, and insolation minima to low sea-level, increasing erosion of detrital materials and their transport to the basin. This involves a competition between multiple variable fluxes, and is supported by a multi-proxy study (magnetic susceptibility, weight percent carbonate, manganese content coupled with cathodoluminescence analysis, oxygen and carbon stable isotopes, and calcareous nannofossil analysis) on a 8.5-m thick interval from a ~ 40 m-thick section at Châteauneuf-d'Oze (SE France). This differs substantially from previous models suggesting that detrital input to this part of the Vocontian Basin constitutes ‘background noise’, and that the main mechanism inducing the marl–limestone rhythms was orbitally forced carbonate mud export from the Jura Platform.

Comment on "Orbitally forced climate and sea-level changes in the Paleoceanic Tethyan domain (marl–limestone alternations, Lower Kimmeridgian, SE France) " by S. Boulila, M. de Rafélis, L. A. Hinnov, S. Gardin, B. Galbrun, P.-Y. Collin [ Palaeogeography Palaeoclimatology Palaeoecology 292 (2010) 57–70

A recent paper by Boulila et al. (2010) reports on orbitally-forced cycles recorded as marl-limestone alternations in one Lower Kimmeridgian section, Chateauneuf d'Oze (SE France, Tethyan domain). In an attempt to illustrate the in situ (i.e., pelagic) origin of carbonates in marls and limestones, the authors challenged the results of previous work concerning sections from other Tethyan areas of similar age, which demonstrate that nannofossil abundance is originally higher in marls than in limestones. Boulila et al. (2010) present data acquired with optical and Scanning Electron microscopes estimating nannofossil contribution to the rocks. However, their selected illustrations do not support a high contribution of the nannofossils to the carbonate fraction of their samples or their interpretation of the finest fraction of the rocks (micarbs). Furthermore, according to their data on carbonate content of the rocks, they did not analyse true marl-limestone alternations, but slightly argillaceous limestones and limestones. Boulila et al. (2010) also criticize earlier work providing a sequence stratigraphy interpretation of the Chateauneuf d'Oze section. However, contrarily to these earlier works, they did not consider platform-to-basin correlations that are essential in order to understand the sedimentary dynamics at the basin-wide scale. This comment challenges the work by Boulila et al. (2010) that largely underestimates the role of carbonate production on shallow platforms and its subsequent export basinwards as a major mechanism controlling the sedimentation in epicontinental basins.

Turonian Rudist Facies from Abu Roash area, North Western Desert, Egypt

Five radiolitid rudist species are described from the Turonian sequence of Abu Roash area. They are recognized in three rudist biostromes that occur in two informal members of Abu Roash Formation; the Rudist- and the Actaeonella-bearing limestone–marl members. The three biostromes show autochthonous and parautochthonous fabrics and moderate to high packing potential. The first rudist biostrome at the base of the Rudist-bearing limestone–marl member (Middle Turonian) contains Durania gaensis, Praeradiolites ponsianus and Bournonia fourtaui. The second biostrome in the same member consists of Bournonia roashensis. The third biostrome that recognized in the Actaeonella-bearing limestone–marl member (Late Turonian) consists of Durania arnaudi. Rudist biostromes in the Rudist-bearing limestone–marl member were deposited on subtidal rudist shoals with moderate to high energy versus that of the Actaeonella-bearing limestone–marl member that deposited in low to moderate energy on deeper part of subtidal rudist shoals. The exposed Turonian succession at Abu Roash area could be divided into three depositional sequences bounded by three sequence boundaries (paleosols and angular unconformity). The first rudist biostrome in the Rudist-bearing limestone–marl member represents the lower part of the transgressive systems tract of the first depositional sequence. The deepening upward trend of the transgressive systems tract is due to increase of accommodation space in transgressive context during relative sea-level rise episode. On the other hand, the second rudist biostrome in the Rudist-bearing limestone–marl member and the third rudist biostrome in the Actaeonella-bearing limestone–marl member are in shallowing-upward set sequence forming the highstand systems tract of the first and third depositional sequences. This indicates that, the accommodation space was being filled more rapidly than was being created during the highstand stage.

High-resolution intra- and interbasinal correlation of the Danian–Selandian transition (Early Paleocene): The Bjala section (Bulgaria) and the Selandian GSSP at Zumaia (Spain)

The Danian–Selandian (D–S) boundary has been identified for the first time in the Black Sea coast at Bjala (Bulgaria) based on a new integrated bio-, magneto- and cyclostratigraphic study. Several correlation criteria as established for the basal Selandian GSSP from Zumaia (Basque Basin) are evaluated. Noteworthy, is the almost complete lack of calcareous nannoplankton species Braarudosphaera bigelowi in the Bulgarian sections, a sharp decrease of which was indicated as suitable criteria for defining the D–S boundary as it occurred both at Zumaia and in the classical locations of the North Sea basin. Conversely, the second evolutionary radiation of the calcareous nannofossil genus Fasciculithus together with the occurrence of Fasciculithus tympaniformis that define the NP4/NP5 zonal boundary seem to be reliable criteria to approximate the D–S boundary. In detail, however, the best approach is to integrate biostratigraphic data within a magnetostratigraphic and/or cyclostratigraphic framework. Refinements on the placement of chron C27n at Zumaia and robust bed-by-bed correlation between several Basque sections and Bjala indicates that the D–S boundary is located 30 precession cycles (~ 630 ky) above C27n. In addition to the precession-related marl–limestone couplets and 100-ky eccentricity bundles recognized in the studied sections, expression of the stable 405-ky long eccentricity allows direct tuning to the astronomical solutions. A correlation of the land-based sections with previously tuned data from ODP Site1262 from the Southern Atlantic is challenged. Our choice is consistent with original tuning at Zumaia but shifts one 100-ky cycle older previous tuning from Site 1262 along the interval above C27n. Under the preferred tuning scheme the D–S boundary can be given an age of 61.641 ±0.040 Ma on the La04 orbital solution.

Orbitally forced climate and sea-level changes in the Paleoceanic Tethyan domain (marl–limestone alternations, Lower Kimmeridgian, SE France)

High-resolution analysis of multiple climatic proxies was carried out on a ∼ 8.5-m Lower Kimmeridgian interval of a pelagic marl–limestone succession at Châteauneuf-d'Oze (Vocontian Basin, southeastern France). The aim of the study was to characterize the orbitally controlled sedimentary cyclicity and to decipher paleoclimatic and paleoceanographic changes. All analysed proxies (magnetic susceptibility, carbonate content, manganese content, and bulk carbon and oxygen stable isotopes ( δ 13C and δ 18O) respond in synchrony to orbitally forced climate change. Precession index cycles modulated by short (∼ 100 kyr) eccentricity cycles are strongly expressed in magnetic susceptibility, carbonate and Mn contents but less so in δ 18O. Obliquity cycles are expressed in manganese and particularly in δ 13C. The orbital forcing was conferred to these rhythmic pelagic sediments via paleoclimatic and paleoceanographic changes as follows. Detrital input and marine carbonate production are recorded by the magnetic susceptibility and carbonate signals with strong precession cyclicity. Calcareous nannofossil analysis shows the omnipresence of coccoliths and debris of coccoliths in the marls and limestones, suggesting that the carbonate production was in largest part in situ. Precession index may exert oscillations in the antagonist marine surface productivity and detrital flux processes via solar radiation change. Oxygenation of the seafloor leading to redox cycles is detected in the manganese signal and supported by the δ 13C record, with a strong expression of the obliquity. Finally, paleotemperature variations were deduced from the δ 18O record, although this latter has been in part perturbed by diagenesis. In sum, the variations in the studied proxies suggest a depositional model of combined climate and sea-level cycles forced in concert by Earth's orbital parameters. In particular, the limestones represent warmer climates and higher sea-levels and the marls colder climates and lower sea-levels. This depositional model concurs with a previous depositional model of deep-water carbonates of the German Kimmeridgian, while is in opposition to a previous one of the same section in the Vocontian Basin.

Characterization and astronomically calibrated age of the first occurrence ofTurborotalia frontosain the Gorrondatxe section, a prospective Lutetian GSSP: implications for the Eocene time scale

The Gorrondatxe section, a prospective Lutetian Global Stratotype Section and Point (GSSP), has recently been used as the master reference section to reassess the correlation between Eocene magnetostratigraphic and calcareous planktonic biostratigraphic scales. However, the exact calibration of some events remained ill defined, as they were thought to be missing in Gorrondatxe due to a fault. The most important missing events were the first occurrence of the planktonic foraminifera Turborotalia frontosa and the C22n/C21r chron boundary. Either might be a reliable correlation criterion for the Ypresian/Lutetian boundary, as both approach the age of the original Lutetian Stratotype. New studies allowed the identification of the former event 9 m above the Gorrondatxe fault, within magnetic polarity Chron C21r and calcareous nannofossil Zone CP12a. Distinctive test features that characterize the most primitive morphotype of T. frontosa are described. Despite the high turbidite content, recurrent pelagic limestone–marl couplets and bundles occur, whose formation was driven by precession and eccentricity astronomical cycles. The first occurrence of T. frontosa was found 27 couplets and 5.5 bundles (60 m) below the first occurrence of the calcareous nannofossil Blackites inflatus, which is dated at 48 Ma. Hence, the age of the first occurrence of T. frontosa is estimated at 48.55 Ma, confirming that it is the most suitable planktonic foraminiferal correlation criterion for the Ypresian/Lutetian boundary. These results show that the stratigraphic interval missing due to the Gorrondatxe fault cannot be greater than a few metres and reinforce the value of this section as a prospective Lutetian GSSP.

Evaluation of bulk carbonate δ13C data from Triassic hemipelagites and the initial composition of carbonate mud

AbstractBulk carbonate samples of hemipelagic limestone–marl alternations from the Middle and Upper Triassic of Italy are analysed for their isotopic compositions. Middle Triassic samples are representative of the Livinallongo Formation of the Dolomites, while Upper Triassic hemipelagites were sampled in the Pignola 2 section, within the Calcari con Selce Formation of the Southern Apennines in Southern Italy. Triassic hemipelagites occur either as nodular limestones with chert nodules or as plane‐bedded limestone–marl alternations which are locally silicified. In the Middle Triassic Livinallongo Formation, diagenetic alteration primarily affected the stable isotopic composition of sediment surrounding carbonate nodules, whereas the latter show almost pristine compositions. Diagenesis lowered the carbon and oxygen isotope values of bulk carbonate and introduced a strong correlation between δ13C and δ18O values. In the Middle Triassic successions of the Dolomites, bulk carbonate of nodular limestone facies is most commonly unaltered, whereas carbonate of the plane‐bedded facies is uniformly affected by diagenetic alteration. In contrast to carbonate nodules, plane‐bedded facies often show compaction features. Although both types of pelagic carbonate rocks show very similar petrographic characteristics, scanning electron microscopy studies reveal that nodular limestone consists of micrite (< 5 μm in diameter), whereas samples of the plane‐bedded facies are composed of calcite crystals ca 10 μm in size showing pitted, polished surfaces. These observations suggest that nodular and plane‐bedded facies underwent different diagenetic pathways determined by the prevailing mineralogy of the precursor sediment, i.e. probably high‐Mg calcite in the nodular facies and aragonite in the case of the plane‐bedded facies. Similar to Middle Triassic nodular facies, Upper Triassic nodular limestones of the Lagonegro Basin are also characterized by uncorrelated δ13C and δ18O values and exhibit small, less than 5 μm size, crystals. The alternation of calcitic and aragonitic precursors in the Middle Triassic of the Dolomites is thought to mirror rapid changes in the type of carbonate production of adjacent platforms. Bioturbation and dissolution of metastable carbonate grains played a key role during early lithification of nodular limestone beds, whereby early stabilization recorded the carbon isotopic composition of sea water. The bulk carbonate δ13C values of Middle and Upper Triassic hemipelagites from Italy agree with those of Tethyan low‐Mg calcite shells of articulate brachiopods, confirming that Triassic hemipelagites retained the primary carbon isotopic composition of the bottom sea water. A trend of increasing δ13C from the Late Anisian to the Early Carnian, partly seen in the data set presented here, is also recognized in successions from tropical palaeolatitudes elsewhere. The carbon isotopic composition of Middle and Upper Triassic nodular hemipelagic limestones can thus be used for chemostratigraphic correlation and palaeoenvironmental studies.

Sedimentary vs. diagenetic control on rhythmic calcareous successions (Pliensbachian of Asturias, Spain)

AbstractThe diagenetic overprint in rhythmic hemipelagic successions can either enhance or change the original distribution of CaCO3, but it is difficult to evaluate its effect because in most field examples, it is not possible to distinguish between sedimentary and diagenetic features. The rhythmic succession of the Pliensbachian of Asturias (Spain) shows alternation of bioclastic and laminated/burrowed intervals. The original content of carbonate brought by storms from shallow areas was larger in the bioclastic horizons. However, there is a widespread mismatch between facies alternations and the observed lithological rhythms. The diagenetic redistribution of CaCO3 resulted in successive limestone–marl/clay couplets that do not match (either in number or in the location of the boundaries) the sedimentary cycles defined by facies alternations. We conclude that interpreting the limestone–marl rhythms as a direct response to primary changes is highly questionable, unless there is unequivocal proof of a sedimentary origin of the alternation of the two lithologies.

The biostratigraphy of the Upper Pliensbachian-Toarcian (Lower Jurassic) sequence at Ilminster, Somerset

Abstract. Temporary road sections for the A303 bypass at Ilminster, Somerset, revealed Upper Pliensbachian and Toarcian sediments from an alternating limestone–marl facies, in marked contrast to the limestone-dominated Dorset coast succession. The lithostratigraphy is described, with the standard ammonite zonation providing chronostratigraphical correlation. The uppermost Pliensbachian (Spinatum Chronozone) and much of the Toarcian (Serpentinum, Bifrons, Variabilis, Thouarsense and Pseudoradiosa chronozones) of the classic Dorset coast Lower Jurassic sequence are represented by the Beacon Limestone Formation, formerly the ‘Junction Bed’ (a highly condensed carbonate deposit). The Tenuicostatum Chronozone is largely missing from the sequence. Elsewhere in southern and eastern England this interval is poorly or rarely exposed. The argillaceous units have yielded rich microfaunas and -floras (foraminifera, ostracods, calcareous nannofossils and palynomorphs), the biostratigraphical distributions of which are analysed and discussed in relation to contemporary evolutionary patterns and other UK records. This study provides an insight into the micro-biostratigraphy of the Late Pliensbachian to Toarcian interval for onshore southern England.

Geology of Neogene basins of Buldan-Sarıcaova region and their importance in Western Anatolia neotectonics

The Buyuk Menderes and Gediz (Alasehir) grabens are two significant segments of the Western Anatolian extensional province. They merge around Buldan-Saricaova in the east. Outcropping Neogene sedimentary units in this area display a rather complex structure. This paper summarizes the importance and meaning of the data obtained during a detailed investigation of the Neogene units and aims to improve our understanding of the neotectonic evolution of Western Anatolia. The Buldan-Saricaova Neogene sequence is composed of three different sedimentary units: (1) the Lower Unit, (2) the Middle Unit, and (3) the Upper Unit. The Lower Unit crops out on the Buldan horst which is located between the Buyuk Menderes and Alasehir grabens. The sequence starts as a coarse conglomerate and sandstone (Isalar Formation) and continues as lagoonal-lacustrine mudstone, interbedded with coal seams and shales (Bostanyeri Formation) and also with lacustrine limestones. The age of this succession is Lower-Middle Miocene. The development of the basin is structurally controlled by NNW-trending normal faults. The Middle Unit begins with a conglomerate–mudstone sequence (Kizilburun Formation), followed by a sandstone–mudstone–marl sequence (Saraykoy Formation). A lacustrine limestone–marl unit occurs at the top (Aktepe Formation). Some thin gypsum lenses and layers are observed in the Saraykoy Formation. The unit contains some brackish-water fossils. The rocks of the Middle Unit crop out mostly at the low altitudes of the Buldan horst, i.e. the southeast piedmont, around the junction of the Buyuk Menderes and the Gediz grabens. The Middle Unit was deposited in fluvial and lacustrine environments during the Late Miocene–Pliocene period. These rocks were formed in response to the uplift of the Buldan horst. The Upper Unit, which is composed of conglomerates, was deposited within the Buyuk Menderes Graben–Gediz Graben depressions as alluvial fill.

Methane seeps on an Early Jurassic dysoxic seafloor

The rhythmically bedded limestone–marl–shale succession of the Blue Lias Formation (Lias Group, Early Jurassic age) of Kilve in Somerset (SW England) preserves a suite of large conical concretions that formed around methane seeps. These are 1–2 m high, and elliptical in plan (axes 2–4 m), with an outer limestone shell forming the flanks of the cone. The cone flank is composed of micritic carbonate (20–30 cm thick), which locally includes sheets and pods of intraclasts and bioclasts. The cycle-forming limestone beds of the host strata are composed of dark grey micrite with carbon-isotope values ( δ 13C = 0.6 to 0.8‰) consistent with carbon sourced from a mixture of seawater and by sulphate reduction, and oxygen-isotope values ( δ 18O = − 6‰) suggesting some degree of later diagenesis. The pale grey micrite that forms the sides of the mounds includes three-dimensional ammonites and intraclasts, and thus cemented close to the sediment–water interface prior to compaction. The mound-forming carbonate is markedly isotopically light with respect to carbon, but not with respect to oxygen ( δ 13C = − 24.3 to − 26.4; δ 18O = − 2 to − 3.5‰). The isotope signature indicates that cements were probably derived from a mixture of sources that included biogenic methane. The intraclasts within the limestone suggest that syn-depositional physical brecciation and mixing of cements had occurred, and thus mixing of methane rich-fluids with the overlying surface waters is likely also to have occurred. The relatively heavy oxygen-isotope values may be indicative of anaerobic oxidation of methane. The mound-bearing interval of the Blue Lias Formation is benthos-poor and comprises predominantly laminated black shales, characteristic of poor bottom water oxygenation. The largest of the mounds is however, capped with fossiliferous breccias. Thus the mounds either formed benthic islands that elevated the biota into an oxic zone or, alternatively, they may have supported a chemotrophic community. Although cold seep deposits have been documented previously they are still comparatively rare. This example is one of the oldest in Europe, and is unusual amongst described ancient seeps in preserving relief that extended above the ancient seafloor.

Time series analysis of magnetic susceptibility variations in deep marine sedimentary rocks: A test using the upper Danian–Lower Selandian proposed GSSP, Spain

It has been clearly demonstrated that past climate fluctuations are recorded in marine sedimentary rocks. However, for many reasons, extracting the climate signature is difficult. Initial low-field mass-specific magnetic susceptibility (MS) data can potentially provide a measure of climate variability and thus become a proxy characterizing climate cyclicity in a wide range of marine sediments. This is due to the fact that climate change (warm, wet versus cold, dry) drives cyclic weathering and erosional variations that are recorded as the detrital components of marine sediment that dominates the MS. To test the utility of MS to yield climate proxies in marine sediments showing major changes in lithology, we have sampled the well-studied Danian/Selandian boundary interval (Lower Paleocene) at Zumaia (Zumaya), Spain. This interval represents a dramatic, rapid lithologic change from Danian carbonate-dominated limestone–marl couplets to a detrital-dominated marl-shale sequence in the Selandian, indicating onset of a major regression-erosional event beginning in the lowest Selandian. Sampling included a continuous sequence from the uppermost Danian Stage (3.71 m) into the lowermost Selandian Stage (5.2 m), a suite of 175 samples collected at 5 cm intervals. Our results indicate that MS measurements reflect changes in detrital sediment at the site, first by closely tracking high-frequency limestone–marl couplets, second with a large, rapid shift toward higher MS values beginning at the Danian/Selandian boundary resulting from a major regression, and third by tracking low-frequency climate-controlled variations known to have occurred during deposition of these sediments. MS zones developed from the cyclicity observed throughout the sequence, supported by time series analysis using Fourier Transform (FT) methods applied to the MS results, exhibit Milankovitch cyclicities in the precessional (19–24 kyr), obliquity (41–54 kyr) and eccentricity bands (100 kyr). This is in excellent agreement with previous FT work on the section using measured variations in cyclic bed thicknesses. With the new MS data set and FT results, we then developed a Floating Point Time Scale (FPTS) for the sequence sampled (covering ∼ 550 kyr through the Danian/Selandian boundary interval), yielding a time-scale resolution for the uppermost Danian to ∼ 10,000 years. However, only the ∼ 100,000 year eccentricity band for the Selandian is sufficiently well developed for an FPTS estimate, and yields a time-scale resolution of ∼ 50,000 years. Our test of the utility of MS data sets in this varying depositional setting demonstrated that these data can provide a climate proxy that is not disrupted by large lithologic changes.

Facies distribution and coral-microbialite reef development on a low-energy carbonate ramp (Chay Peninsula, Kimmeridgian, western France)

The Chay Peninsula (western France) succession corresponds to a Kimmeridgian shallow and low-energy ramp system that developed in the northeast of the Aquitaine Basin. The sedimentation shows a transition from a carbonate-dominated to a mixed carbonate–siliciclastic depositional environment that takes place at the base of a major, third-order sea-level transgression. Facies partitioning and coral-microbialite reef levels (i.e., reef windows) suggest higher-frequency (fourth-order) sea-level oscillations. Reef-growth phases (i.e., high-frequency reef windows) are interpreted to have been controlled by fifth-order sea-level oscillations. In the mixed siliciclastic–carbonate system, successive prograding units of shoreface sediments pass into thick marl–limestone alternations over a distance of tens of metres. Such remarkable short-distance lateral facies variations suggest a complex sedimentary system made of juxtaposed deposits that rapidly shift through time. Terrigenous fluxes, in modifying the light intensity and the nutrient level in the water column strongly controlled the facies distribution and notably the reef composition and growth. Identified coral genera suggest mixed photo-heterotrophic mode of nutrition and low-mesotrophic conditions in marine waters. For each reef-growth phase, nutrient increase favoured large amounts of microbialites to develop, forming a crust that entirely capped the coral reef surface. Thus, high-frequency fluctuations in nutrient input in tune with fifth- (or higher) order sea-level fluctuations controlled the reef development at a millennial scale. The Chay Peninsula reef distribution and development provide insight into understanding the response of reef communities under climatically-induced environmental changes.

Provenance of microcrystalline carbonate cement in limestone–marl alternations (LMA): aragonite mud or molluscs?

Aragonite derived from marine molluscs is evaluated as the source for microcrystalline carbonate cements of limestone–marl alternations (LMA). Calculations demonstrate that extremely low levels of mollusc-derived aragonite, well below the production rates of molluscs in modern marine settings, could have provided sufficient carbonate to cement examples of Ordovician, Silurian and Jurassic LMA in non-tropical or tropical settings. It is likely that even in the Palaeozoic molluscs provided sufficient carbonate entirely to source microcrystalline cements of LMA. Autochthonous molluscan aragonite is the only viable aragonite precursor for LMA microcrystalline cements of cool-water settings where temperatures precluded calcified algae and abiotic carbonate precipitation. In ‘calcite seas’ where Mg:Ca ratios inhibited both abiogenic aragonite precipitation and aragonite generation by calcified algae, molluscan aragonite was again the most likely main contributor. In some epeiric seas where brackish wedges switched off the shallow-water carbonate mud factories molluscan aragonite is the parsimonious source of carbonate for LMA microcrystalline cements.

High-resolution cyclostratigraphic analysis from magnetic susceptibility in a Lower Kimmeridgian (Upper Jurassic) marl–limestone succession (La Méouge, Vocontian Basin, France)

High-resolution magnetic susceptibility (MS) analysis was carried out on a Lower Kimmeridgian alternating marl–limestone succession of pelagic origin that crops out at La Méouge (Vocontian Basin, southeastern France). The aim of the study was to characterize the strong, dm-scale sedimentary cyclicity of the succession at a very high resolution, and to analyze the cycles for evidence of astronomical forcing. From marl to limestone, MS varies progressively and closely tracks the highest frequency cyclicity corresponding to the basic marl–limestone couplets. Long-term wavelength cycling modulates the high-frequency cyclicity (couplets), and appears to be controlled by clay content. Spectral analysis of the MS record reveals the presence of the complete suite of orbital frequencies in the precession, obliquity, and eccentricity (95–128 ka and 405 ka) bands with very high amplitude of the precession index cycles originating from dm-scale couplets. 405 ka-eccentricity cycles are very pronounced in the MS maxima of the marl members of the couplets, suggesting eccentricity-driven detrital input to the basin. 405 ka-orbital tuning of the MS maxima further sharpens all of the orbital frequencies present in the succession. These results are similar to those of previous studies at La Méouge that used carbonate content observed in field. Our results are also in accordance with cyclostratigraphic studies in Spain and Canada that report dominant precession index forcing. By contrast, in the Kimmeridge Clay (Dorset, UK), obliquity forcing dominates cyclic sedimentation, with weaker influence from the precession index. Ammonite zone duration estimates are made by counting the interpreted precession cycles, and provide an ultra-high resolution assessment of geologic time. In sum, this study demonstrates the power of the MS as a proxy in characterizing the high-resolution cyclostratigraphy of Mesozoic sections, particularly in alternating marl–limestone successions, and for high-resolution correlation and astronomical calibration of the geologic time scale.

Marine microplankton and calcareous nannofossil palaeoecology of the Toarcian stratotype

In the Bifrons to Aalensis Ammonite Zones of the Vrines section (Toarcian stratotype), woody phytoclasts, nonsaccate pollen grains, and marine assemblages (dinoflagellate cysts, acritarchs, and foraminiferal linings) dominate the palynofacies. The dinoflagellate cyst assemblage is cosmopolitan with minor Boreal influences, characterized by relatively high quantities of Micrhystridium, Baltisphaeridium, Mendicodinium spinosum, Nannoceratopsis, and the Parvocysta suite, dominating in turn the marine assemblages. Marine assemblage compositions, both dinoflagellate cysts and acritarchs, and calcareous nannofossil abundances are different in marl and limestone lithotypes of the Vrines section. Calcareous nannofossils are generally more abundant in marls than in limestones, they display however a cyclic pattern of semiquantitative abundances in phase with lithological cycles. Although a diagenetic overprint cannot be completely excluded to explain such a difference, it seems likely that these dissimilarities are in part primary, the results of variations in terms of proximality–distality, and climatic fluctuations. A mean duration of 117.6 Kyr per marl–limestone alternation, and the stacking of four marl–limestone alternations for 470.6 Kyr, suggest a control by the Earth's two orbital eccentricity cycles. It is likely that the palaeoenvironmental conditions, which influenced the formation of marl–limestone alternations, also controlled the variations in marine phytoplankton assemblages.

Ammonoid taphonomy, palaeoenvironments and sequence stratigraphy at the Bajocian/Bathonian boundary on the Bas Auran area (Subalpine Basin, south‐eastern France)

Bas Auran, in south-eastern France, is the candidate area for Global Boundary Stratotype Section and Point (GSSP) of the base of the Bathonian Stage (Middle Jurassic). In the Bas Auran area, upper Bajocian and lower Bathonian deposits are made up of limestone beds alternating with marls that correspond to the ‘Marno-calcaires a Cancellophycus’, below the ‘Terres Noires’ Formation. Taphonomic analyses of the successive ammonoid fossil assemblages provide new and complementary data for biostratigraphic completeness, palaeoenvironmental setting and sequence stratigraphy. Lithologic differentiation between limestone and marl intervals resulted from alternating episodes of carbonate input and starvation. Both lithologic phases may contain evidence for sedimentary and taphonomic reworking, associated with scours, that reflects low rates of sedimentation and stratigraphic condensation. Three successive types of elementary cycles resulted from increasing rates of stratigraphic condensation, sedimentary condensation and substrate stabilization during early Bathonian. The occurrence of reelaborated ammonoids (i.e. exhumed and displaced before their final burial) implies that tractive current flows or winnowing affected the burial of concretionary internal moulds. In the lower Bathonian strata, the dominance of homogeneous concretionary internal moulds of phragmocones, completely filled with sediment, is indicative of low rates of sedimentation and sediment accumulation, respectively associated with low degrees of stratigraphic and sedimentary condensation. However, at the Bajocian/ Bathonian transition, hemipelagic, bed-scale limestone–marl alternations show a maximum value of biostratigraphic completeness and there is no evidence for taphonomic condensation in the ammonoid fossil assemblages. Taphonomic analyses of the successive ammonoid fossil assemblages and taphofacies confirm the development of a deepening phase associated with sedimentary starvation, which characterizes the last episode within the deepening half-cycle of third and second order cycles, in the Bas Auran area of French Subalpine Basin during early Bathonian.