Lower Cretaceous Shallow-water Carbonates Research Articles

Synsedimentary tectonics vs paleoclimatic changes across the Aptian-Albian boundary along the Southern Tethyan margin: The panormide carbonate platform case history (NW Sicily)

The Aptian-Albian boundary is a deeply investigated time interval on the basis of the deep oceanographic, environmental and climatic changes, globally recorded along the sedimentary rock successions.A regional latest Aptian extensional pulse in the Southern Tethyan margin is here envisaged as being responsible for different events of sedimentation and erosion in the Panormide carbonate platform as highlighted by the outcropping sections studied from the NW Sicilian fold and thrust belt.Integrating facies and stratal pattern analysis with backstripping methods has permitted to define the interaction between tectonics and sedimentary factors of the Lower Cretaceous shallow-water carbonates.Based on geometric configuration, lateral facies relationships, and thicknesses variations of the Lower Cretaceous Requienid limestone, is here envisaged a paleophysiography characterised by banks separated by deep channels. The margins of the isolated carbonate platform were oriented towards both the present-day west, where originally the Cala Rossa intraplatform basin developed driven by transtensional movements, and to the present-day east, where the continental slope was spreading.Including subsidence analysis and significance of the top-unconformity of the Requienid limestone a tectonic event at the end of the Aptian was detected as responsible for the fragmentation of the platform in various rotating fault-blocks and reworking of shallow-water carbonates towards the slope. The variable subsidence rates correspond to the different responses, both emersion and drowning, of the various platform-blocks to the extensional tectonics.Paleoceanographic changes, detected around the world during the Aptian-Albian, are well recorded also from the Sicilian sections, where continental clays – as the product of weathering of crystalline rocks – metallic crusts (hardgrounds) and condensed phosphatized pelagic deposits – as the product of sediment starvation, condensation and drowning – cover the Lower Cretaceous shallow-water carbonates respectively in the different faulted blocks of the segmented carbonate platform. Chemical and mineralogical analyses confirm for these deposits an influence in their formation caused by variations in oceanographic and environmental conditions, as increasing of metal ions in the surficial sea-water and high rates of groundwater precipitation. Paleoclimate evaluation reveals changes from the greenhouse arid conditions, developed during the lower Cretaceous and permitting the sedimentation of the shallow-water carbonates, to warm-humid climate condition during the latest Aptian-lower Albian boundary.

Taxonomic revision of Dictyoconus? vercorii Arnaud-Vanneau, 1980 (Foraminifera, Orbitolinidae) from the Lower Cretaceous (lowermost Barremian–lower Aptian) of southeastern France, Switzerland and Spain

The new genus Vanneauina belonging to the family Orbitolinidae is described herein from Lower Cretaceous shallow-water carbonates of southeastern France. Its type-species, here referred to as Vanneauina vercorii (Arnaud-Vanneau, 1980) comb. nov., has been described originally as belonging with some reservations to Dictyoconus Blanckenhorn. Vanneauina gen. nov. differs from Dicytoconus above all by its simple exoskeleton (lacking rafters), triangular-shaped primary beams, pillars laterally fusing at the base and top, and a simple and slightly eccentric embryo. So far, Vanneauina is a monospecific genus. In its type-area, it is reported from the lowermost Barremian (upper part of the hugii zone) to the lower Aptian (lower part of the weissi zone). After the present taxonomic revision, there is no representative of Dictyoconus in the orbitolinid assemblages from the upper Hauterivian to lower Aptian interval in the Urgonian type area of southeastern France and Switzerland.

Fracture simulation parameters of fractured reservoirs: Analogy with outcropping carbonates of the Inner Apulian Platform, southern Italy

This work focuses on the nature, relative timing, and multi-scale dimensional properties of both background and fault-related fracture networks that crosscut the Lower Cretaceous shallow-water carbonates exposed at the Monte Alpi, southern Italy. Away from the major fault zones, fractures mainly consist of stratabound and non-stratabound joints and sheared joints, which form two main bed-perpendicular cross-orthogonal sets. Both sets are characterized by a Poissonian spacing distribution, they do not form well-defined clusters, and formed during either burial diagenesis or subsequent Upper Miocene foreland flexure of the Inner Apulian Platform. In contrast, the study fault damage zone is mainly crosscut by non-stratabound fractures and through-going slip surfaces, which form clustered conjugate systems characterized by power law spacing distributions. Results of Discrete Fracture Network modelling of geocellular volumes representative of the surveyed outcrops show that the fractured carbonates of the study fault damage zone form the main storage volume for underground fluids. There, non-stratabound fractures act as the main control on fracture porosity and contribute towards determining isotropic horizontal fluid flow properties. These isotropic conditions are interpreted as a factor of the profound control exerted by the pre-existing cross-orthogonal fracture sets, which underwent to an extreme uplift during the Plio-Quaternary tectonic evolution of the area. Data presented in this work also highlight the importance of scan area methods for the quantitative analysis of stratabound fracture sets.

Early dolomitization in the Lower Cretaceous shallow-water carbonates of Southern Apennines (Italy): Clues about palaeoclimatic fluctuations in western Tethys

A multidisciplinary study of the dolomitized bodies present in the Lower Cretaceous platform carbonates of Mt. Faito (Southern Apennines – Italy) was carried out in order to explore the connection between early dolomite formation and fluctuating climate conditions.The Berriasian-Aptian investigated succession is 466m thick and mainly consists of shallow-water lagoonal limestones with frequent dolomite caps. The dolomitization intensity varies along the succession and reaches its peak in the upper Hauterivian-lower Barremian interval, where it is present a completely dolomitized interval about 100-m-thick. Field relations, petrography, mineralogy, and geochemistry of the analyzed dolomite bodies allowed identifying two populations of early dolomites, a fine-medium crystalline (FMdol) and a coarse crystalline dolomite (Cdol), both interpreted as the product of mesohaline water reflux. According to our interpretation, FMdol precipitated from concentrated brines in the very early stage of the reflux process, producing typical sedimentary features as dolomite caps. In the successive step of the process, the basin-ward ‘latent’ reflux precipitated Cdol from less concentrated brines.A peculiar feature of the studied succession is the great consistency between stratigraphic distribution of dolomite bodies and their geochemical signature. The completely dolomitized Hauterivian-Barremian interval, in fact, is characterized by geochemical values suggesting an origin from distinctly saltier brines. Considering that the observed near-surface dolomitization process is controlled by physical and chemical parameters reflecting the paleoenvironmental and paleoclimatic conditions during dolomite formation, we propose that the stratigraphically controlled dolomitization intensity reflects periodic fluctuations in the salinity of dolomitizing fluid, in turn controlled by long-term climate oscillations.The present work highlights that the stratigraphic distribution of early diagenetic dolomite may be used as proxy to define the climatic fluctuations that have influenced the sedimentary dynamics in the Early Cretaceous. Moreover, considering that a comparable early dolomite distribution is present also in the Dinaric Platform, we suggest that a regional scale climate control acted on early dolomite formation and distribution. Refining the knowledge of such a key control may have a significative impact on hydrocarbon reservoir characterization and exploration in the Periadriatic area.

An extraordinary single-celled architect: A multi-technique study of the agglutinated shell of the larger foraminifer Mesorbitolina from the Lower Cretaceous of southern Italy

Orbitolinids are larger foraminifera widespread in Lower Cretaceous shallow-water carbonates of the Tethyan realm. They are among the most important fossil groups used for Biostratigraphy. Despite this and although the structural features of the group have been described in detail, very little is known about the composition of their agglutinated test and the process by which they selected foreign grains. In this study, the test of Orbitolina d'Orbigny, 1850 (subgenus Mesorbitolina Schroeder, 1962) from Aptian shallow-water carbonate deposits of southern Italy has been studied in detail. We combine petrographic techniques (optical microscope and SEM) with energy-dispersive x-ray spectrometry (EDS), electron probe microanalyzer (EPMA), X-ray diffraction and Raman spectroscopy analyses. The results show that the test of Mesorbitolina is composed of carbonate and non-carbonate agglutinated grains with the latter distributed across the test with a specific pattern, moving from the marginal to the central zone. In the marginal zone, non-carbonate grains are found only in the epidermis and along the septa which are composed of quartz, with smaller amounts of illite/muscovite and K-feldspar grains. In the central zone of the test, non-carbonate grains are distributed in two ways. Coarse grains of quartz and K-feldspar are abundant and randomly placed in the endoskeleton embedded in a mosaic of minute carbonate grains. Flat grains, mainly of illite/muscovite constitute the external part of the septa. Our observations indicate that Mesorbitolina did select and place agglutinated grains across its test, mainly according to their shape, whereas it did not select particles according to grain size. The distribution of agglutinated particles according to their mineralogical composition shows some contradictory evidence and therefore, at the moment, grain selection in function of mineralogy cannot be completely confirmed or ruled out. Analogies in the test composition of Mesorbitolina specimens from coeval deposits from different areas of southern Italy indicate that the features of their agglutinated test are typical characters of the genus Mesorbitolina . However, it is still unclear what advantage was obtained by the foraminifer by the described test features.

Eustatic cycles and tectonics in the Cretaceous shallow Tethys. Central-Southern Apennines

Carbonate platforms bordering the Tethyan margins carry a distinct periodic signal that can be related to Jurassic-Cretaceous climate and eustatism. They yield a rich archive of information, including the tectonics affecting the platforms at regional (subsidence) as well as at «local» (uplift) scale, at a time scale between 104 and 105/106 years (order of magnitude). In Central and Southern Italy, we have analyzed at centimetre scale, along well exposed sections and in bore cores, textures and early diagenetic features of Cretaceous carbonate platform deposits that evidence oscillations in which a hierarchy of cycles (elementary cycles, bundles and super bundles) has been recognized. Eustatic-climatic, high-frequency changes, linked to the Earth’s orbital perturbations, have been considered at the origin of this hierarchy, where the elementary cycles record the precession and/or the obliquity periodicities, while the bundles and super bundles record the short- and long-eccentricity, respectively. These orbital cycles are, in turn, superimposed on lower-frequency cycles (Trangressive/Regressive Facies Trends, T/RFTs). Adopting a sequence stratigraphy approach, the super bundles and the T/RFTs have been interpreted in terms of depositional sequences and used for high-resolution, long-distance (regional to supraregional) correlation, as well as for assembling orbital chronostratigraphic diagrams which quantify the minimum time required for each succession to stack up. Moreover, we have observed that a number of gaps are randomly intercalated in the various sections so that the high-precision correlation of distant intervals rises the problem of explaining their local absence. To reconcile these discrepancies, we propose a tectonic mechanism, already used to explain the stratigraphic gaps related to bauxitic horizons intercalated up section in the same stratal successions. Namely we postulate that the action of transient lithospheric bulges (few meters to tens of meters in elevation, few to several tens of kilometres across), arising from distant compressional or extensional tectonics, may give an explanation for the stratigraphic architecture characterizing the cyclic organization of the lower Cretaceous shallow-water carbonates. In conclusion, we assume that while the eustatic oscillations, driven by orbital and allied climatic variations, follow high-frequency composite rhythms of few tens to few thousand years (Milankovitch periodicities), the regularity of the subsidence is locally modified by transient lithospheric bulges, that result in increase (lows) and decrease (ups) of subsidence (and hence variation of rate of sediment deposited), up to the emersion of more or less large areas evidenced by omission of strata as well as by paleokarst and bauxites.

Foraminifera diversity changes and paleoenvironmental analysis: the Lower Cretaceous shallow-water carbonates of San Lorenzello, Campanian Apennines, southern Italy

A high-resolution stratigraphic study, carried out on the carbonate platform strata of the San Lorenzello section (Matese Mountains, southern Italy), Valanginian–Hauterivian in age, has allowed to: recognise lithofacies and their associations; assign the lithofacies associations to specific environments and individuate early meteoric diagenetic modifications, recurring at specific horizons. In this frame the vertical variation of benthic foram diversity has been analysed. On the whole, foraminiferal genera diversity decreases from open to restricted marine environments. Moreover, a climatic control on carbonate sedimentation is suggested by a Milankovitch cyclicity organised in elementary cycles, bundles and superbundles as well as by diagenetic characteristics testifying that humid and arid conditions alternated during the Early Cretaceous times. The orbital cyclicity is also documented by foraminiferal diversity changes, even if some discrepancies between the lithofacies and the diversity locally occur. Therefore, the above diversity changes do not appear to provide sufficient information for the sequence-stratigraphic interpretation of shallow-water carbonates.

Geomagnetic long-term secular variations in Italian Lower Cretaceous shallow-water carbonates

SUMMARY The remanent magnetic properties of an 88 m bore core are unrelated to either the dolomite content or the sedimentological textures and are considered to be carried primarily by biogenetic magnetite that was cemented in during very early diagenesis. Individual readings represent time intervals of c. 720±32 yr and, after 40 mT partial demagnetization, they provide an almost continuous record of averaged geomagnetic secular variations over a period of some 3.17 Myr. The magnitude of directional secular variation is twice that of the present day, despite being smoothed, and the secular variations appear to grade into polarity transitions, suggesting no diVerence in their mechanisms. The rates of change in direction between subjacent levels in the core have a log-normal distribution which extends smoothly beyond 90° and has a median value of 13°/700 yr, the same as for unsmoothed European secular variation during the last 2000 yr. The intensity of remanence, after 40 mT partial demagnetization, appears to provide a reasonable approximation to geomagnetic field intensity. This tends to be weaker when the direction is moving faster, reflecting averaging, but is unrelated to the distance of the vector from the mean direction; that is, it depends on the rate of change and not on the virtual pole latitude. The virtual poles, after correction for tectonic rotations about horizontal and vertical axes, have latitudes that form a strongly platykurtic Fisherian distribution, while their longitudes have a circular distribution on which are superimposed two Gaussian peaks, 180° apart. This bore core thus provides detailed information of smoothed geomagnetic secular variation in the Lower Cretaceous (127±3 Ma) which shows clear regularities in behaviour, some related to changes in the Earth’s orbital parameters.