Early Aptian Times Research Articles

Petrogenesis of Early Cretaceous volcanic rocks in Santos Basin, offshore SE Brazil: evidence for the Tristan da Cunha plume

ABSTRACT The Santos Basin comprises the most voluminous hydrocarbon accumulations in its ‘Pre-salt’ reservoirs. Magmatic rocks have been recorded in the basin mostly by the tholeiitic basalts (Camboriú Formation) within its rift megasequence at Hauterivian time. Recent studies have shown that magmatic rocks are also found interbedded with Aptian sedimentary rocks in the basin, related to its post-rift (mostly Aptian) megasequence. This paper presents petrological data obtained from sidewall core samples of magmatic rocks interbedded with Early Aptian coquina in a well drilled in the Santos Basin. The well comprises six magmatic sections of alkaline basalts being overlain and underlain by the Barra Velha limestones. The basalts are mostly plagioclase-phyric, amygdaloidal rocks but vary in texture and modal compositions. Magmatic compositions seem to have been slightly affected by alteration processes. Basalts comprise an alkaline, miaskitic series with relatively evolved compositions (MgO = 5.55 ± 1.50 wt%; Ni < 220 ppm; Cr < 400 ppm). The patterns in the Primitive Mantle normalized multielement diagram displays the typical Nb peak (0.3 < La/Nb(N)<0.7) of the OIB. Geochemical modelling showed that basalts within five magmatic sections are not cogenetic by differentiation, representing basaltic flows emitted from different vents. Modal batch partial melting model indicates that it is possible, although unlikely, to generate the differences in La/Yb(N) and Zr/Y ratios of the least evolved samples in the studied well by different partial melting degrees (0.5–11%) from the same mantle source. Binary mixing models using lithogeochemical datapoint to a role for the Tristan da Cunha plume tail in the petrogenesis of the alkaline basalts with no clear evidence for a possible role for the local subcontinental lithospheric mantle. The results of the petrogenetic models are consistent with a shallow MOHO and a high-temperature mantle below part of the Santos Basin in Early Aptian time.

The controlling factors of the spatio-temporal distribution of the upper Barremian to the upper Albian sedimentary succession in the Zagros folded belt, SW Iran

• Initiation of the Kazhdumi intrashelf basin started during the late Barremian . • Drowning of the Dariyan carbonate platform happened in the latest early Aptian . • Paleohighs re-activation evidenced from the early Aptian time. • More exploration potential expects in the Aptian-Albian sediments of Dezful Embayment. • Stratigraphic traps are predicted in the upper Barremian to the upper Albian sedimentary succession. The current study highlights the controlling factors of the spatio-temporal distribution of the upper Barremian to the upper Albian sedimentary succession which comprises a petroleum system in the Zagros folded belt, SW Iran. Combination of field surveys, seismic data (2D and 3D lines), cores and petrographic investigations, wireline logs, paleologs, as well as sequence stratigraphy analysis led to identification of four third-order depositional sequences. Sequence stratigraphy analysis delineates interaction of several factors in which controlled the spatio-temporal distribution of lithofacies. Interaction of an inherited paleotopography, regional tectonic activity together with reactivation of paleohighs, sea level fluctuations, climatic changes, and siliciclastic influx accompanied by nutrient levels played dominant role. Creation of the Kazhdumi intrashelf basin started during the late Barremian in the Dezful Embayment, which is much older than previously thought the late early Aptian. From petroleum geology point of view, our findings are expected to bring more opportunities for hydrocarbon exploration and production in the upper Barremian to the upper Albian deposits in the Dezful Embayment and Abadan Plain. Lateral and vertical lithological changes together with clinoform geometries and progradation associated with forced regressive wedges have provided suitable conditions for stratigraphic traps/plays in the study area.

Kovadúsulásos és alumíniumdúsulásos mállási és üledékképződési folyamatok a kora-albai folyamán a Csehbányai-medencében

The tectonically controlled uplift affecting various sectors of the Csehbanya Basin in early Aptian times seems to have been very inhomogeneous. As a result, individual, tectonically controlled blocks show very different denudation histories reflected by the basal layers of the Albian cover sequence (Tes Clay-Marl Formation) postdating the uplift-cumerosion event. At places where the Tes Fm is underlain by Jurasssic to Aptian formations these basal layers developed in the form of the clastic.Kepekő Member rich in chert fragments. At other places where the immediate bedrock of the Tes Fm is Late Triassic limestone or limestone with intercalated dolomitic layers, at the unconformity surface there are often bauxites or Al-rich clays belonging to the Alsopere Bauxite Formation. This suggests that the pre-Tes Fm. relief must have been highly variegated. Therefore both the depositional and the early diagenetic environments of the Alsopere Bauxite Fm and the clastic, cherty Kepekő Member could have been quite different. The differences may reflect different climates and/or different paleoenvironmental conditions. On the other hand since they both belong to one and the same major regional unconformity they have also some characters in common. These are: (1) the mostly local source material,(2) the predominantly oxidized nature of the sediment, (3) the abundance of “slack-water” microfacies, (4) sphericity, roundness and sorting of the coarse detrital material. However, unlike the bauxite, the material of the Kepekő Member is obviously of local origin. It is essentially an in situ dissolution residue without any signs of transport-related concentration/separation of even the chert fragments. Limestone-fragments in the bauxite on the other hand must have been added to the fine-grained sediment in the course of some transport/redeposition, to the distance of which we have no clue. The original source of the prebauxitic material is not known, either. Had it been of local origin it could not have been of the same age as the cherty Kepekő Member since the climatic and/or hydrologic conditions favourable for bauxitisation are substantially different from those favouring the accumulation of the silica-rich sediments, particularly because the latter show even some degree of silica mobilization and reprecipitation.

High resolution δ13C stratigraphy through the Selli Oceanic Anoxic Event (OAE1a) in the Apulia carbonate platform: the Borgo Celano section (western Gargano Promontory, Southern Italy)

We show that the global-scale environmental perturbations leading to the Selli OAE1a were clearly recorded in the middle Bedoulian Apulia carbonate platform at Borgo Celano, in the western Gargano Promontory (Southern Italy). The studied section (Bari Limestones), which is 72.45 m thick and spans the Late Barremian p.p. -latest Early Aptian time interval, has been logged with an average resolution of 3 samples per metre for chemostratigraphic analyses and up to 7 samples per metre for facies analysis in the aim of detailed paleocologic and paleoceanographic investigation. It contains the complete biosedimentary and chemostratigraphic record across the Selli OAE1a, as suggested by a consistent match of the observed isotope excursions with those documented in the Cismon pelagic reference section and the lack of significant non-depositional or erosional hiatuses within the chemostratigraphic Selli level: biostratigraphic and sedimentary-diagenetic analyses document a continuous record of fully subtidal conditions. Furthermore, the Borgo Celano section allows correlation of the shallow- and deep-water records without intervening stratigraphic breaks. Upper Barremian-Lower Bedoulian, chlorozoan-type carbonates showing subtidal-intertidal Milankovitch-type cyclicities characterize the lower part of the Borgo Celano section (C2 pro parte isotopic segment). Starting from the latest C2 segment, a remarkable Crisis Interval (CI) about 4 m thick shows an increasing frequency of bivalves ( Chondrodonta and requieniids), the disappearance of green algae and a reduced diversity of foraminiferal assemblages (foramol-type carbonates). This interval is sharply overlain by an interval, about 3.70 m thick, that is largely dominated by “out-of-balance” microbial limestones almost devoid of metazoans, apart from sparse, small-sized requieniids and Chondrodonta , which accompanies the C3-middle C4 isotope segment. It is interpreted to reflect a sharp hyperthermal and acidification event induced by excess p CO 2 . Starting from the upper C4 segment, less severe but still threatening environmental conditions permitted the deposition throughout the lower C5 segment of peculiar shallow-water tropical-like carbonates, which are dominated by oncoidal floatstones and microbial wackestone/packstone emphasizing the enduring influence of benthic cyanobacteria during the Selli OAE1a. Upsection guilds are dominated by foraminiferal and peloidal wackestones/packstones with sparse bivalves and green algae (upper C5-lowermost C7 isotope segments). Similarly to other carbonate platforms of the central Tethyan Realm (e.g., Apennine and Adriatic), the resilience of the Apulia carbonate platform inner shelf to the environmental perturbations that led to the coeval drowning of its margin, the starvation of its slope and to the deposition of black shales in the bounding Ionian basin is a striking supraregional feature. The inception of the resilience of the Apulia carbonate platform inner shelf is coincident with the lithostratigraphic boundary in between the Borgo Celano and Chondrodonta and requieniids members of the Bari Limestones and physically corresponds in the field with the base of a significant, 1 m thick, Chondrodonta -rich marker bed. These observations confirm the value of routinely applied field and laboratory studies for precise chemostratigraphic and bio-chronostratigraphic correlations on a regional scale. Furthermore, chemostratigraphic data presented here corroborate recent paleoceanographic and stratigraphic models proposed for the Lower Aptian across the transition between the Apulia carbonate platform and the bounding Ionian basin, and provide further elements for regional reconstructions of historical patterns.

Mapping the rise and demise of Urgonian platforms (Late Hauterivian - Early Aptian) in southeastern France and the Swiss Jura

Lower Cretaceous carbonate deposits historically called “Urgonian limestones” are widely exposed around the margins of the Vocontian basin in southeastern France and in the adjacent Swiss Jura. This paper presents the history of their rise, growth and sudden demise. Eleven maps were constructed for deposits ranging in age from the Late Hauterivian pro parte to the Early Aptian (Bedoulian) pro parte. Based on sequential interpretations, they illustrate the present geographical distribution of the inner platform facies (Urgonian limestones stricto sensu, with rudists), the outer platform facies (essentially bioclastic deposits) and the basinal facies (slope, hemipelagic, pelagic deposits). These maps depict only the final terms of each successive sequence (the late highstand intervals). Chronostratigraphy is constrained by ammonites found mainly in basinal deposits, by echinoids, by rudists and to a lesser extent by dinoflagellates and calcareous nannoplancton. Inner platform, outer platform and slope (talus) deposits are dated by rich assemblages of orbitolinids and dasycladalean algae. Currently 39 species of orbitolinids have been recognized and their ranges collated with those of the ammonites in the area.In the Jura and in Provence the oldest Urgonian deposits are dated early Late Hauterivian, thus showing the synchroneity of the onset of platform carbonates development on both the southern and northern margins of the basin. Thereafter, growth of the platforms led a clearly regressive shallowing-upward trend, resulting from a stepwise progradation toward the center of the Vocontian area, coordinated with cyclical exposures in the inner platform areas. The maximum reduction of the platform deposits occurred early in Late Barremian times, coeval with a noticeable turnover in the orbitolinids assemblages.Thereafter, carbonate platform deposition shifted toward the margins of the Vocontian basin. In Early Aptian time, a well-dated discontinuity of regional extent marks the sudden, almost synchronous disappearance of the Urgonian deposits.

Lead isotopic record of Barremian–Aptian marine sediments: Implications for large igneous provinces and the Aptian climatic crisis

We present initial isotopic ratios of lead for Early Cretaceous (Barremian–Aptian) sections from Shatsky Rise (Pacific) and Gorgo a Cerbara (Italy). Our Pb isotopic data track an interval representing Oceanic Anoxic Event (OAE)-1a, which is characterized by quasi-global deposition of organic carbon-rich black shale. Pb isotopic compositions of sediments from Shatsky Rise decrease at the end of Barremian time, from radiogenic continental values to unradiogenic values, and subsequently remained less radiogenic until the end of early Aptian time. We explain the isotopic shift by a significant increase in supply rate of unradiogenic Pb, most likely due to massive volcanism. In contrast, the Pb isotopic compositions from the Italian section, which was situated at the western end of Tethys, are mostly identical to those of upper continental crust, showing no significant change in supply rate of unradiogenic Pb. The discrepancy between two sites is attributed to quiescent deep-submarine eruptions of Pacific large igneous provinces (LIPs) such as the Ontong Java Plateau (OJP), which severely limited dispersion of Pb-carrying particles out of the Pacific Ocean. Published Os isotopic data from the Italian section indicate two episodes of massive eruptions of OJP or contemporaneous Manihiki and Hikurangi plateaus starting from earliest Aptian time, slightly later than that indicated by the sedimentary Pb isotopic record from Shatsky Rise. Differences in isotopic variations between Pb and Os likely reflect differences in their chemical behaviors in the oceans, i.e., Pb isotopic compositions would have varied in response to local or regional changes in sediment provenances, whereas large-scale changes in Os inputs are required to explain variations in seawater Os isotopic compositions. Our Pb isotopic data, together with the published Os isotopic record, provide new evidence for the eruptive history of OJP together with contemporaneous Pacific plateaus and its environmental consequences, starting from end-Barremian time and extending through early Aptian time.

Tectonosedimentary evolution related to extreme crustal thinning ahead of a propagating ocean: Example of the western Pyrenees

In this paper we describe the tectonosedimentary evolution and its subsequent inversion of a basin that underwent extreme crustal thinning in a transtensional setting ahead of a propagating ocean in the western Pyrenees. The Labourd‐Mauléon area situated in the western Pyrenees, at the termination of the V‐shaped Bay of Biscay, is an ideal natural laboratory to study how such complex basins evolve in time and space. Because of a mild inversion of the basin during Pyrenean compression, the rift structures and their relations to basement rocks and sediments are exposed and can be directly studied in the field. The basin shows a complex polyphase evolution that starts with left‐lateral dominated transtension in latest Jurassic–early Aptian time. This event is overprinted by a late Aptian–early Albian extension that is related to the counterclockwise rotation of Iberia away from Europe leading to the opening of the Bay of Biscay. During this stage, the Late Triassic to Jurassic carbonate platform was stretched, salt migrated, and detachment faults exhumed upper and lower crustal and mantle rocks to the seafloor. The final structure of the basin resembles a sag basin floored by exhumed rocks overlain by extensional allochthons and compartmentalized by N40° to N60° transfer faults. The sedimentary architecture is characterized by late Aptian synrift sediments (e.g., Urgonian limestones) that were deposited in fault‐bounded basins and are overlain by thick latest Aptian to Albo‐Cenomanian sediments (e.g., Flysch noir) that define a sag sequence. The complex tectonosedimentary evolution of the basin is associated with salt tectonics and overprinted by a major magmatic/thermal event that postdates mantle exhumation.

Discussion of: Development of carbonate platforms on an extensional (rifted) margin: the Valanginian–Albian record of the Prebetic of Alicante (SE Spain), by J.M. Castro et al., Cretaceous Research 29 (2008), 848–860

The original reconstruction (Granier, 1987, and subsequent revisions) of the Early Cretaceous tectono-sedimentary evolution of the northeastern border of the Citrabetic basin in the Alicante province (Spain) was made possible by the integration of sedimentological and palaeontological data. Its findings are based on direct evidence (synsedimentary fractures, reworking, etc.) that differentiates it significantly from the model proposed by Castro et al. (2008). Errors in their analysis of both the sequence and timing of events are corrected. In addition, their negligence with regard to citations of previous work in the region is discussed. In the Citrabetic basin a thick dominantly-siliciclastic wedge developed at the start of the Valanginian, as a consequence of the major transgression that flooded the "Upper Jurassic" carbonate platform. A rift phase occurred later in Valanginian times; the related block-faulting and fracturing persisted up to early Aptian times. At the edges of the blocks and in the starved basins they delimit, the synrift series is characterized by condensed sedimentation (ferruginous oolites, glauconite). This block-faulting initiated the migration of Triassic salts which possibly reached an apogee in Albian times.

Neocomian to early Aptian syn-rift evolution of the normal to oblique-rifted North Gabon Margin (Interior and N'Komi Basins)

The North Gabon coastal rift basins consist of a set of 130–150 long-segment asymmetrically tilted half grabens (Interior Basin) and 000–020 short-segment en échelon half grabens (N'Komi Basin) separated by 040–060 major transverse faults. Tectono-sedimentary analysis of field and subsurface data reveals the control exerted by extensional tectonism over continental sedimentation. During Berriasian to early Barremian times, uniform uniaxial 040–060 extension was responsible for the stretching of the brittle upper crust over a 100-km wide domain. During late Barremian–early Aptian times, the main locus of extension stepped westward resulting in severe end-rift uplift and erosion of the failed Interior and N'Komi rift basins. Early Cretaceous coastal rifts in North Gabon display a wide range of styles from oblique rifting (N'Komi Basin), normal rifting (Interior Basin) to transform rifting. The pre-existing Precambrian tectonic fabric exerts a strong control over the mode and over the 100–300 km-scale segmentation of the rifting.

Destruction of a fluvial reservoir by hydrothermal activity (Cameros Basin, Spain)

This study provides an example of a high-quality fluvial hydrocarbon reservoir that was completely destroyed by hydrothermal processes. The reservoir unit was deposited in the Cameros Basin, located in the NW sector of the Iberian Chain (Spain). The basin was filled with clastic fluvial deposits (sandstones and conglomerates) between Late Berriasian and Early Aptian times. Provenance of sands was mainly from coarse crystalline rocks. A humid tropical climate produced intense weathering of K-feldspar during transport from source to basin. Thus, a mineralogically mature rigid framework with high porosity existed at the time of deposition, which would have constituted a high-quality hydrocarbon reservoir. At present however, the porosity of the reservoir is negligible. Porosity was reduced by a sequence of diagenetic processes: (1) mechanical compaction (i.e. crushing of metamorphic lithic grains) and chemical compaction, (2) kaolinite and siderite cementation, and (3) early quartz cementation. Hydrocarbon emplacement probably occurred between phases (2) and (3). A low-grade metamorphic (hydrothermal) event, reaching greenschist facies, took place during the Late Cenomanian. It dramatically reduced the remaining porosity of the reservoir and destroyed the hydrocarbon charge. Hydrothermal processes which affected the sandstones include (1) re-compaction; (2) late quartz cementation and silicification of remaining feldspars; (3) carbonate cementation; (4) chloritization of feldspars, metamorphic lithic fragments and intrabasinal argillaceous grains; and (5) growth of pyrite and chloritoid crystals on argillaceous material of intrabasinal, extrabasinal or even diagenetic origin. Hydrocarbons that migrated to the margins of the basin escaped these hydrothermal modifications and were preserved. The results of this study may be used to predict the diagenetic and hydrothermal evolution of other potential reservoirs in similar tectonic settings.

Platform–slope transition during rifting: The mid-Cretaceous succession of the Amasya Region (Northern Anatolia), Turkey

The mid-Cretaceous rocks of the Amasya region, North Central Anatolia record the geological processes that occurred during the break up of the eastern Pontide carbonate platform which existed from mid-Jurassic to Early Cretaceous. The mid-Cretaceous sedimentary rocks consists of benthic foraminiferal packstones/grainstones of Ferhatkaya Formation, and limestone breccias, turbidites, pelagic foraminiferal wackestones/packstones and radiolarites of Sarilar Formation corresponding to platform, and slope/apron environments, respectively. The lithological development of these units was controlled mainly by active subsidence due to block-faulting affecting the Eastern Pontide carbonate platform. Due to the break-up of the platform during Late Barremian–Early Aptian times, slope and basin sediments were deposited on the deeper parts of the platform. Re-deposited sediments such as limestone breccias and calciturbidites document the infilling of asymmetric grabens during the mid-Cretaceous rifting phase. Re-deposited sequences, which were formed as a fault slope-apron sedimentary prism related to this rifting, were deposited during thinning-and-fining upward megacycles. A typical megacycle commences with breccias deposited at the base of a fault-scarp association. In addition, the upper part of the platform carbonates (Ferhatkaya Formation) includes neptunian dykes composed of red mudstones with pelagic fossils, indicating a period of extension.

Controls on the Evolution of Carbonate Mud Mounds in the Lower Cretaceous Cupido Formation, Northeastern Mexico

ABSTRACT The Lower Cretaceous Cupido Formation in northeastern Mexico represents part of an extensive post-rift carbonate platform system that stretched across much of the Gulf of Mexico passive margin. Numerous sponge-microbial mud mounds are found within the lower part of the Cupido Formation; four mound types were identified at Bustamante Canyon and other localities in the State of Nuevo Leon, Mexico. Type 1 siliceous sponge-microbial mounds contain boundstone core facies, are the oldest mounds, and are associated with adjacent deep-ramp to basinal lime mudstone facies. Type 2 sponge-microbial mounds also have boundstone core facies, are associated with outer-ramp wackestone-packstone facies, and overlie strata that contain Type 1 mounds. Type 3 sponge-microbial-coral mounds contain only 40-50% boundstone core facies and overlie or are locally interstratified with Type 2 mound-bearing strata, although Type 3 mounds are associated with outer-ramp packstone facies. Type 4 calcisponge-microbial-coral biodetrital (dominantly skeletal packstone) mounds are the youngest mounds and are associated with outer-ramp packstone and ramp-crest rudist packstone facies. Both the length and height (i.e., synoptic relief) of individual Types 1, 2, and 3 mounds range from a few meters to tens of meters. Only Type 4 mounds reach several hundred meters in length and are up to 90 m thick, although synoptic relief on Type 4 mounds was minor. Systematic changes in the biota, morphologies, dimensions, abundance, stratigraphic distribution, and facies associations of the lower Cupido mounds were largely controlled by the initial ramp-like depositional profile of the lower Cupido platform and the long-term accommodation trends that affected ramp evolution during Early Cretaceous time. The ramp-like profile was not conducive to transport of large volumes of allochthonous sediment from updip source areas to mound locations on the outer ramp. This allowed slowly accumulating Types 1, 2, and 3 mounds to build synoptic relief because adjacent outer-ramp facies accumulated at even slower rates. In contrast, Type 4 mound cores contain abundant coarse-grained skeletal sediment and also developed the least synoptic relief of all lower Cupido mounds because of higher accumulation rates associated with adjacent grain-rich ramp-crest lithofacies. We suggest that these systematic changes in mound attributes largely reflect long-term (second-order) accommodation loss and regional progradation of the lower Cupido ramp during Early Cretaceous time. Accordingly, the upward transition from Type 1 to Types 2 and 3 mounds, and ultimately to Type 4 buildups, can be explained by the progressive shallowing of outer-ramp environments as more updip facies tracts prograded eastward from late Hauterivian to early Aptian time. There are few stratigraphic successions in the rock record that provide better evidence for the links between large-scale stratigraphic relationships and mound evolution along low-gradient carbonate ramps.

Organic geochemistry and depositional history of Upper Albian sediments from the Kirchrode I borehole, northern Germany

In the framework of a multidisciplinary research program, an organic geochemical study was carried out on a drill core which comprises a 245 m thick sequence of light-colored, Upper Albian marlstones that were deposited in the central part of the Lower Saxony basin (northern Germany). For part of the Upper Albian sequence, high-resolution measurements of carbonate contents reveal cycles which can be related to earth orbital forcing. Based on these data, sediment accumulation rates were calculated to be in the order of 15 g/m 2yr 1. These high accumulation rates contrast with very low organic carbon contents and an extremely poor preservation of the autochthonous organic matter. Most of the sedimentary organic matter is of terrigenous origin and mainly derived from the erosion of older sedimentary rocks. Organic petrography reveals only a very small fraction of marine organic particles. Carbon/sulphur ratios, pristane/phytane ratios as well as the predominance of resedimented organic particles over autochthonous organic particles suggest that aerobic degradation processes rather than anaerobic processes (sulphate reduction) were responsible for the degradation of the organic matter. Furthermore, the scarcity of terrigenous organic particles (vitrinite) indicates that there was little vegetation on nearby land areas. To explain these analytical results, a depositional model was developed which could explain the scarcity of organic matter in the Upper Albian sediments. This model is based on downwelling of oxygen-rich, saline waters of Tethyan origin, which reduces the nutrient content of surface waters and thus primary bioproductivity while degradation of primary organic matter in the water column is enhanced at the same time. These conditions contrast to those which existed in Barremian and early Aptian times in this basin, when limited water exchange with adjacent oceans caused oxygen deficiency and the deposition of numerous organic carbon-rich black shales. The thick, organic matter-poor Upper Albian sequence of northern Germany also contrasts with comparatively thin, time-equivalent, deep-sea black shales from Italy. This discrepancy indicates that local and regional oceanographic factors (at least in this case) have a greater influence on organic matter deposition than global events.

Upper Permian-Lower Cretaceous clay mineralogy of East Greenland: provenance, palaeoclimate and volcanicity

AbstractThe clay mineralogy of Upper Permian–Lower Cretaceous mudstones from East Greenland has been investigated by X-ray diffraction (XRD), atomic force microscopy (AFM) and thermal analysis in order to evaluate long-term trends in provenance and palaeoclimate and to detect possible volcanic events. The Upper Permian–Lower Triassic mudstones contain illite, chlorite, vermiculite, kaolinite and illite-smectite (I-S), whereas the Rhaetian–Sinemurian mudstones are dominated by kaolinite. Aalenian–Albian mudstones contain kaolinite and large amounts of I-S with ˜80% illite layers. Exceptions are three Kimmeridgian samples, which contain mainly I-S with 30% illite layers, and three Upper Barremian–Lower Aptian samples with large amounts of smectite layers. Discrete clay minerals in the Upper Permian–Jurassic mudstones are largely detrital. The smectite-rich I-S probably reflects episodes of volcanic activity in late Jurassic and late Barremian–early Aptian times. This is the first indication of Mesozoic volcanism from the Mesozoic rift basin of East Greenland. The main sediment source during late Permian–early Cretaceous times was weathered Precambrian and Caledonian crystalline basement. The only possibly climate-induced change is a change from chlorite, illite, vermiculite and kaolinite in Upper Permian–Lower Triassic mudstones to kaolinite and I-S in the Jurassic mudstones and is probably due to an increase in precipitation.

40Ar/39Ar geochronology and palynology of the Cerro Negro Formation, South Shetland Islands, Antarctica: A new radiometric tie for Cretaceous terrestrial biostratigraphy in the Southern Hemisphere

The upper part of the Jurassic‐Cretaceous Byers Group, exposed on Byers Peninsula in the South Shetland Islands, Antarctica, consists of 1.4 km of non‐marine strata assigned to the Cerro Negro Formation. Silicic pyroclastic units close to the base of the formation have yielded new 40Ar/39Ar ages of 120.3 ± 2.2 Ma on plagioclase from one horizon, and 119.4 ± 0.6 and 119.1 ± 0.8 Ma on biotite and plagioclase from a second horizon. Plagioclase from a welded ignimbrite close to the topmost exposed part of the formation has given an 40Ar/39Ar age of 119 ± 3.0 Ma. These ages indicate that the Cerro Negro Formation was deposited during a relatively short period in early Aptian times. The identification of palynomorph taxa has enabled us to propose correlations for the Cerro Negro Formation with spore/pollen zonations of South America and Australia. The presence of Interulobites pseudoreticulatus, Appendicisporites and F. wonthaggiensis in the Cerro Negro Formation supports correlation with the Interulobites‐Foraminisporis and the lower part of the tectifera‐corrugatus zones in southern South America. The presence of Foraminisporis asymmetricus and other palynomorphs suggests correlation with the Cyclosporites hughesii Interval Zone of early to late Aptian age in Australia. These data represent a valuable addition to the few radiometric ties available for Mesozoic terrestrial palynostratigraphy in the Southern Hemisphere.

The Barremian–Aptian turnover of biota in northwestern Europe: evidence from belemnites

`Neocomian' (Valanginian–Barremian) and `Gault' (Aptian–Albian) are lithostratigraphic terms used in the last century to divide the sub-Chalk Cretaceous of northern Europe. The division is based on significant palaeoceanographic and palaeogeographic changes in the early Aptian. These changes are linked to a major turnover of marine floras and faunas, which occurred simultaneously in the Boreal Realm, the Tethys and the Indo-Pacific Ocean. Various floral and faunal elements became extinct in the Aptian both in the Tethys and in the Boreal Realm, while on the other hand new genera and species evolved. Marked changes in the diversification rate were observed for organisms with a different mode of life (plankton, nekton, benthos). Apart from calcareous nannofossils, foraminifera and ammonites, this turnover also occurs in belemnites, and the suborder Belemnitina was completely replaced by the suborder Belemnopseina. The northwestern European Barremian is characterised by the very successful belemnite family Oxyteuthididae (Belemnitina), which was completely replaced by the genus Neohibolites (Belemnopseina) in early Aptian times. This Aptian turnover, which is expressed by a homogenisation of marine floras and faunas, is best explained by significant palaeoceanographic changes occurring in the early Aptian. The sea-floor spreading rate, which increased at the Barremian–Aptian boundary significantly, peaked in the late early Aptian. A mantle plume in the Pacific possibly caused an additional production of oceanic crust. Both processes caused a major sea-level rise. This in turn resulted in a major change of the palaeoceanographic setting and restricted marginal seas, like the northwestern European basins in Barremian times, became drowned. The more open oceanic setting of the Aptian enabled cosmopolitan floras and faunas to spread worldwide. The Aptian saw the radiation of planktonic foraminifera, which played only a minor role in pre-Aptian times. These palaeoceanographic changes, from restricted marginal seas to a more homogeneous pelagic setting, are considered to be the main cause for the significant changes in the composition of belemnite faunas.

The Jurassic–Lower Cretaceous Byers Group, South Shetland Islands, Antarctica: revised stratigraphy and regional correlations

The Byers Group, exposed in the western South Shetland Islands, is a thick (at least 2.7 km) succession recording Late Jurassic–Early Cretaceous sedimentation and volcanism on the Pacific flank of the Antarctic Peninsula magnetic arc. At least 1.3 km of marine clastic rocks are overlain by 1.4 km of non-marine strata. Penecontemporaneous, mainly intrusive, igneous rocks are present in much of the succession. At the base of the Byers Group, radiolarian-rich, hemipelagic mudstones of the Kimmeridgian–Tithonian Anchorage Formation are succeeded by terrigenous, slope-apron mudstones and sandstones of the Berriasian President Beaches Formation. These deep-marine strata are overlain by shallower-marine conglomerates and mudstones of the Berriasian–Valanginian Chester Cone Formation. The upper part of the marine succession also includes the volcanic breccias of the Start Hill Formation, interpreted as part of a debris-apron flanking a substantial volcanic edifice. Finally, following a Valanginian–Aptian erosional hiatus, non-marine volcaniclastic rocks of the Cerro Negro Formation were deposited in early Aptian times. Comparison with time-equivalent successions elsewhere in the Antarctic Peninsula region suggests that within the overall regressive trend, the Byers Group records regionally significant pulses or arc uplift in Tithonian–earliest Berriasian, latest Berriasian, and late Valanginian–Earliest Aptian times.

El cretácico medio y superior de la parte meridional del Bloque Maya (Guatemala)

Study of the Cretaceous strata of the Altos Cuchumatanes (western Guatemala) and in Alta Verapáz (central Guatemala) has revealed a number of foraminifers never before reported from these areas. These data permit a similar nomenclature to be applied to both areas in regard to the division of the sequence into formations. Thus, we distinguish in both areas: a) the Cobán Formation, a shallow platform carbonate of early Aptian to early Senonian age; b) the Campur Formation, of Campanian age, composed of a lower carbonate member with rudists and large benthonic foraminifers, and an upper carbonate member with pelagic foraminifers; c) the Sepur Formation, of Maastrichtian age, with flysch-like sediments, including intercalated blocks of ophiolite complex obducted during the Maastrichtian. We conclude that in early Aptian time, a shallow carbonate platform developed along the southern margin of the Maya Block. The platform subsided and was drowned during the terminal Campanian. This subsidence led to the formation of a flysch-like basin that received turbiditic sedimentation from the carbonate platform to the north and fragments of serpentinite from the ophiolite complex to the south.

Stratigraphie et paleogeographie du Cretace inferieur de l'anticlinal de Gourdan et du bassin du Var moyen de Daluis a la Mescla

Abstract The Var basin (Provence Alps, southeastern France) is separated from the Esteron basin to the south by the Gourdan anticline. During much of early Cretaceous time the geography of this area closely resembled the present, with two basins separated by a fault scarp at the position of the present Gourdan anticline. In the Var basin a thick Vocontian-type section of dark pelagic marls to the west graded into a neritic section of thicker Berriasian sediments, very calcareous Valanginian sediments, and glauconitic Hauterivian deposits to the east. In the Esteron basin to the south, sediments were subneritic, with numerous interruptions of sedimentation and abundant glauconite. After Bedoulian (early Aptian) time the scarp disappeared so that the basins coalesced and sedimentation became uniform.