Middle Aptian Research Articles

Lithostratigraphy and biostratigraphy of some Lower Cretaceous outcrops from Northern Sinai, Egypt

Lower Cretaceous successions of the Risan Aneiza, El Tourkumanyia and Manzour areas (Northern Sinai, Egypt) were studied for biostratigraphic purposes. These successions were subdivided into three rock units, from base upwards: the Malha, the Risan Aneiza and the Halal formations. Each formation included enriched cephalopod horizons. Fifty cephalopod species were identified. The age and stratigraphic distribution of these cephalopods allowed the recognition of eight ammonite zones ranging in age from Barremian to late Albian. These zones are, from base to top: Subpulchellia oehlerti Zone (late Barremian), Barremites difficilis Zone (late Barremian), Deshayesites deshayesi Zone (early Aptian), Aconeceras nisus Zone (early middle Aptian), Epicheloniceras tschernyschewi Zone (late middle Aptian), Acanthohoplites nolani Zone (late Aptian), Knemiceras gracile Zone (early Albian) and Mortoniceras inflatum Zone (late Albian). The identified ammonite zones were correlated with national and standard international ammonite zones as proposed in previous studies of the same time interval.

Holostratigraphy of the Kahmah regional Series in Oman, Qatar, and the United Arab Emirates

The stratigraphic framework of the uppermost Jurassic - Lower Cretaceous interval of the Gulf area is revised using both historical and recently acquired paleontological (ammonites, calpionellids, foraminifers, 'calcareous' algae), sedimentological and sequential information. The Kahmah regional Series ranging in age from Late Tithonian to Gargasian (= middle Aptian) times is subdivided into regional stages, named from bottom to top: Rayda (with two substages, Bu Haseer and Belbazem), Salil, Zakum, Lekhwair, Kharaib, Hawar, and Shu'aiba. The Kahmah rests either on strata representing the Habshan regional Stage, which is the last term of the Sahtan regional Series (locally absent due to a stratigraphic hiatus in basinal areas), or on much older strata; it is followed either by the Bab regional Stage (locally absent due to a stratigraphic hiatus on platform areas), or by the Sabsab regional Substage (the lowermost part of the Nahr Umr regional Stage), both of which are assigned the Wasi'a regional Series. The Kahmah succession (as well as those of the Sahtan below and the Wasi'a above) is discontinuous, i.e. punctuated by sedimentary hiatuses due to forced regressions, some of significant importance (in the Zakum or in the Bab, and those bounding the Hawar and the Shu'aiba).

The Douvilleiceratidae (Ammonoidea) of the Lower Aptian historical stratotype area at Cassis-La Bédoule (SE France)

Recent biostratigraphic research in the marly limestones of the Cassis-La Bedoule area (SE France) provided a rich macrofauna of Douvilleiceratidae Parona & Bonarelli, 1897. From the uppermost Barremian (Pseudocrioceras waagenoides Subzone) to the middle Aptian (Parahoplites melchioris Zone), specimens of Procheloniceras, Cheloniceras, Roloboceras, Megatyloceras and Epicheloniceras were collected in succession. In this paper we describe the various genera and species from this material and delimit precisely their stratigraphic positions. Our study shows that each genus or subgenus characterizes a discrete stratigraphic interval. In addition, the Cheloniceras meyendorffi (upper Bedoulian), Epicheloniceras debile, Epicheloniceras gracile, and Epicheloniceras buxtorfi (Gargasian = middle Aptian) subzones, originally defined in England by Casey (1961a), are identified for the first time in the Lower Aptian stratotypic area of Cassis-La Bcdoule.

Early Cretaceous nonmarine bivalve assemblages from the Jehol Group in western Liaoning, northeast China

The Jehol Group in western Liaoning yields 13 species of nonmarine bivalves belonging to seven genera. Three assemblages are identified: Arguniella yanshanensis-Sphaerium anderssoni, Margaritifera ( Mengyinaia) mengyinensis-Nakamuranaia subrotunda and Arguniella liaoxiensis-Musculiopsis yixianensis, their ages ranging from late Barremian to early Albian. The Arguniella yanshanensis-Sphaerium anderssoni Assemblage is shown to be late Barremian–early Aptian in age on the basis of radiometric dating. The other two assemblages range from middle Aptian to early Albian in age according to both marine intercalations and radiometric dating.

Fluctuations of sea-water chemistry during Gargasian (Middle Aptian) time. Data from trace-element content (Mg, Sr, Mn, Fe) in hemipelagic carbonates from La Marcouline Quarry (Cassis, SE France)

In the Lower Aptian historical stratotype area (Cassis-La Bedoule, SE France), a geochemical study of the Gargasian (Middle Aptian) marl-limestone alternations of the La Marcouline quarry complements data already obtained from Bedoulian (Early Aptian) sediments there. Nannoconids are the main carbonate producers in both limestones and marls. Although diagenetic minerals, such as ankerite (2.5%) are present in small amounts, the trace-element content of bulk carbonate is very close to that of Nannoconus spp. so geochemical sequences can be defined. The long-term evolution of trace-element content was not affected by diagenetic processes, variations in carbonate mineralogy, or a change of carbonate producers. An increase of around 500 ppm in the strontium content of bulk carbonate occurs between the base of the Cabri zone (late Bedoulian) and the Algerianus zone (late Gargasian). This evolution is linked to fluctuations in seawater Sr/Ca ratios caused by variability in the influx of hydrothermal and river waters, by changes in the ratio of aragonite/calcite production and by shifts in sea level. The eustatic sequence Aptian 4, its parasequences and its key surfaces (sequence boundaries, maximum flooding surface) are clearly reflected in the evolution of the bulk-carbonate contents of manganese.

Environmental changes during marl-limestone formation: evidence from the Gargasian (Middle Aptian) of La Marcouline Quarry (Cassis, SE France)

Limestone-marl alternations are usually interpreted to reflect cyclic paleoenvironmental fluctuations linked to MILANKOVITCH-scale climate variations. However, the impact of diagenesis on lithological differentiation can be overprinted on the primary signal. In order to evaluate environmental variations during the deposition of the Gargasian hemipelagic limestone-marl alternations from the quarry (SE France), we have applied a multi- discipline approach (carbonate geochemistry, micropaleontology and mineralogy). The work uses of a method of granulometric separation of the main carbonate particles which (1) allows the characterization and quantification of the compositions of the carbonate phases of both lithologies and (2) gives access to the geochemical signatures of similar carbonate (bio-) particles in each lithology. The quantifications show that micarb constitutes a significant portion (around 40 per cent) of the sediments. Their geochemical signatures indicate that most of them were formed by fine fragmentation of nannoconid tests. Based on these interpretations, our results indicate that nannoconids are the major constituents of both the marly limestone and the marl layers and that both lithologies are similar in composition. The coccolith assemblages (mainly Watznaueria spp.) of the limestone and marl layers are similar and equally well-preserved. Changes in oxygen and carbon isotopic ratios express cyclic fertility and salinity/temperature variations in surface waters. The marls indicate higher fertility and lower salinity (or higher temperature) conditions than those denoted by the marly limestones. Although diagenesis has not altered these rhythmites no strong evidence has been found for linking classical processes such as dilution or productivity cycles to the composition of the sediments (carbonate constituents and clay mineralogy). This study demonstrates that La Marcouline succession has retained, as recorded by geochemical proxy, its primary environmental variations. Nevertheless, the homogeneity in both lithotypes of sedimentological parameters (clay mineralogy, sediment composition, …) seems to indicate that these fluctuations did not affect the ecosystem strongly enough to cause marked differences in the composition of marls and marly-limestones.

The Gargasian (Middle Aptian) of La Marcouline section at Cassis-La Bédoule (SE France): Stable isotope record and orbital cyclicity

Bulk rock stable isotope analysis of La Marcouline section (Cassis-La Bedoule area, SE France) revealed a general trend of decreasing δ13C and δ18O values from the bottom to the top of the section. The decrease in δ13C values reflects a global trend in Middle Aptian times, namely a return to pre-excursion values of δ13C values following a major, positive excursion in the Early Aptian, which is a reflection of the Oceanic Anoxic Event (OAE) 1a. Power spectra of the bedding rhythms suggest that precessional and long eccentricity cycles of the Milankovitch band controlled the deposition of marl-limestone bundles. This can be interpreted as the result of a forcing by insolation at low latitudes which resulted in a monsoon-controlled precipitation pattern, that in turn induced the deposition of clay-rich beds. Conversely, limestone beds were formed in periods of dryer climate. Higher sea-surface productivity during wet periods may have been caused by an increase in continental runoff and a consequent enhancement in the delivery of nutrients to epicontinental basins. An orbital cyclicity in the sedimentary patterns in La Marcouline section provides the opportunity to calibrate the duration of the well-established G. ferreolensis foraminiferal Zone with that of orbital chronology. The G. ferreolensis Zone at Cassis-La Bedoule spans 33 precessional cycles and its duration is thus estimated to be approximately 760 ka. This length of time is significantly shorter than the estimates of published timescales for this zone and thus may be an argument for proposing that the Gargasian substage is significantly shorter than its currently accepted range.

The Gargasian (Middle Aptian) of La Tuilière - St-Saturnin-lès-Apt (area of the Aptian historical stratotype, Vaucluse, SE France): geographic setting and outcrop correlation

A stratonomic and micropaleontological analysis of the Aptian marls cropping out in the La Tuiliere area near Saint-Saturnin-les-Apt (Vaucluse, SE France), enabled us to reconstitute a continuous succession almost 120 m thick, that includes the upper terms of the Lower Aptian (Bedoulian) and the lower part of the Middle Aptian (Gargasian). These levels had never before been observed with such continuity in the Gargas region, the Aptian historical stratotype.

The Gargasian (Middle Aptian) strata from Cassis-La Bédoule (Lower Aptian historical stratotype, SE France): planktonic and benthic foraminiferal assemblages and biostratigraphy

This paper presents a thorough analysis of foraminiferal assemblages ranging in age from the Bedoulian-Gargasian transition to the middle Gargasian in the Cassis-La Bedoule area (SE France), the historical stratotype of the Lower Aptian substage. This region is particularly suitable for detailed studies of Aptian foraminifera owing to rapid and continuous sediment depositional rates and well- diversified microfaunas. The ranges of benthic forms appear to be fairly stable but some species (Praedorothia praeoxycona, Lenticulina cf. nodosa, Astacolus crepidularis, Globorotalites bartensteini) become extinct at the end of the Bedoulian and thus can be used to separate this substage from the Gargasian. The stratotypic area also offers an opportunity to follow the evolution of planktonic forms step by step at a crucial period of their history, when modalities of speciation and phylogenetic relationships appear to be particularly complex. The important morphologic variability of Aptian planktonic foraminifera does not help finding stable stratigraphic markers; nevertheless, we are able to propose a biozonation comprising five zones (Cabri, Luterbacheri, Ferreolensis, Barri, Algerianus) for the interval under consideration, usually subdivided into three zones. Our study of foraminiferal assemblages and species took into account the range of variability among populations and not just the characteristics of a single specimen, such as the holotype. As a result this paper provides new taxonomic precisions on certain planktonic species hitherto controversial or possibly of doubtful validity. ––– Le Gargasien (Aptien moyen) de Cassis-La Bedoule (stratotype historique de l'Aptien inferieur, SE France) : associations et biostratigraphie des Foraminiferes benthiques et planctoniques.- L'analyse approfondie des associations de Foraminiferes depuis les termes du passage Bedoulien-Gargasien jusqu'au Gargasien moyen a ete realisee dans le secteur de Cassis-La Bedoule (SE France), stratotype historique du Bedoulien (Aptien inferieur), particulierement favorable a ce niveau en raison de la continuite et de la dilatation de la serie ainsi que de la diversification de la microfaune. L'eventail des formes benthiques se revele assez stable mais quelques especes (Praedorothia praeoxycona, Lenticulina cf. nodosa, Astacolus crepidularis, Globorotalites bartensteini) s'eteignent a la fin du Bedoulien et peuvent etre utilisees pour differencier ce sous-etage par rapport au Gargasien. L'aire stratotypique offre aussi la possibilite de suivre pas a pas l'evolution des formes planctoniques a un moment crucial de leur histoire, lorsque les processus de speciation et les liens phylogenetiques s'averent particulierement complexes. La grande variabilite morphologique des formes planctoniques durant l'Aptien ne facilite pas le choix de marqueurs stratigraphiques stables; il est cependant possible de proposer une biozonation comportant cinq zones (a Cabri, Luterbacheri, Ferreolensis, Barri, Algerianus) pour l'intervalle considere, habituellement subdivise en trois zones. L'etude populationnelle, et non pas seulement typologique, des associations nous a enfin permis d'apporter quelques precisions taxinomiques nouvelles, en particulier sur certaines formes planctoniques d'acception controversee ou meme de validite douteuse.

Palynological dating of the Alturaia Arkose (Balagne, northern Corsica): geological implications

In Alpine Corsica, the Balagne Nappe displays the best-developed sedimentary succession associated with an ophiolite sequence. This sedimentary succession includes the Alturaia Arkose, whose age is still unknown. Several shale horizons cropping out in the Cima di Alturaia area were studied for palynological analyses. In this paper, a new palaeontological find in the Alturaia Arkose is reported and the related geological implications are discussed. The collected data indicate the occurrence of a palynological assemblage of Late Barremian to Middle Aptian age. The Alturaia Arkose can be regarded as a clastic deposit of Late Barremian–Middle Aptian age derived from rocks cropping out in Hercynian Corsica. To cite this article: M. Marroni et al., C. R. Palevol 3 (2004) .

New paleomagnetic data for the Hauterivian-Aptian deposits of the Middle Volga region: A possibility of global correlation and dating of time-shifting of stratigraphic boundaries

The aim of this project was to study the paleomagnetic stratigraphy of the Hauterivian-Aptian deposits in the Middle Volga region, namely in the areas of the Saratov, Samara, and Uliyanovsk right-hand Volga areas. Original data were obtained for the first time for the magnetic polarity of the Hauterivian and Barremian rocks of the Russian Platform. The results obtained for the Aptian rocks agree very well with the results of the previous paleomagnetic studies which had been carried out in the Uliyanovsk region earlier (Baraboshkin et al., 1999) and in the vicinities of Saratov City (Grishanov, 1984). Based on the complex correlation of the bio- and magnetostratigraphic data for the rocks from the Middle Volga region, the North Caucasus, the North Mediterranean area, and South England, it is shown that the stage and substage boundaries of the Hauterivian, Barremian, and Aptian stages dier in terms of the absolute age from the similar boundaries in the Boreal Belt by a value of some 10 5 -10 6 years, which is comparable with the duration of the Early Cretaceous ammonite zones. The diachronism of the Lower Aptian ammonite zones (Deshayesites volgensis - D. forbesi) and of the Middle Aptian Parahoplites melchioris zone in dierent regions. The results of this study called for a complex substantiation and tracing of the units of the General Stratigraphic Scale (GSS). The basic GSS unit must be a stage, rather than a zone, because where boundaries were traced over a large territory, the asynchronism of the boundaries between the zones is comparable with the duration of the zones, whereas in the case of a stage it is 1-2 orders of magnitude lower. Geomagnetic inversions or isotopic data can be used to test the isochronism of stratigraphic boundaries.

The Gargasian (Middle Aptian) substage in the Aptian historical stratotypes (SE France): General introduction

In 1998 a double volume of the Journal Geologie Mediterraneenne (t. XXV, N° 3-4) was devoted to a detailed inventory of the Lower Aptian (Bedoulian substage) historical stratotype located in the vicinity of Cassis and Roquefort-La Bedoule, near Marseille (Bouches-du-Rhone, SE France) (...)

The Gargasian (Middle Aptian) of Cassis-La Bédoule (Lower Aptian historical stratotype, SE France):geographic location and lithostratigraphic correlations

In the middle of the last century Gargasian strata overlying the historical stratotypic beds of the lower substage of the Aptian (Bedoulian) were still well exposed in a number of quarries that extended in a NNE-SSW trending belt from the village of Roquefort-la Bedoule to the vicinity of the Cassis railway station (...)

Dinosaurs from the Early Cretaceous Murtoi Formation in Buryatia, eastern Russia

ABSTRACT Six dinosaurian taxa, Therizinosauridae indet., Ornithomimosauria indet., Dromaeosauridae indet., Titanosauridae indet., cf. Mongolosaurus sp., and Psittacosaurus sp., are identified and described from late Barremian to middle Aptian strata of the Murtoi Formation at the Mogoito locality in Buryatia, western Transbaikalia, Russia. The Mogoito vertebrate assemblage is most similar to the Early Cretaceous assemblages from Khuren Dukh (Mongolia) and Elesitai (Inner Mongolia, China).

Development of a Lithocodium (syn. Bacinella irregularis)-reef-mound- A patch reef within Middle Aptian lagoonal limestone sequence near Nova Gorica (Sabotin Mountain, W-Slovenia)

A Middle-Aptian (zone of Palorbitolina lenticularis) „patch reef“ of about 40 m maximum thickness with marked morphology was analyzed at the Sabotin Mountain near Nova Gorica. It is developed above an basal unit of superficial oolites within lagoonal sediments and is overlain by shallow subtidal to intertidal sediments which reveal short time periodically subaerial exposure and early diagenetic freshwater influxes (birds eyes, vadose silt, characeans). Above these sediments within the zone of Salpingoporella dinarica finebedded to platy, laminated bituminous limestones occur. The central part of the reef structure reveals an alternation of individual lenses of Lithocodium-boundstones and of rudist-beds (up to 4 m thickness) which are separated by coarse- to medium-grained, moderately to poorly sorted bioclastic sands. These facies types show also lateral interfingering and are concentrated in the central part of the buildup where the greatest thickness can be observed and where packstones, grainstones and boundstones prevail. The neighbouring lagoonal sediments which consist of mudstones and wackestones predominantly were analyzed in the so-called Sabotin-standardprofile which is located north of the patch reef at a lateral distance of about 300 m. Three vertical profiles (A = 90 m, B = 100 m, C = 64 m thickness) were analyzed. The middle profile B of greatest thickness is taken as reference profile and documents best the vertical facies development within the patch reef directly overlying a basal unit of peloidal packstones with superficial ooids. The patch reef itself is characterized by the faunal associations within the unit rich in Lithocodium and rudists. It is overlain by a subtidal unit of peloidal mudstones with very minor biogenic allochems. An intertidal unit above rich in birds-eyes and vadose silt is followed by fine-laminated black shales which are covering the general seqeunce of interfingering patch-reef - lagoonal sediments. Using different time lines (marker horizons) for correlation it can be shown that already very early differential compaction of fine-grained uncemented sediments in comparison to the core area of the patch reef is of great importance. The greater thickness of the patch reef itself is caused by an intensive early diagenetic marine phreatic cementation within the core zone and by the early fixing of sediment by Lithocodium aggregatum (syn. Bacinella irregularis) resulting in a greater resistivity against compaction. Furthermore a lagoonal side and an more open marine side of the patch reef can be determined. Lithocodium aggregatum is the main constructing organism within the buildup investigated, beginning with the incrustation of varying substrates and biogenic particles. Finally, a dense network of encrustation-sequences is formed interfingering with the general sedimentary textures intensively and resulting in larger „biogenic-cemented“ patches within the sediment. The correlation of the three profiles A, B, and C allows to reveal the history of differential compaction of the associated lagoonal sediments in comparison to the more massive patch reef limestones. It becomes obvious that the main compaction must have occurred within the time span of about 50 m sediment-deposition overlying the patch reef. Differential compaction resulted in differences in thickness of about 10 m from the patch reef (profile B) to the more lagoonal influenced sediments (profile A) within a lateral distance of about 50 m.

NEW ANGIOSPERM POLLEN FROM THE UPPER BARREMIAN-APTIAN OF THE WESTERN DESERT, EGYPT

The Lower Cretaceous Alam El Bueib and Alamein formations in the Ghazalat-1 (GTX-1) well, Western Desert, Egypt, consist of marginal marine siliciclastic sediments, overlain by the Aptian evaporites (dolomite of the upper Alamein Formation). Angiosperm species diversity is reasonably high, and account for 8.0–22.0% of the total palynomorphs. Most of these species might belong to monosulcate, zonasulculate, and inaperturate pollen. The genus Afropollis is emended herein to include forms with monosulcoid aperture. The following two new subspecies and seven new species are described: Afropollis operculatus operculatus, Afropollis operculatus microreticulatus subsp. nov., Afropollis schrankii sp. nov., Arecipites microfoveolatus sp. nov., Retimonocolpites bueibensis sp. nov., Retimonocolpites ghazalii sp. nov., Stellatopollis dejaxii sp. nov., Stellatopollis doylei sp. nov. and Stellatopollis limai sp. nov. In addition, one new combination is proposed: Arecipites pereensis (Juhasz and Goczan) comb. nov. Three palynomorph assemblages (A1–A3) are assigned to the upper Barremian and lower–middle Aptian. These assemblages show close correlation and similarity with the palynozones C-VI, C-VIIa and C-VIIc of the “Cocobeach System”, West Africa and Zone I of the Potomac Group, USA. Sediments of the Alam El Bueib and basal Alamein formations accumulated in a marginal shallow marine environment under arid to semiarid climatic conditions.

Clockwise rotations recorded in Early Cretaceous rocks of South Korea: implications for tectonic affinity between the Korean Peninsula and North China

Recent interest has focused on whether South Korea may have undergone variable tectonic rotations since the Cretaceous. In an effort to contribute to the answer to this question, we have completed a palaeomagnetic reconnaissance study of Early Cretaceous sedimentary and igneous rocks from the Kyongsang basin in southeast Korea. Stepwise thermal demagnetization isolated well-defined characteristic magnetization in all samples. The palaeomagnetic directions reveal patterns of increasing amounts of clockwise (CW) rotation with increasing age for Aptian rock units. Palaeomagnetic declinations indicate clockwise vertical-axis rotations of R = 34.3° ± 6.9° for the early Aptian rock unit, R = 24.9° ± 10.6° for the middle Aptian, and R = −0.9° ± 11.8° for the late Aptian relative to eastern Asia. The new Cretaceous palaeomagnetic data from this study are consistent with the hypothesis that Korea and other major parts of eastern Asia occupied the same relative positions in terms of palaeolatitudes in the Cretaceous. An analysis of and comparison with previously reported palaeomagnetic data corroborates this hypothesis and suggests that much of Korea may have been connected to the North China Block since the early Palaeozoic. A plausible cause of the rotation is the westward subduction of the Kula plate underneath the Asian continent, which is inferred to have occurred during the Cretaceous according to several geological and tectonic analyses.

Controls on carbonate-platform evolution on active fault blocks; the Lower Cretaceous Castro Urdiales Platform (Aptian-Albian, northern Spain)

ABSTRACT Four contrasting carbonate sequences developed in a tectonically active, extensional setting in the Lower Cretaceous (Aptian-Albian) Castro Urdiales platform of north Spain. The four carbonate phases (P1-P4; early Aptian, late Aptian, earliest Albian, and early-late Albian, respectively) are identified across the platform-to-basin transition on the basis of sedimentary geometries and internal facies patterns. They are vertically separated by four incipient drowning events (D1-D4; middle Aptian, Aptian-Albian boundary, early Albian, and late Albian). The first carbonate stage (P1) corresponds to a low-relief carbonate platform characterized by widespread shallow-marine depositional environments with a nearly flat organization. The second platform stage (P2) developed as a rimmed carbonate platform. The third carbonate stage (P3) evolved from a low-relief aggradational platform (substage A) to a progradational offlapping platform (substage B), and back to a backstepped, aggradational rimmed platform (substage C), responding to changes in differential subsidence and carbonate production. At the beginning of the Albian the Castro Urdiales carbonate platform was broken up by extensional tectonic movements with formation of fault blocks bounded by NE-SW and NW-SE trending faults. On the crests of uplifted blocks meteoric processes were active, whereas grabens and half grabens acted as channel conduits for terrigenous-rich deposits. Subsidence rates increased during the late early Albian-late Albian. The crests of rotated blocks formed paleohighs that nucleated small residual carbonate platforms (carbonate stage P4) that were surrounded by deeper-water basins. This study suggests that the evolution of carbonate platform sequences in the Lower Cretaceous of Castro Urdiales was primarily controlled by changes in accommodation space, which were most likely driven by intraplate stresses related to extension in the north Spanish continental margin during the opening of the North Atlantic. Drowning events in the Castro Urdiales platform coincide with major crises of Cretaceous carbonate production worldwide, suggesting that regional tectonism and global oceanic changes interacted to cause phases of platform demise.

Observations on the lithostratigraphy and ammonite succession of the Aptian (Lower Cretaceous) Lower Greensand of Chale Bay, Isle of Wight, UK

A revised lithostratigraphy is given for parts of the Atherfield Clay and Ferruginous Sands Formations (Lower-Upper Aptian) of the Lower Greensand Group of the standard section in Chale Bay, Isle of Wight, retaining in essence the classic nomenclature of Fitton. Changes in zonal and subzonal boundaries are proposed on the basis of new collections of ammonites obtainedin situ. The Upper Lobster Beds Member of the Atherfield Clay is detached from the Subzone ofDeshayesites callidiscusand placed in a separate Subzone ofDeshayesites annelidusat the top of the Zone ofD. forbesi. The influx ofMegatyloceras, Cheloniceras, PseudosaynellaandSinzoviaat this level has palaeogeographic implications, pointing to the opening of a marine link with the Paris Basin and confirming the Upper Lobster Beds as a transgressive horizon. The upper part of the Scaphites Beds Member of the Ferruginous Sands, previously included in thedeshayseiZone, is transferred to the Subzone ofDufrenoyia transitoria, at the base of the Zone ofTropaeum(T.)bowerbanki, on the strength of the occurrence therein of species ofTropaeumandDufrenoyia.Tropaeumis not present below thebowerbankiZone as now defined; its primitive ancyloceratoid (hooked) forms such asT. (T.)hillsiihave limited biochronological value, persisting alongside the criocone (spiral) species of thebowerbankiZone and giving rise to a local horizon ofT. (T.)pseudophillsiat the bottom of the Upper Crioceras Beds (basal Upper Aptian). A more precise delimitation of the Walpen Clay & Sands from the Upper Crioceras Beds is made and the junction of these two members of the Ferruginous Sands is reaffirmed as the Lower/Upper Aptian boundary, coinciding with the junction of the Zones ofT. (T.)bowerbankiandEpicheloniceras martiniodes. Bed-by-bed collecting across the boundary shows thatEpichelonicerascan no longer be taken as the prime biostratigraphical marker for the base of the Upper Aptian; greater significance is given to the extinction ofDufrenoyia. The sources of the genusVectisitesand some species ofAmmonitocerasandCaspianites, hitherto fixed only vaguely in the column, are pinpointed in themartinioidesZone. The horizon ofTropaeum(T.)drewiis confirmed as high in thebowerbankiZone (meyendorffiSubzone) and the validity of a ‘drewiZone’ at the base of the German Upper Aptian (=base of Middle Aptian, Kemper, 1995) is questioned. In a review of the Lower/Upper Aptian boundary, it is suggested that the German stratum be called the Horizon ofCicatrites?tenuinodosusand placed tentatively at the top of the Lower Aptian. These and other changes in ammonite nomenclature are explained in the systematic notes.

Cyclicity in pelagic sediments of an epeiric sea - Cyclostratigraphic interpretation of Early Cretaceous (Middle Aptian) sediments of NW Germany Abb:7

Cyclicity in pelagic sediments of an epeiric sea - Cyclostratigraphic interpretation of Early Cretaceous (Middle Aptian) sediments of NW Germany Abb:7