Stratigraphic Stages Research Articles

Comparison of taxonomic diversity, strontium isotope and sea-level patterns

Sepkoski’s compendium of 28,730 marine genera was analyzed with a view toward comparing taxonomic diversity, strontium isotope stratigraphy and sea-level changes. The fossil record analyses discriminated between origination, extinction and pass through genera and collectively delineated changes in number of genera with time at the largely stratigraphic stage level. With notable exceptions of the Cretaceous and Tertiary, taxonomic diversity generally paralleled both strontium isotope stratigraphy and sea-level changes. Sampling bias was suggested in the Tertiary where an increase in genera numbers toward the Recent opposed the downward trends of both strontium isotope and sea level curves. The removal of all Recent genera from the Sepkoski database confirmed the bias in producing complimentary downward trends in all curves. Regression analysis of taxonomic diversity with respect to sea level extent and time duration explained 82% of the variability of the remaining data. Sea-level fluctuations are not always accompanied by significant changes in taxonomic diversity but strontium isotope analyses offers the potential of constraining the extent to which the magnitude of changes are associated with time duration.

Integrated chronostratigraphy of the Pliocene-Pleistocene interval and its relation to the regional stratigraphical stages in the southern North Sea region

AbstractTime-stratigraphic interpretations of Late Pliocene to Early Pleistocene sediments from onshore locations and from marginal marine settings of the North Sea Basin often refer to the subdivision of the Dutch and British ’Quaternary’ regional stratigraphic stages. Since age control for these stages and their stage boundaries are based on relative dating methods, in this study pollen, dinoflagellate cysts and foraminiferal assemblages were investigated to correlate the regional stratigraphic stages independently to the global chronostratigraphy and the paleomagnetic timescale. The data were obtained from eight boreholes located in the depocentre setting of the Late Pliocene North Sea Basin comprising a 1000 m thick sedimentary succession. The British Gedgravian and Waltonian stages, the Dutch Reuverian to Brunssumian as well as published foraminiferal zones (NSB 14, FB and the lower part of the FA2 zone) fall within the Zanclean and Piacenzian. The lower boundaries of the Pre-Ludhamian and Pretiglian stages and of the NSB 14 to 15 zones are close to the paleomagnetic Gauss-Matuyama boundary. The Pre-Ludhamian, Ludhamian, Thurnian and the Pretiglian, Tiglian A and Tiglian B stages presumably cover the marine isotope stages 103 to 95. It is proposed that the Ludhamian, Thurnian and the Tiglian A were short lasting, warm, periods during which sea level highstand facilitated sedimentary deposition at the marginal areas of the North Sea Basin. The lower boundary of the paleomagnetic Olduvai subchron is situated in the Tiglian Cl-4b stage while the TC4c stage is found within the Olduvai subchron. Foraminiferal NSB 15 and NSB 16 zone as well as the upper part of the FA2 and FA1 zone fall within the Gelasian and cover the Matuyama chron as well as the lower part of the Olduvai subchron. Comparison with formerly dated North Sea sediments shows a good agreement between foraminiferal zonations on a broader scale but significant differences in absolute ages occur. Strontium isotope values indicate approximately 1 Ma younger ages as expected from our chronostratigraphic model. This discrepancy is explained by the dominance of freshwater from river discharge contributing high amounts of eroded material to the basin, leading to an increase of the87Sr/86Sr ratio in the shelf-sea water.

Mass extinctions and macroevolution

Mass extinctions are important to macroevolution not only because they involve a sharp increase in extinction intensity over “background” levels, but also because they bring a change in extinction selectivity, and these quantitative and qualitative shifts set the stage for evolutionary recoveries. The set of extinction intensities for all stratigraphic stages appears to fall into a single right-skewed distribution, but this apparent continuity may derive from failure to factor out the well-known secular trend in background extinction: high early Paleozoic rates fill in the gap between later background extinction and the major mass extinctions. In any case, the failure of many organism-, species-, and clade-level traits to predict survivorship during mass extinctions is a more important challenge to the extrapolationist premise that all macroevolutionary processes are simply smooth extensions of microevolution. Although a variety of factors have been found to correlate with taxon survivorship for particular extinction events, the most pervasive effect involves geographic range at the clade level, an emergent property independent of the range sizes of constituent species. Such differential extinction would impose “nonconstructive selectivity,” in which survivorship is unrelated to many organismic traits but is not strictly random. It also implies that correlations among taxon attributes may obscure causation, and even the focal level of selection, in the survival of a trait or clade, for example when widespread taxa within a major group tend to have particular body sizes, trophic habits, or metabolic rates. Survivorship patterns will also be sensitive to the inexact correlations of taxonomic, morphological, and functional diversity, to phylogenetically nonrandom extinction, and to the topology of evolutionary trees. Evolutionary recoveries may be as important as the extinction events themselves in shaping the long-term trajectories of individual clades and permitting once-marginal groups to diversify, but we know little about sorting processes during recovery intervals. However, both empirical extrapolationism (where outcomes can be predicted from observation of pre- or post-extinction patterns) and theoretical extrapolationism (where mechanisms reside exclusively at the level of organisms within populations) evidently fail during mass extinctions and their evolutionary aftermath. This does not mean that conventional natural selection was inoperative during mass extinctions, but that many features that promoted survivorship during background times were superseded as predictive factors by higher-level attributes. Many intriguing issues remain, including the generality of survivorship rules across extinction events; the potential for gradational changes in selectivity patterns with extinction intensity or the volatility of target clades; the heritability of clade-level traits; the macroevolutionary consequences of the inexact correlations between taxonomic, morphological, and functional diversity; the factors governing the dynamics and outcome of recoveries; and the spatial fabric of extinctions and recoveries. The detection of general survivorship rules—including the disappearance of many patterns evident during background times—demonstrates that studies of mass extinctions and recovery can contribute substantially to evolutionary theory.

Extinction and recovery patterns of scleractinian corals at the Cretaceous-Tertiary boundary

Abstract The extinction and recovery of scleractinian corals at the Cretaceous–Tertiary (K–T) boundary was analyzed based on a global database of taxonomically revised late Campanian to Paleocene coral collections. In contrast to earlier statements, our results indicate that extinction rates of corals were only moderate in comparison to other marine invertebrates. We have calculated a 30% extinction rate for Maastrichtian coral genera occurring in more than one stratigraphic stage and more than one geographic region. Reverse rarefaction suggests that some 45% of all coral species became extinct. Photosymbiotic (zooxanthellate) corals were significantly more affected by the extinction than azooxanthellate corals; colonial forms were hit harder than solitary forms, and among colonial forms an elevated integration of corallites raised extinction risk. Abundance, as measured by the number of taxonomic occurrences, had apparently no influence on survivorship, but a wide geographic distribution significantly reduced extinction risk. As in bivalves and echinoids neither species richness within genera nor larval type had an effect on survivorship. An indistinct latitudinal gradient is visible in the extinction, but this is exclusively due to a higher proportion of extinction-resistant azooxanthellate corals in higher-latitude assemblages. No significant geographic hotspot could be recognized, neither in overall extinction rates nor in the extinction of endemic clades. More clades than previously recognized passed through the K–T boundary only to become extinct within the Danian. These failed survivors were apparently limited to regions outside the Americas. Recovery as defined by the proportional increase of newly evolved genera, was more rapid for zooxanthellate corals than previously assumed and less uniform geographically than the extinction. Although newly evolved Danian azooxanthellate genera were significantly more common than new zooxanthellate genera, the difference nearly disappeared by the late Paleocene suggesting a more rapid recovery of zooxanthellate corals in comparison to previous analyses. New Paleocene genera were apparently concentrated in low latitudes, suggesting that the tropics formed a source of evolutionary novelty in the recovery phase.

Extinction and recovery patterns of scleractinian corals at the Cretaceous-Tertiary boundary

The extinction and recovery of scleractinian corals at the Cretaceous–Tertiary (K–T) boundary was analyzed based on a global database of taxonomically revised late Campanian to Paleocene coral collections. In contrast to earlier statements, our results indicate that extinction rates of corals were only moderate in comparison to other marine invertebrates. We have calculated a 30% extinction rate for Maastrichtian coral genera occurring in more than one stratigraphic stage and more than one geographic region. Reverse rarefaction suggests that some 45% of all coral species became extinct. Photosymbiotic (zooxanthellate) corals were significantly more affected by the extinction than azooxanthellate corals; colonial forms were hit harder than solitary forms, and among colonial forms an elevated integration of corallites raised extinction risk. Abundance, as measured by the number of taxonomic occurrences, had apparently no influence on survivorship, but a wide geographic distribution significantly reduced extinction risk. As in bivalves and echinoids neither species richness within genera nor larval type had an effect on survivorship. An indistinct latitudinal gradient is visible in the extinction, but this is exclusively due to a higher proportion of extinction-resistant azooxanthellate corals in higher-latitude assemblages. No significant geographic hotspot could be recognized, neither in overall extinction rates nor in the extinction of endemic clades. More clades than previously recognized passed through the K–T boundary only to become extinct within the Danian. These failed survivors were apparently limited to regions outside the Americas. Recovery as defined by the proportional increase of newly evolved genera, was more rapid for zooxanthellate corals than previously assumed and less uniform geographically than the extinction. Although newly evolved Danian azooxanthellate genera were significantly more common than new zooxanthellate genera, the difference nearly disappeared by the late Paleocene suggesting a more rapid recovery of zooxanthellate corals in comparison to previous analyses. New Paleocene genera were apparently concentrated in low latitudes, suggesting that the tropics formed a source of evolutionary novelty in the recovery phase.

Origin, phylogeny, faunal composition, and stratigraphical significance of the Albian engonoceratidae (pulchelliaceae, ammonitina) of Peru

Recent study of the Albian ammonite fauna from Peru provides a major revision of the systematic position and taxonomy of the family Engonoceratidae, which we place in the superfamily Pulchelliaceae. We consider the oldest south Tethyan representatives, specifically the mahmoudii species described in Algeria, ancestors of the Andean taxa. Peruvian specimens show a monophyletic evolution marked by four morphological and stratigraphical stages, from unornamented Platiknemiceras to evolute Glottoceras and Engonoceras. The weakly ornamented Parengonoceras haasi, a late lower Albian index species, is considered the ancestor of all Peruvian engonoceratids. The family reaches its highest diversity (15 species) in the upper part of the Lower Albian. The genus Glottoceras, which comprises the majority of the microconchs, is considered an Andean paraphyletic equivalent to the Tethyan genus Knemiceras. Globally, Peruvian engonoceratids are dominated by representatives of the genera Parengonoceras and Glottoceras. We also report several engonoceratids, including Engonoceras and Platiknemiceras, for the first time from Peru.

Le « créationnisme » d’Alcide d’Orbigny et les fondements de la théorie biostratigraphique

D’Orbigny was more interested in the discontinuities of life history than in the origin of species. He looked for reference points to correlate geological strata and to reconstruct their history. This allowed him to formalise the concept of stratigraphic stage and to propose the first geological time scale. Therefore, one of the results was that he was unjustly considered as one of the leaders of the creationism school, because he used the ambiguous term ‘creation’ to designate the renewal of fauna after catastrophic events, but without providing a definition and with a meaning that was probably different from those of his successors.

Survival without recovery after mass extinctions.

Because many survivors of mass extinctions do not participate in postrecovery diversifications, and therefore fall into a pattern that can be termed "Dead Clade Walking" (DCW), the effects of mass extinctions extend beyond the losses observed during the event itself. Analyses at two taxonomic levels provide a first-order test of the prevalence of DCWs by using simple and very conservative operational criteria. For four of the Big Five mass extinctions of the Phanerozoic, the marine genera that survived the extinction suffered approximately 10-20% attrition in the immediately following geologic stage that was significantly greater than the losses sustained in preextinction stages. The stages immediately following the three Paleozoic mass extinctions also account for 17% of all order-level losses in marine invertebrates over that interval, which is, again, significantly greater than that seen for the other stratigraphic stages (no orders are lost immediately after the end-Triassic or end-Cretaceous mass extinctions). DCWs are not evenly distributed among four regional molluscan time-series following the end-Cretaceous extinction, demonstrating the importance of spatial patterns in recovery dynamics. Although biotic interactions have been invoked to explain the differential postextinction success of clades, such hypotheses must be tested against alternatives that include stochastic processes in low-diversity lineages-which is evidently not a general explanation for the ordinal DCW patterns, because postextinction fates are not related to the size of extinction bottlenecks in Paleozoic orders-and ongoing physical environmental changes.

Miocene–Pleistocene tectono-sedimentary evolution of Bahı́a Concepción region, Baja California Sur (México)

Bahı́a Concepción is one of the largest fault-bound bays in the Gulf of California. It is one of the area's best examples of an extensional basin, and an accommodation zone related to the Late Miocene extension in the Gulf of California region. Extensional tectonics in the Gulf region initiated from middle to early Late Miocene, in a simple east–west manner. The units within the Comondú Group are tilted between 14 and 45° in opposite directions (E/W), as a direct result of a single main extensional episode. This event produced subsidence that created the depocenters for nearshore marine basins. The oldest sedimentary units present in the region are assigned to the Late Miocene–early Pliocene Tirabuzón Formation. Up-faulted granodiorite basement occurs on the Peninsula Concepción, and at Punta San Antonio as a direct result of the Late Miocene extensional episode. Extension on the Bahı́a Concepción zone was responsible for development of a divided half-graben structure first flooded in Late Pliocene time. The tectono-sedimentary evolution of Bahı́a Concepción is recorded by three stratigraphic stages: (1) pre-rift strata represented by the Comondú Group, (2) a syn-rift unconformity, and (3) a post-rift strata represented by the Late Miocene to Pliocene flat laying sedimentary units. This triad of stratigraphic stages clearly varies from other basins along the margin of the Gulf of California. Upper Pleistocene terrace deposits generally conform to the 6–12 m elevation regionally associated with substage 5e. Their uniformity in elevation inside and outside Bahı́a Concepción indicates that tectonic uplift was locked in step throughout the immediate region at least since that time.

Lithological expression of Milankovitch cyclicity in carbonate-dominated, pelagic, Barremian deposits in central Italy

In continuous sedimentary sequences, cyclostratigraphy can provide good time control on the duration of stratigraphic stages and palaeomagnetic chrons. This approach has been applied to the Barremian pelagic marl-limestone alternations of the Umbria-Marche Basin. Two criteria have been used: variations in the thickness and carbonate content of limestones and the vertical distribution of intercalated black marls. Marl–limestone alternations are organized in bundles of generally four or five couplets. The cyclicity expressed by the vertical stacking of couplets and bundles has been interpreted as the sedimentary expression of the Earth's orbital parameters, specifically the precession of the equinoxes and the eccentricity.This breakdown of the Barremian Stage into astroclimatic cycles allows the calibration of the duration of the stage and of the magnetic chrons M-3 to M-0. The lower and upper stratigraphic boundaries of the Barremian Stage are picked at the base of the M-3 and M-0 chrons, respectively. The duration of the Barremian Stage is estimated at 5.13±0.34Myr by adding up contained cycles. The duration of related magnetic chrons varies considerably from 0.43 to 2.3Myr.

Evidence for periodicity and nonlinearity in a high-resolution fossil record of long-term evolution

The application of new signal analysis techniques provides increased insight into the study of the fossil record and processes of evolution. The fossil record of 622 planktic foraminifera contains data from 200 stratigraphic stages of the past 127 m.y. Time-series analyses (wavelet and Fourier transform) of the planktic foraminifera fossil record were used to discern periodic components in long-term evolution. The correlation function analysis was used to distinguish between random and deterministic behavior of the fossil record. The analyses show that stationary ~30 m.y. periodicity and complex deterministic patterns occur in the long-term planktic foraminifera evolution, in particular in the extinction record. Our results suggest that the occurrence of intense diversity fluctuations with 3–10 m.y. periodicity after major extinction events may be attributed to nonlinear, self-organized evolutionary response to the availability of new ecospace. This coupled nonlinear-periodic scenario may explain the repetitive appearance of similar morphotypes in ~30 m.y. intervals.

Geochemistry of igneous rocks of Shimane Peninsula, formed within a Miocene back-arc rifting zone at the Japan Sea margin.

In the San'in district (back-arc side of the Southwest Japan arc), violent subalkalic magmatism took place related to the opening of the Japan Sea during the Miocene. In the northeastern part of Shimane Peninsula, thick middle Miocene submarine volcanigenic strata crop out. This middle Miocene volcanism took place along with intense subsidence probably related to rifting in a mature arc. In the late Miocene, this volcanism and subsidence were followed by eruption of subaerial alkalic basalt (Matsue basalt). Thirty seven samples of these igneous rocks have been analyzed by XRF and INAA. Middle Miocene igneous activity is divided into three stratigraphic stages, and each stage shows a wide variation in chemistry. Intra-stage chemical variations result from crystal differentiation, whereas variations between Stages II and III rocks are interpreted in terms of various degrees of melting from a common source mantle. Stage I rocks were generated from a more depleted mantle. Most middle Miocene rocks have an arc signature, which resulted from the melting of subarc lithospheric mantle which had been metasomatized by a subduction-related fluid. Associated felsic rocks were formed by magmatic fractionation of basaltic magma.

Syn-extension rotations in the Permian St Affrique basin (Massif Central, France): paleomagnetic constraints

We present the results of a paleomagnetic study of the Permian sedimentary basin of St Affrique (southern Massif Central, France). This study was devised to test for tectonic rotations in the Montagne Noire area during the Stephano-Permian extension in the Hecynian chain and to complete a preliminary study [1] in which we suggested an counterclockwise rotation of St Affrique basin with respect to the neighbouring Lodève basin. This rotation was attributed to the domino-like rotation of the basement of St Affrique (i.e., the Paleozoic Monts de Lacaune imbricate units) as expected from the tectonic interpretation of the Montagne Noire area made by Van Den Driessche and Brun [2,3]. However, the rotation of the basin during Permian should induce declination differences between the basal and the upper part of the stratigraphic sequence. We therefore conducted vertical sampling of the basin throughout the Permian sequence. Two sites of Stephanian age gave no results, but twelve sites from the lower to the upper Permian have been sampled in the St Affrique basin and did provide results. Paleomagnetic analysis using thermal procedures confirms our previous analysis and allowed us to demonstrate three magnetization components. The low temperature A component is unblocked by 300°C and conforms to the present-day dipole field direction. An intermediate temperature component (B) has also been isolated, with unblocking temperatures of 300° to 550°–600°C, and this component was possibly carried by magnetite. Between 600° and 680°C a third component (C) may be suspected at some sites, but it has not been isolated with confidence. At a site showing a tilted-block geometry produced by syn-sedimentary normal faulting (site p7), it is shown that the B component is an early magnetization acquired during faulting of the succession, and we are therefore able to give it a stratigraphic age. The mean directions of the B component, which are computed at the stratigraphic stage level, show the following counterclockwise deviations from the reference magnetic field direction for stable Europe: 21° ± 4.5° for the upper Autunian, 10° ± 7° for the Saxonian, 10° ± 9° for the lower Thuringian and 4° ± 6° and 3.5° ± 4.5° for two successive stages of middle/upper Thuringian age. We thus consider that (1) the comparison of the lowest units of St Affrique and Lodève basins [1], and (2) the evolution in paleomagnetic declination during the Permian within the St Affrique basin itself provide evidence, in space and time, of a rotation of the basin during its formation. We attribute this rotation to the domino-like rotation of its basement (the Monts de Lacaune imbricate units) during the Stephano-Permian extension in the Hercynian chain [2,3].

Mass Extinctions as Statistical Phenomena: An Examination of the Evidence Using χ2 Tests and Bootstrapping

Although much natural historical evidence has been adduced in support of the occurrence of several mass extinctions during the Phanerozoic, unambiguous statistical confirmation of the mass extinction phenomenon has remained elusive. Using bootstrapping techniques that have not previously been applied to the study of mass extinction, we have amassed strong or very strong statistical evidence for mass extinctions (see text for definitions) during the Late Ordovician, Late Permian, and Late Cretaceous. Bootstrapping therefore verifies three of the mass extinction events that were proposed by Raup and Sepkoski (1982). A small amount of bootstrapping evidence is also presented for mass extinctions in the Induan (Triassic) and Coniacean (Cretaceous) Stages, but high overall turnover rates (including high origination) in the Induan and uncertain estimates of the temporal duration of the Coniacean force us to conclude that the evidence is not compelling.We also present the results of more liberal X2 tests of the differences between expected and observed numbers of familial extinctions for stratigraphic stages. In addition to verifying the mass extinctions identified using bootstrapping, these analyses suggest that several stages that could not be verified as mass extinction stages using bootstrapping (including the last three in the Devonian, and the Norian Stage of the Triassic) should still be regarded as candidates for mass extinction. Further analysis will be required to test these stages in more detail.

Probabilities of origination, persistence, and extinction of families of marine invertebrate life

In this paper we model the process of taxonomic evolution as a Galton-Watson branching process in discrete time and, using maximum likelihood, develop methods to estimate the probabilities of origination, persistence, and extinction of fossil taxa. We use the methods to estimate the probabilities of origination, persistence, and extinction of families (1) within 135 orders of marine invertebrate organisms, (2) within 12 phyla, and (3) within all marine invertebrate life (independently of the suprafamilial classification).Most orders, including the arcoid bivalves, the dentaloid scaphopods, the orders of chitins, and many others, have relatively low probabilities of familial origination and extinction. The various ammonoid and trilobite orders, and some others, have high probabilities of origination and extinction. Among the phyla, the Archaeocyatha have the highest probabilities of familial origination and extinction, and the Annelida the lowest, with the more typical phyla of shelly organisms clustering near the high end of the probability scale. The Porifera and Protozoa also have low probabilities but not as low as the Annelida. The estimated origination and extinction probabilities for families within all marine invertebrate life are 0.470 and 0.452 per stage, respectively, values that are at the high end of the probability scale. We have also estimated the probabilities of ultimate extinction (extinction of all families) of the supertaxa.By analyzing the changes of the diversity during each stratigraphic stage separately, we have also determined the trajectories of the estimated origination and extinction probabilities for families within all marine invertebrate life. The estimated origination probability is relatively high in association with the expansion of the Cambrian and Paleozoic evolutionary faunas and declines to more normal levels for the remainder of the Phanerozoic. The trajectory of the estimated extinction probability is from nearly zero early in the Phanerozoic to more normal levels later, showing clearly defined peaks in association with the five Phanerozoic mass-extinction events. The terminal Cretaceous mass extinction is the only one of the five that was not preceded by a monotonic decline of origination probability or by a series of stages with low origination probability. It appears to have been a unique, singular event.Because the mathematical theory we employ as a model corresponds so closely to the processes of taxonomic evolution as we understand them, we believe that the theory provides a reasonable model of biological reality.

Periodicity in extinction and the problem of catastrophism in the history of life.

The hypothesis that extinction events have recurred periodically over the last quarter billion years is greatly strengthened by new data on the stratigraphic ranges of marine animal genera. In the interval from the Permian to Recent, these data encompass some 13,000 generic extinctions, providing a more sensitive indicator of species-level extinctions than previously used familial data. Extinction time series computed from the generic data display nine strong peaks that are nearly uniformly spaced at 26 Ma intervals over the last 270 Ma. Most of these peaks correspond to extinction events recognized in more detailed, if limited, biostratigraphic studies. These new data weaken or negate most arguments against periodicity, which have involved criticisms of the taxonomic data base, sampling intervals, chronometric time scales, and statistical methods used in previous analyses. The criticisms are reviewed in some detail and various new calculations and simulations, including one assessing the effects of paraphyletic taxa, are presented. Although the new data strengthen the case for periodicity, they offer little new insight into the deriving mechanism behind the pattern. However, they do suggest that many of the periodic events may not have been catastrophic, occurring instead over several stratigraphic stages or substages.

Times of crisis in the evolution of the cephalopoda

The evolutionary history of the ectocochlian cephalopods is punctuated by a number of severe crises during each of which this class came very close to extinction. The crisis events follow each other at intervals of from seven to almost 300 million years and, with one exception, were not synchronous for Nautiloidea and Ammonoidea. Only at the end of the Triassic period did both groups simultaneously face the danger of extinction. Generally, the survivors of crisis situations have simple shell forms and are strikingly similar to each other. To trace the details of cephalopod evolution, the family on the taxonomic level and the stratigraphic stage on the chronological level do not provide scales fine enough to reconstruct the true course of this process. The causes of crises and “mass extinctions” are not yet understood. Most authors have approached this problem in a simplistic manner, searching for a single cause for any, or all, events of this kind. It seems that we do not even have begun to understand what the problems are.

Les héritages du quaternaire moyen méditerranéen dans leur cadre régional (Inheritances from the mediterranean middle quaternary within their regional framework)

Abstract. The very high degree of intricacy with regard to the problems of the Middle Quaternary in the Mediterranean can be the result, more particularly, of the increasing inadequacy of the existing chronological data to the inheritances which are, as it is commonly observed, quite different according to the regional framework : not only by the number of the landforms inherited (such as the systems of terraces and glacis), but also by their types. Geomorphologists used to extrapolate (too often mistakenly ?) from the systems of terraces, with regional significance, to stratigraphical stages, with general value. The opposition between continental and oceanic chronological data is likely to be connected with the peculiar circumstances of geomorphological developments in continental environments (crossing of very subtle thresholds of response which are involved in the trend of the climatic changes). But the tectonics are also very important in the Mediterranean, absolutely prevalent in the piedmont systems developing a very high instability in the eastern basin ; in spite of their visible moderation in the western basin, they can however play a decisive role in some regional discrepancies.

Neogene subsidence and sedimentation of the Middle Danube depression system

Thickness analysis, proposed by Beloussov already in 1937, but virtually unknown in the English literature, is a graphical method for the study of sedimentation and tectonism. It is applied here to the thick sediment cover of an intramontane basin. The Neogene depression system embraced by the Alps, Dinarids and Carpathians consists of two types of structural regions: 1. (a) Central elevation with a thin Neogene sediment cover and outcrops of basement rock. 2. (b) Marginal depressions in the foreland of the embracing mountains. They form a double row west and east of the central elevation. All the formations and stratigraphical stages found in the marginal depressions are also represented in the central elevation. They are, however, five times thinner here on average. Thickness analysis gives evidence of gradual subsidence in equilibrium with sedimentation. As shown by the diagrams, sediment thickness is proportional to subsidence in each of the regions studied. Miocene formations are at variance locally in this respect because unconformities and extensive volcanic activity overprint the main trends of the basin. Miocene sedimentation was more variable both in space and time than that of the Pliocene.