Marc Floquet

Mass mortality or exceptional fossilization ? The case of the early and middle Toarcian fossiliferous beds from the Digne-Les-Bains area (southeastern France)

The latest Domerian to late Toarcian sedimentary series (from −190 Ma up to −180 Ma) from the « Reserve Geologique de Haute-Provence » (southeastern France) yields two kinds of remarkable fossiliferous beds. The greatest interest of the early Toarcian type is the occurrence of ichtyosaur remains (at least in six sites) among many other fossils such as ammonites, belemnites, bivalves, wood. The middle Toarcian type is specifically rich in ammonites and nautiluses. Litho- and biostratigraphical, palaeontological, sedimentological and geochemical analyses allow us to determine whether these fossiliferous beds are the results of mass mortalities, linked or not to biological crisis, or of exceptional fossilizations of organisms after normal mortality. The early Toarcian accumulations of fossils have been accurately dated from the middle part of the Serpentinum zone, Exaratum sub-zone pro-parte and Falciferum sub-zone pro-parte , strangewaysi and falciferum horizons. Thus, they were not connected to the so called Toarcian biological crisis which occurred previously (during the Semicelatum sub-zone). Organisms likely died according to a normal mortality rate during a time span that lasted around 700 to 800 k.y. All organisms appear to have been removed from their life environments and buried within siliciclastic sediments. Organisms and sediments were trapped owing to the creation of hemigrabens in an extensional tectonic regime, as evidenced by synsedimentary normal faults. Burying and good preservation, i. e. exceptional fossilization, were favoured by the hypoxic or anoxic conditions which prevailed at the sea floor, specifically during these times. High organic carbon content (up to 2.49 per cent) in the fossiliferous silty-quartzose marls proves hypoxia if not anoxia. All these facts were linked to a deepening and transgressive systems tract which succeeded a relative sea-level lowstand at the Pliensbachian-Toarcian boundary. The marine transgression probably reworked and shifted basinwards the siliciclastic sediments and wood remains which formed previously on the exposed land. Thus, continental remains were mixed with the marine fossils. Small scale hummocky cross stratifications in the terrigenous deposits show episodic occurrence of high hydrodynamics events. Such events, likely storms and associated currents, may have provoked the accumulations of dead marine organisms. The middle Toarcian accumulations of cephalopods are dated from the upper part of the Bifrons zone, Bifrons sub-zone pro-parte , lusitanicum and bifrons horizons. They are contained within 3 to 4 calcareous beds, 60 up to 85 centimetres thick in the whole. Since the corresponding deposition time span lasted around 850 k.y. up to 1 m.y., the sedimentation rate was very weak : about 1 centimetre or less each 1 k.y. Subsequently, these accumulations are regarded firstly as faunal condensations. The weak sedimentation rate is considered as linked to a major deepening and flooding event which led to sedimentary starvation in the involved Vocontian basin. Moreover, this phenomenon was probably favoured by a well known crisis of the carbonate production at that time. High values of manganese at the base of these condensed limestones are useful in correlating lithostratigraphic units of the same age in the entire basin. Previous studies gave rise to similar interpretations of these units, i. e. records of major flooding and wide connection to the oceanic domain. In addition, the taphonomic study indicates that many fossils were reworked due to episodes of high hydrodynamism, as indicated by erosion of internal molds of ammonites, and in addition to bioturbation. So, the accumulations may come also from faunal concentrations. Finally, this study shows that the inventory of the local and general conditions that govern fossilization must be done before interpreting all exceptional and widespread fossiliferous beds in terms of mass mortality or extinction.

The Leiza palaeo-fault: Role and importance in the Upper Cretaceous sedimentation and palaeogeography of the Basque Pyrenees (Spain)

Abstract New analysis of the Upper Cretaceous deposits of the Central Depression, a syncline within the Basque Pyrenees, shows that this structure was a deep marine basin analogous to the regional flysch troughs. It was bounded by active faults, including the Leiza palaeo-fault, which sustained erosion of the partly subaerially exposed margins. The Leiza palaeo-fault and its western counterpart, the Kalamua palaeo-fault, are thought to constitute the former plate boundary between Iberia and Europe, and a westward continuation of the North Pyrenean Palaeo-Fault.

Numerical Modelling of Carbonate Platforms and Reefs: Approaches and Opportunities:

This paper compares different computing procedures that have been utilized in simulating shallow-water carbonate platform development. Based on our geological knowledge we can usually give a rather accurate qualitative description of the mechanisms controlling geological phenomena. Further description requires the use of computer stratigraphic simulation models that allow quantitative evaluation and understanding of the complex interactions of sedimentary depositional carbonate systems. The roles of modelling include: (1) encouraging accuracy and precision in data collection and process interpretation (Watney et al., 1999); (2) providing a means to quantitatively test interpretations concerning the control of various mechanisms on producing sedimentary packages; (3) predicting or extrapolating results into areas of limited control; (4) gaining new insights regarding the interaction of parameters; (5) helping focus on future studies to resolve specific problems. This paper addresses two main questions, namely: (1) What are the advantages and disadvantages of various types of models? (2) How well do models perform? In this paper we compare and discuss the application of five numerical models: CARBONATE (Bosence and Waltham, 1990), FUZZIM (Nordlund, 1999), CARBPLAT (Bosscher, 1992), DYNACARB (Li et al., 1993), PHIL (Bowman, 1997) and SEDPAK (Kendall et al., 1991). The comparison, testing and evaluation of these models allow one to gain a better knowledge and understanding of controlling parameters of carbonate platform development, which are necessary for modelling. Evaluating numerical models, critically comparing results from models using different approaches, and pushing experimental tests to their limits, provide an effective vehicle to improve and develop new numerical models. A main feature of this paper is to closely compare the performance between two numerical models: a forward model (CARBONATE) and a fuzzy logic model (FUZZIM). These two models use common data sets, thereby permitting one to obtain similar results (Norlund, 1999). The geological model we use to test the validity of these two numerical models comes from a Holocene coral reef located at the island of Mauritius, Indian Ocean. A detailed description of the stratigraphy and general geological setting is given by Montaggioni and Faure (1997). The general stacking pattem and facies distribution obtained are similar to the Holocene reef section used as an example (Dalmasso, 2001). The results here include: (1) an enhanced understanding of similarities and differences between models and modelling philosophies; (2) critical evaluation of applications and assessment of how models have been utilized; and (3) improvements and refinements in techniques for generating and describing model inputs and outputs. The models have various drawbacks, most of which are due to the lack of knowledge about the systems we are trying to model, but also due to the incapability to make effective use of available knowledge. There is no perfect system for modelling carbonate platforms. Numerical modelling can help with both of these problems because it provides effective methods for making qualitative data available for numerical modelling and allows effective exploration of the behaviour of systems through experimentation, thereby increasing our understanding.

Relation accommodation–production carbonatée dans le développement de séquences de dépôt élémentaires de plate-forme carbonatée : la série d'âge Tithonien–Berriasien de basse Provence occidentale

Abstract The Tithonian–Berriasian series of the western Basse Provence carbonate platform (southeast France) is made of the stacking of very high frequency elementary depositional sequences, of metric scale, generally shallowing upward and frequently with emergent caps. The study of this series brings a better understanding of the controlling factors of these sequences. Fischer plots built from the sequence thicknesses and analogous diagrams built from the palaeodepths data suggest that the variations of positive accommodation do not exclusively control the development of these sequences. Variations of neritic carbonate production and/or accumulation have a considerable importance as far as they directly modify the ratio positive accommodation/sedimentary supplies.

Tectonique synsédimentaire distensive dans les calcaires aaléno-bajociens du Seuil de Bourgogne (France)

The analysis of Late Aalenian and Early Bajocian deposits from the Burgundy High and its surroundings shows that: the boundaries between the emerged areas or shallow marine carbonate platforms and the deeper marine marly depositional areas were determined by major faults; coral buildups formed at the top of blocks bounded by normal faults; and marine sediments have filled tension fractures within the limestones. Such features result from extensional synsedimentary tectonics of Middle Jurassic age linked to the Liguro-Tethyan rifting.

Praeradiolites ciryi, a polymorphicrudist from Upper Santonian and Campanian carbonate formations of the Castilian ramp (Northern Spain)

Abstract On the basis of the analysis of more than one hundred specimens of Praeradiolites ciryi Floquet , 1991 , it is shown that this rudist yielded by Upper Santonian and Campanian carbonate formations of Northern Spain, presents a wide range of variability in size, shape and ornamentation of the shell. Given that all morphological intermediates exist and the architectural pattern and shell structure appear identical as well, this variation is regarded as truly intraspecific. Part of this polymorphism is probably related to the available space for growing within a biostrome on the one hand, and to the nature of the substrate on the other hand.