Sequence stratigraphy allows elucidation of the time-space relationships of deposits in sedimentary basins. However, the relative distribution of unconformities and condensed sections can neither be used as diagnostic criteria for particular palaeogeographic settings, nor as indicators of maximum regional transgressions. Condensation processes also occur in the shallowest portions of platforms and during maximum regression episodes. Three separate processes of condensation are here distinguished: stratigraphic, sedimentary and taphonomic. These three categories of condensation can be used to analyze patterns of sediment distribution, and also to estimate lateral and vertical variations in sedimentation rates. Contrary to the usual interpretation, the maximum values of rate of sedimentation and degree of accommodation of sediments, as well as the highest relative sea level, occur in the lowest levels of the shallowing-upward sequences. The distinction between these condensation processes allows one to predict that the degree of sedimentary and stratigraphic condensation will be higher towards the distal portions of the platforms, whereas the stratigraphic condensation processes without sedimentary condensation will show the maximum intensity and frequency in the shallowest portions of the platforms.
The oolitic ironstone level occurring at the Callovian-Oxfordian boundary across wide areas in the central Iberian Range (Eastern Spain) is interpreted here as formed on an extended, very shallow to temporarily emerged, uniform carbonate platform. Main evidence to support this interpretation comes from both sedimentological analysis of facies and taphonomic analysis of the ammonites. The sedimentological analysis gives support to the idea of iron oolites being formed on, or in the surroundings of, emerged areas. On the other hand, the taphonomic analysis shows that some inner moulds of ammonites from this level display evidence of taphonomic reworking, such as the presence of ellipsoidal abrasion facets on the final part of the last preserved whorl, or annular abrasion furrows carved on the external region. These features would have developed by the action of directional currents under extremely shallow conditions. Bathymetric implications are relevant for the interpretation of the sea level fluctuations at this stratigraphic interval: A relative lowstand of sea level is proposed for the Upper Callovian-Lower Oxfordian interval in the studied area. KEY-WORDS: FACIES ANALYSIS, TAPHONOMIC ANALYSIS, EUSTATIC CHANGES, CALLOVIAN-OXFORDIAN BOUND-ARY, IBERIAN RANGE. RESUMELe niveau d'oolites ferrugineuses situe it la limite Jurassique moyen-superieur, rencontre sur une grand partie de la Chame lberique (Est de l'Espagne), est interprete comme ayant ete forme sur une plate-forme carbonatee uniforme, tres etendue et peu profonde, voire localement emergee. Cette interpretation est basee sur l'etude sedimentologique des facies et l'analyse taphonomique des associations d'ammonites. L'etude sedimentologique etaye bien l'hypothese de la formation du niveau oolitique sur, ou proches, des aires emergees. D'autre part, l'analyse taphonomique a montre la presence dans ce niveau de moules internes d'ammonites qui presentent notamment des facettes ellipsoldales d'usure recoupant le dernier tiers de spire ou des sillons annulaires d'usure, interpretes comme preuves de remaniement des moules internes, et probablement dus it l'action des courants directionnels en milieux extremement peu profonds. Ces implications sur la profondeur des eaux au-dessus de la plate-forme et les variations de la bathymetrie sont d'importance primordiale pour l'interpretation des fluctuations du niveau marin pendant cet intervalle stratigraphique. Une phase de bas niveau relatif est proposee pour l'intervalle Callovien superieur-Oxfordien inferieur dans la region etudiee. MOTS-CLES : ANALYSE DE FACIES, ANALYSE TAPHONOMIQUE, CHANGEMENTS EUSTATIQUES, LIMITE CALLO-VIEN-OXFORDIEN, CHAINES IBERIQUES.
During the Middle Jurassic, the domain of the Iberian and Catalan Coastal ranges of eastern Spain was occupied by a system of fault-controlled carbonate platforms that flanked the Iberian Massif to the East. This platform system marked the transition between the shelves of the Alpine Tethys and the Central Atlantic Ocean. The palaeogeographic reconstruction of the Iberian Middle Jurassic platfOlm system is based on more than 199 surface sections and 37 wells. From southwest to northeast, eight main palaeogeographic elements with associated characteristic facies are recognized. These represent a system of horsts and grabens. In
Bas Auran, in south‐eastern France, is the candidate area for Global Boundary Stratotype Section and Point (GSSP) of the base of the Bathonian Stage (Middle Jurassic). In the Bas Auran area, upper Bajocian and lower Bathonian deposits are made up of limestone beds alternating with marls that correspond to the ‘Marno‐calcaires àCancellophycus’, below the ‘Terres Noires’ Formation. Taphonomic analyses of the successive ammonoid fossil assemblages provide new and complementary data for biostratigraphic completeness, palaeoenvironmental setting and sequence stratigraphy. Lithologic differentiation between limestone and marl intervals resulted from alternating episodes of carbonate input and starvation. Both lithologic phases may contain evidence for sedimentary and taphonomic reworking, associated with scours, that reflects low rates of sedimentation and stratigraphic condensation. Three successive types of elementary cycles resulted from increasing rates of stratigraphic condensation, sedimentary condensation and substrate stabilization during early Bathonian. The occurrence of reelaborated ammonoids (i.e. exhumed and displaced before their final burial) implies that tractive current flows or winnowing affected the burial of concretionary internal moulds. In the lower Bathonian strata, the dominance of homogeneous concretionary internal moulds of phragmocones, completely filled with sediment, is indicative of low rates of sedimentation and sediment accumulation, respectively associated with low degrees of stratigraphic and sedimentary condensation. However, at the Bajocian/Bathonian transition, hemipelagic, bed‐scale limestone–marl alternations show a maximum value of biostratigraphic completeness and there is no evidence for taphonomic condensation in the ammonoid fossil assemblages. Taphonomic analyses of the successive ammonoid fossil assemblages and taphofacies confirm the development of a deepening phase associated with sedimentary starvation, which characterizes the last episode within the deepening half‐cycle of third and second order cycles, in the Bas Auran area of French Subalpine Basin during early Bathonian.
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