Cretaceous Coral-Rudist Buildups of France
Coral-nidist buildups are known from three outcrop areas of Cretaceous age (Berriasian to Maestrichtian) in France. These are: 1. Paris Basin, 2. Sud-Est Basin, and 3. Aquitaine-Pyrénées Basin. During Berriasian time coral-rudist buildups were limited to the Sud-Est Basin, and here they only occur locally in relation to “Purbecktype” sediments. The Vaianginian Stage corresponds to an extensive phase of marine shallow water carbonate sedimentation containing coral-rudist buildups. These deposits are recorded from both the Jura Subalpine and Provence-Pyrénées Platforms, while isolated coral beds have been reported from the Paris Basin. During the Hauterivian, regional tilting of former carbonate platforms caused a reduction of coral-rudist buildups, nevertheless isolated coral beds have been reported from the Paris Basin. Barremian and lower Aptian times were marked by a considerable extension of platform-type carbonates in the Sud-Est Basin and in the Pyrénées. In these areas “Urgonian” limestones are better represented than coral beds. During the upper Aptian deepening of the Sud-Est Basin led to the disappearance of coral-rudist buildups in most areas, except in the Aquitaine-Pyrénées region. Albian tectonism caused the disappearance of most shallow water carbonates. The Cenomanian transgression followed, leading to reestablishment of coral-rudist buildups, especially in the Aquitaine-Pyrénées and Provence regions. In lower Turonian time another deepening phase occurred and caused the disappearance of shallow water carbonates. During upper Turonian time a new surge of rudist buildups occurred coincident with an increasingly more important influx of terrigenous sediments. Only relatively small carbonate platforms containing lenses of rudists have been recorded from the lower Senonian of the Pyrénées-Provence region. The upper Senonian is mainly a regressive sequence characterized by an extension of deltaic deposits and restriction of coral-rudist lenses primarily to the Aquitaine-Pyrénées Basin.
Two groups of coral-rudist formations have been distinguished: 1. those associated with off-shore “highs,” and 2. those associated with carbonate platforms. Three types of off-shore “highs” have been recognized. These are: 1. coral “highs,” known only from the Lower Cretaceous, 2. oobioclastic/coral “highs” from the lower Barremian of the subalpine area, and 3. rudist banks present only in the Upper Cretaceous. We regard “highs” as topographic units of limited lateral extent that can be divided into a small number of ecological and sedimentological zones, and are surrounded by deeper water sediments. Platforms are regarded as morphological units of regional extent composed of several adjacent biofacies and lithofacies related to the hydrodynamic properties of their aquaeous environment, and which may laterally grade into a continental facies. A platform may be subdivided into two zones: 1. an outer zone with high energy deposits and organic buildups (mainly corals in the Lower Cretaceous and corals and rudists in the Upper Cretaceous), and 2. an inner zone of quiet to moderate energy deposits characterized by abundant rudists.
Development of coral-rudist formations appears to be governed by six important factors. These are: 1. shallow water conditions, mainly infralittoraJ, 2. relative basement stability (although in some instances tectonism may create “highs” on which organisms may thrive), 3. eustatic stability or transgression, 4. low terrigenous influx, 5. absence of organism restricting océanographie conditions, and 6. a warm climate of tropical to subtropical nature.
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The voluminous amount of information presented in this Special Publication not only fills a gap in understanding the European approach to reef studies but also provides the necessary data base to allow us (in particular the North American geologist) to incorporate this information in our overall interpretive studies. These studies should serve as an impetus for new investigations and will broaden our understanding of the complex interrelationships that operate in the reef environment.