Conodont-based quantitative biostratigraphy for the Eifelian of the eastern Anti-Atlas, Morocco

The measured ranges of 52 conodont taxa present in 7 Eifelian sections of the eastern Anti-Atlas in southern Morocco have been as- sembled by graphic correlation method into a chronostratigraphic framework. The Anti- Atlas regional composite developed in this study provides much higher stratigraphic res- olution than the traditionally used conodont zonation. It allows a subdivision of the Eifelian into 65.7 composite standard units, derived from the Jebel Ou Driss section, which was se- lected as the standard reference section. The framework appears to have a linear fit to time, giving the first image of the relative duration of the conodont zones in the upper Emsian to lower Givetian interval. The costatus, kockeli- anus, and Lower varcus chrons appear to have lasted more than two times longer than the partitus, australis, and/or hemiansatus chrons. An alternative upper Eifelian-lower Givetian conodont zonation with zones of more equal duration is proposed. Several conodont index taxa in the Anti-Atlas area display an almost synchronous appearance, independent of fa- cies conditions and shelf paleobathymetry. The analysis by graphic correlation reveals significant differences in rate of sediment accu- mulation between the Tafilalt Platform, which had condensed sedimentation (2.5 m/m.y.), and the Mader Basin, characterized by rates up to 40 m/m.y. area, the Devonian rocks offer excellent condi- tions for detailed biostratigraphical studies. Their depositional history reflects the process of disin- tegration of the northern margin of the Sahara cra- ton (Wendt, 1985, 1988). In contrast to the rather uniformly developed Upper Silurian and Lower Devonian sedimentary rocks, differential subsi- dence affected the area during Eifelian time, and thus a platform to basin topography of the shelf began to originate. This phase is biostratigraphi- cally relatively well constrained. Conodonts were found in many sections, providing a base for cor- relation of Eifelian rocks in the Anti-Atlas (Bul- tynck and Hollard, 1980; Bultynck and Jacobs, 1981; Bultynck, 1985, 1987, 1989, 1991). The fauna dominated by Polygnathus elements is lo- cally very abundant, but some stratigraphically important Eifelian index taxa are rare or absent, so that the position of some zonal boundaries (e.g., the base of the Eifelian) cannot be precisely rec- ognized. This is why Bultynck (1987) proposed two alternative conodont zonations for the upper Eifelian-lower Givetian interval. In order to provide a more accurate basis for correlation of Eifelian rocks and events in north- ern Africa, we used the graphic correlation method to develop a composite standard section based on conodont data from the eastern Anti-At- las. Our regional chronostratigraphic framework (Anti-Atlas regional composite) provides a much more linear biostratigraphic subdivision than any previously used conodont zonation. It offers, moreover, the best possible stratigraphic resolu- tion, useful for detailed analysis of the sedimen- tary evolution of the study area during the Middle Devonian. This contribution constitutes a first ap- proach in the construction of the Eifelian com- posite standard with worldwide applicability. In conventional biostratigraphy, fossil zones are used to achieve correlation. The graphic cor- relation technique, however, produces a frame- work that is founded on biostratigraphy, but inte- grates all the biostratigraphic events onto a composite standard. This method was developed by Shaw (1964), and was subsequently extended (e.g., Miller, 1977; Sweet, 1979; Edwards, 1984, 1989) to be applicable to nonpaleontologic and nonunique, but chronostratigraphically signifi- cant, events (including magnetic polarity data, key lithological marker beds, various geophysical log- type data, and stable isotope logs). It constitutes a powerful tool for the analysis of synchroneity or nonsynchroneity of geologic events. Integrated with sequence stratigraphy and sedimentological data, Shaw's method of graphic correlation is one of the standard techniques applied in modern basin analysis (e.g., Neal et al., 1994). GEOLOGIC SETTING