Ecostratigraphy and sequence stratigraphy in high frequency sea level fluctuations: examples from Jurassic macroinvertebrate assemblages

Abstract

Sequence stratigraphy is a method for stratigraphic analysis providing interpretative models with the aim of integrating the subsurface and surface information for correlations, and therefore biostratigraphical data has become increasingly valuable in this context. Given the foregoing, unsolved problems still arise out of the difficulty in obtaining local biostratigraphical data, which can certainly be related to the fact that the linkages of contemporaneous depositional systems are facies independent, and also because of the comparatively broad time span accepted for the cycles of high frequency sea level fluctuations (3rd order and lesser ones). Other restrictions have to do with the comparative complexity of ancient (e.g. Mesozoic) platform systems whose extent and subdivision lead to deviations in the stratal patterns as regards those proposed by the existing synthetic models of sequence stratigraphy. Moreover, at present the attempts at correlation of data obtained in ancient platforms with those from distal areas of continental margins, which are normally characterized by low sedimentation rates, are made indirectly (Olóriz et al., 1991) and are still at a comparatively early stage of development. We here propose an ecostratigraphical approach for correlation and characterization of system tracts in the context of high resolution stratigraphy. The interpretation of the geological record must of course be undertaken with regard to recognizable local determining factors. The observable trends in the record of benthic-epibenthic marine macroinvertebrate assemblages are analyzed in this context. Examples of the Upper Jurassic ammonite record, and especially bivalves among benthic faunas are analyzed on the South and West Iberian margins. Using the sequence stratigraphy frame proposed for this margin by Marques et al. (1991), the data obtained are compatible with the contraction-expansion of ecospaces which could presumably be related to the evolution of Highstand, Transgressive and Shelf Margin Wedge System Tracts. We also comment on published data by others authors concerning fluctuations in ammonite assemblages and ammonite phenotypes, chronological aspects related with sequences stratigraphy analyses, and the record of benthic assemblages within the framework of high frequency fluctuations in relative sea level. The approach proposed here proves to be a useful complement in sequence stratigraphy analyses and will also be of use in more profound study of the ecostratigraphic characterization of system tracts and new interpretations of the fossil record.