Depositional sequences in shallow carbonate-dominated sedimentary systems:: concepts for a high-resolution analysis

Oxfordian and Berriasian sections representing shallow-water, carbonate-dominated sedimentary systems have been studied in the Swiss and French Jura, in Spain, and in Normandy. They all display a hierarchical stacking of depositional sequences. Facies evolution and stacking pattern allow to define elementary, small-scale, medium-scale, and large-scale sequences. Some depositional sequences display well-marked sequence boundaries, others are limited by transgressive or maximum-flooding surfaces. The hierarchical organisation of such sequence-stratigraphic elements implies that sea-level fluctuations were an important factor in their formation, and that these fluctuations had different frequencies. The superposition of high-frequency sea-level changes on a long-term sea-level trend leads to repetition of diagnostic surfaces, defining sequence-boundary and maximum-flooding zones wherein the corresponding high-frequency surfaces are well developed. Chronostratigraphic tie points permit us to estimate the duration of large-scale sequences. This time control and the observed hierarchical stacking suggest that the high-frequency sea-level changes were controlled by climatic cycles in the Milankovitch frequency band. The variability of stacking pattern and facies evolution between sections illustrates the complexity of the studied environments. Furthermore, because of the minimal accommodation space available in these shallow-water settings, much of the geologic time is not recorded. Nevertheless, detailed analysis of the depositional sequences allows the interpretation of the evolution of the sedimentary system with a high time resolution. Thus, there is a potential to monitor sedimentological, ecological, and diagenetic processes on a time scale of 20 to 100 ka. (C) 1999 Elsevier Science B.V. All rights reserved.