Abstract: Lower Cretaceous Stratigraphic Framework Interplay Between Tectonics and Eustasy

Thierry Jacquin

Within the Lower Cretaceous series of the Western European basins, we identified one sequence boundary related to a first-order (>50 m.y.) transgressive-regressive cycle, six second-order (3-50 m.y.) transgressive-regressive facies cycles, and 45 third-order (0.5-3 m.y.) depositional sequences with an average duration of 0.9 m.y. All the cycles have been dated in the areas where the sediments are thick. By places the strata are removed by erosion or are condensed due to sediment starvation and the sequences cannot be identified.

The first-order sequence boundary is a major onlap surface which becomes conformable within the Campilotoxus Zone of the lower Valanginian. Strata ranging in age from upper Valanginian to upper Cenomanian onlap the Upper Jurassic and the Hercynian basement in a landward direction.

Six second-order transgressive-regressive facies cycles have been identified within the Lower Cretaceous series. The peak transgressions occur at third-order maximum flooding surfaces. The maximum regressions occur at third-order top lowstand or top shelf-margin systems tracts. These second-order transgressive and regressive facies cycles appear to be controlled by intraplate stress fluctuations. They are also recorded in different tectonic settings, such as the North Sea, Bay of Biscaye, Galicia, offshore Africa, Maracaibo basin (Venezuela), and Gulf of Mexico. These large-scale correlations indicate there may have been a tectono-eustatic effect created by the change in size of the ocean basins during Atlantic rifting.

Third-order depositional sequences result from high-frequency cycles within the accommodation space controlled by tectonic subsidence. An example from the Vercors (France) shows how tectonics governed the development of the Urgonian carbonate platform and large-scale stratal patterns, and how eustasy induced alternation of lowstand and highstand deposits.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994