Palynological variations and sedimentary cycles in the Jurassic from the Paris basin

Abstract

Two studies, one on Hettangian dolomitic cap rocks over “Gaz de France” gas storage reservoir rocks at Sologne, and the other on two separate sections in Dogger formations in Burgundy, conducted at the Palynology Laboratory of Strasbourg, illustrate that a palynological approach can lead to a considerable refinement of knowledge in sequence stratigraphy. The palynological study of the dolomitic cap rocks at Sologne dates them as Hettangian, distinguishes microfloristical assemblage types within the sequence, and establishes palynofacies variations with basin evolution. The evidence suggests that palynological variations in the Hettangian rocks are attenuated, reflecting restricted marine conditions and weak eustatic changes. The two sections in Dogger formations in Burgundy, one in Buffon with Callovian sediments deposited in an inner neritic environment, and the other in Varzy with Bathonian sediments deposited in an outer neritic environment, show an alternation between a sapropelic palynofacies, with abundant amorphous organic matter and numerous acritarchs, and a humic palynofacies, rich in dinoflagellate cysts, and spores and pollen. Such alternations indicate a cyclic palynological evolution even within the sedimentary parasequences (4th–6th order), as recorded in the section at Buffon.