Formation of laminite deposits in anaerobic—dysaerobic marine environments

Abstract

Vraconian sediments from the Bandol area (Var, southeastern France) are characterized by alternations of thin, dark laminae, in which organic matter is concentrated, and light, calcareous, silt-grained laminae. The dark laminae (black shales) are regarded as the result of autochthonous (i.e. in place) hemipelagic sedimentation. The light laminae (calcareous silt) result from the partial reworking of turbiditic influxes by deep marine currents.Comparison with modern marine environments suggests that the Vraconian laminite deposits from the Bandol area were formed by sedimentation from deep marine circulations. The laminated structures formed in a deep marine environment subjected to periodic variations in the physicochemical conditions, organic productivity and thus the nature of organic matter. The dark laminae were deposited in an anaerobic marine environment in which the organic matter was preserved. During the deposition of the light laminae, the depositional environment changed to dysaerobic conditions, allowing the development of a peculiar benthic fauna responsible for the decreasing organic matter content. Source rock characteristics of these deposits are inferred from their laminated structure and fine texture coupled with the occurrence of organic-rich layers.