FORMATION OF LAMINITE DEPOSITS IN ANAEROBIC DYSAEROBIC MARINE ENVIRONMENTS

Vraconian sediments from the Bandol area (Var, southeastern France) ar e characterized by alternations of thin, dark laminae, in which organi c matter is concentrated, and light, calcareous, silt-grained laminae. The dark laminae (black shales) are regarded as the result of autocht honous (i.e. in place) hemipelagic sedimentation. The light laminae (c alcareous silt) result from the partial reworking of turbiditic influx es by deep marine currents. Comparison with modem marine environments suggests that the Vraconian laminite deposits from the Bandol area wer e 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 prese rved. During the deposition of the light laminae, the depositional env ironment changed to dysaerobic conditions, allowing the development of a peculiar benthic fauna responsible for the decreasing organic matte r content. Source rock characteristics of these deposits are inferred from their laminated structure and fine texture coupled with the occur rence of organic-rich layers.