Black shales on the basin margin: a model based on examples from the Upper Jurassic of the Boulonnais, northern France

Abstract

Abstract Organic-rich shales were deposited during two phases of relative sea-level rise in the Kimmeridgian (Late Jurassic) basin margin sections of the Boulonnais (N. France). Pyrite framboid analysis indicates that bottom waters were euxinic during substantial periods of organic-rich shale formation, and Th/U ratios and degree of pyritisation of Fe suggest severe oxygen-restriction, whilst palaeoecological and sedimentological evidence suggests that seafloor oxygenation events were frequent. Interbedded, hummocky cross-stratified sandstones indicate deposition above storm wave base. Correlation of lateral facies changes reveals that the black shales record deposition beneath a nearshore zone of oxygen-poor waters that contracted in lateral extent during parasequence formation. The Boulonnais black shales are similar to other varieties of transgressive black shale, in that they are developed upon surfaces of marine starvation, but they differ in that they pass down-dip into better oxygenated facies. They are therefore classified as transgressive, nearshore (TN) black shales that do not record the ‘feather edge’ of basinal conditions. Deposition is interpreted to have occurred beneath a small volume of sub-thermocline bottom waters that were readily de-oxygenated, whilst anoxia was less readily developed in more offshore settings due to the greater volume of sub-thermocline water. Why such conditions should pertain only during initial stages of base-level rise remains obscure.