Introduction and Overview

The term "black shale" is a common expression widely used to describe any dark~olored, fine-grained sedimentary rock relatively rich in organic matter. As defined by Weissert (1981), the range of organic carbon in black shales is between 1 and 30%, but values outside this range exist. The term usually connotes at least partial lithification, yet this may not be true in young, shallow sediments. The most important characteristics conveyed by this descriptive expression are the dark color and the high content of organic matter in black shales, two features which distinguish these rocks from other modern and ancient fine-grained sedimentary deposits. Episodes of black shale deposition have occurred sporadically throughout geological time. The Cretaceous Period contains the most extensive record of black shale formation in both shallow-water and deep ocean locations (Arthur and Schlanger, 1979; Weissert, 1981). Much of the world's oil production is sourced from Cretaceous rocks (Schlanger and Cita, 1982), and so there are economic reasons for interest in understanding the conditions leading to deposition of black shales. This interest, moreover, transcends the role of black shales as hydrocarbon source rocks. Knowledge of the factors involved in their deposition provides many clues as to the paleoceanographic conditions favorable to their formation. Geochemical and sedimentological data contribute to this understanding.