The offshore basins near the coast of California are sedimentation areas that are the best-documented, in recent times, in the entire world. They are known as the California Continental Borderland. The area is a model environment for the formation of Paleozoic and Mesozoic black shales.
The California Continental Borderland is the area off the southern California coast consisting of over 15 deep basins. Plankton productivity in surface water is high and the renewal of deep water proceeds at a very slow rate. This means that oxygen depletion of the sea bottom environment is severe, particularly in basins near the coast.
Between 1975 and 1977, more than 800 core samples were retrieved for sedimentological and geochemical analysis. Bottom fauna was sieved out from the cores, identified and counted. The analysis of this extensive dataset is of special importance, since the basins are modern examples of the formation of black shales.
Benthic fauna is present, even where the oxygen content is less than 0.1 ml O2/litre. They include crabs, annelids, snails and mussels as well as fish that live close to the ocean floor. Contradicting older models for patterns of fauna development in an oxygen gradient, the number of species does not decrease continually. Rather, it remains fairly high, even well into the dysoxic area, albeit with far fewer individual numbers.