The depth significance of shales with bituminous laminae

Abstract

The shales under discussion contain regularly-spaced thin bituminous laminae alternating with mineral layers, which probably represent varves. The distribution of particulate organic matter in the sea is briefly reviewed. Phytoplankton productivity is highest in regions of upwelling and in continental shelf waters, where in addition there is generally a significant proportion of detritus from terrestrial plants and benthonic algae. Only a small fraction of this organic matter reaches the sea floor and of this very little is ultimately preserved in marine sediments, principally as kerogen. The proportions of terrestrial and marine components of the organic matter in sediments can be determined in several ways. The organic content tends to be highest in fine-grained sediments laid down in poorly-oxygenated or anaerobic waters, characteristically in the deeper parts of nearshore basins partly isolated from normal oceanic circulation by sills. Certain sediments laid down in very shallow lagoons may also be rich in organics. No simple relationship of organic content and depth of sea is discernible. In interpreting bituminous shales in the stratigraphic record, comparisons with the present may be somewhat misleading. For long periods in the past the world climate was apparently more equable than today and shallow shelf seas more extensive. It is argued, taking examples from the Jurassic of Europe and the Devonian of North America, that the familiar “barred basin” model may be inapplicable to many bituminous shales in the past, which were relatively shallow-water deposits laid down in quiet but not invariably stagnant water below wave base. Certain geosynclinal graptolitic shales may have been laid down in deep water, though perhaps shallower than the associated greywackes.