Britannica
Sources used in this answer
Why did black shales form in many Cretaceous oceans?
Created from Britannica articles
using AI.
AI can make mistakes, so verify using Britannica articles.
Cretaceous Period
Distribution of landmasses, mountainous regions, shallow seas, and deep ocean basins during the late Cretaceous Period. Included in the paleogeographic reconstruction are the locations of the interval's subduction zones.
During the Cretaceous Period, the formation of carbon-rich black shales was primarily driven by a severe deficiency of oxygen in the bottom waters of the oceans.1
While the exact cause of this widespread anoxia remains a subject of scientific debate, two primary theories have been proposed:
- Poor Ocean Circulation: Many researchers attribute the oxygen decline to diminished vertical and latitudinal ocean circulation. This reduced mixing is thought to have resulted from the generally warmer climate of the Cretaceous, which minimized temperature differences between the poles and the Equator, combined with the physical restriction of major basins like the North Atlantic, South Atlantic, and the Tethys Sea.213
- Undersea Volcanism: Some authorities suggest that extensive undersea volcanism, occurring approximately 93 million years ago, triggered the oxygen deficiency.1
These anoxic conditions prevented the normal decomposition of organic matter, allowing it to accumulate and eventually form the black shale deposits found today across various continental areas and ocean floors.415