What conditions may account for the formation of widespread black shales in seas that spread over continental surfaces at certain times during the Cretaceous Period?

The Cretaceous Period is a geologic time period that lasted from about 145 million years ago until about 66 million years ago. During this time, there were significant changes in the Earth's climate, sea levels, and life. The Earth's continents were still moving toward their present-day positions, and large seas covered many of the continental areas.

Black shales are sedimentary rocks formed from the accumulation of organic-rich, fine-grained sediments, typically in low-oxygen environments. These environments can include deep marine settings or shallow seas with restricted water circulation. The organic material preserved in black shales comes from both aquatic and terrestrial sources, such as algae and land plants. As these organic materials accumulate and become buried, they undergo compaction and lithification, eventually forming the black shale rock layers.

One possible condition contributing to the formation of widespread black shales during the Cretaceous Period is the fluctuation in sea levels. Sea levels were much higher during the Cretaceous than they are today, leading to the flooding of large continental areas by shallow seas. The tectonic movement of the continents and the presence of high sea levels allowed for the formation of these shallow seas over vast areas, creating an ideal environment for the deposition of organic-rich sediments.

Changing climate conditions could also play a role in the formation of widespread black shales. The Cretaceous Period experienced a warm climate with high temperatures at both the poles and the equator. This created a relatively stable, stratified ocean with reduced vertical mixing and circulation. These conditions restricted the supply of oxygen to deeper ocean layers, leading to the formation of low-oxygen environments in which black shales could form.

Another factor that might contribute to the widespread formation of black shales during the Cretaceous Period is high marine productivity. The warm climate and high levels of carbon dioxide gas in the atmosphere during the Cretaceous Period likely led to increased photosynthetic activity by marine organisms, such as algae and phytoplankton. The resulting high levels of organic material in the water led to an increase in the deposition of organic-rich sediments that can form black shales. In summary, the conditions that may account for the formation of widespread black shales in seas covering continental surfaces during the Cretaceous Period include fluctuating sea levels, warm climate, restricted ocean circulation creating low-oxygen environments, and high marine productivity resulting in the accumulation of organic-rich sediments.