Question

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?

Short answer

During the Cretaceous Period, widespread black shales formed in seas covering continental surfaces due to a combination of factors such as sea level changes, anoxic events, high productivity, and volcanic activity. High sea levels allowed warm waters to create vast shallow epicontinental seas, while anoxic events, high productivity caused by global warming, and volcanic activity contributed to the accumulation and preservation of organic matter necessary for the formation of black shales.

Step-by-step solution

Understanding Black Shales

Black shales are a type of sedimentary rock that is rich in organic matter and have a dark color due to their high carbon content. These shales form when organic matter (mainly from algae and plankton) accumulates and becomes preserved in deep-sea environments with low oxygen levels. In the Cretaceous Period, certain environmental conditions led to the widespread deposition of black shales.

Sea Level Changes

During the Cretaceous Period (145-65 million years ago), global sea levels experienced significant fluctuations. Higher sea levels allowed warm waters to spread over continental surfaces, creating vast shallow epicontinental seas. These seas favor the accumulation and preservation of organic matter, an important factor for the formation of black shales.

Anoxic Events

Anoxic events refer to periods when seawater becomes depleted in oxygen, resulting in the accumulation of organic matter in seafloor sediments. These low-oxygen environments are ideal for preserving organic matter, which ultimately forms black shales. In the Cretaceous Period, temperature gradients, water stratification, and nutrient supply contributed to the occurrence of anoxic events.

High Productivity

The Cretaceous Period also saw high productivity in marine ecosystems due to global warming and an increase in atmospheric carbon dioxide (CO2). This elevated temperature allowed for increased photosynthesis and favored the growth of various types of algae and plankton. As a result, large quantities of dead organic matter settled to the seafloor, creating an environment conducive to the formation of black shales.

Volcanic Activity

The Cretaceous Period was marked by extensive volcanic activity. Volcanoes released large amounts of nutrients and trace elements, which stimulated high biological productivity in surrounding marine environments. This resulted in a rapid deposition of organic material, contributing to the formation of black shales in adjacent seas. In conclusion, widespread black shales formed in seas that spread over continental surfaces during the Cretaceous Period due to a combination of factors such as sea level changes, anoxic events, high productivity, and volcanic activity. These factors led to the accumulation and preservation of organic matter necessary for the formation of black shales.