What does the term 'shaly chalk' describe in relation to aquifers?

The term 'shaly chalk' specifically refers to a geological formation that contains a combination of clay and chalk. In the context of aquifers, it is significant because the presence of clay can influence the permeability and porosity of the rock, thus affecting its ability to store and transmit groundwater. The Niobrara formation is known for its unique properties, where shaly chalk can serve as both a source rock for hydrocarbons and a formation that may hold water, impacting aquifer quality and behavior. Furthermore, the mixture of these materials can create a stratified system that plays an essential role in groundwater flow dynamics within the aquifer.