Minecraft wiki entry for Development_versions, associated text: The cavern now generates in 2 layers of different stone bases depending on the depth.
Cavern layer above Y=0 is made up of regular stone.
Stone transitions to deepslate from Y=0 to Y=-7.
Deepslate now completely replaces stone from Y=-8 to bedrock.
Certain types of ores and structures still generate in the deepslate layer.
Dirt, diorite, granite and andesite do not generate in this layer.