Here are some more specific terms depending on the context:
* Basal conglomerate: A layer of coarse-grained sediment, often containing pebbles and boulders, that forms at the base of a sedimentary sequence.
* Basal sandstone: A layer of sand-sized sediment that forms at the base of a sedimentary sequence.
* Basal shale: A layer of fine-grained sediment that forms at the base of a sedimentary sequence.
* Basal till: A layer of unsorted glacial sediment that forms at the base of a glacial deposit.
* Basal unconformity: A surface of erosion or non-deposition that separates older rocks from younger rocks.
The specific term used to describe the thick sedimentary accumulation at the base will depend on the type of sediment and the geological context.
