Complex sentence: The 6.4 km thick Cretaceous limestones, which predominately make up The Bahamas, were deposited in shallow seas, assumed to be a stretched and thinned portion of the North American continental crust.
Split into simpler sentences: The 6.4 km thick limestones, which predominately make up The Bahamas, date back to the Cretaceous. These limestones would have been deposited in shallow seas, assumed to be a stretched and thinned portion of the North American continental crust.