The Delaware Basin is a geologic depositional and structural basin in West Texas and southern New Mexico, famous for holding large oil fields and for a fossilized reef exposed at the surface. Guadalupe Mountains National Park and Carlsbad Caverns National Park protect part of the basin. It is part of the larger Permian Basin, itself contained within the Mid-Continent oil province. Geology By earliest Permian time, during the Wolfcampian Epoch, the ovoid shaped subsiding Delaware Basin extended over 10,000 square miles (26,000 km²) in what is now western Texas and southeast New Mexico. This period of deposition left a thickness of 1600 to 2200 feet (490 to 670 m) of limestone interbedded with dark-colored shale. A narrow outlet that geologists call the Hovey Channel periodically supplied new seawater from the Panthalassa Ocean to the west. The somewhat smaller and shallower Midland Basin was just east and the much smaller Marfa Basin was to the southwest. All three basins were south of the equator, north of the Ouachita Mountains of mid-Texas, and part of the northern continent Laurasia. Structurally the Delaware, Midland and Marfa were foreland basins created when the Ouachita Mountains were uplifted as the southern continent Gondwana collided with Laurasia, forming the supercontinent Pangea in the Late Carboniferous (Pennsylvanian). The Ouachita Mountains formed a rainshadow over the basins, and a warm shallow sea flooded the surrounding area. On the other side of the equator, the Ancestral Rocky Mountains formed a large mountainous island. The Delaware Basin temporarily stopped subsiding in the Leonardian Epoch at the start of the mid-Permian. Small banks along its margin developed along with small discontinuous patch reefs in the shallow water just offshore. The first formation that resulted was the Yeso and consists of alternating beds of dolomitic limestone, gypsum, and sandstone. The sediments responsible for creating the Yeso were deposited in nearshore areas that graded into the carbonate banks of the Victorio Peak Formation in the deeper waters. Thin-bedded limestones of the Bone Spring Formation accumulated as limy ooze in the stagnant deepest part of the basin. Subsidence of the Delaware Basin restarted later in the mid Permian and by the Guadalupian Epoch of the upper Permian the patch reefs had grown larger. Sediments deposited close to the shore are now the cherty dolomites of the San Andres Formation while deposition a little further out forms the quartz sandstone and scattered patch reefs of