The Cenozoic: Sea Level Changes Shape the Continent

The Cretaceous - Paleogene boundary marks one of the most significant physical and biological events in Earth’s history. The boundary, which is about 66 million years old, marks the contact between the Mesozoic and Cenozoic eras. It represents a time during which many marine and terrestrial animals and plants, from microscopic varieties to massive dinosaurs, suddenly became extinct. Many scientists accept that the Cretaceous extinctions resulted from the impact of a large meteorite that produced the Chicxulub impact crater along the northern coast of Mexico’s Yucatan Peninsula. The extinctions may also have been coupled with the regression of shallow seas and massive volcanism in India.

Because the North American plate was (and still is) drifting away from the Mid-Atlantic Ridge during the Cenozoic, mountain building along the plate’s eastern margin ceased. Instead, sediment eroding from existing mountains was gradually deposited along North America’s passive continental margin. Despite minimal tectonic activity in the Coastal Plain area throughout the last 140 million years, the face of the land continued to change significantly due to erosion, deposition, sea level fluctuations, and the ice age.

The Coastal Plain of the South Central US is the largest and generally youngest region. Its geology is also relatively straightforward: sediment eroded in the north and west since the Cretaceous has been transported and deposited on the shores of the Gulf of Mexico. In fact, the Coastal Plain’s outermost portion marks the approximate shoreline of the Gulf during the Cretaceous. Over the subsequent 70 million years, sediment from the west and north has filled in the Gulf, pushing the shore roughly 300 to 800 kilometers (190 to 500 miles) east and south to its current position, with sediment deposits 15,000 to 18,000 meters (49,000 to 59,000 feet) thick. The depositional settings that formed the Coastal Plain are similar to those seen today, including river, floodplain, shoreline, delta, and shallow marine environments. The region is still an active environment of deposition, and deposits become increasingly younger toward the Gulf (Figure 1.15).

In Texas, the Coastal Plain extends west to the Balcones Fault and Escarpment, commonly regarded as the division between upland and lowland Texas. This relatively low-stress fault is thought to have formed 15 million years ago as a result of the Coastal Plain having “settled” relative to the bedrock of the upland (part of the Great Plains region), but it occasionally still generates small earthquakes. The entire state of Louisiana is found within the Coastal Plain, where Cenozoic deposits formed as the sea advanced and retreated on several occasions. The huge volume of sediment deposited by the Mississippi River helped to build the greatest breadth of the Coastal Plain, which stretches approximately 800 kilometers (500 miles) from southeastern Missouri to the Mississippi Delta.

Figure 1.15: Shoreline positions along the Coastal Plain during the past 70 million years. The shoreline reflects the regression that resulted from the last significant glacial advance of the modern ice age.

By the Neogene, the Farallon plate lay shallowly under the North American plate for hundreds of kilometers eastward of the West Coast. Now situated more fully beneath what are now the South Central, Southwestern, and Rocky Mountain States, this extra layer of crust caused uplift and extension of the region, as the added thickness of buoyant rock (relative to the mantle) caused the entire area to rise isostatically. The Farallon plate was subjected to increasing temperatures as it subducted, causing it to expand; as heat dissipated to the overlying North American plate, that rock expanded as well. Finally, the high temperatures in the upper mantle caused the Farallon plate to melt, and the resulting magma was injected into the North American plate, destabilizing it. These processes caused the surface of the North American plate to pull apart and fault into the mountainous blocks of the huge Basin and Range province, a portion of which is found in West Texas.