November 14. The Atlantic and Gulf Coastal Plains

Today, November 14, is the day the Precambrian would have ended if this calendar were at a proper time scale. And everything we have covered since February 1 would have to be squeezed into the next 6 weeks. That’s why I chose to organize the original book and these podcasts as I have, so I could give more time to each of the more recent periods. But geologic time, deep time, is really hard to grasp. I hope this helps a bit with that.

* * *

Geologic map of the US (from USGS) showing the Atlantic and Gulf Coastal Plains south and east of the heavy black line. The Cretaceous rocks that crop out in the most inland part of the plains (green colors) underlie the Cenozoic (Tertiary) rocks in orange and yellow, and the Cretaceous rocks continue in the subsurface far offshore as well.

Our Cretaceous topic today is the Atlantic and Gulf Coastal Plans of the United States. These relatively flat lands are underlain at depth by all sorts of complicated geology, reflecting the various orogenies that were parts of the assembly of Pangaea, as well as the break-up to produce the present continents. But the rocks closer to the surface of these plains have been virtually undisturbed since they were laid down, beginning in the Cretaceous.

The sedimentary rocks of the coastal plains are from times when sea level was higher. In Late Cretaceous time, the shoreline was located well inland from where it is today, crossing through east-central North Carolina, central South Carolina and Georgia, and central to northwestern Alabama. Almost all of what is now the state of Mississippi was under water, and there was a huge embayment that extended up what is now the Mississippi River valley all the way to southern Illinois. Southeast Arkansas, all of Louisiana, and a wide swath of eastern Texas were also under the Cretaceous sea. The positions of the shoreline in the Cretaceous, and later into the Cenozoic era, shifted many times. In our earlier discussions, that kind of transgression and regression of the sea was often attributed to glacial periods, alternately locking up water into ice sheets and melting. There’s no evidence for Cretaceous glaciation, so we have to call on some other rationale for the changing sea levels. Probably the most popular possible cause relates to changes in ocean water level because of increases and decreases in tectonic activity. More high-standing mid-ocean ridges would displace more water, raising sea level. But once established, the mid-ocean ridges were more or less still there over time, so that explanation doesn’t work too well.

Sedimentation on the coastal plains and the adjacent continental shelves of eastern and southern United States has been more or less continuous for 100,000,000 years or more, making for a huge pile of sediment – as much as 30,000 feet in places. The sediments are mostly clastics, sand, silt, mud, and gravel, washed off the continent into the adjacent sea. This is continuing to this day, with the Mississippi River the leading source of sediment into the Gulf of Mexico.

Throughout the US East Coast, a topographic break marks the boundary between the coastal plain sediments and the complex igneous and metamorphic rocks of the Appalachian Orogenic belt. It’s called the fall line, because the escarpment the boundary forms results in waterfalls in many places. The Great Falls of the Potomac River is one such waterfall. During colonial times, the fall line was typically the head of navigation for the coastal rivers, so cities like Richmond, Virginia, and Columbia, South Carolina, sprang up along it.

All that material washing into the Gulf of Mexico, especially the porous sand, created a great many reservoirs for oil and natural gas. Both the Cretaceous and Tertiary sediments in the Gulf of Mexico hold many oil and gas fields. Although the general setting was similar on the east coast, there are no oil and gas fields there – probably because of differences in organic content of the source rocks, or in the thermal maturation of the sediments, but the reason for the paucity of oil on the east coast is not completely clear.

* * *

Charles Lyell was born November 14, 1797, in Kinnordy, north of Dundee, Scotland. He is best known as the author of the Principles of Geology, first published in the early 1830s. The multiple volumes of the Principles were among the most influential books in the early development of the geological sciences. It was published continuously, in 12 revised editions, until 1872. Lyell effectively popularized the geological concepts of James Hutton, including uniformitarianism.

—Richard I. Gibson

Atlantic coastal plain