November 16. Transgressions and Regressions

The other day, when I was talking about the Atlantic and Gulf Coastal Plains of the United States, I mentioned sea-level changes that produced various locations for the Cretaceous shoreline. And I left it pretty much unexplained.

These transgressions and regressions of the sea were mostly world wide, and there were at least five major transgressions or sea-level rises. An analysis of the facies, or rock types and structures in the rocks, suggests that the rate of sea-level rise could have been on the order of 10 to 90 meters per million years, which is obviously slow in human terms, as we wrestle with possible sea-level rises of a few meters in the next century, but it’s really a lot in a short time, in geological terms. At the end of the early Cretaceous, the Albian Age, the global sea level was probably similar to what it is today, but at times during the late Cretaceous, it was as much as a whopping 650 meters higher, although other estimates put the maximum at about 270 meters higher. That’s why the ocean was in Kansas and Illinois. 

And while the late Cretaceous transgressions and regressions are probably the best known, there were similar events during the early Cretaceous as well.

Sea-level change graph by Robert Rhode and Global Warming Art,
by way of Wikimedia. Used under Creative Commons license.

So if, as I suggested the other day, we can’t call on glacial changes to account for these dramatic rises and falls of sea level, what is the cause? Tectonic activity, especially sea-floor spreading that produced mid-ocean ridges, was high during the Cretaceous, and is typically cited as a reason for the generally increasing sea level through the late Cretaceous, and reduced tectonic activity near the end of the Period might be a rationale for the dramatic regression, or drop in sea level, seen at and after the end of the Cretaceous. But those reasons are hard to use to explain the alternating rises and falls seen during the late Cretaceous.

Maybe it was more specific tectonic events such as collisions and mountain building, which might take place over a few million years, and then they were done. Erosion then would start to actively send sediment into the oceans, perhaps in enough volume to displace enough water to increase sea level. Another tectonic way to increase relative sea level is to lower the land. Subsidence of the continents, induced by things like sediment loading and structural pushing of great slabs of rock on top of others, could have allowed the sea to transgress across the land. When we talk about the mountain building in western North America later this month, this idea will come into play

I haven’t found any reports on a good correlation between specific tectonic events and the record of transgressions and regressions, but such work might be out there. If any listeners or readers know of such research, please let me know!

Having an accurate record of sea-level changes is valuable in terms of our general understanding of earth history, of course, but it also has practical applications for oil exploration, as a predictive tool for knowing where in the sequence of rocks you might expect a beach sand, with good porosity as an oil reservoir, to be found. And there are also obviously implications for our understanding of modern climate change and sea levels.

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Today’s geological birthday is Edward Salisbury Dana, born November 16, 1849, in New Haven, Connecticut. His father was the prominent geologist James Dwight Dana, but the son E.S. Dana made monumental contributions to the fields of mineralogy and crystallography. His books, the System of Mineralogy, Textbook of Mineralogy, and Manual of Mineralogy defined the way geoscientists look at minerals and crystals. Although revised, edited, and updated by others, Dana’s Manual of Mineralogy is still the basic text for mineralogy students today.

—Richard I. Gibson

Late Cretaceous transgressions 

Correlating transgressions and regressions 

Cretaceous sea levels 

Sea-level change graph by Robert Rhode and Global Warming Art,  by way of Wikimedia. Used under Creative Commons license.