February 5. The Cambrian transgression

During the early Cambrian, sea levels were rising. This produced a near-global transgression – which is not a sin, but rather it just means that the seas were advancing, covering more land area than they had previously. The opposite is a regression, when the seas become relatively smaller.

So how does that happen? With minor exceptions, the volume of water on earth has been more or less constant, at least since pretty early in the earth’s history, back in early January sometime. But the volume of the ocean basins that hold the water can change and does.  One way that can happen is by rifting apart continents, as Rodinia was splitting into several smaller continents in late Proterozoic and early Cambrian time. The volume change comes about because of the mid-ocean ridges, the point where oceanic crust is pulling apart. Today, the oceanic ridge system is the longest mountain range on earth, and added together, it takes up a pretty notable volume of oceanic water. Enough that if there are a lot more oceanic ridges, it can result in sea level rise. Likewise, if there were a lot more oceanic trenches, very deep water, that could also accommodate at least a little more water than a flat ocean floor.

We know from concerns today about sea-level rise that melting and freezing ice caps can contribute to sea level changes. And a small effect might even come about because of water temperature. Warmer water expands, if only a bit, but when the entire ocean expands, it can make a difference in sea level, especially on very low, flat shores.

Paleogeographic map by Ron Blakey via Wikipedia under CC-BY-SA & GFDL

In North America during the Cambrian, shallow seas covered a vast amount of the continent, all except high area on the Superior Craton and a long narrow peninsula called the Transcontinental Arch, extending from Minnesota southwest to what is now Colorado and New Mexico. Abbreviations in the middle Cambrian (about 500 million years ago) paleogeographic map above: NA-North America. B-Baltica (Europe). S-Siberia. SA-South America. AF-Africa. AUS-Australia. CH-China. ANT-Antarctica. Here is another version of a map showing the arch, and another one.  

The sea took millions of years to transgress across North America during the Cambrian. As it progressed further and further, the shoreline beach also changed position. Consequently there was a lot of sand – sandstone today – that marks the base of the Cambrian across much of North America, but don’t think of it as one big beach – think of it more like a continuously migrating beach that, over millions of years, ended up depositing sand across many hundreds or even thousands of square miles of the continent. We’ll talk about some of those sandstones as we work our way through February.

—Richard I. Gibson

Paleogeographic map by Ron Blakey via Wikipedia under CC-BY-SA & GFDL.