During the Mississippian, a special kind of carbonate build up evolved, called Waulsortian Mounds. They were not reefs – they did not contain the kinds of organisms like corals that could build large complex structures. You may recall that the end-Devonian extinction decimated the reef-building organisms, and we don’t find true reefs in the geologic record for a hundred million years after the Devonian extinction. These mounds are the only constructions known during the interval until reef builders recovered. They are not simple piles of sediment – they can reach 200 meters in height with very steep flanks. They seem to really be constructions of some kind.
And they are found in a restricted geographic setting – the southern margin of the Laurussian Continent, formed by the collision of North America and Baltica, the core of Europe. Today, this zone is in present-day Belgium – the community of Waulsort, Belgium, gives its name to these mounds – southern England, Wales, and Ireland, Illinois, Missouri, and Kansas, and north up to North Dakota and Alberta. During the Mississippian, that margin was very near the paleo-equator, so the mounds probably relate to processes and life in warm, tropical seas. The entire southern margin of Laurussia was probably a relatively narrow but continuous ocean, in part the Tethys Ocean between Gondwana and Eurasia and in part the ocean that remained between Gondwana and southeastern North America.
Waulsortian mounds are mostly lime mud, including some broken skeletal fragments of things like crinoids and bryozoans. It is not clear exactly why the mounds formed, but there may be a connection to the likely storm tracks of hurricanes at the time they formed in the Mississippian Period. Or more specifically, the lack of storms. The mounds seem to be in regions that would likely have been protected from the most common tracks of hurricanes, so that they would have had time to grow by whatever mechanism. David King, at Auburn University, shows a connection between the mounds and low wave energy, another way of suggesting that major storms were rare or absent. I have a link on the blog to King’s interesting report on Waulsortian mounds.
Were these mounds constructed by life processes such as sediment binding by bacteria, or by some inorganic process of selective cementation? As near as I can tell, the answer is not certain. It may have been a combination of processes, made possibly by the fortuitous absence of large storms.
—Richard I. Gibson
David T. King, Jr.’s report