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Mar 29

Guest Post: A Muirkirk Dilemma

Posted on March 29, 2018 at 11:17 AM by Bonnie Man

An interesting geological dilemma exists at the nearby Dinosaur Park, located just south of Laurel, in Prince George's County, Maryland. The Park's geology is from a slice of time, roughly 112 millions of years ago, in what is known as the Cretaceous Period. Within the boundaries of the Park there exists two formations, both the same age but with vastly differing characteristics. These are the Arundel and the Patuxent Formations. 

The present fenced-in area of the Park was once a meandering stream much like the Mississippi River of today, as it flows through the bayou of what is now southern Louisiana. Periodically, this meandering river did as the present Mississippi, forming an almost complete curve, known as an oxbow. Over time, the curve becomes narrower and narrower, until the river cuts across the bend during a flood. The curve becomes separated from the river, becoming an oxbow lake. At Dinosaur Park, a prehistoric oxbow lake is now preserved in time and is in what we now know as the Arundel Formation. 

What is unusual about the Arundel Formation is the preserved fauna found there: it is mostly vertebrate material and mostly dinosaurs, and all are known from their teeth and various disarticulated bones. Four species of turtles are known, but crocodilian teeth and armor are abundant as well as gar fish scales teeth from freshwater sharks. The most common plant material is wood preserved as lignite and ironstone molds of Metasequoia (a kind of cedar tree) cones. Trace fossils also are found as impressions in ironstone slabs and include plant leaves and animal tracks.

Now for the interesting part: the dilemma is in the close proximity of the Arundel Formation with the Patuxent Formation. Their boundaries touch. Whereas the Arundel Formation deposits are almost exclusively clay and silt with large amounts of tree lignite, ironstone nodules and preserved (but disarticulated) bones and teeth of vertebrates, the Patuxent Formation is primarily unconsolidated sand and clay, but with tree, fern, and cycad fossils preserved as silicon replacement and often containing considerable cellular detail - but, not always. More often than not, these Patuxent Formation plant fossils are preserved in a state of decomposition. And, even more interesting, is that the vertebrate fossils found in the Arundel is not found in the Patuxent, and vice versa, although their boundaries meet. As Peter Kranz in his 1998 paper on the Arundel Formation found “Mostly Dinosaurs,” in my studies with the Patuxent Formation, I have found, “Mostly Cedars.” 

My conclusion is that cypress is a decay resistant wood and therefore has been preserved more intact than other plant material in an environment that favors decay and incomplete preservation while undergoing petrification.

I would like to acknowledge and thank Dr. Shelly Jaye, a professor at the Annandale Campus of the Northern Virginia Community College, for providing access to the geology lab and geological thin section machine, with which I was able to make the histology sections so important in the identification of the plant material from Muirkirk and to Dr. Peter Kranz, without whom the Dinosaur Park would not exist. He has been a valued mentor providing insight and direction.

References

Kranz, P. 1998. Mostly Dinosaurs: A Review of the Vertebrates of the Potomac Group (Aptian, Arundel Formation), USA. New Mexico Museum of Natural History and Science Bulletin No. 14.

Hoadley, Bruce. 1988. Identifying Wood: Accurate Results with Simple Tools. 

Tidwell, W. D. 1975. Common Fossil Plants of Western North America.

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This guest post was contributed by Dinosaur Park volunteer and veteran fossil hunter Gerald Elgert. It has been lightly edited.