A 300 Million Year Old Pennsylvanian Age Mire Forest
John Nelson and Scott Elrick in the Coal Section of the Illinois State Geological Survey, in cooperation with Bill DiMichele of the National Museum of Natural History Smithsonian Institution, Howard Falcon-Lang of the Univeristy of Bristol and Phil Ames of Peabody Energy recently published a paper about the ecology of a fossil forest in the journal Geology, entitled (along with the abstract):
Ecological gradients within a Pennsylvanian mire forest
Pennsylvanian coals represent remains of the earliest peat-forming rain forests, but there is no current consensus on forest ecology. Localized studies of fossil forests suggest intermixture of taxa (heterogeneity), while, in contrast, coal ball and palynological analyses imply the existence of pronounced ecological gradients. Here, we report the discovery of a spectacular fossil forest preserved over ~1000 ha on top of the Pennsylvanian (Desmoinesian) Herrin (No. 6) Coal of Illinois, United States. The forest was abruptly drowned when fault movement dropped a segment of coastal mire below sea level. In the largest study of its kind to date, forest composition is statistically analyzed within a well-constrained paleogeographic context. Findings resolve apparent conflicts in models of Pennsylvanian mire ecology by confirming the existence of forest heterogeneity at the local scale, while additionally demonstrating the emergence of ecological gradients at landscape scale.
The location of this fossil forest is just to the south and west of Danville, Illinois, about 30 miles to the east of the ISGS in Champaign, Illinois (see picture to the right). This 300 million year old fossil forest was found directly on top of the Herrin coal seam in the Riola and Vermillion Grove coal mines, and represents the last stages of the peat mire forest responsible for forming the Herrin coal.
If you were walking down a hiking trail through a forest today, looking closely at the vegetation, you might notice that as the landscape changed, gradual changes in the types of plants and trees occured as well. For present day plant biologists examining the ecology of a modern day forest, this is a luxury that is easily taken for granted.
When fossil plant researchers (paleobotanists) want to look at the ecology of an ancient forest, it is rather difficult to simply go walking through an ancient forest! Instead, much study must go into looking at many plant fossils from multiple locations and times and settings, getting one puzzle piece of the ecology here... one puzzle piece of the ecology there. Through diligent study, a picture of ancient environments can be slowly be pieced together and good guesses about ancient ecologies can be made.
However, every once in a while a unique opportunity presents itself where a paleobotanist has an opportunity to do what his plant biologist colleagues do all the time. Walk through a forest... a fossil one! This paper represents the largest study of the plant ecology of a Pennsylvanian age forest to date and shows that subtle ecologic changes in the make-up of the forest were present 300 million years ago.
In the practice of geology in the field, scientists regularly look at vast spreads of time in small geographic slices. For example, standing at the rim of the Grand Canyon and peering across to the other side, your eye takes in millions of years of geologic time... but you are only able to see a thin 'slice' of each unit in profile. Do you want to know what a particular rock unit looks like 500 feet into the side of the canyon walls? The only way to find out is to drill a hole and take a core sample.
Geologic research in an underground mine such as these coal mines turns this notion on its ear!
In this study, we were able to look up at preserved trees and ferns in the mine ceiling, peering upwards at a single slice of time over a huge (relatively speaking) geographic area. This is a unique way to get a 'snapshot' in time look at the forest landscape of 300 million years ago. It's the 'worms eye' view of a fossil forest... allowing a veritable 'walk' through the forest and a chance to see first hand what plant species were present and how they were distributed across the landscape. In other words, a chance to see the ecology of a fossil forest!
If you would like to see a piece of this fossil forest in person, you are in luck! The coal mining exhibit at the Museum of Science and Industry in Chicago has a fossil covered slab of gray roof shale from the Riola mine on display. Now you can take a (mini) walk of your own.How was this fossil forest discovered? One of the responsibilities of the ISGS is to try to understand the geology of the state of Illinois. For the Coal Section at the ISGS, that means trying to visit the coal mines in the state on a regular basis. When the Riola mine opened in 1996, geologists from the ISGS visited and noted the presence of fossil plants in the roof of the mine. Plant fossils are not uncommon in Illinois coal mines, so while notes were made, nothing exceptional was thought of the discovery. As time went on, more coal was mined, more of the mine roof was uncovered and the plant fossils didn't stop! Fossils were numerous and showed excellent preservation.
Adding visits to the Vermillion Grove mine, Survey geologists soon realized that a very interesting story was waiting to be told about the fossil plants, and in 2004, contacted Bill DiMichele from the Smithsonian and Howard Falcon-Lang of the University of Bristol, both experts in paleobotany. With the assistance of Phil Ames of Peabody Energy, a large study was then undertaken to try to understand the mosaic of preserved plant fossils presented just over our heads in the gray shale of the mine roof.Why was it preserved? By examining the relative thicknesses of the rock units at the mine, such as the Herrin coal and overlying Energy shale, as well as noting the nature of the contact (either gradational or abrupt) between the afore mentioned rock units and adding in other observations from the mine, ISGS geologists John Nelson and Scott Elrick and Peabody geologist Phil Ames discovered that the ancient forest had grown in an estuary that was tectonically fault controlled. In other words, the fossil forest was growing in a low lying area that was 'low lying' because a fault was allowing part of the land to slowly sink.
More importantly however, was the realization from examining deposits above the fossils and the fossils themselves that this area had not only been sinking slowly, but the fault responsible for the slow tectonic sinking had also drowned the forest by way of an abrupt earthquake... quickly dropping the land surface below sea level. The extreme southern end of a nearby known fault called the Royal Center fault in Indiana was hypothesized to be the culprit.
An intriguing modern day analogue to this kind of phenomenon of earthquake created lowlands is Reelfoot lake in Tennessee.
Like many other fortunate fossil finds, especially ones that have such great preservation, this locality had the happy coincidence of being in the 'right place' at the 'right time'. A catastrophic event (earthquake) provided the opportunity for a large segment of forest to be preserved and reveal to us ecologic subtleties of Pennsylvanian age peat mires that we had guessed and inferred before, but can now see.
Click below for selected images of the fossil plants species that made up this 300 million year old fossil forest. In the Riola Mine in east central Illinois, such plant fossils were abundant and consisted of a mixture of large trunks and the leaves and reproductive organs of a variety of plants.
Text by Scott Elrick, Image captions by Bill DiMichele, Images by Howard Falcon-Lang, Bill DiMichele and Scott Elrick
Updated 03/15/2012 SDE