This is Mazon Monday post #112. What's your favorite Mazon Creek fossil? Tell us at email:[email protected].
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Lately, we've been getting a bunch of questions about concretion shape, probably due to the Braceville field trip this past weekend. It's an age old question that repeats again and again. Back in March 1952, Stevens T. Norvell, a founding member of ESCONI and its first treasurer, wrote "Our Fossil Ferns - Why They Occur in Form-Fitting Pebbles" for the newsletter. The information is still mostly relevant today.
Our Fossil Ferns - Why They Occur in Form-Fitting "Pebbles" by Stevens T. Norvell
Why you may very well ask is it that the fossil ferns which we collect in the ' nearby strip mines are always inside of pebbles, and why is it that the size of the enclosing pebble is proportional to the size of the enclosed leaf, that is big fronds are inside of big pebbles, little leaves are inside of little pebbles and medium size leaves are in medium size pebbles. Likewise, long leaves are in long pebbles and round leaves are in round pebbles. This situation is indeed peculiar to say the least. It almost looks like the leaf itself had some thing to do with the size and shape of the pebble. Well, that is exactly right, the leaves did have a lot to do with the formation of the enclosing pebbles.
In the first place let us dispose of that word "pebble". These are not pebbles at all. Pebbles are pieces of rock broken from a large rock mass and then rolled along a stream-bed until they are nicely rounded. What we are dealing with here are not pebbles because they never were part of a large rock mass, and they did not become rounded by rolling along a stream-bed. So in stead of calling them pebbles we shall call them "concretions", meaning that they are masses of natural concrete. This is not a figure of speech. The rock crypts in which the fossils have reposed for millions of years are actually produced by silt or clay being cemented.
Now let us study this phenomenon by following in our minds eye, a leaf or a frond from the time it falls or is blown from the parent plant until we have it, in fossil form safely in our specimen cabinet. The falling leaf lands en or is blown onto the water of the nearby swamp. Here it floats for a day or so until it becomes "waterlogged" and then it sinks to the bottom. At that particular time the bottom of our swamp was clay or silt and more clay or silt was constantly settling from the roilly water. Soon our leaf was covered, buried in a layer of mud which was quite homogeneous. Probably there. was no definite mark to show where the surface of the mud happened to be when our leaf came to rest upon it. Now, in strictly fresh water the leaf would soon have decayed. But this was swamp water which is as you know quite another matter. Being deficient in oxygen as all swamp water is, this water was a pretty good bactericide, especially for bacteria that require much free oxygen, and the bacteria of decay are just such aerobic organisms. So decay did not take place. However certain anaerobic bacteria, bacteria that could get along without free oxygen, did have some effect on the leaf but not very much, just enough to cause the production of a comparatively small amount of organic acid. This acid gradually dispersed outward from its source, and as it did so it came into contact with the clay and with the iron-disulfide, FeS2 (one atom of iron, F, to 2 atoms of sulfur, S) dissolved in the water. Between them they form a cement, and of course, the more acid there was the larger was the ball of cement that resulted. In other words a large frond produced more acid than a small one could produce and therefore, a larger ball of cement formed around a large frond than formed around a small leaf. This accounts for not only the size of the concretion but also for its shape which as we have already noted, pretty well follows the outline of the leaf. In fact, the margin around a leaf as it is exposed when the concretion is opened is a sort of a dispersion diagram. By studying it closely you can easily visualize and can mark with a pencil the various paths followed by the acid as it dispersed outward from the leaf.
Another question: Why is it that most of the concretions split just right to expose the fern to the best advantage? Well, since the leaf formed a sort of a partition between the cement above the leaf and that below, it created a plane of weakness. When you tap the concretion on the side (edge), the break occurs along this natural cleavage plane.
At the time the shovels bring the fossils up from the pit and dumps them on the tailing piles they are bluish-gray in color, same as all the rest of the debris. After being exposed to the elements for a few months the concretions begin to turn red. The reason for this is that the iron which was locked up at the time the concretions were formed is now turning to that well known form, the mineral hematite, Fe203. The concretions are readily seen now because their color makes them stand out against the blue-gray background. At the time the concretions formed at least 250,000,000 years ago there was just as much iron in the mud between the concretions as there was in the concretions, but that in the concretions was "locked up", chemically speaking, while that in the rest of the mud was not. As a result, down through the millions of years since then most of the free iron-disulfide has been leached out, carried away by percolating ground waters. That in the concretions could not be leached as it was bound up too firmly in the chemical compounds of the concretion.
For a more modern view try Gordon Baird, Steven Sroka, Charles Shabica, and Gerald Kuecher "Taphonomy of Middle Pennsylvanian Mazon Creek Are Fossil Localities, Northeast Illinois: Significance of Exceptional Fossil Preservation in Syngenetic Concretions" in Palaios in 1986.
Abstract
Mazon Creek area fossil localities (Pennsylvanian: Westphalian D) in northeast Illinois provide an extraordinary preservational "window" for the study of Late Paleozoic terrestrial, fresh-water, and estuarine marine organisms. Marine animals were killed by episodic pulses of turbid fresh water associated with flooding from distributaries, and terrestrial organisms were introduced into coastal waters from upstream sources. Rapid burial entombed remains, commonly before significant decomposition had occurred, and very early diagenesis ensured fossil preservation as molds and composite impressions within sideritic concretions. The taphonomy of these deltaic, fossil-bearing beds is complex. Episodic engulfment of organisms is indicated by animal escape activity and by the presence of upended plant fragments. Numerous, distinctive, cyclic repetitions of siltstone-claystone laminae, each recording a single tidal (flood-ebb) event, are observed in areas yielding marine organisms; these indicate that rapid but extremely regular deposition was also important. Concretion formation was probably influenced, but not necessarily triggered, by decay processes. Siderite precipitation reflects three key conditions: the availability of iron, rapid burial of organic material, and a low to nonexistent supply of sea-water sulfate to centers of interstitial microbial activity. A regional (seaward) decrease in the quality of fossil preservation is observed away from the coastal depocenter, and an interval of sparsely fossiliferous facies (a taphonomic discontinuity) occurs between the delta complex and shell-rich, normal marine deposits. The preservational significance of early diagenetic siderite is discussed; we argue that paleoenvironmental interpretations of analogous or similar nearshore deposits lacking fossiliferous concretions must be made cautiously. The potential value of Mazon Creek-type facies in the study of other Pennsylvanian nearshore deposits, including many superficially "barren" shales, is stressed. The association of the fossil-bearing concretions with inferred estuarine-deltaic fades points to the need for future actualistic studies of comparable modern coastal environments.
Or, Thomas Clements, Mark Purnell, and Sarah Gabbott in "The Mazon Creek Lagerstatte: a diverse late Paleozoic ecosystem entombed within siderite concretions" in 2018.
Abstract
One of the best records of late Paleozoic ecosystems, the Mazon Creek Lagerstätte is world famous for its striking flora and fauna preserved within siderite concretions. Distinct from other late Carboniferous concretionary Lagerstätten because of the remarkable fidelity of soft tissues and pigments that are frequently preserved, the Mazon Creek has seen a revival in investigations during the last 10 years using modern palaeontological techniques. However, many of these modern investigations build upon a literature that incorrectly interprets the palaeoenvironment of the Mazon Creek and the separate biotas: there is a lack of evidence to support a distinct freshwater fauna. Here, we present a detailed overview of the Mazon Creek Lagerstätte, including the palaeoenvironmental conditions, organisms present and the complex taphonomic processes involved in fossil formation. Investigation into the formation of siderite concretions and the complex taphonomic processes controlling soft-bodied preservation are still continuing but are reviewed in detail.
Or, S. Cotroneo, J. D. Schiffbauer, V. E. McCoy, U. G. Wortmann, S. A. F. Darroch, Y. Peng, and M. Laflamme in "A new model of the formation of Pennsylvanian iron carbonate concretions hosting exceptional soft-bodied fossils in Mazon Creek, Illinois" in GeoBiology from 2016.
Abstract
Preservation of Pennsylvanian-aged (307 Ma) soft-bodied fossils from Mazon Creek, Illinois, USA, is attributed to the formation of siderite concretions, which encapsulate the remains of terrestrial, freshwater, and marine flora and fauna. The narrow range of positive δ34S values from pyrite in individual concretions suggests microenvironmentally limited ambient sulfate, which may have been rapidly exhausted by sulfate-reducing bacteria. Tissue of the decaying carcass was rapidly encased by early diagenetic pyrite and siderite produced within the sulfate reduction and methanogenic zones of the sediment, with continuation of the latter resulting in concretion cementation. Cross-sectional isotopic analyses (δ13C and δ18O) and mineralogical characterization of the concretions point to initiation of preservation in high porosity proto-concretions during the early phases of microbially induced decay. The proto-concretion was cemented prior to compaction of the sediments by siderite as a result of methanogenic production of 13C-rich bicarbonate—which varies both between Essex and Braidwood concretions and between fossiliferous and unfossiliferous concretions. This work provides the first detailed geochemical study of the Mazon Creek siderite concretions and identifies the range of conditions allowing for exceptional soft-tissue fossil formation as seen at Mazon Creek.
