Reworking

[Tompaleo@aol.com]

Could someone explain exactly what "reworking" means? It's always confused me. 

Reworking is the word used to refer to fossils that are found "out of place" per se.  In other words, a particular type of fossil(s) that is/are known only to occur above or below a certain horizon (and by definition) a certain time period is found on the _othe_ side of that boundary. A fossil can be reworked up or down.  Reworking thus also implies an energetic mechanism -- either biotic or abiotic to displace the fossil material and redeposit it elsewhere, out of context. A number of geologic and taphonomic processes can and do rework material. The best known and probably most notorious reworking process as far as dinosaur research is concerned is by fluvial, i.e., riverine processes. However, reworking is well known from the marine realm both littoral and pelagic where wave action, tsunami deposits, submarine debris flows and heavy infaunal and/or epifaunal activity churn up pr!
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eviously deposited material (say for example on the "bottom") and rework it to where the material is now on "top" and vice versa. Similarly, fossils from two different time periods can be commingled as well but this is an obvious case. In the absence of this however, it takes some work to definitively determine whether or not something has been reworked up or down or not.

In the terrestrial realm the primary mechanism of reworking are rivers, especially those prone to frequent flooding and channel shifting, mass debris slides, and even bioturbation by tree roots can churn up their substrate to displace previously deposited material. If this reworking occurs at a major stratigraphic boundary it can make things difficult to sort out (no pun intended).

A prime example is the famous Hell Creek and Tullock Formations of Montana which I had the privilege to explore during summer 2000. This area boasts the famous K-T boundary that has been studied extensively.  I spent a week there working with Rich Cifelli who had arranged for us to get THE tour of the area by both Bill Clemens and Don Lofgren. We toured (and prospected) Brownie Butte, the Bug Creek Ant Hills, and the famous "10 meter gap" which has a road kill Triceratops skull lying in the road nearby. As most now know, that 10 meter gap has been shrunk to about 2 meters thanks to better strat resolution, and during Clemens' talk there at site, he alluded to even more recent work that shrinks this gap even further. I believe Nan Arenas is leading that charge with her palynological study.

Back to the point, the upper Hell Creek (Cretaceous) and most of the overlying Tullock (Tertiary) Formations are a geologist's dream and a tangled puzzle for the stratigrapher. It is the epitome of channeling. Channeling is so pervasive and extensive throughout the sequence that even the famous Z-coal (and the famous Ir anomaly) at one locality has been dissected and trisected, truncated and reunited-all by constantly shifting energetic streams and rivers over several million years before, during and after the K-T event! Even the channels are crosscut by other channels.

More to the point and from personal experience: While prospecting in the area known as the Bug Creek Ant Hills, where I happened to be poking around in the upper Hell Creek Formation, I noticed rather large croc scute and some bone frags that I bent over to collect. Lo and behold right next to the scute was a beautiful mammal jaw! It was soon identified by Cifelli and Lofgren as a Condylarth- a _Paleocene_ Condylarth (protungulate). I found it in a cross bedded cut and fill deposit that was definitely at the top of the late K Hell Creek Fm. The jaw was clearly reworked _downward_ in this case. A further testament to the fluvial transportation of the jaw is evidenced by the rounded edges where the ramus had broken away from the symphysis and where the rostral end too was smoothed where it had been previously broken. 
Thus an Early Paleocene stream channel carrying the condylarth jaw cut into the upper Hell Creek formation where the jaw had come to rest. The reverse is true as well. Virtually all of the dinosaur teeth and very, very few isolated and transported bones recovered from above the Z-coal (the "official" K-T boundary) are reworked by the same Paleocene streams that gouged out the Late K deposits, this time reworking the Cretaceous material upward.

Oh, before I forget. The Bug Creek Ant Hills themselves are a form of biotic reworking. I forget the name of the species of ant that resides there but they have a predilection for collecting fossils to build their ant hills with. Thus they commingle fossils from both the Hell Creek and Tullock Fms! The hills make for easy collecting except for the nasty stings of the ants!

Man do I love that place!

Hope this helps.

Tom

Thomas R. Lipka
Geobiological  Research
2733 Kildaire Drive
Baltimore, Md. 21234 USA