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Reworked fossils, was: Paleogene Titanosaurs in South America? Paleogene ammonoids in the Persian Gulf? Not quite, but analogous...



Fossils you find in clasts are (more or less) by definition in a material different than the surrounding matrix otherwise it would be difficult to interpret the clast as "reworked" The Hell Creek mud pebbles I mentioned are obviously rounded (sometimes even cigar sized/ shaped by rolling down the smooth sandy Hell Creek River Bed and are an obviously different lithology. I have little doubt on the penecontemporaneous association of these sediments though. The reworking was from adjacent sedimentary environments (mud flat to river channel) and not reworking across and unconformity of some kind. The Hell Creek Formation in particular is full of reworked sediments from adjacent environments because of stream action moving mass quantities (barge loads!) of previously deposited sand around by the cut and fill process over and over again. (I look for these pebble/sands to find microsites.)

While some of the HC sands that you find are almost loose like modern sediments, others are fairly well lithified and quite hard. There is a continuous spectrum in between these two end members of consolidation as well. Younger Fort Union (Tert) sediments are known to have an occasional dino bone contained within ( EG: Evidence of Reworked Cretaceous Fossils and Their Bearing on the Existence of Tertiary Dinosaurs by Jeffrey G. Eaton, James I. Kirkland and Kentaro Doi, PALAIOS, Vol. 4, No. 3 (Jun., 1989),l). I see no reason why blocks of lithified sand with fossils within couldn't move as well as just the bone. I have seen whole bone and even wood chunks in very hard cemented sandstone that would move very nicely down a younger Fort Union river channel assuming that the channel cut into older Hell Creek sediments and dislodged the clast that contained the bone. Some of the older sediments look (composition and texture) identical to the younger sediment but differ in their fossil content. (Having Cretaceous plant/animal remains in a Tertiary formation).

There is no reason that two formations separated by great time but representing similar re-occuring sedimentary environments couldn't have an obfuscating tendency of the upper formation to contain reworked fossils from the lower (older) formation. Having an unfortunate location/sample selection could really throw off your analysis. I think this is pretty rare though I could take you right to a spot where Hell Creek faunal fossils from a rich microsite up slope are fairly common in the Holocene loess blanket below. This phenomena would only be obvious to the trained eye, or when lithologies/associations between the clasts and the matrix are significantly different.

I have little question that sediments within the Hell Creek have been reworked from lower positions in the formation to more upper aspects by some of the fairly large rivers that were sweeping through the region at the time. To what degree is the question, a big river might cut 20 or 30 feet down and move that sediment to a river bank and then repeat the process 20 times over the long length of a river and the time span of the formation. This could lift a particular fossil quite a distance in the formation hypothetically. This is a consideration to those trying to create zones of fossils is a hyper-active fluvial system like HC.

This could relate to seed ferns easily as the number of seeds would certainly suggest a greater probability of reworking. Sample size, Sample size, Sample size.....I could attribute complete dinosaur bones to the Holocene taking only that one microsite into account (and not finding the real the source uphill). The key is the mixed taxa present in the rocks or sediments.

Frank (Rooster) Bliss
MS Biostratigraphy
Weston, Wyoming
www.wyomingdinosaurs.com


On Apr 11, 2008, at 5:35 PM, David Marjanovic wrote:
You can find reworked seeds and leaves in clasts (chunks)
of older sediments.

Yes, but no such things are mentioned in the paper, and they should be pretty easy to recognize, shouldn't they?