[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]

Re: The Final Days

Andrew A. Farke wrote:
>Mountain habitats have very poor preservational potential--usually there is 
>much more erosion than deposition going on. Are there really *any* mountainous 
>habitats preserved? I guess you might get the occasional terrace or cave, but 
>that's about it. <
Like with many generally accepted "truths" things are not quite that simple. 
High-altitude fossil sites from the Pleistocene and Pliocene are fairly common, 
particularily in Western North America and Central Asia. Indeed in the Western 
US it is absolutely necessary to factor in the altitude of Pleistocene sites to 
make sense of distributional or ecological data (for a good example of this see 
Harris, A. H. 1985. Late Pleistocene vertebrate paleoecology of the west. 
University of Texas Press, Austin, 293 pp.).
Older high-altitude sites are rare but not unknown. Florissant for example is 
usually stated to have been at approximately the same altitude as today when 
the fossil beds were formed. While the youngest sites (say <5 Ma old) are often 
in true high-mountain terrain older ones are typically found on high plateaus, 
often with interior drainage where large-scale deposition can occur even at 
quite high altitudes.
Whether any Mesozoic high-altitude sites are preserved is more uncertain. It 
seems unlikely that any would have survived if they had remained att high 
altitude ever since then. However what comes up can also come down. There are 
plenty of evidence for large scale vertical motion of large continental areas. 
Again some of the most striking examples are in the Western US where the 
Colorado Plateau and the Basin and Range Province have been elevated by several 
thousand feet since the Neogene. The same is true for much of eastern and 
southern Africa ("High Africa"). In the case of the Western US this is due to 
the continent overriding part of  an ocean ridge while the African "bulge" is 
centered on the Great Rift, an incipient ocean. 
Such areas might preserve fragments of high altitude deposits when they return 
to lower altitudes, as they presumably ultimately will do. That such processes 
were active in the Mesozoic too is clear from e. g. Australia. Much of 
Australia was flooded by the sea during early/middle Cretaceous but then the 
sea *receded* during the Campanian transgression when sealevels elsewhere in 
the world reached record levels. Most of the Australian continent must have 
risen for at least a several hundred meters to accomplish this.
The best places to search for Mesozoic highland sites might be in ex-rift 
marginal areas which were presumably also high "bulges" at one time. To judge 
from Tertiary  fossil sites in High Africa, deposits protected from erosion by 
volcanics have the best chance for long-term survival. The Inter-Trappan beds 
of India come to mind. Since India was right on top of the Reunion hotspot at 
the time they were formed they were quite possibly at a higher altitude then 
than they are now. 
Tommy Tyrberg