Posted by: paulgarner | September 21, 2009

Petrified forest or petrified log jam?

SDC10755While I was in Arizona during the summer, I had the opportunity to visit Petrified Forest National Park, a locality known worldwide for its extraordinary concentrations of fossilized wood. Here are just a few of the many photos I took during my brief visit. The colourful badlands expose sediments of the Chinle Formation (Upper Triassic), littered with petrified logs and wood fragments, mostly of the gymnosperm species Araucarioxylon arizonicum.

SDC10710My visit reminded me of a modest study of the petrified forest published some years ago by Art Chadwick and Leonard Brand in which they measured the orientations of over 700 prone logs at eight locations in Arizona and Utah, and were able to demonstrate very strong orientation of logs at most sites (Chadwick and Brand 1974). Everyone agrees that these logs must have been transported into the depositional basin by extensive flash flooding. However, Chadwick and Brand were also able to excavate the bases of two upright stumps, in an area where trees putatively in positions of growth had been previously reported. They found no evidence of palaeosols, suggesting that even these trees might be allochthonous.

SDC10745In fact the evidence for the rapid transport and deposition of these trees is so ubiquitous that some think the name ‘petrified forest’ is a misnomer and that ‘Petrified Log Jam National Park’ might be more accurate!

Reference

Chadwick A. V. and Brand L. R. 1974. Fossil tree orientation in the Chinle Formation. Origins 1(1):22-28.

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