Friday, December 7, 2012

Petrified But Not Frightened

It can be considered a bit of a stretch to look around today at the arid, rocky, shrub–covered desolation of southern Utah and see lakes edged by luxuriant green vegetation, swiftly–moving rivers, sinuous streams, and fetid swamps. Scenic? Absolutely. Frighteningly desolate? Perhaps. But luxurious and verdant? That would be a definite stretch of anyone’s imagination. 

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Southern Utah mesas capped by Shinarump Conglomerate

Nevertheless, around 220–210 million years ago during the Late Triassic and Early Jurassic Periods, that’s exactly what we would have observed. During millions of years, rivers and streams that washed away earlier sediments deposited newer sands and gravels of the Shinarump Conglomerate across an area of approximately 100,000 square miles. Overlying this resistant cliff–forming rock is a softer layer of purple, white, and gray clay–rich rock appropriately named the Petrified Forest member. It is within these strata of the Chinle Formation that we find petrified wood. 


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Varied clay-rich beds of the Petrified Forest member atop the Shinarump

During these millions of years of geologic history, mosses and ferns grew in profusion within freshwater marshes. Gymnosperms, which do not have flowers, were common, too, found in abundance in river floodplains where they had thrived since the Permian Period some 60 million years earlier. 

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Horizontal petrified tree in lower center of image

Angiosperms, or flowering trees and flowers, are believed to have come into common existence around 140 million years ago, their ancestors having diverged from gymnosperms starting around 245 million years ago.  

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Tree was exposed for about 12-15 feet


Finding logs in a horizontal position within the Shinarump (as opposed to standing upright) strongly suggests that they were transported by high velocity rivers and streams, possibly from highlands as far away as 200 miles, and deposited as driftwood on gravel bars. 

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Horizontal tree (middle of image) eroding out of gully


The ancient conifers of Chinle time could grow as high as 150 feet high with nine–foot diameters. As these ancient trees were brought down by storms, floods, or old age, they were buried rapidly by the sediments they found themselves trapped in. Lacking the oxygen required for rapid decay the cellular tissue was gradually replaced by different minerals.  Minerals such as soluble silica from volcanic ash in the upper Chinle also are responsible for the petrification and often vibrant hues of the fossil wood. 
 
Over time these ancient trees hardened to stone and became the multi–colored petrified wood we see scattered about on the mesas in this corner of the Colorado Plateau.
 
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Close-up of petrified tree - length about two feet


On the expansive mesas of southern Utah topped by the resistant Shinarump Conglomerate and the overlying Petrified Forest member, petrified wood can be found scattered nearly everywhere. On the surface much has been collected over the decades. Still, people continue to come out and dig for this treasure, thinking perhaps that they might actually be able to haul out an intact tree or at least a substantial section of one. 

The Bureau of Land Management (BLM) and the Forest Service have limits on how much petrified wood an individual can collect in a year. The mesas are pockmarked with depressions where folks have exposed massive specimens with what must have been a pick and shovel. 

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We never did see the rest of this tree nearby


Considering that none of us had thought to drive a backhoe up onto the mesa we were content to collect smaller fist–sized samples. 

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