Bill DiMichele, Curator of Fossil Plants, is posting from the field in north-central Texas, where he and Museum Specialist Dan Chaney are collecting plant fossils. This trip is part of a long-running study of how plant communities responded to major environmental changes caused by periods of rising and cooling temperatures about 275 million years ago.
Bill writes: What a change in the weather! Yesterday’s storm front came ahead of plummeting temperatures and increasing winds. None of us brought the right clothing for wind chills in the 30s, and it certainly slowed us down. Hopefully this kind of weather slows down the ticks and rattlesnakes, too!
We did three things today:
First, we headed west to continue orienting our colleagues Martin and Sharane to the Texas Permian section. During the Early Permian, the modern continents were all pushed together into a single, gigantic landmass, which geologists call Pangea. North-Central Texas was in the western part of this ‘supercontinent’, very near to the equator. We visited upper Clear Fork Formation outcrops – rocks that may have been deposited during a period of hyper-warming in central and western Pangea. They contain no fossils of any kind, as far as we have been able to determine. The hyper-warming period would have been a bad time to be living on land in the western tropics (which is maybe why everything was living somewhere else at the time, leaving no remains here to fossilize).
After our visit to the upper Clear Fork outcrops, we continued west to near Dickens, Texas, where an ancient volcanic ash bed is exposed in several small road cuts just to the east of town. The Dickens ash has been dated as very earliest Triassic in age (the Permian-Triassic boundary is dated at about 252.17 million years ago). The layer is found within what appear to be a lake deposit – the kind of sediments in which we often have found plant fossils in older Permian rocks. Somewhere below the ash there should be sediments (and fossils) dating from the Permian-Triassic boundary, but we have had no luck finding fossils here, though we have looked closely several times.
The Dickens ash is full of small black crystals of the mineral biotite, aka “black mica.” Biotite commonly occurs in volcanic rocks of various kinds and is a useful mineral for radiometric dating.
We ended the day back at the "Know Where to Park" area (see yesterday’s post) where Sharane and Martin photographed the bluffs in preparation for a detailed sedimentological study.
Hoping for warmer weather sometime tomorrow.