Brazil 2012 Fieldwork Diary Entry 5: Ex Libro Lapidum, Historia Mundi
Roger Smith and Christian Kammerer study rocks in the Pedra de Fogo Formation in the Parnaíba Basin, Brazil. Photo by Ken Angielczyk.
The latin phrase that's the title of this entry means “from the book of rocks, the history of the world,” and it is a motto that a geology professor I had in graduate school adopted for himself. It's also a good description for our activities today, our first full day of fieldwork in the Parnaíba Basin.
We arrived last night at one of the main areas we'll be focusing on this year, near the town of Pastos Bons(meaning “good pastures”). This area is where the first specimen of Prionosuchus (an archaic amphibian that lived at the time the Pedra de Fogo Formation was being deposited) was found, and it's an area that we worked in last year during our reconnaissance trip to the basin. However, Roger Smith wasn't on last year's trip and we were quite interested in his interpretation of the rock sequences in the area. Therefore, we spent most of the day looking at the structure and composition of the sedimentary rocks preserved in the area to learn about the environmental conditions under which they were likely. Also important was the order in which different rocks occur, which provides information about how the environment was changing over time.
We saw a lot of evidence today that suggests that many of the rocks were formed in conditions in which water was present, but in which the climate was likely quite dry. For example, the picture below shows very large shrinkage cracks that likely formed as mud submerged in shallow water shrank as it started to dry up. Later the cracks were filled in with slightly younger sediments, preserving them as structures in the rock that eventually formed from the sediments. We also saw rocks that were derived from sand dunes, minerals like gypsum that normally form when water evaporates leaving its dissolved salts behind, and evidence of mats of cyanobacteria that were living on the surface of the mud. Taken together, the book of rocks we were reading today suggest that the environment was likely a sabkha, a setting commonly found along ocean margins in hot, dry environments. Good examples of sabkhas can be found along the Red Sea today. In this particular area, the environment probably became more terrestrial as time passed, with rocks representing the dunes along the margin of the sea occurring above rocks formed in the shallow water just at its edge (in most situations rocks occurring higher in a sequence formed after those lower in the sequence).
Casts of large shrinkage cracks exposed in the Pedra de Fogo Formation. The cracks initially formed as mud submerged under a shallow layer of water began to shrink as it dried up. Later, the cracks were filled in with slightly younger sediments, preserving the cracks as casts in the rock that eventually formed from the sediments. Photo by Ken Angielczyk.
Although this environment might sound hostile, we did find some fossils at the end of the day, which demonstrate that animals were living in the area. A lot of the fossils are fragmentary remains of fish, but there is also a specimen that we think is part of a vertebra (backbone) that may be from a terrestrial vertebrate (also known as a tetrapod). We're not sure if its from an archaic amphibian, a reptile, or synapsid, but it gives us hope of finding other tetrapod specimens in the area.
Photo of a fossil that the team thinks may be a vertebra (or backbone) of a terrestrial vertebrate. The scale in the picture is in centimeters. Photo by Ken Angielczyk.
Now that we have a better idea of what the rocks are telling us, our plan for tomorrow is to explore some new outcrops near where we we found today's fossils. Part of what we'll do is to see how the rocks there fit in with the interpretation that Roger started developing today. Our main goal, though, will be to devote more effort towards finding fossils. Hopefully we'll have good results.