Two West Texas outcrops: which looks more inviting?

November 15th, 2009

texasoutcrop111409albanyoutcrop111409COLLEGE STATION, TEXAS–The upper one is the base of the Valera Formation on US Highway 84 (N31.88196°, W99.47115°) and the lower one is the lower Bead Mountain Formation on Route 6 near Albany; both are Permian and both have delicious microconchid fossils along with much else.  You can imagine which is the more pleasant to work on.

I have been very impressed with the Permian geology of this part of Texas.  The fossils and sedimentary rocks are very accessible and sufficiently mysterious to generate at least two paleontology and sedimentology projects, including future Independent Study work by Wooster students.  Sure there are fire ants, rattlesnakes, and very fast country road driving, but it wouldn’t be Texas without them!  (And the barbecue … all beef, dry-rubbed barbecue …)

The puzzle of gypsum

November 15th, 2009

Our Permian sections on this Texas trip have had thick beds of gypsum only a meter or three beneath our fossiliferous limestones and shales.

An outcrop of sedimentary gypsum below the Valera Formation (Permian).

An outcrop of sedimentary gypsum below the Valera Formation (Permian).

Gypsum (calcium sulfate) is an evaporite mineral, indicating when the Permian shallow sea in this case was much saltier than normal (hypersaline).  Our fossils show a restricted nature (lower diversity than normal, and generally smaller shells), but they were still living in at least close to normal salinities.  This is especially the case with our numerous echinoids.  We even have evidence of some evaporites within our fossiliferous limestones.  It is a curious juxtaposition of depositional environments.

You’re never alone on an outcrop

November 13th, 2009

Molted skin layer from a rattlesnake apparently in the cavity underneath the rock.

Molted skin layer from a rattlesnake apparently in the cavity underneath the nice piece of Permian limestone. I didn't poke around in there to wake him up, and I let that limestone stay where it was.

A West Texas outcrop

November 13th, 2009

BROWNWOOD, TEXAS–It was nearly a five hour drive from College Station, Texas, through the Hill Country to our first Permian exposure in West Texas. (We passed, by the way, through Killeen and Fort Hood.) It was worth the trip for all the strange features we found on this outcrop of the Valera Formation.

Tom Yancey, a paleontologist at Texas A&M University, seated on our little outcrop of the Valera Formation in West Texas (N31.48454°, W99.69368°).

Tom Yancey, a paleontologist at Texas A&M University, seated on our little outcrop of the Valera Formation in West Texas (N31.48454°, W99.69368°).

We spent several hours measuring, describing and sampling this outcrop in ideal weather. We found plenty of examples of what we came here for: fossil microconchids, otherwise known as “worm tubes”.

Microconchid tubes from the Valera Formation at the above outcrop.

Microconchid tubes from the Valera Formation at the above outcrop.

We were surprised to also find abundant sea urchin (echinoid) spines in one of the limestone units here. These usually indicate normal marine salinity, but they are unaccompanied by other indicators such as brachiopods and bryozoans. A thick gypsum below our exposed rocks shows that we are likely dealing with elevated seawater salinity during the Permian in this area. A mystery. (And we love mysteries in this business.)

A barbed echinoid spine from the Valera Formation (Permian).  In the lower left with the apparent hole in it is an echinoid test plate.

A barbed echinoid spine from the Valera Formation (Permian). In the lower left with the apparent hole in it is an echinoid test plate.

Tomorrow we visit a similar outcrop with microconchids. Now we have some hypotheses to test. Fieldwork is such a joy!

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