Wooster Geologists

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).

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.

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°).

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.

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.

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