Summary: Eighty million years ago, while the Laramide Orogeny was getting underway in western North America, central and south Texas were parts of the shallow carbonate bank that developed on the North American side of the Gulf of Mexico. The Gulf had begun to form during the Jurassic as Yucatan pulled away from what is now Texas and Louisiana. These Cretaceous rocks are part of the Gulf Coastal Plain that we talked about a few days ago. Remember the rudists? The tubular clams a meter high that trapped sediment to help form reefs? They grew in this area too. It was overall a quiet, shallow sea, perhaps something like today’s Florida Shelf. The setting was not one where you’d expect volcanoes, but that’s what we got. In a long, linear zone east of a line from Waco to Austin and on south to San Antonio and Uvalde, Texas, there’s a string of more than 200 little volcanoes that erupted into the carbonate sediments. They’re basically piles of volcanic ash together with some basaltic flows, and some of them today are resistant enough to form mounds on the land surface. One well-known example is at Pilot Knob, not too far from Austin. Pilot Knob is about two miles across. Most of these volcanic bodies are buried in the subsurface and have no expression that we can see on the land, but most of them are basaltic and contain a lot of magnetite. This gives them a distinct expression in a magnetic map. That characteristic was useful in oil exploration, because the volcanics and the surrounding carbonate rocks contain oil in 35 or 40 of the known volcanoes. Something like 50 million barrels of oil have been produced from several accumulations since the first field was discovered in the 1910s. Magnetic map of Uvalde and Medina Counties, Texas (from USGS). Most of the little pimple-like bumps represent igneous plugs of Cretaceous age. The margin of the old Texas Craton, the Precambrian core of this region, is called the Balcones Escarpment, a topographic feature that follows a fault zone separating the older rocks of central Texas from the Cretaceous and younger rocks of the Gulf Coastal Plain. The Balcones Escarpment is followed by Interstate 35 from Waco to Austin to San Antonio, just about the same zone where the volcanic centers are found. It’s not completely clear why these volcanoes erupted into this tectonically quiet region. Probably the best explanation is that there was a pulse of extension, pulling apart, that opened deep-seated faults and fractures through which the magma rose. The weakest zone was along the old break between the strong craton and the stretched crust to the south and east that was pulled and stretched by Yucatan’s departure in the Jurassic. The magmas are not all basaltic, which makes it difficult to say, for example, that they simply came from the mantle – there must have been some melting of other rocks and mixing of magmas to get the kinds of igneous rocks we see. —Richard I. Gibson Cretaceous volcanism in South Texas and oil Pilot Knob Magnetic modeling Magnetic map (USGS OFR-02-0049) and this one
