Nothing brings out those feelings of awe and wonder like the study of volcanics, at least from a geologist’s perspective. Did you know that in southwest Texas, there are several examples of a rare type of caldera formation called a laccocaldera? I didn’t. While searching the Datapages Archives, I found several West Texas Geological Society publications about magmatism in the Big Bend Region (also known as the Trans-Pecos region) of Texas.
Laccocaldera
What is a laccocaldera?
In 1989 it was a newly recognized caldera type. Instead of a regular caldera formed over a deep-seated magma body; the laccocaldera forms above a thin laccolithic magma body. In the Trans-Pecos area there are three domes with laccocalderas: the Black Mesa Dome, the Solitario Dome and the Christmas Mountains Dome.
Location
Near the northern border of the Big Bend National Park and situated within the Terlingua Ranch of Brewster County, Texas, is the Christmas Mountains caldera complex. It was created approximately 42 million years ago during the Eocene. The area was a low coastal floodplain with wetlands. Unlike the desert climate of today, the climate in the Eocene was temperate with heavy rainfall.
Eocene Fauna
Fossil evidence shows that were early primates (phenacolemur), early rhinoceros relatives the size of modern African elephants (brontotheres), and primitive horse relatives (hyracotherium) the size of a house cat, and more living in the region in the Paleocene and Eocene epochs.
Volcanics
Cretaceous sedimentary rocks of the Trans-Pecos region consisted of limestones, shales, sandstones and mudstones. During the late Paleocene and Early Eocene, a magma body pushed into the Cretaceous rocks inducing doming. The Christmas Mountains caldera complex started with a big pyroclastic eruption of rhyolitic ash flow tuff; then lava flows of quartz trachyte. The emptying of the laccolithic magma body caused the collapse of the dome, creating four laccocalderas in the Christmas Mountains.
To learn more, visit the Datapages Archives at Archives.Datapages.com.