GEOLOGY AT THE CROSSROADS: BIG BEND RANCH STATE PARK

Big Bend Ranch State Park (BBRSP), is the largest of the Texas State Parks covering an area of over 280,000 acres (~1100 square kilometers). The scenery is magnificent and the landscape varies from river lowlands, through deep canyons, across high plateaus, and up steep mountains. Ultimately all of this is controlled by the character and variety of the underlying geology and the processes that created it over millions of years.

But why a Crossroads of Geology?

Because, the park’s southern boundary follows the Rio Grande from about 10 miles below Presidio through a series of Basin and Range grabens for some 40 miles down-river to Lajitas at the northwest corner of Big Bend National Park. Extensive Tertiary-age volcanics and intrusives make up the Bofecillos Mountains in the central part of the Park, while the Solitario Dome covers the northeastern corner where early Cretaceous-age limestone mark the flank of the dome and Paleozoic-age chert, sandstone, limestone, novaculite, and shale are exposed in its core. And finally, in southeastern BBRSP early Cretaceous-age shale, marl, and limestone are exposed along the Fresno-Terlingua Monocline.

Furthermore, four of the major orogenic events that mark the development of North America converge in BBRSP. 1) The Ouachita/Marathon foldbelt extends through the Marathon Basin and can be seen in the interior of the Solitario Dome, where exceptional exposures of highly deformed Siluro-Devonian Caballos Novaculite can be seen. 2) The Laramide foldbelt is also represented in the Park, particularly where the lower Cretaceous Santa Elena, Del Rio, and Buda Formations are asymmetrically folded along the Fresno-Terlingua monocline. 3) Undoubtedly, the most dramatic geological event represented in the Park is the mid-Tertiary volcanism that built up the Bofecillos Mountains as part of the very extensive Trans-Pecos volcanic province. 4) And finally, along the south edge of BBRSP, the route of the Rio Grande follows Basin and Range age sediment-filled grabens, marked by long, continuous normal to oblique slip faults exposed along very prominent fault scarps.

So come on out to Big Bend Ranch State Park and watch for the evidence of all four events. Revive your appreciation for the geological enormity of time and scale and the resulting variety of landscape and scenic beauty here at the Crossroads.