Abstract

The Solitario is a circular, 14- to 16-km-diameter, mid-Tertiary laccolithic dome and caldera in southern Trans-Pecos Texas; it is among the world’s largest laccoliths. A recent addition to the Texas parks system, the Solitario lies largely within the Big Bend Ranch State Natural Area, and west of Big Bend National Park (see Erdlac’s Figure 10, this volume). Rocks ranging in age from Cambrian through Quaternary crop out in and around the laccolithic dome. The oldest rocks, exposed in the core of the dome, are Cambrian through Pennsylvanian clastic, carbonate, and silicic sedimentary rocks, deposited in the ocean basin that lay south of the Paleozoic North American continent. A relatively thin sequence of Cambrian through Mississippian shale, sandstone, chert, novaculite, and limestone was deposited slowly in a passive-margin setting. Cambrian-Ordovician shale commonly contains blocks of the other rocks that are either olistoliths or boudinage structures. A much thicker sequence of upper Mississippian-lower Pennsylvanian shale and sandstone was deposited rapidly in the Ouachita foreland basin. All these rocks were folded and thrust northwestward during late Paleozoic Ouachita-Marathon deformation. Individual folds in well-exposed areas that are held up by resistant novaculite and chert are asymmetric to overturned and verge northwestward. A complex, folded thrust in the eastern part of the Solitario is cut by numerous northwest-striking cross faults that parallel the direction of transport. The northern part of the Paleozoic outcrop consists mostly of poorly exposed but, where observed, intricately folded Ordovician shale.

Cretaceous sedimentary rocks were deposited over the region following a long hiatus during which the Paleozoic foldbelt eroded. The basal Cretaceous conglomerate is a terrestrial deposit developed on a pediment surface cut in the Paleozoic rocks. Subsequent Cretaceous clastic and carbonate rocks are shallow marine deposits that accumulated on the eastern flank of the Chihuahua trough, a deep basin related to rifting that formed the Gulf of Mexico. Alternation of massive limestone with marl or shale indicates deposition in intertidal to subtidal environments with varying contributions of fine terrigenous material.

The Solitario lies at the eastern edge of Laramide (early Tertiary) deformation and is superposed upon the Laramide Terlingua uplift and Fresno-Terlingua monocline. The monocline probably overlies a high-angle reverse fault in basement, and both the monocline and uplift are cut by numerous east-northeast-striking strike-slip faults. The southwestern rim of the Solitario follows the northern end of the northwest-striking Fresno monocline. Cretaceous rocks within the Solitario were largely flat lying before doming but show evidence of Laramide deformation, including east-northeast-striking strike-slip faults, calcite-filled veins and fractures, and tectonic stylolites.

During the late Eocene, the Solitario underwent a complex sequence of laccolith, sill, and dike injection, doming; ash-flow eruption; caldera collapse; and intracaldera volcanism and sedimentation. ⁴⁰Ar/³⁹Ar ages demonstrate that Solitario magmatism occurred in three pulses at 36.0, 35.4, and 35.0 Ma. During the first pulse of magmatism, abundant rhyolitic to trachytic sills, dikes, and small laccoliths were emplaced into both the relatively undeformed Cretaceous sequence and into the structurally complex Paleozoic rocks. The main laccolith that uplifted the dome was emplaced during the second, most voluminous pulse. The laccolith probably intruded Paleozoic rocks along the sole thrust of the Marathon-Ouachita system at a depth of 3 to 4 km. The dome is generally doubly hinged, similar to the geometry of many laccoliths worldwide. Cretaceous rocks dip 15° to 50° radially outward in a continuous rim; however, the southwestern rim of the Solitario is substantially steeper than other sides, probably because doming was superposed upon the Laramide monocline. Cretaceous rocks flatten away from the dome along a concave-upward hinge. Steep dips were maintained well into the interior of the dome; probably only an area 2 to 3 km wide in the center of the dome was flat. Radial faults and fractures developed over the dome as it rose, commonly reactivating east-northeast-striking Laramide faults. Quartz-phyric dikes intruded along these fractures on the north and west flanks of the dome. Ash-flow eruption along faults over the roof led to collapse of a 6 x 2-km, north-trending caldera over the south and central parts of the dome. Collapse was greater in the southern part of the caldera, where ponded ash-flow tuff buries older rocks, than in the northern part, where exposed Paleozoic and Cretaceous rocks dip steeply southward. ⁴⁰Ar/³⁹Ar ages averaging 35.4 Ma on the quartz-phyric, radial dikes and the ash-flow tuff probably indicate the age of the laccolith and doming. The caldera was filled by a complex sequence of debris-flow and debris-avalanche deposits, megabreccia blocks up to 200 m in diameter, trachyte lava, and small-volume ash-flow tuff. Late subsidence within the caldera produced a north-striking syncline. The final pulse, at 35.0 Ma, consisted of several small laccoliths or stocks and numerous dikes in caldera fill and along the ring fracture.

The Solitario is nearly surrounded by Tertiary volcanic, volcaniclastic, and intrusive rocks that are representative of magmatism in Trans-Pecos Texas. These rocks erupted from numerous local and distant sources between 47 and at least 24 Ma ago. Most postdate 36- to 35-Ma Solitario magmatism, but a group of peralkaline rhyolite lava domes erupted north and west of the Solitario at 35.4 Ma ago, contemporaneous with the main pulse of magmatism in the Solitario. A thick, coarse conglomerate shed from the Solitario dome is interbedded with the volcanic and volcaniclastic rocks. Igneous rocks emplaced before about 30 Ma ago are part of a subduction-related volcanic arc related to convergence off the west coast of North America. Igneous rocks emplaced after that time are related to Basin and Range extension, including distinctive alkali basalts emplaced contemporaneously with initial faulting about 25 Ma ago. The Solitario lies in the eastern part of the Basin and Range province. It occupies a relatively unfaulted block surrounded by Basin and Range normal and strike-slip faults.

Editor’s note: This is a part of a more complete report on the Solitario to be released by the Bureau of Economic Geology. Preprint is by permission of the senior author.