KARST REGIONS OF TEXAS

by A. Richard Smith and George Veni

(with minor additions by William R. Elliott)

Carbonate Regions (map)

Carbonates include limestone (calcium carbonate or CaCO₃) and dolomite (calcium magnesium carbonate or CaMg(CO₃)₂). Dolomite dissolves slower than limestone, but is very similar.

1. Balcones Fault Zone

The BFZ is narrow and runs south from Dallas to San Antonio, then turns west and nearly reaches Del Rio. It contains thick-bedded to massive Cretaceous limestone and some dolomite beds from the Edwards Group, Glen Rose, Buda, Georgetown, Austin, and Anacacho formations. However, the Buda, Georgetown, and Anacacho are poorly cavernous in this region; the upper member of the Glen Rose is significantly cavernous only in the Bexar-Comal County area. Abundant faults, all related to Miocene-age structural activity, are generally downthrown toward the Gulf of Mexico. Despite their prevalence, faults guide the development of only about 1% of the caves in this region. Joints are more numerous and determine the orientation of most caves. Major caves form by groundwater which flows parallel to the BFZ and discharges at distant and large artesian springs (e.g., Valdina Farms Sinkhole, Medina County; Indian Creek Cave, Uvalde County). Vertical caves, some quite deep, route vadose water to deep phreatic groundwater conduits (e.g., Genesis Cave, Bexar County). For the most part, cave fill is either old, pre-entrance red clay and kaolinite, or modern black soils eroded from the surface probably because of overgrazing. The BFZ is closely related to but separated from the Edwards Plateau by a strip where the Edwards Group limestones have been erosionally removed.

2. Edwards Plateau

The Edwards Plateau is well-known as a broad, gently rolling upland locally incised by moderate-size streams; few other perennial streams exist. It is one of the largest continuous karst areas in the United States. The resistant rocks at the surface of the plateau are thin- to thick-bedded Cretaceous limestone with some dolomite and belong to the Edwards (or Fredericksburg equivalent) Group. The rocks dip very gently to the south and southeast; fractures appear to be related to regional uplift rather than local faulting and folding. Many of the caves, some quite extensive, are confined to small vertical intervals, either by lithology of the confining beds or by still-stands of nearby rivers. Vertical caves are less common. The Edwards Plateau can be divided into subregions principally on the basis of stratigraphy and hydrology.

2a. Devils River Trend

This narrow band of very thick-bedded to massive Devils River Limestone is not easily correlated to laterally adjacent formations, and appears to be the result of carbonate bank-construction that lasted through most of the Middle Cretaceous. Vertical caves are common in the western section because of the thick vadose zone that recharges deep phreatic loops and the few lithologic barriers to vertical flow (e.g., Emerald Sink and Langtry Lead, Val Verde County). In the subregion's eastern section, vadose flow is relatively recent, so most caves are old, large high-elevation, phreatically-formed rooms and passages. Their large size is structurally supported by the limestone's thick to massive bedding, although substantial collapse is common (e.g., Frio Bat Cave, Uvalde County). Fractures may be related to underlying fault zones.

2b. Maverick Basin

South of the Devils River Trend, the upper and lower portions of the Edwards Group comprise the Salmon Peak Limestone and the West Nueces Formation, thick-bedded limestone sequences that are separated by the McKnight Formation, a thin-bedded shaley limestone. Most caves are limited vertically by lithology (e.g., Webb Cave, Kinney County); groundwater outlets were apparently at the bases of the upper and lower sequences. Passage orientations are determined by the local hydraulic gradients and are weakly guided by fractures along those flowpaths.

2c. Central Edwards Plateau

North of the Devils River Trend, cavernous rocks become progressively thinner to the north. In the southern part of this subregion, some caves attain significant vertical extent because of the good vertical continuity of limestone beds (e.g, 0-9 Well, Crockett County), but in the northern part the total soluble limestone is thin and limits vertical development (e.g., Powell's Cave System, Menard County). Stream incision into the soluble carbonates is much younger in most of this area, and fewer well-integrated caves have formed.

2d. Stockton Plateau

The deeply incised Pecos River separates the Stockton Plateau on the west from its counterpart, the Edwards Plateau. The Pecos and Rio Grande have provided outlets for groundwater at the base of the thick, soluble limestone sequence and have promoted development of some of Texas' deeper caves (e.g., Sorcerer's Cave, Troll Cave, and Wizard's Well, Terrell County). Many cave entrances are near the contact of the cave-forming carbonates with the overlying Boquillas Flags or the Del Rio Clay where present.

2e. Lampasas Cut Plain

Although the cavernous Edwards Group and related limestones are not more than 25 m thick in this subregion, laterally extensive caves have developed in the broad uplands (e.g., Rocket River Cave, Coryell County). Limits to cave development are the abrupt eroded edges of the uplands, which range in size from small plateaus to buttes. Most of the extensive caves or cave remnants are small-diameter conduits. Erosional exposure of some caves suggests that most pre-date current incision.

2f. Isolated Edwards Outliers

Erosional remnants of the Edwards Group encircle much of the Edwards Plateau. To the north and west where the limestone thins and becomes marly, caves are small and apparently unrelated to caves in neighboring outliers. Long caves in this subregion result as foci of groundwater flow (e.g., Comanche Springs Cave, Pecos County) or from maze development (e.g., Amazing Maze Cave, Pecos County). The southeastern portion of this subregion occurs at the Plateau's dissected margin along the Balcones Escarpment and includes the few caves in the upper member of the Glen Rose Formation. Most caves in this area are small, recently formed features; large caves are generally relicts of hydrologic regimes that pre-date the incision of the Plateau margin.

Mike Burrell at the entrance of Spring Creek Cave, Kendall County, Texas. - photo George Veni

3. Lower Glen Rose

The lower member of the Cretaceous Glen Rose Formation is a thick-bedded to massive fossiliferous limestone that contains many of the longer caves in Texas. It is exposed along the most deeply dissected southeastern margin of the Edwards Plateau. Caves in the Cibolo Creek watershed ultimately drain into the artesian Edwards (Balcones Fault Zone) Aquifer, steeply descending toward the water table where they either end in sumps or sediment fill (e.g., Cascade Caverns, Kendall County). Caves in the Guadalupe River watershed are long, dendritic stream systems which discharge to the rivers. The caves are strongly guided by fractures, and many of the longer caves were initiated as routes for the piracy of water across river meanders (e.g., Prassell Ranch Cave, Kendall County) or between drainage basins (e.g., Honey Creek Cave, Comal and Kendall counties). Caves in the Blanco and Medina River watersheds are not well-studied, but appear similar to those along the Guadalupe River.

Wayne Peplinski and Ernest Parker in the entrance of Hill's Gate Cave, Palo Pinto County, Texas. - photo Butch Fralia

4. North Texas

Thin limestone beds with rare thick limestone banks (such as the Winchell) provide limited opportunities for speleogenesis. The few caves known in the Cretaceous and Pennsylvanian-Permian rocks do not have much lateral extent and are very limited vertically by lithology (e.g., Eagle Creek Cave and Manley Water Cave, Palo Pinto County).

5. Llano Region

This structural basin contains Paleozoic rocks, including very thick carbonates of Cambrian and Ordovician age that are exposed along its outer margin (see the discussion on Igneous and Metamorphic Rocks for information on the basin's central section). Abundant normal faulting and related jointing provide numerous avenues for dissolution. Where the thick, Ellenburger Group (Ordovician) carbonates are well above nearby river valleys, cavern development is sometimes vertically extensive (e.g., Polish Cave, San Saba County). Dissolution-widened fissure systems are abundant in this Ordovician group, where jointing is pervasive. The size of certain caves and the type of fill hint that they may at least partly date from an earlier erosional episode (e.g., Gorman Cave, San Saba County). The entire Llano Region was covered by Cretaceous rocks, now mostly removed by erosion.

6. Permian Reef

The Capitan Reef is well-known to geologists and speleologists as the host for Carlsbad Caverns and related caves in New Mexico. In Texas, the same massive carbonate reef/bank rocks are exposed in the Guadalupe Mountains, Sierra Diablo, Apache Mountains, and Glass Mountains. Relatively few caves are known (e.g., Upper and Lower Sloth Caves, and Majestic Ice Cave, Culberson County; 400 Foot Cave, Brewster County), possibly because of difficulty of exploration, very large ranches, and distance to populated areas. The rarity of major caves compared to the Carlsbad Caverns area may be related to the very high geomorphic position with respect to groundwater or possibly indicates that little deep-seated hydrogen sulfide gas rose to mix with the groundwater to aggressively dissolve caves, as occurred at Carlsbad.

7. Block-faulted Ranges.

The basin-and-range province includes far west Texas and parts of the Big Bend area. Several carbonate rock units are exposed in many of the mountainous, block-faulted ranges and are thus bounded by steep, near-vertical, normal faults. Caves are guided by jointing related to abundant faulting. Few caves are known in this region because of the difficulty of access and distance from population centers; few of the known caves are extensive (e.g., Mesa de Anguila Sinkhole, Brewster County). For example, very thick limestone beds are exposed in the block faulted Sierra del Carmen in Big Bend, but almost no caves are known in them. Additional study of the West Texas karst should reveal characteristics that subdivide and better define this region.