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
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
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
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
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