LOCATION BLACKGAP TXTentative Series
TAXONOMIC CLASS: Loamy-skeletal, carbonatic, hyperthermic Lithic Ustic Haplocalcids
TYPICAL PEDON: Blackgap very cobbly loam--rangeland. (Colors are for dry soil unless otherwise stated.)
A--0 to 10 cm (0 to 4 in); pale brown (10YR 6/3) very cobbly silt loam; brown (10YR 4/3) moist; weak fine granular structure; slightly hard, friable; common very fine and fine roots; 45 percent limestone gravel and 15 percent limestone cobbles; violently effervescent; moderately alkaline; abrupt wavy boundary. (10 to 25 cm [4 to 10 in] thick)
Ak--10 to 23 cm (4 to 9 in); pale brown (10YR 6/3) extremely cobbly silt loam, brown (10YR 4/3) moist; weak fine granular structure; hard, friable; common fine and medium roots in cracks; 15 percent limestone gravel and 45 percent limestone cobbles; calcium carbonate coats on rock fragments; violently effervescent; moderately alkaline; abrupt wavy boundary. (13 to 42 cm [5 to 17 in] thick)
R--23 to 203 cm (9 to 80 in); indurated, slightly fractured limestone bedrock.
TYPE LOCATION: Brewster County, Texas; from the junction of U.S. Highways 90 and 385 just east of Marathon; 38.2 miles south on U.S. Highway 385 to intersection with RM 2627; 19.7 miles southeast on RM 2627 to ranch road; 0.9 mile east on ranch road; 1.7 miles northwest on ranch road; and 30 feet south of road in range. (Latitude: 29 degrees, 32 minutes, 56.40 seconds N; Longitude: 102 degrees, 55 minutes, 2.87 seconds W; NAD27. UTM coordinates: 701761 meters E, 3270733 meters N, NAD 83, Zone 13. Black Gap USGS 7.5 minute topographic quadrangle)
RANGE IN CHARACTERISTICS:
Soil moisture: The moisture control section is dry in all parts more than three fourths of the time the soil temperature exceeds 5.0 degrees C (41 degrees F). Intermittently moist in some part of the soil moisture control section during June to September. More than 60 percent of the annual rainfall occurs during that period. The soil does not receive significant amounts of moisture during winter months. Ustic aridic soil moisture regime.
Mean annual soil temperature: 22.0 to 25.6 degrees C (72 to 78 degrees F), based on the summary of a 5-year soil temperature study near La Linda, Texas.
Depth to limestone bedrock: 15 to 50 cm (6 to 20 in)
Particle-size control section (weighted average):
Clay content: 18 to 30 percent
Rock fragment content: 35 to 80 percent; 10 to 35 percent gravel; 20 to 45 percent cobbles; 0 to 20 percent stones
Calcium carbonate equivalent: More than 40 percent
A horizon
Hue: 7.5YR to 2.5Y
Value: 5 to 8, 4 to 7 moist
Chroma: 2 to 4
Clay content: 15 to 27 percent
Calcium carbonate on rock fragments: faint discontinuous coats to pendants 3 cm (1 in) thick
Effervescence class: violent
Reaction: moderately alkaline
Bk horizon (where present)
Hue: 7.5YR to 2.5Y
Value: 5 to 8, 4 to 7 moist
Chroma: 3 to 5
Clay content: 15 to 27 percent
Effervescence class: violent
Reaction: moderately alkaline
R layer:
Fractures are greater than 10 cm (4 in) apart.
Secondary calcium carbonate coats on fracture surfaces.
COMPETING SERIES: This is the
Iceberg (NV) series. Similar soils include
Ector (TX),
Langtry (TX),
Lozier (TX), and
Mariscal (TX) series.
Iceberg soils have less than 18 percent clay content in the control section. In addition Iceberg soils receive significant winter precipitation, and are intermittently moist in the soil moisture control section during winter and spring, typical of the
Mohave Desert (MLRA 30).
Ector and Langtry soils have a mollic epipedon.
Mariscal soils do not have a calcic horizon, and contain channers and flagstones.
Lozier soils have a thermic soil temperature regime.
GEOGRAPHIC SETTING:
Parent material: residuum and colluvium derived from thick-bedded limestone of the Cretaceous Glen Rose, Del Carmen, Telephone Canyon, Santa Elena, Sue Peaks, Buda, and Austin formations
Landscape: dissected plateau
Landform: nearly level mesas or divides to very steep hillslopes and mountainsides.
Slope: 0 to 60 percent
Mean annual precipitation: 254 to 33 cm (10 to 13 inches)
Precipitation pattern: Most precipitation occurs as high intensity rain during afternoon convective thunderstorms from June to September. Precipitation during the months of January, February, and March is less than 13 percent of the annual total.
Mean annual air temperature: 20.0 to 22.2 degrees C (68 to 72 degrees F)
Frost-free period: 240 to 280 days
Elevation: 548 to 1,067 m, ranging to 1,219 m (1,800 to 3,500 ft, ranging up to 4,000 ft)
GEOGRAPHICALLY ASSOCIATED SOILS: These are the
Mariscal,
Seferino,
Stillwell, and the proposed
Strawhouse (TX) series.
Mariscal soils occur on hills, are dominated by channers and flagstones, and overly flaggy limestone of the
Boquillas formation.
Seferino and Stillwell soils are very deep and formed in alluvium. Seferino soils are fine-silty and occur on alluvial flats.
Strawhouse soils are shallow to a petrocalcic horizon.
Stillwell and Strawhouse soils occur on pediments.
DRAINAGE AND PERMEABILITY: Well drained. Permeability is moderate above very slowly permeable bedrock. Runoff is low on slopes less than 1 percent, medium on 1 to 3 percent slopes, high on 3 to 5 percent slopes, and very high on slopes greater than 5 percent.
USE AND VEGETATION: Rangeland used for livestock grazing, wildlife habitat, and recreation. Vegetation physiognomy is desert shrubland. Dominant woody plants are creosotebush, ocotillo, lechuguilla, and dalea species. Grasses include Chino grama, red grama, and tridens species. Other characteristic plants include leatherstem, candelilla, and false agave.
DISTRIBUTION AND EXTENT: LRR I, MLRA 81D, Southern Edwards Plateau. The series is extensive.
MLRA SOIL SURVEY REGIONAL OFFICE (MO) RESPONSIBLE: Temple, Texas
SERIES PROPOSED: Brewster County, Texas; 2004. The name is taken from Black Gap, a pass between basalt hills, near the type location in southern Brewster County.
REMARKS: Blackgap soils were formerly included in the Lozier series.
Diagnostic horizons and features recognized in this pedon are:
Ochric epipedon: 0 to 13 cm (1 to 5 in); (A horizon)
Because the epipedon has calcium carbonate equivalent greater than 40% and color value of 5 or less, it meets the color requirements for mollic epipedon. However, the soil does not meet the moisture requirement for a mollic epipedon because it is moist in some part of the epipedon for less than 90 days (cumulative) in normal years during times when the soil temperature at a depth of 50 cm (20 in) is 5 degrees C (41 degrees F).
Calcic horizon: 0 to 23 cm (0 to 9 in); (A and Ak horizons); secondary calcium carbonate accumulations on fragments and in fractures
Lithic contact: 28 cm (11 in); upper boundary of R layer
Classified according to Keys to Soil Taxonomy, Tenth Edition, 2006.
ADDITIONAL DATA: TAMU Soil Characterization Laboratory data are available for the typical pedon, S90TX043-003.