Mount Bonnell and the Balcones Escarpment
1
Spherical, aerial (drone) photo credit: Chris Zahm, Bureau of Economic Geology SOUTH NORTHWEST WEST COLORADO RIVER / LAKE AUSTIN COLORADO RIVER / LAKE AUSTIN Pennybacker Bridge Tom Miller Dam Edwards Plateau Remnants River Terrace River Terrace River Terrace Albert R. Davis Water Treatment Plant Barton Springs Hill Country Mount Bonnell Fault UP DOWN Mount Barker Hill Country Edwards Plateau Remnants Pennybacker Bridge Tom Miller Dam Edwards Plateau Remnants River Terrace River Terrace River Terrace Albert R. Davis Water Treatment Plant Barton Springs Hill Country Mount Bonnell Fault UP DOWN Edwards Plateau Remnants Mount Bonnell YOU ARE HERE Mount Barker Hill Country Geologic History EROSION of the landscape by the Colorado River and its tributaries began around 15 million years ago. The Edwards Plateau once extended unbroken across Central Texas. With faulting, aggressive down-cutting by the Colorado River began. Streams sculpted the landscape, leaving isolated remnants of the Edwards Plateau and forming the Balcones Escarpment. DISPLACEMENT of rock strata across the Mount Bonnell Fault occurred about 20 to 15 million years ago. Rock layers were displaced upward west of this break relative to the down-dropped strata to the east. DEPOSITION of strata (layers of rock) in the Austin area occurred about 66 to more than 100 million years ago. During this period, the sea covered all of Central Texas. Variable marine conditions, such as depth, water temperature, and salinity produced sand, limy ooze, and clay deposits that eventually hardened into the bedrock strata that we see today. Mount Bonnell’s geologic history includes three major events that took place between 100-plus million years ago and the present, and that shaped today’s landscape. 2 3 1 Age (Ma) = million years ago CENOZOIC MESOZOIC P ALEOZOIC PRECAMBRIAN Mississippian, Devonian, Silurian, Ordovician, Cambrian Pennsylvanian 323 Ma Permian 299 Ma Jurassic T riassic 252 Ma Lower Cre t aceous 145 Ma Upper Cre t aceous ~ 100 Ma Paleogene 66 Ma Neogene 23 Ma Older Quaternary 2 Ma Younger Quaternary ~100,000 y 4600 Ma 541 Ma Mississippian, Devonian, Silurian, Ordovician, Cambrian Pennsylvanian 323 Ma Permian 299 Ma Jurassic T riassic 252 Ma Lower Cre t aceous 145 Ma Upper Cre t aceous ~ 100 Ma Paleogene 66 Ma Neogene 23 Ma Older Quaternary 2 Ma Younger Quaternary ~100,000 y 4600 Ma 541 Ma Source: Geologic Society of America, modified from Geologic Time Scale v. 5.0 GEOLOGIC TIME Older Younger } } } Present Tom Miller Dam Barton Springs Mount Bonnell C O L O R A D O R I V E R C O L O R A D O R I V ER 0 5 mi 8 km 0 N Source: Topographic depiction from Garner & Young, 1976, Plate VII Contour interval 20 ft (~6.1 m) Mount Bonnell Fault Mount Bonnell Fault Downtown Austin Downtown Austin Hill Country Hill Country Pennybacker Bridge L A K E A U S T I N L A K E A U S T I N L A K E A U S T I N L A K E T R A V I S L A K E T R A V I S Balcones Escarpment Balcones Escarpment Balcones Escarpment Balcones Escarpment Mount Bonnell Fault Mount Bonnell Fault GREAT PLAINS GREAT PLAINS GULF COASTAL PLAIN GULF COASTAL PLAIN Hill Country Hill Country E d w a r d s P l a t e a u R e m n a n t s E d w a r d s P l a t e a u R e m n a n t s An escarpment is an area where the elevation of the earth changes suddenly forming a line of hills or scarps. At Mount Bonnell, you are atop the Balcones Escarpment. The Balcones Escarpment runs through Central Texas from Austin, southwest through San Antonio, and from there to the Rio Grande (Big River) near Del Rio. It marks the boundary between the Great Plains stretching west, and the Gulf Coastal Plain to the east. The break in the landscape defines major differences in earth resources, weather and climate, soils, vegetation, and animal habitats. East of the escarpment are the prime farmlands of the Blackland Prairies; to the west are the rangelands and woodlands of the Hill Country and the Edwards Plateau. The Balcones Escarpment marks the beginning of the American West in terms of landscape, ecology, geologic processes, and human endeavors. Mount Bonnell (785 ft. above sea level and 300 ft above Lake Austin) overlooks the Balcones Escarpment and provides a panorama of downtown Austin to the east and the eroded edges of the Edwards Plateau to the west. These very different landscapes were sculpted by the Colorado River and its tributaries to form the Balcones Escarpment along the main fault line at the foot of Mount Bonnell. The Mount Bonnell Fault is one of many faults in the Balcones Fault Zone. In this area, rock formations east of the Mount Bonnell Fault are 600 ft lower than the rock formations to the west of the fault. Physiographic maps show the locations of major landforms-- plateaus, plains, and mountain ranges. What is an Escarpment? Mount Bonnell Fault Mount Barker Edwards Limestone cap Balcones Escarpment Mount Bonnell Fault Up-faulted base of Edwards Limestone Mount Bonnell (elevation 785 ft above sea level) Projected elevation at base of down-faulted Edwards Limestone Edwards Limestone Edwards Limestone Fault displacement ~ 600 ft Fault displacement ~ 600 ft Water treatment plant Simplified Cross Section Down-faulted base of Edwards Limestone UP DOWN UP DOWN } WEST EAST Did You Know? The Balcones Fault Zone was active between 20 and 15 million years ago. Although no major earthquakes along the Balcones Fault Zone have occurred in recorded history, this does not mean that earthquakes could not happen again! In fact, two small historic earthquakes (1873 and 1902) in Travis County may have been caused by small movements along the Balcones Fault Zone.* * Source: Frohlich, C., and S. D. Davis, Texas Earthquakes, Univ. Texas Press, 275 pp., 2002 R i o G r a n d e Waco San Antonio Del Rio BP R e d R i v e r Balcones Escarpment 100 mi 0 0 150 km N G U L F C O A S T A L P L A I N Austin Fort Worth Dallas G R E A T P L A I N S G R E A T P L A I N S HC EDWARDS PLATEAU Physiographic Provinces Great Plains Edwards Plateau Hill Country ( HC) Gulf Coastal Plain Blackland Prairies ( BP) BP BP HC HC Physiographics Regions and Subregions Great Plains Balcones Escarpment Boundaries CENTRAL LOWLANDS BASIN AND RANGE BASIN AND RANGE Visit the Texas GeoSign website to view the English and Spanish-language versions of this sign. http://www.beg.utexas.edu/geosign. Visite el sitio web de Texas GeoSign para ver las versiones en Inglés y Español de este letrero. http://www.beg.utexas.edu/geosign. En Español El Monte Bonnell (785 pies sobre el nivel del mar y 300 pies sobre el lago Austin) domina la Escarpa de Balcones y ofrece un panorama del centro de Austin hacia el este y los bordes erosionados del Edwards Platueau hacia el oeste. Groundwater in the Fault Zone Barton Springs Springs are natural sites where groundwater flows to the surface. About 4 miles south of Mount Bonnell, Barton Springs, the fourth-largest spring system in Texas, has an average flow of approximately 22,400 gallons per minute from the Edwards Aquifer. This spring water, issuing from faulted limestone, supplies water to Barton Springs Pool in Austin’s Zilker Park. The cavernous aquifer provides a habitat for an endangered species of salamander that lives in these springs and nowhere else. TEXAS G EO S IGN Mount Bonnell and the Balcones Escarpment Established in 1909, the Bureau of Economic Geology is the oldest organized research unit of The University of Texas and is the Texas State Geological Survey. Scott W. Tinker serves as its director and as State Geologist of Texas. Our mission is to serve society by conducting objective, impactful, and integrated geoscience research on relevant energy, environmental, and economic issues. Our vision is to be a trusted scientific voice to academia, industry, government, and the public, whom we serve. The Bureau is part of the Jackson School of Geosciences at The University of Texas at Austin. QAe7258
Transcript of Mount Bonnell and the Balcones Escarpment
Spherical, aerial (drone) photo credit:Chris Zahm, Bureau of Economic Geology
SOUTH NORTHWESTWEST
COLORADO RIVER / LAKE AUSTINCOLORADO RIVER / LAKE AUSTIN
PennybackerBridge
Tom MillerDam
Edwards PlateauRemnants
River Terrace
River Terrace
River Terrace
Albert R. DavisWater Treatment
Plant
BartonSprings
Hill Country
Mount BonnellFault
UP
DOWN
MountBarker
Hill Country
Edwards PlateauRemnants
PennybackerBridge
Tom MillerDam
Edwards PlateauRemnants
River Terrace
River Terrace
River Terrace
Albert R. DavisWater Treatment
Plant
BartonSprings
Hill Country
Mount BonnellFault
UP
DOWN
Edwards PlateauRemnants
Mount BonnellYOU ARE HERE
MountBarker
Hill Country
Geologic HistoryEROSION of the landscape by the Colorado River and its tributaries began around 15 million years ago. The Edwards Plateau onceextended unbroken across Central Texas. With faulting, aggressive down-cutting by the Colorado River began. Streams sculpted thelandscape, leaving isolated remnants of the Edwards Plateau and forming the Balcones Escarpment.
DISPLACEMENT of rock strata across the Mount Bonnell Fault occurred about 20 to 15 million years ago. Rock layers were displacedupward west of this break relative to the down-dropped strata to the east.
DEPOSITION of strata (layers of rock) in the Austin area occurred about 66 to more than 100 million years ago. During this period, the seacovered all of Central Texas. Variable marine conditions, such as depth, water temperature, and salinity produced sand, limy ooze, and claydeposits that eventually hardened into the bedrock strata that we see today.
Mount Bonnell’s geologic history includes three major events that took place between 100-plus million years ago and the present, and that shaped today’s landscape.
2
3
1
Age (Ma) = million years ago
CE
NO
ZO
ICM
ES
OZ
OIC
PA
LE
OZ
OIC
PRECAMBRIAN
Mississippian, Devonian, Silurian,
Ordovician, Cambrian
Pennsylvanian323 Ma
Permian299 Ma
JurassicTriassic
252 Ma
Lower Cretaceous
145 Ma
Upper Cretaceous
~ 100 Ma
Paleogene66 Ma
Neogene23 Ma
OlderQuaternary
2 Ma
YoungerQuaternary
~100,000 y
4600 Ma
541 Ma
Mississippian, Devonian, Silurian,
Ordovician, Cambrian
Pennsylvanian323 Ma
Permian299 Ma
JurassicTriassic
252 Ma
Lower Cretaceous
145 Ma
Upper Cretaceous
~ 100 Ma
Paleogene66 Ma
Neogene23 Ma
OlderQuaternary
2 Ma
YoungerQuaternary
~100,000 y
4600 Ma
541 Ma
Source: Geologic Society of America,modified from Geologic Time Scale v. 5.0
GEO
LOG
IC T
IME
Old
erYo
unge
r
}
}
}Present
Tom Miller Dam
Barton Springs
Mount Bonnell
COLO R AD O RIVER
COLO R AD O RIVER
0
5 mi
8 km
0
N
Source: Topographic depiction from Garner & Young, 1976, Plate VII Contour interval 20 ft (~6.1 m)
Mount
Bon
nell
Fault
Mount
Bon
nell
Fault
DowntownAustin
DowntownAustin
HillCountry
HillCountry
Pennybacker Bridge
LAKE A USTIN
LAKE A USTIN
LAKE A USTIN
LAKE TR
AV
I S
LAKE TR
AV
I S
Balco
nes
Escar
pmen
t
Balco
nes
Escar
pmen
t
Balc
ones
Esc
arpm
ent
Balc
ones
Esc
arpm
ent
Mou
nt B
onne
ll Fa
ult
Mou
nt B
onne
ll Fa
ult
GREATPLAINSGREATPLAINS
GULFCOASTALPLAIN
GULFCOASTALPLAIN
HillCountry
HillCountry
Edwards Plateau Remnants
Edwards Plateau Remnants
An escarpment is an area where the elevation of the earth changes suddenly forming a line of hills or scarps. At Mount Bonnell, you are atop the Balcones Escarpment.
The Balcones Escarpment runs through Central Texas from Austin, southwest through San Antonio,and from there to the Rio Grande (Big River) near Del Rio. It marks the boundary between the GreatPlains stretching west, and the Gulf Coastal Plain to the east. The break in the landscape defines majordifferences in earth resources, weather and climate, soils, vegetation, and animal habitats. East of theescarpment are the prime farmlands of the Blackland Prairies; to the west are the rangelands andwoodlands of the Hill Country and the Edwards Plateau. The Balcones Escarpment marks the beginning of the American West in terms of landscape, ecology, geologic processes, and human endeavors.
Mount Bonnell (785 ft. above sea level and 300 ft above Lake Austin) overlooks the BalconesEscarpment and provides a panorama of downtown Austin to the east and the eroded edges of theEdwards Plateau to the west. These very different landscapes were sculpted by the Colorado Riverand its tributaries to form the Balcones Escarpment along the main fault line at the foot of Mount Bonnell.
The Mount Bonnell Fault is one of many faults in the Balcones Fault Zone. In this area, rock formations east of the Mount Bonnell Fault are 600 ft lower than the rock formations to the west of the fault.
Physiographic maps show the locations of major landforms--plateaus, plains, and mountain ranges.
What is an Escarpment?
Mount Bonnell Fault
Mount BarkerEdwards Limestone cap
Balcones Escarpment
Mount Bonnell FaultUp-faulted baseof Edwards Limestone
Mount Bonnell(elevation 785 ft above sea level)
Projected elevation atbase of down-faultedEdwards Limestone
EdwardsLimestoneEdwards
Limestone
Faul
t dis
plac
emen
t ~ 6
00 ft
Faul
t dis
plac
emen
t ~ 6
00 ft
Water treatment plant
Simplified Cross Section
Down-faultedbase of
Edwards Limestone
UP
DOWN
UP
DOWN
}
WESTEAST
Did You Know?The Balcones Fault Zone was active between
20 and 15 million years ago. Although no majorearthquakes along the Balcones Fault Zone
have occurred in recorded history, this does not mean that earthquakes could not happen again!
In fact, two small historic earthquakes(1873 and 1902) in Travis County may have been caused by small movements along the
Balcones Fault Zone.*
* Source: Frohlich, C., and S. D. Davis, Texas Earthquakes,Univ. Texas Press, 275 pp., 2002
Rio G
rande
Waco
SanAntonioDel Rio
BP
Red River
BalconesEscarpment
100 mi0
0 150 km
N
G U L F CO
AS
TA
L
PL
AIN
Austin
FortWorth Dallas
GR
EA
T P
LA
I NS
GR
EA
T P
LA
I NS
HC
EDWARDSPLATEAU
PhysiographicProvinces
Great Plains
Edwards Plateau Hill Country (HC)
Gulf Coastal Plain
Blackland Prairies (BP)BPBP
HCHC
Physiographics Regionsand Subregions
Great PlainsBalcones Escarpment
Boundaries
CENTRALLOWLANDS
BASINAND
RANGE
BASINAND
RANGE
Visit the Texas GeoSign website to view the English and Spanish-language versions of this sign. http://www.beg.utexas.edu/geosign.
Visite el sitio web de Texas GeoSign para ver las versiones en Inglés yEspañol de este letrero. http://www.beg.utexas.edu/geosign.
En EspañolEl Monte Bonnell (785 pies sobre el nivel del mar y 300 pies sobre el lago Austin) domina la Escarpa de Balcones y ofrece un panorama del centro de Austin hacia el este y los bordes erosionados del Edwards Platueau hacia el oeste.
Groundwater in the Fault Zone
Barton SpringsSprings are natural sites where groundwater flows to the surface. About 4 miles south of Mount Bonnell, Barton Springs, the fourth-largest spring system in Texas, hasan average flow of approximately 22,400gallons per minute from the Edwards Aquifer. This spring water, issuing from faultedlimestone, supplies water to Barton Springs Pool in Austin’s Zilker Park. The cavernous aquifer provides a habitat for an endangered species of salamander that lives in these springs and nowhere else.
T E X A S
GEOSIGN
Mount Bonnell and the Balcones Escarpment
Established in 1909, the Bureau of Economic Geology is the oldest organized research unit of The University of Texas and is the Texas State Geological Survey. Scott W. Tinker serves as its director and as State Geologist of Texas. Our mission is to serve society by conducting objective, impactful, and integrated geoscience research on relevant energy, environmental, and economic issues. Our vision is to be a trusted scientific voice to academia, industry, government, and the public, whom we serve. The Bureau is part of the Jackson School of Geosciences at The University of Texas at Austin.
QAe7258
