Geology of Pedernales Falls State Park and

Transcription

Texas Academy of Science, 2011, Field Trip:
Geology of Pedernales Falls State Park
and Enchanted Rock State Natural Area
March 5, 2011
Field Trip Leaders:
R. LaRell Nielson and Chris A. Barker
Department of Geology
Stephen F. Austin State University
Nacogdoches, Texas
Acknowledgements
The writers are grateful to the Geology Department at Stephen F. Austin State
University for providing the opportunity to conduct geological research and for the
opportunity to lead field trips to the Hill Country area for many years. Thanks goes to the
Department of Geology for partially funding these field trips and research projects and
to our students who have helped us learn more about the area through their questions.
A great deal of thanks and appreciation should be given to our spouses (Sylvia and
Anne) for their support and encouragement to complete this project.
The cover photo was taken by one of our Hill Country Field Trip students.
Pedernales Falls, Texas
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Table of Contents
Acknowledgements ................................................................................................i
Introduction ...........................................................................................................1
Field Trip Stops: LOCATION ONE: Pedernales Falls State Park .......................2
Stop 1 – Flooding of the Falls .....................................................................2
Stop 2 – Overview of Pedernales Falls.......................................................3
Stop 3 – Joints and Tension Gashes..........................................................7
Stop 4 – Crinoid Columnals and Chert Layers............................................8
Stop 5 – Fluvial Cut Slots and Potholes .....................................................9
Stop 6 – Caves ...........................................................................................9
Stop 7 – Micro-karst Features ....................................................................9
Stop 8 – Springs .........................................................................................9
LOCATION TWO: Enchanted Rock State Natural Area...................................10
Stop 9 – View of Enchanted Rock in the distance ....................................11
Stop 10 – Sandy Creek Packsaddle Schist metamorphic complex ..........13
Stop 11 – Sandy Creek modern sediments ..............................................14
Stop 12 – Breccia and fault zone below the pavilion ................................14
Stop 13 – Third of the way up Enchanted Rock........................................15
Stop 14 – Half way up Enchanted Rock ...................................................19
Stop 15 – Highest point on Enchanted Rock ............................................19
Stop 16 – Caves on the Northeast side of Enchanted Rock.....................16
Stop 17 – Cooper’s Old Time Bar-B-Que .................................................21
References..........................................................................................................21
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Table of Figures
Figure 1a – Geologic Map of Texas ......................................................................4
Figure 1b – Geologic Map Pedernales Falls Area .................................................5
Figure 2 – Stratigraphic column for Pedernales Falls............................................6
Figure 3 – Pennsylvanian Aged Basins of West Texas .........................................7
Figure 4 – Crinoid diagram....................................................................................8
Figure 5 – Origin of potholes .................................................................................9
Figure 6 – Geologic Map of Enchanted Rock Area .............................................12
Figure 7 – Generalized Geologic Map of Enchanted Rock..................................13
Figure 8 – Fault and fracture zones near Enchanted Rock ................................15
Figure 9 – Exfoliation domes and fault systems ..................................................15
Figure 10 – Tent blisters......................................................................................16
Figure 11 –Joints and weathering .......................................................................17
Figure 12 – Spheroidal weathering .....................................................................17
Figure 13 – Rock pedestal development .............................................................18
Figure 14 – Rock doughnuts ...............................................................................18
Figure 15 – Honeycomb weathering....................................................................19
Figure 16 – Weathering pits ................................................................................20
Figure 17 – Exfoliation sheets .............................................................................20
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Geology of Pedernales Falls and Enchanted Rock–Texas Academy of Science 2011
Introduction
This year's Texas Academy of Science Geology Field Trip will take you to two of
the most scenic and interesting locations in the Texas Hill Country. At
Pedernales Falls State Park you will see a spectacular unconformity between
tilted Paleozoic strata and flat-lying Cretaceous rocks, observe a gushing spring
near the banks of the Pedernales River, stroll through Pennsylvanian-age crinoid
shoals, determine stress orientation from tension gashes in carbonate rocks,
peer into potholes large enough to swallow a geologist, examine a variety of
karst features and ponder local drainage reorganization from dendritic to trellis.
Our second location is at Enchanted Rock State Natural Area. Enchanted Rock
is a Precambrian granite batholith that formed during the Grenville orogeny and
the assembly of the supercontinent, Rodinia. As we climb this spectacular
outcrop of pink granite, we will discuss the tectonics involved in the creation of
the Llano granites, episodic uplift of the plutons, fault brecciation zones, xenolith
inclusions, magmatic foliation, rapakivi textures, hoodoos, vernal pools, unusual
minerals in adjacent pegmatite dikes and the genesis of contact migmatites in the
Packsaddle schist on the edge of the pluton. We will also crawl through
exfoliation slab caves at the back of Enchanted Rock.
Those who want the full Central Texas experience can join us for the last stop at
Cooper’s Bar-B-Que in Llano.
1
Field Trip Stops
Location 1:
Pedernales Falls State Park, Texas
Introduction
Pedernales Falls State Park is located 10 miles (16 km) east of Johnson City and
30 miles (48 km) west of Austin. Before 1970, the area that is now Pedernales
Falls State Park was a working ranch known as the Circle Bar Ranch. The state
of Texas purchased the land for the park in 1970 and the park opened in 1973.
Pedernales Falls State Park contains a spectacular unconformity between tilted
Paleozoic strata and flat-lying Cretaceous rocks as well as many other features
of geologic interest. Students from our weekend field trips consistently rate this
as one of their favorite stops. As you will see, it is also one of the most scenic
locations in Texas.
Stop 1: Flooding of the Falls:
At the end of the parking lot, stop and observe the warning sign about sudden
flash floods on the Pedernales River. During periods of torrential downpours
major flooding can occur at the falls. It can take as little as five minutes for the
river to go from a normal flow to a high flood stage. Often the park doesn’t
receive any precipitation when flooding occurs; instead, all of the precipitation to
produce the flood can occur up river in the Pedernales drainage basin.
2
Stop 2: Overview of Pedernales Falls:
At this scenic overlook we see the Pedernales River below us. Pedernales “Falls”
is actually a group of small waterfalls that cascade down a series of tilted beds of
Paleozoic rock. From this vantage point we can also see evidence of a
spectacular unconformity between the tilted Pennsylvanian Marble Falls
Limestone exposed in the river bed and flat-lying Cretaceous rocks of the
Sycamore, Hammett, Cow Creek, Hensell and Glen Rose Formations in the
surrounding hillsides.
Figures 1a, 1b and 2 show the geology and stratigraphy of the area. On the
stratigraphic column (Figure 2), equivalent Cretaceous units are listed as the
Travis Peak Formation.
In the late Paleozoic, the Ouachita Orogeny created a foldbelt to the southeast of
this area (Figure 3) and may have been responsible for some or all of the tilting
of the Pennsylvanian strata seen at this stop.
Later erosion during the Mesozoic truncated the tilted Paleozoic rocks and
produced a relatively flat erosional surface. Cretaceous seas during the Zuni
transgressive cycle deposited clastic and carbonate strata horizontally over the
tilted Paleozoic beds. Later uplift of the area during the Tertiary allowed the
Pedernales River to cut down through and expose the angular unconformity. This
resulted in a reorganization of local drainage from a dendritic to a trellis pattern.
This is occurring because there is dendritic drainage on the flat-lying Cretaceous
rocks, but when the underlying tilted Paleozoic beds are exhumed by erosion,
then the drainage changes to a trellis pattern.
3
Figure 1a. Geologic map of Texas. The two main locations for this field trip are on the
south edge of the Llano Uplift, which is the pink “bull’s-eye” in the center of Texas. The
main field trip locations are labeled 1 and 2 on the map above and their positions are
indicated with small ‘x’s. Location 1 is at Pedernales Falls State Park. Location 2 is at
Enchanted Rock State Natural Area. Note the green bands of Cretaceous rocks that
surround the Llano Uplift on three sides. (Texas Bureau of Economic Geology; Geology
of Texas map, 1992)
4
Figure 1b. Geologic map of the Pedernales Falls area. An angular unconformity between
the Paleozoic and Mesozoic strata can be seen at the falls.
5
Figure 2. Stratigraphic column for Pedernales Falls State Park area (McCrary 2003).
6
Figure 3. Map showing the locations of Pennsylvanian and Permian basins of Texas.
The Marble Falls Limestone was deposited on a shelf between the Kerr and Fort Worth
Basins. The Llano uplift was to the west (Ewing 1991).
Stop 3: Joints and Tension Gashes:
Joints and tension gashes are common in the Marble Falls Carbonate exposed in
the bed of the Pedernales River. They were probably produced by the
deformation of the Ouachita Orogeny, or by stresses generated by later uplift of
the Llano area. The tension gashes have been filled with white calcite and are
therefore mineralized veins. We will look at stress orientations indicated by the
tension gashes.
The ubiquitous joints in the area form conduits for water to flow into the
subsurface; the effect of this aquifer recharge system will be seen at a later stop
in this area.
7
Stop 4: Crinoid columnals and Chert Layers:
Deposition of the Marble Falls Limestone occurred on a Pennsylvanian shelf
which contained abundant crinoids and sponges. Study of the limestones
indicates that they are petrographically mudstones and wackestones deposited
on a low energy Pennsylvanian carbonate shelf. Chert layers found throughout
the park in the Marble Falls Limestone were produced by silica rich solutions
migrating through the limestone after deposition. Some of these chert layers
contain sponge spicules. Circular, or nodular, patterns in the chert may represent
the outline of sponges. Many individual crinoid columnals and segments of
crinoid stems are found in the Marble Falls Limestone and indicate a low energy
deposition environment (Figure 4).
Figure 4. Diagrams showing the crinoid and sponge parts. Columnals or pieces of the
stalk are found in the Marble Falls Limestone. Upon death the crinoid becomes
disarticulated and only columnal segments and plates remain (www.tolweb.org/Crinoidea,
www.daviddarling.info/encyclopedia/S/sponge.html).
8
Stop 5: Fluvial Cut Slots and Potholes:
Roughly planar slots have been eroded down into the Marble Falls Limestone by
the Pedernales River are common in the Falls area. These slots are produced by
preferential weathering and stream erosion along joints. Pot holes have also
developed along some of the slots by circular flow of water rotating clasts to cut
the holes (Figure 5).
Figure 5: Potholes are produced by river current spinning clast in circles wearing a hole
in the bottom of the channel (http://img.geocaching.com/cache/c5b41ded-69fd-400d8ea8-a9d50e5a90e7.jpg ).
Stop 6: Caves:
Several caves have developed in the Paleozoic limestone of the Falls area by
dissolution related to water moving through the limestone toward the base of the
falls. The water reappears as springs below the falls.
Stop 7: Microkarst Features:
Rough and irregular surfaces have developed on the Marble Falls Limestone that
represent micro-karst or micro-dissolution pits. These occur as the present
limestone surface is slowly dissolved.
Stop 8: Springs
Springs are present at the base of Pedernales Falls. These springs are
considered to be primarily charged by river water from above the falls.
Groundwater mainly moves along joints and bedding and emerges as artesian
springs at the base of the Falls.
9
Location 2:
Enchanted Rock State Natural Area
Introduction
The main feature at Enchanted Rock State Natural Area is a Precambrian granite
pluton in the Llano Uplift of central Texas. The park is located approximately
15 miles (24 km) north of Fredericksburg, Texas, and 24 miles (39 km) south of
the town of Llano. Enchanted Rock State Natural Area consists of 1,643 acres on
Big Sandy Creek, north of Fredericksburg, on the border between Gillespie and
Llano Counties. Humans have visited here for over 11,000 years.
Enchanted Rock is a large, pink granite exfoliation dome that rises 425 feet
above the local ground surface. It is 1,825 feet above sea level and covers 640
acres. It is one of the largest batholiths (underground rock formation uncovered
by erosion) in the United States. Enchanted Rock was designated a National
Natural Landmark in 1970 and was placed on the National Register of Historic
Places in 1984. It was obtained from the Moss family in 1978 by the Nature
Conservancy of Texas. The state acquired Enchanted Rock in 1984, completely
rebuilt the camping facilities and reopened the park.
DIRECTIONS: In Fredericksburg, Texas, set your vehicle’s trip
mileage indicator to zero at the intersection of W. Main Street (Hwy
290) and Milam Street (FM 965). This intersection is 3 to 4 blocks
west of the county courthouse. Turn north onto FM 965 toward
Enchanted Rock. Go 11.3 miles to Keese Road, but do not turn.
Continue north on FM 965 for another 3 miles to a high overlook
where Enchanted Rock is visible in the distance to the north for the
first time. Quickly pull completely off the road and park. Do not stop
in the road! This is on a blind curve with high speed traffic.
10
Stop 9: View of Enchanted Rock in the distance:
At this stop you are on the south edge of the Llano Uplift (Figure 1a). Enchanted
Rock is visible in the distance to the north as a broad, round pink dome.
Enchanted Rock is part of the Town Mountain Pluton which was emplaced into
the Packsaddle Schist about 1.2 billion years ago. It is a series of three
exfoliation domes of which the largest is Enchanted Rock. After roughly half a
billion years of erosion, the pluton was exposed during the Cambrian Period. At
various times in the Paleozoic and Mesozoic, the area was inundated by
transgressing seas and sediments were deposited across the granites. Feldsparrich sediment in nearby basal Cretaceous units indicates that the granite was
again exposed in the late Mesozoic. The area was subsequently covered in part
or fully by Cretaceous seas and sediment, and finally reexposed by the current
Tertiary and Quaternary Erosional cycle (Figure 6 and 7).
At this location we are standing on Cretaceous rocks of the Edwards Group. Note
that we are topographically higher than the Precambrian and Paleozoic rocks of
the Llano “Uplift” (the low area in front of you). The region has this name because
the older rocks have been pushed upward compared to other areas, but these
older, uplifted rocks are topographically lower than the surrounding Cretaceous
limestones. Why are the limestones higher? It may be due to differences in
weathering properties of various rocks. In this relatively dry climate where the
average annual rainfall is only about 27 inches
(http://www.homefacts.com/weather/Texas/Llano-County/Llano.html), carbonates are more
resistant to weathering than many other rock types and thus form topographic
highs. This phenomenon is apparently responsible for some outcrops within the
Llano Uplift where large fault-bounded blocks of Paleozoic carbonate strata form
hills that stand above surrounding plains of Precambrian rocks.
Alternatively, the Cretaceous Edwards limestone may have been deposited
completely over the Paleozoic and Precambrian rocks of the Llano Uplift, and
thus always have been topographically higher.
11
Figure 6. Geologic map of Enchanted Rock area. Enchanted Rock batholith is the large
area of light pink (pCtm) that fills most of the center of the map (compare with figure 7).
The approximate location of Enchanted Rock State Natural Area is shown in yellow.
Approximate width of map is 13.5 miles (21.7 km). (Texas Bureau of Economic,
Geologic Atlas of Texas: 1:250,000) The main geologic units are:
Greens: Cretaceous (Kft = Fort Terrett Fm.; Kh = Hensell Fm.)
Browns: Paleozoics (Crh = Hickory Sandstone; Crc = Cap Mtn. Limestone)
Pinks: Precambrian (pCvs = Valley Springs Gneiss; pCtm = Town Mountain Granite;
pCps = Packsaddle Schist)
12
Figure 7. Generalized geologic map of the Enchanted Rock State Natural Area showing
the outline of the pluton and age of rocks associated with the pluton (map from Petersen
1988 p.11).
DIRECTIONS: From the overview, drive 2.7 miles north on FM 965 to
Enchanted Rock State Natural Area. Turn in at the big sign and check
in at the park entrance building; we will eventually assemble at the
lot on the northeast end of the Park.
13
Stop 10: Sandy Creek Packsaddle Schist
metamorphic complex (East parking area):
Exposed along the southeast side of Sandy Creek are older metamorphic rocks
of the Precambrian Packsaddle Schist. These rocks were regionally
metamorphosed prior to emplacement of the Enchanted Rock pluton. They have
a strong foliation and relatively uniform amphibolite-grade metamorphic character
wherever they are exposed. In this area, the Packsaddle Schist was also contact
metamorphosed by emplacement of the Enchanted Rock pluton. Note the
swirling, migmatitic texture and partial anatexis of some of the schists. Also
present at this location are pegmatite dikes, some with unusual occurrences of
millimeter-scale crystals of magnetite. North of the creek is the highly brecciated
contact between the Pack Saddle Schist metamorphic complex and the Town
Mountain Pluton.
Stop 11. Sandy Creek modern sediments:
Sediments found in Big Sandy Creek contain high amounts of feldspar, quartz,
biotite, hornblende and magnetite. These sediments were derived from erosion of
Enchanted Rock granites and the surrounding Packsaddle Schist. Lithification of
such sediments produces arkose or feldspathic arenite. Similar feldspar-rich
sediments are seen in the Sycamore Formation of lower Cretaceous age in
outcrops along the eastern edge of the Llano uplift, which indicates that the
granite had been reexposed by Cretaceous time after having been buried
periodically by Paleozoic sediments.
14
Stop 12: Breccia and fault zone below the
pavilion:
Below the pavilion in a dry stream bed is a breccia zone created by a series of
faults that cut the southern margin of the pluton. Streams often follow or parallel
such breccia zones, and Sandy Creek seems to do so in this area. The map
below shows the location of the major fault zones associated with Enchanted
Rock (Figure 8 and 9).
Figure 8. Map showing fault and fracture patterns around Enchanted Rock (map from
Petersen 1988, p. 23)
Figure 9. Relationship between exfoliation domes and fault systems. Domes appear to
form between the fault zones (drawing from Petersen 1988, p. 23).
15
Stop 13: Third of the way up Enchanted Rock:
At the beginning of the climb up Enchanted Rock you will see large sheets and
blocks of granite scattered about. These slabs of rock are referred to as
exfoliation sheets and are produced when erosion slowly removes the
overburden about a pluton. Removal of the overlying rock decreasing the load on
the granite and this results in expansion of the pluton. Eventually, it beaks into
sheets producing a cracking sound called a rock burst. The rock separating into
sheets is referred to exfoliation and the sheets are called exfoliation sheets. At
certain locations sheets separate to form a “v”. These “v” shaped structures are
referred to as tent blisters (Figure 10). Along the trail differential weathering along
joint sets and exfoliation sheets has produced spheroidal weathering blocks,
corestones, tors, castle tors and rock pedestals (Figure 11, 12, 13 , 14, and 15).
Weathered boulders that have human or animal like figures are referred as
hoodoos. Spheroidal weathering produces rounded boulders or corestones.
Mushroom shaped rocks, called tors, castle tors and rock pedestals (Figure 13)
are produce by precipitation of minerals such as calcite, quartz, manganese
oxide or iron oxide along the weathered surface. Differential weathering removes
the mineral-enriched surface granite more slowly than non mineral-enriched
granite to produce the tors, castle tors and rock pedestals. Rock doughnuts are
also present where differential erosion has produced circular or tire shape
weathering patterns (Figure 14). Holes or small cave-like feathers are produce by
differential weathering and are referred to as honeycomb weathering, tafoni or
rock lattices (Figure 15). Desert varnish has developed on the surface of the
granite at a number of locations. This represents the deposition of clay, iron and
manganese oxide on the surface of the rock. These components are thought to
have been transported by the wind to these surfaces.
Figure 10. Tent blisters form where exfoliation sheets buckle up leaving a cavity under
the sheet (drawing from Peterson 1988, p 32).
16
Figure 11. Joints in this area develop in part due to gradual diminishing of overburden
pressure as overlying rocks are eroded away. Preferential weathering along joints then
produces slots in the rock and joint-bounded boulders (drawing by Petersen 1988, p.
34).
Figure 12. Weathering along joints produce spheroidal weathered clasts, corestones,
tors, castle tors and rock pedestals (drawing from Petersen 1988).
17
Figure 13. Rock pedestals develop by mineralization and cementation of the granitic
surface by the addition of calcite, silica and iron oxide during the weathering process.
Differential weathering then removes the less cemented materials below the surface
zone and along joints to produce the pedestals or cap. (Drawing from Petersen 1988, p.
40).
Figure 14. Rock Doughnuts are the result of differential erosion of the surface of the
granite (drawing from Petersen 1988, p.46).
18
Figure 15. Differential weathering of rock surfaces sometimes produces small holes;
these features have been referred to as honeycomb, taphoni (or, tafoni), rock lattice or
cavernous weathering patterns (drawing from Petersen 1988, p. 42).
Stop 14: Half way up Enchanted Rock:
Half way up Enchanted Rock the trail crosses dikes that were produced by late
stage magmatism. These dikes formed during the final cooling process by
magma under great pressure being injected into the granite that had already
formed. Two types of dikes are present. These are: aplite (fine grained dikes)
and pegmatite (coarse grained) dikes. Xenoliths are present in the granite and
are formed by fragments of the country rock (Packsaddle Schist) detaching and
settled into the pluton as the magma cooled and moved up through the country
rock.
Stop 15: Highest point on Enchanted Rock:
Approaching the summit of Enchanted rock, rill-like channels can be seen that
developed on the surface of the granite by water running down and producing
parallel stream channels. At the summit there are large depressions referred to
as weathering pits when dry and vernal pools when filled with water (Figure 16).
These pits form when water collects in a depression and causes accelerated
weathering of the minerals present. Wind then deflates the weathering products
(such as clays minerals, feldspars and quartz) from the pits to produce and
deepen these depressions. Differential weathering of the surface minerals in the
granite produces small depressions that represent a form of micro-karst.
Crystalline micro-relief develops on the exfoliation surface when feldspar grains
weather more rapidly than more resistant minerals such as quartz to produce a
rough surface texture.
19
Figure 16. Weathering pits develop in low areas where water collects on the surface and
weathers the minerals present. When the area dries wind then deflates the loose grains.
Rain from the next storm fills the low areas again with water to begin the next cycle of
weathering (drawing from Petersen 1988, p. 45).
Stop 16: Caves on the northeast side of
Enchanted Rock:
On the north side of Enchanted Rock caves have developed along the edge of
an exfoliation surface. The caves have developed as the result of exfoliation
sheets sliding down the exfoliation surface and stacking up on top of each other
leaving space between the sheets as holes for people to crawl through. Blistering
of the exfoliation sheets to form tent blisters can be found in the area.
Figure 17. Cave development is the result of exfoliation sheets sliding off the dome and
collecting in this location (Petersen 1988, p. 25).
20
That concludes our visit to Enchanted Rock State Natural Area. We hope
you enjoyed the geologic part of the field trip!
Stop 17: Coopers Old Time Bar-B-Que in Llano,
Texas:
Lunch will be at Cooper’s Old Time Barbeque in Llano. It is consistently rated as
one of the best barbeque restaurants in Texas by Texas Monthly and other
periodicals.
References
Ewing, Thomas E., 1991 The Tectonic Framework of Texas: Text of accompany
“The Tectonic Map of Texas”: Bureau of Economic Geology Austin.
http://www.homefacts.com/weather/Texas/Llano-County/Llano.html
McCrary, Joseph Mark, 2003, Sequence Stratigraphy of the Marble Falls
Limestone, Pedernales Falls State Park Blanco County, Central Texas: Master
Thesis, Stephen F. Austin State University, p. 130.
Petersen, James F., 1988, Enchanted Rock State Natural Area: A Guide to the
Landforms: Terra Cognita Press, San Marcos, Texas, p. 56.
21

Geology of Pedernales Falls State Park and

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