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Home , Llano Uplift

Fractures Caused by North-South Compression, Eastern Llano Uplift, Central ; A Field Guide

David Amsbury, Russell Hickerson, and Walter Haenggi

Gulf Coast Association of Geological Societies, Austin, Texas, October 8, 1994 The field trip leaders gratefully acknowledge support for this trip from: Conoco Inc., Finding Functional Excellence,for defraying costs for refreshments; and Union Texas Petroleum for subsidizing costs of printing the guidebook. 1

Preface

The leaders of the field trip want you, the participants, to consider three ideas:

The fracturing that shattered the crust during mid- Pennsylvanian time may have been strike-slip, not extensional;

Compression responsible for the fracturing may have been oriented north-south, not east-west; and

The implications of these two ideas include hypotheses about plate- tectonic history and predictions about the distribution of rock bodies on the surface and within the subsurface, that can be tested by future observation.

We will visit a handful of exposures where fracture patterns are consistent with strike-slip movement in response to north-south compression. Our main "evidence" consists of

1) regional patterns of mapped faults plus

2) very local features such as sub-horizontal slickensides and mullions, and diamond-shaped sets of vertical fractures.

Neither type of evidence is conclusive, only suggestive. We believe that if you look at the evidence with an open mind, you will find ways to test our ideas, to negate them, corroborate them, or extend them in ways we have not envisioned. 2

Contents

PREFACE p. 1

CONTENTS p. 2

INTRODUCTION p. 3 Figure 1 Regional outline map p. 5

RQADLOG p. 6

STOP1- HIGHWAY 281 OVERLOOK p. 8 Figure 2 Stratigraphic section p. 9 Figure 3 Marble Falls fault block p. 10

STOP2 -DOWQUARRY p. 11 Figure 4a Map of the quarry p. 12 Figure 4b, c Sketches of flower structures . p. 12

LUNCH -LONGHORNCAVERN STATEPARK p. 14

STOP3 -HOOVERPOINTOVERLOOK p. 15 Figure 5 Photos of fractures, Hoover Point p. 16

STOP4 -LEFT ABUTMENT OF WIRTZ DAM p. 18 Figure 6 Sketch, location map p. 19 Figure 7 Photo, fractured xenolith; diamond in aplite p. 20

OPTIONALSTOP -MARBLE FALLS FAULT p. 21 Figure 8 Fractures in Marble Falls beds p. 22 Figure 9 Jolly map p. 23

STOP5 -RIGHTABUTMENTOFWIRTZDAM p. 24 Figure 10 Photo, right steppers p. 25 Figure 11 Photo, displaced pegmatite dike p. 26 - STOP6 HIGHWAY 281 WRAPUP p. 27 Figure 12 Plate tectonic summary p. 28

REFERENCES p. 29 3

Introduction

The conventional interpretation of the Ouachita belt - before and after plate tectonic theory became generally accepted - assumes primary westward movement of the orogen (present coordinates) in Central Texas (Flawn, et al., 1961; Viele and Thomas, 1989, Fig. 1; Ewing,

1991). Pindell (1985, Figure 2) and Sherbet and Cebull (1987, Figure 3D) proposed northwestward plate movement parallel to the axis of the San Marcos Arch. The kinematic scheme of Sacks and Secor (1990) called for rotation of the compression direction from north- south during the "middle " to east-west in the Permian.

The plate-tectonic implications of westward orogenic movement in Central Texas had always struck Haenggi and Amsbury as odd; the conventional explanation requires:

1) a northward shove by an impacting continent (or microplate) that bypassed Central

Texas to create the Ouachita Mountains during the Late ;

2) a southward movement into the present Basin to disengage the impactor;

3) a westward shove by the mass during the mid-Pennsylvanian to form the Ouachita

System alongthe western side of the East Texas Basin;

4) eastward movement to disengagethe mass again, create such extension faulting as had

not been created already by flexure into the Ouachita foredeep during east-west

compression; and finally

5) southward movement of the mass out of the region to allow subsidence of the Ouachita

fold belt below sea level.

When Llanoria was in vogue (Miser, 1921; Schuchert, 1930; Dunbar, 1949, Figs. 74c, 154, p. 252) differential vertical uplift of different parts of the continent could plausibly cause gravity sliding of material into the geosyncline at different times. A mechanism that could cause later subsidence of Llanoria (and Appalachia) beneath the deep oceans was recognizedto be a problem. But after plate-tectonic theory solved that conundrum, a continent-sized plate wobbling about like a cam on an automobile camshaft was still a little difficult to envision. The lack of a neat fit of the rocks and structures to the ruling paradigm was puzzling. 4

- Haenggi was tasked in 1989 with solving a feedstock problem erratic silica in Ellenburger that was being burned to dololime at the Brownlee Ranch Quarry for Dow Chemical

Company. He involved Amsbury in early 1990 because of previous experience with the rocks in the region. The silica problem was seen first as a chert problem, either as hydrothermal chert along fracture patterns (Haenggi), or detrital chert grains in early sinks

(Amsbury). Preliminary field work demonstrated that neither was correct - the chert is early-diagenetic in peritidal dolomite that overlies the pure dolomite intended to be produced.

The peritidal chert was dropped down into the underlying material along complex fracture zones

(not karst ).

Detailed mapping showed a complex pattern of fractures, but a pattern. Flower structures, and subhorizontal slickensides and mullions, were discovered on the overwhelmingly near-vertical fractures. A new look at the 1:250,000 scale geological maps (Barnes, 1976, 1981) resulted in the interpretation that the dominant mid-Pennsylvanian fracture pattern throughout the

Llano Uplift (Figure 1) is compatible with strike-slip faulting. Haenggi believed that north- south compression was most probable, but at the time the 1991 GCAGS Abstract was due

(Amsbury and Haenggi, 1991) Amsbury still thought that east-west compression would serve.

By the time of the meeting, both authors were in agreement that north-south compression is much more likely (Amsbury and Haenggi, 1993).

Hickerson was discussing thesis topics with W. R. Muehlberger, who had reviewed the

Geological Society article. They realized that detailed examination of the fracture patterns at other in the eastern Llano Uplift could support or falsify the hypothesis of strike-slip faulting. If the hypothesis were supported, further study might provide unequivocable evidence of the dominant direction of compression. Hickerson's work, as demonstrated during the field trip, did support both the strike-slip hypothesis and probable north-south compression.

What we have not done is study the vast remaining reaches of the Uplift. There are many more theses to be written on this subject, and many more pleasant Spring and Fall weekend trips to be experienced. 5

1)Fault patterninthe outcropping pre-middlePennsylvanian rocks of the Llano uplift, central Texas. Solid lines are faults shown on Barnes (1976, 1981); dotted lines are possible faults inferred from patterns. Thepattern previously was assumed to reflect foreland normal faulting, either 1) inresponse to "relaxation" of the Ouachita eastward compression, or 2) byforeland downwarpingduringthe ; but the patterniscompatible withpervasive strike-slip faulting,mostly right-lateral,during the orogeny. 6

Road Log

Begins at Stop 1 - Roadside park overlook on the west side of U. S.281, south of Marble Falls

Drive north on U. S. 281

0.6 0.6 Pass through Marble Falls

5.8 6.4 Turn right into Dean Word ("Dow Brownlee Ranch") Quarry.

1.4 7.8 After Stop 2 in quarry, turn right (north) onto U. S. 281.

2.7 10.5 Turn left (west) on Park Road 4 toward Longhorn Cavern State Park.

5.9 16.4 Turn left into Longhorn Cavern State Park entrance; lunch.

0.5 16.9 Leave Park, turn left (west) onto Park road 4.

1.7 18.6 Overlook from the top of Backbone Ridge.

0.2 18.8 Turn left onto Park Road 4 again.

0.7 19.5 Turn left (south) on Ranch Road 2342 toward Kingsland.

4.2 23.7 Turn left (east) on Ranch Road 1431 toward Marble Falls.

1.0 24.7 Stop 3 - Hoover Point road cut. Park on right.

0.2 24.9 Leave parking lot; continue east toward Marble Falls.

6.9 31.8 Turn right on road marked with a (small) sign to Wirtz Dam. Look for large

power transmission towers as a landmark.

2.2 34.0 Follow curve of road to left.

0.7 34.7 Just past the power substation,turn left and proceed down the hill past the

LCRA-NIORB sign. Park near the outcrop for Stop 4. 0.2 34.9 Drive up the road out of the dam area.

0.1 35.0 Turn right on road at the top of the hill.

0.4 35.4 Veer right.

2.5 37.9 Turn right (east) on Ranch Road 1431 toward Marble Falls. 7

2.1 40.0 Marble Falls fault (Marble Falls against Town Mountain Granite) just

east of the Granite View Roadside Park. Time and weather permitting we will

make a brief stop at this roadside park to discuss Jolly's mapping of fractures in

the granite a quarter-mile north of here, and Hickerson's analysis of the

fractures in the Marble Falls along Ranch Road 1431.

1.2 41.2 Intersection of RR 1431 and U.S. 281 in Marble Falls. Turn right (south) on U.

S. 281,and cross the bridge to the traffic light.

1.0 42.2 Turn right on Ranch Road 2147.

4.1 46.3 Turn right at sign to Boat Ramp.

0.4 46.7 Enter Cottonwood Resource Area; look for a fence cut by a pedestriangate near a

parking area. Walk down to river for Stop 5.

0.4 47.1 Exit Resource area, turn left (east) on Ranch Road 2147. 4.1 51.2 Turn right (south) on U. S. 281.

0.2 51.4 Turn right into roadside park for Stop 6, wrapup. 8

- Stop 1 Highway 281 Overlook

The rest stop on the west side of U.S. 281,south of Marble Falls,provides enough elevation above for a view of the landscape. For orientation with a geological map (e.g., Barnes,

1981, 1982) the granite quarry in Town Mountain Granite can be seen across the Colorado

River at about N4OW. We will travel north-northeast to the ridgetop, traverse along a ridge to the northwest, return to Marble Falls past the granite quarry, and then visit the south abutment of Alvin Wirtz Dam to our west.

Ridges to the north and west are held up by Cambro- carbonates,more resistant than granite and metamorphic rocks in the paleoclimates of Central Texas. Paradoxically, the early

Paleozoic carbonates occupy down-dropped fault blocks, so that lowlands between ridges have had a kilometer or so of rocks removed (Figure 2; Barnes and Bell, 1972; Barnes and

Cloud, 1972), plus enough material to create the present topography.

The ridge north of Marble Falls consists of the Marble Falls Block (Figure 3), merging to the north-northwest into Backbone Ridge. Ridgetop rocks are dominantly Ellenburger dolomite.

Packsaddle Mountain, composed of Upper ,is prominent to the west. Late

Cambrian and early Ordovician shallow-water rocks covered the Llano region in a thick blanket, the top of which remained essentially at or slightly above sea level from the middle Ordovician through the Mississippian (Barnes and Cloud, 1972).

Flat-topped ridges to the northeast are composed of early Cretaceous carbonate rocks, which at one time completely covered the Llano Uplift. The present landscape is a composite of Hill's

(1901, p. 363-367) Wichita Paleoplain, whose hills and valleys were gradually covered during the basal Cretaceous transgression, and late Tertiary/Pleistocene exumation and of the old surface. Traces of a mid-Pleistocene landscape,mantled by thick caliche deposits, can be found throughout the Llano Uplift 50 to 150 feet above the streambeds. The combination of pre-middle Cretaceous weathering and middle Pleistocene soil formation explains the near- absence of extensive,naturally clean outcrops in the Llano region; of course, outcrops here are better than in most parts of Texas! 9

Figure 2. Stratigraphic column of units discussed in the text in the Llano Uplift. Shaded bars show theparts of the section studied in the field. 10

3)The Marble Falls fault block (between arrows), at the eastern edge of outcropping pre-middle Pennsylvanian rocks of the Llano uplift. pC - Precambrian (Town Mountain Granite), Crh - Hickory , Cm - Cambrian marine strata (including San Saba Formation), Ot - Tanyard Formation undivided,Ott - Threadgill Member, Ots- Staendebach Member, Og - Gorman Formation, Oh - Honeycutt Formation, Cmf - Marble Falls Limestone, and Csw - Smithwick . Compiled on a 1:24,000 USGS topographic base from Cloud and Barnes (1946, Figure 7), Barnes and Bell (1977, Figure 20, Plate 7), Barnes (1981,1982),andfield mapping by Amsbury and Haenggiin1990and1991. 11

Stop 2 - Brownlee Ranch Quarry

The Brownlee Ranch Dolomite Quarry is excavated in a pocket of pure dolomite of the Threadgill

Member of the Tanyard Formation, mapped by Virgil Barnes during World War II (Cloud and

Barnes, 1946; Barnes and Bell, 1977). Dolomite is quarried by the Dean Word Company,

crushed and screened,then burned to dololime by the ChemLime Company. The dololime is shipped by rail to Freeport, Texas, where Dow Chemical Company utilizes it as a neutralizing base in their magnesium-from-seawater process.

Marble Falls Fault Block - The quarry (Figure 4a) is in gently-dipping beds at the crest of a complex fault slice centered on Marble Falls (Figure 3). The eastern bounding fault at

Mormon Mills has a throw of about 500 meters (Marble Falls Limestone and Smithwick Shale against Tanyard Dolomite). The western boundary fault has a throw of at least 1 km at Marble

Falls (Smithwick Shale and Marble Falls Limestone against Town Mountain Granite) but to the north splinters into several left-stepping blocks. Some splinter faults die out northward

within a few hundred meters. The western bounding fault of the dominant splinter block some

10 km north of Marble Falls (not the same fault trace as that on Ranch Road 1431 west of

Marble Falls - see Figure 3) has only 300-350 meters of throw. Much of the differential displacement clearly was accommodated by folding of strata within the fault block, although cross-faults are abundant.

Fractures Near the Quarry - We will concentrate on two portions of the quarry (Figure

4): the northwest corner north of the entrance near the conveyor belt; and the east wall near the southeastern corner. The Threadgill Member and the overlying Staendebach Member of the

Tanyard Formation are shattered by fractures in sets that trend N2OW, NBOE, N4O-50E, and

N3O-40W. The N2OW and NBOE sets are most prominent on 1940 and current aerial photographs; these sets bounded the quarry on the west and north in 1990-91. Randomly oriented blocks of chert-bearing Staendebach dolomite were dropped into breccia zones along northeast- and east-trending fracture sets ("collapse zones") within Threadgill dolomite. No younger material was structurally incorporated into the breccia. 12

4a) Brownlee Ranch dolomite quarry (labled "Dow Quarry" on Figure 3), 5.8 miles north of Marble Falls, Burnet County. The short bars are fracture (joint or fault) strike directions.

4. Near-vertical fractures in Threadgill dolomite, northwestern corner of the Brownlee Ranch Quarry. Sketches from photographs. b) View westward of the N80E set. c) View northward of a flower structure formed by the N20W fracture set. 13

Where fractures are well exposed in solid rock between breccia zones, little vertical

displacement could be discerned across most individual fractures. Nearly horizontal to

shallowly dipping slickensides and mullions occur on some faces. Some fractures of each set

display "flower structures" within the quarry, cutting dolomite into vertical blocks (Figures

4b, 4c). In pasture outcrops these features can be mistaken easily for vertically dipping beds 5

cm to 1 m thick. We believe that the N2OW/NBOE and the N4O-50E/N4O-50W fracture sets

constitute pairs that formed at different times under different stress conditions, but could not

determine unequivocable temporal relationships.

Post-fracture Diagenesis - Saddle dolomite druse in single to multiple layers coats open fracture faces of all sets. No vertical or steeply dipping stylolites were found, although

stylolites parallel to bedding are abundant.

Two distinctly different types of cave deposits occur in the quarry (Amsbury and Haenggi,

1992), in addition to the breccias composed of Staendebach and Threadgill dolomite that are

present in many fault zones. One type of cave fill consists of laminated to cross-bedded, coarse

dolomite sand and angular chert grains; it postdates dolomite druse that, in turn, coats tectonic fracture voids. The detrital-dolomite cave fill is much younger than the widespread post-

Ellenburger, pre-middle Paleozoic exposure (Barnes and Cloud, 1972) and the resulting karst

known from subsurface occurrences (Kerans, 1990). The detrital dolomite occurs in mid-

Pennsylvanian fracture voids that were exposed to the surface after erosion of whatever upper

and middle Paleozoic rocks may have been present,in addition to 300+ meters of the Gorman

and Honeycutt Formations.

The second type of cave fill is composed of basal Cretaceous (Hensel/Gillespie) subarkose and

paleocaliche; this fill clearly formed during the mid-Cretaceous transgression into the Colorado

River valley, as local stream base level rose prior to of the peritidal Glen Rose

Formation. 14

Lunch Stop - Longhorn Cavern State Park

Longhom Cavern is open for tours for about 1/3 mile to the north from the Sam Bass Entrance,

at the visitor's center. The nearly level surface here is the top of Backbone Ridge, planed off

prior to deposition of basal Cretaceous rocks. The cavern formed in limestone and dolomite of

the Gorman Formation (mid-Ellenburger Group), presumably after several hundred feet of

Honeycut (uppermost Ellenburger) were removed by erosion.

The original cave was the result of solution, of course, but much modification by frequent flash floods is evident (Matthews, 1963). After all, the base of Backbone Ridge is more than 500 feet

below us. Some flowstone is present within the cave, much of it eroded by stream action.

Matthews' map (his Figure 1) suggests at least partial control of dissolution and erosion by fracture sets trending NW/SE, NE/SW, and maybe N/S (cfr. Jolly's map, Figure 9). A fracture-mapping study within the cavern might prove rewarding.

Much of the original cave fill was removed before any could be salvaged, but no fossils earlier than late Pleistocene (less than 14,000 b.p. or so) are known (Lundelius, 1967). The

view of the outcrops near the entrance enhances one's wonder at the accomplishments of Dr.

Barnes in mapping the stratigraphy and structure of this countryside. 15

- Stop 3 Hoover Point

Hoover Point overlooks Lake LBJ from the southern end of Backbone Ridge. The Ridge consists of a wedge of Upper Cambrian through Lower Ordovician rocks, faulted down into Precambrian granite and metamorphics. The early Paleozoic rocks are tilted northward, preserving a more- or-less complete outcrop section for the eastern Llano Uplift (Cloud and Barnes, 1946; Barnes,

1981). The south-facing road cut exposes Lion Mountain greensand containing concretions of trilobite hash, and Welge Sandstone (Figure sa), overlain by Cap Mountain Limestone.

Major faults bound the sides of the sedimentary-rock wedge, so it is not surprising to find this thin end of the wedge fractured. Not all fractures show displacement. Those that do, display left-slip or right-slip components, mostly by slickensides or mullions (Figure sb). Most fault planes are steep,having dips from vertical to 65 degrees. Exposed slickensides have plunges from 8 to 65 degrees; most are less than 45 degrees. Slickensides indicate both left-lateral and right-lateral movement (Tables 1 and 2); right-lateral faults have slickensides bearing about 190 degrees, parallel to one of the dominant fault trends within the uplift (N10E). 16

Figure 5a. Photograph ofa short section ofthe Hoover Point roadcut wallshowing one ofthe near vertical faults. The wall shows Lion Mountain (LM) and Welge Sandstone (WS) members. The fault strikes N anddips 68°E at the location marked bythe star. The view is to the north and is about 10 m high.

Figure 5b. Photograph ofslickensides from the south side of the Hoover Point roadcut. The slickensides trend S2O°W and plunge 22°. See rock hammer near center of photograph for scale. 17

Fault Slickenside Location Strike Dip Bearing Plunge Motion sense NSW 90 175 Left Slip NB2E 90 82 12 Left Slip 10 N34E 90 214 11 Left Slip 21 NI2W 78E 168 28 Left Slip

4oA Q N6OE 65W 240 65 Left Slip

Table 1. Listing of faults showing slickensides with a component of left slip.

Fault Slickenside Location Strike Dip Bearing Plunge Motion sense 35 NBE 90 188 45 Right Slip 52 70W 180 50 Right Slip 56 NIOE 78E 190 45 Right Slip 58 N2OE 85E 200 45 Right Slip 61 N2OE 90 200 30 Right Slip 65 N2OE 70E 200 40 Right Slip Table 2. Listing of faults showing slickensides with a component right slip. 18

- Stop 4 North Side of Colorado River, Wirtz Dam

The Town Mountain Granite below Wirtz Dam (Figure 6) has been scoured of overburden by former floods on the Colorado River as well as by construction work. Wirtz Dam is 600 meters or so west of the Marble Falls fault Gust off the lower edge of figure 6). The outcrop north of the river contains several large fracture zones, up to 75 cm wide, composed of 2-3 mm angular fragments of and in a microcrystalline matrix. The zones are vertical and trend

NSOE, which is parallel to the trace of the Marble Falls Fault downstream (Barnes, 1982). An outcrop 800 meters upriver (west of the dam) shows no fractures at all, suggesting that the fracturing below the dam is related to the Marble Falls Fault.

Markers that provide a sense of displacement are scarce north of the river and downstream from the dam. A fractured xenolith (Figure 7a) shows right-slip displacement of a few millimeters on a NSOE fracture. A vertical dike is cut by a N3OE fracture, with about 13 cm left slip. And two vertical N2SW fractures display horizontal slickensides having an indeterminate sense of slip. 19

6 ) Locations of significant structural features in Town Mountain Granite outcrops below Wirtz Dam. 20

Figure 7b. On top is a photograph of the fracture pattern in an aplite dike in the Town Mountain Granite near Wirtz Dam, see 3 in Figure 6. The compass is pointed north. The bottom figure shows how the strike of the acute bisector ofthe faults inthe photograph wasdetermined.

Figure 7a. Photograph ofa fractured xenolith in the TownMountain granite. The location is on the north side of the Colorado River near the Wirtz Dam. The fracture is N50°E and vertical. The xenolith showsright lateral separation of a few mm. Nikon lenscap = 5 cm. 21

Optional Stop - Marble Falls Fault

The Marble Falls Fault has a throw of at least a kilometer at the Ranch Road 1431 crossing, based on displacement of Smithwick/Marble Falls strata against Town Mountain granite. The fault trace strikes N37E here. The actual fracture zone is buried by alluvium, but Barnes1

(1982) mapping indicates a vertical fault because the fault trace is a straight line across stream valleys and hills. Fault drag appears to show normal motion in the south-facing road cut

(Figure 8a). But 11 different limestone beds have a strike of NS3E, 16 degrees or so east of the vertical fault trace (Figure 8b), which is compatible with left-oblique dip slip. Some left- slip faults at Hoover Point have a strike of N34E.

Along Buckhorn Creek on the Snifflett Ranch about 1/4 to 1/2 mile north of Ranch Road 1431,

W. T. Jolly mapped fractures in Town Mountain Granite west of the Marble Falls Fault as a class project. The NW/SE and NE/SW fracture sets (Figure 9) are entirely compatible with north- south compression. There are even offsets suggesting limited strike-slip motion, and some north-south fractures that suggest incipient through-going fractures parallel to the Marble Falls Fault. 22

Figure 8a.Photo of Marble Fallslimestone beds on RM 1431on the western edge of thecity ofMarble Falls, Texas, the view is facing north. Thebeds show tilt dueto fault drag from the Marble Falls fault, whichis justto theleft ofthe photo. The average strike ofthe beds is N53°E and the dips range from 26° E to 45° E.

Figure 8b. Rose diagram ofthe strike ofthe Marble Falls fault and strikes of limestone beds near the fault. The diagram shows that the strike of the beds arenot parallel to the strike of the falult, which would be expected if the fault were purely dip slip. Shifflet Creek, Buckhorn along1431. Road GraniteRanch of Mountain north Town and inFault patternsFalls Marble Fracture the of

9. west FigureRanch, 24

Stop 5 - South Side of Colorado River, Wirtz Dam

Outcrops on the south side of the Colorado River below Wirtz Dam lack the large fracture zones seen on the north side, but instead contain numerous small fracture zones. Fractures are

vertical to near-vertical; many trend NSOE, which is parallel to the trace of the Marble Falls

Fault downstream (Barnes, 1982). En echelon steps showing a right-lateral sense of motion occur on a few fracture faces (Figure 10). Pegmatite and aplite dikes are common; some display offsets by faulting (Figure 11).

Aplite dikes are uniformly fine-grained, are as thick as 20 cm, and do not all trend in the same direction. These provide local homogeneous environments. Several show a characteristic diamond fracture pattern (Figure 7b) that can be used to indicate the stress orientations at the

time of fracture. Five different areas across the outcrop were measured to determine the maximum principal stress direction. The directions shown are all within 5 degrees of north

(Table 3).

Location number 26 8 -j direction 60 N4W 75 N3E 58 N3E 69 N4E 70 NSE 25

Figure 10. Photo ofa fault surface inthe TownMountain Granite near point 4inFigure 6, showing en-echelon steps. The steps show a right lateral sense of motion. The fault strikes N25°E and is vertical. 26

Figure 11. Photo of displaced pegmatite dike in the Town Mountain granite. seeDinFigure 6. Thedike is located on thesouth side of the Colorado River near the Wirtz Dam. The dike in nearly vertical and the apparent separation is 1 m right lateral. The compass is pointing North. 27

- Stop 6 Marble Falls Overlook

We think that the evidence for North-South compression at the time of faulting is pretty impressive. Some of these faults have more than a kilometer of throw, they run for tens of miles, and there are a lot of them. They formed during a short time within the mid-

Pennsylvanian, late Morrow through Atoka, perhaps (Grayson and Trice, 1988; Freeman and

Wilde, 1964). Map patterns suggest near-vertical faulting, which gave us the idea of strike- slip movement in the first place.

Plate tectonic implications of the north-south compression are interesting. The main outcrop belt of the Ouachita Mountains (Figure 12) was formed by north-south compression, though over a much longer time than the fracturing in Central Texas. If our hypothesis is correct, we would not expect to find large west-directed nappes along the buried Ouachita Trendbetwen San

Antonio and . Instead, we would expect northwest-directed Ouachita-facies splays, and splinters of blocks, extending into the Fort Worth Basin.

Paleotopographic speculations are interesting, too. Great slices of early Paleozoic rocks must have pushed into the Strawn Basin north of the Llano Uplift, where their disruptive effect on sedimentation is evident. There must have been some dandy earthquakes for several million years. Within the Uplift, the upthrown blocks would have towered over today's "mountains" by several hundred meters or more. At some time between mid-Pennsylvanian and mid-

Cretaceous, these topographic eminences were not only eroded away, but turned into today's valleys! Some of the rocks went west to add to the Dockum, and some went east to add to the

Cotton Valleyand Hosston.

As Mark Twain said in "Life On The Mississippi": "That's what Ilike about science. You get such a wholesale return of conjecture from such a trifling investment in fact!" 28

Figure 12. Locationof the Ouachita orogenic front taken from Viele and Thomas, 1989. Arrow 1shows theproposed direction ofcompression by previous workers. Arrow 2 shows the maximum compression direction proposed by this thesis and by Amsbury and Haenggi, 1993. 29

References

Amsbury, D. L., and Haenggi, W. T., 1991, ABS., Middle Pennsylvanian strike-slip faulting on

the eastern edge of the Llano Uplift, Central Texas:Gulf Coast Association of Geological

Societies Transactions vol. 41,p. 11.

Amsbury, D. L, and Haenggi, W. T., 1992, Detrital dolomite in tectonic fracture voids, Burnet

County, Texas: Abstracts with programs, South-central Section, Geological Society of

America Meeting, Houston, Texas February 24-25, 1992, p. 1.

Amsbury, D. L, and Haenggi, W. T., 1993, Middle Pennsylvanian strike-slip faulting in the

Llano Uplift, Central Texas: Houston Geological Society Bulletin, June, 1993, p. 20-58

reprinted in the Bulletin of the Geological Society, vol. XXXIV, September,

1993, p. 9-16.

Barnes, V. E., 1976, Geological Atlas of Texas, Brownwood Sheet: Bureau of Economic Geology

The University of Texas at Austin, map + 21 p. text.

Barnes, V. E., 1981, Geological Atlas of Texas,Llano Sheet: Bureau of Economic Geology,The

University of Texas at Austin, map + 15 p. Text.

Barnes, V. E., 1982, Geology of the Marble Falls Quadrangle, Burnet and Lampasas Counties,

Texas: Bureau of Economic Geology,The University of Texas at Austin,Geologic

Quadrangle Map No. 48, map + 15 p. text.

Barnes, V. E.9 and Bell, W. C, 1977, TheMoore Hollow Group of Central Texas:Bureau of Economic Geology, The University of Texas at Austin, Report of Investigations No. 88,

169 p.

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Economic Geology, The University of Texas at Austin, Guidebook No. 13, p. 30-37. 30

Bell, W. C, and Barnes, V. E., 1972, Cambrian history, Llano region, in Barnes, V. E., Bell, W.

C., Clabaugh,S. E., Cloud, P. E., Jr., McGehee, R. V., Rodda, P.U., and Young,Keith,

Geology of the Llano region and Austin area: Bureau of Economic Geology, The University

of Texas at Austin, Guidebook No. 13, p. 24-29.

Cloud, P. E., and Barnes, V. E., 1946, The Ellenburger Group of Central Texas: University of

Texas Publication No. 4621, 473 p.

Dunbar, C. 0., 1949, Historical Geology: New York, John Wiley and Sons, 573 p.

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