Fractures Caused by North-South Compression, Eastern Llano Uplift, Central Texas; 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 Central Texas 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 fault 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 Carboniferous" 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 Mississippian; 2) a southward movement into the present East Texas 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 dolomite 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 Cretaceous karst 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 sinkholes). 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 Houston Geological Society article. They realized that detailed examination of the fracture patterns at other outcrops 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 outcrop patterns. Thepattern previously was assumed to reflect foreland normal faulting, either 1) inresponse to "relaxation" of the Ouachita eastward compression, or 2) byforeland downwarpingduringthe orogeny; 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 granite 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 Limestone 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 Colorado River 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.