COLLINS 77

FRACTURE ZONES BETWEEN OVERLAPPING EN ECHELON FAULT STRANDS: OUTCROP ANALOGS WITHIN THE

BALCONES FAULT ZONE, CENTRAL TEXAS

Edward W. Collins Bureau of Economic Geology, The University of Texas at Austin

Box X, University Station, Austin, Texas 78713-7508

ABSTRACT

This study describes two types of fault overlap within the Balcones Fault Zone: a relay ramp between overlapping master faults dipping in the same direction and a structural bridge between overlapping faults dipping in opposite directions. Cretaceous limestone outcrops within these structural zones have been described for this study, and the outcrops reveal a variety of fracture characteristics, including fracture type, geometry, spacing, and connectivity, that are important in understanding the framework of fractured strata. Areas between overlapping normal faults con­tain abundant fractures and therefore are potential targets for hydrocarbons in fractured reservoirs, as well as potential areas for preferential ground-water recharge and flow in fractured aquifers.

The fault overlap areas are up to 0.6 mi (1 km) wide, and the en echelon master faults may over­lap by as muchas 1.2 mi (2 km). Strata within these fault overlap zones are cut by joints and by abundant small-displacement normal faults, commonly having throws of less than 1.6 ft (0.5 m). These deformed areas consist of a mosaic of intermingled fracture sets that have multiple strikes; thus, fracture connectivity is locally high.

Fracture spacing is variable within fault overlap areas. Sorne individual beds and multiple-bed packages are more fractured than other beds of similar thickness and composition. Also, fractures do not have uniform spacing within any given unit. Spacing of single small faults and fracture swarms is commonly between 6.5 and 150 ft (2 and 46 m) along traverses perpendicular and oblique to the master overlapping faults. Swarms of small faults, commonly as muchas 20ft (6 m) wide, may contain as many as 15 faults. Joint swarms that are as much as 40 ft (12 m) wide have fracture spacings of 2 to 5 ft (0.6 to 1.5 m).

INTRODUCTION

Location and characterization of high-density frac­ture zones are important to petroleum geologists exploring for fractured hydrocarbon reservoirs. Hydrologists, environmental geologists, and engineer~ ing geologists also study these high-density fracture zones in evaluating ground-water recharge and flow in fractured aquifers or in evaluating the effects of urban development and construction within ground-water recharge areas. Exploration for and characterization of hydrocarbon-bearing reservoirs containing high-den­sity fractures have increased in Texas (Finley and oth­ers, 1990) and elsewhere in the United States (Schmoker and others, 1992) since the mid-1980's, when horizontal drilling technology created new exploration opportunities. With the development of horizontal drilling technology, a better understanding of fracture systems has been sought by petroleum geol­ogists (Corbett and others 1987, 1991a, b; Reaser and Collins, 1988; Wiltschko and others, 1991; Collins and others, 1992; Friedman and Wiltschko, 1992). Hydrologists and geologists are concerned with ground-water recharge and flow in fractured recharge zones and aquifers in areas of steadily increasing urban development and construction such as the Edwards aquifer recharge zone in Central Texas (Woodruff and Slade, 1984; Woodruff and others, 1985; Maclay and Small, 1986; Collins, 1987; Yelderman, 1987; Senger and

others, 1990; Johns, 1991). Sorne sites for critica! facili­ties such as the Superconducting Super Collider in North-Central Texas also require fracture characteriza­tion (Reaser and Collins, 1988; Collins and others, 1992). Because of this need for more knowledge of frac­ture systems, several fracture investigations have been conducted since the early 1980's on limestones within the Balcones Fault Zone (Fig. 1) and the Cretaceous outcrop belt of Central Texas (Corbett, 1982; Collins, 1987; Corbett and others, 1987, 1991a, b; Reaser and Collins, 1988; Collins and Laubach, 1990; Wiltschko and others, 1991; Collins and others, 1992; Friedman and Wiltschko, 1992). These previous researchers studied fractured Austin Chalk, an important hydrocarbon reservoir in Texas, and fractured Lower Cretaceous limestones that compase the Edwards aquifer, an important source of water for Central Texas. Local structural settings that have been found to contain abundant fractures include anticlines, monoclines, listric normal faults, graben, downward-steepening faults, and synclines on the hanging-wall blocks of nor­mal faults.

One type of structural setting that has not been described in much detail is the overlap area of en eche­Ion normal fault strands. This paper discusses the frac­ture characteristics of a relay ramp between overlap­ping normal faults dipping in the same directíon and a horst bridge between overlapping faults dipping in opposite directions (Fig. 2a and 2b). The high density of

78 GULF COAST ASSOCIATION OF GEOLOGICAL SOCIETIES

TEXAS

Study area

N

~

o 1 o

200 mi 1 1

300 km

GULF OF MEXICO

QAa2130c

Figure 1. Regional setting and location of study areas along the Balcones Fault Zone.

(a)

fractures in these areas is probably due to strains caused by interacting faults and bending of rock (Larsen, 1988; Peacock and Sanderson, 1991). Fracture characteristics, including fracture types, geometries, density, and connectivity within overlap areas of en echelon normal fault strands, are presented in this paper.

METHODS

Interpretations reported in this paper are based on field observations of faults and joints in road-cut, stream-bed, and stream-bank outcrops. Two sites were studied in detail: a site in western Comal County, which consists of fractured Glen Rose Limestone, and a site in northern Ellis County, which consists of frac­tured Austin Chalk. Four road-cut outcrops studied in western Comal County are located along U.S. 281 about 26 mi (42 km) north of San Antonio (Fig. 1). Stream-bed and stream-bank outcrops studied along Brushy Creek in northem Ellis County are west of Sta te Highway 983, which crosses the creek about 8 mi (13 km) northwest of Waxahachie. Photomosaics were used to measure and map fracture characteristics. Fracture spacing was measured along traverses up to 1,640 ft (500 m) long, and fracture occurrences were plotted along horizontal sean lines.

STRUCTURAL SETTING

Normal faults described in this study are part of the

(b) / /

Figure 2. Plan view and block diagram of (a) relay ramp between overlapping master faults dipping in the same direction and (b) horst bridge between overlapping faults dipping in opposite directions.

COLLINS

Balcones Fault Zone (Fig. 1). From Dallas this fault zone extends southward to San Antonio, where it bends west-southwestward toward Del Rio. This exten­sional fault zone partly coincides with the Cretaceous outcrop belt of Central Texas. Cretaceous rocks dip gently east-southeastward into the Gulf Coast Basin. The Balcones Fault Zone also closely follows the trend of the buried Paleozoic Ouachita fold and thrust belt (Flawn and others, 1961). The Comal County study site is within the central part of the Balcones Fault Zone, and the Ellis County study site is at the northem exten­sion of the fault zone.

Most of the movement on the Balcones Fault Zone is thought to have occurred during the late Oligocene or early Miocene (Weeks, 1945). Weeks (1945) interpreted the existence of Cretaceous fossils and limestone and chert pebbles in the lower Tertiary Catahoula and Oakville Formations of the Gulf coastal plain to be evi­dence of fault uplift that exposed Cretaceous source rocks. Fault movement could have resulted from flex­ure along the perimeter of the Gulf of Mexico (Murray, 1961).

RELAY RAMP BETWEEN OVERLAPPING FAULTS DIPPING IN THE SAME

DIRECTION

A relay ramp (Larsen, 1988; Peacock and Sanderson, 1991) is a structure that may form between the tips of two en echelon normal faults dipping in the same

Study

O 20mi

r------r O 30 km

EXPLANATION

_..1J.- Fault (U=upthrown D D=downthrown)

/c Outcrop

O 1 mi

O 1 km

0Aa2132c

Figure 3. Location of outcrops within overlapping en eche­Ion faults in western Comal County. Outcrops a-d corre­spond to cross sections a-d in Figure 4.

79

direction. The ramp connects the hanging-wall and footwall blocks of the faults. Displacement from one fault is transferred across the relay ramp to the other fault. Because relay ramps are strained zones produced by shearing and block rotation during slip along the overlapping faults (Larsen, 1988), they may be cut by additional smaller faults.

Within the Balcones Fault Zone, Grimshaw (1976) and Grimshaw and Woodruff (1986) recognized large (approximately 6-mi-wide [10-km-wide]) ramp struc­tures, which form steps in the throw of the fault zone. This study focuses on smaller sized (up to 0.6-mi-wide [1-km-wide]) ramps, which connect individual fault strands that are commonly between 3.7 and 12.4 mi (6 and 20 km) long. A unique series of road-cut outcrops in westem Comal County provides an oblique, cross­sectional view of the fractures within a ramp that con­nects two en echelon faults (Fig. 3). The large master faults strike N60-65°E and dip 60° to 70° southeastward (Figs. 3 and 4). Precise fault throw within the outcrop­ping Glen Rose Limestone is unknown because of the absence of subsurface data; however, throw is esti­mated to be between 20 and 98 ft (6 and 30 m). The faults overlap about 1.2 mi [2 km] along strike, and the fault tips are separated by a ramp that is up to 0.6 mi (1 km) wide.

Strata adjacent to the master faults are intensely frac­tured, and distinct fracture zones, or halos, have formed. These fracture zones are up to 10 ft (3 m) wide and consist of anastomosing fractures. The fracture zones are better developed on the ramp side of the master faults. The fracture zone adjacent to the north­westem master fault is better developed on the hang­ing-wall block, whereas the zone adjacent to the south­ern master fault is preferentially developed on the footwall block. Striations on the master faults indicate that slip was parallel to the fault plane dip, at S25-30°E (Fig. 5). Strata adjacent to the master faults are gently folded and dip as muchas 10°. At both master faults, faulted beds dip gently into the ramp area. Beds adja­cent to the northwestern master fault dip southeast­ward, and faulted beds at the southeastem master fault dip northwestward. Fracturing is very intense locally within the folded beds.

Within the ramp, small normal faults with throws commonly less than 0.6 ft (0.2 m) are very abundant. Striations are well developed on most fault surfaces, and they aid in the recognition of slip on small faults that only have centimeter- to millimeter-scale displace­ment. Calcite coats most of the fault surfaces. On the downthrown, southeast side of the ramp, a 0.2- to 0.25-mi-wide (0.3- to 0.4-km-wide) graben has formed. This graben probably is a ramp-related structure, and although graben are not associated with all ramps, detailed mapping in other parts of the Balcones Fault Zone (Garner and others, 1976) indicates ramps with graben are present elsewhere.

Small faults within and adjacent to the ramp have strikes that span the compass (0-180°). Of the two best developed fault sets, one strikes northeastward at N25-700E, similar to the two master faults, and the other strikes northwestward at N30-60°W (Fig. 5). Another fault set striking eastward at N80-l10°E is well devel-

80 GULF COAST ASSOCIATION OF GEOLOGICAL SOCIETIES

(a) South

1) \

(b)

Small faults North

~" Abundant joints and joints 100 m to ; /.,,, ~~~P;¡~P 11\fíf@! ljr}.;'-'-'f'l/: t:\ ,¿Y./ ,_ "---+ section b

'\- Figure 5a

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(d) South Abundant small taults North [$''?,,•·":" ;,1/1;2',_,q·-:-,Z::Y"~~· /fkci[i\dXt'''-'"'·~.! ,. ;i'/!o~ Smallfaults

- - - :..:..:::...._ -J'~ - ,_ ~Y%•>'• ,, '''>1'5i3 , Figure 5e Fault / Figure 51

surface QAa2133c

Figure 4. Cross sections of fractured Cretaceous Glen Rose Limestone showing fracture intensity within overlapping en ech­elon faults. Cross sections a-d correspond to outcrop locations a-d in Figure 3. Locations for collection of fracture data that are presented in equal-area net plots of Figure 5 are indicated as 5a-5f.

oped within the graben adjacent to the ramp. Sorne small east-striking faults also occur within the ramp. The small faults dip mostly between 50° and 70°. Striations on the surfaces of the small faults in all three sets indicate that slip has been parallel to and slightly oblique to the fault dip direction. Rakes on oblique-slip faults are usually greater than 70°.

The extensional slip on the small northeast-striking faults appears to have varied slightly from S30-35°E at the northwest part of the ramp to S10-20°E at the southeast side of the ramp. These small faults dip both northwestward and southeastward. Small northwest­striking faults, which have formed as cross faults within the ramp area, dip northeastward and south­westward. Striations on these faults show that exten­sion was N55-65°E, parallel and slightly oblique to the dip of the fault surfaces. The small east-striking faults are most abundant in the graben, and they dip north­northwest and south-southeast. Striations that are par­allel to and slightly oblique to the dip of these east­striking fault surfaces indicate that extension was southward at Sl0°E to S20°W. The shift in small fault orientations from northeast to east-northeast across the

Figure 5. Lower hemisphere equal-area net plots of fracture geometry data collected at western Comal County study area. (a) Stereographic projection of joint surfaces and potes (dots) to joint planes. (b)-(f) Stereographic projection of fault surfaces, potes (dots) to fault planes, and striations/ lineations (arrows) on fault surfaces. Locations for collec­tion of fracture data are shown in Figure 4.

COLLINS

ramp appears to be gradual. The master antithetic fault of the graben also strikes eastward and dips about 55° toward the north. Fracture style changes abruptly across the ramp and associated graben. Small faults are most abundant in the ramp and at the northern part of the graben, whereas joints are the main type of fracture within the southern part of the graben. These joints strike N65-100°E, similar to many of the small faults. Most joints dip greater than 70° south-southeastward. Many of these joints cut through the entire height of the outcrop, about 16 ft (5 m). Dissolution has widened sorne of the joints, indicating that they have been con­duits for ground-water flow.

HORST BRIDGE BETWEEN EN ECHELON FAULTS DIPPING IN

OPPOSITE DIRECTIONS

Within the Balcones Fault Zone, horst bridges between en echelon faults dipping in opposite direc­tions (Figs. 2b and 6) are not as common as relay ramps between overlapping faults dipping in the same direction. However, outcrops of upper Austin Chalk along the bed and banks of a creek that tran­sects a fault-overlap zone in northern Ellis County enabled the study of a horst type of structural bridge between overlapping faults dipping in opposite direc­tions. Both types of structural settings contain frac­tures with similar characteristics.

At the Ellis County study site, the two en echelon master normal faults strike northeastward at N30-400E. One fault dips about 63° northwestward (Reaser, 1961), and the other fault dips southeastward at an unknown angle. Throw across the faults is as much as 90 ft (27 m), although fault throws near the fault ter­minations at the overlap area are probably less than 90 ft (27 m). Vegetation and stream alluvium cover most

Northwest

Northwest

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Figure 6. Location of study outcrop within overlapping en echelon faults in northern Ellis County and cross section through area. Kau = Austin Chalk, Qal = alluvium.

(a)

(b)

Contour interval

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Figure 7. Lower hemisphere, equal-area net plots of fault geometry data and rose diagram of joint data collected at northem Ellis County study area. (a) Stereographic projec­tion of fault surfaces and potes (dots) to fault surfaces. (b) Contoured stereographic projection of poles to fault sur­faces. (e) Rose diagram of nearly vertical joint azimuths.

82 GULF COAST ASSOCIATION OF GEOLOGICAL SOCIETIES

of the bridge area between the faults. Core and electri­cal logs of borings drilled for studies of the Superconducting Super Collider aided in recognizing the northeastern trace of the southeast-dipping master fault. The horst bridge between the faults is about 0.8 mi (1.3 km) wide. Poor exposures at the master faults prevent precise identification of the master fault tips and the amount of fault overlap. lt is possible that these faults could be underlapped (Larsen, 1988, p. 7) rather than overlapped. In either case the bridge area is a strained zone because of shearing during slip along the bounding offset faults (Larsen, 1988, p. 7).

Small normal faults with throws commonly less than 1.6 ft (0.5 m) and joints are very common within the horst bridge. The bridge area is a mosaic of inter­mingled fault and joint sets having multiple strikes (Fig. 7). Most small faults strike in one of three direc­tions, N40-65°E, N75-100°E, and N5-45°W. A less com­mon set of small faults strikes N10-25°E. Dips are mostly between 50° and 70°. Northeast-striking faults dip southeastward and northwestward, northwest­striking faults dip northeastward and southwestward, and the east-northeast-striking faults dip north-north­westward and south-southeastward. Striations along fault surfaces of each set are nearly parallel to dip, similar to faults in other areas along the Balcones Fault Zone (Collins, 1987; Reaser and Collins, 1988; Collins and others, 1992). Nearly vertical joints also occur, mostly within three sets that strike N30-50°E, N75-100°E, and N1-30°W. Most fracture heights and lengths exceed the outcrop dimensions, although the length of one swarm of severa} faults with a cumula-

(a) .-L 1 1

tive throw of about 3 ft (1 m) was traced for over 130 ft (40 m).

FRACTURE SPACING AND CONNECTIVITY IN OVERLAP AREAS

Areas between overlapping traces of en echelon faults locally contain numerous fractures with variable spacing. Sorne individual beds and multiple-bed pack­ages may be more fractured than other beds, and the fractures do not have uniform spacing within any given unit. Fault overlap areas are a mosaic of individ­ual fractures (small faults and joints) and swarms of dosel y spaced faults and/ or joints. Variations in the spacing and strikes of individual fractures and fracture swarms cause fracture connectivity to be high locally.

A traverse across 1,800 ft (550 m) of Glen Rose Limestone in western Comal County within the ramp between overlapping master faults dipping in the same direction intersected over 130 small faults and many joints (Fig. 8a). Spacing of single small faults and frac­ture swarms is commonly between 8 and 50 ft (2.5 and 15m). Swarms of small faults are as muchas 20ft (6 m) wide and contain as many as 12 faults. Joints domínate the graben associated with the ramp, and spacing between the joints is between 2 and 15 ft (0.6 and 4.5 m).

A sean line along a 1,640-ft (500-m) traverse within the northern Ellis County horst bridge between over­lapping master. faults dipping in opposite directions intersected more than 70 small faults and numerous swarms of closely spaced joints in upper Austin Chalk

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i JOLilt Small tault * Continuat;on of scanline

o o

tOO ft

30 tt

0A<l2137c

Figure 8. Sean lines showing fractures encountered along outcrop traverses. Line a is within Cretaceous Glen Rose Limestone in an overlap area between two en echelon master faults dipping in the same direction. Outcrop locations are shown in Figure 3. Line b is within Cretaceous Austin Chalk in an overlap area between two en echelon faults dipping in opposite directions. Outcrop location is shown in Figure 6.

COLLINS 83

(a)

0/U

(b)

(C)

--------------~-+----------------------------------~~----------~--------------*

*~------~~-+rr--------~~~----------------------

U/0 Large fault (U=upthrown: O=downthrown)

Jornt Small fault * Contrnuation of scanline

o o

100 f1

30 f1

0Aa2'3Bc

Figure 9. Sean lines showing fractures encountered along outcrop traverses in Austin Chalk of Ellis County. Line a is adja­cent to a fault having about 60 ft (18 m) of throw. Lines b and e do not cross large faults or folds and represent fracture inten­sity away from large faults. Modified from Collins and others (1992).

(Fig. 8b ). Spacing of single small fractures and fracture swarms is between 6.5 and 150 ft (2 and 46 m). Swarms of small faults are as muchas 20ft (6 m) wide and con­tainas many as 15 faults. Joint swarms that are as much as 39 ft (12 m) wide have fracture spacing of 2 to 5 ft (0.6 to 1.5 m).

Areas bctween en echelon faults have about twice as many fractures per unit traverse length as do areas having only regional fracture abundance patterns (Figs. 8 and 9), but fault overlap areas may have fewer fractures than are found in local areas directly adjacent to large faults. Howcver, fractured areas in the overlap between en echelon faults may be larger than in the fracture zones adjacent to large faults. The multiple strikes of intermingled fracture sets within fault overlap areas may cause high fracture connec­tivity locally. Vertical and cross-strike fracture con­nections result mainly from intersections among small faults. The best fracture connectivity occurs within swarms of small faults. Although joints within joint swarms are often poorly connected, local cross joints cause sorne cross-strike connectivity. Joints are poorly interconnected vertically. In shallow near-sur­face beds, separation of the limestone beds along bed­ding planes may also cause joints and faults to be connected.

SUMMARY

1. Two types of fault overlap exist within master en echelon faults of the Balcones Fault Zone: (a) relay ramps between overlapping master faults dipping in the same direction and (b) horst bridges between overlapping faults dipping in opposite directions. Overlap areas are about 0.6 mi (1 km) wide, and en echelon master faults may overlap by as much as 1.2 mi (2 km).

2. Fault overlap areas are a mosaic of individual fractures and swarms of dosel y spaced faults and 1 or joints. Spacing of single small faults and fracture

swarms is commonly between 6.5 and 150 ft (2 and 46 m). Fault swarms are as much as 20ft (6 m) wide and may contain as many as 15 faults. Joint swarms are up to 40 ft (12 m) wide and have fracture spacings of 2 to 5 ft (0.6 to 1.5 m). Fractured overlap areas consist of a mosaic of intermingled fracture sets having multiple strikes, and fracture connectivity is locally high. Heights of fractures commonly exceed the limits of the outcrop, approximately 16 ft (5 m) locally. Most fracture lengths are unknown because they exceed the outcrop dimensions; however, one swarm of several small faults was mapped for more than 130ft (40 m).

ACKNOWLEDGMENTS

This research was conducted during mapping inves­tigations of the 30- by 60-minute New Braunfels Quadrangle for the ongoing U.S. Geological Survey COGEOMAP project, funded under contract no. 1434-92-A-1085, and during geologic investigations of the Superconducting Super Collider, funded by the Texas National Research Laboratory Commission (TNRLC) under interag.ency contract IAC(90-91)1650. C. D. Henry helped collect sorne of the fault data in western Comal County. This paper benefited from useful com­ments by J. A. Raney and T. F. Hentz. Helpful editorial comments were made by B. S. Duncan.

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84 GULF COAST ASSOCIATTON OF GEOLOGICAL SOCIETIES

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