COBISS: 1.01 A REVIEw OF COALESCED, COLLAPSED-PALEOCAVE SySTEMS AND ASSOCIATED SUPRASTRATAL DEFORMATION MEDSEBOJNO ZDRUŽENI PORUŠENI PALEOKRAŠKI JAMSKI SISTEMI IN DEFORMACIJE NAD NJIMI LEŽEČIH PLASTI – PREGLED Robert G. LOUCKS1 Abstract UDC 551.44 Robert G. Loucks: A Review of Coalesced, Collapsed-Paleo-cave Systems and Associated Suprastratal Deformation Coalesced, collapsed-paleocave systems and associated supra-stratal deformation appear to be prominent diagenetic/struc-tural features in carbonate sections at/near composite uncon-formities. Te basic architecture of the system can be divided into two sections. Te lower karsted section, where high-den-sity cave formation took place, is preserved as massive breccias commonly displaying a rectilinear pattern in map view. Te overlying suprastratal deformation section is characterized by large, circular to linear sag structures containing faults and fractures. Regional distribution of coalesced, collapsed-cave systems commonly appears as large-scale (hundreds to thou-sands of square kilometers in area), rectilinear patterns with areas of concentrated, coalesced breccias separated by relatively undisturbed host rock. Tis pattern may refect development of the paleocave system along fracture swarms. Collapsed-paleocave systems are large, complex features that show broad-scale organization. Te complete paleocave system may need seismic data or large, mountain-scale outcrops to de-fne their architecture and distribution. Key Words: Paleocaves, Paleokarst, karst, suprastratal deformation, cave systems. Izvleček UDK 551.44 Robert G. Loucks: Medsebojno združeni porušeni paleokraški jamski sistemi in deformacije nad njimi ležečih plasti – pregled Medsebojno združeni porušeni paleokraški jamski sistemi in deformacije nad njimi ležečih plasti predstavljajo izrazite dia-genetsko/strukturne oblike karbonatnih zaporedij v bližini sestavljenih geoloških nezveznosti. Osnovno zgradbo posameznega sistema lahko razdelimo na dva dela. Spodnji zakraseli del, kjer je gostota jam velika, je ohranjen v obliki masivnih breč, ki pogosto kažejo v tlorisu vzorec sestavljen iz ravnih odsekov. Za deformirane plasti, ki prekrivajo porušene jamske sisteme, so značilne velike skledaste do škatlaste uleknine, ki jih sekajo prelomi in razpoke. Regionalno gradijo združeni paleokraški jamski sistemi tega tipa vzorec velikega merila (zajemajo območja velika stotine do tisoče kvadratnih kilometrov), sestavljen iz ravnih odsekov in vključuje območja zgoščenih združenih brečastih teles, ločenih z relativno neprizadeto prikamnino. Tak vzorec lahko kaže na razvoj paleokraškega jamskega sistema vzdolž razpok-linskih con. Porušeni paleokraški jamski sistemi predstavljajo velike kompleksne pojave, ki odražajo organiziranost velikega merila. Za opredelitev zgradbe in razprostranjenosti popolnega paleokraškega jamskega sistema teh dimenzij potrebujemo podatke seizmičnih raziskav ali izdanke dimenzij gorovja. Ključne besede: pelokraški jamski sistemi, paleokras, deformacije, jamski sistemi. 1 Bureau of Economic Geology , John A. and Katherine G. Jackson School of Geosciences, Te University of Texas at Austin, Uni-versity Station Box x, Austin, Texas 78713-8924 U.S.A., Fax: 512-471-0140 , email: bob.loucks@beg.utexas.edu Received/Prejeto: 27.11.2006 TIME in KARST, POSTOJNA 2007, 121–132 ROBERT G. LOUCKS INTRODUCTION At several composite unconformities in the stratigraphic record, carbonate sections display extensive karsting that leads to multiple development of cave systems (Esteban, 1991). Tese cave systems underwent extensive collapse and mechanical compaction with burial. Deformation of the overlying strata is associated with burial collapse of the cave system. Te efects of this suprastratal deformation can be noted 700+ m up section above the karsted interval. Tis review will describe the evolution of cave systems during burial and what the characteristics of the cave systems are at diferent stages of burial. Also the characteristics of suprastratal deformation will be de-scribed. Paleocave systems have been investigated by sev-eral authors including Lucia (1968, 1995, 1996), Loucks and Anderson (1980, 1985), Kerans (1988, 1989, 1990), wilson et al. (1991) wright et al. (1991), Candelaria and Reed (1992), Loucks and Handford (1992), Lucia et al. (1992), Kerans et al. (1994), Hammes et al. (1996), Maz-zullo and Chilingarian (1996), McMechan et al. (1998), Loucks (1999, 2001, 2003), Loucks et al. (2000, 2004), Loucks and Mescher (2001), McMechan et al. (2002), and Combs et al. (2003). Te review will mainly synthe-size material from these studies. CLASSIFICATIONS OF CAVE PRODUCTS AND FACIES Loucks (1999) and Loucks and Mescher (2001) pro-duced classifcations of cave products and cave facies. Loucks (1999) used a ternary diagram (Fig. 1) to show the relationships between crackle breccias, mosaic bre-ccias, chaotic breccias, and cave sediments. Crackle breccias are highly fractured rock, with thin fractures separating the clasts and only minor displacement ex-isting between the clasts. Mosaic breccias show more displacement than crackle breccias, but the clasts can still be ftted back together. Chaotic breccias are com- m §3 Crackle breccia I gravel | *ä •• % »!_•• Cave-sediment fill Matrix-rich: clast-supported: chaotic breccia Matrix-supported chaotic breccia cd Cave sediment with chips, slabs, and blocks Fig. 1: Cave-sediment flls and breccias can be separated into three end members: crackle breccia, chaotic breccia, and cave-sediment fll. modifed from Loucks (1999) and reprinted by permission of the AAPG whose permission is required for further use.” posed of mixtures of clasts that have been transported vertically by collapse or laterally by fuvial or density-fow mechanisms. Clasts show no inherent association with their neighbors. Chaotic breccias grade from ma-trix-free, clast-supported breccias; through matrix-rich, clast-supported breccias; to matrix-rich, matrix-sup-ported breccias. Cave-sediment fll can consist of any material, texture, or fabric. Loucks and Mescher (2001) proposed a classifcation of six common paleocave facies (Fig. 2): (1) Undisturbed strata, which are interpreted as un-disturbed host rock. In this facies bedding continuity is excellent for tens of hundreds of meters. (2) Dis-turbed strata that are disturbed host rock around the collapsed passage. Bedding continuity is high, but it is folded and ofset by small faults. It is commonly overprinted by crackle and mosaic brecciation. (3) Highly disturbed strata, which is collapsed host rock adjacent to or immediate-ly above passages. (4) Coarse-clast chaotic breccia that is interpreted as collapsed-breccia cavern fll pro-duced by ceiling and wall collapse. It is characterized by a mass of very poorly sorted, granule- to boulder-sized chaotic-breccia clasts approxi-mately 0.3 to 3 m long that form a ribbon-to tabular-shaped body as much as 15 m across and hundreds of meters long. It is commonly clast 122 TIME in KARST – 2007 A REVIEw OF COALESCED, COLLAPSED-PALEOCAVE SySTEMS AND ASSOCIATED SUPRASTRATAL DEFORMATION supported, but can contain matrix material. (5) Fine-clast chaotic brec-cia interpreted as laterally (hydro-dynamically) sorted, transported-breccia cavern fll. Characterized by a mass of clast-supported, mod-erately sorted, granule- to cobble-sized clasts with varying amounts of matrix. Clasts can be imbricated or graded. Resulting bodies are ribbon-to tabular-shaped and are as much as 15 m across and hundreds of meters long. (6) Cave-sediment cavern fll that can be carbonate and/or silici-clastic debris of any texture or fabric and commonly displaying sedimen-tary structures. Fig. 2: Six basic cave facies are recognized in a paleocave system and are classifed by rock fabrics and structures. modifed from Loucks and mescher (2001) and reprinted by permission of the AAPG whose permission is required for further use.” EVOLUTION OF CAVE PASSAGES Knowledge of the processes by which a modern cave lapsed paleocave passage in the subsurface is necessary passage forms at the surface and evolves into a col- to understand the features of paleocave systems. Loucks (1999) described this evolutionary process (Fig. 3), and the review pre-sented here is mainly from that in-vestigation. A cave passage is a product of near-surface karst processes that in-clude dissolutional excavation of the passage, partial to total breakdown of the passage, and sedimentation in the passage (Fig. 4). During lat-er-burial cave collapse, mechanical compaction takes place. Cave-ceiling crackle breccia Burial cave-ceiling crackle breccia Crackle/mosaic breccia Burial cave-wall crackle breccia Burial chaotic _ breakdown breccia Cave-ceiling collapse and further dissolution Transported breccia and sediment Mechanical compaction Sag, faults, and fractures Fig. 3: Schematic diagram showing evolution of a single cave passage from its formation in the phreatic zone of a near-surface karst environment to burial in the deeper subsurface. modifed from Loucks (1999) and reprinted by permission of the AAPG whose permission is required for further use.” TIME in KARST – 2007 123 ROBERT G. LOUCKS Initial passages form in phreatic and/or vadose zones (Fig. 3). Passages are excavated where surface recharge is concentrated by preexisting pore systems, such as bed-ding planes or fractures (Palmer, 1991), that form a con-tinuous link between groundwater input, such as sink-holes, and groundwater output, such as springs (Ford, 1988). Cave passages are under stress from the weight of Karst towers Vadose canyon Dohne (passage) Cave-sediment fill Solution-enlarged fractures Vadose zone Phreatic / / „. I~ Cave-floor crackle breccia zone | Phreatic tube Stream Chamber breakdown (passage) sediment (room) breccia Fig. 4: block diagram of a near-surface modern karst system. Te diagram depicts four levels of cave development (upper-right corner of block model), with some older passages (shallowest) having sediment fll and chaotic breakdown breccias. modifed from Loucks (1999) and reprinted by permission of the AAPG whose permission is required for further overlying strata. A tension dome, or zone of maximum shear stress, is induced by the presence of the passage or cavity (white, 1988). Stress is relieved by collapse of the rock mass within the stress zone. Tis collapse produces chaotic breakdown breccia on the foor of the cave passage (Figs. 3 and 4). Te associated stress release around the cavity produces crackle and mosaic breccias in the adjacent host rock. As cave-bearing strata are bur-ied, extensive mechanical compac-tion begins, resulting in collapse of the remaining void (Fig. 3). Multiple stages of collapse occur over a broad depth range. Meter-scale bit drops in wells (indication of cavernous pores) are not uncommon down to depths of 2,000 m and are observed to occur to depths of 3,000 m (Loucks, 1999). Te collapsed passages become pods of chaotic breccia (Fig. 3). Te areal cross-sectional extent of brec-ciation and fracturing afer burial and collapse is greater than that of the original passage because the ad-jacent fractured and brecciated host rock has become part of the brec-ciated pod. Sag features, faults, and fractures (Fig. 3) occur over the col-lapsed passages. Sediment-filled passages , Breakout dome Breakdown pile EVOLUTION OF COALESCED, COLLAPSED-PALEOCAVE SySTEMS A coalesced, collapsed-paleocave system can be divided into two parts: (1) a lower section of strata that contains collapsed paleocaves and (2) an upper section of strata that is deformed to varying degrees by the collapse and compaction of the section of paleocave-bearing strata (Fig. 5). Te deformed upper section of strata is termed suprastratal deformation (Loucks, 2003) and is discussed in a later section. Cave systems are composed of numerous passages. If the areal density of passages is low, the collapsed cave system will feature isolated, collapsed passages (nonco-alescing paleocave system; Fig. 6). If the cave system has a high density of passages, as is common at composite third-order unconformities (Esteban, 1991; Lucia, 1995; Fig. 6: Schematic diagram showing burial and collapse of low-density cave system (noncoalescing, collapsed-cave system) and reprinted by permission of the AAPG whose permission is required for further use.” 124 TIME in KARST – 2007 A REVIEw OF COALESCED, COLLAPSED-PALEOCAVE SySTEMS AND ASSOCIATED SUPRASTRATAL DEFORMATION Active cave systems Unconformity Phase 1: Modern cave system Phase 2: Multiple near-surface cave systems developed below composite S^2k Composite unconformity Phase 3: Coalesced, collapsed-paleocave system 100 to >1000 m Loucks, 1999), then upon burial and collapse the system can form large-scale, coalesced, brecciated and fractured breccia bodies upon burial and collapse that are the amalgama-tion of many passages and interven-ing disturbed host rock (coalescing paleocave system; Fig. 5). Te bodies are hundreds to several thousands of meters across, thousands of meters long, and tens of meters to more than 100 m thick. Internal spatial complexity is high, resulting from the collapse and coalescence of nu-merous passages and cave-wall and cave-ceiling strata. Fig. 5: Schematic diagram showing the stages of development of a coalesced, collapsed-paleocave system. modifed from Loucks et al., (2004) and reprinted by permission of the AAPG whose permission is required for further use.” SUPRASTRATAL DEFORMATION Collapse and compaction of cave systems provide potential for development of large-scale fracture/fault systems that can extend from the collapsed interval upward to more than 700 m (Kerans, 1990; Hardage et al., 1996a; Loucks, 1999, 2003; McDonnell et al., in press). Tese fracture/fault systems are not related to regional tectonic stresses. Large-scale suprastratal deformation occurs above the collapsed-cave system. As the cave system collapses during burial, overlying strata will sag or subside over the collapsed area. Tis phenomenon is well documented in mining literature (Kratzsch, 1983; wittaker and Red-dish, 1989). Kratzsch (1983, p. 147) presented a diagram (Fig. 7) that shows the stress feld above a collapsed mine passage and associated subsidence. Te overlying stress feld widens from the edges of the excavation, and the overlying strata are under compression directly over the excavation. Near the edges of the excavation, between a vertical line extending from the edge of the cavity and the limit line, strata are under extension (tension). within this zone of stress the overlying strata have the potential to sag, creating faults and fractures for some distance upward, depending on the mechanical properties of the strata and the thickness of the beds within the strata. Fig. 8 is a scatterplot showing a number of examples of the magnitude of subsidence over coal mines. Te graph in-dicates that subsidence is recorded at horizons more than 800 m above the cavity. Tese data indicate the magni-tude of the efect that the collapse of a cavity can have on overlying strata. Extension Compression Extension -Suprastratal deformation zone! Collapsed mine Fig. 7: diagram of a collapsed mine showing collapsed breccia zone and suprastratal deformation. Te center of the subsidence trough is under compression, whereas the wings are under extension. modifed from Kratzch (1983). Applying the above concept of stress felds over cav-ities to the collapse of a cave passage during burial sug- TIME in KARST – 2007 125 ROBERT G. LOUCKS gests that similar stress felds will develop. As the cave passage collapses, it has the potential to afect a consider- 1000 500 400 300 200 100 50 40 30 20 10 Mm Wittaker and Reddish (1989) U^ + •^ • • • •- • « « ^• • si • 2 3 Subsidence (m) able number of overlying strata. within a cave system, numerous passages will collapse with burial. Each passage will develop a stress feld above it, and these stress felds will interact to create a larger, combined stress feld. Tis concept was presented by wittaker and Reddish (1989; p. 47), who detailed instances in which multiple mining excavations are collapsing. Te stress feld above a collapsing cave system will be complex because the dif-ferent cave passages do not collapse and compact uni-formly over time. As local areas collapse, diferent stress felds will develop, producing fractures and faults related to that individual stress feld. Resulting suprastratal deformation will show variable fracture and fault patterns within an overall subsidence sag. A unique circular fault pattern above collapsed cave systems is recognized by cy-lindrical faults (Hardage et al., 1996a; Loucks, 1999; Mc-Donnell et al., in press). Fig. 8: Scatterplot showing thickness of overburden that can be afected by mine collapse. Graph shows a trend of greater subsidence with less overburden. MEGASCALE ARCHITECTURE PATTERNS OF COALESCED, COLLAPSED-PALEOCAVE SySTEMS Coalesced, collapsed-paleocave systems are megascale diagenetic/structural features that can afect more than 700 m of section and be regional in scale. As discussed earlier, the karsted section refects the coalescing of col-lapsed breccias that formed by collapse of passages and associated disturbed host rock. Te vertical extent of the breccias commonly afects the upper 100 m of section (Loucks and Handford, 1992; Loucks 1999) and as much as 300 m of the total section (Lucia, 1996). Te intensity of brecciation can vary throughout the afected interval. Kerans (1990), Loucks (1999), Loucks et al., 2004), and many others have published descriptions of collapsed, brecciated paleocave zones. Fig. 9 shows examples of cave facies from the Lower Ordovician Ellenburger Group in central Texas (Loucks et al., 2004). Te regional pattern of the collapsed paleocave system is commonly rectilinear (Loucks, 1999). Tis rectilinear pattern is probably an artifact of the original cave system developing along an early-formed fracture system. In a detailed study of a paleocave system in the Fig. 10: Slice map through a collapsed-paleocave system in the Lower Ordovician Ellenburger Group in central texas. modifed from Loucks (2004) and reprinted by permission of the AAPG whose permission is required for further use.” 126 TIME in KARST – 2007 Core locator] Ground-penetrating radar line V Quarry wall Undisturbed host rock 1000 ft 300 m Fractured disturbed host rock Fractured and brecciated rock (coalesced, collapsed cavern) 0 i 4 5 A REVIEw OF COALESCED, COLLAPSED-PALEOCAVE SySTEMS AND ASSOCIATED SUPRASTRATAL DEFORMATION Lower Ordovician in central Texas, Loucks et al. (2004) presented maps (Fig. 10) and cross sections of the three-dimensional, fne-scale architecture of a coalesced, col-lapsed-paleocave system. Te coalesced, collapsed-pas-sage breccias range in size to as much as 350 m and are separated by disturbed and undisturbed host rock ranging in size up to 200 m. Lucia (1995) also presented a map of brecciated collapsed passages (Fig. 11) from out-crops in the Franklin Mountains of far west Texas, which displays a crude rectilinear pattern. Tis rectilinear pattern can be seen on seismic data as well. Loucks (1999) presented seismic-based maps from Benedum feld in west Texas that display a rectilin-ear pattern of sags and circular faults induced by collapse of the Ellenburger paleocave system below (Fig. 12). A similar rectilinear pattern is evidenced on seismic data in Boonsville feld (Fig. 13) in the northern Fort worth Basin in Texas (Hardage et al., 1996a; McDonnell et al., in press). In both the Benedum and Boonesville datasets, suprastratal deformation afects up to 700 m of section above the karsted interval (Figs. 12 and 13). (a)' TJ • •¦ ^1 1 ¦ä _^2 ¦ ^- 1 ' -* k mm 1 _t * , 1 * ¦ 5 err d ~ (c) i ». 5 cm Fig. 9: Representative cores from paleocave facies. (a) Crackle-fractured disturbed host rock. (b) Collapsed chaotic breccia with large slabs and cave-sediment fll. (c) transported chaotic breccias in carbonate cave-fll matrix. Sample on right is under Uv light. Samples from Lower Ordovician Ellenburger Group in central texas. modifed from Loucks (2004) and reprinted by permission of the AAPG whose permission is required for further use.” TIME in KARST – 2007 127 ROBERT G. LOUCKS (a) Sag (suprastratal deformation) Upper Ordovidan Montoya Silurian Fusselman Upper Orctovidsn Montoya (b) 3000 ft 900 m Approximate area of Great McKelligon Sag (above photograph) ¦ Brecciated McKelligon Canyon, Cindy, and Ranger Peak Formations ] Brecciated Ranger Peak Formation ___ Unbrecciated El Paso Group Fig. 11: (a) Photograph of the Great mcKelligon Sag in the Franklin mountains of far West texas. Photograph and general interpretation are from Lucia (1995) but have been modifed by current author. Tis outcrop is an outstanding example of a collapsed-paleocave system with associated overlying suprastratal deformation. (b) map produced by Lucia (1995) of several paleocave systems within the Franklin mountains. Paleocave trend lines are by current author. CONCLUSIONS Coalesced, collapsed-paleocave systems are megascale Te overlying strata were generally lithifed, but the sag diagenetic/structural features that can afect more than also afected concurrent sedimentation patterns (Hard-700 m of section and be regional in scale. Te architec- age et al., 1996b). Te deformation in the deformed su-ture of the complete system can be divided into the lower prastratal zone consists of normal, reverse, and cylindri-collapsed zone, where the dense system of caves formed cal faults and fractures (Loucks, 1999; McDonnell et al., and collapsed with later burial, producing a complex in press). It is important to emphasize that large-scale zone of brecciation. Te upper, suprastratal deformation structural features can develop above karsted zones and section formed during the collapse of the karsted section. not be related to regional tectonic stresses. 128 TIME in KARST – 2007 0 0 A REVIEw OF COALESCED, COLLAPSED-PALEOCAVE SySTEMS AND ASSOCIATED SUPRASTRATAL DEFORMATION (a) (b) Tan, L k. Seismic line ^in vm fusse -EUenburger Marker Collapsed-paleocave zones -<----------» (shown by missing reflections) ~300 m Fig. 12: 3-d seismic example over an Ellenburger paleocave system from benedum feld in West texas. (a) Second-order derivative map in the Fusselman interval displaying sag zones produced by Ellenburger paleocave collapse. (b) Seismic line showing missing sections (collapse in Ellenburger section), cylindrical faults, and sag structures. Suprastratal deformation is >1,000 f thick in this section. modifed from Loucks (1999) and reprinted by permission of the AAPG whose permission is required for further use.” Coalesced, collapsed-paleocave systems and associated suprastratal deformation are complex systems, and large-scale outcrops or datasets are necessary to defne them. However, with the model presented in this paper, individual data points can lead to recognition that the system is a coalesced, collapsed-paleocave feature. TIME in KARST – 2007 129 ROBERT G. LOUCKS Fig. 13: Suprastratal deformation sag features in post-Lower Ordovician Ellenburger strata in Fort Worth basin in north texas. (a) Curvature map at mississippian Forestburg Limestone horizon displaying sag features and faults produced by collapse in the Ellenburger interval. From mcdonnell et al. (in press). (b) 3d seismic line at 1:1 scale showing sag features produced by paleocave collapse in the Ellenburger section. Line-of-section location is shown by dashed line in Fig. 13a. ACKNOwLEDGEMENTS I would like to express my appreciation to Angela Mc- rector, Bureau of Economic Geology, John A. and Kath-Donnell for reviewing this manuscript. Lana Deiterich erine G. Jackson School of Geosciences, Te University edited the text. Published with the permission of the Di- of Texas at Austin. 130 TIME in KARST – 2007 A REVIEw OF COALESCED, COLLAPSED-PALEOCAVE SySTEMS AND ASSOCIATED SUPRASTRATAL DEFORMATION REFERENCES Candelaria, M. P. & C. L. Reed, eds., 1992: Paleokarst, karst related diagenesis and reservoir development: examples from Ordovician-Devonian age strata of west Texas and the Mid-Continent.- Permian Basin Section SEPM Publication No. 92-33, p. 202. Combs, D. M., R. G. Loucks, & S. C. Ruppel, 2003: Lower Ordovician Ellenburger Group collapsed paleocave facies and associated pore network in the Barnhart feld, Texas.- in T. J. Hunt & P. H. 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