COBISS: 1.01 CLASTIC SEDIMENTS IN CAVES – IMPERFECT RECORDERS OF PROCESSES IN KARST KLASTIČNI SEDIMENTI V JAMAH – NEPOPOLNI ZAPIS KRAŠKIH PROCESOV Ira D. SASOwSKy1 Abstract UDC 552.517:551.7 Ira D. Sasowsky: Clastic sediments in caves – imperfect recorders of processes in karst Clastic sediments have played an important role in deciphering geologic history and processes since the inception of the discipline. Early studies of caves applied stratigraphic principles to karst deposits. Te majority of cave deposits are breakdown and alluvium. Te alluvial materials have been successfully investi-gated to determine ages of caves, landscape evolution, paleoen-vironmental conditions, and paleobiota. Rapid stage changes and the possibility of pipe-full fow make cave deposits diferent than surface deposits. Tis and other factors present difculties with interpreting the cave record, but extended preservation is aforded by the “roofng” of deposits. Dating by magnetism or isotopes has been successful in many locations. Caves can be expected to persist for 10 Ma in a single erosive cycle; most cave sediments should be no older than this. Key words: clastic sediments, paleoclimate, sedimentology, stratigraphy, dating. Izvleček UDK 552.517:551.7 Ira D. Sasowsky Klastični sedimenti v jamah – nepopolni zapis kraških procesov Že od nekdaj so klastični sedimenti pomembno orodje pri razbiranju geološke zgodovine. V zgodnjih študijah so uporabili načela stratigrafje tudi pri raziskovanju jamskih sedimentov . Glavnino jamskih sedimentov sestavljajo podori in aluvij. Raziskave aluvija so se uspešno izkazale pri dataciji jam, določanju razvoja površja, paleookolja in paleontologije. Zaradi možnega tlačnega toka in hitrih sprememb stanj, so jamski sedimenti drugačni od površinskih. To, poleg ostalih dejavnikov, predstavlja težave pri interpretaciji zapisov, ki jih hranijo jame. Po drugi strani pa je obstojnost jamskih sedimentov daljša zaradi zavetja, ki jim ga nudi jama. Po vsem svetu poznamo številne uspešne datacije jamskih sedimentov z magnetizmom ali izotopi. Jame znotraj erozijskega cikla vzdržijo do10 milijonov let, zato naj jamski sedimenti ne bi bili znatno starejši. Ključne besede: klastični sedimenti, paleoklima, sedimen-tologija, stratigrafja, datiranje. INTRODUCTION Geology is undeniably a science of history, and since the earliest practice of the discipline, that history has been revealed in clastic sedimentary deposits. william Smith, for example, created maps of the sedimentary rocks in England in the late 1700’s, and established a relative chro-nology of their deposition using stratigraphic position and fossils. It has been natural, therefore, that karst scien-tists examine clastic deposits in caves, in order to explore geologic time. In doing so, they are in large part applying the same principles and techniques developed by classi-cal stratigraphers. An early example of this was a study by Kukla and Ložek (1958) examining the processes of cave sediment deposition and preservation. In the present day, work such as that by Granger et al. (2001) and Polyak et. al. (1998) builds upon those classical techniques and ap-plies laboratory methods to develop absolute chronolo- 1 Ofce for Terrestrial Records of Environmental Change, Department of Geology and Environmental Science, University of Ak-ron, Akron, OH 44325-4101, USA. Received/Prejeto: 24.01.2007 TIME in KARST, POSTOJNA 2007, 143–149 IRA D. SASOwSKy gies. Tese chronologies in turn have allowed insight to such processes as river incision, water-table lowering, and landscape/climate linkages. Tis paper is a brief evaluation of clastic sediments as they apply to deciphering historical processes and events MATERIALS A Te processes that result in clastic sedimentation in caves are quite varied. Reviews and details including classif-cation of deposits are presented in several texts (white, 1988; Ford and williams, 1989; Sasowsky and Mylroie, 2004). A perspective is given here. A useful broad-level classifcation is genetic, and based upon whether the clastic material originated within the cave (autogenic) or was carried in from the surface (allogenic). Te former class is mainly bedrock breakdown (incasion), but encompasses fne grained sediments sourced from insoluble residue during phre-atic enlargement, collapse of secondary mineralization (speleothems), and so forth. Allogenic sediments include alluvium, windblown material, animal feces, fossil matter, till, etc. In practice, the most commonly occurring materials by far are bedrock breakdown and alluvium. Conse-quently, autogenic cave sediments are mainly limestone. Allogenic sediments are usually resistant siliciclastics, because carbonates do not typically persist in the fuvial environment. Tere is no satisfying overall term for the clastic de-posits found in caves. Te word “soil” has been applied to the fne grained deposits, but this is a misnomer by most defnitions, and is not recommended. Cave fll and cave earth have also been used. Regolith seems applica-ble in spirit, but, because this material does not strictly “….form(s) the surface of the land ....” (Jackson, 1997) some may object to such usage. BREAKDOwN Te collapse of cave bedrock walls and ceilings results in material that is angular, and ranges in size from sand to boulders. It is possible many times to visually ft larger blocks to their point of origin on the adjacent cave walls and ceilings. Te process of breakdown is not a common occurrence on human timescales. Only a few cases of present-day natural failure have been documented. For example, in Mammoth Cave, Kentucky only one large col-lapse was noted in 189 years of mining and tourism (May et al., 2005). However, on geologic timescales, the proc- in karst terrane. Advantages and problems of working with these unique deposits are presented. For purposes of this paper, the “age” of a given cave sediment refers to the time of deposition of the material in the cave. D PROCESSES ess is pervasive and evident in most caves. Failures occur along existing planes of weakness (joints, faults, bedding planes). Causes of collapse can include removal of under-lying support (particularly loss of buoyancy caused by the transition from phreatic to vadose conditions), removal of overlying arch support, cryoclastism (wedging by ice), and secondary mineral wedging (white and white, 2003). Triggering by earthquakes has also been observed, for example in Sistem Zeleške Jame-Karlovica (personal communication, F. Drole). Davies (1951) published an early analysis of expected collapse parameters in the cave environment. Tis was expanded on by white (1988, p. 232) to evaluate stability of ceilings relative to limestone bed thickness. Greater spans can be maintained by thick-er beds. Jameson (1991) provides a comprehensive overview and classifcation of breakdown. Breakdown is frequently most prolifc at 1) the intersections of cave passages, presumably due to the greater span lengths present at such points, and 2) where the cave is close to the surface, due to lack of thinning of the span and resulting decreased competency. In evalua-tions of causes for passage terminations (white, 1960) it was noted that many cave passages ended in breakdown blockage (referred to by explorers as “terminal break-down”). Although pervasive, breakdown has not found sig-nifcant utility for deciphering earth history in karst ter-ranes. ALLUVIUM Alluvium enters caves by sinking stream, and occasion-ally by colluvial mechanisms. Te transport processes are for the most part similar to those in surface channels. Te full range of sediment sizes are seen, structures such as cross-bedding and pebble imbrications develop, and cut-and-fll stratigraphy is possible. However, there are two important diferences exhibited for stream fow in caves when compared to most surface channels. First, channel width is severely constrained by bedrock walls. Tis promotes rapid stage increase during fooding, akin to that of slot canyons in surface streams (Fig. 1). Second, 144 TIME in KARST – 2007 CLASTIC SEDIMENTS IN CAVES – IMPERFECT RECORDERS OF PROCESSES IN KARST Fig. 1: Subterranean stream channels are typically narrow, and have no foodplain (a). Tis leads to rapid stage changes. Similar conditions in the surface environment are only seen in slot canyons such as the virgin River, Utah, USA (b). because the channel is roofed over, it is possible to have confned (pipe-full) rather than open channel fow. Tak-en in conjunction, the results of these two conditions are the likelihood of high fow velocities, and the possibil-ity of upwards phreatic fow. A striking example of rapid stage change is seen in Hölloch, Switzerland, where rises of 250 m in a single food have been recorded (wildberger and Preiswerk, 1997; Jeannin, 2001). Cases of phreatic lifs are seen in many cave systems. In Castleguard Cave (Rocky Mountains, Canada) a seasonally active lif of 9 m is observed (Schroeder and Ford, 1983). In that situation well-rounded cobbles are accumulated at the base, where they reside until communition reduces them suf-ciently to allow transport up the lif tube. Te composition of the alluvium refects the source of the material, as well as some other factors. It is interesting to note that a high proportion of clay sized material found in cave alluvium is actually fne-grained silica, not a clay mineral (white, 1988). Te residuum found on the surface of many karst terranes frequently contains high amounts of clay and chert. Te clay results from insoluble residues of the weathered limestone. Te chert behaves in a very persistent way, being found throughout cave passages. INFORMATION REVEALED In the investigation of clastic sedimentary deposits, either cave related or not, answers are sought to such questions as: How old? what was the paleoenvironment? what was the fow direction? what organisms were present? Tese in turn allow an understanding of geologic history, environments of deposition, past climates, and potential for sedimentary deposits to act as mineral and fuel reservoirs. In the case of cave studies, it is primarily the frst question which has been addressed. Caves can only be numerically dated by the deposits that they hold, and this age is usually reported as a minimum value. Alluvial materials are considered superior to speleothems in this undertaking, because they are emplaced much earlier in the existence of the cave. Once a date has been obtained, subsequent inferences such as rates of river incision, denudation, and so forth, can then be made based upon the relation of the cave to the landscape. Dating has been accomplished by radiocarbon, magnetism, and cosmo-genic isotopes. Paleoenvironmental information is revealed through studies of sedimentary structures and sequences, as well as via analyses of clay mineralogy and environmental magnetism. Paleohydrology can be deduced using tra-ditional stratigraphic indicators such as cross-bedding, pebble imbrication, etc. Fossil deposits of organisms are actually rather rare within caves – most cave depsits are barren of these materials. Signifcant deposits are known, though, and many excavations made in caves (particu-larly in the entrance facies) serve as irreplaceable records of terrestrial fauna. TIME in KARST – 2007 145 IRA D. SASOwSKy LIMITS ON TIMESCALE Caves are erosional landforms, which have a limited period of existence. Excluding those caves which have been subjected to burial, this places a practical limit on their duration as potential recorders of nearby processes. In any case, the cave sediments can be no older than the cave they are emplaced in (Sasowsky, 1998). Terefore, the ultimate limit on preservation of sediments within a cave is the persistence (lifetime) of the cave in the environment. In most limestone terranes epigenetic processes occur, with dissolution taking place both at the surface (forming pavements, dolines, etc.) and in the subsur-face (forming caves). As base level lowers, denudation of the upland surfaces is also occurring and uppermost caves are eventually breached and destroyed. In certain settings examples of various states of decay can be seen in the landscape, and the sedimentary flls of breached (unroofed) caves may even be observed (e.g. Šušteršič, 2004). In settings such as the Appalachian Valley and Ridge, hundreds of meters of carbonate have been de-nuded from anticlinal valleys (white, 1988), and one may imagine extensive systems of caves which have been obliterated with no remaining trace. Bounds on the expected lifetime of an epigene cave may be evaluated by considering the two main control- Fig. 2: Teoretical persistence of caves in an erosional environment. Te length of time that a given cave will exist depends upon the initial depth of formation (position on y-axis) and the denudation rate (slope of line). Gray regions envelope a range of reasonable denudation pathways for two examples. In case A, a cave formed at 200 m depth, the expected lifetime is 2.5 to 10 ma. For a cave formed at 100 m depth (case b), the lifetime is reduced to 1.25 to 5 ma. Solid sloping lines are the average denudation rate, 69 m/ma, for 33 major drainage basins (calculated from data in Summerfeld and hulton, 1994). 146 TIME in KARST – 2007 ling factors: initial depth of formation and rate of land surface lowering (denudation, Fig. 2). Although caves may form at any depth, a practical limit of 300 m is rea-sonable, and the majority of caves are much shallower (Milanovic, 1981). Note that this “depth” is not correla-tive to the frequently reported mapped depth of caves, which refers to the maximum vertical extent of survey. In the context of the present evaluation, depth is the position below surface (thickness of overlying rock) at a given point in the cave. Denudation rates can be quite variable, and tend to correlate with rainfall (white, 1988, p. 218). Envelopes of expected cave persistence can be construct-ed (Fig. 2) using these 2 parameters. Based upon this cal-culation, epigene caves would usually exist in the erosive environment for up to 10 Ma. In practice, dating has not yet resulted in identifca-tion of caves this old within the present erosional cycle. Paleomagnetic dating has been used back to 4.4 Ma (Cave of the winds, Colorado, USA; Luiszer, 1994). Cosmogen-ic isotope dating has documented cave sediments as old as 5.7 (±1.1) Ma (Bone Cave, Tennessee, USA; Anthony and Granger, 2004). Te absence of older values may be a consequence of limitations of dating methods, or refect the relative dearth of older caves in the environment, or both. Te challenges of paleomagnetic dating include ab-sence of fne-grained sediments, lack of uninterrupted sedimentation, and uncertainties of correlation with the global magnetic polarity scale. Cos-mogenic dating is constrained by the absence of quartzose sediments, un-certainties in parent isotope values, and the cost/efort of analyses. If consideration is extended beyond the present erosional cycle, flled and buried caves (paleokarst) are found in the rock record. Such materials have been recognized in many places, and the flls described in some detail (e.g. Loucks, 1999). Interest has been strong in the con-text of exploration for minerals or petroleum. Tese deposits also rep-resent a potential trove of information on far past hydrologic and en-vironmental conditions because of their capacity to preserve. CLASTIC SEDIMENTS IN CAVES – IMPERFECT RECORDERS OF PROCESSES IN KARST RESOLUTION, CONTINUITy, AND VERACITy Stratigraphers have traditionally examined marine or paralic sediments because of their resolution, continuity, and veracity. Compared to terrestrial deposits, marine/ paralic strata are much more laterally and vertically extensive, they are of economic interest, and they potential-ly function as continuous recorders for long periods of time. Terrestrial deposits are of interest though, particu-larly because they contain information about the on-con-tinent setting. within the terrestrial environment lacus-trine deposits and fuvial terraces have seen the greatest attention as recorders of Cenozoic paleo-conditions. Lakes probably represent the highest quality records in the terrestrial environment – their environment many times is one of high preservation potential. Lacustrine deposits can be sampled by coring; duplication of cores can serve as a quality control; accumulation rates can be rapid; sediment properties are well tied to local environ-mental conditions; and spatial variability is usually well understood. Terraces tend to preserve a partial record of the fuvial environment, depending upon regional uplif or down-cutting of the stream. In comparison, most caves contain spatially irregu-lar deposits that can be afected by factors such as plugging of swallets, extreme fow events, and back-fooding. Hydrologic complexity is common (Bosák et al., 2003), even more so than surface fuvial environments. Analysis of the paleohydrology of the depositional setting through cave passage morphometry is usually necessary, and may be quite time consuming if detailed maps are not avail-able. Stratigraphic sections may be discontinuous, and require compilation. Caves are difcult sampling loca-tions, due to logistics, remoteness, lack of light, and con-straints on sampling equipment transport. Nevertheless, the cave environment is one that pro-vides some advantages in recording the history of a region. Te greatest advantage is that of potential preserva-tion. Because caves are “roofed over” deposits are likely to be protected (at least on intermediate time scales), from Fig. 3: Comparison of sedimentary records from Lake baikal, Russia (3 columns on lef), and Cave of the Winds, USA (3 columns on right). baikal data used with permission from King and Peck, 2001. Cave of the Winds data used with permission from Luiszer, 1994. TIME in KARST – 2007 147 IRA D. SASOwSKy Fig. 4: Episodic inflling and removal of sediments is commonly observed in caves. In this section of Windy mouth Cave (West virginia, USA) a diamict was almost completely removed afer being covered with fowstone. Te conduit is presently dry. surfcial erosion. Tis is particularly germane for the fu-vial deposits. weathering and erosion of surface fuvial terraces is commonplace. In the cave, such materials may sit undisturbed for years. For example, in xanadu Cave, Tennessee, USA, a pristine, non-indurated fuvial deposit that is greater than 780 ka was sampled (Sasowsky, et al., 1995). Although rare, in exceptional settings the quality of the cave record may approach that of lakes (Fig. 3). Conditions amenable to this are stable recharge confguration, diffuse recharge, minimal variation of discharge, and deep circulation. In Figure 3 two exceptional records are compared. Te Lake Baikal record was constructed from cores taken on watercraf. In that setting, about 40 m of sediment accumulate in 1 Ma. In contrast, at Cave of the winds the accumulation rate is slower by more than an order or magnitude. In many settings caves appear to undergo episodic flling and exca-vation (Fig. 4). In certain cases this may be locally controlled by cata-strophic storms (e.g. Doehring and Vierbuchen 1971). However, the presence of broadly similar depos-its/incisions within many caves in a region supports the idea that cave clastic materials refect regional paleoclimatic conditions. Tese deposits hold much information that will be revealed with continued advances in conceptual frameworks and improved labo-ratory methods. REFERENCES Anthony, D.M. & D.E. 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