trace fossils biology and taphonomy 1990 special topics in palaeontology 3 280 pp e17 95 pbk e40 00 hbk unwin hyman london isbn 004445 6867 pbk 3035 hbk

flash flood sediments and ichnofacies of the late pleistocene homeb silts kuiseb river namibia abstract the homeb silt formation consists of eroded remnants of fine grained alluvium that were deposited within the kuiseb river valley in southern namibia during the late pleistocene in holocene times the river deeply incised its valley fill leaving steep walled castles of relict alluvium the homeb silts this study is based on the 25 m thick type exposure of homeb silts at homeb and describes flash flood sediments that were colonised by successive generations of opportunistic arthropods the stacked flood units each comprise a massive tabular siltstone bed overlain by a narrower interval of rapidly alternating sandstone and siltstone with claystone veneers these flood units are interpreted as having accumulated under semi arid climatic conditions by episodic back flooding of the kuiseb river into embayments and tributary mouths soon after deposition as the floodwaters subsided the sediment was colonised by burrowing sediment ingesting organisms mostly arthropods that produced a taenidium ichnofacies after the floodwaters had drained the exposed sediment was colonised by grasses and burrowed by terrestrial arthropods probably ants and termites resulting in an overprint of termitichnus ichnofacies with associated pelletal chambers thirteen horizons of rhizocretions and root tubules occur toward the top of the succession and indicate a gradual reduction in the frequency of flooding these immature calcic palaeosols suggest that the climate in the central namib desert 20 000 years ago was semi arid being wetter and more seasonal than the hyperarid conditions that prevail today

trace fossils as paleoenvironmental indicators in the taylor group devonian of antarctica abstract the taylor group of southern victoria land antarctica contains varied and abundant trace fossils which have been interpreted by palaeontologists as indicating marine deposition however sedimentologists have largely favoured non marine deposition on the basis of desiccation polygons fining upward cycles unidirectional paleocurrents small channels red beds and paleosols the debate is reconciled by a reinterpretation of the trace fossil assemblage it is shown that widespread and well preserved cruziana diplichnites rusophycus and skolithos which have previously been used as marine indicators comprise an ichnofacies scoyenia which is entirely typical of fresh water

animal burrowing and associated formation of large desiccation cracks as factors of a rapid restoration of soil cover in flooded farmlands the burrowing activity of annelids controls the spatial pattern and rapid formation of large desiccation cracks in the river flood derived mud cover in recently inundated farmland areas in northwestern anatolia turkey the phenomenon is confirmed by laboratory experiments imitating the natural floodplain conditions

reconnaissance of upper jurassic morrison formation ichnofossils rocky mountain region usa paleoenvironmental stratigraphic and paleoclimatic significance of terrestrial and freshwater ichnocoenoses seventy five types of ichnofossils documented during a four year reconnaissance study in the upper jurassic morrison formation demonstrate that highly diverse and abundant plants invertebrates and vertebrates occur throughout most of the morrison or equivalent strata invertebrate ichnofossils preserving the most environmentally and climatically sensitive in situ behavior of morrison organisms are in nearly all outcrops terrestrial ichnofossils record biotic processes in soil formation indicating soil moisture and water table levels freshwater ichnofossils preserve evidence of water depth salinity and seasonality of water bodies ichnofossils categorized as epiterraphilic terraphilic hygrophilic and hydrophilic new terms reflect the moisture regime where they were constructed the ichnofossils are vertically zoned with respect to physical chemical and biological factors in the environment that controlled their distribution and abundance and are expressed as surficial shallow intermediate and deep the sedimentologic stratigraphic and geographic distribution of morrison ichnofossils reflects the environmental and climatic variations across the basin through time marginal marine tidal to brackish water ichnofossils are mainly restricted to the windy hill member very large to small termite nests dominate the salt wash member similar size ranges of ant nests dominate the brushy basin member soil bee nests dominate in the salt wash decreasing in abundance through the brushy basin deeper and larger insect nests indicate more seasonal distribution of precipitation and rainfall shallower and smaller insect nests indicate either dry or wet substrate conditions depending on the nest architecture and paleopedogenic and sedimentologic character of the substrate trace fossil indicators of flowing or standing water conditions are dominant in the tidwell member and in fluvial sandstones of the salt wash and brushy basin members large communities of perennial freshwater bivalve traces are abundant in the tidwell and brushy basin members but to a lesser extent in the salt wash member shallow crayfish burrows indicating a water table level close to the surface 1 m are restricted to channel bank and proximal alluvial deposits in the salt wash recapture and brushy basin members sauropod theropod pterosaur and other vertebrate tracks occur throughout the morrison formation associated with alluvial lacustrine and transitional marine shoreline deposits ichnofossils and co occurring paleosols in the morrison reflect the local and regional paleohydrologic settings which record the annual soil moisture budget and were largely controlled by the climate in the basin contributions to near surface biologic systems by groundwater from distant sources were minor except where the water table perennially seasonally or ephemerally intersected the ground surface the jurassic morrison formation in the southern portion of the basin experienced a mosaic of seasonal climates that varied from a drier tidwell windy hill deposition to a wetter lower and middle salt wash deposition and slightly drier upper salt wash deposition tropical wet dry climate returning to a wetter tropical wet dry climate near the end of morrison deposition brushy basin deposition the northern part of the basin experienced similar trends across a mosaic of mediterranean climate types the range and mosaic pattern of wet dry morrison climates is analogous to the range of climates and their seasonal variability that dominates the african savanna today

trace fossils from carboniferous floodplain deposits in western argentina implications for ichnofacies models of continental environments abstract floodplain deposits from the carboniferous tupe formation of western argentina contain a low diversity ichnofauna of invertebrates and plants the assemblage consists of archaeonassa fossulata didymaulichnus lyelli helminthoidichnites tenuis palaeophycus tubularis planolites isp and root traces this ichnofauna is characterized by the dominance of very simple forms superficial or very shallow trace fossils combination of locomotion grazing and dwelling structures and production by arthropods and vermiform organisms primary sedimentary fabrics are disturbed only by plant trace fossils while animal trace fossils are mostly restricted to bedding surfaces resulting in the virtual absence of bioturbation the poorly defined morphology of the ichnofossils and the absence of backfilled striated trace fossils and structures indicative of subaerial exposure suggest subaqueous production the envisaged depositional environment is a freshwater body developed in a floodplain and overfilled by overbank deposits a review of floodplain ichnofaunas documented from the stratigraphic record indicates that two recurrent assemblages may be recognized the first assemblage is characterized by low to rarely moderate diversity of invertebrate trace fossils moderate to high diversity of vertebrate structures and common presence of meniscate backfilled structures bilobate trace fossils with scratch marks arthropod trackways and tetrapod trackways it is regarded as an example of the scoyenia ichnofacies developed in desiccated floodplains the second type of floodplain assemblage illustrated by the example documented in this paper is characterized by grazing trails locomotion trails and dwelling burrows low to rarely moderate ichnodiversity simple trace fossils and superficial to very shallow structures although formed in floodplain environments also this type of assemblage is remarkably different from the scoyenia ichnofacies it is here regarded as an example of the mermia ichnofacies basic features of these ichnofaunas reflect subaqueous production of the assemblage the lower ichnodiversity of these floodplain assemblages in comparison with their equivalents from lacustrine basins is probably an expression of the less stable conditions and temporary nature of floodplain water bodies our study supports the view that ichnofacies are not indicators of sedimentary environments but reflect sets of environmental factors

trace fossils from marginal lacustrine deposits of the cretaceous jinju formation southern coast of korea abstract trace fossils from the cretaceous jinju formation of the south coast of korea occur in interbedded medium to fine grained sandstone of a moderate energy lake shoreface and mudstone shales of a periodically exposed low energy shallow lake and interfingerings of both of the above the trace fossils are represented by beaconites antarcticus beaconites coronus cochlichnus anguineus diplichnites octopodichnus cf didactylus diplichnites type a diplichnites isp helminthopsis hieroglyphica palaeophycus sulcatus palaeophycus tubularis planolites annularis planolites beverleyensis protovirgularia dichotoma skolithos magnus taenidium barretti and sauropod tracks the diplichnites specimens are among the first arthropod trackways recorded from the cretaceous two of the four trackway bearing ichnocoenoses present in the study area are considered representative of composite ichnofacies sinusoidal burrows or trails and simple irregular interface burrows or trails probably formed in wet possibly submerged soupground softground conditions in low or medium energy lake margins and represent the mermia ichnofacies trackways and meniscate burrows probably formed on the subsequently subaerially exposed lake margins following lake regression and represent an overprinting by the scoyenia ichnofacies the other two trackway bearing ichnocoenoses are from the scoyenia ichnofacies alone

life and death in a late cretaceous dune field nemegt basin mongolia for more than 70 years red sandstones of the gobi desert have yielded abundant articulated skeletons of late cretaceous dinosaurs lizards and mammals at ukhaa tolgod structureless sandstones are the only fossiliferous facies and we present new evidence for deposition on dune sand sourced alluvial fans that were built at the margins of stabilized eolian bedforms during mesic climatic episodes in laterally and vertically adjacent large scale eolian cross strata in which skeletons are absent we have found abundant tracks of dinosaurs that walked on sparsely vegetated dunes that were active under xeric conditions our study of calcareous concretions in vaguely bedded eolian sandstones suggests that bedding was nearly destroyed by burrowing invertebrates and trampling dinosaurs the accumulation of illuvial clays and pedogenic calcite in these sediments reduced infiltration of rainwater and with attendant climatic change and heavy rainfall events led to fan development

neogene avian and mammalian tracks from death valley national monument california their context classification and preservation abstract the copper canyon formation consists of over 3000 m of basalt and alluvial fan and lacustrine strata deposited in a small neogene basin lacustrine strata are of two typs fine grained dolomitic carbonates deposited in a playa lake environment and calcitic bioclastic carbonates deposited in a freshwater lake environment bedding surfaces of the playa lake carbonates deposited in shoreline environments commonly contain avian and mammalian footprints tracks of artiodactyls are most abundant those of birds horses carnivores and proboscideans are less common using the binomial avian and mammalian footprint classification scheme of vialov 1966 four different mammalian ichnogenera containing thirteeen ichnospecies as well as six ichnospecies of avian tracks are recognized environmental factors of the ancient copper canyon playa lake which contributed to good footprint preservation included 1 periods of flooding and intense evaporation which resulted in receding shorelines that presented wet fine grained sediment surfaces for impression and subsequent subaerial exposure and 2 periodically high sedimentation rates which insured quick burial of the tracks while vertebrate footprints also occur in sandstones and conglomerates deposited in alluvial fan environments the morphological distinctness of tracks is best preserved in fine grained dolomitic carbonate and siltstone strata deposited in the playa lake environment distinctness of preserved tracks also increases with apparent decrease within a certain range of sediment water content at the time of impression an examination of preserved morphologies has allowed several sediment water content states important to footprint preservation to be deduced changes in sediment water content at the time of impression acted to produce a wide range of track morphologies for each individual trackmaker an examination of track abundance and occurrence within the formation suggests increasing visitation by larger vertebrates to the playa lake as lake water freshened the rare ocurrence of tracks of small mammals and low amount of vertebrate bioturbation within the playa deposits also suggests that the ancient copper canyon playa lake lacked a resident vertebrate population of its own probably receiving only migrating visitors seeking food and water the footprints in the copper canyon formation reflect part of the wide diversity of vertebrates present in southern california during the hemphillian

trace fossils of talitrid sandhoppers in interglacial littoral calcareous sandstones cornwall u k small 6cm isolated cylindrical shafts observed in carbonate cemented current bedded interglacial littoral sands sandrock in cornwall have previously been interpreted as intrasedimentary stalactites an alternative explanation as trace fossils of the ichnogenus skolithos is proposed probable attribution to talitrid amphipoda sandhoppers in particular talitrus saltator montagu is supported by the similarity of the features to the modern burrows of this organism the facies dependence of the features and by independent evidence on the backshore frontal dune environment of deposition the ecological requirements of t saltator support the interpretation of the raised beaches as interglacial in status based on thermoluminescence tl dating and aminostratigraphy and conflict with a recent suggestion that they may be of cold stage origin

short paper fossil vertebrate tracks and indenter mechanics a reconnaissance study is outlined showing how an improved understanding of the formation and character of fossil vertebrate tracks in soft sediments is provided by the application of the mechanical engineer s indenter theory supported by complementary scaled laboratory experiments only the footprint at the bottom of the shaft cut by the animal9s limbs in general offers the best preservation of the shape of the underside of the foot the character of the foot is less well preserved in the deformed bedding beneath the footprint and among laminae deposited in the shaft after the passage of the animal this understanding will help to reduce errors in the use of tracks in taxonomic biostratigraphic palaeoecological behavioural and environmental studies when a limbed vertebrate traverses yielding sediment the limbs deform the substrate into tracks which vary with the nature size and gait of the animal the forcefulness of its limb movements and the sediment properties most previous geological work on tracks and trackways has emphasized their taxonomic biostratigraphical palaeocological and behavioural significance lockley 1986 leonardi 1987 but without an adequate understanding of their environmental implications lockley 1986 and in particular their genesis and preservation as trace fossils the response of an elastic plastic material to an indenter or punch both in terms of theory calladine 1969 hill 1971 johnson et al 1982 and in the light of qualitative laboratory experiments provides a general mechanical model for track formation and a basis for the assessment of track preservation in sediments the indenter represents the animal9s

trace fossil analysis of lacustrine facies and basins abstract two ichnofacies are typical of lacustrine depositional systems the scoyenia ichnofacies characterizes transitional terrestrial nonmarine aquatic substrates periodically inundated or desiccated and therefore is commonly present in lake margin facies the mermia ichnofacies is associated with well oxygenated permanent subaqueous fine grained substrates of hydrologically open perennial lakes bathymetric zonations within the mermia ichnofacies are complicated by the wide variability of lacustrine systems detected proximal distal trends are useful within particular lake basins but commonly difficult to extrapolate to other lakes other potential ichnofacies include the typically marine skolithos ichnofacies for high energy zones of lakes and substrate controlled still unnamed ichnofacies associated to lake margin deposits trace fossils are useful for sedimentologic analysis of event beds lacustrine turbidites are characterized by low diversity suites reflecting colonization by opportunistic organisms after the turbidite event underflow current beds record animal activity contemporaneous with nearly continuous sedimentation ichnologic studies may also help to distinguish between marine and lacustrine turbidites deep marine turbidites host the nereites ichnofacies that consists of high diversity of ornate grazing traces and graphoglyptids recording highly specialized feeding strategies developed to solve the problem of the scarcity of food in the deep sea deep lacustrine environments contain the mermia ichnofacies which is dominated by unspecialized grazing and feeding traces probably related to the abundance and accessibility of food in lacustrine systems the lower diversity of lacustrine ichnofaunas in comparison with deep sea assemblages more likely reflects lower species diversity as a consequence of less stable conditions increase of depth and extent of bioturbation through geologic time produced a clear signature in the ichnofabric record of lacustrine facies paleozoic lacustrine ichnofaunas are typically dominated by surface trails with little associated bioturbation during the mesozoic bioturbation depth was higher in lake margin facies than in fully lacustrine deposits while significant degrees of bioturbation were attained in lake margin facies during the triassic major biogenic disruption of primary bedding in subaqueous lacustrine deposits did not occur until the cretaceous

a decadally resolved paleohurricane record archived in the late holocene sediments of a florida sinkhole article i nfo a 4500 year record of hurricane induced storm surges is developed from sediment cores collected from a coastal sinkhole near apalachee bay florida recent deposition of sand layers in the upper sediments of the pond was found to be contemporaneous with significant historic storm surges at the site modeled using slosh and the best track post 1851 a d dataset using the historic portion of the record for calibration paleohurricane deposits were identified by sand content and dated using radiocarbon based age models marine indicative foraminifera some originating at least 5 km offshore were present in several modern and ancient storm deposits the presence and long term preservation of offshore foraminifera suggest that this site and others like it may yield promising microfossil based paleohurricane reconstructions in the future due to the sub decadal 7 years resolution of the record and the site s high susceptibility to hurricane generated storm surges the average local frequency of recorded events approximately 3 9 storms per century is greater than that of previously published paleohurricane records from the region the high incidence of recorded events permitted a time series of local hurricane frequency during the last five millennia to be constructed variability in the frequency of the largest storm layers was found to be greater than what would likely occur by chance alone with intervals of both anomalously high and low storm frequency identified however the rate at which smaller layers were deposited was relatively constant over the last five millennia this may suggest that significant variability in hurricane frequency has occurred only in the highest magnitude events the frequency of high magnitude events peaked near 6 storms per century between 2800 and 2300 years ago high magnitude events were relatively rare with about 0 3 storms per century occurring between 1900 and 1600 years ago and between 400 and 150 years ago a marked decline in the number of large storm deposits which began around 600 years ago has persisted through present with below average frequency over the last 150 years when compared to the preceding five millennia

trace fossils and rhizoliths as a tool for sedimentological and palaeoenvironmental analysis of ancient continental evaporite successions abstract recognition of evaporite formations from continental tertiary basins of spain provides evidence that trace fossils including rhizoliths can be abundant in some saline lake systems and their study helps in palaeoenvironmental interpretation of ancient continental evaporite sequences six main types of trace fossils have been distinguished and include 1 networks of small rhizoliths 2 large rhizoliths 3 tangle patterned small burrows 4 isolated large burrows 5 l shaped traces and 6 vertebrate tracks rhizoliths were related to both marginal areas of hypersaline lakes and lakes of moderately high saline waters in these settings pedoturbation resulted from colonization by grasses and bushes of distinct lake subenvironments the activity of burrowing invertebrate faunas was especially intense in lakes of moderately concentrated brines from which gypsum was the main evaporite mineral deposited a specific gypsum lithofacies bioturbated gypsum deposits forming thick massive beds has a widespread occurrence in many of the basins tangle patterned small burrows and minor isolated large burrows constitute the typical trace fossil types within the gypsum the traces are interpreted as having been caused by burrowing insect larvae probably chironomids coleopterans and annelids the behaviour of these organisms in recent lake environments yields information about the salinity range of lake waters from which gypsum precipitated concentration values averaging 100 150 g l may be thus deduced though some organisms involved in the formation of the traces can tolerate higher salinities the combined analysis of lithofacies and trace fossils from the lacustrine evaporite sequences contributes to the study of distinct saline lake subenvironments as well as changes in the sedimentary evolution of the lake systems consequently trace fossils can provide valuable insight for palaeoenvironmental analysis of at least some evaporite formations that accumulated in continental settings

faunal burrows as indicators of paleo vegetation in eastern washington usa abstract the palouse loess deposits in the pacific northwestern united states contain buried paleosols with distinctive biological fabrics the washtucna soil formed 40 000 15 000 yr ago during the last full glacial contains up to 90 by volume distinctive cylindrical pedotubules that are 1 2 cm in diameter formed by burrowing fauna the overlying post glacial loess and modern surface soils that support perennial grasslands lack these cylindrical burrows we investigated the paleoecological significance of the burrows observations of several types of burrowing fauna under native vegetation indicated that nymphs of cicadas homoptera cicadidae are responsible for cylindrical back filled burrows that match those in the washtucna soil we measured the abundance of active cicada burrows in soils in four native vegetation zones soil sites establishing that cicada burrows comprise 19 of the rooting zone volume in sagebrush steppe but just 1 3 of the rooting zone volume in bunchgrass steppe meadow steppe and coniferous forest this suggests strongly that abundant cicada burrows in paleosols are a proxy for the geographic extent of plant communities that contained sagebrush to assay the activity of cicada nymphs through time we measured burrowed volume by depth from the soil surface down through the washtucna soil i e from the present to ca 40 000 yr bp at five stratigraphic research sites paleosol sites cicada burrowed volume is generally less than 5 in the rooting zone of the surface soils and throughout holocene loess but is up to 94 in the rooting zone of the washtucna soil at paleosol sites that currently support bunchgrass steppe outside the present day sagebrush steppe zone cicada host preferences in vegetation zones today suggest that sagebrush was the dominant shrub as the washtucna soil formed from this reconstruction combined with information from studies of pollen and plant opal we infer that periglacial steppe plant communities dominated by sagebrush spread from 40 000 to 15 000 yr bp into areas of the columbia plateau that support bunchgrass steppe meadow steppe and even some coniferous forest today trace fossils of soil fauna in paleosols are indicators of fluctuations in biological activity in response to climatic changes of the quaternary period in palouse loess deposits

trace fossils in fluvial deposits of the duchesne river formation eocene uinta basin utah abstract a diverse suite of nonmarine trace fossils is present in a late eocene sandstone siltstone mustone sequence in the duchesne river formation of northeastern utah on the basis of primary sedimentary structures and stratification sequences the beds are interpreted as deposits in meandering and braided stream environments the trace fossils include ancorichnus muensteria m palaeophycus scoyenia skolithos two unidentified ichnotaxa and rhizoliths insects and oligochaetes apparently made most of the animal traces in moistor wet cohesive stream sediment during subaerial exposure or under thin water cover

vertebrate tracks in pleistocene eolian sand sheet deposits of alaska abstract deformation structures interpreted as vertebrate hoof and foot tracks occur within upper pleistocene eolian sand sheet deposits in the stabilized kantishna sand sea of central alaska and in the nushagak lowland of southwestern alaska exposures of tracks are generally limited to cross sections which reveal concave up deformation structures in which displacement of preexisting strata diminishes downward deposits in both areas contain tracks that are 6 to 16 cm in diameter and are divided by a central ridge reflecting formation by artiodactyl even toed ungulates larger 21 34 cm tracks without a central ridge observed in the nushagak lowland were formed by proboscideans probably woolly mammoth large vertebrate tracks occur within irregularly stratified sand and silty sand that accumulated upon partially vegetated sand sheets and within thin even wind ripple laminae of unvegetated sand sheets the presence of tracks at multiple stratigraphic levels and preservation of roots and rhizocretions within the eolian deposits suggest that vegetated sand sheets may have formed a locally important grazing habitat for large herbivores during at least part of the last glaciation recognition that vertebrate tracks are preserved in eolian sand sheet deposits and in deposits of other environments as well opens a new source of stratigraphic and paleoecological data to aid reconstruction of the vanished ecosystems of beringia

ichnofossils of the alluvial willwood formation lower eocene bighorn basin northwest wyoming u s a abstract the ichnofossil assemblage of the lower eocene willwood formation consists of at least nine distinct endichnia that are preserved in full relief four forms three ichnogenera and four ichnospecies are new and represent fodinichnia and domichnia of oligochaete worms an insect or spider an unknown vertebrate probably a mammal and domichnia of an unidentified organism other potential trace makers of the ichnofauna include insects mollusks and decapods in contrast to an egyptian oligocene fluvial ichnofauna produced largely by animals that burrowed in stream channel deposits the willwood assemblage is principally of flood plain origin though the ichnofauna occurs in a variety of paleosol types most of the fossils are restricted in distribution to specific sediment and soil types and within paleosols to specific identifiable horizons this attribute will make them valuable indiced of paleoenvironment once they are better known in other ancient alluvial sequences the environment suggested by the willwood trace fossils damp but not wet soils with fluctuating water tables is consistent with the warm temperate to subtropical possibly monsoonal conditions that are interpreted for the willwood formation by independent evidence of body fossils and paleopedology

paleoearthquakes on the anninghe and zemuhe fault along the southeastern margin of the tibetan plateau and implications for fault rupture behavior at fault bends on strike slip faults abstract fault bends can serve as fault segment boundaries and are used in seismic hazard assessment recent studies addressing whether rupture propagation would be arrested at such structural complexities have commonly focused on computational modeling however multi cycle paleoseismic rupture observations through fault bends have seldom been reported in this study we used trenching and radiocarbon dating to reveal paleoseismic rupture histories on the southern segment of the anninghe fault anhf along the southeastern margin of the tibetan plateau to explore multi cycle surface rupture behavior at an extensional fault bend with an angle of about 30 at xichang between the anhf and zemuhe fault zmhf specifically nine trenches were opened in a fault depression at maoheshan site and five paleoseismic events were identified these have been named e1 through e5 respectively corresponding to events at 1400 935 bce 420 875 ce 830 1360 ce 1295 1715 ce and 1790 ce present after comparison with the historical records of earthquakes around xichang and previous paleoseismic results we suggest that the five seismic events are constrained at 1365 bce 935 bce 814 ce 950 ce 1145 ce 1536 ce and 1850 ce respectively the average recurrence interval of earthquakes along the southern segment of the anhf is about 700 800 yr furthermore the evidence indicates that surface faulting events along the southern segment of the anhf appear to be unevenly spaced in time moreover based on comparisons of seismic events along the anhf and zmhf we find that two fault segments simultaneous ruptured during the 814 ce and 1850 ce earthquakes event e3 and the 1536 ce earthquake ruptured the anhf but not rupture the zmhf we suggest that the xichang fault bend is not a persistent fault boundary indicating that extensional fault bends with an angle of about 30 may not effectively terminate seismic ruptures on strike slip faults