Paleoecological and paleoenvironmental implications
Trace fossils are relatively diverse and abundant in the wave-dominated shallow-marine deposits of the Alfarcito Member, reflecting the establishment of shallow-marine conditions that terminated estuarine deposition in the area. Alternating and contrasting energy conditions due to repeated storm events were among the key controlling factors for trace fossil distribution and preservation. Storm events involve erosion followed by rapid deposition. The initial phase of rapid deposition is in turn followed by a waning phase and the re-establishment of fairweather sedimentation under lower energy conditions. Accordingly, these sedimentary events impose a stress factor on the benthic communities inhabiting wavedominated, shallow-marine areas.
The ichnology of storm-influenced, shallow-marine successions has been addressed in a number of studies ( e.g . Pemberton and Frey, 1984; Vossler and Pemberton, 1989; Frey, 1990; Frey and Goldring, 1992; Pemberton et al ., 1992; MacEachern and Pemberton, 1992; Pemberton and MacEachern, 1997). These authors noted that storm-influenced deposits display two contrasting trace fossil assemblages that reflect the behavioral response of the benthic fauna inhabiting two successive and different habitats. The resident, fairweather trace fossil assemblage records the establishment of a benthic community developed under stable and rather predictable conditions. This assemblage commonly belongs to the Cruziana ichnofacies and has been attributed to populations displaying K-selected or climax strategies (see also Bromley, 1996). By contrast, the storm-related trace fossil assemblage records colonization after storm deposition. This assemblage reflects the establishment of an opportunistic community displaying r-selected population strategies in an unstable, physically-controlled environment. Opportunistic colonizers are usually, though not always, represented by the Skolithos ichnofacies.
These two trace fossil assemblages have been recognized in the Alfarcito Member. The fairweather trace fossil assemblage is the most diverse and includes more varied behavioral strategies, representing an example of the Cruziana ichnofacies. Ethologically, this assemblage includes locomotion ( Archaeonassa fossulata , Cruziana semiplicata , C. problematica , Cruziana isp. and Diplichnites isp.); resting ( Rusophycus moyensis , R. carbonarius , Rusophycus issp. and Bergaueria aff. B. hemispherica ); pascichnia ( Dimorphichnus aff. D. quadrifidus ); feeding ( Arthrophycus minimus , ? Gyrolithes isp., Gyrophyllites isp., ? Phycodes isp. and Planolites reinecki ); and dwelling ( Palaeophycus tubularis , P . striatus ) trace fossils. Most of these ichnotaxa are preserved at the base of sandstone layers. However, some occur at the top of the event beds ( Archaeonassa fossulata , Diplichnites isp., Gyrophyllites isp.). Ichnotaxa preserved at the top of sandstone layers may record the activity of infaunal organisms that burrowed through the fairweather and waning mudstone down to the mudstone-sandstone interface ( Gyrophyllites isp.). These structures are clearly part of the community that established under normal conditions after the storm disturbance. Other biogenic structures preserved at the top of event layers record the activity of vagile organisms that produce simple locomotion trails and trackways ( Archaeonassa fossulata , Diplichnites isp.). This suite may reflect the activity of the first incomers after the storm, which may in fact have survived the mild effects of a storm in a distal setting. Distinction of pre- and post-storm assemblages in these cases is less straightforward.
The typical storm-related trace fossil assemblage in the Alfarcito Member is monospecific and includes vertical dwelling traces ( Skolithos linearis ), recording an example of the Skolithos ichnofacies. This assemblage occurs in low to moderate densities and records opportunistic colonization after storm deposition. Vertical burrows are preserved as relatively deep, endichnial structures that penetrate into stormemplaced sandstone layers. Because these vertical burrows commonly do not occur in profuse densities, degree of bioturbation remains relatively low and internal physical structures of the tempestites ( e.g . hummocky cross stratification, combined- flow ripple lamination) are well preserved. Skolithos linearis records a distinctive onshore-offshore trend. Specimens of S. linearis in distal settings (upper offshore) characteristically display smaller size and occur in even lower densities than those from more proximal environments (offshore transition and shoreface).
Integration of ichnologic and sedimentologic data allows reconstruction of proximal-distal trends in shallow-marine trace fossil assemblages along a nearshore-offshore transect (figura 8). High-energy conditions prevailed in lower and middle shoreface environments, commonly precluding the preservation of biogenic structures. Most shoreface beds are unbioturbated. Repeated storm-wave erosion precluded the establishment and/or preservation of the fairweather assemblage in the shoreface deposits. Shallow- to mid-tier biogenic structures were most likely removed by erosion due to deep scouring. Bioturbation is therefore restricted to vertical burrows ( Skolithos linearis ) of the Skolithos ichnofacies. This postdepositional assemblage records environmental changes related to episodic sedimentation, such as the input of organic particles that were kept in suspension in the water column by storm wave agitation.
Environmental conditions in the offshore transition are more variable and reflect the alternation of high-energy storm events and low-energy fairweather mudstone deposition. The storm-related Skolithos trace fossil assemblage is present, but alternates with the fairweather trace fossil assemblage illustrating an archetypal Cruziana ichnofacies. Hummocky sandstones having intensely bioturbated tops and covered by the radial feeding trace Gyrophyllites isp. Are common in the offshore transition deposits. As previously discussed, these sandstone tops are palimpsest surfaces, recording the storm-related assemblage and the subsequent fairweather assemblage.
As in the offshore transition facies, upper offshore deposits display the alternation of the resident fairweather and storm-related colonization trace fossil assemblages. The fairweather trace fossil assemblage reaches a diversity maximum in the upper offshore (archetypal Cruziana ichnofacies). Undoubtedly, the less erosive nature of these more distally emplaced tempestites coupled with the overall lower energy of the upper offshore favor development and preservation of the fairweather suite. Additionally, there is a tendency for the preservation of shallow-tier trace fossils ( e.g . Arthrophycus minimus ). Additionally, preservation of trace fossils in upper offshore deposits may have been favored by the presence of abundant sandstone/mudstone interfaces. The Cruziana ichnofacies reaches a climax in the upper offshore deposits studied. Here, however, the stormrelated assemblage becomes less and less distinctive, with some sandstone beds recording small and dispersed Skolithos linearis or occasionally only the presence of horizontal locomotion structures.
Trace fossils are scarce in lower offshore deposits. The storm-related trace fossil assemblage is not present and the fairweather assemblage is restricted to a few non-descript burrows, mostly Palaeophycus tubularis (representing the distal Cruziana ichnofacies). The scarcity of trace fossils in lower offshore facies probably reflects oxygen-depleted conditions. This hypothesis is also supported by the overall small size of the trace fossils studied, which suggests a stressed environment. Additionally, the lower abundance of sandstone interbeds may have inhibited preservation and visibility of biogenic structures.
Analysis of shallow-marine clastic deposits of the lower fine-grained interval of the Alfarcito Member shows that few ichnotaxa are restricted to particular zones of the nearshore to offshore transect. This case study reveals, therefore, the pitfalls of the checklist approach, as previously noted by Howard and Frey (1975). An integrated approach, which takes into account several characteristics, such as degree of bioturbation, abundance of individual ichnotaxa, ethological and ecological significance of the biogenic structures, ichnofabrics and tiering structure, is more useful to delineate environmental subdivisions of shallow-marine clastic successions ( e.g . Buatois et al ., 2002).
MacEachern and Pemberton (1992) analyzed shoreface variability in the Cretaceous of the North American Western Interior Seaway and characterized three types of shoreface based on the intensity and frequency of storm events. This model has proved to be useful to characterize shoreface deposits in the stratigraphic record. The weakly stormaffected shorefaces (low energy) are characterized by relatively minor amounts of tempestites. These shorefaces are dominated by fairweather trace fossil assemblages and thin storm beds are commonly obliterated by biogenic reworking or thoroughly bioturbated. A transition from the Cruziana to the Skolithos ichnofacies is coincident with the lower to middle shoreface transition. Examples of ichnofaunas in weakly storm-affected shorefaces have been documented recently from the Upper Ordovician Santa Gertrudis Formation of northwestern Argentina (Mángano and Buatois, 2003), the Lower Pennsylvanian Morrow Sandstone of subsurface Kansas (Buatois et al ., 2002), the Upper Carboniferous Hoyada Verde Formation of San Juan (Mángano et al ., 2003) and the Miocene Chenque Formation of Patagonia (Buatois et al ., 2003). The moderately storm-dominated shorefaces (intermediate energy) display an alternation of laminated storm beds and intensely bioturbated fairweather deposits, resulting in the so-called ?Lam-Scram? pattern. These deposits commonly show the alternation of elements of the Skolithos ichnofacies as opportunistic pioneers colonizing sandstone tempestites and the Cruziana ichnofacies recording the activity of the fairweather resident community. Examples of ichnofaunas in moderately storm-dominated shorefaces are known from the Permian Rio Bonito and Palermo formations in southern Brazil (Netto, 1998; Buatois et al ., 2001, 2005). The strongly storm-dominated shorefaces (high energy) commonly consist of amalgamated hummocky sandstones showing little or no bioturbation; only the deepest representatives of the post-storm Skolithos ichnofacies are present and development of the fairweather assemblage is inhibited. Examples of ichnofaunas in strongly storm-dominated shorefaces have been documented also from the Rio Bonito and Palermo formations (Netto, 1998; Buatois et al ., 2001, 2005). Shoreface deposits from the Alfarcito Member compare favorably with the strongly storm-dominated type of MacEachern and Pemberton (1992).
Sequence stratigraphic implications
Integration of ichnologic data within a sequence stratigraphic framework allows delineation of wavedominated parasequences, parasequence sets and alostratigraphic surfaces in the lower fine-grained interval of the Alfarcito Member. A wave-dominated parasequence coarsens upward, recording shoreline progradation (Van Wagoner et al ., 1990). In terms of associated environmental factors, each parasequence reflects a progressive increase in hydrodynamic energy, degree of oxygenation, sand content, amount of organic particles in suspension, and mobility of the substrate (Pemberton et al ., 1992; Mángano et al ., 2002a). These environmental factors undoubtedly controlled vertical distribution of trace fossils in stratal packages of the Alfarcito Member. An idealized wave-dominated parasequence in the Alfarcito Member is characterized, from base to top, by a distal Cruziana ichnofacies (lower offshore), an archetypal Cruziana ichnofacies associated with a stormrelated Skolithos ichnofacies (upper offshore to offshore transition) and a Skolithos ichnofacies (lower/ middle shoreface). However, this ideal parasequence is the exception rather than the rule because not all subenvironments are represented in each parasequence. This is understood if individual parasequences are analyzed in the context of their stacking pattern (figure 9).
In the Alfarcito Member, regional parasequences are well developed recording short-term seaward migrations of the shoreline separated by drowning events. This is in marked contrast with the estuarine units, whose stratal packages are areally restricted to the paleovalley. The lowermost beds of the Alfarcito Member records the drowning of the underlying estuarine deposits. Open-marine deposits locally interfinger with the seaward face of the subtidal sandbar complex that characterizes the outer region of the estuarine valley (Buatois and Mángano, 2003). The passage from estuarine to open-marine conditions is commonly expressed by a low-energy flooding surface or, more appropriately, a drowning surface ( sensu Posamentier and Allen, 1999) rather than an erosive wave ravinement surface (Buatois and Mángano, 2003). Zaitlin et al . (1994) suggested that wave ravinement surfaces are typically absent in tide-dominated estuaries. However, the local presence of channels scoured into the tidal sandbar complex and transgressive lags suggests some influence, albeit limited, of wave ravinement at the top of the estuarine unit. The transgressive systems tract at the lower interval of the Alfarcito Member is poorly developed. Only the lowermost strata of the Alfarcito Member are included in the transgressive systems tract; the bulk of this systems tract is preserved within the underlying incised valley. Transgressive strata culminate with the development of a maximum flooding surface that lies within lower offshore deposits at the lower interval of the Alfarcito Member. At this moment of depositional evolution, the Alfarcito sea reached its maximum depth, coastal plains were flooded and sediment was trapped in shoreline areas, resulting in condensation and starvation in offshore settings. Maximum flooding intervals are currently associated with oxygen-depleted conditions and display limited development of trace fossil assemblages. This condensed section is overlain by lower offshore to offshore transition deposits forming a progradational parasequence set, which represents a highstand systems tract that characterizes part of the lower interval of the Alfarcito Member.
Another sequence boundary is present at the top of this highstand package. This sequence boundary is a co-planar surface (flooding surface/sequence boundary), which marks the vertical replacement of a progradational to a retrogradational parasequence set. This retrogradational parasequence set represents the establishment of a transgressive systems tract that includes parasequences that environmentally range from upper to lower offshore deposits and reflects that the accommodation rate exceeds the rate at which sediment is supplied during deposition of this interval of the Alfarcito Member. A maximum flooding surface separates these transgressive deposits from the subsequent highstand. As in the case of the maximum flooding interval of the lower sequence, bioturbation is restricted and oxygen-depleted conditions were dominant. The lower parasequences in this progradational parasequence set include lower to upper offshore facies, while offshore transition and lower/ middle shoreface deposits are restricted to the upper parasequences of the unit. The progressive shallowing reflected by stratal architecture is paralleled by vertical changes in trace fossil distribution as discussed above. Overall, the Cruziana ichnofacies is gradually replaced by the Skolithos ichnofacies.
Evolutionary implications
Because most of the classical ichnologic models of storm-dominated shallow-marine seas are based on Mesozoic examples ( e.g . MacEachern and Pemberton, 1992), the main peculiarities of the lower Paleozoic example are worth analyzing to evaluate evolutionary trends and secular changes in bioturbation in wave-dominated shallow-marine clastic rocks (cf. McIlroy and Logan, 1999).
Although there seems to be gross similarities between Paleozoic and Mesozoic shallow-marine ichnofaunas, some differences in both fairweather and storm-related trace fossil assemblages are apparent under closer inspection. While Skolithos is the dominant ichnotaxa in lower Paleozoic storm-related trace fossil assemblages, Ophiomorpha is the dominant trace fossil in their Mesozoic counterparts. The decline of Skolithos piperocks through the Paleozoic has been noted by Droser (1991). The replacement of Skolithos by Ophiomorpha in high-energy shallow-marine environments is most likely related to the Mesozoic radiation of decapod crustaceans (see Carmona et al ., 2004). However, this is certainly a long-term evolutionary trend because the storm-related Skolithos assemblage remains virtually unchanged during the lower Paleozoic. As noted by Mángano and Buatois (2003), opportunistic storm-related assemblages are less sensitive to evolutionary events than fairweather, climax ichnofaunas.
Mángano and Droser (2004) noted that while trilobite trails and trackways are dominant in Cambrian to Early Ordovician shallow-marine fairweather assemblages, Middle to Late Ordovician ichnofaunas generally show more varied behavioral patterns. Also, lower Paleozoic fairweather assemblages are dominated by shallower-tier structures than younger, particularly post-Paleozoic shallowmarine assemblages. An increase in burrowing depth and intensity through the Phanerozoic has been suggested ( e.g . Thayer, 1979, 1983). In lower Paleozoic clastic rocks of northwestern Argentina, some temporal changes in fairweather ichnofaunas have been detected (Mángano and Buatois, 2003). Late Cambrian to early Tremadocian fairweather assemblages are dominated by shallow-tiered community structures, defining an essentially two-dimensional ichnofabric. By the late Tremadocian, threedimensional ichnofabrics became more common and produced significant disruption of the primary sedimentary fabric, reflecting more efficient ecospace utilization by deposit feeders. This is clearly evidenced by upper offshore deposits of the Rupasca and Humacha members, which are characterized by the establishment of a pervasive Trichophycus ichnofabric.
The increase in depth and extent of bioturbation through geologic time has implications with respect to preservation of event layers. Wheatcroft (1990) noted that if the transient time ( i.e . time required to advect the signal through the biologically active zone) is less than the dissipation time ( i.e. time required to destroy the event bed), then some evidence of the event layer should be preserved in the stratigraphic record. The pristine preservation of thin tempestites in the Alfarcito Member most likely reflects the restriction of the benthic fauna to the uppermost tiers of the sediment and the absence of true sediment bulldozers in these ancient seas.
Biostratigraphic implications
Although most trace fossils display long temporal ranges, there are some ichnotaxa that reflect particular kinds of animals in which body morphology and behavior underwent closely related evolutionary transformations through geologic time (Seilacher, 2000). Accordingly, these trace fossils are useful in biostratigraphic studies. Recent research in Cambrian-Ordovician clastic rocks of northwest Argentina underscores the potential of the Cruziana and Arthrophycid stratigraphy proposed by Seilacher (1970, 1990, 1992, 1994, 1996, 2000).
The Cruziana stratigraphy is based on ribbon-like bilobate structures ( Cruziana ) and coffee beanshaped structures ( Rusophycus ) identified at ichnospecies level; other trilobite ichnotaxa ( e.g . Dimorphichnus ) were added to the scheme subsequently (Seilacher, 1990). Cruziana ichnospecies are based on fine morphological features, particularly the so called ?claw formula' ( i.e. the fingerprint left by the tips of the endopodites displaying groupings of claws or setae), and secondarily on the presence and morphology of exopodal brushings, pleural or genal spine impressions, and cephalic and coxal marks reflecting burrowing behavior. The attempt to establish a biostratigraphic zonation based simply on the size of Cruziana and Rusophycus (Aceñolaza, 2003, figure 4) is flawed because it ignores taxonomy at ichnospecies level and fails to reflect the complexities of trilobite trace fossils. The biostratigraphic utility of trilobite trace fossils in lower Paleozoic rocks of northwest Argentina has been discussed by Mángano and Buatois (2001, 2003). One of these biostratigraphically useful Cruziana ichnospecies is C. semiplicata , which is present in the Alfarcito Member (and coeval and slightly older units of Cordillera Oriental as well). Cruziana semiplicata ranges from the Upper Cambrian to the Tremadocian (Crimes, 1969, 1975; Baldwin, 1977; Fillion and Pickerill, 1990) and represents a valuable stratigraphic indicator in northwestern Argentina. Rusophycus moyensis , also present in the Alfarcito Member, is a local diagnostic element that also characterizes Upper Cambrian-Tremadocian strata of northwestern Argentina and is apparently absent in younger and older strata (Mángano et al ., 2002b). Seilacher (1990) suggested a Middle Cambrian age for D. quadrifidus . Dimorphichnus cf. D . quadrifidus occurs in the younger Alfarcito Member. However, further material should be analyzed to extend the stratigraphic range of this ichnotaxon.
The Arthrophycid stratigraphy is based on the analysis of selected ichnotaxa ( i.e . Arthrophycus , Daedalus and Phycodes ) produced by worms of unknown taxonomic affinity and has been recently proposed by Seilacher (2000). Arthrophycus minimus , which occurs in the Alfarcito Member, is less complex and remarkably smaller than Ordovician and Silurian Arthrophycus ichnospecies. As noted by Mángano et al . (2005), it may be considered a primitive ichnospecies of Arthrophycus , a sort of link between the ichnogenera Phycodes and Arthrophycus .
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