Taphonomic controls

The mode of preservation of trace fossils is controlled by intrinsic biological determinants, but also by extrinsic ones as the type or consistency of the substrate (Bromley, Reference Bromley1996), as well as other subsequent sedimentologic factors, such as diagenetic processes and penecontemporaneous erosion and agradation (Goldring et al., Reference Goldring, Pollard and Taylor1997). The morphological features resulting from external parameters must be taken into account during ichnotaxonomic determination (Goldring et al., Reference Goldring, Pollard and Taylor1997; Minter et al., Reference Minter, Braddy and Davis2007) to avoid oversplitting. Here, the relationship between some morphological features of the studied trace fossils and the extrinsic parameters are evaluated.

One of the principles of ichnology is that the same behaviour of an organism can produce various morphologies in different substrates (Bromley, Reference Bromley1996). A well-known example of trace fossil controlled by substrate consistency are the different morphologies of the chevron-shaped locomotion trace Protovirgularia, which is produced by bivalves with a bifurcated foot moving across the sediment (e.g., Maples and West, Reference Maples and West1990; Seilacher and Seilacher, Reference Seilacher and Seilacher1994; Mángano et al. Reference Mángano, Buatois, West and Maples1998; Gibert and Domènech, Reference Gibert and Domènech2008; Carmona et al., Reference Carmona, Mángano, Buatois and Ponce2010). The two morphologic types of Taenidium barretti in the Tremp Formation can be explained as well as controlled by the characteristics of the substrate. Substrate consistency and physical properties are mainly controlled by water content, compaction, and cementation (Ekdale, Reference Ekdale1985; Lewis and Ekdale, Reference Lewis and Ekdale1992) and these characteristics control burrowing styles. For example, as compacted sediment is stiffer, this results in a decreased burrowing rate and penetration depth (Ansell, Reference Ansell1962; Trueman, Reference Trueman1966). The same happens with cementation, which increases shear strength, making penetration difficult. The vertical profile through the substrate is zoned in terms of water content because the weight of overburden causes dewatering (Bromley, Reference Bromley1996). Consequently, a more compacted and/or cemented substrate has less water content and increased shear strength, offering additional resistance to burrowing (Mángano et al., Reference Mángano, Buatois, West and Maples1998). Less compacted or cemented substrates tend to favour the generation of variably oriented burrows, being the tracemakers able to change the direction of burrowing and to penetrate deeper (Fig. 6.1). Otherwise, the path followed by infaunal burrowers where the substrate is stiffer tends to be shallower and mainly oriented horizontal to the bedding plane since the underlying sediment is more compacted, dewatered or/and cemented (Fig. 6.2). Following this line of reasoning, the two types of Taenidium barretti do not represent differences in behaviors, but essentially reflect changes in the properties of the substrate.

On the other hand, diagenesis implies physicochemical processes of preservation and alteration that may enhance or diminish trace fossils (Goldring et al., Reference Goldring, Pollard and Taylor1997; Buatois and Mángano, Reference Buatois and Mángano2011). This is due, in part, to the different characteristics of the sediment within burrows and in the host rock. For instance, the content in organic matter may act as focus for mineral precipitations in burrow linings (Fürsich, Reference Fürsich1972) or linked to the organic detritus excreted by feeding organisms (McIlroy et al., Reference McIlroy, Worden and Needham2003; Needham et al., Reference Needham, Worden and Cuadros2006). In addition, the differences in porosity and permeability of the burrows play an important role during diagenesis (Pemberton and Gingras, Reference Pemberton and Gingras2005). Taenidium barretti shows Fe oxides precipitated along the discontinuities between the menisci (Fig. 7), most likely during diagenesis. These oxides enhance the meniscate fill, favouring menisci visibility (Fig. 7). Because the precipitation of oxides is controlled by the oxidation/reduction conditions, a change in these conditions would affect the mode of preservation of the burrows. Although Planolites (Fig. 5) may be regarded as a preservational variant of Taenidium barretti where the meniscate fill is not visible, subtle evidence of the meniscate fill, such as the array of the quartz grains or visible meniscate shaped discontinuities (Fig. 7), are not observed in the specimens assigned to Planolites. In addition, Planolites is consistently smaller than Taenidium (Fig. 8) in the studied collection, arguing against a taphonomic control.

The large variation in size of Loloichnus and Arenicolites is most likely due to intrinsic biologic determinants, but external parameters, such as erosion and lithification cannot be disregarded. The preservation potential of the thick-walled Loloichnus is higher than that of unlined burrows; even smaller burrows produced by juvenile crustaceans may be preserved. In addition, the depth of penetration of the burrowing crustaceans producing Loloichnus is larger than that of the other burrows described in this study (Fig. 3), increasing preservation potential as well. These features, intrinsic to the Loloichnus tracemaker, may have contributed to the large range of sizes recorded for Loloichnus (Fig. 8, Table 2). Arenicolites is similar in width to Loloichnus (Fig. 8, Table 2) but because of the type of preservation of the burrows as negative epirelief, the large variation in their size may reflect an overprint by erosion. Preservational style of the Tremp Arenicolites is remarkably similar to U-shaped burrows present in intertidal sediment covering beach rocks in the Bay of Fundy (Fig. 10.1). There, on the sedimentary surface, openings of U-shaped burrows are visible as paired large oval depressions preserved as negative epirelief (Fig. 10.2). In cross-sectional views, small U-shaped traces of incipient Arenicolites produced by the amphipod crustacean Corophium volutator (Pallas, Reference Pallas1766) are preserved as full relief (Fig. 10.3, 10.4). Corophium volutator has the ability to switch from suspension feeding to deposit feeding (Gerdol and Hughes, Reference Gerdol and Hughes1994), as well as to feed from detritus by scraping (Meadows and Reid, Reference Meadows and Reid1966). The larger size of Arenicolites preserved on bed tops may reflect currents during ebb-flood tides, washing up sediment previously manipulated and accumulated by the organisms. Thus, the contrast in size between the openings preserved as negative epirelief and the U-shaped burrows preserved as full relief (Fig. 10) may be a taphonomic feature controlled by sedimentary processes, such as erosion and partial lithification and not by differences in behavior. Thus, the high dispersion of Arenicolites width in the Tremp Formation can be of taphonomic origin. Differences in substrate conditions (e.g., water or organic matter content or consolidation) even in an isochronous surface result in variable resistance of the substrate to erosional processes.

Figure 10

Incipient Arenicolites produced by Corophium volutator (Pallas, Reference Pallas1766) in intertidal deposits of the Bay of Fundy: (1) paired large oval openings preserved as negative epirelief at the top of the layers; (2) close-up of paired openings; note the similitude with the trace fossil in Figure 4.2; (3) cross-section in the intertidal deposit, showing small U-shaped tubes preserved as full relief (black arrow); (4) detail of the tracemaker, the amphiphod Corophium volutator, scale bar represents 0.5 mm.

Ichnofacies model

The dominant elements of the Tremp Formation ichnofauna are meniscate backfilled trace fossils typical of the Scoyenia Ichnofacies, an archetypal ichnofacies first proposed by Seilacher (Reference Seilacher1967) and refined subsequently by Frey et al. (Reference Frey, Pemberton and Fagerstrom1984b) and Buatois and Mángano (Reference Buatois and Mángano1995, Reference Buatois and Mángano2002). The meniscate trace fossils of the Tremp Formation belong in Taenidium, mostly Taenidium barretti. Most of the characteristics associated with the Scoyenia Ichnofacies are present in the Tremp ichnofauna. For example, the abundance of meniscate burrows produced by mobile organisms, the mixture of invertebrate, vertebrate and plant trace fossils and the common occurrences of monospecific suites of meniscate ichnotaxa are all characteristics of this ichnofacies (Frey et al., Reference Frey, Pemberton and Fagerstrom1984b; Buatois and Mángano, Reference Buatois and Mángano2004). The Scoyenia Ichnofacies indicates that sediments were periodically exposed to air or periodically inundated (Frey et al., Reference Frey, Pemberton and Fagerstrom1984b; Frey and Pemberton, Reference Frey and Pemberton1984, Reference Frey and Pemberton1987). A low-energy setting is suggested by the abundance of horizontal trace fossils (Buatois and Mángano, Reference Buatois and Mángano2011), whereas the lack of ornamentation characterizes the soft substrate suite of the Scoyenia Ichnofacies (Buatois et al., Reference Buatois, Mángano and Aceñolaza1996; Savrda et al., Reference Savrda, Blanton-Hooks, Collier, Drake, Graves, Hall, Nelson, Slone, Williams and Wood2000; Buatois and Mángano, Reference Buatois and Mángano2002, Reference Buatois and Mángano2004). Dwelling structures, such as Skolithos Haldeman, Reference Haldeman1840 and Cylindricum Linck, Reference Linck1949, may occur, but tend to be subordinate in the Scoyenia Ichnofacies (e.g., Buatois and Mángano, Reference Buatois and Mángano1995). In the Tremp Formation, dwelling structures, such as Arenicolites and Loloichnus, are subordinate to the meniscate trace fossils, but they may be abundant in some paleochannel deposits characterized by higher-flow velocities. Finally, Planolites in the lower paleochannel of the first paleochannel cluster (Fig. 9) most likely illustrates a poorly developed aquatic suite that cannot be included within the Scoyenia Ichnofacies.

Implications for the characterization of tide-influenced meandering channels

Trace fossils of tide-influenced meandering channels have been described in delta plains (e.g., Rebata et al., Reference Rebata, Gingras, Räsänen and Barberi2006a; Hovikoski et al., Reference Hovikoski, Lemiski, Gingras, Pemberton and MacEachern2008a; Sisulak and Dashtgard, Reference Sisulak and Dashtgard2012) and inner zones of estuaries (e.g., Gingras et al., Reference Gingras, Pemberton, Saunders and Clifton1999, Reference Gingras, Räsänen and Ranzi2002; Pemberton et al., Reference Pemberton, Spila, Pulham, Saunders, MacEachern, Robbins and Sinclair2001; Mángano and Buatois, Reference Mángano and Buatois2004; Rebata et al., Reference Rebata, Räsänen, Gingras, Vieira, Barberi and Irion2006b; Hovikoski et al., Reference Hovikoski, Räsänen, Gingras, Ranzi and Melo2008b). In these marginal-marine environments, the ichnofauna is controlled by the stressful conditions affecting the benthic biota, such as salinity fluctuation, increased sediment discharge, and high clay content (e.g., Dörjes and Howard, Reference Dörjes and Howard1975; Pemberton and Wightman, Reference Pemberton and Wightman1992; MacEachern and Pemberton, Reference MacEachern and Pemberton1994; Buatois et al., Reference Buatois, Mangano and Maples1997; Mángano and Buatois, Reference Mángano and Buatois2004; MacEachern and Gingras, Reference MacEachern and Gingras2007; Carmona et al., Reference Carmona, Buatois, Ponce and Mángano2009; Dashtgard, Reference Dashtgard2011). For instance, tide-influenced deposits from brackish-water deltaic or estuarine systems show low-diversity marine trace-fossil assemblages (Pemberton and Wightman, Reference Pemberton and Wightman1992; Mángano and Buatois, Reference Mángano and Buatois2004), whereas the freshwater inner parts may display relatively high diversity of assemblages produced by a terrestrial to freshwater fauna (Buatois et al., Reference Buatois, Mangano and Maples1997). However, examples exhibiting a continuous ichnologic record from the open to the inner part of the estuarine/deltaic system are virtually inexistent.

The ichnofauna from the Tremp Formation does not display marked changes in ichnodiversity through the stratigraphic section, but it shows a subtle change in the trace-fossil assemblages from a poorly developed aquatic suite in the lower part of the first cluster of paleochannel deposits to a relatively well-developed Scoyenia Ichnofacies in the third one (Fig. 9). In addition, small variations in burrow sizes have been detected between the elements of these two assemblages (Fig. 8, Table 2). Therefore, the two trace-fossil assemblages may reflect differences in salinity during the development of the meander loops. The assemblage of the lower part of the first cluster could reflect higher stress conditions due to extreme salinity fluctuations, and brackish-water. This interpretation agrees with the presence in the first cluster of authigenic glauconite (Díez-Canseco et al., Reference Díez-Canseco, Arz, Benito, Díaz-Molina and Arenillas2014).

The Scoyenia Ichnofacies is typical of non-marine, inundated environments, such as lake margins and floodplains (Buatois and Mángano, Reference Buatois and Mángano2011, p. 75 and references therein). However, the Scoyenia Ichnofacies is also present in marginal-marine environments (Buatois et al., Reference Buatois, Mangano and Maples1997), particularly in the fluvial-tidal transition where the ichnofauna distribution is controlled by the limits of maximum salinity and tides. Between both limits, a freshwater tide-influenced area is formed, commonly tens of kilometres wide (e.g., Buatois et al., Reference Buatois, Mangano and Maples1997; Dalrymple and Choi, Reference Dalrymple and Choi2007; Shiers et al., Reference Shiers, Mountney, Hodgson and Cobain2014). The occurrence of elements of the Scoyenia Ichnofacies in the tidal-fluvial transition has been also reported by Netto and Rossetti (Reference Netto and Rossetti2003), Mángano and Buatois (Reference Mángano and Buatois2004), and Hovikoski et al. (Reference Hovikoski, Gingras, Räsänen, Rebata, Guerrero, Ranzi, Melo, Romero, del Prado and Jaimes2007, Reference Hovikoski, Räsänen, Gingras, Ranzi and Melo2008b). Netto and Rossetti (Reference Netto and Rossetti2003) attributed monospecific assemblages of Taenidium in this setting to inner mangrove areas. Thus, the upper part of the section of Tremp Formation was deposited most likely under freshwater conditions within the fluvial-tidal transition.

On the other hand, Arenicolites is more abundant in the lower part of the section, particularly in the second cluster of paleochannel deposits (Fig. 9). The higher abundance of Arenicolites may indicate areas occasionally subjected to higher-energy tidal currents and/or brackish-water conditions. Arenicolites in the Tremp Formation closely resembles the enlarged Arenicolites observed in the intertidal areas of Bay of Fundy, where it is produced by the crustacean Corophium volutator (Pallas, Reference Pallas1766) (Fig. 10). Corophium volutator inhabits mud flats, salt-marsh pools, and brackish-water ditches, and tolerates a wide range of salinities from seawater to almost freshwater (Kluijver and Ingalsuo, Reference Kluijver and Ingalsuo1999; Percy, Reference Percy1999). The differences in abundance of Arenicolites in the studied section can be explained as a result of salinity fluctuations, as well as by tidal fluctuations. Salinity fluctuations may control the distribution of the producer, whereas tidal fluctuations may act as a control factor for the erosion and enlargement of burrow entrances. The second cluster of paleochannel deposits most likely represents an intermediate situation within the fluvial-tidal transition characterized by the Scoyenia Ichnofacies and areas with high density of Arenicolites.

The ichnofaunal variation through the stratigraphic section agrees with the shallowing-upwards tendency detected in the Tremp Formation, based on the decrease in abundance of planktonic foraminifera transported by tidal currents (Díez-Canseco et al., Reference Díez-Canseco, Arz, Benito, Díaz-Molina and Arenillas2014). In the same vein, the ichnofaunal changes reflect the landward location of the younger meandering loop deposits since a decrease in salinity and a passage to freshwater conditions is inferred (Fig. 11). The higher abundance of root trace fossils, paleosols and vertebrate tracks in the third cluster of paleochannel deposits, together with the widespread Scoyenia Ichnofacies indicates formation under freshwater conditions in a landward location. The ichnofauna of the Tremp Formation records deposits within the fluvial-tidal transition of a delta (Díez-Canseco et al., Reference Díez-Canseco, Arz, Benito, Díaz-Molina and Arenillas2014), particularly located in the mixed-energy area and the fluvial-tidal channel (sensu Dalrymple and Choi, Reference Dalrymple and Choi2007) and controlled by the maximum salinity limit (Fig. 11).

Figure 11

Summary model of ichnofaunal distribution in the fluvial-tidal transition of the Tremp Formation. The mixed area and the fluvial-tidal channel are marked in terms of current dominance, whereas ichnology responds to salinity and is controlled by the salinity limit (modified from Buatois et al., Reference Buatois, Mangano and Maples1997; Dalrymple and Choi, Reference Dalrymple and Choi2007; Díez-Canseco et al., Reference Díez-Canseco, Arz, Benito, Díaz-Molina and Arenillas2014; and Shiers et al., Reference Shiers, Mountney, Hodgson and Cobain2014).