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Geological Society, London, Special Publications doi: 10.1144/GSL.SP.2004.228.01.16 p355-382. 2004, v.228; Geological Society, London, Special Publications    Jorge F. Genise, E. S. Bellosi and M. G. Gonzalez   ichnofabrics in palaeosols An approach to the description and interpretation of service Email alerting new articles cite this article to receive free e-mail alerts when here click request Permission part of this article to seek permission to re-use all or here click Subscribe Colle
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  Geological Society, London, Special Publicationsdoi: 10.1144/GSL.SP.2004.228.01.16p355-382.2004, v.228; Geological Society, London, Special Publications Jorge F. Genise, E. S. Bellosi and M. G. Gonzalez   ichnofabrics in palaeosolsAn approach to the description and interpretation of serviceEmail alerting new articles cite this article to receive free e-mail alerts whenhereclick requestPermission part of this article to seek permission to re-use all orhereclick Subscribe Collection London, Special Publications or the Lyell to subscribe to Geological Society,hereclick Notes © The Geological Society of London 2013  at CAPES on April 9, 2013http://sp.lyellcollection.org/ Downloaded from    n approach to the description and interpretation of ichnofabrics in palaeosols JORGE F. GENISE l, E. S. BELLOSI 2 & M. G. GONZALEZ 2 1CONICET, Museo Paleontol6gico Egidio Feruglio, Av. Fontana 140, 9100 (Trelew), Chubut, Argentina (e-mail: jgenise@mef org.ar) 2 CONICET, Laboratorio de Icnologia, Museo Argentino de Ciencias Naturales, Av. Angel Gallardo 470, (1405) Buenos Aires, Argentina Abstract Studies on ichnofabrics have focused mainly on marine environments. Attempts to apply the ichnofabric methodology and theoretical framework to continental deposits bearing palaeosols are few and poorly developed. Methodologies analysed in this contribution include the applicability of current ichnofabric indexes and diagrams, the assessment of the destruction of the srcinal bedding by ichnofabrics and by other soil characters separately, and the relation- ships between different stages of palaeosol and ichnofabric development. Many soil features may be formed without the intervention of bioturbation, or may be the result of interactions of physical, chemical and biological processes, in which traces of organisms may have only a subsidiary role. Ichnofabrics can be well developed in palaeosols devoid of other soil characters and, conversely, palaeosols showing a well-developed soil structure can bear almost no trace fossils. This fact adds a third component to classical methods that normally consider only srcinal bedding and ichnofabrics. Theoretical analysis includes the possibility of recording composite ichnofabrics in palaeosols, and the value of individual ichnotaxa as possible indicators of subaerial conditions and environmental changes. The ecological preferences and requirements of trace-makers provide the key to understanding composite ichnofabrics; however, only complex traces can be certainly attributed to particular modern taxa. Insect nests, pupal chambers and earthworm burrows are the most reliable indicators of subaerial exposure and, in many cases, very particular environmental conditions. Ichnofabric analysis has become an increasingly important tool in ichnological analysis, provid- ing a methodology for documenting and compar- ing the extent of bioturbation and relative chronology of infaunal tiering (Ausich & Bottjer 1982; Ekdale & Bromley 1983, 1991; Bromley & Ekdale 1986; Droser & Bottjer 1986; Bottjer & Droser 1991, 1994; Taylor & Goldring 1993; Taylor et al. 2003). Most studies of ichnofabrics have been focused on marine environments (Taylor & Goldring 1993; Bottjer & Droser 1994). Few, poorly developed, attempts have been made to apply ichnofabric methodology and its concepts to continental deposits bearing palaeosols. The few references involve the utiliza- tion of the ichnofabric index (Droser & Bottjer 1986) as a comparative scale to evaluate the degree of development of soil structure (Retal- lack 1990), tiering in palaeosols (Hasiotis et al. 1993; Hasiotis & Dubiel 1994; Gonzalez et al. 1998), the proposal of informal names for ichnofabrics produced by particular groups of arthropods (Hasiotis 1997), and the use of the term terrestrial ichnofabries to describe trace fossil assemblages in palaeosols (Miller & Mason 2000). The aim of this contribution is to present examples of palaeosol ichnofabrics in order to analyse the utility of different aspects of ichno- fabric in continental deposits bearing palaeosols. In order to apply the ichnofabric method to palaeosols the effects of bioturbation must first be distinguished from physical and chemical pedogenic processes. Other aspects required for complete palaeosol description are the interrela- tionships between different stages of palaeosol and ichnofabric development. Another particu- lar point to be considered herein in detail is the value of the individual components of palaeosol ichnofabrics as possible indicators of subaerial conditions and changes in environment, and the possibility of recording composite ichnofabrics (sensu Bromley & Ekdale 1986) in palaeosols. Methodology Review of ex ting methodology ichnofabric analysis There are different methods for measuring the extent of bioturbation and describing ichno- fabrics, which are reviewed elsewhere (Taylor & Goldring 1993; Bottjer & Droser 1994; Miller & Smail 1997; Netto 2000). Two basic philosophies are recognized: those that use a semi-quantitative From: MClLROY, D. (ed.) 2004. The Application of lehnology to Palaeoenvironmental and Stratigraphic Analysis. Geological Society, London, Special Publications, 228, 355-382. 0305-8719/04/$15.00 9 The Geological Society of London. at CAPES on April 9, 2013http://sp.lyellcollection.org/ Downloaded from   356 J.F. GENISE ET AL approach (e.g. Reineck 1963; Droser Bottjer 1986; Miller Smail 1997) and those that are mostly descriptive (e.g. Ekdale Bromley 1991; Wetzel Uchman 1998). Semi-quantitative methods recognize different categories of ichnofabric (ichnofabric indices or bioturbation indices) based on the degree of disruption of the srcinal bedding by biotur- bation, which are represented in schematic dia- grams developed for different sedimentary settings (Droser Bottjer 1986; Bottjer Droser 1994; Miller Smail 1997). Most of them have been developed for studying ichno- fabrics in vertical sections, whereas that of Miller Smail (1997) was elaborated for bedding planes. In turn, descriptive diagrams (tiering diagrams, TD) were proposed to illus- trate relationships of ichnofabrics with palaeoen- vironmental factors, rate of sedimentation, and biological activity (e.g. Ekdale Bromley 1991; Pollard et al 1993; Wetzel Uchman 1998 and references therein) at particular environments or localities. A comprehensive approach to com- bine in a single analysis the evaluation of the extent of bioturbation (bioturbation index, BI), as in semi-quantitative methods, with a visual representation of ichnofabrics (ichnofabric con- stituent diagram, ICD) was proposed by Taylor Goldring (1993). The ICD considers the ichno- taxa present, ichnodiversity, density and order of emplacement. However, no single method has been widely accepted. Critical analysis of the applicability of different methods has been made elsewhere (e.g. Frey Wheatcroft 1989; Ekdale Brom- Icy 1991; Pemberton et al 1992; Miller Smail 1997). Some authors stated that ichnofabric indices may be useful for describing simple ichnofabrics, whereas descriptive methods are more suitable for complex ones (e.g. Ekdale Bromley 1991). Others concluded that semi-quantitative methods are more appropri- ate for large-scale studies, involving hundreds of box cores or thousands of metres of strati- graphic section, whereas the descriptive method has been used with core material or outcrops of limited size (e.g. Miller Smail 1997). In addition, Taylor Goldring (1993) claimed that descriptive methods provide the possibility of assessing and comparing a more complete set of data, including also the degree of bioturbation contemplated in the semi-quantitative method. In practice, however, the methodology is so time consuming that its use is restricted, in most cases, to describing key stratigraphic surfaces or summarizing recurring ichnofabrics within a stratigraphic unit (e.g. McIlroy 2004b). Methodology for palaeosol ichnofabric analysis The case studies presented herein involve com- plex ichnofabrics, small-scale, high-resolution observations and detailed palaeoenvironmental analysis. The inherent complexity of soil fabrics is such that conventional methodologies were deemed inadequate, and some modifications to previous descriptive methods are introduced for their description and as an aid to their analysis. Additionally, the bioturbation indices proposed by Taylor Goldring (1993) have been used to complete the descriptions, as well as a semi- quantitative approach to quantification of bio- turbation of each tier used for composite ichnofabrics (as suggested by Ekdale Bromley 1991). Seven grades of BI, based on Reineck (1963), were recognized by Taylor Goldring (1993), which were adapted herein for descrip- tion of palaeosols according to burrow density and the amount of burrow overlap. These BI are: 0, no bioturbation; 1, sparse bioturbation, few discrete traces; 2, low bioturbation, low trace density; 3, moderate bioturbation, traces discrete, overlap rare; 4, high bioturbation, high trace density with overlap common; 5, intense bioturbation, later burrows discrete; and 6, complete bioturbation, repeated over- printing. The previous authors also utilized the sharpness of the primary sedimentary fabric to define the grades (Taylor Goldring 1993). However, in palaeosols the srcinal bedding can also be heavily disrupted by other soil processes that are non-biological, or only par- tially related to bioturbation: thus this disruption of the sedimentary fabric is not exclusively related to bioturbation as in the classical marine examples. In both the semi-quantitative and descriptive methods only two variables are considered: the srcinal bedding and the ichnofabric. In palaeo- sols, soil structures other than ichnofabrics should be also considered as destructive agents of the srcinal bedding. Different soil features that can be recognized in palaeosols, such as horizons, peds, cutans, glaebules, crystals, and root and animal traces, modify the srcinal sedimentary fabric of the deposits in which soils develop (Teruggi 1971; Yaalon 1971; Andreis 1981; Bown Kraus 1987; Retallack 1990; Nettleton et al 2000). Root and animal traces are the direct products of the activity of organ- isms, and as such they are considered trace fossils and deserve an ichnotaxonomical treat- ment, whereas other soil characters are the result of interactions of purely physical-chemical processes, or mixed physical-chemical-biological at CAPES on April 9, 2013http://sp.lyellcollection.org/ Downloaded from   ICHNOFABRIC ANALYSIS OF PALAEOSOLS 357 processes, in which traces of organisms may only be involved to different extent (e.g. Brewer 1976; Fitzpatrick 1984; Retallack 1990). Regarding Ekdale Bromley's (1983) definition of ichno- fabrics, it would be possible to consider the whole soil structure as an ichnofabric because it may be a direct or indirect result of bioturbation. However, soil features such as peds, cutans, glaebules and crystals may be formed as well, without the necessary intervention of biotur- bation (e.g. Buol et al. 1990; Retallack 1990). For instance, Blokhuis et al. (1990) stated that Vertisols are typical cases of ped formation by physical processes of shear failure, along inclined surfaces, and cracking. Repeated wetting and drying of a dispersed cracking clay soil results in the fragments parting into fine wedge-shaped aggregates, because of the stress-strain regime of the swelling soils (Blokhuis et al. 1990). Even when the traces of organisms would have been involved in the srcin of soil characters (e.g. granular and crumb peds), they might be unpreserved or so distorted as to become recog- nizable, thereby precluding an ichnotaxonomical approach. Apart from its formal definition, the common usage of the ichnofabric concept involves fabrics completely produced by traces of organisms, more commonly than fabrics in which traces are only partially involved. Finally, the development of an ichnofabric may be inde- pendent of other soil characters. Ichnofabrics can be well developed in immature palaeosols, obliterating the srcinal bedding almost com- pletely (e.g. Genise Bown 1994b). Conversely, palaeosols showing a well-developed ped struc- ture may preserve almost no trace fossils (Melchor et al. 2001). In conclusion, the fabric directly and completely produced by root and animal traces is considered herein to be the ichno- fabric whereas the fabric produced by other soil characters, resulting from the interactions of physical, chemical and biological processes, is termed herein the pedofabric. It should be noted that pedofabric is distinct from the terms 'soil fabric' and 'soil microfabric' (e.g. Brewer 1976; Fitzpatrick 1984; Bullock et al. 1985; Retallack 1990; Buol et al. 1990), which are applied mostly to the microstructure of the fine-grained part of the soils. The stages of palaeosol develop- ment may be used to evaluate the degree of pedofabric development. There are two scales for estimating stages of palaeosol development, which were proposed by Bown Kraus (1987) and Retallack (1988) respectively. Both scales are broadly based on similar criteria. The first three stages are characterized by (1) no horizon formation; (2) A horizon and incipient B hori- zon, and (3) A and B horizons well defined, whereas the last two stages (4, 5) are defined mostly by the increasing thickness of A and B horizons. In terms of BI, a highly developed palaeosol will comprise volumetrically high percentages of pedofabric and, accordingly, low to moderate percentages of primary sedimentary fabric and ichnofabrics. Hence the single BI value does not necessarily indicate the percentage of disruption of the srcinal bedding (by animals and plants), and conversely the single percentage of disruption is not a true reflection of the degree ofbioturbation. In palaeosols, the proportions of all three processes must be properly described in order to analyse meaningfully the fabric of the deposits in which they occur. A comparison of five study cases is presented herein utilizing a pedofabric/ichnofabric ternary diagram (PITD). The PITD (Fig. l) includes the bioturbation index (BI), and percentages of srcinal bedding (OB), ichnofabrics (IF) and pedofabrics (PF). BI and percentage of IF were measured following Taylor Goldring's (1993) adapted table based on burrow density and overlap. The percentage of the srcinal bedding (OB) was measured according to the preservation of primary sedi- mentary structures. The pedofabric (PF) was calculated considering the development of soil features other than the recognizable trace fossils. Two types of diagram have been proposed for representing descriptive methodologies: tiering diagrams (TD) (e.g. Ekdale Bromley 1991) and ichnofabric constituent diagrams (ICD) pro- posed by Taylor and Goldring (1993). The ICD represents events consecutively, starting with the primary fabric and then ichnotaxa in order of emplacement. This order reflects the infaunal succession produced by changes in the environ- mental conditions (e.g. Bromley Ekdale 1986; Buatois et al. 1997). In turn, the original tiering the vertical partitioning of the habitat in response to environmental gradients (e.g. Ausich Bottjer 1982; Bromley Ekdale 1986) - may be not represented if it is not the result of the order of emplacement. In marine environments, where colonization of substrates may progress from the surface downwards, the deepest tier is also the last emplaced (Bromley Ekdale 1986; Ekdale Bromley 1991; Brom- ley 1994). In these cases ICDs may represent tier- ing and order of emplacement simultaneously. However, in palaeosols the first colonizers of the deposit may belong to the deepest tier, as shown herein. Both srcinal tiering (Hasiotis Dubiel 1994; Gonzalez et al. 1998) and infaunal succession (Hasiotis et al. 1993) have been recorded from palaeosols, and so descriptive diagrams such as TDs, which can illustrate both features of ichnofabrics simultaneously, at CAPES on April 9, 2013http://sp.lyellcollection.org/ Downloaded from 
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