Realistic interpretation of ichnofabrics and palaeoecology of the pipe-rock biotope

The meticulous study of ichnofabrics and palaeobiological interpretation of behaviour are challenges that lie at the heart of rigorous ichnological analysis. Recent study of Cambrian Skolithos ichnofabrics from the north-west Scottish Highlands has attempted to shed light on the palaeoecology and evolution of the early Cambrian endo-benthos (Davies et al. 2009). The work presented herein highlights some of the difficulties in attempting nearest neighbour analysis on ancient bedding surfaces, and provides some suggestions for how palaeoecological analyses might be approached in pipe-rock facies in future. The greatest challenge to serious study of ichnofabrics in the Eriboll Formation is the lack of lithological contrast between the burrows and host sediment (Fig. 1A). This is less pronounced in the ‘trumpet pipe’ (Monocraterion-rich) interval due to diagenetic enhancement of the fabric (Fig. 1B). At most stratigraphic levels in the Eriboll Formation, pervasive quartz cementation requires that special techniques be employed to study ichnofabric in detail (Garton & McIlroy 2006; Fig. 1D, E). The recognition of the upper burrow terminations that could be considered colonization surfaces is, in our experience, complicated by the observed tendency of some Skolithos in the Eriboll Formation to deviate from the vertical through short oblique kinks (cf. Fig. 1C), and for outcrop surfaces to be variably and irregularly oblique to the vertical plane (see Davies et al. 2009, fig. 3C). In our experience, upper and lower terminations of burrows within pipe-rock ichnofabrics are difficult to determine in the field (cf. Davies et al. 2009, fig. 3A, C) but may be determined with careful observation (Bromley 1996) and can be demonstrated in the laboratory through creation of large thin slices (Garton & McIlroy 2006; Fig. 1F). At sites with net deposition, the Skolithos trace makers are forced to extend their burrow upward to compensate for aggradation of the sediment–water interface. Lower Palaeozoic pipe-rock ichnofabrics have been studied by a number of authors (e.g. Pemberton & Frey 1984; Droser & Bottjer 1989; Droser 1991; McIlroy 2004; McIlroy & Garton 2004). These studies include discussion of bioturbation intensity and its estimation in Skolithos ichnofabrics using flashcards (Droser & Bottjer 1986, 1989). The importance of Skolithos pipe-rock stems from: (1) its potential to provide evidence for the early colonization of marginal marine settings; but also (2) the presence of significant hydrocarbon reserves in such facies throughout northern Africa, the Middle East and in other peri-Gondwanan regions (McIlroy & Garton 2004). Conceptual models for the interpretation of piperock ichnofabrics have been proposed to focus observations of ichnofabric development with respect to colonization, sedimentation rate and changing palaeoenvironmental stress and model ichnofabrics (McIlroy 2004; McIlroy & Garton 2004; Figs 2, 3). These models can be used to augment descriptions of bioturbation intensity (cf. Droser & Bottjer 1989). Application of these ichnofabric-based models to the Eriboll Formation requires careful documentation of ichnofauna and sedimentary facies over large lateral distances, and is a work in progress.