Trace Fossils and Trolls: Report of the First International Ichnofabric Workshop in Norway

Abstract

Ichnofabric is a relatively new concept in paleontology and sedimentary geology, having been introduced in the literature less than a decade ago. Ichnofabric is an important component of ichnology, as it deals with how organisms affect sediment properties, produce preservable sedimentary structures, and create biogenic sedimentary fabrics. Ichnologic research traditionally has centered on identifiable trace fossils, although the processes of bioturbation and bioerosion are quite important and have received significant attention by some workers. The advent of the ichnofabric concept during the 1980's has added a new dimension to the field of ichnology, because it provides a more comprehensive picture of the original depositional environment by emphasizing biogenic aspects of the fabric of sediments. In September of 1991, a very enthusiastic group of ichnofabric researchers from all over the world gathered in Oslo, Norway, for the First International Ichnofabric Workshop, which was sponsored by Norsk Hydro and several other oil companies that operate in the North Sea region. The meeting was organized by Fredrik Bockelie (Norsk Hydro, Stabekk, Norway) and Tony Ekdale (University of Utah, Salt Lake City). Other participants in the workshop included David Bottjer (University of Southern California, Los Angeles), Richard Bromley (Institute for historisk Geologi, Copenhagen, Denmark), Allen Curran (Smith College, Northampton, Massachusetts), Mary Droser (University of California at Riverside), Christian Gaillard (University Claude Bernard, Lyon, France), Roland Goldring (Sedimentology Institute, Reading, England), Niels-Martin Hanken (University of Tromso, Norway), Chris Maples (Kansas Geological Survey, Lawrence), Tom Mason (University of Natal Durban, South Africa), Molly Miller (Vanderbilt University, Nashville, Tennessee), John Pollard (University of Manchester, England), Charles Savrda (Auburn University, Auburn, Alabama), Ronald Steel (University of Bergen, Norway), Andrew Taylor (University Manchester, England), Guanzhong Wang (Sedimentology Institute, Reading, England), and Andreas Wetzel (University of Basel, Switzerland). The purpose of the workshop was to examine the concept of ichnofabric, to come to a consensus regarding its terminology, and to develop applications of ichnofabric in facies recognition, event correlation, reservoir prediction, sedimentologic modeling, and paleontologic reconstruction. Major themes that were addressed at the conference included the manner in which the tiered structure of infaunal organism communities is reflected in composite ichnofabrics, the application of empirical ichnofabric indices in assessing the degree of bioturbation of sediment packages, the utility of ichnograms in basin wide reconstructions, the recognition and correlation of recurrent ichnofabrics and strata in both the subsurface and outcrop sections, and the potentially widespread use of ichnofabric correlation in petroleum exploration and reservoir prediction. The workshop began in the Vaekero Research Center of Norsk Hydro Oil Company in Oslo. A long first day of oral presentations and free-wheeling discussions by all participants set the stage and established the major themes of the conference. On the second and third days, a field trip from Oslo to Bergen allowed the group to examine a variety of lower Paleozoic ichnofabrics in outcrop settings. The group went to Bygdoy Peninsula southwest of Oslo to examine Lower Ordovician limestone and shale sequences, where the Huk Formation contains three suites of trace fossils at successive omission surfaces. Complex, composite ichnofabrics are well displayed in these rocks. A pre-omission episode of biogenic homogenization caused chaotic orientation of trilobite fragments. This event was followed by development of an omission surface, including an omission suite of Thalassinoides-like firmground burrows, upon which a postomission set of light-colored burrows penetrating the tops of the firmground and the omission suite burrow fills was emplaced. A second stop was at Kalvoya, near Sandvika, where the group observed Upper Ordovician nodular limestones in the Husberoya Formation. Abundant trace fossils occur in positive hyporelief, and there are three main trace fossil associations occurring in distinct ichnofabrics representing mid to upper-shoreface, lower to mid-shoreface, and offshore facies. At Kolsas Hill along Tyri Fjord the participants observed rippled sandstones of Silurian age that contained epichnial arthropod trackways (Steinfjordichnus). At Langodden on the shores of Lake Mjosa, the group examined ichnofabrics in Lower Cambrian sandstones of the Ringsaker Quartzite. Trace fossils are patchy but locally well displayed, and the Skolithos Ichnofacies is exemplified by Skolithos, Monocraterion and Diplocraterion. Discussions on the outcrop