Anconichnus horizontalis; a pervasive ichnofabric-forming trace fossil in post-Paleozoic offshore siliciclastic facies
Anconichnus horizontalis; a pervasive ichnofabric-forming trace fossil in post-Paleozoic offshore siliciclastic facies (in 13th international sedimentological congress, Ichnologic symposium, David J. Bottjer (editor))
Palaios (June 1991) 6 (3): 250-263
The fabric of often intensely mottled, upper offshore to lower shoreface siliciclastic sediments of post-Paleozoic age is analyzed and the principal trace described from material collected from outcrop (U.K.) and the North Sea Basin (core). The characteristic trace of these mottled zones is Anconichnus horizontalis, which is a narrow, discontinuous, twisting, muddy fecal string within a poorly defined burrow fill depleted in mud and inertinite. This trace formed endogenically, principally in association with small-scale cross-stratification and thin (cm) event beds. Seven taphonomic-sediment associations (ichnofabrics) are recognized: 1, In siltstone to very fine-grained sandstone in thin (cm) event beds, often as the only trace; 2, In association with small-scale cross-stratification; 3, In dense concentrations in fine-grained sediment where the primary structures have been obscured; 4, Similar to (3) but with a patchy distribution of the trace; 5, Siltstone to very fine-grained sandstone, often associated with heterolithic stratification, with a higher ichnodiversity, including Phoebichnus, Palaeophycus, Thalassinoides, Rhizocorallium, Cylindrichnus and Diplocraterion; 6, As (3) but with muddy patches and sand-filled Chondrites sp.; 7, Mud dominated, with silty horizons and associated with Terebellina. Associations 1 to 4 were produced by opportunistic shallow tier burrowers penecontemporaneous with deposition of mud-depleted event beds in an offshore environment. Association 5, with diverse, later mid-tier burrowers, suggests a more equilibrium endobenthic community in aggrading sedimentation conditions, under fair weather, in the offshore transition zone to lower shoreface, while associations 6 and 7 indicated muddier offshore situations. Although Anconichnus horizontalis is often assigned to Helminthopsis in the literature, a detailed ichnotaxonomic review of Helminthopsis and similar burrows confirms that the burrow's mantle, discontinuous fecal core and non-planar aspect justify its referral to A. horizontalis.