Ichnofabrics containing <i>Ophiomorpha:</i> significance in shallow-water facies interpretation

Abstract

In an attempt to interpret Ophiomorpha ichnofabrics observed in core, three ichnofabrics are described from outcrops where O. nodosa is a conspicuous element. These ichnofabrics enable sandy shoreline sedimentary environments to be characterized and differentiated: (1) shoreface with mottled- Ophiomorpha—Planolites ichnofabric generally without primary lamination; (2) offshore tidal shelf sand wave facies with Macaronichnus-Ophiomorpha ichnofabric associated with primary, mainly cross-laminated or cross-bedded sands; (3) estuarine facies with Ophiomorpha ichnofabric associated with primary lamination and, commonly, heterolithic sands and mudstones. Distinctions between the ichnofabrics are attributed to differences in primary stratification, the total ichnocoenoses, morphological features (such as burrow attitude, shaft restriction, pellet wall lining), to the nature of the substrate and, particularly, to the time available for colonization (larval settlement or relocation) and burrow construction, referred to here as the colonization window The analysis is applied to an interval of core (Upper Jurassic, Central Graben, North Sea) and a sequence in Eocene sediments in southern England.