The role of trace fossil (ichnofabric) analysis in the development of depositional models for the Upper Jurassic Fulmar Formation of the Kittiwake Field (Quadrant 21 UKCS)

Abstract

Abstract The sedimentology and sequence stratigraphy of the Upper Jurassic (Upper Oxfordian — Middle Kimmeridgian) Fulmar Formation of the Kittiwake Field, Western Platform of the north Central Graben are investigated through an integrated study of core material and wireline logs. The Fulmar Formation, in this area, comprises largely finegrained sandstones which are intensely bioturbated such that the use of primary sedimentary structures for the identification of depositional environments is impractical. By using the approach of ichnofabric analysis presented here, the information provided by trace fossils can be fully utilized in the formulation of a depositional model for the Fulmar Formation. A depth and substrate-related succession of ichnofabrics was determined for the Fulmar Formation from more complete and progradational successions. This attached shoreface succession extends from a Chondrites ichnofabric (offshore), through Anconichnus (upper offshore), Anconichnus and spreiten burrows and ‘ Teichichnus zigzag ’ (upper offshore-offshore transition zone), bivalve tube ichnofabric (offshore transition zone to lower shoreface), Ophiomorpha irregulaire (lower shoreface), Ophiomorpha nodosa (middle shoreface) to a burrow mottling ichnofabric and associated high-energy laminated sandstones (upper shoreface). Anomalies in this succession form the basis for the identification of bounding surfaces, particularly omission surfaces ( Thalassinoides and Diplocraterion habichi ichnofabrics) and sequence boundaries. The distribution and evolution of the essentially retrogradational succession of the Fulmar sandstones is illustrated by analysis of a detailed core log of well 21/18-3, a general cross section through the Kittiwake Field and an Early-Mid-Kimmeridgian time slice facies distribution map.