The Ichnofabric Concept - Trace Fossils as Indicators of Sedimentary Environments - page 146

FIGURE 7 Ichnofabrics in slabbedwell cores fromtheOseberg Field, offshoreNorway (Block 30/9),
with deep-marine, redeposited Shetland Chalk (Upper Maastrichtian-Danian). The wavy to nodular
fabric is enhanced by the abundant occurrence of dissolution seams. UV light (A, B) and white
light (C, D) ( 60 deviated well). (A) The overall degree of bioturbation is high (approx. 85%), and
the ichnofabric comprises a monoichnospecific suite of branched and partly lined networks with a
meniscate or passive fill assigned to Thalassinoides / Ophiomorpha , probably produced by burrowing
shrimp. Two colonization phases can be recognized: (1) predeformation ichnofabric consisting of
relatively homogeneous, oil-stained chalk (yellow color) of the background sediment, and (2) post-
deformation ichnofabric resulting from subsequent massive colonization, which led to the destruction
and in-situ brecciation of the primary sedimentary fabric and the incorporation of gray mud.
(B) Higher up in the succession, sediment destruction during the second colonization phase is increased
and contributes to subsequent sediment reworking, which again leads to considerably reduced res-
ervoir properties. (C) Mass-transport complex consisting of debrite units (D) with well-rounded
chalk intraclasts (some with bioerosion traces, arrows) and larger fragments of semiconsolidated
rocks. The rock fragment in the upper portion of the core shows a gradual transition from original soft
(S) to firm (F) sediment and preserves a diffuse ichnofabric with Planolites , Palaeophycus, and
Chondrites from the pelagic background. The debris-flow deposit is weakly bioturbated ( 15%)
and only comprises discrete Thalassinoides / Ophiomorpha filledwith graymud and partly incorporated
chalk clasts. (D) Detail of (C), showing a Thalassinoides burrow with an active (meniscate) fill in
the debrite and softground (left and middle portion of the image), gradually changing to a passive
fill in the firmground (right portion of the image).