Trace fossils and ichnofabric in the Kjølby Gaard Marl, Upper Cretaceous, Denmark

The Kjølby Gaard Marl (Late Maastrichtian) is a 30 cm-thick, grayish brown, clay-rich, pelagic carbonate rich unit (75 to 85% CaC03) exposed in the Limfjord region of northern Jylland, Denmark. Trace fossil suites and ichnofabric (the sedimentary fabric resulting from all phases of bioturbation) reflect a complex depositional and post-depositional history of the marl unit, which is sandwiched between comparatively pure chalk strata above and below. Initiation of marl deposition occurred gradually and episodically, as indicated by a micro-styolitic fabric resulting from solution-compaction of finely alternating chalk and clay laminae in the basal portion of the bed. During the major phase of marl deposition represented by the middle and upper parts of the unit, the sea floor apparently was very soft and was colonized by an active infauna which produced a low-diversity suite of trace fossils dominated by horizontal burrows. These are now preserved in a highly compacted ichnofabric which is totally bioturbated but contains no easily identifiable trace fossils. As the sedimentary mode returned to chalk deposition, the marl was buried and subsequently strengthened by compaction sufficiently to allow its colonization by a deeper-burrowing infauna that normally preferred somewhat stifter chalky substrates. Thus, the original ichnofabric was modified by the introduction of late-generation, sharply defined and relatively uncompacted "chalk" trace fossils (Thalassinoides, Zoophycos and Chondrites, probably appearing in that order). These are superimposed directly on top of the earlier, highly deformed burrows of the initial trace fossil suite.