ABSTRACT: Ichnology of Pelagic Carbonate in New Zealand and Denmark: Shelf Sea or Deep Sea?

A. A. Ekdale

Today, pelagic carbonate ooze is an exclusive feature of deep-sea environments 1 km or more in depth. In contrast, the extensive epicratonic seas that characterized the Late Cretaceous and early Tertiary received great thicknesses of pelagic carbonate sediment in environments only a few hundred meters deep. Distinguishing between deep-sea and shelf-sea carbonate is not an easy task, but ichnologic investigation helps.

Biogenic sedimentary structures, ichnofacies, and ichnofabrics in Cretaceous--Tertiary epicratonic pelagic deposits display many similarities to those in deep-sea sediment. Ichnologic features in shelf-sea chalk and limestone in both New Zealand and northern Europe reveal some interesting paleobathymetric trends. These trends include a general decrease in crustacean traces (Thalassinoides, etc.) and bioerosion traces (Trypanites, etc.) with increasing water depth, accompanied by a concomitant increase in worm burrows (Zoophycos, etc.).

Maastrichtian-Oligocene pelaic limestone in New Zealand and Maastrichtian-Paleocene chalk in Denmark, neither of which represent a truly deep-sea setting, exhibit similar ichnofacies and ichnofabrics. Some notable differences exist because the New Zealand platform was less extensive and more tectonically active than the northern European shelf. Pelagic strata in New Zealand are associated with shallow-water quartzose sandstone beneath and fossiliferous calcarenite above, as well as deep-water bedded chert within, the pelagic carbonate sequence. In New Zealand strata, Zoophycos-rich facies dominate Thalassinoides-rich facies, and bored hardgrounds are uncommon; in Danish chalk sequences, the opposite is true in both cases.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990