Abstract

A laterally extensive Danian (early Paleogene) bryozoan mound complex is well exposed in 14 km of nearly continuous outcrop in the coastal cliff Stevns Klint in eastern Denmark. A NW–SE oriented seaway covered the Danish Basin in Danian time. It widened towards the NW, where it passed into the deeper North Sea Basin and narrowed towards the SE, where it was connected with the Polish Trough. The exposed mound succession is up to 20 m thick. Individual mounds are oval in plan view, internally asymmetrical, 5–11 m high, 45–110 m long in NNE–SSW oriented dip sections parallel to the direction of mound migration, and 50–300 m in strike sections. The southern flanks are steep and dip 15–30° mainly towards the SSW, whereas northern flanks have gentle dips of 0–15° towards the NNE. Four facies associations are distinguished: (1) the initial mound association, consisting mainly of bryozoan packstone–rudstone; (2) the steep flank-mound crest association, consisting of bryozoan rudstone with subsidiary octocoral and encrusting bryozoan rudstone and bryozoan grainstone; (3) the gentle-flank association, characterized by bryozoan packstone, and incipient to fully developed hardgrounds; and (4) the intermound association, consisting of echinoid rudstone, packstone, and minor occurrences of bioclastic grainstone and argillaceous wackestone–packstone beds and laminae.

Facies analysis and mesoscale mapping indicate that the bryozoan mound complexes were formed in relatively deep, cool water below the photic zone and swell wave base. Deposition was influenced by along-slope currents, with prominent seasonal and long-term variations. Mound growth was governed by primarily in-place benthic carbonate production of mainly delicate branching bryozoans which baffled and trapped pelagic and detrital benthic carbonate mud. Gentle currents rich in particulate nutrients flowing towards the WNW deflected by the Coriolis force, and possibly a tidally or wind-induced upslope component towards the NNE, resulted in mound growth mainly towards the SSW. High production in the surface waters combined with an effective tidally induced mixing of the water column was important for transporting particulate nutrients to the bottom waters. Periodic intensification of the mean WNW-ward flowing bottom current system combined with a storm-associated downslope component towards the SSW resulted in winnowing and erosion of the northern mound flanks. Long-term periodic waning or cessation of bottom currents and possibly stratification of the water column resulted in very limited benthic carbonate production and mound growth associated with formation of extensive Thalassinoides burrow galleries mirrored by the rhythmic occurrences of flint nodule bands. Mound development thus reflects the delicate balance between biogenic growth towards particulate nutrient carrying currents, current winnowing, and erosion. The lack of framework buildup and the poorly cemented nature of the mounds is remarkable. In this respect they deviate from most other mound types known from the geological record, and may—possibly together with the sub-recent analogues on the Great Australian Bight—represent a new class of noncemented skeletal mounds.

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