An early Campanian rocky shore at Ivö Klack, southern Sweden

Abstract

Well-exposed, ancient rocky shores are rare and the associated shelly faunas are normally strongly worn and fragmented due to erosion during both sea-level rise and fall. An early Campanian rocky shore with a rich fauna is preserved at Ivö Klack in the Kristianstad Basin, southern Sweden. Ivö Klack is situated on the small, hilly island of Ivö situated in lake Ivösjön with a lake level about 6 m above present-day sea level. The rocky shore was formed during a major late early Campanian transgression caused by a sea-level rise up to about 100 m above present sea level. An archipelago was formed along the northern basin margin during the transgression with numerous small islands and peninsulas. The steep palaeo-coast at Ivö Klack consists of gneiss overlain in the lower part by more than 30 m of kaolin in places containing large boulders of gneiss with a kaolinized outer crust. The kaolin wedges out upslope and is absent above 30–35 m above lake level. The kaolin is overlain by up to 5 m of muddy quartz sand with highly angular grains, representing a residue of the kaolin formed during transgressive wave and current reworking. The steep gneiss coast was rapidly transgressed by the sea in the latest early Campanian and at least 25 m of onlapping coarse-grained skeletal sand, gravel and whole fossils were deposited on the clean gneiss surface during the Belemnellocamax mammilatus belemnite biozone. Large boulders of gneiss are common in the carbonates and formed distinct boulder beds at some levels but these are now quarried away. The top 6 m of the preserved carbonate succession are free of boulders. The rocky surface is fairly even on a large scale, but highly irregular in detail with numerous metre-sized hummocks and boulders formed during terrestrial weathering and kaolinization. The lowest part of the shore is developed as a bench, topped by a sub-horizontal irregular platform, 20–30 m wide, with large hummocks and rounded boulders. The platform passes upwards into a steep boulder-strewn slope. A well-developed glauconitized hardground with Thalassinoides burrows and several incipient hardgrounds occur in the carbonates draping the bench and lower part of the platform. They dip gently outwards and probably represent a kind of beach rock. A highly diverse benthic fauna with more than 200 shell-bearing species lived between, below and on the hummocks and boulders. It comprises bivalves, brachiopods, polychaetes, gastropods, echinoids, asteroids, bryozoans and is dominated by large oysters and includes the northernmost Late Cretaceous rudists and hermatypic corals. The remaining benthic invertebrate groups are represented by only a few species each. The non-benthic invertebrate fauna comprises four belemnite species and one rare ammonite species. The lower Campanian carbonates of the small basin have yielded an unusually diverse vertebrate fauna totalling more than 60 species, including mosasaurs, plesiosaurs, crocodiles, turtles, birds, and as many as 44 species of sharks and rays. Ivö Klack thus provides information on virtually all trophic levels in the rocky shore ecosystem and represents the most diverse rocky shore fauna known from the geological record. This paper aims at providing the framework for a series of papers on the palaeoecology of the main benthic faunal groups.