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Latest Cretaceous to earliest Paleocene scleractinian coral-stromatoporoid patch reefs on periodically emerging carbonate platform, the Island of Brač (Croatia)

The Cretaceous–Palaeogene (K/Pg) transition is marked by significant palaeoenvironmental changes resulting with biotic crisis and one of the most severe global mass extinctions. Studied uppermost Maastrichtian–lowermost Palaeocene (Danian) section in the NW part of the Island of Brač (Likva Cove) comprises shallow marine platform carbonates with continuous K/Pg transition and several levels of exceptionally well preserved in situ scleractinian coral and stromatoporoid patch reefs. The Maastrichtian part of the section is characterized by co-occurrence of rudists and corals: within predominantly micritic limestones with rudist and echinoid fragments, ostracods, small benthic foraminifera (discorbids, Rhapydionina sp., Dicyclina cf. schlumbergeri, Moncharmontia apenninica, Laffiteina sp., Fleuryana adriatica) several levels with scleractinian coral and stromatoporoid reefs poor in other fossil remains have been found. Immediately below already determined K/Pg boundary (Korbar et al., 2017), last rudists and a concentration of small benthic foraminifera (mostly miliolids) occur. Oldest Palaeocene strata record the input of planktonic foraminifera in otherwise shallow marine micrites with discorbids. The rest of the Palaeocene comprises micrites with ostracods, discorbids, occasional charophyta and Bangiana hanseni, overlain by the biggest and best preserved patch reef. Cretaceous and Palaeocene successions show certain differences: Cretaceous shallow-marine restricted environments with rudist and bioclastic micrites and tidal flats with laminated fenestral limestones were followed by Palaeocene very shallow restricted marginal marine and freshwater to brackish environments with ostracod–discorbid micrites with charophyta. However, Cretaceous and Palaeocene patch reefs appear to be very similar if not almost identical. A 50-m-thick succession contains three well-marked and several less prominent reef levels in the Cretaceous part of the succession and at the very top of the section the most pronounced, Palaeocene reef level topped by regional emergence. All four main reef levels follow short-lasting subaerial emersion surfaces indicating that corals and stromatoporoids preferred colonization of hard substrates. Scleractinian corals are present with globular and domal growth forms forming small (up to 2 m in diameter) coral knobs, which formed low relief (at least 2 m) above the surrounding bottom. Generic determination of corals was not possible due to the intense recrystallization, but they resemble Recent finger-like coral colonies such as Porites sp. Massive domal and bulbous stromatoporoids occur both together with corals and individually in the youngest reef level. Wavy-laminar stromatoporoids usually overgrow/encrust(?) coral colonies. Recrystallized micrite, occasionally with very rare ostracods and discorbids, fills the spaces available between the knobs. It occurs also as an internal sediment partly filling the reef cavities within the youngest reef level, while the rest of the cavities are often filled by botryoidal calcite crystals. Well-developed coral and stromatoporoid patch reef in the earliest Palaeocene suggests rapid recovery of reefs after K/Pg boundary and survival of same reef-builders as in the latest Cretaceous until the regional emergence.

coral-stromatoporoid patch reefs ; late Maastrichtian ; early Paleocene ; Brač

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