engleski

Latest Maastrichtian to earliest Paleocene platform carbonates with coral-stromatoporoid patch reefs, the Island of Brač (Croatia)

Deposits comprising the Cretaceous–Paleogene (K/Pg) transition reveal evidence of biotic crisis and one of the most severe global mass extinctions. Studied uppermost Maastrichtian–lowermost Paleocene (Danian) section on the NW part of the Island of Brač (Likva Cove) comprises shallow marine platform carbonates with continuous K/Pg transition and exceptionally well preserved in situ scleractinian coral and stromatoporoid patch reefs. The Cretaceous part of the section is characterized by co-occurrence of rudists and corals: micritic limestones with rudist and echinoid fragments, ostracods, small benthic foraminifera (discorbids, Rhapydionina sp., Dicyclina cf. schlumbergeri, Moncharmontia apenninica, Laffiteina sp., Fleuryana adriatica) alternate with scleractinian coral and stromatoporoid reef levels, which are poor in other fossil remains. Immediately below the K–Pg boundary, which is confirmed by other researchers, last rudists and a concentration of small benthic foraminifera (mostly miliolids) occur. Oldest Paleocene strata record the input of planktonic foraminifera in otherwise shallow marine micrites with discorbids. The rest of the Paleocene comprises micrites with ostracods, discorbids, occasional charophyta and Bangiana hanseni, which are overlain by the best exposed patch reef. Although, the Cretaceous and the Paleocene strata show certain differences: shallow marine restricted environments with rudist and bioclastic micrites and tidal flats with laminated fenestral limestones in the Cretaceous in opposition to Paleocene very shallow restricted marginal marine and freshwater to brackish environments with ostracod-discorbid micrites with charophyta, the Cretaceous and Paleocene reef levels are very similar, albeit almost identical. A 50-m-thick succession contains three well-marked and several less prominent reef levels in the Cretaceous part and at the top of the section the most pronounced, one Paleocene reef level. All four main reef levels follow after 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 above the surrounding bottom. Generic determination of corals was not possible due to the recrystallization, but they resemble to Modern finger-like coral colonies such as Porites sp. Massive domal and bulbous stromatoporoids occur together with corals, but also 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 between the knobs. It occurs also as internal sediment partly filling the reef cavities in 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 Paleocene suggests that recovery of reefs was rapid, and already in the Danian. Support by the Croatian Science Foundation (IP-2014-09-9541) is acknowledged.

Cretaceous–Paleogene (K/Pg) transition ; corals ; stromatoporoids ; Brač ; Croatia

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