The Lower Badenian (Middle Miocene) coral patch reef at Grobie (southern slopes of the Holy Cross Mountains, Central Poland), its origin, development and demise

Abstract

The Lower Badenian (Middle Miocene) patch reef of Grobie (southern slopes of the Holy Cross Mountains, Central Poland) is the only coral buildup in the Polish Miocene. It contains four hermatypic coral taxa, of which Tarbellastraea reussiana (MILNE EDWARDS & HAIME) and Porites collegniana MICHELIN dominate, whereas Montastraea sp. and Stylophora reussiana MONTANARO-GALLITELLI & TACOLI are subordinate. Wide diversities of colony shapes in this coral assemblage reflect natural development of the reefal structure and/or energy of environment; it varies from platy colonies (first settlers on an unconsolidated, pebble-strewn substrate), through branching, to massive forms. Wave activity was the most significant factor that influenced the reef core and distribution of associated facies. Disintegration and removal of reef core sediment led to the formation of crack crevices in freshly lithified deposits. Redeposited sediment formed the back-reef talus, where its composition varied with distance from the reef core. Reef-associated molluscs and decapods are abundant, the bivalves being represented by high-energy resistant borers (Lithophaga sp., Jouannetia (J.) semicaudata DESMOULINS) and the squatter Sphenia (S.) anatina (BASTEROT). Among 21 decapod taxa, the species Dardanus hungaricus (LÖRENTHEY, 1929) is reported from Poland for the first time. Recognition of the diversity of facies and their distribution enabled reconstruction of Grobie area during the Early Badenian transgression. Slowing and/or stopping of sealevel rise encouraged development of the patch reef, while the subsequent, rapid transgressive pulse caused its demise. Shortly after all reefal deposits were buried, diagenetic processes of neomorphisation and/or dissolution of aragonitic skeletal elements took place. This diagenesis could have continued till the Late Miocene and Pliocene, synchronously with rapid erosion that progressed since the Early Sarmatian until the Pleistocene glaciations.