Development of the Turonian/Coniacian hardground boundary in the Cracow Swell area (Wielkanoc quarry, Southern Poland)

Abstract

During the Turonian and Coniacian, up to the early Santonian, the present-day Polish Jura Chain composed a positive submarine palaeotectonic feature referred to as the Cracow Swell, separating the deeper Opole Trough to the SW from the Danish-Polish Trough to the NE. At present the Turonian and Santonian deposits at the margin of the Polish Jura Chain and the Miechów Trough are fragmentarily preserved. They are characterised by numerous stratigraphic hiatuses and the occurrence of many unconformity surfaces. One of the most spectacular unconformities is a hardground at the Turonian/Coniacian boundary described herein from the vicinity of Wielkanoc. Its development took place in several stages. Three main stages can be distinguished with a composite middle stage. In the first stage during the early late Turonian, a gradual drowning of the carbonate Cracow Swell took place followed by eutrophication of the environment. The second stage from the latest Turonian to the earliest Coniacian was linked with a crisis of a carbonate sedimentation leading to its cessation. A firmground with Thalassinoides traces was formed, followed by a hardground with bivalve borings and ?Trypanites. Carbonate-clastic sedimentation recommenced at least twice (with quartz arenites), followed by rejuventation of burrows and/or borings, lithification of the sediment, glauconitization and phosphatization, as well as the development of microbial mats undergoing early phosphatization. This led to the formation of phosphatic stromatolites. In consequence a composite hardground was formed. The third stage took place in the late early Coniacian. Carbonate-clastic sedimentation resumed. Deposits, developed as carbonate arenites with quartz and glauconite admixtures (non-phosphatized), filled the last generation of the rejuvenated burrows and finally covered the hardground.