Borehole Gressenich BK1: new insights into the early Mississippian Kohlenkalk Platform of the Aachen region, Germany

The early Mississippian of the Aachen region is characterized by a tropical shallow-water carbonate succession, the so-called Kohlenkalk Platform. Establishing a second phase of intensive carbonate production, it follows on the Givetian to Frasnian biostromal buildups around the southeastern edge of the London-Brabant Massif. The timing and patterns of the development of these Carboniferous carbonates were studied, based on drill core Gressenich BK1. It was sunk by the Geological Survey of North Rhine-Westphalia in the core of the Burgholz Syncline and reached a depth of 100 metres. The borehole is located in the Vygen/Gressenich Quarry, which exposed and strongly tectonized Devonian strata were previously investigated by Reißner (1990). In contrast to the nearby active Hastenrath Quarry, the Palaeozoic part of the succession shows an inverted stratification. As Holocene sediments comprise a thickness of 7.30 m within the drill core, subsequent Devonian sedimentary rocks (mostly reefal limestones) occur up to a depth of 27.00 m. Followed by 12.40 m of black shale of unclear stratigraphic age, possibly the Frasnian Matagne Shale, all formations and subformations of the drilled Kohlenkalk succession can be easily subdivided by lithostratigraphy. Considering the regional stratigraphic schemes in the Vesdre Massif of eastern Belgium and referring to the formal lithostratigraphic concept of Amler & Herbig (2006), these are the Vesdre and Terwagne formations. The latter contains the Hastenrath (= Vaughanites Oolith), Bärenstein, and Bernardshammer subformations. Unfortunately, conodonts are rare or absent. In the case of the Vaughanites Oolith, reworking processes cannot be ruled out.