In my last blog I started to write about Alum Shale and more specifically about the Alum Shale of southernmost Sweden, which is about 500 million years old. It is fascinating that geologists can read these old rocks and reconstruct when, where and how these sedimentary layers formed originally, how they were transformed into rocks and how many deep burial phases these rocks had experienced. But it is even more surprising that a Quaternary geologist (me) suddenly finds such old rocks so very interesting. But then geology is geology, and the only differences are time and the environment in which the sediments were deposited.
Geologists from Lund University and from the Geological Survey of Sweden have done a great amount of work to reconstruct the depositional environment of the Alum Shale and its underlying and overlying rocks. To make these reconstructions, they worked on outcrops, but also with a number of deep drill cores, which were analysed using different techniques. The drill cores helped estimating the thickness of the Alum Shale and showed the contacts between the underlying sandstone and the overlying shale and limestone.
The first drill cores, Andrarum-1 and Andrarum-2 were made in the years 1941-1942 and were later complemented by core Andrarum-3. Ahlberg and co-authors (2009) describe this latter drill core in detail and also provide a good description of what the Alum Shale looks like. During the years 2009-2010, several new boreholes were drilled by Shell: Lövestad A3-1, Oderup C4-1 and Hedeberga B2-1, plus Fågeltofta-1, Flagabro-1, Gislövshammar-1 and Gislövshammar-2. Some of these were examined as part of a MSc thesis by Eriksson (2012), are for example summarized in Calner & co-workers (2013) and in Schovsbo & co-workers (2015; DOI: 10.3997/2214-4609.201413170), among many other publications.
But what exactly is the Alum Shale? It is a dark grey to black laminated mudstone and shale that contains strongly smelling carbonate lenses (a really smelly stinkstone – imagine rotten eggs, but much worse). These latter formed during early diagenesis, i.e. when the mud and clay started to turn into rocks. The mudstones and shales, which are up to 100 m thick in southern Sweden, are rich in organic matter, in pyrite (an iron sulfide), phosphate and in trace elements.
The mud and clay, which later turned into Alum Shale, were deposited in a shallow marine environment and under almost anoxic conditions, meaning that there was little oxygen available at the sea floor and that organic matter could therefore be preserved (i.e. it was not eaten up by for example bacteria). The mud and clay reaching the ocean floor (did rivers transport all the sediments to the sea shore?) were most likely weathering products from the surrounding mountain chains and maybe also ash particles from volcanoes, but could also have been at least partly derived from submarine volcanoes or hydrothermal vents.
What makes the Alum Shale so special is its high amount of organic matter, the frequent occurrence of iron sulfides and the many trace elements, such as vanadium and uranium. More about these another time.
Here are some references in case you would like to read more:
Ahlberg, P., Axheimer, N., Babcock, L.E., Eriksson, M.E., Schmitz, B. & Terfelt F. (2009): Cambrian high-resolution biostratigraphy and carbon isotope chemostratigraphy in Scania, Sweden: first record of the SPICE and DICE excursions in Scandinavia. Lethaia, Vol. 42, pp. 2–16.
Calner, M., Ahlberg, P., Lehnert, O. & Erlström, M. (eds.) (2013): The Lower Palaeozoic of southern Sweden and the Oslo Region, Norway. Field Guide for the 3rd Annual Meeting of the IGCP project 591. Sveriges geologiska undersökning, Rapporter och meddelanden 133, 57–85.
Eriksson M. (2012): Stratigraphy, facies and depositional history of the Colonus Shale Trough, Skåne, southern Sweden. Dissertations in Geology at Lund University, No. 310, 37 pp.
Barbara Wohlfarth 