Outcrop at Djupadal of lapilli tuff. South exposure.
Last week I ordered a print of a paper about one interesting location not that far from me: “Om basalttuffen vid Djupadal i Skåne”, which translates to “About the basaly-tuff at Djpadal in Scania”. This is one of just a few tuffs of the Triassic to Cretaceous volcanism seen in outcrops here. Tuff has been discovered in several drilling cores and while digging, but outcrops are rare. As far as I can remember there are two, of which Djupadal is the best.
The outcrop are over 10 meters high and several tens of meters wide and are heavily weathered. When I visited the location it was a rather spontaneous visit and I forgot to bring a map with me. Because of that I only looked at the southern exposure and completely missed the northern exposure. I suspect the northern exposure will be less weathered and have less moss so a further visit are planned.
Anyway, this short paper was written by Fr Eichstädt in 1883 and are written in really archaic Swedish. Not an easy read. A funny point is that the cost of this paper in 1883 was 0.25 SEK, compared to the 100 SEK I paid for it last week. Now I have got access to most – if not everything – publicized about this outcrop, and I will here write its story.
In 1877 Angelin publicized a geological map of Skåne where several basaltic localities were described, unfortunately I have not got my hands on this map. On this map a weathered diabase was described at Djupadal.
Two years later Tullberg and Nathorst was working on a map “Trolleholm” at a larger scale and one location they investigated was Djupadal. They did not agree with the earlier definition and noted that it did not fit a common basalt either. Their interpretation was that it was a heavily weathered basalwacke1or tuff with bombs of gneiss and mineralized wood. This rock was in the beginnings described as a basaltwacke or tuff, later they just described it as wacke. They also describe the outcrop as 170 meters wide and ~15 meters high with loose soil above it and gneiss below, even if they could not find any lower contact.
Fresh rocks are described as beautiful blue-green and can be scratched with a nail. Heavily weathered rock at the south exposure are more gray, brown or brown-yellow and falls apart to the touch. The western exposure has visible bombs that can be mistaken for conglomerate and mineralized pieces of wood. The rock are vesicular and the vesicles are filled with calcite and other minerals. It was hoped that the age of the wacke could be defined from the wood found in it, an age of Tertiary was thought probable since it would correlate with volcanism in Germany. Since the wood was conifers this was not possible. (Tullberg & Nathorst, 1880)
Somewhat later in 1883 Eichstädt write a paper criticizing the use of wacke to describe this outcrop, his point were instead that it would rightly be described as a tuff. Eichstädt pointed this out in an earlier paper “Skånes basalter” written 1882, but did in that paper not explain why he came to this conclusion Instead this are the point of this later publication. Eichstädt described this rock as a metamorphed and cemented tuff of ash, sand and lapilli. It was concluded that the rock was vesicular from macroscopic to microscopic scale wand were primary glass which had been secondary changed. The only crystals that could be found were some olivine that tended to be serpentinized, the odd augite could be found but in so small amounts that they were judged inconsequential. Since only olivine could be found it was concluded that no other minerals could have existed since olivine are most easily changed. The ash are mostly cemented by calcite, but others minerals, like zeolites, are possible. Furter analysis by Eichstädt showed the existence of xenoliths that might be from the local gneiss and sandstone based on textural analysis. He also found many examples of wood in the tuff, the largest at 15 cm width. In some cases you could identify the bark and other parts of the tree. These pieces was identified as at least two types of conifers.
Eichstädts interpretation are that the olivine crystallized in the magma chamber and the magma was later erupted explosively due to the lack of any other crystallized minerals. Since the tuff is unsorted, the existence of wood and the location close to several basaltic necks his most probably conclusion are that the tuff is deposited by ashfall by a close-by volcano. He did not rule out sedimentary deposition in a fast flowing river, but think this is unlikely. (Eichstädt, 1883a)
The microscopic analysis in the paper by Tullberg and Nathorst was done by Svedmark. In 1883 he wrote a paper defending his analysis which Eichstädt had criticized. Svedmarks main point is that it is not possible to determine if this rock are tuff or a lavaflow, since it according to his analysis has elements of both. He also compared the thin sections from Djupadal with others from close by basaltic necks. In this analysis he could not conclude if there were any connection between any of the basaltic necks and the formation at Djupadal. It should instead be interpreted as an independent formation. (Svedmark, 1883) Eichstädt responded to this and concluded that the only new information that Svedmark added was the existence of xenolith bombs and lapilli, which according to Eichstädt rather are characteristic for tuff. He also notes that it should not be interpreted as an independent formation. (Eichstädt 1883b) These papers are an excellent example of researchers disagreeing and trying not to be to rude to each other, a rater humorous read.
More than a century later we return to Djupadal for a more advance analysis. Augustsson notes in her paper that the underlying layer are sandstone, and not gneiss as has been determined a century earlier. This seem to be based on several more outcrops a few hundred meters to the west of the main outcrop. The rest of her paper supports parts of Eichstädts and Svedmarks analysis. It supports Eichstädts conclusion that it is tuff, at the same it supports Svedmarks petrological analysis of altered pyroxenes and olivine. Augustsson also concludes that the tuff was deposited close to the vent in a strombolian eruption and that it might have been reworked in a marine environment at a later point. She believes the underlying sandstone layer are of the Höör formation and that the tuff was deposited close to this time period due to specific alterations of the tuff. It would make the tuff early Jurrasic. (Augustsson, 2001)
It is fascinating to follow the research and ideas on a specific topic from inception to current thoughts over a time period of 130 years. Some facts about science and research that tend to be overlooked stand out are important lessons to remember.
- Science change! Just because someone at some point came to a conclusion does not make it correct still. It might even be a contested conclusion when it was made.
- Basic research matters! No one would know of the tuff in Scania Sweden if no one went out and did the basic research almost 150 years ago. No one would know that it was an interesting find if someone did not analyze it. There might exist more old interesting finds that has not yet been analyzed closer.
- Get a better perspective! Following a subject from inception through several decades of research, discussion and conflict gives you a better perspective on the subject and science. Choose a small subject, otherwise you will be overwhelmed.
- Get a wider perspective! Reading all research on a very small subject will give a fuller understanding of all different subjects that really matter, especially if some conclusions are uncertain and there exists a vigorous scientific discussion taking up all relevant arguments from a wide selection of subjects.
- And last, my favorite: This was fun!
Augustsson, C. (2001). Lapilli tuff as evidence of Early Jurassic Strombolian-type volcanism in Scania, southern Sweden GFF DOI: 10.1080/11035890101231023
Eichstädt, Fr. (1883a). Om basalttuffen vid Djupadal i Skåne GFF DOI: 10.1080/11035898309444082
Eichstädt, Fr. (1883b). Ytterligare om basalt-tufjen vid Djupadal i Skåne GFF DOI: 10.1080/11035898309444119
Svedmark, E. (1883). Mikroskopisk undersökning af dé vid Djupadal i Skåne förekommande basaltbergarterna GFF DOI: 10.1080/11035898309444104
Tullberg, S. A., & Nathorst, A. G. (1880). Meddelande om en växtlemningar innehållande basaltvacka – vid Djupadal i Skåne GFF : 10.1080/11035898009444984
1Wacke are here defined as a completely or almost completely weathered basalt.