There is certainly some flowage of material, also alteration of fragments, a prominent pink colour from fine hematite throughout (probably from altered mafics).
These rocks were found in dug-out pits of what has been called locally breccia. On surface it looks like a conglomerate, but not after close inspection.
Location: inside the St. Martin meteorite crater, the 13th biggest on Earth.
Yes !
See the attached images with meltig textures, flow an degasing textures and at outcrop in contact with the colored breccia (backfill of the crater)
H.G.Dill
Dear Michael. In my opinion, the rocks are so much similar to melt glass than a suevite. In fact suevites are semi-fussed rocks with clasts embeeded in a glassy semi-melted matrix, and other debris. But yes, It could be interesting to analyze for impact origin.
Thank you Harald G. Dill for providing the photos of similar material from the Ries crater. They resemble the rocks from St. Martin's, broadly speaking.
Thank you José García for your valuable contribution, especially pinpointing the difference between suevite and "melt glass" or "melt rock". May I ask, what type of analysis would you do if you had encountered the above samples in your research?
Petrographic, basically. Indeed determining an impact structure depends on many factors, not only suevites, but it is an interesting field of research. Petrography will help determine proper characteristics in impact rocks; suevites, impact glasses, micro - macro tektites, shatter cones, impactites (which can be monomict, polymict... ), impactor remains. In this sense, the SEM or micro raman analysis can be very useful to determine these remains and to know their composition.
But first of all, the fastest and most economical, prepare thin sections and petrography. Determine planar deformation features, textures, etc.
I attached a picture in thin section of an impact melt glass from Ries. If you need documentation, feel free to ask me. I have hundred of micropictures from dozens of thin sections of several impact structures around the world.
Prior to any mineralogical study it is a must to map the facies changes within the crater and around and try and get an overview of the facies zonation versus depth. The facies association comprises impactite, shock-metamorphites, backfill-sediment, and lacustrine sediments. A good support to the common silica marker modifications is provided by the clay mineral log (see Dill 2020): Extract below
A peculiar geological event took place at around 14.6 ± 0.2 Ma at
the boundary between the present-day Alpine Foredeep (section 7) and
the Late Jurassic (Malm) limestone platform in the South German
Basin-this section (Buchner et al., 2010). It is the impact of a meteorite
which created a vast crater measuring 24 km in diameter around the
town of Nördlingen (Shoemaker and Chao, 1961). Salger (1977) provided
a clay mineral log of the research drill hole Nördlingen 1973
which may be subdivided into two principal sections the Miocene impactites
between 324m and the final depth at 1200m and the postimpact
clay-sand lacustrine crater fill. The lowermost part of the drill
hole hit a chaotic basement breccia of red and grey clay with illite,
550 °C - montmorillonite, chlorite, and corrensite. Chlorite and corrensite
increase towards depth within the deformed crystalline basement
rocks. Kaolinite, illite, montmorillonite-mixed layers are widespread
besides little chlorite in the lake sediments, while in the
transitional suevite zone 660° C- montmorillonite is present almost
exclusively in the clay mineral assemblage. The temperature data in
context with montmorillonite refer to endothermic reactions at 550 °C
which typical of phyllosilicates are found in marlstones of the southern
Alpine Foredeep and from the weathering loam blanketing the Jurassic
karst platform. The 660° C – montmorillonite has been derived from
devitrification of glass in the suevite.
DILL, H.G. (2020) A geological and mineralogical review of clay mineral deposits and phyllosilicate ore guides in Central Europe - A function of geodynamics and climate change.- Ore Geology Reviews 119 (on-line)
Dear Michael,
thanks for posting these pictures. Given the context, this seems to be a nice example of a flow-banded impact melt rock with some relict digested target rock clasts. The rock should be referred to as an impact melt rock rather than suevite (suevite has its impact melt component in discrete glass/melt fragments and typically possesses a particulate groundmass). Similar red-stained impact melt rocks also occur, e.g., at the Ries crater, Germany (Polsingen quarry) and at Rochechouart, France (e.g., Montoume quarry; outcrops around Valette). The Lake St. Martin impact structure is about 228 million years old.
However, I'd like to stress that - based on remote image analysis alone - one cannot reliably tell impact melt rock from volcanic (or, in some cases, even metamorphic) lithologies, e.g., welded ignimbrites (or, e.g., ultramylonites). Microscopic inspection of impactites should always reveal the presence of shock metamorphism in mineral grains, most notably quartz and zircon. In impact melt rocks, those grains commonly exhibit shock deformation at all levels of shock because the rock is usually a mix of highy shocked and weakly shocked target material.
Best, Martin
Thank you José García for providing the picture of a melt glass and for your guidance to proceed to identify rock units.
The government study on this crater did thin sections and petrography from their drilling project, but they did not include the location where I took my samples.
Your thin section collection will be valuable for meteor crater investigators and I will contact you again for further assistance.
Thanks again!
Thank you Harald G. Dill for your suggestions as well as providing the example of the mineral distribution at depth within the Ries crater. As you say, this crater is peculiar and has many unique aspects. The drilling provided an excellent opportunity to see the formation of (alteration?) clays with depth.
In the case of the St. Martin crater the outcrops are rare and most of the information comes from drilling.
Thank you Martin Schmieder for your contribution and specifically for not referring these rocks as suevites, but impact melt rocks.
You are very right, we need to find the final proof, which is examples of shock metamorphism on suitable grains of quartz (which is rare here) and zircon. Do you think the material in these rocks would show shock metamorphism?
This crater has no shatter cones, but has a central peak and an annular ring.