In an upper Miocene lacustrine sequence, we found very small volumes of volcanic-like rocks (see the picture) that nevertheless have not any crystal or any volcanic geochemical signature. It looks like in situ melting of narrow clay-bearing layers between limestones. Volcanic-like rocks reach a volume of only few cubic centimeters, along a discontinuous layer, but limestone above them show 2-3 meters of a strong rubefaction. There is not a heat source under the layer (only the unaltered lacustrine sequence). There is not any evidence of impact, and rocks reached only a few meters deep in his young history. Thus the only thing we can suppose is that the sedimentary rocks, that are very rich in organic matter, burnt below ground. Please, someone knows published examples that could be similar to this one?