Lignite and pyrite

While out drilling not so long ago, I happened upon a unexpected layer of lignite and pyrite in the overburden.

The lignite chips were from ~160m below the current surface and came up as small, black spongy rodlike shapes with a clear woody texture. A number of the chips had positive buoyancy and would float in my rinse water. Samples above and below the lignite, appear to be a form of brown or red dolomitic mudstone as it fizzed on contact with acid. I'm leaning away from a siderite classification, as i also recovered the following:

Imbedded in the mudstone, and proximial to minor lignite, were small grains of pyrite. They sparkled like gold in the sun and had a dark streak. Siderite is prized as it is low in sulfur however the presence of pyrite indicates sulfer enrichment.

I hypothesize these rocks are the result of a significantly more humid climate where water was able to pool at the surface. In this particular location, a swamp has formed. Dead vegetation was covered by water and sediment before significant decay could occur. The anoxic environment of the swamp has preserved some of the vegetation prior to burial. The water chemistry probably has also contributed to the precipitation of sulfur as pyrite.

Sedimentary structures - mud cracks and rain droplets

Very occasionally, it's possible to find ancient raindrops preserved in sedimentary rocks. These imprints tell us a couple of things, a) the host sediment was either under very shallow water or at the Earths surface so it's highly likely to be of terrestrial (as opposed to marine) origin and b) the overlying sediment was deposited in a very low energy environment so as not to disturb the imprints.

 

The image above displays two lovely modern sedimentary features, mudcracks and rain drop craters. Interestingly, we can see that in the middle of the images the raindrops imprints get fuzzy before stopping. We can therefore assume this is the border of a puddle that existed either before the rainfall or soon after as either the water absorbed the impact of the raindrop creating no mini-crater or the water encroached and wore the imprint away.

Think about how fragile these mudcracks and raindrop craters are, they'd be erased by the next large rainfall if not covered over and protected. The likelyhood of raindrop preservation is very low, so if you do happen to find some take a moment to marvel at the numerous process that had to occur to create that rock!