In addition to searching out evidence for Archean microbial
mats, my revisitation of the Pongola sandstones gave me the
chance to look a bit more closely at their lithology. When I
last posted pictures from this sequence, there was a bit of
discussion about why the beds appeared to be quite dark –
sandstones are generally lighter in colour (being composed
mainly of quartz). Is this due to some weird mineralogy? Or just
an effect of modern day weathering? Here’s a close-up of one of
the dark beds: It does appear that the dark colour does indeed
seem to be a result of minerals actually in the rock, rather
than formed by weathering on the surface. However, I’m still not
sure what these minerals are; presumably there’s just enough
squeezed in between this quartzite’s cemented quartz grains to
change it’s optical properties. You’d probably have to make a
thin section to know for sure what they are – any guesses? A bit
further up section, you find much more familiar-looking, pale
quartzites: Two things of interest