What’s up with those Archean sandstones?

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:

WMpyrite2.jpg

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:

WMpyrite1.jpg

Two things of interest here*. Firstly, the appearance of much coarser gravel-rich horizons indicates a more high energy environment: stronger currents are afoot that can move around larger sediment grains. Secondly, just as in the first picture, there are numerous squarish holes surrounded by rusty weathering halos, which mark the former location of iron-rich minerals that have now been oxidised. From their shape, I suspect that these were detrital pyrite grains, which are one of the bits of evidence for an anoxic atmosphere in the Archean; it is only in a reducing, anoxic atmosphere that pyrite can be eroded, transported, and redeposited, rather than being chemically destroyed as it has been on exposure to the modern elements. The fact that these pyrites occur in both in the pale and the dark quartzites seems to rule out weird anoxic geochemistry as the explanation for the latter. Perhaps the stronger currents evident in the light quartzites have just winnowed away any darker minerals.
*Other than the use of my mobile phone as a scale bar. And the primitive nature of said phone.

Categories: Archean, fieldwork, geology, geopuzzling, past worlds

Comments (7)

  1. BrianR says:

    yeah, cut a thin section of two …
    How systematic is the change from darker to lighter sandstones? Does it always go from one to the other in the stratigraphic succession? Or, does it alternate?
    If there’s a systematic stratigraphic pattern maybe there’s a change in source area (?). That’s just one of many hypotheses. Again, a thin section would be key.

  2. How about some heavy minerals accumulating in the darker sand? Again, best thing to do, get a thin section.

  3. Ron Schott says:

    I’ll guess pyrite, Fe-Ti oxides, and possibly uraninite.

  4. Lab Lemming says:

    I think your grainsize and rounding (or lack of ) argues against detrital pyrite.
    As for the color, our Xian arenites are white sandstones that appear black or purple in outcrop until you whack off the outer 1-5 cm- there’s a ferrugenous weathering rind. So find a less photogenic part of the outcrop- maybe dig out a buried section- and apply the sledgie.

  5. octopod says:

    I’m going to guess that Lab Lemming’s right here, probably because I’m trained in the desert. It might just be a section that attracts, for whatever reason, the black weathering rind (possibly slightly elevated Fe/Ti/Mn content would do it — I’m thinking desert varnish). Did you try cracking off a piece of the outside yet?

  6. christie says:

    I’m with Lab Lemming on the detrital question. I bet those little cubes are authigenic. I bet the dk gray quartzites might have tiny micro-graphite xtals in the cement? If so, you’ll have a hard time finding them in thin section anyway.

  7. Chris Rowan says:

    I’m going to have to respectfully disagree on the pyrite – although I am by no means 100% certain. Two reasons:
    – Although the grains (or more technically, the holes) are squarish, many do actually have rounded corners (this is especially evident in some of the smaller ones in the first photo), which is possibly consistent with a little bit of transport.
    – Where the large holes appear, you also get larger detrital grains of other lithologies, like chert, of similar size. In the second photo, this is the case in the gravels towards the bottom, and (less obviously) in the prominent pyrite/hole-rich layer towards the top.
    As for the possibility of weathering rind – although I didn’t hammer the outcrop to be sure, you can see grain boundaries on these surfaces if you look closely, which (to me) suggested not.
    Feel free to refute me, though!