Re: Loess, sandstone or tephra?


<peba@xxxxxxxxxxxxxx> skrev i en meddelelse
news:1119019037.866918.303880@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
> I'm grappling with the origin of a peculiar "sandstone". Here are some
> photos and a macroscopic description:
>
> http://www.users.totalise.co.uk/~peba/sst1.jpg
> http://www.users.totalise.co.uk/~peba/sst2.jpg
> http://www.users.totalise.co.uk/~peba/sst3.jpg
>
> Characterised by a friable, fine-grained, homogeneous, creamy/beige
> matrix of quartz with randomly oriented, angular clasts of various
> lithologies and variable size that constitute less than 1% of the rock
> volume.

I've considered a couple of option that hasn't been mentioned, but I don't
think that it's at work: 1) Gypsum crystals in a desert/evaporitic
environment may form somewhat angular minerals that could be interpreted as
misplaced clasts. Naa. 2) A muddy deposit that's exposed to an increasing
salt-consentration may develope cracks and fractures (syneresis) that fills
with mineral precipitates. Naa.
The clasts look like they are allochthonous. I'm not very familiar with
fluvial structuring in desert-environments, where there is both an excess of
silty/sandy sized sediment, but foremost a general excess of sediment
compared to the run-off that occationally carries it.

> The colour of the matrix is locally red-brown, presumably due
> to infiltration by Fe-bearing fluid. The distribution of the clasts is
> highly variable. A very fine, barely discernible lamination was
> observed in the matrix at a couple of localities. Adjacent to small
> hydrothermal veins, the matrix is usually indurated (cemented) to a
> hard quartzite.
>
> Other possibly useful clues: the rock occurs in the Naboomspruit area
> of South Africa. it is believed to be part of the sedimentary Karoo
> package (140-300 Ma) that overlies and is faulted against the
> volcanosedimentary Rooiberg Group (2050 Ma). The Rooiberg Group is
> believed to represent the initial volcanism associated with the
> emplacement of the mighty Bushveld Complex.
>
> My problem is the randomly oriented, angular clasts in this otherwise
> homogeneously textured rock. How did they get there? When I first saw
> the rock, I dismissed a sedimentary origin and thought it might be some
> kind of tephra.

> And when I say friable, I mean the rock comes away as
> powder when you rub it - it doesn't break into small chunks.

It sounds like a somewhat immature sediment that lack precipitation to
forward the chemical processes to develope a through induration. That would
be consistent with the above comment.

Carsten


.

Relevant Pages

  • Re: Smooth inclusions in Wishstone rock.
    ... inside the rim of the RAT hole in this image of Wishing ... Wishstone rock. ... clasts", but. ... > and incorporated into a new unit of sediment. ...
    (sci.astro)
  • Re: Angles and Erosion Rates - For John Harshman
    ... that their thickness was no greater than if they were flat. ... the WWW-type folded sheet, will be less, at every point, compared to ... rock under the bottom of the sheet of sediment is also folded into a ... remains will be over the depressions in the underlying crystalline rock ...
    (talk.origins)
  • Re: R. Forrest: common rat
    ... The geological column is a theoretical construct based on rock ... thickness as well as the numbers of layers present, ... field geology can tell you. ... formed, overlain by a large amount of additional sediment, and ...
    (talk.origins)
  • carefully scatter this solar disadvantage
    ... in a crime beside the sediment is too wide for Jonathan to house it. ... Tell Haji it's sexual admiring amid a gang. ... learn the bad rock. ...
    (sci.crypt)
  • Re: The Thickness of Richard Forrest
    ... That's what you get when you put all the layers together. ... crystalline base rock. ... Why is it still covered by it's layered sedimentary "skin" ... The rocks below the sediment are not associated ...
    (talk.origins)