2700 million years in one outcrop

This was the first outcrop we stopped at on our last field trip:

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Most of the outcrop consists of coarse sandstones, complete with down-cutting channels and cross-bedding, and the occasional conglomerate horizon.

BFU-2.jpg

At the top, there’s a contact with an unconsolidated conglomerate, which (from the red colour) appears to have been fairly pervasively weathered over long periods of exposure. So far, so not-particularly-interesting (although Brian or Kim may beg to differ) – but only until you start to investigate the age of the two different units. The lower sandstone and congolmerate unit can be traced into other areas, where it is sandwiched between lava units which have radiometric dates of about 2.7 billion, or 2700 million, years (they’re part of the Ventersdorp Supergroup). In contrast, minerals formed by weathering reactions at the contact have also been dated, and have an age of only about 15 million years.
So the contact is in fact a massive unconformity: the time between the deposition of the two units in this outcrop is somewhere in the region of 2680 million years, which is a fairly sizeable chunk of geological history. But the really mind- blowing part is exactly how little seems to have happened, geologically, in that missing time. When I think of big uncomformities, I tend to think of something like Hutton’s Uncomformity, found at various places in north-west Scotland:

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(image source)
Here, the older, underlying unit (which is, if I recall correctly, somewhere in the region of 600-700 million years old) has been severely warped during the process of uplift and erosion, before the deposition of flat-lying 400 million year-old sediments on top (which have also been tilted slightly by a later tectonic episode). And this is in a time gap of ‘only’ 300 million years or so! Yet here in central South Africa, in almost an order of magnitude more time, not much has happened at all; the Ventersdorp rocks don’t even seem to have been buried very deeply underneath younger sediments that have then been removed by later erosion, because there’s no real signs of the metamorphic minerals that would have formed at the increased temperature and pressure that would have resulted.
So, when your hear geologists talking about ‘stable cratonic interiors’, this is what they mean: very old rocks just sitting there, letting all the geology happen somewhere else. On this particular field trip, this starting outcrop was a useful point of comparison to the numerous belts of much more deformed rocks we passed through on our journey to the west coast, testaments to multiple bouts of continental collision that went unnoticed further inland.

Categories: deep time, fieldwork, geology

Comments (6)

  1. Kim says:

    Wow. Your unconformity beats my Great Unconformity (Cambrian, so 500-million-year-ish, sitting on top of 1700 million year old granite – 1200 million years missing). Though in my case, there’s also a significant bit of crust missing – I don’t know exactly how deep the granite was when it intruded, but mid-crust is probably a reasonable guess, given the texture and the metamorphic grade of other rocks in the area.
    There’s a reason why people go to South Africa to try to understand what the Earth’s surface was like back in the Really Old Days.

  2. Bob O'H says:

    One probably shouldn’t anthropomorphize rocks unduly, but I do like the idea of the older sandstone looking around thinking “Quiet here, isn’t it?”

  3. Lassi Hippel‰inen says:

    Here is Scandinavia we have similar formations, except that we don’t have that 15 My old dirt on the rock. The Ice Age took care of it.

  4. Lockwood says:

    Zowie! The unconformity alone is nearly as old as the oldest rocks I’ve ever seen in the field. The oldest rocks here in Oregon are roughly 1/10th of that; in other words, my entire state could evolve from scratch 10 times over during that span of time. I’m not much of one most of the time for “plain old seds,” (with respectful apologies to the strat and sed types out there) but that’s a pretty special outcrop.
    Not that you’d know by just looking at it.

  5. PaulG says:

    Yeah, I like that one.

  6. Stephen says:

    A little off-topic: I’m off to South Africa with the family next week. We’re doing a fairly standard circuit: Jo’burg – Lydenburg – Kruger – Swaziland – Mkuze – Durban – Royal Natal NP – Jo’burg. We’ll be spending most of the available time on mammals, birds and general interest subjects, but your blog made me wonder if a closer look at some of the rocks might also be rewarding. Are there any geological features you’d recommend as of interest to the layman on that route? (Apart from dear old Bourke’s Luck Potholes.)

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