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24 January 2012

Paleoproterozoic stromatolites from the Malmani Dolomite (Transvaal Supergroup)

After our safari, Lily and I were taken up onto the Great Escarpment in northern South Africa. The escarpment is supported by sedimentary strata of the Transvaal Supergroup that overlie the Archean basement rock of the Kaapvaal Craton. The Transvaal strata are Paleoproterozoic in age, somewhere between 2.5 and 2.0 billion years old. They are a mix of siliciclastic sediment and carbonates. Here’s the view from an overlook dubbed “God’s …


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21 March 2011

The Big Bentonite just got smaller

The Shenandoah Valley of western Virginia records the switch in late Ordovician time from passive margin sedimentation associated with the Sauk and Tippecanoe epeiric seas, to active margin sedimentation associated with the onset of the Taconian Orogeny to the east. Higher up in the stack, a similar pattern is seen: a return to passive margin sedimentation with the deposition of the Helderberg Group of limestones, and then more active margin …


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22 January 2011

Let’s coin a name for this phenomenon

I didn’t mention it yesterday, but there was one other structure that I saw at my newest outcrop on New 55. This is it: That’s a bunch of fractures. The broken rock is being altered by preferential fluid flow through the fractures. The fluid is not inert; it’s chemically active, and reacting with the rock. These reactions produce the color and weathering differences that you note in the previous images. …


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28 December 2010

GoSF8: Fractures

The 8th edition of the ongoing “Geology of San Francisco” series examines brittle fractures and the chemistry they host along their planar surfaces.


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4 December 2010

Reduction rims?

Another look at Konnarock Formation diamictite, showing colorful reaction “halos” around some clasts.


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