25 May 2017

Chert-slab conglomerate from the Fig Tree Group, Barberton

Posted by Callan Bentley

A return to the Barberton Greenstone Belt in South Africa today….

Let’s examine a fascinating chert-slab flat-pebble conglomerate in the (Archean aged) Fig Tree Group’s Mapepe Foramtion, exposed at one of the stops along the R40 “GeoTrail”:

These shots show close-up images of the texture of a single 6m thick unit that cuts into (and is overlain by) deep-water ferruginous and tuffaceous shale. It was probably deposited in a single violent moment in geologic time. It may have been triggered by a tsunami, which itself may have been triggered by an earthquake or meteorite impact.

I visited it on a terrific field trip I took prior to the International Geological Congress meeting I attended in Cape Town last summer. The field trip was led by Don Lowe, Chrisoph Heubeck, and Gary Byerly.

The clasts are slab-like in shape: roughly 1:15 axial ratios as exposed on these bedding-perpendicular outcrop surfaces. The largest slab has roughly the dimensions of a family-sized pizza!

The clasts are oriented roughly (paleo-) subhorizontally, though there’s plenty of imbrication to be observed, too.

It’s intriguing to think of the story implied by these clasts: chemical deposition of chert in calm waters below the wave base, for years and years. Then, a sudden violent event (tsunami from a distant meteorite impact??) fractures the chert and triggers a turbidity current of the unsettled debris. This tumbles its bits and pieces down a submarine channel en masse, to be dumped in a huge pile. Wow: sudden enough and violent enough to qualify as downright catastrophic!

And then, it was over, and the debris rained out, and the water cleared, and a more gradual deposition of sediment resumed in its same old uniformitarian way.