5 June 2018

Cross-bedding in Archean komatiitic ash deposits

Posted by Callan Bentley

When I was learning about the Archean from the perspective of Barberton Mountain Land in South Africa, I was expecting to see komatiite lava flows. Early on in Earth history, the planet was hotter: (1) it was closer to the many thermokinetic impacts that built the planet from stone-cold meteorites, and (2) there were many more unstable radionuclides around, decaying and releasing their energy into the young planet. As a result, some minerals that don’t melt readily at modern volcanoes were able then to turn to liquid and ooze out of volcanoes at temperatures much higher than modern eruptions. As they crystallized, these lava flows grew “chandeliers” of spinifex textured olivine and pyroxene crystals.

However, I was not expecting to see primary sedimentary structures in these ash deposits. Some of the ultramafic volcanoes blew up, sending ash into the Archean atmosphere, from where it rained down as itty-bitty particles. In addition to accretionary lapilli and tsunamites (with accretionary lapilli!), the komatiite ash deposits show current flow indicators such as cross-beds, indicating they were moved around by currents of either air or water prior to final deposition:

These cross-beds are concave-up (like smiley faces), and if you’re astute, you’ll be able to find a few instances here where the top of the cross-beds are truncated by an overlying bed. Here’s a GIGAmacro example of that, with cross-bedding annotated in blue, and the bottom of the overlying bed in gray:

Note also the rusty patina of these ultramafic volcanic/sedimentary rocks – the olivine therein is ready to rust, even in the arid climate of South Africa.