22 July 2015
Oolitic soft sediment deformation in Helena Fm. limestone
Posted by Callan Bentley
Another gem from the Grinnell Glacier cirque:
Zooming in on the contact, showing the concentrically-zoned ooids:
Near the tip of the flame structure (?), I noted alignment of longer platy / flaky components within the oolitic layer:
This looks like a loading structure – soft sediment deformation due to a density inversion – perhaps when some high-energy event (a storm?) dumped a bunch of relatively coarse ooids atop some squishy micrite mud. The upper oolitic layer glooped down into the micritic lower layer, creating this convoluted contact.
Very nice images indeed. Not a load structure though but a scour surface. The underlying lime mud was undergoing incipient cementation after the formation of the molar-tooth structure veins. These are seismic injection features, whereby granular lime mud was ejected during dewatering, and they cemented quickly. The matrix also started to cement away from the veins. That is why the areas in between the vertical veins were scoured more deeply. The oolite is allochthonous and I have also argued that these beds may be tsunami-generated, washed in from shoals in shallower water.