24 November 2014
Saturday I posted some images of bedding-parallel stylolites from one member of the Devonian-aged Helderberg Formation (or one formation in the Helderberg Group; I’m not sure whose stratigraphy is preferable in this case). Here we are, further up-section, and you can see both bedding-parallel and non-bedding-parallel stylolites overprinting the limestone:
Bedding-parallel stylolites can be understood readily in terms of sedimentary loading (compression from above), but non-bedding-parallel stylolites imply a maximum pressure direction that came not from above — but instead, from the sides. In other words, it’s a tectonic stress. These are likely the signatures of Alleghanian mountain-building in the late Paleozoic.
Interestingly, this same outcrop includes extensional veins filled with white calcite – these rocks have been both squeezed and stretched in more or less the same direction. I should assign an Honors student to go measure their orientations in detail, and check for overprinting relationships (cross-cutting relationships) between the veins and the stylolites.
Happy Monday – back at work!