On Day 4 (Wednesday, 17 October), we examined older rocks of the Iberian Massif: a thick Paleozoic sequence that had been deposited on the passive margin of Gondwana, and the angular unconformity separating Proterzoic from Cambrian rocks.
Paleozoic Rocks Near the Luna Dam
We began the day by driving from Barrios de Luna, over the Luna Dam, to a thick sequence of Paleozoic sedimentary rocks.
At the top of the sequence outcropped the Armorican Quartzite– a highly resistant quartzite that can be found all across Europe. The Armorican likely formed on the continental margin of Gondwana during the Lower Ordovician, just after the rifting of Avalonia and opening of the Rheic.
Embedded in the Armorican, we found a 1-foot-thick layer of kaolinite, an aluminum-rich clay that forms from weathering of feldspars. The kaolinite has been dated to 456 million years and is found across Europe (the old northern edges of Gondwana). It is likely that this continent-wide kaolinite layer was originally a continent-wide layer of volcanic ash, rich in feldspars. Such an ash layer could result from a super-volcano eruption, hundreds of times more powerful than the Mount St. Helen’s eruption. Such a volcano might have formed on the margin of Gondwana, as the Avalonia rifting event caused crustal extension and volcanism.
Below the Quartzite and ash layer, we found a Middle Cambrian sedimentary sequence, including trilobite-bearing shales, sandstones with cross-bedding, and a red nodular limestone that forms the main detachment layer for the thrusts we saw in the Picos Mountains on Day 2.
The Precambrian-Cambrian Unconformity
After visiting the Luna Dam outcrop, we stopped at a road cut that exposed the angular unconformity between the Precambrian and Cambrian. The Precambrian sandstones and shales had been tightly folded and faulted, then uplifted and exposed to erosion. The Cambrian sediments had then been deposited on top of them, making a spectacular angular unconformity at the base of the Paleozoic.