27/10/2022
Blackchurch rock .... is an impressive sea stack found on the North Devon coast, UK. It is composed of marine sandstone, shale and mudstone that have been separated from the mainland cliffs by sea erosion. Wave action has also attacked the less resistant layers of mudstone, opening up two windows or arches within the stack (comment schematic). The rocks dip ~60o with the axes trending parallel to the coast.
The formation is Carboniferous in age (~320 Mya) and was dated using index fossils. The rocks contain goniatites (comment photo), ammonoid cephalopods that lived in tropical to subtropical seas from the Middle Devonian until their extinction at the end of the Permian (The Great Dying). Their sutures are smooth saddles and lobes, which gives the name "goniatitic" to this particular pattern, and these sutures often have a distinctive "zigzag" pattern. Goniatites are useful as index fossils for age determination and stratigraphic correlation.
From the Late Devonian to the Early Permian, rocks in the SW UK were subjected to a major mountain building episode, the Variscan Orogeny. This tectonic event was caused by the northward convergence and collision of the continent of Gondwana (present day South America, Africa, Antarctica, Australia and India) with the continent of Laurasia (present day Europe, Russia and North America). This collision eventually resulted in the closure of the ancient 'Rheic Ocean' and the formation of the supercontinent Pangea in the early Permian (~290 Ma). This area was part of a foreland basin as the ocean was closing.
Rivers flowed from areas of high land in Wales southwards into the basin, carrying fine sediment which settled out on the sea floor as muddy layers, eventually becoming shales. Periodically, submarine ‘avalanches’, triggered by earthquakes, carried flows of much coarser sediment mixed with water down onto the ocean floor. These are called turbidity currents and they deposited layers of fining-upward sands across the steep slopes and floor of the basin, eventually becoming the interbedded rocks we see today.