Sunday, December 28, 2008

Unconformity

A geological term. It defines a break in the sequence of sedimentary rocks, usually with a significant time break between the older and newer sediments, often with a significant structural modification of the older rocks prior to deposition of the younger rocks. Such is the case in the Brian Gorge near Minerve in southern France:
Brian Gorge, the Unconformity
When one bed of rock is deposited directly on top of the preceding bed, we geologists say this is conformable. So when the beds have an angular discrepancy, as is the case here, we start looking for the unconformity between the two sequences. This photo shows the obvious rather well, with the gently dipping package of Cretaceous limestones resting across a series of steeply dipping Paleozoic rocks. Another view, below, shows that the surface of the unconformity is uniform, suggesting that the older sequence of rocks was eroded to a flat surface (peneplaned in geological vernacular) prior to the later deposition of limestones. The younger series of limestones has itself been tilted during the formation of the Alps.
Brian Gorge, the unconformity
Vegetation is also a key guide to defining the unconformity. The older rocks are generally impervious to water so plants and trees exist quite easily despite the long hot dry summers. The limestones are not so hospitable. The rivers in the area typically have water all year round where they flow over the Paleozoic rocks but are dry in the summer months over limestone. At one location we found a small spring of water issuing out along the unconformity - almost certainly a regular source of water for wildlife:
Spring at base Cretaceous unconformity, Cesse Gorge, Minerve