Question

What is the difference between a geological disturbance and an unconformity

Answer 1

Answer:

Basically both indicate a break in deposition, either by nondeposition or erosion. An unconformity describes this phenomena (break in the rock record: hiatus). There are several types of unconformities and of course each has a specific name (eg. nonconformity, disconformity, angular unconformity). A disconformity discribes an unconformity at which the same general layering is present above and below the disconformity. An disconformity is thus much harder to recognize on seismic than for example an angular unconformity (in which layering above and below differs in dip).

A nonconformity is also more easily recognized as it occurs where sedimentary rocks are deposited on igneous or metamorphic rocks. These contacts are usually easy to spot since sedimentary rocks are layered and igneous rocks are not generally layered. Metamorphic rocks can also have layering, therefore close examination will allow one to distinguish metamorphic textures and minerals from sedimentary textures and clasts.

Anyway, disconformities mark the contact between two sedimentary units that are quite different in age but have parallel layers. Whenever the younger overlying sedimentary rocks fill channels eroded into the older sedimentary layers, the contact can more easily be identified. Sometimes, paleosol formation can alos be recognized if prolonged exposure to the atmosphere/environment has caused leaching weathering of the subsoil. If however, no significant topographic highs and lows or soil horizons are preserved at the top of the underlying older sedimentary rocks, the contact can usually only be recognized by a gap in the paleontologic record (dating (micro) fossils and checking whether a gap is present).

Also, on some occasions, there are igneous rocks intermixed with the sedimentary rocks (from vulcanic eruptions for instance). These can sometimes be dated by radiometric dating techniques. If these dates point toward a large gap in ages of the two sedimentary bodies, it could be very well possible that the boundary between them is a disconformity.

A disconformity is thus a specific type on unconformity.

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