Unconformities: Gaps in the Record
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| The unconformity at Siccar Point, Scotland. |
To find good exposures of rock, James Hutton sometimes boated along the coast of Scotland, where waves of the stormy North Sea have stripped away soil and shrubbery. He was particularly puzzled by an outcrop at Siccar Point, where two distinct sequences of sedimentary rock lie in contact (figure above a, b). In the lower portion of the outcrop, beds of gray sandstone and shale dip nearly vertically, whereas in the upper portion, beds of red sandstone and conglomerate display a dip of less than 20 degree. Further, the gently dipping layers seem to lie across the truncated ends of the vertical layers, like a handkerchief lying across a row of books. We can imagine that as Hutton was examining this odd geometric relationship, the tide came in and deposited a new layer of sand on top of the rocky shore. With the principle of uniformitarianism in mind, Hutton suddenly realized the significance of what he saw. The gray sandstone-shale sequence had been deposited, turned into rock, tilted, and truncated by erosion before the red sandstone–conglomerate beds had been deposited.
Hutton deduced that the surface between the gray and red rock sequences represented a time interval during which new strata had not been deposited at Siccar Point and the older strata had been eroded away. We now call such a surface, representing a period of non-deposition and possibly erosion, an unconformity. The gap in the geologic record that an unconformity represents is called a hiatus. Geologists recognize three kinds of unconformities:
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| The three kinds of unconformities and their formation. |
- Angular unconformity: Rocks below an angular unconformity were tilted or folded before the unconformity developed (figure above a). Thus, an angular unconformity cuts across the underlying layers, and the orientation of layers below an unconformity differs from that of the layers above. (The outcrop at Siccar Point exposes an angular unconformity.)
- Nonconformity: A nonconformity is a type of unconformity at which sedimentary rocks overlie generally much older intrusive igneous rocks and/or metamorphic rocks (figure above b). The igneous or metamorphic rocks underwent cooling, uplift, and erosion prior to becoming the substrate, or basement, on which new sediments accumulated.
- Disconformity: Imagine that a sequence of sedimentary beds has been deposited beneath a shallow sea. Then sea level drops, exposing the beds for some time. During this time, no new sediment accumulates, and some of the pre-existing sediment gets eroded away. Later, sea level rises, and a new sequence of sediment accumulates over the old. The boundary between the two sequences is a disconformity (figure above c, d). Even though the beds above and below the disconformity are parallel, the contact between them represents an interruption in deposition.
The succession of strata at a particular location provides a record of Earth history there. But because of unconformities, the record preserved in the rock layers is incomplete. It’s as if geologic history is being chronicled by a tape recorder that turns on only intermittently when it’s on (times of deposition), the rock record accumulates, but when it’s off (times of non-deposition and possibly erosion), an unconformity develops. Because of unconformities, no single location on Earth contains a complete record of Earth history.
Credits: Stephen Marshak (Essentials of Geology)
