Unconformities: Gaps in the Geological Record

A 2005 research cruise in the remote Pacific found something surprising: nothing. The scientific team aboard the research vessel Melville, mapping and drilling in the central South Pacific seafloor, traced out a region of bare rock that's bigger than Alaska. It had none of the mud, clay, ooze, or manganese nodules that cover the rest of the deepest sea. This was not freshly made rock either, but oceanic crustal basalt that was 34 to 85 million years old. In other words, the researchers discovered a strange 85 million year gap in the geological record. The finding was important enough to be published in the October 2006 Geology, and Science News also took note. 

Unconformities Are Gaps in the Geological Record

Gaps in the geological record, like those discovered in 2005, are called unconformities because they do not conform to typical geological expectations. The concept of an unconformity arises from two of the oldest principles of geology, first stated in 1669 by Nicholas Steno:

1. The Law of Original Horizontality: Layers of sedimentary rock (strata) are originally laid down flat, parallel to the Earth's surface. 
2. The Law of Superposition. Younger strata always overlie older strata, except where the rocks have been overturned. 

So in an ideal sequence of rocks, all the strata would stack up like the pages in a book in a conformable relationship. Where they don't, the plane between the mismatched strata—representing some sort of gap—is an unconformity. 

The Angular Unconformity

The most famous and obvious kind of unconformity is the angular unconformity. Rocks below the unconformity are tilted and sheared off, and rocks above it are level. The angular unconformity tells a clear story:

1. First, a set of rocks was laid down.
2. Then these rocks were tilted, then eroded down to a level surface.
3. Then a younger set of rocks was laid down on top.

In the 1780s when James Hutton studied the dramatic angular unconformity at Siccar Point in Scotland—called today Hutton's Unconformity—it staggered him to realize how much time such a thing must represent. No student of rocks had ever contemplated millions of years before. Hutton's insight gave us the concept of deep time and the corollary knowledge that even the slowest, most imperceptible geologic processes can produce all the features found in the rock record.

The Disconformity and Paraconformity

In disconformity and paraconformity, strata are laid down, then a period of erosion occurs (or a hiatus, a period of nondeposition as with the Pacific Bare Zone), then more strata are laid down. The result is a disconformity or parallel unconformity. All the strata line up, but there is still a clear discontinuity in the sequence—maybe a soil layer or rugged surface developed on top of the older rocks.

If the discontinuity is visible, it is called a disconformity. If it is not visible, it is called a paraconformity. Paraconformities are harder to detect, as you might imagine. A sandstone in which trilobite fossils suddenly give way to oyster fossils would be a clear example. Creationists tend to latch onto these as proof that geology is mistaken, but geologists see them as evidence that geology is interesting.

British geologists have a slightly different concept of unconformities that is based purely on structure. To them, only the angular unconformity and the nonconformity, discussed next, are true unconformities. They consider the disconformity and paraconformity to be non-sequences. And there's something to be said for that because the strata in these cases are indeed conformable. The American geologist would argue that they are unconformable in terms of time.

The Nonconformity

Nonconformities are junctions between two different major rock types. For example, a nonconformity may consist of a body of rock that is not sedimentary, upon which sedimentary strata are laid down. Because we aren't comparing two bodies of strata, the notion of them being conformable doesn't apply. 

A nonconformity might mean a lot or not much. For instance, the spectacular nonconformity at Red Rocks Park, in Colorado, represents a gap of 1400 million years. There a body of gneiss 1700 million years old is overlain by conglomerate made of sediment eroded from that gneiss, that is 300 million years old. We have almost no idea of what happened in the eons between.

But then consider fresh oceanic crust created at a spreading ridge that is soon covered by sediment settling down from the seawater above. Or a lava flow that goes into a lake and is soon covered with mud from local streams. In these cases, the underlying rock and the sediment are basically the same age and the nonconformity is trivial.