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Geology of the Pacific Northwest

Basics -- Stratigraphy & Relative Ages


Stratigraphy is the study of rock layers and reconstruction of the original sequence in which they were deposited. The stratigraphy of an area provides the basis for putting together the geologic history of an area.

What is the Role of Stratigraphy in Understanding Geologic History?

The details of a region's stratigraphic story are revealed by:

Ask yourself how the things that are happening in the world today might end up being recorded in the sediments that are now or soon will be deposited. How would today's sediments appear to a geologist millions of years in the future examining outcrops of sedimentary rock that originated in our time? What would the geologist be able to deduce about the world we live in, based on what was left in the strata?

Stratigraphy started to become a formal science due to the work of a man who published under the name Nicolaus Steno in the 17th century. Steno made careful geologic observations and illustrations. He published the results of his work and established a basic set of principles for interpreting sedimentary strata. Geologists still use Steno's principles, with some refinements and additions. They are summarized as the Principles of Relative Geologic Age Determination, sometimes referred to as the Principles of Relative Dating.

What are the Principles of Relative Geologic Age Determination?

  1. The principle of original horizontality - sedimentary strata are initially deposited as horizontal or nearly horizontal layers.
    Note: If sedimentary strata dip at an angle other than horizontal, or are folded into various angles of tilt, then the layers of rock have been tilted or folded after the layers originally formed.

  2. The principle of lateral continuity - sedimentary strata extend sideways for some distance.
    Note: If a sedimentary stratum occurs on one side of a stream valley and a seemingly identical stratum occurs at a corresponding level on the other side of the valley, then presumably they were once a single, laterally continuous layer that was later partly eroded away as the valley was eroded.

  3. The principle of superposition - In a sequence of sedimentary strata, the stratum that is underneath is older, the stratum that is on top is younger.
    Note: This is probably the simplest and yet most powerful principle of relative age determination. However, to make sure it correctly applied, you need to be sure which way was up when the sediments were initially deposited, because in some geologic structures (faults or folds) it is possible for a layer of rock to be turned completely upside-down.

  4. The principle of inclusions - A piece of rock that is included in (completely surrounded by) sedimentary rock is older than the sedimentary rock in which it is included.
    Note: If rounded pieces of granite are pebbles in a layer of conglomerate that lies on top of the granite, then the granite must have been exposed, weathered and eroded prior to the conglomerate being deposited.

  5. The principle of cross-cutting relationships - A rock body or geologic structure that cuts off other layers or structures that would otherwise tend to continue is younger than the layers or structures that it cuts off.
    Note: If sedimentary beds are cut off by a fault, then the fault must be younger than the layers of sediment.

  6. Principle of faunal succession - Within a geologic era, period, or epoch there are certain fossil types that occur in strata of that age that are not found in strata of other ages.
    Note: This principle is a powerful tool for determining the age of sedimentary rocks. Index fossils are ones that only occur within limited intervals of geologic time. Much geological research has been done to determine the extent of geologic time through which particular index fossils occurred.

By the end of the 19th century, geologists had used these principles to put together an outline of the geological history of the world, and had defined and named the eons, eras, periods, and epochs of the geologic time scale. They did not know how many thousands, millions, or billions of years ago the Cambrian period began, but they knew that it came after the Proterozoic Eon and before the Ordovician Period, and that the fossils unique to Cambrian rocks were younger than Proterozoic fossils and older than Ordovician ones.

In the 20th century, radiometric methods of absolute age determination were developed. These methods allow the ages of certain types of rocks and minerals to be quantified in terms of years. By the 1960s absolute dating methods had been used to determine the ages of many rocks from all the continents and ocean floors. Repeatedly, the absolute age determinations confirmed what geologists already knew, for example that the Cambrian period occurred before-is older than-the Ordovician period. The absolute dating methods proved that the relative dating methods had been correct, and now geologists can say not only state the sequence of geologic time, they can also estimate fairly accurately how many years ago each division in the sequence occurred.

How Do Unconformities Mark Missing Time?

Another essential concept in stratigraphy is the unconformity. An unconformity is a surface upon which no new sediments were deposited for a long geologic interval. During this interval, erosion may have occurred before more deposits of sediments covered the surface. An unconformity marks a "gap in geologic time" because the rocks below and above it come from widely separated geologic times. There are no sedimentary strata to record what happened during the intervening interval. Instead, there is just an unconformity, a buried erosional or non-depositional surface.

Unconformities separate chapters in the geologic history of a given region. For instance, an orogenic episode (a long geologic episode of mountain building) may finally come to end and the eroded mountains may be buried beneath a new sequence of sediments. A major unconformity would mark the change from the building up of mountains to the wearing down of those same mountains and the subsequent blanketing of the area with sediments.

There are several specific types of unconformities. The three major, specific types of unconformities are included here.

The key to identifying each specific type of unconformity is recognizing what the unconformity is on top of. The possibilities for what is in the rocks immediately beneath the unconformity are (1) layers of sedimentary or volcanic rock (strata) that have been tilted or folded prior to development of the unconformity; (2) a stratum is parallel to the unconformity and parallel to the stratum above the unconformity; or (3) plutonic or metamorphic rocks, which originated much deep in the earth's crust rather than at its surface.

An angular unconformity is an unconformity beneath which the strata were tilted or folded before deposition of the younger layers of sediment above the unconformity. After being tilted or folded, the older layers of sediment were eroded. Then younger layers of sediment were deposited on them. The angular unconformity is the contact between the younger layers of sediment and the older, tilted strata beneath.

angular unconformity diagram

A nonconformity is an unconformity with sedimentary or volcanic strata on top and, beneath it, either plutonic rock such as granite or metamorphic rock such as schist. Because granitic and metamorphic rocks form deep in the earth's crust, a significant amount of time is required for uplift and erosion to expose them. Nonconformities mark major chapter breaks in the geologic history of an area.

In the example below, the contact between the conglomerate and the granite beneath it appears likely to be a nonconformity. However, it is possible that the granite may have intruded as a magma within the crust, beneath conglomerate, after the conglomerate formed. If so, the granite is younger and the boundary between the granite and the conglomerate is an intrusive contact rather than a nonconformity. To determine the nature of the contact - whether it is an intrusive contact or a nonconformity - further evidence from field investigations would be needed. Evidence such as angular pieces of conglomerate surrounded by the granitic intrusion, and contact metamorphism of the conglomerate adjacent to the granite, would indicate that the granite is younger and intruded the older conglomerate. Evidence such as rounded pebbles of the granite within the conglomerate would indicate that the granite is older and underwent erosion prior to the conglomerate forming, and the contact is a nonconformity.

nonconformity diagram

A disconformity is an unconformity with a sedimentary stratum beneath it that is not folded or tilted relative to the unconformity. Because there is a layer of sedimentary rock below a disconformity that is parallel to the layer above it, a disconformity may be difficult to recognize. The existence of a disconformity is indicated by the geologic ages of the sedimentary strata. If there is a significant gap in geologic time between the two layers - for example, if the layer beneath is Cambrian in age and the layer above is Devonian in age - then it can be inferred that the contact between the layers is a disconformity. Confirming evidence of a disconformity may include signs of erosion into the lower layer, and soil development on top of it, prior to deposition of the sediment of the upper layer.

In the example below, it appears that the contact between layers b and c may be a buried erosional surface. If the geologic ages of the strata show a significant gap in geologic time between stratum b and stratum c, then the contact between them is a disoncormity.

disconformity diagram

Web Links
University of California Museum of Paleontology description of principles of relative dating and some of the geologists who developed them
UCMP interactive geologic time scale

Glossary terms that appear on this page: sedimentary structure; fault; intrusion; tectonic plate; accreted terrane; sedimentary rock; granite; conglomerate; weather; erode; radiometric; metamorphic rock

Geology of the Pacific Northwest
Basics--Stratigraphy & Relative Ages
© 2001 Ralph L. Dawes, Ph.D. and Cheryl D. Dawes
updated: 9/11/13