5.1: Relative Dating

Placing Events in Geologic Order Using Principles of Relative Dating

Relative dating is the process of determining whether one rock or geologic event is older or younger than another rather than determining a specific age. The fundamental principles of relative time were developed from the mid-1600's to the early 1800's by key players, including Nicolas Steno (1638-1686), James Hutton (1726-1797), and Charles Lyell (1797-1875), among many others. Once geologists realized that many geologic processes are slow, the concept that “the present is the key to the past” provided a basis for interpreting rocks of the Earth using scientific principles.

An important concept is the fact that strata (layers of rock) equate to events. 

Stratigraphic Cross Sections: Interpreting the Geologic History

This video describes the way in which concepts of geologic time can be applied to the interpretation of a geologic region.

The following section reviews concepts that are applied to resolve the relative order of geologic events in the field. The key thing to remember here is that events = time (in other words, sequences of events take time, so an event can be equated to a period of time).

Relative Dating Principles

The following relative dating principles are useful in studying layered sedimentary rocks and volcanic units that are deposited at the Earth’s surface. 

Principle of Superposition

In an otherwise undisturbed sequence of sedimentary strata, or rock layers, the layers on the bottom are the oldest and layers above them are younger. In Figure \(\PageIndex{1}\), the horizontal layers have been uplifted (in this case via the nearby Garlock fault) and eroded, but there is no indication of any other events affecting their position. 

Figure \(\PageIndex{1}\): Layered sedimentary units of the Ricardo Group at Red Rock Canyon State Park in California. The oldest are on the bottom and the youngest are on the top. "Redrock Canyon" by Martha House, is licensed under CC BY-SA-NC.

Principle of Original Horizontality

Layers of rocks deposited from above, such as sediments and lava flows, are originally laid down horizontally. The exception to this principle is at the margins of basins, where the strata can slope slightly downward into the basin. The lower to middle Pliocene, lacustrine rocks of the Anaverde Formation in Figure \(\PageIndex{2}\), for example, were originally deposited in a horizontal fashion, but have contorted due to interactions with the nearby San Andreas and Little Lake Faults. Relative dating cannot determine the exact age or duration of the contortion, but we know it took time.

Figure \(\PageIndex{2}\): Folded Late Cenozoic sedimentary units are exposed on the northeast side of Highway 14 in Palmdale, CA. These rocks have been tilted due to their proximity to the San Andreas Fault. This work by Jens Bludau is licensed under CC BY-SA 4.0.

Principle of Lateral Continuity

Within the depositional basin, strata are continuous in all directions until they thin out at the edge of that basin. Of course, all strata eventually end, either by hitting a geographic barrier, such as a ridge, or when the depositional process extends too far from its source, either a sediment source or a volcano. Strata that are cut by a canyon later remain continuous on either side of the canyon. The Grand Canyon strata in Figure \(\PageIndex{3}\), for example, can be seen to continue on the far side of the canyon.

Figure \(\PageIndex{3}\): Kaibab Fm., Mather Point, Grand Canyon National Park. Light units in the foreground are correlative with light units in the background, even though they are separated by the canyon. "Mather Point at Grand Canyon National Park in October 2015" by Tim Adams, is licensed under CC BY 3.0.

Not all geologic materials are deposited horizontally at the Earth’s surface. For example, plutonic rocks and dikes do not form horizontal layers; they intrude into the rocks around them. And faults cut across older units. In order to recognize these events and the time they take, additional concepts are needed: 

Principle of Cross-cutting Relationships

Deformation events like folds, faults and igneous intrusions that cut across rocks are younger than the rocks they cut across. Younger things cut older things. Figure \(\PageIndex{4}\) shows normal faults that cut the older Noonday Dolomite in Death Valley National Park. The faults are younger than all units they cut through. 

Figure \(\PageIndex{4}\): Normal faults in Noonday Dolomite, Death Valley National Park. The dark layers in the center are truncated by the two inward dipping normal faults that drop the large tan block of Noonday Dolomite downward. The faults post-date the sedimentary units. "Normal faults in Noonday Dolomite, Death Valley National Park" by Marli Miller, by permission.

Principle of Inclusions

Inclusions are pieces of an older rock that are contained within a younger rock. These are often found in sedimentary rocks, but can also be found when igneous rocks intrude pre-existing rocks surrounding them (pieces of the country rock might be found within the intrusive rock. In general, the younger unit contains or envelops the older unit. The glacial dropstone shown in Figure \(\PageIndex{5}\), for example, is from an original rock that is older than the Kingston Peak Fm. 

Figure \(\PageIndex{5}\):  Glacial dropstone in Proterozoic diamictite of Kingston Peak Formation, SE California. "Glacial dropstone in Proterozoic diamictite of Kingston Peak Formation, SE California" by Marli Miller, by permission.

In addition to these concepts which apply to relative ordering of rock units, fossil remnants preserved in sedimentary rocks provide a means to link chronostratigraphic (time-correlative) units across large, sometimes global, distances. 

Principle of Faunal Succession

Assemblages of fossils contained in strata are unique to the time they lived, and can be used to correlate rocks of the same age across a wide geographic distribution (Figure \(\PageIndex{6}\)). Assemblages of fossils refers to groups of several unique fossils occurring together. When assemblages are found in short sequences, the overlap of portions of the sequences from place to place can be used to extend relative time constraints.

Figure \(\PageIndex{6}\): Fossil succession in two outcrops that are exposed on either side of a canyon. In each exposure of layered rocks, a distinctive sequence of fossils can be used for comparison of the outcrop sequences. The correlation of the units in the two areas can be done by comparing these sequences and noticing where a layer might be missing (based on the lack of a specific fossil). "Faunal Succession" by Paul Webb is licensed under CC BY-NC-SA.

Principle of Chilled Margins

When an igneous intrusion cools and crystallizes in the Earth, the portion that is on the edges, adjacent cold surrounding rock will form smaller crystals than the portion of the intrusion that cooled more slowly deeper in the intrusion. Sometimes, the smaller crystals appear darker in color than larger crystals, so a chilled margin appears as a darkening of the intrusive rock towards the surrounding rock. This principle can be used to distinguish between an igneous sill, which will have a chilled margin at top and bottom, and a lava flow, which will have a chilled margin only at the bottom.

Principle of Baked Contacts

Rocks that surround igneous intrusions are affected by the heat of these intrusions. As a result, rocks surrounding the intrusions, sometimes called the country rock or host rock, may be metamorphosed. The presence of a metamorphic aureole (zone) or “baked contact” indicates the intrusion is younger than the rocks around it. In contrast, if an intrusive igneous rock is exposed via erosion, then later buried by sediments, the surrounding rocks will not be baked, as the intrusion was already cold at the time of sediment deposition. 

Query \(\PageIndex{1}\)

Query \(\PageIndex{2}\)

References

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