Completing the Puzzle:Rocks, Fossils, and Time
Stratigraphy is the branch of geology that is concerned with the composition, origin, relationship, and age of sedimentary rocks. Although this branch of geology is primarily concerned with sedimentary rocks its principles are used to also understand any layered earth material such as igneous and metamorphic rocks.
As we look at sedimentary rocks their most obvious features are their layers or stratification. The boundary between each strata is a surface known as the bedding plane; where no bedding planes are found sedimentary rocks will grade from one texture to another. The rocks above and below bedding planes will differ in composition, texture, or color. As discussed in chapter 5, the features of sedimentary rocks are a direct result of their sedimentary environment and source, therefore, since sediment on either side of the bedding plane must be visibly different in composition/texture/color the bedding plane represents changes in environmental conditions. This change may have occurred over a relatively short period of time or after an extended period of nondeposition or erosion. In contrast, slow changes in environmental conditions would cause sediment to grade vertically from one type to the other.
As previously discussed in chapter 5, the principle of superposition states that in an undisturbed sequence of sedimentary rocks the older rock is on the bottom. Using this principle it is quite simple to determine the relatively date of each strata by its position within a sedimentary sequence. We can also use the principle of inclusions to determine relative ages of strata. The principle of inclusions states that an inclusion, or fragment of another material, must be older than whatever it is included in.
The principles of superposition and inclusions become extremely important when studying sedimentary strata that also includes igneous bodies. For example, the igneous rock basalt is produced in or near earth's surface. On the surface basalt forms as a result of a lava flow. Basalt can also form near the surface as it squeezes its way through sedimentary rock; this structure is known as a sill. Once cooled both the lava flow and the sill would look very similar however their position within the rock record and their relative ages when compared to surrounding material would be very different and after further deposition and burial they would both look almost identical. However, in terms of relative dating, only the lava flow would obey the principle of superposition; the sill would be younger than the surrounding layers. So how can the nature of these structures be determined in order to accurately determine the relative ages of geologic events?
Lava flows occur on Earth surface. As a result lava flows across previously deposited material and therefore obeys the principle of superposition; the lava flow would be younger than the layers below it. As the lava flowed across the surface it would also flow over any other loose material already on the surface. If this material is not destroyed it would be incorporated at the base of the lava flow as an inclusion. The heat of the lava could also char any organic matter that was present at the surface causing the lower layer to appear to be baked. Any new sediment deposited would be deposited after the lava cooled and could then include weathered pieces of the lava flow.
|Image showing the various types of unconformities|
A sill cools beneath the surface from magma that forces itself through cracks in other rock. As a result pieces of rock from above and below the magma layer may be incorporated as inclusions in the magma before it cools. Since, in this case magma squeezes between previously deposited layers, layers on either side of the sill will appear to be baked and the sill would contain inclusions of both the upper and lower surrounding layers.
Unconformities: Recognizing whats not there
Up to this point we have only discussed conformable strata, that is strata that is deposited in nearly continuous fashion through time. Although bedding planes may record small breaks in deposition lasting a few minutes to thousands of years, these time frames are inconsequential when viewed relative to Earth time. In many areas, however, bedding planes may represent much longer intervals of time--on the order of millions of years. In these cases the rock on top of the bedding plane is much younger than the rock below; this boundary represents an unconformity: long periods of nondeposition and/or erosion. Since the amount of material eroded is a function of rock type and environmental factors the amount of time represented by an unconformity does not necessarily correspond to the length of time during which the unconformity formed. In other words, erosion occurring over the course of a million years may erode hundreds of millions of years deposition. Because of these processes we must understand that the geologic record is not a complete record of all events and conditions experienced on Earth. Furthermore a complete record of events in one location may need to be inferred from rock preserved in other nearby locations.
Unconformities are categorized based on the rock types they affect as well as the orientation of layers above and below the unconformity. A paraconformity is an unconformity that separate younger sedimentary rock from much older sedimentary rock both of which are parallel to the unconformity. Because layers above and below the disconformity are parrallel--just like all other bedding planes--disconformities may be difficult to recognize purely through visual observations. These types of unconformities may only be recognized only after the use of fossils or absolute dating techniques are used to date the sediments. Similar to a paraconformity, a disconformity also separates younger sedimentary rock from much older sedimentary rock however, unlike a paraconformity there is a clear erosional surface present between the older and younger beds.
An unconformity that cuts into an igneous or metamorphic body and which is overlain by sedimentary rock is called a nonconformity. Nonconformities form only after metamorphic or igneous rock is exposed at the surface and allowed to be buried by younger sediments. Finally, an angular unconformity is produced when sedimentary rock is deposited on tilted or angled beds. Angular unconformities form after beds that have been deformed through oregenic (mountain building) proccesses have been eroded horizontal allowing new sediment to be deposited on top of the much older tilted strata.