Clastic Sedimentary Rocks: A Collection of Fragments
* Formation: Clastic sedimentary rocks are formed by the accumulation and cementation of fragments (clasts) derived from pre-existing rocks. These clasts can be of various sizes, from sand grains to boulders.
* Mineral Composition: The mineral composition of a clastic sedimentary rock is primarily inherited from the source rocks that were weathered and eroded to produce the clasts. This means the minerals within the rock are often older than the rock itself.
Relationship between Mineral Age and Rock Formation:
1. Source Rock Age: The minerals within the clasts will be as old as the source rocks from which they originated. For example, if a sandstone contains quartz grains derived from granite, the quartz grains will be as old as the granite.
2. Limited Information: While the minerals tell us about the age of the source rocks, they don't directly indicate the age of the sedimentary rock itself. The rock's formation is determined by the time of deposition and cementation of the clasts.
3. Diagenetic Alterations: During the process of diagenesis (transformation of sediment into rock), new minerals can form within the rock. These minerals will be younger than the rock, but they won't necessarily reflect the age of the original clasts.
4. Radiometric Dating: To determine the age of the clastic sedimentary rock itself, researchers might use radiometric dating techniques on other materials within the rock, such as fossils or volcanic ash layers interbedded with the sediment.
Example:
Imagine a sandstone with quartz grains, feldspar fragments, and mica flakes. The quartz grains may have originated from a Precambrian granite, making them billions of years old. The feldspar fragments might have come from a Paleozoic sandstone, making them hundreds of millions of years old. The mica flakes could be derived from a Mesozoic shale, making them tens of millions of years old. The sandstone itself might have formed in the Cenozoic era, meaning it's only a few million years old.
In Conclusion:
The age of minerals within a clastic sedimentary rock reveals the age of its source rocks. This information can be used to understand the geologic history of the area. To determine the age of the sedimentary rock itself, other dating techniques need to be employed.
