The Basics
* Sedimentary rocks form in layers: They are deposited over time, with older layers at the bottom and younger layers on top.
* Fossils tell time: Different types of fossils are characteristic of different geological periods. Cambrian fossils are older than Ordovician fossils.
The Unconformity
1. Original sequence: Imagine a sequence of sedimentary rocks being deposited, with Cambrian fossils in the lower layers and Ordovician fossils in the upper layers.
2. Erosion and uplift: Over millions of years, the landscape can change. The land might be uplifted, causing the rock layers to be exposed to erosion. The upper layers (with Ordovician fossils) might be eroded away completely.
3. Subsidence and deposition: The area then subsides (sinks) again, and new sedimentary layers are deposited on top of the eroded surface. These new layers will contain fossils from a later time period, like Cambrian fossils.
The Result
You are left with a horizontal sedimentary rock layer containing Cambrian fossils lying on top of Ordovician fossils. This is a disconformity because:
* The layers are parallel: The layers above and below the unconformity are parallel, indicating that there was no major tilting or folding.
* There is a missing time: The unconformity represents a period of time during which erosion occurred and no new sediment was deposited.
In Summary
This scenario shows how geological processes like erosion and uplift can disrupt the original order of rock layers, leading to seemingly unusual fossil arrangements. Studying unconformities helps geologists understand the complex history of our planet.
