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How are bentonite beds used to determine the ages of Cretaceous rock layers?
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Cretaceous Period
Distribution of landmasses, mountainous regions, shallow seas, and deep ocean basins during the late Cretaceous Period. Included in the paleogeographic reconstruction are the locations of the interval's subduction zones.
Bentonite beds, which are formed from the alteration of volcanic ash, serve as critical tools for determining the ages of Cretaceous rock layers because they act as "key beds" that provide precise radiometric dates.123
These beds are used in the following ways:
- Radiometric Verification: Because volcanic ash is deposited nearly instantaneously over vast areas, bentonite layers allow geologists to obtain absolute radiometric ages. These dates are used to independently verify the synchronicity of fossil-based zones, such as the ammonite zones used to correlate Cretaceous strata.13
- Precise Dating: Bentonite often contains unaltered mineral crystals, such as zircon, which are ideal for isotopic dating methods like the uranium-lead technique. By analyzing these crystals, geologists can establish precise absolute ages for the volcanic event and, by extension, the surrounding sedimentary layers.43
- Correlation Across Environments: Unlike many fossils that are restricted to specific environments or latitudes, a volcanic ash layer settles across diverse settings, allowing for physical correlation between contrasting sedimentary environments.13
This integration of radiometric data from bentonite with fossil-based biostratigraphy provides a highly detailed resolution for dating Cretaceous geologic history.1