JonF
Posts: 634
Joined: Feb. 2005
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Thought I'd answer a few questions. Not that Davie's gonna pay any attention, but others may be interested.
| Quote (afdave @ July 16 2006,07:16) | | 1) First, we have Circular Reasoning ... i.e. Fossil Age = Age of Rock in the "Fossil" article, and Age of Rock = Age of Fossil in the "Paleontology" article. |
This is a common canard, and is totally false. Relative dates of many rocks were worked out from fossils and stratigraphy long before radioactivity was discovered. The relationships discovered in that effort are still useful and used, e.g. in exploring potential new oilfields.
The discovery of radioactivity led to the ability to derive absolute dates for many formations for which we only had relative dates before, and to date many formations for which relative dates could not be derived.
The date of a formation can often be derived in two ways, radiometrically or from stratigraphy or index fossils. These are independent determinations. They often agree. When they do not agree, scientists investigate until they figure out the source of the disagreement.
Much more detail at Radiometric Dating and the Geological Time Scale: Circular Reasoning or Reliable Tools?.
| Quote | | 2) I thought sedimentary rocks could not be dated radiometrically ... Deadman told me that this is true of the GC layers that contain fossils. But this article says you CAN date sedimentary layers radiometrically. What's up with that? |
Yes and no.
Most sedimentary layers cannot be dated radiometrically today, but stay tuned. Typical sedimentary layers consist of accretions of the remains of rocks that are much older than the sedimentary formation. If you blindly take a large chunk of sedimentary rock and date it, you are going to get an age that is pretty close to the average age of the rocks (weighted by percentage composition) that eroded to form the sediment which then lithified to form the sedimentary rock. IOW, not a particularly useful or meaningful result.
However, advances have been made in reducing sample size (SHRIMP systems regularly sample a disk about 10 micrometers diameter and 1 micrometer thick:
 ) and in studying the materials that form between grains when the rock lithifies. If we can reliably date the material that formed between grains when the rock became rock, we can date the rock. One very promising such material is xenotime, which can be dated by U-PB concordia-discordia analysis in SHRIMP instrumentation. See SHRIMP Uranium-Lead Dating of Diagenetic Xenotime in Siliciclastic Sedimentary Rocks (requires free subscription, or see BugMeNot).
There are other techniques of dating sedimentary rocks, such as K-Ar dating of glauconite (which forms as part ot the "cement" in some environments) and fission track dating of any of several other minerals found in the "cement" such as apatite. These are difficult to apply to tiny samples and are constrained by some other technical issues, and are not widely used.
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