<p>Fossils are only in very rare cases dated directly, because they do not contain sufficient radioactive isotopes for dating.
Most geological periods are first and foremost defined by biostratigraphy- fossil assemblages, typically. 
The Cambrian period starts with the Fortunian stage - this stage is defined by the appearance of a certain trace fossil in Fortuna peninsula, eastern Canada, but is correlable with similar formations world wide. The whole discussion on how this stage is defined is available <a href="https://stratigraphy.org/gssps/fortunian" rel="nofollow noreferrer">here</a>.
As outlined in this paper, in addition to fossils, the Fortunian stage hosts various volcanic rocks which contain zircons, which can be dated by U-Pb. The age of the actual boundary is then interpolated from these data.</p>

CC BY-SA 4.0
2020-03-17T14:15:38.107Z
2020-06-17T12:54:08.343Z
2020-06-17T12:54:08.343Z
Geochron
<p>Just to give some complementary informations from the other answer: the base of the Fortunian is indeed defined in eastern Canada on the basis of a <strong>trace</strong> fossil (which obviously cannot be itself dated since it is just an imprint), <em>Trichophycus pedum</em>. This trace fossil however being spread worldwide help correlating various sections throughout the world. One of such section is the  Birba-5 well in Oman where the radiometric age was actually measured (at least the one currently considered as the proper age of the base of the Cambrian): volcanic ash bed BB5 could be dated using <span class="math-container">$\ce{^{206}Pb/^{238}U}$</span> radiometric dating at 541 +/- 0.63Ma, based on the analysis of 8 different zircon grains.</p>

<p>It also corresponds to a negative carbon isotope excursion.</p>

<p>See the Cambrian chapter and the Appendix 2 on radiometric ages in <a href="https://www.sciencedirect.com/book/9780444594259/the-geologic-time-scale" rel="nofollow noreferrer">Gradstein et al. 2012</a>. Or <a href="http://nora.nerc.ac.uk/id/eprint/3262/1/Bowring_et_al_AJS_2007.pdf" rel="nofollow noreferrer">Bowring et al. 2007</a> for the original data.</p>

CC BY-SA 4.0
2020-06-17T13:20:47.557Z
Note that the margin of error given here is the one from Gradstein et al. 2012 where they recalculated it apparently (though given i am not a specialist I don't think i can explain correctly that computation).
plannapus
CC BY-SA 4.0
2020-06-17T13:02:07.640Z
2020-06-17T13:02:07.640Z
plannapus
<p>What dating method was used to determine the beginning of the Cambrian, and what was its margin of error? Was only one fossil dated, or were a number of similar fossils used?</p>

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2020-03-13T13:35:12.380Z
It doesn't answer my question. I already knew the dating was radiometric.
Michael Walsby
CC BY-SA 4.0
2020-03-15T14:39:12.223Z
@MichaelWalsby other people don't know that until you say so. Can you add some more detail to your question? I think I can see what you are asking. While radiometric dating can generate the age of a given object, to determine the beginning or end of a period in geological history requires much more than that.
uhoh
CC BY-SA 4.0
2020-03-15T17:02:47.477Z
@uhoh i removed my comment,i still think the question needs some work.
trond hansen
<earth-history><fossils><stratigraphy><dating>
How was the onset of the Cambrian period dated?
Michael Walsby
# How was the onset of the Cambrian period dated?

(Asked by: username_0 on 2020-03-13)

What dating method was used to determine the beginning of the Cambrian, and what was its margin of error? Was only one fossil dated, or were a number of similar fossils used?

**Tags:** earth-history, fossils, stratigraphy, dating

> **Comments:**
> **username_0**: It doesn't answer my question. I already knew the dating was radiometric.  
> **username_1**: @username_0 other people don't know that until you say so. Can you add some more detail to your question? I think I can see what you are asking. While radiometric dating can generate the age of a given object, to determine the beginning or end of a period in geological history requires much more than that.  
> **username_2**: @username_1 i removed my comment,i still think the question needs some work.  

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## 2 answers

### Answer by username_3 on 2020-03-17. Score: 4

Fossils are only in very rare cases dated directly, because they do not contain sufficient radioactive isotopes for dating.
Most geological periods are first and foremost defined by biostratigraphy- fossil assemblages, typically.
The Cambrian period starts with the Fortunian stage - this stage is defined by the appearance of a certain trace fossil in Fortuna peninsula, eastern Canada, but is correlable with similar formations world wide. The whole discussion on how this stage is defined is available [here](https://stratigraphy.org/gssps/fortunian).
As outlined in this paper, in addition to fossils, the Fortunian stage hosts various volcanic rocks which contain zircons, which can be dated by U-Pb. The age of the actual boundary is then interpolated from these data.


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### Answer by username_4 on 2020-06-17. Score: 0

Just to give some complementary informations from the other answer: the base of the Fortunian is indeed defined in eastern Canada on the basis of a **trace** fossil (which obviously cannot be itself dated since it is just an imprint), *Trichophycus pedum*. This trace fossil however being spread worldwide help correlating various sections throughout the world. One of such section is the Birba-5 well in Oman where the radiometric age was actually measured (at least the one currently considered as the proper age of the base of the Cambrian): volcanic ash bed BB5 could be dated using $\ce{^{206}Pb/^{238}U}$ radiometric dating at 541 +/- 0.63Ma, based on the analysis of 8 different zircon grains.

It also corresponds to a negative carbon isotope excursion.

See the Cambrian chapter and the Appendix 2 on radiometric ages in [Gradstein et al. 2012](https://www.sciencedirect.com/book/9780444594259/the-geologic-time-scale). Or [Bowring et al. 2007](http://nora.nerc.ac.uk/id/eprint/3262/1/Bowring_et_al_AJS_2007.pdf) for the original data.

> **Comments:**
> **username_4**: Note that the margin of error given here is the one from Gradstein et al. 2012 where they recalculated it apparently (though given i am not a specialist I don't think i can explain correctly that computation).  

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