When did the cretaceous end?

[Top Trilo]

I know the cretaceous period ended 65-66 million years ago but when exactly did it end? Did it end as soon as the meteor touched the Earth's surface? As soon as all dinosaurs went extinct? After a certain amount of species went extinct? There may not be a definitive answer but if it was up to you where would you decide where the end of the cretaceous meets the beginning of the paleogene?

[Troodon]

The cretaceous ended 66mya not 65.  Its identified by the  K-T boundary found around the world which contains a higher concentration of iridium than normal.  The layer was deposited shortly after the impact.

 

ChronostratChart2020-03.pdf

[Thecosmilia Trichitoma]

From what I know, the mass extinction is what defines the K-T boundary, and the impact of the asteroid was the cause. People knew about the extinction event before they new about the asteroid that caused it. I would assume there was a transition period, as the effects of the asteroid killed off the species. 

[Thecosmilia Trichitoma]

I would assume there was a transition period in between. For me, I consider the Cretaceous to end when the asteroid’s effects ended, as that (as Troodon says) is when the iridium layer would have stopped being deposited too.

[Tidgy's Dad]

I think it is now known as the K-Pg boundary (Cretaceous-Paleogene) and was 66.043 mya, plus or minus 0.011 million years and is marked by the iridium anomaly, a thin layer that is also associated with the mass extinction. 

[Top Trilo]

Yes I know of the iridium layer deposited from the meteor so from what I've heard it is at the top of the layer or the iridium layer is still part of the creteacous. Thanks guys very helpful

[piranha]

18 minutes ago, Top Trilo said:

Yes I know of the iridium layer deposited from the meteor so from what I've heard it is at the top of the layer or the iridium layer is still part of the creteacous.

 

 

The iridium layer is placed at the base of the Paleogene.

 

 

Gradstein, F.M. et al. 2020.

Geologic Time Scale 2020.

Elsevier Publishing, 1357 pp.

[DPS Ammonite]

21 minutes ago, piranha said:

 

 

The iridium layer is placed at the base of the Paleogene.

 

 

Gradstein, F.M. et al. 2020.

Geologic Time Scale 2020.

Elsevier Publishing, 1357 pp.

Forgive my asking, is the iridium layer fully within the Danian?

[Jeffrey P]

In New Jersey there is the Pinna Layer which lies right above the iridium. This roughly foot thick layer of clay contains a "Cretaceous" fossil assemblage that includes ammonites and baculites (well over a 100 invertebrate species in all) that appear to have survived the asteroid impact for at least a few years though it is unclear how long. It has been theorized that during this time due to the sharp reduction in sunlight, micro organisms were rapidly dying off and sinking down through the water column creating a temporary food surplus for the macro fauna that survived until this food source was exhausted and the entire ecosystem collapsed.  Above this layer, fossils are scarce, the beginning of the Paleogene. 

[piranha]

24 minutes ago, DPS Ammonite said:

Forgive my asking, is the iridium layer fully within the Danian?

 

 

Gradstein et al. 2012 showing the same info.

 

 

Gradstein, F.M. et al. 2012
The Geologic Time Scale 2012.
Elsevier Publishing, 1144 pp.

[DPS Ammonite]

7 minutes ago, piranha said:

 

 

Gradstein et al. 2012 showing the same info.

 

 

Gradstein, F.M. et al. 2012
The Geologic Time Scale 2012.
Elsevier Publishing, 1144 pp.

So the answer to my question is yes? The boundary, (a plane), between the Cretaceous and the Paleocene is the bottom side of the iridium layer as apposed to the middle or top of the iridium layer.

[piranha]

6 minutes ago, DPS Ammonite said:

So the answer to my question is yes? The boundary, (a plane), between the Cretaceous and the Paleocene is the bottom side of the iridium layer as apposed to the middle or top of the iridium layer.

 

 

The answer appears to be self-evident.

[DPS Ammonite]

I disagree. To say that the iridium rich layer is a “marker” bed or the Cretaceous/Paleocene boundary is “at” an iridium layer does not specifically tell you where the boundary is. It could be below the iridium layer, in the middle of the iridium layer or on top of the iridium layer. I think that the boundary is placed just below the iridium layer; is that correct?

[piranha]

32 minutes ago, DPS Ammonite said:

I disagree. To say that the iridium rich layer is a “marker” bed or the Cretaceous/Paleocene boundary is “at” an iridium layer does not specifically tell you where the boundary is. It could be below the iridium layer, in the middle of the iridium layer or on top of the iridium layer. I think that the boundary is placed just below the iridium layer; is that correct?

 

 

I have no idea what you're arguing.  The charts and the info in the cited publications appear to indicate the the iridium layer is defined at the base of the Paleogene. 

 

Sounds like you need to contact a specialist that can elaborate further.

[FossilDAWG]

Presumably the iridium layer was deposited after the impact, as fine dust from the vaporized meteor and bedrock was distributed around the world and settled out.  Since the impact itself defines the end of the Cretaceous, it seems to me the iridium layer would be entirely in the Paleocene, at least as originally deposited.  I think there is evidence that some iridium has migrated down into underlying layers with groundwater movement, so it is important to take this possibility into account.  The associated extinction event was likely more complex, as different taxa may have persisted for a shorter or longer time depending on their ability to tolerate the severe environmental disruption. 

 

Don

[FranzBernhard]

2 hours ago, piranha said:

The answer appears to be self-evident.

 

1 hour ago, DPS Ammonite said:

I disagree.

 

It is not totally self-evident and not explicitly stated in that table (I am a fan of redundancy!). The Ir-layer, while very thin, is a 3D-object. The boundary between C/Pg is 2D.

 

But this makes it clear:

1 hour ago, FossilDAWG said:

Presumably the iridium layer was deposited after the impact, as fine dust from the vaporized meteor and bedrock was distributed around the world and settled out.  Since the impact itself defines the end of the Cretaceous, it seems to me the iridium layer would be entirely in the Paleocene, at least as originally deposited.

 

However, all this depends on definition.

 

Franz Bernhard

[Norki]

3 hours ago, Jeffrey P said:

This roughly foot thick layer of clay contains a "Cretaceous" fossil assemblage that includes ammonites and baculites (well over a 100 invertebrate species in all) that appear to have survived the asteroid impact for at least a few years though it is unclear how long.

This has interested me for awhile, and from what I've read there seems to be pretty good evidence for some ammonoid survival into the Danian, especially among scaphitids. Apparently the main thing that eventually killed them off was ocean acidification, because the the shells of freshly hatched, planktonic ammonoids would reliably dissolve once ocean pH levels got low enough. This theory also accounts for why nautiloids survived and they didn't - nautiloids hatch as subadults and skip the vulnerable planktonic stage altogether.

Anyway, the acidification process probably would have taken a fair amount of time, just like the delayed food chain collapse, so I suspect that at least a few species of ammonoids were around for a couple thousand years into the Danian.

[Ludwigia]

Not to forget of course that the conclusions leading to the above described definition followed up on years of intensive field study and deliberation by the International Commission on Stratigraphy and its pertinent Subcommissions. In other words, human scientific conclusions, which, as far as dating is concerned, are relatively specific, but nevertheless always approximate given the inevitable +/- factor. A border is 2D, as Franz has pointed out (when do you actually leave or enter your house?), at least when viewed and defined by the human intellect, and the members of the Commission decided in this case to place the iridium layer in the Danian and the impact in the Cretaceous. It would be practically impossible to define a younger world-wide layer where the dinosaurs and "a certain amount of species" went instinct as the border between the Cretaceous and the Paleogene. However, the "no man's land" in front of and behind a border is of no set magnitude, depending on the circumstances, which can often give arise to doubt as to exactly where the border is.

[LabRatKing]

Late to the game as usual...makes my snide comment of "It ended when the Paleogene began..." moot.

[Al Dente]

As Don pointed out, the iridium layer has a tendency to migrate. There are some who think there were multiple impacts thousands of years apart and the iridium layer could be from a different event than the impact that created the tektites. Here's one opinion on the Brazos River boundary section. Taken from "The Cretaceous-Paleogene boundary on the Brazos River, Falls County, Texas: is there evidence for impact-induced tsunami sedimentation?"

 

 

[siteseer]

I always understood the iridium layer to be neither Cretaceous nor Paleocene but THE boundary - the line/zone between them.  If there was more than one impact over hundreds or thousands of years and the layer/layers was/were deposited over that amount of time, it would still be virtually instantaneous from a geologic standpoint.  It's like asking what time of day the Cretaceous ended (relative vs. absolute as we learned in geology class).  We will likely never reach that level of resolution so the boundary might still move or get thinner or thicker.  When I was a kid, the number was 60-65 million years ago.

 

Jess

[LabRatKing]

Keep in mind the iridium layer didn’t form overnight. It took thousands of years to form.

[doushantuo]

can recommend:

maybe in conjunction with:

 

and(e.g.)

 

 

[doushantuo]

some of you may like(less than 3 Mb)

Iridium_profiles_and_delivery_across_the.pdf

[jeannie55]

Wow!  I was feeling a little guilty about my plan to skip my doctors appointment today but the second cup of coffee and first gingerbread cookie with this thread totally se t the guilt packing. Of course my non- scientific brain feels stretched. That’s why I love that you big brained guys (male and female) allow those of us who are enjoying our hobby access to this forum.