geologic eras

[Jdeutsch]

I'm curious about assigning ages to various strata.  As far as I am aware, much of what we know was derived from the fact that "fossil species change the deeper one goes in a sedimentary layer, with newer fossils above older fossils".  It makes a nice story.  However, one can take a drive across the USA and find surface deposits from all eras- as noted on any geologic survey.  I assume that means there were always surface finding from all previous eras during every geologic eras, and the mixing always continues to occur.  I die in Utah and could be buried in Ordovician mud in Millard county, or  buried by a  Jurassic diplodocus at Dinosaur.  The amount of mixing in all areas must be tremendous, not to mention continual upheavals.

 

How many observations does it take to establish clean data?  How much wiggle is there? How does one assign error bars to the data?

 

 

[Herb]

generally the Law of Original Horizontality is accurate with the newer deposits on top of the older but there are several factors that can influence that. Coarser particles can deposit on a slope even on a level surface such as sand in a delta deposit can have a 15 degree slope, of if the newer layer is  deposited on a sloped or eroded area, like a cliff.  It can be mechanically altered by mountain building where younger deposits can be overlain by older ones. Folds and fractures and mess with the deposits also. There may be great periods of time where there were no deposits or they were eroded away.  Glacial deposits can deposit older rock on newer rock and mixing up the fossils. There are fewer terrestrial deposits than marine environments  due to erosion an land and the open climate is harsher on potential fossils.  My thoughts, I hope they help a little.

[Sagebrush Steve]

Following up on Herb's comments, I think you have raised an interesting question.  I believe you are asking that since you see exposures of all geologic ages around the earth today, what is the likelihood that some of the fossils from those eras will get mixed in with current sediments so that a million years from now it may look like ammonites and velociraptors lived concurrently with people.  But as Herb points out, most of the exposures you see today are on land.  They will most likely be eroded away before they have a chance to move somewhere they can become covered again and re-fossilized.  The most likely places this could occur is where sedimentary rock exposures meet the sea and can crumble down into it, where they could become mixed with current deposits.  But those would be across very narrow widths, they wouldn't affect most of the marine layer.  And I would imagine that such interfaces would look much different, more like a conglomerate rather than a uniform layer.  But I'm very interested in hearing what others have to say.

[Tidgy's Dad]

These were called "derived fossils" back in my day, but are also now called remanié or reworked fossils. 

Obviously it only occurs with fossils from older beds being eroded free of their original matrix and deposited in a younger bed and not the other way around. They are quite common but can usually easily be told due to remnants of the original matrix, different preservation, and erosion marks on the fossil. Or radioactive dating, where possible, i suppose.  

[ashcraft]

Most organisms don't bury their dead like humans.  In 10 million years, will whatever may pass for a fossil collector find a human grave and think we lived in burrows?

 

Brent Ashcraft

[Johan]

17 hours ago, Jdeutsch said:

How many observations does it take to establish clean data?  How much wiggle is there? How does one assign error bars to the data?

You are raising in important issue here. While reworked/remanieted fossils can indeed often be recognized, usually based on a different preservation state, as Tidgy's Dad explained, there have been quite a number of cases where this process has caused quite some controversy within the scientific community.

 

These processes often become a problem when rapid geological transitions are studied. An example of this is the Cretaceous-Paleogene boundary. While the scientific consensus nowadays is that the impact of a large asteroid caused a rapid and catastrophic mass extinction, it took quite some time before this was generally accepted. One of the reasons why there was scientific debate, was because in most Cretaceous-Paleogene boundary records in the world, you can find quite a number of 'Cretaceous' species above the boundary (ie: in the Paleogene). For a long time, this was considered as indicating a more gradual transition, with species gradually disappearing (in stead of instantly, by one big event). Only when (micro)paleontologists and geochemists started studying the chemical compositions (mostly carbon isotopes and strontium isotopes) of these fossils, they found out that they had a Cretaceous chemical signal, and were thus reworked from the underlying (Cretaceous) strata.