Determining time frame for most recent geological layers

[dmehling]

What significant features are present in the five or six most recent geological layers? In other words what is the basis for determining how much time has elapsed in these layers? Is it mostly based on the types of fossils or the types of soil or rocks in these layers? Also, is it easier or more difficult to establish relative dates for these layers than it is for much older ones, such as those which comprise the dinosaur age?

 

[Andre]

Dating is generally the easiest for the youngest layers. Dating to the exact year is possible counting annual layers (varves) in lakes or tree rings, or coral. This goes back up to ten thousands of years. Next best (accuracy wise) is carbon dating, which goes back to about 50,000 years. Then there are many more dating methods involving varios types of radioactivity cycles, like argon dating which is going back 100nds million years or thermo and opto luminescence dating.

Furthermore volcanic ash layers of know eruptions set a time horizon and also magnetic orientation helps to put sediment cores into a certain time frame. Certain fossils are indeed used for dating, like foraminifera species which have a relatively quick evolution cycle from emerging to extinction but then you're looking at accuracies of thousands of years.

 

[dmehling]

I was more specifically thinking about layers roughly between the present and say 50 MYA. For such distant ages this would require the use of radiometric dating of igneous rocks. However my impression was that such types of rocks are not nearly as plentiful in the higher geological layers. Have many fossils between 100,000 and 50-100 MYA actually been dated by the use of radiometric methods, or does this require relying on less precise methods?

 

[Andre]

My best knowledge mineralized fossils cannot be dated reliably by radiometrics in general, so I guess one still relies mainly on igneous rocks for older layers. Some progress appears to have been made on a combination of uranium series–electron spin resonance method of teeth enamel that may work to 2 Ma.

http://www.science.mcmaster.ca/geo/research/age/dating.htm

 

[dmehling]

So what other methods can be used? Can dates be determined by the types of fossils found in these layers, or rates of sedimentation? Also, how are the most recent layers differentiated: are there different types of minerals or soils in each of the layers?

 

[Andre]

ESR has proven to be useable in some cases, see for instance this http://www.sciencedirect.com/science?_ob=ArticleListURL&_method=list&_ArticleListID=2001396745&_sort=r&_st=13&view=c&_acct=C000228598&_version=1&_urlVersion=0&_userid=10&md5=633448ab3c58a3b877a6ec38c42c2266&searchtype=a .

Also index fossils other than forams can be used indeed to determine the era of the layer where it has been found.

But indirect dating with sedimentation rates or different types of layers is not working due to the large changes in conditions over time as erosion, tectonics and sedimentation do their erratic work.