Right, now I have a better idea of what you're looking for:
I'm not an expert on Paleontology, but I am interested in Science in general. Usually, my first stop would be Wikipedia, just to get a big picture of the information I'm looking for:
http://en.wikipedia.org/wiki/Paleontology#Estimating_the_dates_of_organisms do touch on the subject you're enquiring about, and also refer to some other links, such as an article on Stratigraphy (
http://en.wikipedia.org/wiki/Stratigraphy) and another on Index fosils (
http://en.wikipedia.org/wiki/Index_fossil), which are both related to your query. Based on my cursory inspection, the stratigraphy article should profide you with much information to guide your further online searches.
Also do not forget the related articles ("See also") and external references at the end of each artiicle. Also look at the "Geologic time" section of the Geology article (
http://en.wikipedia.org/wiki/Geology).
I hope this helps.