You are a graduate student in paleontology, studying a fossil site below an anci
ID: 113221 • Letter: Y
Question
You are a graduate student in paleontology, studying a fossil site below an ancient coastal lagoon. A simplified and exaggerated profile of the rocks and sediments at your site is shown here. The profile is not drawn to scale.
Formation 1 is composed of fine-grained sediments in which rodent fossils are abundant, but no dinosaurs are found (assume you would have found some dinosaurs had there been any present on Earth when this layer was formed).
Formation 2 is a layer of red clay in which several theropod and rodent fossils are found.
Formation 3 is composed of gravelly sediments.
Formation 4 Chemical analysis of this layer and a few calculations reveal that X (see below) of the 40K that was originally present in the sample (at the time the lava cooled) has decayed into the two daughter isotopes, 40Ar and 40Ca.
Formation 5 is rich in reef deposits where fossil fish are abundant.
Formation 6 is another surface-deposited layer derived from a volcanic lava flow. Chemical analysis of a feldspar sample from this layer reveals that for 10,000 atoms of 40K originally in the sample at its formation, Y (see below) atoms have decayed into 40Ar and 40Ca.
Formation 7 is composed of fine-grained sediment
Formation 8 is an igneous (granite) intrusion. Your chemical analyses of the granite crystals demonstrate that for every 10,000 atoms of 238U in the sample, there are Z (see below) atoms of 206Pb present.
A) Based on stratigraphic analysis of the profile, list the order in which the formations (1-8) were formed, from most recent to most ancient
B) Estimate the probably age (in terms of millions of years from the present) for each formation. For formations that can’t be dated with radioisotopes, give a maximum and minimum age estimate, if both can be estimated from the data at hand. These estimates can be used to estimate the ages of fossils found in each formation. Based on your calculations (carry out all calculations to 5 significant digits), construct a table showing minimum and maximum age estimates for each formation. You will have to use outside information on known fossil ages and geological history to fill in a few of these maxima and minima!
C) Would it be surprising to also find fossils of rodents in formation 5? Briefly justify your answer.
X = 16.85%, Y = 2,060, Z = 401
Explanation / Answer
A) formartion 1 (youngest) <- formation 2<- formation 8 <- formation 3 <- formation 4<- formation 5<- formation 6 <- formation 7 (oldest)
B) formation 1 - after cretaceous (<66 mya)
formation 2- 100 - 66 mya
formation 8- 114.573 mya
formation 3- between 114- 144 mya
formation 4- 144. 573 mya
formation 5- between 144- 180 mya
formation 6- 180.731 mya
formation 7 - > 180 mya
c) yes it would be surprising to find rodent fossils in formation 5, as this formation was deposited in shallow marine conditions and rodents are land dwellers
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