You are studying a sequence of rocks cut by a basa lt dike, a basalt sill, and b

Question

You are studying a sequence of rocks cut by a basa

lt dike, a basalt sill, and by two strike-slip

faults (F1 and F2). A vertical section of this rock sequence is shown on the accompanying sheet.

Part of the sedimentary section contains dinosaur fossils.

The purposes of your study are: (a) to determine the absolute ages of the igneous rocks and

volcanic ash layers with the K-Ar method, (b) to de

termine the relative ages of the sedimentary rock

layers and of the 2 faults, and (c) to estimate when the dinosaurs became extinct.

Also on the accompanying sheet is a plot showing th

e ratio of daughter to parent versus time for

the

40

K –

40

Ar method of radiometric dating.

To date your rocks and faults you must answer the questions below.

1) What is the youngest sedimentary rock unit, in terms of relative age? _____________

2) What is the oldest rock unit, in terms of relative age? _____________

3a) Is the basalt dike younger or

older than Sandstone 1? __________

3b) Is the basalt dike younger or older than the basalt flow? __________

3c) Is F1 older or younger than ash layer 1___________

3d) Is F2 older or younger than shale unit 2___________

4) How could you tell (in the previous problem) wh

ether the basalt dike is younger or older than the

basalt flow? _________________________________________________________________

5) Laboratory measurements yield the following ratios of

40

Ar to

40

K for the basalt sill, the basalt

flow, the 2 ash layers, and the basalt dike. Using the plot, determine the absolute ages of each, in

millions of years.

40

Ar/

40

K:

absolute age (my)

sill: 0.0058 ________________

dike

0.00028

________________

ash 1

0.0038

________________

ash 2 0.0046 ________________

basalt flow: 0.0052

________________

6) What is the minimum (youngest) age that one can assign to sandstone 4?

Hint: consider which of

the five dated units of the previous problem is/are older than sandstone 4.

_____________________

9) Using your absolute ages for the different i

gneous rock units and ash layers and your knowledge

about relative ages, when do you think the dinosaurs went extinct? _______________

Vertical cross-section of a sequence of rocks cut by a dike and strike-slip faults (F1 & F2). Part of rock sequence contains dinosaur fossils. Present-Day Ground Surface 0.00 Sandstone 1 0.00 0.00 Limestone 1 0.00 Sandstone 2 0.00 ' Sandstone 3 0.001 Basal Flow 0 10 20 30 40 50 60 70 80 90 100 time, my Basalt Sill Basalt Di ' Sandstone 4 F1 F2

Explanation / Answer

1) Youngest unit is Basalt dike but it’s igneous in nature and the question is about sedimentary unit hence the youngest sedimentary unit is Sandstone 1.

2) Now the basalt sill in an intrusion in the sandstone 4 formation and as per geological laws the intrusion are always younger than the formation in which the intrusion is found hence the Sandstone 4 is the oldest rock unit.

3a)Basalt dike is younger than sandstone 1 as the intrusion are always younger than than the parent rock as mentioned in the question above above. This is because in order to get an intrusion in a rock unit we first need a rock unit.

3b)Basalt dike is also cutting the basalt flow and again as per above mentioned law we can say that basalt dike is younger than basalt flow.

As per Chegg’s policy we are only adviced to solve first 4 questions of the questioner, please post the remaining question as a separate questionnaire. Thanks

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