HW 08: Dating of Rocks, Fossils, and Geologic Events

You attend a geology lab where you are asked to estimate the age of a fossil. The ratio of parent to daughter elements in the fossil sample is 1:7. You know that fossils are the remains of living organisms, which have some amount of C-14 isotope. The C-14 isotope, which has a half-life of 5730 years, begins to decay as the organism dies. What would be your estimation of the fossil's age?

17,190

Which set of coordinates most accurately locates the boundary between the Jurassic shales and mudstones and the Cretaceous shales and sandstones?

39°42'27" N, 105°12'07" W

Select the three that apply.

If human-like ancestors have been around approximately 5 million years, it is correct to say this represents 0.1 percent of geologic time. Evidence of abundant fossils date back to 540 million years ago. It is correct to say this represents 10.8 percent of geologic time. Assume the length of recorded history is 5,000 years. It is correct to say recorded history represents 0.0001 percent of geologic time.

Scientists have suggested that the downward bulges in the Jurassic sandstone layers along Dinosaur Ridge are the "mudholes" created when a dinosaur or two walked in mud and squeezed the layers together. In order for the deep bulges shown in the image to have been created, a unique series of events must have taken place. Which of the following must NOT have been part of this series of events?

The footprints remained exposed at the surface for many years.

Which answer most correctly describes the relative ages of the rock units visible in the placemark A popup balloon? Pay your attention that the east is to the left and the west is to the right.

The shales and mudstones to the west are older than the shales and sandstones to the east.

In order for the ripple marks shown in the image to have been preserved and exposed at the surface, a unique series of events must have taken place. Which of the following answers most accurately describes this series of events?

Wave action in shallow water overlying soft, sandy sediments produced ripple marks in the sediments. Growth of blue-green algae mats cemented the ripples in place before the next tide could destroy them. Lithification occurred as the blue-green algae secreted their silica gel and cemented the grains in place. When the next tide came in, the algal mat absorbed iron from the water, which further cemented the ripples in place. Later, the sedimentary rock containing these preserved ripples was uplifted, tilted to the east, and ultimately re-exposed at the surface by weathering and erosion (here helped by a road-building crew). Submit

The rocks steeply dip eastward, which suggests that these rocks have been uplifted and deformed. What "law" of relative dating could be used to establish this observation?

law of original horizontality

What "law" of relative dating did you apply to determine the relative ages of the rocks here?

law of superposition: in most sedimentary sequences, the oldest rocks will be on the bottom

At the road cut, which rocks are the LEAST resistant to weathering and erosion, and where are these rocks located?

shales and mudstones to the west

In which direction do most of the tracks suggest the dinosaurs were moving?

toward the top left of the photo

Over what span of time were the tracks most likely created?

weeks