1) Describe the Forces That Combine to Entrain a Grain. Draw a Picture to Go with Your Answer

Name ______Geology 103

Midterm #1Spring, 2016

Short answer questions. Answer each question. If the question asks for a sketch or diagram, be sure to include it. (5 points each).

1) Describe the forces that combine to entrain a grain. Draw a picture to go with your answer.

2) What does Stoke’s Law describe? Give the equation, and define each variable.

3) What conditions favor formation of oscillation ripples? Why can an oscillation ripple tell us the “up” direction of a sedimentary bed? Draw a picture to illustrate your answer.

4) What is the phi scale, and how are phi numbers calculated? (Give an equation!) What phi size would a 0.25 mm (1/4 mm) grain represent?

5) Draw a cross section of a ripple. Label the lee side, stoss side, bottomset laminae and foreset laminae?

6) Describe the textural and compositional maturity of a medium-grained sandstone that is composed of 50% well-rounded chert grains (SRF), 30% subrounded quartz grains, and 10% each subrounded zircon and tourmaline grains. The sample is well sorted.

7) How does the Wentworth scale subdivide sand? List each category and the size range in mm that it represents.

8) What are the four main families of clay minerals? Why is illite more common in the geologic past than it is today?

9) What colors are common in shales, and what might these colors indicate about depositional environments, sediment sources, or composition of the shale?

10) Describe the particle motion, velocity patterns and erosive force of a fluid that is experiencing laminar flow. Where does this type of flow occur in nature (give examples)?

11) What terms (adjectives) are used to describe beds and bed configuration? Draw a simple sketch of each. (10 points)

12) What types of minerals or rock fragments would you expect to find in a shale, a siltstone, a sandstone, and a conglomerate? Make predictions about the percentages of each grain type that you would expect to find in a typical sample of each rock type. (10 points)

13) Why is it harder to erode a clay particle than a fine sand particle? Use the Hjulstrom diagram to explain your answer (10 points)

14) Describe the progression of bedforms that would occur as a medium sand experiences higher and higher current velocities. List each bedform, draw a simple sketch and give a brief description.Identify the change from lower flow regime to upper flow regime. Which bedforms have the highest preservation potential? (20 points)