The upper part of the Sillem Member is a mudstone and claystone unit. It ranges in color from pink and yellow to gray and green. Some volcanic debris is found. There are also interbedded layers of marlstone and limestone. Some sandstone is present.
The Sillem Member is between 100 and 400 ft thick and most probably rests unconformably on the Bullpen Member of the Wasatch.
BULLDOG HOLLOW MEMBER.
This middle member of the Fowkes Formation has the thickest and most extensive outcrops. The Bulldog Hollow Member is exposed along the west side of the basin.
Included rocks are green, white, and blue-green mudstone with ash beds, green and buff claystone, and tuffaceous, limy sandstone. A high percentage of the iron mineral, magnetite, occurs in the sandstone. Conglomerate occurs as lenses.
The Bulldog Hollow Member has a gradational contact with the underlying Sillem Member. The amount of volcanic material increases upward from the Sillem, indicating an increase in volcanic activity during the deposition of the Bulldog Hollow Member.
GOOSEBERRY MEMBER.
Oriel and Tracey (1970:55) place this uppermost member provisionally within the Fowkes Formation. Most of the Gooseberry Member is a puddingstone, a lithology with well-rounded, spherical pebbles in a marlstone, sandstone, or sandy limestone matrix. The pebbles are too rounded for the rock to be a diamictite, and too separated from each other to be called a conglomerate.
The nature of the Gooseberry-Bulldog Hollow contact is not completely known. It appears to be gradational in some areas and to be an angular unconformity in others.