Like most of the Fowkes Formation, this sequence of rocks has been
eroded extensively and is preserved as erosional remnants, where
protected by faulting, in the western part of Fossil Basin.
The Sillem Member consists of a lower conglomeratic sequence with some
sandstone and mudstone. The conglomerate contains well-rounded clasts of
gray quartzite, chert, and Paleozoic limestone. The sandstone is light
gray, calcareous to muddy, and coarse to medium-grained. The mudstone is
pink, gray, or tan in color.
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.