Athro, Limited Gubik Field Trip Section 20 Cretaceous Bentonite?

You are on the beach, looking in the direction of the green arrow. You are near the base of the cliff as shown by the blue arrow.

Bentonite?

The sedimentary rocks here are interbedded sandstones and siltstones. The sands form hard resistant beds, while the finer grained sediments are still relatively soft. This is typicaly the result of the coarser grain size in the sands. Larger grain size means more pore space between grains, and more ability for water carrying disolved minerals to flow through the sands. Thus the sands become cemented more easily than the surrounding less porus fine grained sediments. There is also a distinctive yellow bed a couple of centimeters thick visible in this section. This could be a bentonite. In loose usage, a bentonite is an odd kind of rock that is sort of both igeneous and sedimentary. They are derived from volcanic ash falls that get incorporated into sediments. Properly, a bentonite is a bed of clay rich in the clay mineral smectite. Bentonites are very slippery when wet and feel waxy to the touch. A fresh surface can be any of a wide range of colors (blue, green, red, yellow), but a weathered surface is yellow. You can collect a sample of this possible bentonite by pressing the button below.

These rocks are Cretaceous in age and on the fringes of the North Slope Oil fields of Alaska.