Charcoal and swelling clay of the Triassic Chinle Formation, Arizona

Last winter I visited Jim and Ellen’s Rock Shop in Cottonwood, Arizona and purchased a large piece of charcoal, 15 cm x 19 cm x 10 cm, which someone had carefully collected. It is mainly a web of quartz holding together charcoal, a wonderful, rarely collected specimen from near the Petrified Forest National Park. We are all familiar with the petrified wood logs preserved with silica of many colours from the Petrified Forest area. Charcoal must be more common, though you have to be lucky as pieces break down easily. Occasional small pieces were washed down the Triassic rivers and have be collected from the sediment and studied. Odd larger pieces are found, one larger piece was found at the Ghost Ranch area of New Mexico where they are found in the same upper Triassic deposits. The presence of charcoal indicates that there were occasional fires started by lightening. Most of the charcoal was washed away as crumbs and larger pieces during heavy rain. Occasional pieces were preserved by a speedy covering of sediment.

The interesting thing about charcoal is that it is inert in this environment and doesn’t interact with minerals as they form and seems to repel them. As silica dissolved in the water from volcanic ash gradually converted logs to stone, some had a portion of the wood burned and the cracks around the charcoal were filled with quartz preserving them. The specimen has to be gently handled so that the charcoal doesn’t fall apart.

Petrified Wood

Reference: Late Triassic charcoal from the Petrified Forest National Park, Arizona, USA.

Timothy P. Jones, Sidney Ash and Isabell Figueiral, Palaeogeography, Palaeoclimatology, Palaeoecology, Volume 118 3-4, 2002.

http://www.sciencedirect.com/science/article/pii/S0031018202005497

The colourful sediments containing the petrified wood and other fossils in the Petrified Forest and Painted Desert are rich in swelling clay. Swelling clay (Bentonite) is formed of the clay mineral montmorillonite. Weathering of volcanic ash in the tropical humid environment, produced the bentonite clay.

Clay minerals are formed of two layers of silica and elements such as magnesium forming a sheet structure of connecting rings like chainmail. Two sheets of these layers attract water like a sponge and expand seven to eight times the dry volume. Clay minerals are members of the phyllosilicate family. The name comes from the Greek name for leaf phyllo, phyllo pastry is also made in thin sheets. Swelling clay is used for kitty litter, drilling mud, a bulk laxative and many other uses.

Stone Tree House was built in the 1920’s and purchased by the National Parks Service in 1936 and renamed The Painted Desert Inn. The building was closed in 1963. Concerned public saved the structure from being torn down in 1975. After being designated a National Historic Landmark they were obliged to restore it. Painted Desert Inn was built on swelling clay, which caused lots of problems keeping the place intact. When I visited in 2003 with a group of students, tests were being carried out to measure the amount of damage to the structure that was being caused by the swelling clay. In 2006 the building opened again after extensive renovations and is a museum and bookstore now.

Measuring movement along cracks in the walls of the building using a crack monitoring gauge. Movement is shown by red cross hairs on a grid which is recorded at intervals onto a chart to shows the displacement.