File:Fossil charcoal in weathered coal (Bedford Coal, Middle Pennsylvanian; Noland Tunnel's north portal outcrop, Tunnel Hill, Coshocton County, Ohio, USA) 1 (30238570444).jpg

Fossil charcoal in coal from the Pennsylvanian of Ohio, USA.

This rock is from the Pottsville Group, a Pennsylvanian-aged cyclothemic succession containing nonmarine shales, marine shales, siltstones, sandstones, coals, marine limestones, and chert ("flint"). The lower Pottsville dates to the late Early Pennsylvanian. The upper part dates to the early Middle Pennsylvanian. The Lower-Middle Pennsylvanian boundary is apparently somewhere near the Boggs Member (?).

Shown above is a sample derived from the Bedford Coal, a horizon that occurs just below the Upper Mercer Limestone (or Upper Mercer Flint). Lithologically, the Bedford ranges from carbonaceous shale to argillaceous coal to bituminous coal to cannel coal. The cannel coal in the Bedford was targeted for mining in the 1800s as a source of fuel. It was particularly useful in the manufacture of kerosene, an illuminating fuel. After the petroleum industry started in the 1860s, production of kerosene from cannel coal essentially ceased.

At this locality, the Bedford Coal consists of cannel coal and bituminous coal. This sample is weathered coal with pieces of compressed fossil charcoal (= lustrous, blackish-colored chunks). The Pennsylvanian was a time of relatively high atmospheric oxygen (O2) levels, and forest fires were relatively common events. Charcoalized fossil wood can be found in some abundance in Pennsylvanian sedimentary successions. The original wood microstructure is usually well preserved, but the charcoal fragments themselves are quite delicate. A gentle rub with a finger turns these fragments into black powder. Sometimes, the fossil charcoal is partially pyritized.

The rainbow-colored areas are thin weathering films of turgite, which is essentially hydrous hematite (2Fe2O3·H2O - hydrous iron oxide). Some geologists do not consider turgite to be a mineral - rather, it's interpreted as a mixture of hematite and goethite resulting from goethite alteration. Turgite often occurs as rainbow-colored iridescent coatings on iron oxide-rich rocks or rocks having surficial iron oxide staining. It can also occur as irregularly botryoidal masses. (see also: www.jsjgeology.net/Turgite.htm and www.flickr.com/photos/jsjgeology/albums/72157661979539290)

Stratigraphy: Bedford Coal, upper Pottsville Group, Atokan Stage, lower Middle Pennsylvanian