Evidence for the occurrence of Gondwana coals in North America as indicated by comparisons of similar deposits in the world
CIM Bulletin, Vol. 97, No. 1084, 2004
P.A. Hacquebard and M.P. Avery
Gondwana coal deposits are widespread and are known to occur on four continents. Gondwana is the southern super continent that resulted from the breakup of the super continent Pangea during Late Carboniferous to Early Permian time. Gondwana was separated from the northern continent Laurasia by an equatorial sea, the Tethys. Gondwana coals are present in South America, Africa, India, and Australia. Laurasian coals occur in the United States, Canada, Russia, and Europe where they constitute the main coal producing fields of the world. From coal petrographic examinations and a detailed literature survey, it is interpreted that Gondwana coals are present in northeastern North America. These deposits are Late Carboniferous and Early Permian in age and differ considerably from the Laurasian coals that are mined extensively. In North America, typical characteristics of Gondwana coals are found in deposits in Nova Scotia, Pennsylvania, Rhode Island, and Massachusetts. The productive Carboniferous coals in Nova Scotia occur in the Avalon Zone related to Laurasia (see figure). The coal deposits related to the Meguma Zone, which formed part of the Gondwana landmass, are very limited. They comprise minor deposits at Debert and Kemptown and the main deposits of the Pictou coalfield in Nova Scotia. In the United States, a similar tectonic development is represented by the anthracite deposits in eastern Pennsylvania and those in the Narragansett Basin in Rhode Island and Massachusetts.
Megascopically, Gondwana coals have a uniform, micro-banded appearance, with a dull lustre; Laurasian coals are bright and have distinct bands of vitrain and clarain. Petrographically, the Gondwana coals are low in vitrinite, but high in inertinite, particularly in semifusinite (averaging 65%, 20%, and 15%, respectively). The mineral matter averages 16%. In the Laurasian coals, vitrinite content averages 74%, inertinite 16%, and semifusinite 5%. The mineral matter averages 5.7%.
Thick seams occur in the Gondwana coals where they vary in thickness between 2.7 m and 13.2 m (9 ft and 44 ft). In the Laurasian deposits, the seams are from 1.5 m to 3.3 m (5 ft to 10 ft) thick. Seam termination in the Gondwana coals is through lithification (Versteinerung), whereby coal passes laterally into shale. The Laurasian seams finish by splitting and digitation.
Vitrinite/inertinite ratio data indicate deposition of the Gondwana coals in dry marshes under strong oxidizing conditions. The Laurasian coals originated in wet marshes in a reducing environment. As deduced from fossil plants and fossil spores, temperate and near arctic conditions apply for the Gondwana coals. A tropical and subtropical climate is indicated for the Laurasian deposits. The Gondwana coals were formed under allochthonous conditions. They derived from vegetation that drifted into the peat bogs. The Laurasian coals are autochthonous and originated from vegetation that grew and was buried in the peat bogs. The Gondwana coals are Late Carboniferous and Early Permian. The Laurasian coals are entirely Late Carboniferous.
Data gathered for this study under the categories of geographical distribution, megascopic appearance, petrographic composition, seam development, environment of deposition, paleoclimate, source of vegetation for peat formation, and geological age indicate the presence of Gondwana coal seams in eastern North America.