The Changuinola peat deposit of northwestern Panama: a tropical, domed, back-barrier coal-forming environment

Abstract

A large (approximately 82 km 2), low-ash, low-sulfur peat deposit has formed on the northwestern coast of Panama (Fig. 1). Its low ash and sulfur contents can be attributed partly to (1) doming, (2) the occurrence of a marine-blocking, beach-barrier shoreline feature, and (3) leaching of the mineral matter. The coal seam formed by this deposit would be roughly rectangular in shape and oriented with its longest axis parallel to the beach-barrier shoreline. It would be bright and finely banded in its thickest central region and more coarsely banded at its margins. Its northwestern margin would be characterized by interfingering of coal with sandstones and siltstones of the Changuinola River flood plain; whereas, its southeastern boundary would be characterized by truncation of the upper part of the seam and replacement with marine to brackish bay rocks. Total sulfur was found to be very low (less than 0.2% [dry wt.]) except in a few places. In these rare cases, nearly all of the sulfur was pyritic and the total sulfur content was anomalously light (approximately 14%). Although the general tendency was for total sulfur to increase wherever the deposit was influenced by marine conditions, one case was found where the sulfur was high a the base of an inland freshwater core, suggesting the need for a new mechanism to explain sulfur enrichment in such a case. Differences in inherent fixed carbon and Btu contents due to changes in peat type tended to mask seam-wide trends in these parameters. However, fixed carbon and Btu showed a slight tendency to increase with depth (especially if plotted for single peat types). Vitrinite reflectance within the hypothetical Changuinola coal seam can be predicted to increase toward the margins of the deposit, toward the bottom and top of the seam, and toward stream channels or tidal inlets. A detailed description of this deposit and discussion of its relevance to coal genesis can be found in Cohen et al. (1989).