Depositional Environment of the Late Paleocene Canyon Coal Seam in Northeastern Wyoming

Nelson, Charles R.¹ (1) Consultant, Golden, CO

The marine and terrestrial geologic records indicate that a global climate warming pulse occurred during the late Paleocene and early Eocene. This was the warmest period of the Cenozoic. During this warm period the climate in northeastern Wyoming was humid and subtropical. Peat swamps were abundant in this area. The coals derived from these peat swamps are among the thickest in the entire geologic record. The late Paleocene-age subbituminous Canyon coal seam originated from one of these peat swamps. The Canyon coal seam is >60 ft thick, lacks clastic partings, and has very low average ash content (3.9%), which suggest that throughout its depositional history the precursor peat accumulated in a rain-fed raised swamp environment unaffected by clastic sediment influx. The Canyon coal is composed of highly variable amounts of huminite (44 to 93%) and inertinite (2.3 to 53.5%) macerals and minor amounts of liptinite (0.2 to 9.5%) macerals. Plant tissue preservation index values indicate that peat deposition occurred in a wet forest-type swamp environment. The Canyon coal seam contains four inertinite-rich (>30%) layers, which is geologic fingerprint-type evidence of periodic subaerial peat exposure and shifts from water-saturated to drier swamp conditions favoring peat desiccation, oxidation, and wildfires. Fusinite is the dominant inertinite maceral. In present day, living subtropical forest-type swamps and terrestrial forests, wildfire frequencies are strongly correlated with climate particularly annual dry seasons and droughts. The four inertinite-rich layers in the Canyon coal seam suggest the occurrence of periodic droughts during the late Paleocene in northeastern Wyoming.