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.