ABSTRACT: Laramide Deformation Front-Effects On Coal Quality In Paleocene Coal Basins, Northern Rocky Mountain Region

ELLIS, M. S., G. D. STRICKER, and R. M. FLORES, U.S. Geological Survey, Denver, CO

Ash and sulfur contents and apparent coal-rank trends from Paleocene basins are related to the evolution of the Laramide deformation front (LDF). The LDF marked the eastern and northeastern front of intensely thrust-faulted and uplifted rocks and extended from Montana through central Wyoming to the Front Range of Colorado. The eastward advancing LDF partitioned the Hanna basin (HB), Bighorn basin (BHB), Powder River basin (PRB), Bull Mountain basin (BMB), and Williston basin (WB) into intermontane basins. The HB (with as much as 11,000-ft-thick Paleocene sediments) and BHB were transformed from a partially closed to fully closed basin with internal fluvial drainage. The PRB, BMB, and WB evolved from open to partly closed basins with interconnected, flowthrough fluvial drainage.

PRB, BMB, and WB coals have low ash and low sulfur contents (arithmetic means of ash = 7.7% and sulfur = 0.61%) and have low apparent ranks (lignite A and subbituminous A and B). BHB coals have high ash and low sulfur contents (arithmetic means of ash = 11.8% and sulfur = 0.54%) and apparent ranks from lignite A to high volatile C bituminous. HB coals have the highest ash and sulfur contents (arithmetic means of ash = 15.6% and sulfur = 1.7%) and high apparent rank of high volatile C bituminous.

High ash and sulfur contents in HB coals are attributed to detrital and solution loads from Cretaceous marine sediments stripped from nearby uplifts. These materials entered internal fluvial pathways and interacted with low-lying mires in the rapidly subsiding basin. In the PRB, BMB, and WB, these materials bypassed and minimally interacted with the raised mires in the flow-through drainage, resulting in low ash and sulfur coals. Apparent coal rank was elevated in the HB from deeper burial due to rapid subsidence related to the LDF.

AAPG Search and Discovery Article #91017©1992 AAPG Rocky Mountain Section Meeting, Casper, Wyoming, September 13-16, 1992 (2009)