ABSTRACT: Domal and Planar Peat Facies in the Dominy Lignite, Montana

John A. Daniel

Eight lignite cores of the Dominy seam, Custer County, Montana, were analyzed petrographically to show variations in peat-forming conditions. R-mode factor analysis and hierarchial clustering of petrographic results indicate three groups (A, B, and C) of macerals that share common characteristics. Group A is comprised entirely of huminite macerals formed from preserved and residual wood tissue. Group B consists of a mixture of resistant and detrital huminite, inertinite, and exinite macerals while oxidized material and secondary resins comprise Group C. Plotting the distribution of samples with respect to Group A, B, and C, a coal facies diagram was constructed and six facies determined. A transition matrix was constructed of the vertical distribution of the coal facies t determine the succession of facies in the Dominy seam. These results show that Facies 2 represent the environment which initiated the peat-forming swamp. Facies that show the strongest relationships for succession are Facies 2 and 5 and Facies 1 and 3. Facies 4 is independently associated with Facies 2 and 5 while Facies 6 is found only with Facies 1 and 3.

Results of the transition matrix combined with facies geometry suggest that Facies 2 and 5 represent subenvironments of a reed-marsh or open forest setting while Facies 1 and 3 are subenvironments of a thick forest setting. Facies 4 and 6 suggest lacustrine conditions associated with each respective environment. The forest environment forms a domal peat deposit temporally located in the central portion of the wetlands which grades laterally into the reed-marsh environment. The reed-marsh environment forms planar peat deposits and is found at the swamp margin, proximal to the fluvial source.

AAPG Search and Discovery Article #91002©1990 AAPG Rocky Mountain Section Meeting, Denver, Colorado, September 16-19, 1990