COMPETING MODELS FOR THE FRUITLAND FORMATION COAL AND COAL-BED METHANE SYSTEM OF THE SAN JUAN BASIN, NEW MEXICO AND COLORADO

The Fruitland Formation coal system of the San Juan Basin was described in 1971 (USGS Professional Paper 676) in the first detailed description of a Western Interior coal basin confirming the elegant concepts of coal deposition first articulated by Sears, Hunt, and Hendricks in 1941(USGS Professional Paper 193-F). Simply put, those authors suggested that in a back-shore, peat-swamp environment adjacent to a regressing shoreline, thicker peat would accumulate when the regressing shoreline temporarily stabilized allowing both shore-face sandstones and adjacent peat swamps to build up vertically. However, when the shoreline regressed at a relatively rapid rate, the longevity of back-shore swamps would be brief allowing little opportunity for thick peats to form. Professional Paper 676 showed that indeed, Fruitland coal beds were thickest adjacent to (shoreward of) large vertical build-ups of the underlying shore-face Pictured Cliffs Sandstone and thinner where the underlying Pictured Cliffs was thinner. This model came to be called the San Juan Basin model of coal deposition.

In the 1980’s, as the San Juan Basin Fruitland coal-bed methane play was rapidly expanding, Fruitland coals were revisited and a radically new model was proposed to explain the area of thick Fruitland coals in the north-central part of the basin (NMBMMR Bulletin 146). This model suggested that thick Fruitland coals did not accumulat according to the San Juan Basin model, but rather had formed as the result of differential tectonism within what is now the central San Juan Basin. This model further suggested that Fruitland coal beds, rather than becoming progressively younger across the basin (per the San Juan Basin model) were deposited in a series of wide-spread, more or less synchronous, peat swamps extending nearly basin-wide.

A recent study of the Fruitland coal system, part of the USGS National Coal Assessment and published in 2000 (USGS Professional Paper 1625-B, Chapter Q) has confirmed the validity of the 1971 San Juan Basin coal-deposition model and precisely dates the Pictured Cliffs Sandstone regression as lasting 2.5 m.y. and shows that the overlying Fruitland coal beds are discontinuous and become progressively younger northeastward across the basin area.