--> Abstract: Late Cretaceous Paleogeography and Patchwork Coalbeds: Fruitland Formation, Northern San Juan Basin, Western Interior Seaway, USA, by H. S. Nance; #90937 (1998).

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Abstract: Late Cretaceous Paleogeography and Patchwork Coalbeds: Fruitland Formation, Northern San Juan Basin, Western Interior Seaway, USA

NANCE, H. S., Consulting Geologist, Leander, TX 78641

Eleven Fruitland Formation (Upper Cretaceous) coal-bearing intervals were mapped with gamma-ray and bulk-density data from 1620 wells distributed over 1360 sq. mi. in the northern San Juan Basin. A paleogeographic map shows the facies occurring beneath a widespread bentonite bed (ashfall). Individual coeval Fruitland coalbeds average 1 ft. to l0 ft. thick and consist of numerous equant to dip-elongate, patch-like sheets (ranging in area from I to over 200 sq. mi.). The sheets are separated laterally by fluvial sandstone. Zones of coeval coal are bounded on the northeast, along relatively straight northwest (strike) trends, by Pictured Cliffs or upper Pictured Cliffs sandstone. Coalbeds are vertically separated by mainly fluvial sand bodies or coeval shaley (often bentonitic) partings. Compactionally folded coalbeds mark positions of sub- and superjacent sandbody thicks.

Fruitland swamps in the northern San Juan Basin occupied dip-elongate (northeasterly) flood basins located between several perennial streams. The streams fed sand to northwest-trending, wave-dominated, marine shorefaces along the western margin of the Western Interior Seaway. Coastal-plain peat formation proceeded within hydrologically stable zones about 30 mi wide that were bounded on the northeast by a marine shore zone. Swamps were locally inundated by channel sand during abrupt and widespread increases in fluvial activity, often attended by volcanic ashfall.

This investigation has implications for flow in Fruitland coalbeds. Communication between coeval patches is either indirect or through fractures in adjacent sandstone. Compactional folds are sites for potential fracture- permeability enhancement.