PALEOGENE UNROOFING OF THE ANACONDA METAMORPHIC CORE COMPLEX AND SEDIMENT DISPERSAL INTO ADJACENT SOUTHERN AND EASTERN INTERMONTANE BASINS OF SOUTHWEST MONTANA: IMPLICATIONS FOR EVOLUTION OF THE DEER LODGE BASIN

Recent studies of Paleogene strata in SW Montana document a paleotopography consisting of intermontane basins with intervening uplifts, similar to that of the modern, as well as a through-going paleodrainage system similar to much of the modern Missouri River headwater system. The Deer Lodge, northeastern Big Hole, and Divide Basins of this study lie within and beyond the northwestern fringe of the above study areas.

Within the Deer Lodge Basin, facies, paleoflow data, sediment composition, and detrital zircon ages in alluvial fan and fluvial facies document unroofing of the Anaconda Metamorphic Core Complex and the exposure of two-mica plutons in the Flint Creek and Anaconda Ranges. A divergent, axial, trunk-fluvial system transported sediment northward toward the Flint Creek Basin and SSE toward the northeastern corner of the Big Hole Basin, indicating a paleodivide within the Deer Lodge Basin. Fluvial conglomerates to the south, within the northeasternmost Big Hole Basin, indicate connectivity with the Deer Lodge trunk system and continued eastward transport through a water gap in the paleoPioneer Mountains into the southern part of the Divide Basin. Southward flowing, axial, fluvial bodies in the northern part of the Divide Basin were confluent with the other system, with the resultant higher-order system flowing southward then eastward into the northward-flowing paleoBeaverhead-Jefferson system. The Divide Basin was flanked by the Pioneer Mountains and Highland Range with the northern Divide Basin isolated from the adjacent Deer Lodge Basin, as opposed to the modern setting.

Tectonic modification of the Deer Lodge Basin was contemporeanous with extensional development of the core complex, suggesting a rift origin for the basin. However, the basin is also part of the regional intermontane basin complex to the east and south. Those basins were incised by headwaters of the paleoMissouri prior to Eocene extension. The paleoMissouri and modern drainage systems are parallel to, and locally transverse to, the Sevier-Laramide infrastructure, forming a pattern that is typical of foreland fold-thrust belts. Entrenchment of a paleoMissouri tributary into the Deer Lodge Basin suggests initial incisional development of this basin with subsequent extensional reactivation and modification.