Environments of Cordilleran Depositional Basins: ABSTRACT

Abstract

The stratigraphy of the cordillera can be interpreted in terms of stratigraphic assemblages that are unique in distribution, gross lithology, and lateral facies variations. Models of depositional basins in which these assemblages accumulated are essential in exploration for mineral deposits whose distribution is controlled by stratigraphy. End_Page 1437------------------------------ Proterozoic and lower Paleozoic strata in the cordillera comprise an assemblage of clastic and carbonate with minor volcanic rocks that appears to represent a continental-terrace wedge built along the margin of an earlier Precambrian continent. All units show a distinct polarity of facies distribution and thickness relative to the source area. This assemblage contains most of the known stratiform mineral deposits of gypsum, iron, copper, zinc, and lead in the cordillera. The distinctive elements of a Late Devonian and Early Mississippian assemblage suggest, at least in the northern cordillera, the presence of a foredeep and related source areas in the west and northwest. In the southern and eastern parts of the cordillera, however, the rocks reflect a continuing shelf-platform environment linked to the craton. The mineral potential of these rocks has been considered low but needs further study in view of an important zinc-lead deposit in eastern Selwyn basin. Distinctive rocks of oceanic character ranging in age from Mississippian to Middle Triassic underlie parts of the cordilleran intermontane belt. The important mineral deposits in these rocks include asbestos deposits in ultramafic rocks in the northern cordillera. Generally, however, mineral discoveries, other than those in ultramafic rocks have been few. In the eastern cordillera a shelf environment prevailed. The association of copper with volcanic rocks of Late Triassic and Early Jurassic ages is well known. The volcanic rocks, together with spatially and temporally associated plutons, are thought to outline a system of evolving island arcs probably roughly coincident with the mapped distribution of these rocks. Between the arcs and the craton, strata were deposited in a marginal basin with little or no evidence of the volcanism that occurred farther west. The remaining stratigraphic units, ranging in age from Early Jurassic to the Cenozoic, are described as successor-basin and foredeep assemblages whose distribution and lithology reflect a close relation to bounding uplifts of metamorphic and plutonic terrains. Because they are a late-stage phenomenon in the evolution of the cordillera these assemblages have potential for a variety of placer deposits. They also contain all of the known coal reserves of the region. End_of_Article - Last_Page 1438------------