HEAVY MINERAL ANALYSIS OF EOCENE SEDIMENTS DEPOSITED ON THE HIGH ELEVATION NORTH AMERICAN CORDILLERAN PLATEAU

The North American Cordilleran hinterland is the product of extensive crustal shortening and rock uplift followed by extension and orogen collapse. During the Paleogene, a high elevation plateau approximately 3-3.5 km was situated between the Sevier thrust front and the magmatic arc. In northeastern Nevada, hinterland rivers draining the eastern side of the Eocene paleodrainage divide fed a large, deep lake basin on this plateau for ~ 10 million years. To the west of this drainage divide, rivers created deeply incised paleovalleys that extended to the Paleogene continental margin. Sediments derived from these rivers provide records of drainage system evolution, rollback-driven volcanism, and exhumation patterns of sediment source terranes.

The primary goal of this study is to use heavy mineral analysis to reconstruct the fluvial drainage network and differentiate between Eocene sediment sources, as well as provide new information on the exhumation and drainage evolution of the North American Cordillera. This study clarifies uncertainties in existing provenance data from U-Pb geochronology, (U-Th)/He thermochronology, and field counts of clast lithologies.

Heavy minerals were extracted from all nonmagnetic separates denser than 2.8 g/cm³, identified, and counted to compare populations of major and trace minerals. Heavy mineral separation was achieved through acid washing, petrographic and stereomicroscopic identification and grain picking. Unidentifiable grains from each sample were mounted for Scanning Electron Microscopy (SEM), Energy Dispersive Spectroscopy (EDS), and cathodoluminescence (CL). Following grain identification, separates were mounted on glass slides and counted using the ribbon counting method. Point counting has shown changes in fluvial drainage patterns and distinct erosional trends through comparison of igneous and metamorphic minerals eroded from Eocene highlands in the Cordilleran hinterland. Heavy mineral analysis of detrital grain separates differentiates between source regions that have indistinguishable detrital zircon U/Pb age populations and/or thermal histories.