GEOCHEMICAL CONSTRAINTS ON PRE-MIOCENE, CENTRAL WASHINGTON CASCADES TOPOGRAPHY

We analyzed the geochemistry of 18 volcanic clasts collected from the Mashel Formation (MF) to determine whether the high topography and modern drainage divide of the range pre- or postdated the eruption of the Columbia River Basalt (CRB), begun at 17 Ma. The MF is a late-Miocene conglomerate located on the western flank of the central Washington Cascades, approximately 25 km west of Mt. Rainier (46º 45’ N, 122º 40’ W). The MF consists of fluvially transported sand to cobble-sized clasts, many of which are volcanic and basaltic to andesitic in composition. The provenance of these clasts indicates the position and topography of the source basin for the MF. Possible sources include: 1) CRB, at this latitude limited to the eastern flank of the Cascades; 2) pre-Miocene Cascade Arc volcanics, possibly those erupted during the spike in igneous activity during the Miocene; or 3) Lower Eocene members of the Coast Range Basalts. The presence of CRB in the MF would indicate either that the basalt originally flowed further to the west than is preserved today, or that the paleodrainage for the MF extended eastward past the modern drainage divide; either of these possibilities would indicate the lack of a topographic barrier at the time of MF deposition.

Our eighteen chemical analyses by XRF and ICP-MS do not show any evidence of CRB in the Mashel Formation. The chemical characteristics of the clasts are typical of arc volcanism, which precludes inclusion in either the Coast Range Basalts or CRB. In addition, the composition of these clasts is unlike modern High Cascade volcanic rocks in the area today. It is most likely the volcanics in the MF were derived from the now-eroded Western Cascade arc, and that CRB was not in the source region. The large (2-15 cm) size and high degree of rounding of the clasts suggest that there was significant topography during the deposition of the MF, with rivers large and steep enough to carry gravel to cobble-sized clasts. We have not found published data on any other Miocene or pre-Miocene igneous rocks from the area that match these clasts, suggesting perhaps that the source regions for these clasts have eroded. The removal of source rocks is consistent with the conclusions of Reiners et al. (2003), that exhumation rates have been high, removing approximately 2-3 km of rock from the wet western flank of the Cascades in the past 15 My.