EMPLACEMENT OF WILBUR CREEK , LAPWAI , AND ASOTIN FLOWS, COLUMBIA RIVER BASALT GROUP

Detailed field investigations, X-ray fluorescence (XRF) whole rock analysis, and chemostratigraphic correlation of Miocene Columbia River Basalt Group (CRBG) flows permitted identification of previously undocumented complex stratigraphic relationships of flows with Wilbur Creek (Tw), Lapwai (Tl), and Asotin (Ta) chemical types within the Clearwater Embayment, west-central Idaho. Vertical layering of chemical composition was identified in individual cooling units using XRF analyses. These flows featured a wide range of pahoehoe surface structures, such as tumuli and breakouts demonstrating that these are inflated flows. The flows also contain a three-part internal structure with a vesicular basal crust, non-vesicular core, and vesicular upper crust.

We interpret the chemostratigraphic complexity of this group of flows in terms of the inflation model for flood basalt lavas proposed by Self et al. (1996, 1997) and Thordarson and Self (1998). The three-part division of individual cooling units represents the lower and upper vesicular crusts that formed during the initial emplacement of the flow and the non-vesicular core represents the younger lava that was injected during inflation (Thordarson and Self 1996).

Hooper (1985) interpreted the Tw and Ta magmas as end members that mixed to produce the Tl magma based on detailed investigations of mineralogy and petrology of the flows and suggested that the Tw, Tl, and Ta were erupted as separate flows over a short interval of time. Field relations between the three chemical types, however, indicate that they were emplaced during a single eruptive event. The possible sequence of the eruption could be: 1) eruption of Tw magma; 2) mixing of Ta magma with residual Tw magma producing Tl magma; 3) eruption of Tl magma with continued mixing of Tw and Ta magmas until all Tw magma was depleted; and 4) eruption of Ta magma. The younger magmas injected and inflated older magmas during this emplacement sequence.