THE EXCELSIOR PEDIMENT GRAVEL AND DEEP CHEMICAL WEATHERING OF THE NORTHERN COLUMBIA PLATEAU REGION

A highly weathered polymictic gravel layer about a meter thick, which overlies both bedrock and saprolite from Spokane to Reardan and south to Cheney, has been a subject of several disparate interpretations over the past hundred years. This gravel was first thought to be a product of glacial activity, then placed into the Middle Miocene Latah Formation, and finally we interpret it as an extensive post-basalt (≤ 6 my) pediment gravel informally named for an excellent exposure at the top of the abandoned Excelsior clay pit west of Mica, just south of Spokane. Conditions of high rainfall and humidity responsible for deep chemical weathering decreased significantly throughout the Pliocene, thus a 5 m.y. age for the Excelsior gravel appears reasonable.

Deep chemical weathering of Miocene basalts, Eocene to Cretaceous plutonic and Cambrian to Mesoproterozoic basement rock in the northern Columbia Plateau region has also been known and debated for over one hundred years, but problems still remain concerning whether the regionally extensive saprolitic clay is derived from deep chemical weathering of bedrock or is just “old loess,” as discussed as part of the ongoing “Palouse Soil” problem. In practice, geologic mappers have tended to place lithic residuum into bedrock units and map the clays as loess or as the loessal Palouse Formation. We found that the mineralogy of the clays supports the in-situ weathering bedrock origin and no indicators, other than caliche layers, were found suggesting a loessal origin for these clays. These clays are overlain by the Neogene Excelsior gravels locally and thus pre-date known loess deposits in the Pacific Northwest.