GLACIOTECTONICS (GT) OF THE OKANOGAN LOBE (OL) OF THE CORDILLERAN ICE SHEET IN NORTH CENTRAL WASHINGTON STATE

The Okanogan Lobe (OL) terminated on a plateau of flat-lying Columbia River Basalt flows. How the landscape was modified by glacial processes is unresolved. This study documents numerous OL glaciotectonic (GT) features not previously reported. Their relations to a network of crevasse squeeze ridges, eskers, and outwash fans suggest that GT transport was coeval with substrate-water-saturated glacial surging and initial OL retreat.

The flood basalt bedrock, with colonnades and entablatures, has unusual mechanical properties for GT. A fine-grained sedimentary interbed underlies the upper basalt flows. Hanson (1970) and Waitt (1994) previously proposed Pot Hills, Burke Hill, and Lone Butte as standout GT megablocks that moved on a sedimentary interbed basal plane. This study concurs and adds the Wheeler Hills, Piersol Hills, Hook Hills, Chester Butte, and numerous other landforms as GT features.

Prominent GT masses track back to nearby holes of similar volume, forming hill-hole pairs. Some holes have shapes similar to their displaced masses. Flutes and in some cases paraxial ridges connect holes to hills. Some displaced hills show no sign of glacial streamlining or overriding. Others have variable amounts of glacial streamlining and superposed push moraines.

It appears possible that extensive subhorizontal tracts, 10s of km², which begin up-glacier at basalt scarps, were stripped of basalt by GT erosion and the debris transported to the Withrow terminal moraine. This would account for the lack of a fluvial drainage pattern in these zones and the presence of linear drumlins, flutes (verging on megascale glacial lineation size), and debris trains.

Murdlins (Stalker, 1973) breach many push moraines, including those on GT megablocks. Some murdlins, larger GT masses, and linear GT offset tracks cut off parts of, or end at, eskers. The eskers debauched only slightly farther down-flow. This suggests that much of the GT took place during glacial retreat and that many small readvances occurred.

The landscape has similarities to, and informative differences from, the type area of the Evans et al. (2021) GT rafting model. The GT aspects of the landscape do not fit the Shaw et al. (1999) model of subglacial megaflooding and suggest that the Eyles et al. (2018) ice streaming model is incomplete.