The Chumstick basin opened as an extensional half-graben prior to 51 Ma, and was subsequently modified by two episodes of tectonic partitioning of drainage prior to basin deformation. Initially, westward flowing fluvial systems formed a unified depositional system with the Swauk basin. Tectonic partitioning of drainage at 51-49 Ma and at 44-42 Ma was controlled by localized uplift on the Leavenworth (LFZ), Eagle Creek (ECFZ), and Entiat (EFZ) fault zones and led in each instance to the truncation of regional depositional systems, modification and reversal of paleoflow, and internal drainage. Relief on the LFZ at 51-49 Ma may be the result of isostatic uplift of the extensional footwall, producing the Swauk and Chumstick basins as a pair of west facing half grabens. The earliest convincing evidence for the onset of oblique slip in the region is at about 48 Ma (folding in the Swauk basin) or about 44-42 Ma (probable transpressive uplift at left-stepping bends of the LFZ, development of a transtensional step-over basin between the ECFZ and EFZ, horsetail splays in the ECFZ, and possible flower structures in the LFZ and ECFZ in the Chumstick basin). Each episode of tectonic partitioning was followed by proximal onlap and overtopping of fault zones, to reestablish regional flow systems. The Chumstick Formation was deformed by dextral transpression between 37-34 Ma, and is unconformably overlain by the Oligocene Wenatchee Formation. The Chumstick basin is an example of an extensional basin modified by subsequent strike-slip tectonics, thus caution should be used in applying idealized basin models.
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