Late Quaternary Geochronology and Recent Faulting Along the Eastern Margin of the Shukash Basin, Central Cascade Range, Oregon

Publication Date

11-2001

Type of Culminating Activity

Thesis

Degree Title

Master of Science in Geology

Department

Geosciences

Supervisory Committee Chair

Spencer Wood

Abstract

Part I: Seismic Stratigraphy Transecting the Eastern Margin of the Shukash Basin, Central Cascade Range, Oregon

The Shukash Basin is a sediment filled-trough that lies between the Cascade Range and Newberry volcano in central Oregon. A high-resolution seismic reflection survey conducted at Wickiup Dam transected the eastern margin of the basin and showed at least 250 m of sedimentary fill upon a volcanic basement. Three seismic facies were identified in the migrated seismic section: (1) prograded fill, (2) channel fill, and (3) onlap fill. Chronostratigraphy shows (a) the prograded fill unconformably overlies the acoustic basement; (b) the channel fill unconformably overlies the prograded fill; and (c) the onlap fill conformably overlies the channel fill.

Geotechnical boreholes penetrated the prograded fill and onlap fill, and geology was calibrated to the seismic section using velocity data. The Dibekulewe ash bed [~500 ka] and the Pringle Falls D tephra [218 ± 10 ka] were identified in borehole DH-99-1. This boring also determined that the prograded fill and the onlap fill were lake deposits. Furthermore, the prograded fill contained the Dibekulewe ash bed and the onlap fill contained the Pringle Falls D tephra. The age of the tephra layers constrain the age of the channel fill facies between 218-500 ka.

Based on the seismic data and geology, the geochronology of the basin is as follows: (1) eruption of Wickiup Butte [~610 ka]; (2) ponding and sedimentary filling of the basin [425-610 ka]; (3) erosion and aggradation of the basin during the Illinoian-age Abbott Butte glaciation [~425 ka]; (4) ponding and sedimentary filling of the basin [425-140 ka]; (5) erosion and aggradation of the basin during the pre-Wisconsin-age Jack Creek glaciation [~140 ka]; (6) fluvial erosion of Jack Creek outwash, followed by aggradation during the late Wisconsin-age Suttle Lake advance of the Cabot Creek glaciation [18-22 ka]; and (7) Mazama ash mantling the basin [7.6 ka].

Part II: Late Quaternary Vertical Displacement History of the Dilman Meadows Fault, Central Cascade Range, Oregon

The Dilman Meadows fault was discovered about 3 km downstream of Wickiup Dam along the Deschutes River. Numerous Quaternary dating techniques were used to constrain the timing of faulting events and included: (1) basaltic weathering rind dating applied to terrace gravels, (2) tephrochronology, (3) relative ages of terraces, and (4) correlation of terrace gravels to Cascade Range glaciations.

The radiometrically-dated Pringle Falls D tephra [218 ± 10 ka] was identified in the footwall of the fault. This ash provided a maximum age constraint for mapped terrace deposits. Cumulative vertical displacements were determined for each terrace surface. The terraces were then correlated to the Jack Creek glaciation [~140 ka] and the Suttle Lake advance of the Cabot Creek glaciation [18-22 ka] using weathering rinds, soil characteristics, and terrace scarps. The terraces are also mantled with an air-fall deposit of Mazama ash [7.6 ka], but some terraces are covered by reworked ash transported by surficial processes.

Using the net vertical displacement and relative age constraints, six time intervals were determined and the fault's slip-rates were calculated for each interval. With this technique, it is demonstrated that the Dilman Meadows fault was most active between 7.6-18 ka with a slip-rate of 0.26-0.31 mm/yr.

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