UNCONSOLIDATED QUATERNARY DEPOSITS

                                                                                               

                                                                                  

MAIN MENU

 

         Widespread deposits of silt, sand, and gravel are not common on valley floors of the Mitchell area, even adjacent to the John Day River. This is because uplift has been underway and erosion dominates aggredation while most streams flow on or close to bedrock. Summer thunderstorms sometimes cause alluvial fans and sheets of mud to be deposited locally, but they are soon removed by subsequent floods.

deposits of mud

During Pleistocene time, conditions were very different. Although glaciers were not present in the Mitchell area, rainfall was much more abundant than it is now and valleys were characterized by broad alluvial plains covered with gravels.

 

These deposits now survive on isolated terraces and broad pediments where eroded hard-rock detritus (such as PGB) rests upon (and protects) soft rocks such as tuffs and mudstones.

 

 

Subsequent erosion of these pediments has produced many isolated hills and ridges of soft bedrocks capped by basaltic gravels.

 

 

 

It is within adjacent stream deposits that tusks and molars of Pleistocene mammoths have been found in the Mitchell area.

 mammoth-tusk

         Tuffs of John Day and Clarno Formations contain abundant smectite clays and when they become wet, provide a “lubricant” for extensive landslides.

JD mud

In this way, large Pleistocene landsides were produced, especially where tracts of rock fragments rested upon the mobile clays.

land slide map

The valley of Gage Creek and the lower one-half of Meyers Canyon received these landslides in such volume that stream erosion could not remove the accumulation of debris. In both of these valleys, seasonal storms have maintained a deep trench in the alluvial fill and exposed a bed of 7700-year-old Mazama ash in the upper one-half of the deposits.

Meyers trenchtrench with ash

The Mazama ash from the culminating eruptions of Crater Lake is easy to recognize because the ash is composed of fresh glass shards, crystals of black hornblende, and is gritty to the touch.  

I have watched several floods from summer thunderstorms completely fill and re-excavate the Meyers Canyon trench within a single day.

 

Smaller-scale landslides occur locally, such as on the east slope of Sargent Butte where dissection reveals the landslide deposits in cross section.

LS scarlandslide depoaits

      

     On gentle slopes at high elevations, repeated freezing and thawing of the soil has produced a patterned ground where stone stripes are common.

stone stripes

        Streams of the Mitchell area have been superimposed from a much higher topographic level where rivers coursed over broad alluvial plains. Entrenched meanders occur in the course of the John Day River and many tributary streams, such as West Branch, Bridge Creek, and Meyers Creek.

 

They now bisect resistant intrusive bodies instead of following a course on adjacent soft rocks. Consequently, many hillsides are characterized by steep cliffs and aprons of talus.

talus

         Recent volcanic activity in the Mitchell area has occurred only once and is represented by a cinder cone on a small basaltic shield volcano west of the locality of Antone

cinder cone

The cinders have been mostly removed for constructional purposes.

 

RETURN TO THE MAIN MENU