[Next Section] [Previous Section] [Table of Contents]

Geologic Setting of the Study Area

The folds of the study area are in sandstone dominated turbidites (Tuss), above a thin mudstone unit (Tum) of the undifferentiated Umpqua Group (Figure 1). The mudstone overlies another sandstone dominated turbidite unit (Tuss) immediately to the north. In the riverbed 120 m to the north, stratigraphically below the mudstone unit, plane parallel sandstone beds up to 1 m thick (Tuss), dip about 30° S (Figure 1). Along the road above, sandstone beds are oriented 110° /05° S, then abruptly change orientation to 050° /42° SE at the contact with the mudstone unit to the south.

Stratigraphically above the study area unit are mudstone dominated turbidites (Tums) (Figure 1). The undifferentiated units crop out between the Bonanza fault zone and the Cooper Creek fault, a distance of 3 to 4 km. The White Tail Ridge Formation, exposed north of the Cooper Creek fault and south of Woodruff Mountain; and the Bushnell Rock Formation, exposed in the Bonanza fault zone, are both near their northward terminus for this longitude.

A NW-SE cross-section through the study area connecting well #8 and section 23 of the fence diagram (Figure 4), shows the relative position of the study area strata on the south-dipping limb of the Oakland anticline (Figure 6). The cross-section extends from the Camas Valley Formation (uppermost unit of the Umpqua Group) exposed north of Umpqua to Siletz River Volcanics exposed near Roseburg. In constructing the cross-section, two essential elements were addressed, the regional dip of the Oakland anticline and the throw on the Cooper Creek Reservoir fault.

Figure 6. Cross-section of Sutherlin subbasin through study area. Cross-section is from fence diagram section 16 northwest of Umpqua to section 23 near Roseburg (Figure 4), normal to the regional trend of the Oakland anticline. Bedding dip is the general trend of the beds on the Oakland anticline. Local variations, mainly at mudstone contacts, are shown as an overlay to the regional trend. Displacement on the Cooper Creek Reservoir fault has the lower, thick turbidite section brought to the surface to meet unit thickness requirement. The unit containing the outcrop may be a repeat of the lower turbidite unit, also mapped as Tuss (Figure 1).. The upper cross-section shows the spatial relationship of the outcrop and units to the south, and the exposure in the River. The topographic profile shows the local maximum elevation of outcrop unit, Tuss, 240 m to the east.

The regional dip is assumed to be 15° to 30° , based on data beyond areas of local deformation. The areas of local deformation ignored for the base drawing are the vertical beds immediately north of the anticline, a zone of more steeply dipping beds south of the Oakland anticline, vertical beds and small folds near the Cooper Creek Reservoir fault. The regional trend extends east of Sutherlin where Wells describes Umpqua Group strata as dipping 20° to 65° SE with an average of 30° , except for small areas of complex structure. Throw on the Cooper Creek Reservoir fault is taken to meet the need of the map distance of the turbidite unit south of the Cooper Creek Reservoir fault. The required thickness matches well with the lower turbidite unit at well #8. Offset on faults nearer the core of the Oakland anticline is relatively small to match the possible offset seen in the well.

An upper limit estimate of the thickness of the undifferentiated units exposed between the Bonanza fault and Cooper Creek fault (Figure 1) is 1600 m (Figure 7) using the geologic units of Figure 1 and average bedding dip. Including the White Tail Ridge Formation above the undifferentiated units totals 2000 m. Maximum thickness of the Umpqua Group sediments in the Sutherlin area was estimated by Ryberg at 2700 m and Baldwin at 2400 m. In the Umpqua area, Perttu and Benson estimated 1460 m, with a muddier section along Calapooya Creek of 760 m and a sandy unit forming the hills to the south of 700 m. Using two turbidite units as in well #8 of the fence diagram, the thickness estimate of the Umpqua Group in the Umpqua area is about 1600 m (Figure 7).

[Next Section] [Previous Section] [Table of Contents]