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Regional Geologic Setting

Geographic Location

The folds of the study area are exposed in a roadcut between milepost 11 and 12 on Garden Valley Road northwest of Roseburg, Douglas County, southwestern Oregon. They are 4.3 km south of Umpqua (Figure 1). The regional topography is one of domed hills and resistant ridges, up to 700 m in elevation with relief of 600 m, clustered between river and creek valleys. Density of vegetation on hills is thick to sparse depending on the substratum, with mostly madrone, oak, and fir trees. At an elevation of 120 m, the outcrop overlooks a bend in the Umpqua River as it flows from the High Cascades to the Pacific Ocean at Reedsport.

Regional Physiography and Geology

Near the convergence of the Oregon Coast Range, Western Cascades, and Klamath Mountains, the study area is in the Sutherlin subbasin at the southern end of the Oregon Coast Range (Figure 2). The subbasin contains Umpqua Group sedimentary rocks overlying Siletz River Volcanics , the basement rock of the Oregon Coast Range. To the west, Siletz River Volcanics and Umpqua Group strata are buried in the Tyee basin, then resurface in the Myrtle Point subbasin. Tyee sandstone forms higher ridges west and north of exposed Umpqua Group strata. At the eastern edge of the basin is the boundary of the Oregon Coast Range with the late Eocene-early Miocene Western Cascades.

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Figure 2. Location of study area in Sutherlin subbasin. A. Regional geologic map showing extent of the Umpqua Group in southwestern Oregon. B. General geologic map of the Sutherlin subbasin and approximate location of study area .

South of the Sutherlin subbasin, the Oregon Coast Range converges with the Mesozoic Klamath Mountains. An effect of this convergence is seen in the parallel NE-SW structural alignment of the Umpqua arch, Sutherlin subbasin, Bonanza fault zone, and the faulted boundary of the Klamath Mountain province (Figure 2). The Klamath Mountains were the major source of sediment for Umpqua Group rock.

Stratigraphy

The stratigraphy of the Umpqua region consists of Paleocene age Siletz River Volcanics, regarded as basement rock, underlying up to 3700 m of Eocene turbidite sediments and deltaic deposits (Figure 3). The lower to early middle Eocene units, which include the units of the study area, comprise the Umpqua Group. The younger units above the Umpqua Group belong to the Tyee Formation. Following is an overview of these units from oldest to youngest.

Siletz River Volcanics

Underlying the Umpqua Group, the Paleocene to early Eocene Siletz River Volcanics have K/Ar and Ar/Ar dates of 50.73.1 to 62.11.0 Ma with ages typically decreasing from south to north. A date of 62.1 Ma was recorded for volcanics in the Drain anticline in the heart of the Umpqua arch northeast of the study area (Figure 2). Extending from the foothills of the Oregon Cascades east of the study area to the west and northwest, the volcanics form the basement of the Oregon Coast Range. Estimated to be up to 25 km thick in the central Oregon Coast Range, the base of the volcanics has not been observed. The Siletz River Volcanics consist of pillow basalt, tuff-breccia, massive lava flows, and sills with some aerial deposits in its southern extent.

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Figure 3. Stratigraphic nomenclature for the region containing the study area. This study uses the nomenclature of Ryu and others.

Exposure of the Siletz River Volcanics is widespread in the Roseburg area north to the Bonanza fault zone (Figure 1) and in the cores of anticlines east and west of the Tyee basin (Figure 2). Umpqua Group strata lap onto and thin across the Umpqua arch, a volcanic high north of the study area. Northeast of Sutherlin, Mobil Oil Corporation drilled into 3 km of Siletz River basalt without reaching its base.

Umpqua Group

Unconformably overlying the Siletz River Volcanics, the Umpqua Group continental margin deposits are Paleocene to early Eocene. Foraminifera of the Umpqua Group are referable to the Lower Penutian-Upper Ulatisian Stages. Paleoecology indicator fossils show deposition of the units took place in warm shallow waters of sublittoral to neritic depths with occasional connections to the open ocean. The oldest microfauna in the group, in sedimentary beds within volcanics east of Myrtle Point, are of the Paleocene. Planktonic foraminifera of the Tenmile Formation are indistinguishable from those of the lower undifferentiated Umpqua Group. In the uppermost Camas Valley Formation, foraminifera are characteristic of the early Ulatisian Stage of earliest middle Eocene age. Just as fossils do not easily distinguish the formations of the Umpqua Group, neither do contacts within the group. Formation and member contacts are generally conformable with interfingering facies and local unconformities.

Stratigraphic Nomenclature

The interfingering of deposits and complexity of the setting has prompted revision of the stratigraphic nomenclature of the Umpqua Group several times as additional mapping and correlation of the units has been completed (Figure 3). Umpqua Group strata were first described by Diller as the Umpqua Formation. Baldwin upgraded the formation to the group status composed of three formations, the Roseburg, Lookingglass, and Flournoy, with the study area in the Roseburg Formation. The sedimentary part of Baldwin’s Roseburg Formation is based on an exposure of turbidite beds on the south side of the Dickinson Mountain anticline, north of Sutherlin (Figure 2). Baldwin estimated the interbedded sandstone and shale is 2400 m thick.

The basement Siletz River Volcanics were included with the sedimentary strata of the Umpqua or Roseburg Formation by Baldwin and Diller. Molenaar used Umpqua as a formation name with four members but separated the volcanics which were now shown to be equivalent to the type Siletz River Volcanics to the north.

More recently, work of Ryu and others maintains the Umpqua Group status but builds on prior work for redefining the subdivisions of the group. This recent nomenclature divides the differentiated Umpqua Group into the Bushnell Rock, Tenmile, White Tail Ridge, and Camas Valley formations, and units not assigned to formations are referred to as the undifferentiated Umpqua Group.

Differentiated Umpqua Group

In the lower Umpqua Group, the Bushnell Rock Formation unconformably overlies Siletz River Volcanics and in areas, Mesozoic rock of the Klamath Mountains. The formation consists of thick massive units of pebble-cobble conglomerate with minor interbeds of sandstone. Clasts are derived from the Klamath Mountains to the south. Near Woodruff Mountain (Figure 1), an isolated well-bedded sandstone and conglomerate unit about six hundred meters thick, was interpreted by Molenaar as a local deep water channel facies of the Bushnell Rock Formation.

Thickness of the Bushnell Rock Formation varies to 1200 m. Thickest in the south and southeast regions of the Tyee basin, the unit rapidly thins to the north within 8 to 16 km, grading into the undifferentiated Umpqua Group strata. South of the study area, the Bushnell Rock formation is in possible fault contact with undifferentiated Umpqua Group units (Figure 1). A fence diagram of the Umpqua area shows the formation in all columns south of Umpqua to a vertical thickness of 600 m (Figure 4).

Conformably overlying the Bushnell Rock Formation, the Tenmile Formation is mainly composed of thin-bedded turbidites alternating with deep marine mudstone,. In its southern extent, three isolated tongues of pebbly sandstone with mudstone rip-ups and pebble to boulder conglomerate are associated with the turbidites, one of which, the Rasler Creek tongue, has rounded, white calcareous concretions. The turbidite beds consist of poorly sorted, medium to coarse grained, lithic sandstones alternating with dark gray mudstone. Sandstone to mudstone ratios are 1:1 to 2:1.

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Figure 4. Schematic fence diagram of wells and measured sections surrounding the study area. The location of the study area is indicated by the symbol and post in the interior. Lines from well #9 to well #7 connecting the upper and lower boundaries of the Tenmile Formation turbidites cross this interior post at the horizontal line segments on the post.

Up to 450 m thick, the turbidite beds commonly overlie slope mudstone and grade out toward the north and northwest. Locally overlying the turbidite sequence is an upper slope/shelf mudstone. Up to 900 m thick, the deep marine mudstone unit is widespread in the south, southeast, and eastern Tyee basin. In the Umpqua area, the Tenmile Formation occurs as a mudstone, turbidite, mudstone sequence up to a vertical thickness of 850 m (Figure 4).

Conformably above the Tenmile Formation is a deltaic sequence, the White Tail Ridge Formation. Up to 1000 m thick, the formation thins to the north into deep marine mudstone of the undifferentiated Umpqua Group. The fence diagram of Figure 4 has White Tail Ridge Formation in columns south of the study area with vertical thickness up to 750 m.

Consistently above the White Tail Ridge Formation, the widespread Camas Valley Formation, uppermost Umpqua Group unit, consists of massive, shelf to slope mudstone up to 550 m thick. In the Umpqua area, the Camas Valley Formation is about two hundred fifty meters thick (Figure 4) .

Undifferentiated Umpqua Group

The formations of the Umpqua Group are not recognized basin-wide because of poor exposures, lateral facies changes, and complex structure. The category of undifferentiated Umpqua Group accommodates these problematic areas typically in the northern, distal sections of the basin where Bushnell Rock conglomerate and White Tail Ridge deltaic deposits are absent (Figure 4). A sedimentary sequence of turbidites and mudstone, the distal deposits are laterally equivalent facies of formally recognized Umpqua Group formations, though the lower undifferentiated strata in the Sutherlin area might be older than the Bushnell Rock Formation. The undifferentiated name is also applied to sedimentary rocks interfingering with Siletz River Volcanics .

Tyee Formation

Stratigraphically above the Umpqua Group, the widespread middle Eocene Tyee Formation crops out over an area 80 km wide and 280 km long, from a latitude of 4241' N to 4511' N. The thick bedded micaceous arkosic sandstone and minor mudstone sequence ranges from 2000 to 3000 m thick. Provenance of the sandstone is mixed, though Rb/Sr ratios of coarse muscovite flakes indicate the Idaho batholith is the main source of sediments. Column 16 of the fence diagram in Figure 4 shows the Tyee Formation is about one kilometer thick northwest of Umpqua.

Structure

The structure of the Sutherlin subbasin is dominated by a NE-SW trend parallel to the accretionary contact between the Mesozoic Klamath Mountain terrane and Cenozoic Siletz River Volcanics. To the west, structural trends in the Tyee Formation are N-S within an E-W extension regime overprinted by a series of NW-SE anticlinal plays. The larger scale anticlines and synclines in the subbasin accommodate the volcanic highs in the north (Figure 2). Major faults in the south expose Siletz River Volcanics and Umpqua Group strata adjacent to the Klamath terrane.

Folds

Folding is at several scales in the region. First-order folds have wavelengths of 10 km and amplitudes of 1 km. Fold axes parallel the accretionary lineament (Figure 2) south of Roseburg. Perttu and Benson report anticlines are tighter in the hinge than the broader synclines, and north limbs are steeper. Ryu and others describe the folds as broad, and the Oakland anticline in particular, as a symmetrical broad fold with dips of 10 to 30 on its limbs, except near faults. The Oakland anticline and Dickinson Mountain anticline to the north appear to form the skeleton of the Sutherlin subbasin structure (Figure 2). The beds of the study area are on the southeast limb of the first-order Oakland anticline.

Second-order folds occur as syncline-anticline pairs on the limbs of first-order folds. The fold pairs have axes that extend 1 to 5 km and half-wavelengths of 0.4 to 1.1 km. The second-order folds are locally developed; three of the anticlines and synclines are mapped in the Umpqua to Sutherlin area. One pair crosses the Umpqua River south of Umpqua (Figure 1) bordering the Cooper Creek Reservoir fault. The area of the syncline north of the fault is an exposure of the White Tail Ridge Formation.

Third-order folds are highly localized, occurring as sequences of tight chevron-like folds. The folds of the study are third-order, with wavelengths of 10 to 30 m and 3 to 7 m amplitudes. Other third-order folds in the Sutherlin subbasin were reported by Wells and Waters and by Perttu and Benson. These folds are in the Bonanza area east of Sutherlin, and at two sites along the Umpqua River, south of the Cooper Creek Reservoir fault, and south of Woodruff Mountain. The folds south of Woodruff Mountain described by Perttu and Benson have fold axes of more variable azimuth and plunge than the folds in the study outcrop. Perttu (Geology of the Stephens and Calapooya proposed nuclear sites, Douglas County, Oregon: Portland State University, c. 1972-76) reported tight chevron folds in calcareous concretionary siltstone at a fault contact near the currently mapped Cooper Creek Reservoir fault.

The third-order folds of the Umpqua basin are in turbidite beds of the lower Umpqua Group, the Tenmile Formation and its lateral equivalent in the undifferentiated Umpqua Group. These highly localized folds are not generally found in the upper Umpqua Group, the White Tail Ridge sandstone and Camas Valley mudstone. However, a distal facies of the White Tail Ridge Formation, the Coquile River member, is locally infolded with Tenmile Formation turbidites. Away from faults, upper Umpqua units typically have dips of less than 30. In the study area, the turbidite unit containing the folds overlies a less competent mudstone unit (Figure 1).

Faults

In the eastern Umpqua basin, major faults are south of the Sutherlin subbasin. The Bonanza fault on the southern boundary of the subbasin and the Wildlife Safari fault to the south have a NE-SW strike which is similar to the trend of the regional folds (Figure 2). Farther south, the Canyonville fault has a more E-W strike. Dominant motion on the faults varies from reverse, with hanging wall displaced northwest, in the NE-SW Bonanza fault zone, to E-W right-lateral strike slip on the Canyonville fault. The Wildlife Safari fault has elements of both with oblique right-lateral reverse displacement.

In the Bonanza fault zone south of the study area, basement Siletz River Volcanics and Bushnell Rock conglomerate are thrust top-to-the-northwest, adjacent to sedimentary beds of the undifferentiated Umpqua Group (Figure 1). The fault was still active after deposition of the White Tail Ridge Formation. Ryu and others report that reactivation of the Bonanza fault deformed sandstones of the White Tail Ridge Formation and upper Eocene-Oligocene Colestin/Fisher Formation in the foothills of the Western Cascades east of Sutherlin. At the Bonanza mine east of Sutherlin, the fault dips about 45 SE, parallel to bedding. Dip separation is estimated at 1.5 km maximum .

The Wildlife Safari fault brings Mesozoic Klamath Mountain rock adjacent to Umpqua Group units, offsetting the Tenmile Formation but not the White Tail Ridge Formation. The fault dips 70 SE to vertical and has a minimum 5 km right-lateral offset, based on offset of a late Mesozoic thrust fault and associated syncline, and variable reverse offset. The Wildlife Safari and Bonanza faults generally do not displace beds of the Camas Valley and Tyee formations, restricting their age to about 52 Ma .

The near vertical Canyonville fault offsets rocks of the Klamath Mountains at least 30 km in a right-lateral sense. This estimate is based on offset of the intersection of the contact between Riddle and Day Creek Formations with a syncline affecting both units. Gentle folds in the White Tail Ridge Formation are also offset but to a lesser degree suggesting multiple episodes of movement on the fault. The Tyee Formation overlaps the fault in the central Coast Range.

Other faults in the subbasin vary in trend. The Cooper Creek Reservoir fault south of Umpqua has trend and motion similar to the Bonanza fault (Figure 1). It appears to offset the White Tail Ridge Formation, bringing turbidites of the undifferentiated Umpqua Group adjacent to and higher than the formation (Figure 1). West of the study area, faults border Woodruff Mountain. On the northwest side of the mountain is a N-S trending normal fault, west-side down similar to the Tyee structural alignment (Figure 1). Southwest and southeast of the mountain, faults described by Perttu offset Bushnell Rock conglomerate, the southeast fault having motion similar to the Bonanza fault (Figure 1). Perttu also describes a possible fault in the core of the Oakland anticline north of Umpqua, where beds are slightly overturned.

Tectonic History

The rhythmically bedded sandstone and siltstone of the undifferentiated Umpqua Group consists of sequences of inner, middle, and outer submarine fan deposits, and basinal mudstone. Paleogeographic reconstruction of the Northwest done by Heller and others has the Umpqua Group sediments being deposited during a phase of basin development between colliding oceanic islands along a continental margin. Coincident with basin development is uplift of the Klamath Mountains, major sediment source to the basins.

The oceanic islands, source of the Siletz River Volcanics, possibly formed over a hot spot or spreading ridge, or along a rifted continental margin. Geochemical analysis by Pyle indicating an oceanic volcanic setting favors a spreading ridge or hot spot interpretation. Palinspastic reconstruction by Heller and others to 55 Ma places the islands and basins offshore of a continental margin near the present Oregon-Idaho border (Figure 5).

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Figure 5. Paleogeographic reconstruction of the Pacific Northwest at 555 Ma .

The islands and intervening Umpqua basin were accreted and possibly partially subducted as the oceanic plate collided with the North American plate at the margin of the Klamath Mountain terrane. Continuing uplift and erosion of the Klamaths provided sediment to the basins surrounding the volcanic highs. The exact nature of the suture zone between the Umpqua Group and Klamath Mountains is still being investigated to determine if significant thrusting accompanied accretion or there is a more abrupt juxtaposition along the Wildlife Safari fault.

Based on tight folding of the Umpqua Group being confined below the White Tail Ridge Formation, and the Tyee Formation having a N-S structural lineament, Ryberg constrained major faulting and folding of the Umpqua Group to the early Eocene. The change in deformation style within the Umpqua Group is thought to signal a jump in the subduction zone to its current location west of the Coast Range in late early Eocene and a change to N-S structural alignment in the forearc basin. Subsequent subsidence of the forearc basin allows for the great areal extent and thickness of the Tyee Formation with major provenance in the Idaho batholith.

By 40 Ma pre-Basin and Range extension had begun in Idaho, Washington, western Montana, and Oregon causing westward displacement of the Klamaths, accreted islands, and basins. Possibly coupled with extension is some clockwise rotation of these units with additional rotation attributed to dextral shear between the continental and oceanic plates. Paleomagnetism studies indicate the lower Umpqua Group rotated clockwise about 70. Rotation of the Klamath Mountain terranes is 110 to 40 in older to younger terranes, respectively, extending from the Paleozoic Devonian to the Cenozoic .

Superimposed on the NE-SW structural trends paralleling the Klamath-Umpqua suture zone are N-S and NW-SE trends. Major manifestations of the N-S trend are the Oregon Coast Range and the Tyee forearc basin. Some reactivation of the older NE trending faults is possible during this E-W extension. The NW-SE structure is displayed in normal faults and structural culminations of the NE trending anticlines recently described by Ryu and others.

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