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Outcrop Observations

The interbedded sandstone and shale layers of the study area folded into predominantly asymmetric chevron-like folds, accompanied by a symmetric concentric-like fold, and kink-like folds (Figure 8). Folded turbidite beds are also exposed in the bed of the Umpqua River 20 m below the outcrop and downstream to the southeast. In the riverbed 120 m north, stratigraphically below the mudstone unit, Tum, (Figure 1), plane-parallel sandstone beds (Tuss) up to 1 m thick, dip 30° S. Above along the road, sandstone beds exposed for 90 m abruptly change orientation from 110° /05° S to 050° /42° SE at the mudstone contact (Figure 6). Areas to the east are covered.

The exposure of folds is approximately 360 meters long and normal to the fold axes. With plunge of the folds ranging from 2° to 34° west and east, much of the outcrop is close to a down plunge view giving an undistorted view of the folds. Total measured thickness of layers in the outcrop is about 110 m assuming little duplication in covered areas (Figure 9).

Bedding

Bedding relationships (thickness and contrasts in material properties) are extremely important factors in folding. Plane parallel and laterally continuous beds dominate the exposure. The occasional lenticular bed and pinchout in Section I of the outcrop are exceptions. Where bed thickness does vary, the bed is frequently adjacent to a calcareous layer.

Figure 9. Schematic showing thickness of measured fold limbs from north to south. Gray intervals on columns are covered. The scale at left is not meant to portray a continuous vertical section because of covered areas or unrecognized repetition. See Appendix B for detailed stratigraphic sections.

Arenaceous layers in the outcrop are massive to graded with medium to fine grained lithic sandstone. In the classification scheme of Mutti and Ricci Lucci the beds can be classified as turbidites, fitting somewhere between type C, an arenaceous-pelitic facies, and type D, a pelitic-arenaceous facies. Bed thickness is less than the range 50 to 159 cm for facies C classification but the sandstone/shale ratio is greater than the facies D norm of 1:2 to 1:9 (Table 1). These facies imply deposition beyond the distributary system in an outer fan to basin plane setting.

The turbidite beds are up to 70 cm thick with an average of 11 cm. Average thickness increases up section, decreases slightly, then increases again (Table 1). Within this framework, bed thickness is mostly random with minor fining or thickening up sequences of 3 to 5 beds. Average sandstone to shale ratios in the sections varies from 3.9 to 3, but not directly with average thickness.

Table 1. A summary by section of bed thickness and sandstone to shale ratios. Data used are from representative sequences of beds making up the folds in a section.

Current structures vary from north to southeast, though Perttu and Benson determined the dominant paleoflow direction is from the east in the lower Umpqua Group and from the south in the middle and upper Umpqua. Paleoflow in Umpqua Group sections measured by Ryberg in the Sutherlin area is 150° to 210°. In the classification scheme of Bouma , most beds are an ae layer sequence with b, c, and d absent. (The five divisions of the Bouma sequence are from the bottom, a: massive to graded sandstone, b: sandy parallel laminations, c: rippled or convoluted bed, d: laminations of silt and mud, e: mudstone.) Other beds are abae, ce, and ace layer sequences.

Sharp basal and upper contacts characterize most of the beds in the outcrop. Deformation of the planar bed form occurs adjacent to several deeply weathered brecciated shale zones in Sections II, IV, and VI, where beds have a convoluted, wavy appearance (Figure 10). Some beds are also deformed into duplex-like structures (Figure 11), similar in form though not in shear sense, to the flexural slip duplexes described by Tanner.

Figure 10. Bed deformed into low-amplitude sine wave with wavelength of about two meters. Immediately below and 1 m above the deformed bed are weathered shale layers. The bed is on the steep limb of fold 18, the south syncline of Section IV. Scale indicates position in outcrop.

Figure 11. Duplex-like feature. The beds at boundary of Sections I and II (Figure 13) have top-to-the-south sense of layer-parallel shear. The feature appears highly localized, diminishing upward in the exposure and absent at the base of the outcrop. Below the exposed duplex-type feature, the thicker bed is offset in a top-to-the-north sense (arrow).

Deformation of bed form by boudinage is localized in most but not all calcareous layers in the outcrop. Boudins occur on long and short limbs and in at least one hinge. Spacing of the calcareous layers varies from 0.5 to 2 m in Section I, and 1.5 to 3 m in Section VI, a ratio similar to the 0.6 ratio of average bed thickness for the sections (Table 1). The aspect ratio of measured boudins varies from 0.2 to 0.7 with an average of 0.3. Imbrication of boudins occurs at two locations, at another location, boudins are caught up in the hinge of a fold. The sandstone bed closest to the calcareous boudins and layers is frequently conspicuously jointed or thickened, with thickening up to 100 percent over a distance of 3 to 4 meters.

Fractures

Jointing in sandstone layers is typically 1 to 2 times layer thickness but varies considerably, with closer jointing in zones associated with nearby faults or bed movement. Veins or secondary mineralization are rare. A thin layer of calcite partially covers a small fracture in Section I and several hairline veins were found in Section VI. Calcite or quartz slickenfibers in flexural slip movement horizons are also not evident. Shale layers are highly jointed with spacing generally less than layer thickness.

Faulting in the outcrop is common with thrusts across single beds, multiple beds, and limbs. Much of the faulting is at low angle to bedding and consistent with layer-parallel shortening, though occasional higher angle faults cut through hinges or across a fold limb. Ramp faults duplicate beds, producing about five meter displacements in several folds, without preference for north or south-dipping limbs. Where thicker sandstone beds yield in the hinges, thinner beds below commonly repeat by thrusting in the hinge in a top-to-the-south sense.

In the asymmetric chevron-like folds of the outcrop, beds on long limbs are repeatedly thrust in a top-to-the-south sense. Faulting in the kink-like fold is consistent with layer-parallel shortening and flexural slip. Steeply dipping beds bordering the kink-like fold have faults consistent with layer-parallel shortening and with post folding sub-horizontal compression. Faults that are high angle to bedding are mainly confined to Section I, extending across a limb of each fold.

Fold Form

Fold forms vary north to south, from a symmetric concentric-like fold in Section I, asymmetric chevron-like folds in Section II, a kink-like fold in Section III, to asymmetric chevron-like folds in Sections IV and VI (Figure 8). Asymmetry of the folds, defined by (l _L-l _R)/(l _L+l _R) (Figure 12), varies from –0.4 in Section I to near one in Section IV (Table 2).

Figure 12. Asymmetry of folds defined.

The concentric-like fold has a broad anticline bordered by narrow synclines. Chevron-like folds have straight limbs and interlimb angles of 45° to 90°. Wavelengths are 9 to 28 m, amplitudes 3 to 7 m, both greatest in Section VI (Table 2). The kink and asymmetric chevron-like folds have top-to-the-south sense of layer-parallel shear. Plunge of the folds is mainly less than 20° with an average of 13°, and the outcrop face has a slope of 75° to 80°. So much of the outcrop is close to a down plunge view providing a relatively undistorted view of the folds.

Table 2. Summary of fold form by section (Figure 8). Because of cover, values for Sections II and IV are based only on axis position and gross fold form without tracing bedding from one axis to another. The average plunge for Section II is calculated separately for east and west plunging folds.

Bed Form

Thickness of beds varies considerably in zones of Section I. Beds pinch out from the top and bottom on the far north and south of fold 2 (folds are numbered just above the horizontal scale), roughly symmetric to the fold axis, at the 7 to 8 m and 21 to 22 m marks on baseline (Figure 13). On the south limb of fold 2, a bed coming down from the kink-like fold at 17 m thickens then thins again at the base of the exposure. Beds on the north-dipping limb of fold 3 are thicker than those of the adjacent south-dipping limbs (Figure 14). Thicker beds extending into the core of fold 4 become lenticular or deformed with variable thickness (Figure 14).

Figure 14. Folds 3 and 4 of Section I. Beds thicken on the north-dipping limb and in the core of fold 4. A set of beds is repeated the length of the north-dipping limb at the contact below a and again at b. On the south limb of fold 3 these beds are again faulted in a top-to-the-south sense at the top and base of exposure. Stepping offsets in south-dipping, thin beds of fold 4, show the effect of top-to-the-north faults that cut thin beds but indent a larger bed toward the fold interior. Top-to-the-north offset is also seen on the south-dipping limb of fold 3 at lower left and at mid-level. The hatched bed in the core of fold 4, a calcareous layer, appears to influence lateral motion of beds immediately to the south. c, d, and e are points of reference between the photograph and drawing. The black horizontal line crossing the photograph is an overhead wire.

At the south boundary of Section I, a thicker than average bed exhibits layer-parallel shortening with a top-to-the-south sense of shear in the form of duplex-like structures (Figure 11). The feature appears localized above an area of sheared beds to the south and above the kink-like fold south of hinge 4. At base of the outcrop, the duplex-like structures are not present.

Calcareous boudin layers spaced 0.5 to 2 m apart in the section, are more numerous in the limbs of fold 3 than fold 2 (Figure 13). On the south-dipping limb of fold 2 at 17 m, several boudins are thrust one onto another.

Fold Form

The folds of Section I are dominated by a longer wavelength concentric-like fold bordered by shorter wavelength chevron-like folds on each side (Figure 13). North of fold 1 at 0 to 5 m, the exposed chevron-like folds have shorter wavelength and lower amplitude. At the south end of the section, thinner beds are folded into a kink-like feature. A small asymmetric fold with the short limb facing north initiates on the south limb of the concentric-like fold and continues vertically into the upper layers.

Axial planes of the folds in Section I tilt up to 10° N (Figure 15). A bed followed through the section is constrained in an envelope narrowing to the south, with a medial axis dipping 12° S. A distinctive calcareous layer below the bed becomes boudinaged on the south-dipping limb of fold 2, and forms a smooth rounded arc in the core of fold 4 (Figure 14).

Fractures

Most faults of Section I are consistent with layer-parallel shortening. Low-angle thrusts cut beds on the south limb of fold 1 and both limbs of the concentric-like fold (Figure 13). The thrusts form classic wedges as described by Cloos at 11.5 m and 13.5 m. Thrusts across a set of thinner beds form an immature duplex type structure folded into fold 3 with apparent offset of 1.2 to 1.6 m on the upper thrust, 0.3 m on the lower (Figure 16). The 2.6 m spacing of the thrusts between beds E and F (Figure 16) is large for ramp spacing compared to the mature duplex structure as modeled by Cruikshank. A thrust lower in the hinge of fold 2 crosses several beds and merges into a sub-horizontal fault cutting the south limb.

Figure 16. Bedding thrusts and sub-horizontal faults in the concentric-like fold of Section I. Thrusts at a and b cut across a set of eight thin beds between beds E and F. Spacing of the thrusts is 2.6 m. Displacement on the upper detachment from a to c is 1.6 m. The lower slip surface has 1.2 m offset. Bed E below the duplication, is 50 percent thicker from the hinge to point b. Bed G is thickened in the hinge and the area above c, above the thrusted bed repetitions on F. At c the upper detachment joins a thrust on layer F and then cuts across the limb to the south. The upper cross-cutting fault, with offset of 4 to 6 cm, offsets a bedding thrust at d. Extending from a bedding thrust in the interior of the fold, the lower cross-cutting fault has offset up to 16 cm diminishing to the right. The bedding thrust in the core of the fold offsets bed A about one meter.

On the south-dipping limb of fold 4, a bedding parallel fault repeats several beds over the length of the limb with top-to-the-south offset (Figure 14). The same set of beds appears thrust again at the base of the hinge, in a top-to-the-north sense, though exposure is poor.

Faulting in the hinges of Section I is mainly confined to sets of thin beds duplicated with a top-to-the-south sense of offset. Thicker beds in fold 2, the concentric-like fold, show minor thickening in the hinge and are jointed just off the crest of the fold. Thicker beds in the core of folds 3 and 4 folded with little evidence of brittle fractures in the hinge, exhibiting a ductile response. Hinges in fold 1 appear compressed, too broken to determine evidence of hinge accommodation.

Removing bedding thrust faults from Figure 13 leaves the series of sub-horizontal faults as shown in Figure 17. The offsets indicated on the south limb of fold 1 may be due to rotation of joint-bounded blocks as limbs rotated in. The interlimb angle of fold 1 is 45° to 50° , which along with broken hinges, suggests continued lateral compression after folding.

The top-to-the-north faults cutting the south-dipping limbs of folds 3 and 4 are in a zone where the interlimb angle is also small, 50° to 55° (Figure 14). The top-to-the-north sub-horizontal faults on the south-dipping limbs of fold 4 cut sets of thinner beds that are offset into thicker adjacent beds. Offset on the faults is less than 10 cm. In the core of fold 4, the arch of a calcareous layer appears to control lateral motion at the base of the exposure (Figure 14). Immediately above, a bed bulges to the south, and thin beds are squeezed into space above and below the bulging thicker bed.

Figure 17. Section I folds and faults not at low angle to bedding. Removing bedding thrusts from Figure 11 leaves only sub-horizontal faults cutting across the south-dipping limbs of folds 2, 3, and 4, and the north-dipping limb of fold 1. These faults appear consistent with continued axial shortening and rotation of limbs, creating interlimb angles of less than 60° .

Differing from the pattern of sub-horizontal faults on the limbs of folds 1, 3, and 4 that are in vertical sequences, the faults on the south limb of fold 2 form a band of deformation (Figure 16). The band is bounded by upper and lower top-to-the-south faults extending from upper and lower bedding thrusts. These faults are compatible with continued N-S compression, either rotating existing joints, or faulting where the lateral motion is accommodated by space, weakness, or both.

Age relationships are suggested where the upper sub-horizontal fault cuts the bedding thrust at d (Figure 16), implying post-folding N-S compression, and where boudins are imbricated, implying layer-parallel compression after boudinage.

Section II

The fold form and symmetry in Section II is less certain due to poor exposure and variable wavelength (Figure 18). Larger folds have a wavelength 9 to 12 m. Fold form is chevron-like, short limb facing south, except possibly for some shorter wavelength folds which are partly covered. Asymmetry increases to the south. Fold 5 seems to have a small positive asymmetry; fold 10 has high positive asymmetry of 0.8 to 1. Similar to Section I, the folds appear to have a medial plane dipping toward the south. A shear zone at 82 m leads into the next section where steeply dipping beds bound a kink-like fold.

Figure 18. Section II and Section III folds. The asymmetric folds of Section II have bedding thrusts on long limbs and near vertical to overturned short limbs. Bedding is dominated by several 50 to 60 cm strong layers. Shorter wavelength folds are visible above the strong layers. Cover and deformation make it impossible to trace bedding through the section. Section III contains steeply dipping beds bordering a kink-like fold where faults are consistent with layer-parallel shortening. A conjugate fault pattern consistent with post-folding N-S compression is evident in the steeply dipping beds. Offset on these faults is minor, less than 15 cm.

Section III

Exposed between steeply dipping plane parallel beds, the kink-like fold of Section III (Figure 18) has short limb facing north. At 90, 115, and 121 m, steep south-dipping beds north and south of the kink-like fold are cut by conjugate faults consistent with post-folding sub-horizontal compression. Offset on these faults is minor, less than 15 cm. Brecciated shale layers bound the faulted zones.

Within the kink-like fold, beds thicken in the south hinge. This corresponds with the greater inter-limb angle of the hinge (Figure 19). Bed 4 thickens between the hinges where bed separations are duplicated by thrusting (Figure 20).

Variation in bedding interface in the kink-like fold is found above layers 1 and 3 (Figure 20) where 5 to 7 cm thick, soft, tan siltstone forms the upper contact at 102, 104.5, and 107.5 m. At the south end of the siltstone at b and c (Figure 20), a calcareous nodule is welded to the underlying sandstone (Figure 21). Layer 1 (Figure 20) below the nodule at b varies in thickness from 28 cm thick to the north, to 55 cm at the base of the exposure. Adjacent to the siltstone and nodules, bedding is more deformed than higher in the folds. Layer 4 has a bed separation or mechanical layering through both hinges that continues north to the area above a (Figure 20). Also above location a, the otherwise planar lower surface of layer 5 is rough immediately north of layer-parallel fracturing extending from a fault near the north hinge.

Figure 19. Idealized kink form imposed on schematic drawing of kink-like fold. Traces of the walls form angles of 77° and 44° with the layering outside the fold. The interior and exterior angles of 72° and 44° on the south limb of the fold reflects greater thickening on the south limb than on the north limb where interior and exterior angles are nearly equal.

Figure 21. Tan siltstone and associated calcareous nodule at base of kink-like fold of Section III. Welded into sandstone bed, the nodule at b2 is 8 by 29 cm with 5 cm of siltstone above. Tan siltstone layer, b1, extends 80 cm north of nodule. Pen (circled) is 13.5 cm long.

Faults in the kink-like fold are concentrated above layer 3 (Figure 20). Offset is predominantly dip slip and consistent with layer-parallel shortening. The greater offsets of 15 and 11 cm occur on the south-dipping faults through the north hinge. Most other faults have offset of 3 to 6 cm. Normal faults bound a small block on layer 6 just below the south hinge.

Immediately below the main exposure of the kink-like fold, is a fault zone at low-angle to bedding (Figure 22). Poles of a fault plane and fractures above the zone lie in the NW quadrant of the stereonet plot (Figure 20), consistent with layer-parallel shortening. Several meters below the fault are two thick 35 cm beds. Areas above and to the north of the thick curved beds are covered.

Figure 22. Thrust fault zone immediately north of (stratigraphically below) the kink-like fold. Poles to measured fractures and fault planes lie in NW quadrant of the stereonet plot (Figure 20). Interlimb angle of the thick bed at far left is about 155°. Rectangle indicates area of Figure 21, location of a calcareous nodule and associated tan siltstone.

Sections IV and V

Chevron-like folds in Section IV have high positive asymmetry and wavelengths of 12 to 20 m (Figure 23). Beds on long limbs are repeatedly thrust in a top-to-the-south sense, similar to asymmetric chevron-like folds in other sections. Limbs of the south syncline, fold 18, are exposed over a greater distance than other Section IV beds, with straight beds similar to the south end of Section VI. The syncline is bordered by covered areas, greater in extent than areas in Sections I to III. South of the cover in Section V, beds repeat by thrusting into a stack at least 20 m high. Beds at the base of the stack are traceable through Section VI.

Figure 23. Section IV and Section V folds. In Section IV, asymmetric chevron-like folds having wavelengths of 12 to 20 m are separated by cover from fold 18 at the south end of the section. The chevron-like folds have a top-to-the-south sense of shear with beds on the long limbs repeated by thrust faults. Deeply weathered shale zones occur on steeply dipping limbs at 145, 156, 178, and 185 m. The south end of Section V has a stack of beds repeated by faults at low angle to bedding except at the top of the exposure. A strong fracture pattern in Section V with spacing of 0.5 to 2 times bed thickness at the base of the exposure, is oriented 100/51 S. Bed K is traced through Section VI.

The steeply dipping limbs of the chevron-like folds have deeply weathered shale horizons at 142, 154, 179, and 185 m, near the exposed cores of folds 14, 16, and 18. Adjacent to the shale zone at 179 m, a bed is deformed into a low amplitude sine wave having wavelength about 2 meters (Figure 10). Pfaff describes short wave trains of asymmetric chevron-like folds that also exhibit the tendency to have bedding plane faults (indicated by deeply weathered shale layers and slickenlines) more dominantly on the short limbs. The folds described by Pfaff are locally developed on the southeast limb of the Bloomfield anticline, central Pennsylvania.

Figure 24. Jointed hinge of fold 14 in Section IV. Thinner beds at a are folded into kink-like folds (fold pattern highlighted in white). Calcareous layer at b tops a highly jointed sandstone bed.

One of the most closely jointed hinges in the outcrop is fold 14, at 142 m (Figure 24). Small scale kink-like folds are evident in a set of thin beds immediately south of the hinge below a sandstone bed topped by a calcareous layer.

Section VI

Faults in Section VI are typically at low angle to bedding, consistent with layer-parallel shortening (Figure 25). Beds on the long north-dipping limb of fold 21, the north anticline in the section, repeat by thrusting, similar to asymmetric chevron-like folds in other sections. By contrast, the north-dipping limb of fold 23 has a top-to-the-north fault at the base of the exposure that offsets the beds about two meters. The beds are amalgamated without a visible fault plane, but the offset in bed pattern seems to be at low angle to bedding. The top-to-the-north offset is not consistent with flexural slip, but is consistent with layer-parallel shortening before folding as a conjugate to the fault crossing the south-dipping limb of fold 21 (Figure 26).

The top-to-the-south fault crossing the south-dipping limb of fold 21 (Figure 27) is at low angle to the bedding orientation above and below the immediate drag fold, consistent with layer-parallel shortening prior to folding. The steeper south-dipping limbs of the section do not have the thick weathered shale horizons seen in Section IV, though thinner brecciated shale horizons are scattered throughout the exposure. Fold 23 has a bed repetition along one of these shale horizons immediately south of the exposed core at 335 m. Four layers are repeated, two the full 5 to 6 m length of the exposed limb (Figure 28).

Figure 28. Duplication of layers in south anticline of Section VI. Black lines at a and b indicate boundaries of bed duplication. The duplication is above a calcareous layer at c. The top-to-the-south offset in the bed below the boudins at c is not consistent with flexural slip. Jointing normal to bedding at e and f meets but does not cross fractures at low angle to bedding. Another calcareous boudin layer occurs immediately above bed K at d.

Layers with a tendency to form boudins, the calcareous layers, are spaced 3 to 4 m apart in Section VI. Layers adjacent to the boudin layers commonly show increased deformation in the form of thickening, jointing, bed duplication, or shale breccia. The duplication of bedding in fold 23 is in layers immediately above a boudin layer (Figure 28). Conspicuous jointing in a 40 cm thick bed at 337 m, overlies a set of thinner beds having a boudin layer at the base (Figure 28). The near normal joints in the layer do not cut the fractures at low angle to bedding. Near the core of the north anticline, fold 21, at 306 m, boudins fold to the south adjacent to the fault crossing the south limb (Figure 29).

Figure 29. Boudin layer in curve of drag fold in fold 21, Section VI. Layers are near the core of the north anticline at 306 m (Figure 25). The boudins and the upper part of the sandstone layer are drag folded to the south by a fault cutting the south limb (arrow). The lower part of the layer is folded to the north into the anticlinal fold.

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