U.S. DEPARTMENT OF THE INTERIOR U.S. GEOLOGICAL SURVEY








Geologic Map of the Scotts Mills, Silverton, and Stayton Northeast 7.5
Minute Quadrangles, Oregon




By

Terry L. Tolan and Marvin H. Beeson






Open-file Report 99-141















1999


  This report is preliminary and has not been reviewed in conformity
  with U.S.  Geological Survey editorial standards or with the North
  American Stratigraphic Code. Any use of trade, product, or firm names
  is for descriptive purposes and does not imply endorcement by the
  U.S. Government,

Geologic Map of the Scotts Mills, Silverton, and Stayton Northeast 7.5
Minute Quadrangles, Oregon

By

Terry L. Tolan  and Marvin H. Beeson
  ________________________________________________________

INTRODUCTION
  The Scotts Mills, Silverton, and Stayton NE 7.5 minute
quadrangles are situated along the eastern margin of the
Willamette Valley and adjacent lower foothills (Waldo and Silverton
Hills) of the Cascade Range (Fig. 1).  The terrain within this area is
of low to moderate relief, ranging from 100 to more than 1000 ft above
sea level. This area is largely rural, with most of the valley floor
and low-relief foothills under cultivation.  In the last decade, the
rural areas outside the boundaries of established towns have
experienced significant growth in new homes built and the expansion of
housing subdivisions.  This growth has placed an increased demand on
existing geologic resources (e.g., groundwater, sand and gravel,
crushed stone) and the need to better understand potential geologic
hazards within this region.
  Previous geologic mapping by Piper (1942), Peck and others
(1964), Newton (1969), Hampton (1972), Miller and Orr (1984),
Orr and Miller (1984), and Miller and Orr (1986, 1988) established and
refined the general stratigraphic framework of this region.  This
mapping identified few faults or folds; earlier investigators were
hindered by the lack of reliably identifiable marker horizons within
the stratigraphic section.  Werner (1991), using available seismic
profile lines and well data in the Willamette Valley to locate the top
of the Columbia River Basalt Group, was able to identify and map faults
within the subsurface.  Reconnaissance mapping of the Columbia River
Basalt Group (CRBG) units in this region in the early 1980s indicated
that these stratigraphic units could serve as a series of  unique
reference horizons for identifying post-Miocene folding and faulting
(Beeson and others, 1985, 1989; Beeson and Tolan, 1990).
  The major emphasis of this investigation was to identify and
map CRBG units within the Scotts Mills, Silverton, and Stayton
NE quadrangles and to utilize this detailed CRBG stratigraphy to
identify and characterize structural features.


ACKNOWLEDGMENTS
  The authors are very grateful to Karl Wozniak of Oregon Water
Resources for providing a tremendous collection of water well
logs along with maps containing the accurate locations of the wells.
We also want to especially thank Richard Blakely of the U.S. Geological
Survey, Menlo Park, for providing quadrangle-scale aeromagnetic maps to
compare to our surface geologic mapping.  We also appreciate the
landowners for their cooperation in permitting us to access outcrops on
their property.


GEOLOGIC SUMMARY General
  The area encompassed by the Scotts Mills, Silverton, and
Stayton NE 7.5 minute quadrangles lies along the eastern margin
of the Willamette Valley and the adjacent foothills of the Cascade
Range (Fig. 1).  The Tertiary volcanic and sedimentary bedrock units
exposed in the Waldo and Silverton Hills generally dip to the west-
northwest into the Willamette Valley where they are covered by a
substantial (30 to >150 m-thick) Plio-Pleistocene sedimentary basin
fill.  The Waldo and Silverton Hills are not a simple dip-slope, but
are deformed by a series of  northwest- and northeast-trending fault
zones that create numerous fault-bounded blocks (horst and grabens).
This, coupled with unconformable relationships between the Tertiary
volcanic and sedimentary units, creates a much more complex pattern
that the topography alone suggests.

Little Butte Volcanics
  The oldest unit within the map area is the Oligocene Little
Butte Volcanics which is exposed in the northeast portion of
the Scotts Mills quadrangle.  This unit was originally mapped and
described in this area by Harper (1946) as Molalla Formation and
pre-Butte Creek lavas.  In their mapping of the western Cascade Range
in Oregon, Peck and others (1964) assigned Harpers (1946) unit to the
Little Butte Volcanic Series of Wells (1956).  The Little Butte
Volcanics represents an early phase of Cascade arc volcanism (Peck and
others, 1964;  Hammond, 1979, 1989; Priest and others, 1983).  The
total thickness of this unit is not known within the map area, but is
>300 m-thick based upon surface and subsurface data.  Hampton (1972, p.
13) suggests that the Little Butte Volcanics may exceed 2100 m in
thickness in this region based upon data from a petroleum exploration
well (Humble Oil and Refining Company - Wicks Number 1; see Werner,
1991, p. 69) located just off the eastern margin of the Stayton NE
quadrangle (sec.11, T7S, R1E).
  In the Scotts Mills quadrangle, exposures of  the Little Butte
Volcanics reveal that it consists dominantly of  interbedded
basalt, basaltic andesite, and porphyritic andesite flows (Table 1)
with very minor amounts of intercalated volcaniclastic and pyroclastic
deposits.  Little Butte flows appear to have been generally emplaced as
a series of relatively narrow, 5-15 m-thick, shoe-string pahoehoe and
aa flows.  Thick flows (>20 m-thick) are much less common.  Little
Butte flows display a wide range of primary (cooling joint) jointing
patterns, ranging from platy columnar to entablature-colonnade.  Platy
cooling joints can range from horizontal to vertical and impart a
shale-like appearance to many Little Butte outcrops.  No pillow or
hyaloclastite complexes were found associated with Little Butte flows
in the Scotts Mills quadrangle.  No trace of eruptive centers (dikes or
vent complexes) for the Little Butte flows in the Scotts Mills
quadrangle were found during our investigation.  However, the general
nature and characteristics of these Little Butte flows suggest that
they may have been products of shield and composite volcanoes.
  Typically only the blocky and columnar center portions of
Little Butte flows are seen in natural exposures.  This is
because Little Butte vesicular flow-tops and scoriaceous interflow
zones (e.g., flow top breccias, flow levees) are weathered and altered
and, in some extreme cases, these zones have been completely converted
to clay.  The original character of these clay beds can only be
determined from occasionally preserved relic textures.  Primary cooling
joints within the dense interiors of the flows are typically altered to
some extent, displaying reddish to yellow-green colors caused by
secondary mineralization.
  The age of the Little Butte Volcanics exposed within, or
immediately adjacent to, the Scotts Mills quadrangle is poorly
constrained.  Peck and others (1964, p. 8) assigned an Oligocene to
early Miocene age to this unit based on fossil leaf data from a tuff
bed locality that lies northeast of the map area.  Peck and others
(1964) and Hampton (1972) noted that the Little Butte Volcanics
interfingers with, and is overlain by, thick sequence of marine
sedimentary rocks (Scotts Mills Formation of Miller and Orr (1986,
1988)).  Our mapping within the Scotts Mills quadrangle (Garret Creek
area - sec. 2 and 45, T6S, R1E), and water well drillers logs from this
same area, also suggest this interfingering relationship.  Miller and
Orr (1986, 1988) assign a middle Oligocene age to the Scotts Mills
Formation (their Marquam Member) in the Garret Creek area, but indicate
that the Scotts Mills Formation unconformably overlies the Little Butte
Volcanics (Miller and Orr, 1984, 1986, 1988).  No isotopic age
determinations have been made on Little Butte flows in, or near, the
map area.  Potentially correlative flows located both east and south of
these maps have been dated using K-Ar and 40Ar-39Ar methods (Lux, 1981;
Verplanck, 1985) and range from early Oligocene to early Miocene in
age.
  The exposure of Little Butte Volcanics, and continued presence
to the west (>10 km) in the subsurface (Werner, 1991), raises
an interesting question as to the reason for their occurrence away from
the main eruptive axis of the Oligocene Western Cascade Range.  There
appears to be an intriguing similarity in the overall distribution
patterns of the westward extension of the Little Butte Volcanics and
portions of the Plio-Pleistocene Boring Lavas. Boring vents and flows
are found along the western margin of the Portland Basin, many tens of
kilometers from the main axis of High Cascade volcanism.  Faults
associated with the northwest-trending Portland Hills- Clackamas River
structural zone (Fig. 2) provide a pathway for Boring magmas to migrate
away from the axis of High Cascade volcanism.  Perhaps the initial
development of the northwest- trending faults associated with the Gales
Creek-Mount Angel structural zone (Fig. 2) provided similar vertical
pathways along which the more fluid Little Butte basaltic and andesitic
magmas could migrate outward from the main axis of Western Cascade
volcanism.

Scotts Mills Formation
  Overlying the Little Butte Volcanics is a thick (>1000 m)
sequence Oligocene to Miocene marine inner neritic to
terrestrial sediments; these sediments are the oldest exposed rocks on
both the Silverton and Stayton NE quadrangles.   In previously
investigations, these sediments were informally named the Butte Creek
beds by Harper (1946) and subsequently called marine tuff and sandstone
by Peck and others (1964) and marine rocks by Hampton (1972).  As a
result of their mapping and study, Miller and Orr (1986) proposed the
name Scotts Mills Formation for these sediments and designated the type
section as the exposures along Butte Creek, below and above the town of
Scotts Mills.  Miller and Orr (1986) further subdivided the Scotts
Mills Formation into three intercalated, time-transgressive members,
the Marquam, Abiqua, and Crooked Finger Members.  The extent of these
members have been previously mapped for the Scotts Mills (Miller and
Orr, 1984) and Stayton NE (Orr and Miller, 1984) quadrangles.  The
reconnaissance-style maps of Miller and Orr (1984) and Orr and Miller
(1984) do not provide enough detailed information for us to adequately
differentiate members of the Scotts Mills Formation.  Moreover, we
neither had the experience or time to reliably sort out these units
ourselves.  Therefore, we were compelled to map these sediments as
undifferentiated Scotts Mills Formation on all three quadrangles.
  The Scotts Mills Formation consists of a diverse range of
marine sandstones, siltstones, conglomerates, and rare
limestones as well as less common fluvial/paludal deposits consisting
of conglomerates, sandstones, siltstones, claystones, and coals.
Marine sediments range from volcaniclastic to siliciclastic in
composition and are locally highly fossiliferous (Miller and Orr, 1986,
1988).  Scotts Mills sediments are typically moderately to well
indurated, with cementing agents ranging from calcium carbonate to
silica. Despite their relative hardness, they tend to develop
moderately thick soils and vegetative cover on natural slopes.  Within
the map area, the best continuous exposures of the Scotts Mills
Formation are found in stream bottoms (Scotts Mills quadrangle - Butte
Creek, Abiqua Creek; Stayton NE quadrangle- upper Pudding River (sec.
37, T8S, R1W), Drift Creek (sec. 25, T7S, R1W)).  Farm roads on
Missouri Ridge (Scotts Mills quadrangle  secs. 11, 12, 13, T6S, R1E)
and the ridge between Abiqua and Alder Creeks (Scotts Mills quadrangle
- secs. 34, 35, T6S, R1E) also provide intermittent exposures of this
formation.  For a complete description of the sedimentology,
paleontology, and inferred depositional history of the Scotts Mills
Formation see Miller and Orr (1986, 1988).
  Miller and Orr (1986, p. 139-141; 1988, p. 959) state that the
basal portion of the Scotts Mills Formation unconformably
overlies the Little Butte Volcanics, with as much as 100 m of erosional
relief developed on the Little Butte surface prior to the deposition of
the Scotts Mills Formation.  They conclude that the basal portion of
the Scotts Mills Formation (their Marquam Member) was deposited during
a period of marine transgression into a quiescent ancestral Cascade
volcanic arc (Miller and Orr, 1986, p. 143).  This conclusion seems to
contradict the previously reported interfingering of Little Butte
Volcanics and marine rocks (Scotts Mills Formation) by Hampton (1972,
p. 16-17).  Relationships between exposures of Little Butte andesite
flows and Scotts Mills sediments in the upper reaches of Garret Creek
on the Scotts Mills quadrangle could be interpreted to indicate that
these two formations are intercalated or conversely that a series of
faults juxtapose these units and simply give the appearance that they
are interfingered.  Subsurface data obtained from water well drillers
logs from this same area indicate that Little Butte flows are
interbedded with epiclastic sediments which we infer to be Scotts Mills
Formation.  This subsurface data compels us to favor an interfingering
interpretation between the Little Butte Volcanics and Scotts Mills
Formation.  These data support Hamptons (1972) conclusion that the
marine rocks (Scotts Mills Formation) are at least in part
interfingered with Little Butte Volcanics and indicate that the
Oligocene ancestral Cascade volcanic arc in this area was not
magmatically quiescent as Miller and Orr (1986, 1988) suggest.
  As defined by Miller and Orr (1986, p. 148-150), the  Scotts
Mills Formation grades into, and is overlain by, fluvial
volcaniclastic and pyroclastic deposits of their Molalla Formation.
Their Molalla Formation ranges in age from late Oligocene to early
Miocene and also includes younger middle Miocene deposits that
interfinger with, and overlie Columbia River Basalt Group flows in this
area (Miller and Orr, 1986, Fig 3, p. 141). Minor Molalla Formation
deposits had been previously mapped along the southeast  edge of both
the Stayton NE (Orr and Miller, 1984) and the Scotts Mills (Miller and
Orr, 1984) quadrangles.  In our mapping of the Scott Mills quadrangle,
we found that the deposits previously mapped as Molalla Formation by
Miller and Orr (1984) consisted of micaceous, quartzose, arkosic
sandstones and fossiliferous siltstones that, by definition, belongs to
the Scotts Mills Formation (their Abiqua Member).  Our mapping of the
Molalla Formation deposits on the Stayton NE quadrangle indicates that
they stratigraphically lie above CRBG flows (specifically the basalt of
Sand Hollow) and are more properly considered to be Sardine Formation.
In our mapping, we encountered no unequivocal exposures of Miller and
Orrs (1986) Molalla Formation.  However, some water well drillers logs
describe sediments that could possible belong to either Miller and Orrs
(1986) Molalla Formation or their Crooked Finger Member of the Scotts
Mills Formation.


Columbia River Basalt Group (CRBG)
  Middle Miocene CRBG lava flows present within the map area
represent the distal portions of voluminous continental flood
basalt flows that were erupted from linear fissure system located
hundreds of kilometers to the northeast in northeastern Oregon and
southeastern Washington (Beeson and others, 1985, 1989).  More than 300
flood-basalt flows were produced over a 11 million year span of
Columbia River basalt volcanism from 17 to 6 Ma (Swanson and others,
1979; Tolan and others, 1989).  CRBG flows eventually covered more than
163,000 km2 in Washington, Oregon, and Idaho (Fig.  2) and have a total
estimated volume in excess of 174,000 km3 (Tolan and others, 1989).
While Columbia River basalt eruptive activity spanned an 11 million
year period, more than 96 volume-percent of the CRBG was emplaced over
a 2.5 million year span from 17 to 14.5 Ma (Tolan and other, 1989;
Reidel and others, 1989; Swanson and others, 1979).  Flows emplaced
during this peak period of eruptive activity were often of
extraordinary size, exceeding 1,000 km3 in volume and covering many
tens of thousands of square kilometers (Tolan and other, 1989; Reidel
and others, 1989; Reidel and Tolan, 1992).
  The relatively great distances that CRBG flows traveled from
their vent systems is attributable to a combination of both
intrinsic properties (i.e., huge volume and rapid eruption rates) and
external paleotopographic and paleoenvironmental factors (Reidel and
others, 1994; Beeson and Tolan, 1996; Beeson and others, 1985, 1989).
Flow of lava away from the linear fissure systems was directed by
regional-scale tectonic features (i.e., Palouse Slope, Pasco Basin,
Columbia Trans-Arc Lowland; Fig. 2) and continued regional subsidence
(Reidel and others, 1994; Beeson and others, 1989; Reidel and Tolan,
1992) that combined to produce a westward, regional down-gradient
pathway.  The Columbia Trans-Arc Lowland (Fig. 2) provided voluminous
CRBG flows a lowland route across the middle Miocene Cascade Range into
western Oregon and Washington (Beeson and others, 1989; Beeson and
Tolan, 1990, 1996).  Once in western Oregon, the paths of these flows
were strongly influenced by major structural features (Fig. 3) and
paleoriver canyons of the ancestral Columbia River system (Beeson and
others, 1985, 1989; Beeson and Tolan, 1990, 1996).
  Significant variations in the lithologic, geochemical, and
paleomagnetic properties between CRBG flows has allowed  this
sequence of flood basalt flows to be subdivided into a host of
stratigraphic units (Fig. 4) that can be reliably identified and mapped
on a regional basis (Swanson and others, 1979; Reidel and others, 1989;
Tolan and others, 1989; Beeson and others, 1985, 1989; Wells and
others, 1989).  We have employed these criteria to identify and map the
CRBG units within the Scotts Mills, Silverton, and Stayton NE
quadrangles.
  Arrows on Figure 4 denote the CRBG units that are present
within the map area.  The composite CRBG section for the map
area consists of approximately 10 sheet flows; seven flows belonging to
members of the Grande Ronde Basalt and 3 flows belonging to units of
the Frenchman Springs Member of the Wanapum Basalt (Fig. 4).  The
thickness of the CRBG section within the map area varies greatly,
ranging from 0 to > 180 m-thick. This variation in thickness reflects
the existence of pre-CRBG topography (hills and valleys) within the map
area.  This paleotopography appears to have been created by deformation
along a series of northwest- and northeast-trending fault zones that
transect this region.  These fault zones produced an irregular pattern
of up- and down-faulted blocks (horst and graben structures) within the
Little Butte Volcanics and Scotts Mills Formation.  The first CRBG
sheet flows to enter the map area (Ortley and Umtanum Members, Grande
Ronde Basalt; Fig. 4 and 5e, f) were largely confined to these lows and
consequently have a rather restricted distribution; subsequent Grande
Ronde flows (Winter Water member; Fig. 4 and 5d) eventually inundated
these lows thus allowing later CRBG flows to be emplaced over a wider
area.
  One phenomenon commonly found associated with the earliest CRBG
flows (Ortley, Umtanum, and Winter Water members) is the
formation of  thick flow lobes.  We think that the formation of such
flow lobes is a response to paleoground conditions that resulted in the
rapid extraction heat from the advancing lava (e.g., paleoground
surface consisting of wet Scotts Mills sediment).  Such conditions
would inhibit rapid lava flowage and cause the sheet flow to form
energy-conserving tongues that would be inflated as lava tried to
advance.  Occasionally these early flows would also burrow into
(invade) and deform the underlying Scotts Mills sediment.  Excellent
examples of this phenomenon can be found within the map area.
  On the Stayton NE quadrangle in an abandoned quarry (sec. 55,
T7S, R1W), a single Winter Water flow invaded older Scotts
Mills sandstone creating complex series of flow lobes and
discontinuous, contorted sedimentary interbeds.  The presence of a
baked zone along the bottom of the interbeds as well as dikelets
injected upwards into the interbeds makes this a classic example of an
invasive CRBG flow.  Several additional examples of invasion involving
the Winter Water flow can be found on the Scotts Mills quadrangle.  The
first can be found in a small quarry along Butte Creek Road (SW1/4 sec.
24, T6S, R1E).  Here lobes of a Winter Water flow have invaded
sandstones and siltstones of the Scotts Mills Formation and display
similar features and relationships found in the quarry on the Stayton
NE quadrangle.  Perhaps the most interesting example of a CRBG flow
invading Scotts Mills Formation can be found immediately downstream
from the dam on Butte Creek at the town of Scotts Mills.  Here the
Winter Water flow has invaded Scotts Mills siltstones and sandstones
producing many of the same features described above.  However here the
invading Winter Water lava  caused the formation of a small,
asymmetrical anticlinal fold within the Scotts Mills sediments.
  When present, the Ortley and Umtanum Members can consists of up
to four flows and have a collectively thickness that ranges
from 10 to > 60 m.  The Winter Water member consists of up to two flows
and have a collectively thickness that ranges from 5 to >40 m.  Ortley,
Umtanum, and Winter Water flows appear very similar in outcrop, all
generally displaying an entablature/colonnade jointing and only rarely
exhibiting an entirely blocky to columnar jointing pattern.  All of
these flows are very fine-grained to glassy in hand sample; Winter
Water flows can be distinguished from older aphyric Ortley and Umtanum
flows by the presence of small (<0.3 cm in size) plagioclase
glomerocrysts that often display a distinctive radial or spoke-shaped
habit.  Distribution of plagioclase glomerocrysts within Winter Water
flows are often uneven and they tend to be less abundant in the basal
portion of the flows.  Thin (<0.3 m-thick), discontinuous fluvial
volcaniclastic sedimentary interbeds were occasionally found between
these older Grande Ronde flows; the only significant sedimentary
deposits associated with these flows are found in places were they
invaded the older Scotts Mills Formation as noted above.  Vesicular
flow tops of Ortley, Umtanum, and Winter Water flows typically display
only minor evidence of weathering or alteration.  No notable
pillow/hyaloclastite complexes were found at the base of these flows,
despite the fact that they were the earliest CRBG flows to enter this
area.
  A single Sentinel Bluffs flow (Fig. 4 and 5c) represents the
youngest Grande Ronde member within the map area.  This
Sentinel Bluffs  flow has the greatest areal extent of all the Grande
Ronde units present within the map area.  The greater areal extent of
this flow is probably not simply related to its size; data suggest that
Winter Water flows were much more voluminous than Sentinel Bluffs flows
(Reidel and others, 1989).  We think that the greater areal extent of
the Sentinel Bluffs flow is largely the result of earlier Grande Ronde
flows (Ortley, Umtanum, and Winter Water members) that inundated and
leveled the existing paleotopography also created paleoenvironmental
conditions that allowed this Sentinel Bluffs flow to spread evenly and
more thinly in response to the topography. The single Sentinel Bluffs
flow within the map area ranges from 5 to 25 m-thick.  This flow
typically displays a well developed blocky to columnar jointing style
and only exhibits an entablature/colonnade pattern at its flow margins
where it onlaps older topography (e.g., Stayton NE quadrangle - Drift
Creek quarry on Victor Point Road; and in section 34, T5S, R1E on the
Scotts Mills Quadrangle).  This flow is medium-grained, occasionally
diktytaxitic, and rarely  plagioclase phyric with small (<0.5 cm in
size) tabular phenocrysts.  In the field it can be distinguished from
older Grande Ronde flows on the basis of hand sample lithology and
stratigraphic position.  The Sentinel Bluffs flow also has a
distinctive geochemical composition that readily distinguishes it from
older Grande Ronde flows and younger Frenchman Springs Member flows
found within the map area.
  The vesicular flow top of the Sentinel Bluffs flow is often
moderately to deeply weathered with the glassy groundmass
between vesicles altered to clay. This decomposition often imparts a
cream to yellowish white bleached appearance to the Sentinel Bluffs
flow top and gives it the outward appearance of a sedimentary rock.
This relatively severe weathering is the result of a substantially
longer period of exposure to subaerial weathering (>200,000 years vs.
10,000 to 30,000 year hiatus between emplacement of older Grande Ronde
flows) before being covered by the next CRBG flow (Beeson and others,
1985, 1989; Beeson and Tolan, 1996).  This hiatus in CRBG volcanism
permitted soils to develop on Sentinel Bluffs flow tops and rivers and
streams to reestablish drainage systems across areas inundated by
Grande Ronde flows.  Within the map area, a thin (0.3 to >3 m-thick)
interbed overlies the Sentinel Bluffs flow.  This interbed ranges from
a paleosol developed on the Sentinel Bluffs flow top to fluvial
volcaniclastic and siliciclastic siltstones and sandstones.  Springs
and seeps are commonly found along this horizon.  Interpretation of
water well drillers logs suggests that this sedimentary interbed may be
much thicker (3 to >20 m-thick) in the Willamette Valley (Silverton
quadrangle) than in the Waldo and Silverton Hills to the east.
  On the Columbia Plateau, sedimentary deposits found within the
stratigraphic interval defined by the top of the Grande Ronde
Basalt and the base of the Frenchman Springs Member (Wanapum Basalt)
belong to the Vantage Member of the Ellensburg Formation (Swanson and
others, 1979).  This same stratigraphic interval can be recognized in
western Oregon and the term Vantage interbed or, lacking the presence
of sediments, Vantage horizon is often informally employed by CRBG
stratigraphers (Beeson and Moran, 1979; Beeson and others, 1985, 1989;
Beeson and Tolan, 1990, 1996).  In the map area, Miller and Orr (1986)
have defined all sediments interfingered with the CRBG as belonging to
their Molalla Formation.  However sediments found on the Vantage
horizon within the map area are too discontinuous and thin to depict on
the maps as a separate unit; when present, these sediments are included
in the overlying Frenchman Springs unit.
  The long hiatus in CRBG eruptive activity following the
emplacement of the Sentinel Bluffs Member allowed the ancestral
Columbia River system to reestablish and new path through western
Oregon; this hiatus also allowed significant topographic relief to
develop on the Yakima Fold Belt structures within the Columbia
Trans-Arc Lowland and on major northwest-trending fault zones in
western Oregon (Fig. 3).  These features were a significant control
over the eventual areal extent of subsequent CRBG flows that advanced
into western Oregon and Washington (Beeson and others, 1985, 1989;
Beeson and Tolan, 1990, 1996).
  The first Frenchman Springs flow to enter western Oregon was
the basalt of Ginkgo (Fig.  4).  As the Ginkgo flow advanced
into the Columbia Trans-Arc Lowland, it encountered the head of the
ancestral Columbia River canyon (Fig. 3).  The Ginkgo flow used the
ancestral Columbia River canyon as a ready-made conduit to the west,
but in the process totally overwhelmed and destroyed this course of the
ancestral Columbia River (Beeson and others, 1985; Beeson and Tolan,
1996).  Ginkgo lava did over top the ancestral Columbia River canyon
and spread laterally as a sheet flow (Beeson and Tolan, 1996), but did
not reach the map area which lies west and north of this former course
of the Columbia River (Fig. 3).  The next Frenchman Springs flows, the
basalt of Silver Falls (Fig. 4), followed the same generally pathway
into western Oregon (Fig.  5b) as the Ginkgo flows.  However, the
Silver Falls flows were not confined to a deep canyon and were emplaced
as sheet flows.
  In the map area, the basalt of Silver Falls consists of one or
two flows and collectively ranges from 15 to 45 m in thickness.  
Silver Falls flows are commonly blocky to columnar jointed,
with individual columns often exceeding 1 m in diameter. Both flows are
typically medium- to coarse-grained, diktytaxitic, and abundantly
microphyric with acicular laths of plagioclase that readily standout
from the groundmass.  The abundantly microphyric texture of the Silver
Falls flows is its most characteristic lithologic property (Beeson and
others, 1985, 1989).  The upper flow is sparsely phyric with
plagioclase phenocrysts that range from 0.5 to 3 cm in size.  The lower
flow is typically abundantly plagioclase phyric with glomerocrysts
ranging from 0.5 to >3 cm in size.  Silver Falls flows can usually be
distinguished in the field from both the older Sentinel Bluffs flow and
younger Sand Hollow flow on the basis of stratigraphic position and
lithology; chemical composition of the Silver Falls flows is distinct
and can be easily distinguished from either Sentinel Bluffs or Sand
Hollow flows (Table 1).
  The last CRBG unit to enter the map area was a single basalt of
Sand Hollow flow (Fig.  4).  Up to five Sand Hollow flows
reached western Oregon (Fig. 5a), but only a single flow advanced into
the map area.  Best exposures of this Sand Hollow flow are found on the
Stayton NE quadrangle were it underlies much of the Waldo Hills.  Here
the Sand Hollow flow ranges from 10 to 20 m-thick and displays a
entablature/colonnade jointing pattern which is an uncommon jointing
style for Frenchman Springs flows.  This flow is typically glassy to
fine-grained and sparsely plagioclase phyric with glomerocrysts <2 cm
in size.  In the field it can be readily distinguished from older
Silver Falls flows on the combined basis of jointing habit and
lithology - specifically the lack of abundant plagioclase
microphenocrysts.  In hand sample the Sand Hollow flow does bear some
resemblance to the Sentinel Bluffs flow, but can be distinguished by
the presence of occasional, large, plagioclase phenocrysts.  The Sand
Hollow flow can also be readily distinguished from other CRBG units in
the map area on the basis of it chemical composition (Table 1).
  Being the last CRBG flow emplaced within the map area, the Sand
Hollow flow  was subjected to a long period of subaerial
exposure. Consequently this flow is often much more deeply weathered,
especially the vesicular flow top and major vertical cooling joints
within the entablature/colonnade.  This weathering pattern results in
the production of the large, subrounded, hackly boulders that plague
many farm fields on the Stayton NE quadrangle.
  A discontinuous interbed, consisting of fluvial volcaniclastic
siltstones/sandstones or poorly developed paleosols, is present
at the contact between the Silver Falls and Sand Hollow flows in the
map area.  When present, the interbed is typically < 3 m-thick and is
not mapped as a separate unit due to its sporadic occurrence and
thinness.  Numerous springs and seeps emanate from the Sand
Hollow/Silver Falls contact.

Sardine Formation
  In the map area, the middle Miocene to early Pliocene Sardine
Formation is found in the southeastern part of the Stayton NE
quadrangle.  The Sardine Formation ranges from 5 to >30 m- thick on the
Stayton NE quadrangle and unconformably overlies the older CRBG and
Scotts Mills Formation.  The Sardine Formation appears to generally
mantle these older surfaces; no notable erosion or deep incisions into
these pre-Sardine surfaces were found within the map area.
  Exposures of the Sardine Formation on the Stayton NE quadrangle
consists predominately of medial to distal volcaniclastic
deposits produced by, and related to, western Cascade arc volcanism
(Peck and others, 1964; Hampton, 1972; Priest and others, 1983;
Hammond, 1989).  Sardine volcaniclastic deposits consist of poorly
indurated, fluvial pebble/cobble conglomerates, sandstones, and
siltstones that are interbedded with the distal portions lahar run-out
deposits.  The upper surfaces of many of these beds display evidence of
paleosol formation.  The pebble and cobbles within the Sardine
conglomerates appear to be predominately composed of Cascadian basalt,
andesite, and dacite lithologies.  Sardine volcaniclastic deposits are
often moderately to severely weathered and typically develop deep
soils.
  Pyroclastic flows and lava flows are reported by Peck and
others (1964), Hampton (1972), and Hammond (1989) to constitute
a major component of the Sardine Formation in this region.  However, no
notable pyroclastic deposits were found within the map area and only a
single, 10 m- thick, blocky andesite flow was found to be interbedded
with volcaniclastic Sardine sediments on Waldo Hills Drive (sec. 12,
T7S, R1W).
  A middle to late  Miocene age was originally assigned to the
Sardine Formation by (Peck and others, 1964, p. 34-35) on the
basis of fossil flora collected from this formation outside the map
area.  Peck and others (1964, p. 35) inferred that the upper portion of
the Sardine Formation could be as young early Pliocene.  Hampton (1972,
p. 20-21) inferred that the upper age of the Sardine Formation must be
early Pliocene based on interfingering relationships with the Troutdale
Formation.  No isotopic age determinations are reported for this
formation.

Quaternary-Tertiary Sediments
  Undifferentiated late Miocene to Pleistocene fluvial sediments
unconformably overlie the CRBG in the central and northern
Willamette Valley.  Within the map area, this unit is only encountered
in the subsurface; water well drillers logs describe it as consisting
of unconsolidated to poorly indurated sands, silts, clays, and
gravels.  This unit is, in part, contemporaneous with the Sardine
Formation and has been inferred to represent the southern extent of the
Troutdale Formation in the north-central Willamette Valley (Peck and
others, 1964; Hampton, 1972).  These sediments are thought to present
deposits of an ancestral Willamette River and its tributaries.

Quaternary Alluvium
  The Quaternary alluvium within the map area is informally
subdivided into older (Qoal) and younger (Qal) deposits.  The
younger alluvial deposits are generally restricted to recent (Holocene)
stream valleys.  The older alluvial deposits (Pleistocene) consist of
both local fluvial deposits and Cataclysmic (Missoula or Spokane
Floods) flood deposits. Fluvial deposits within the older
alluvium consist of poorly to moderately indurated, fluvial basaltic
conglomerates and sandstones that occur as minor terraces along the
major stream valleys within the map area (e.g., Butte,  Abiqua, Silver
Creeks).  These terrace deposits are typically less than 15 m-thick.
In the map area, deposits produced by cataclysmic (Missoula) flood
events generally mantle much of the topography below the 300
ft-elevation in, and immediately adjacent to the Willamette Valley.
These deposits consist predominately of silt and fine sand (Hampton,
1972; McDowell and Roberts, 1987).  Also associated with the
cataclysmic flood deposits are angular to subrounded, pebble- to
boulder-size rocks of exotic lithologies (e.g., granite, quartzite,
gneiss).  These erratics were transported to this area in icebergs
carried by the flood waters (Allen and others, 1986).

Landslide Deposits
  The only large landslide deposit was found on the Scotts Mills
quadrangle along the southwest side of the Butte Creek Valley.
Here hummocky topography consisting CRBG colluvium denotes the extent
of the slide.  The failure plane appears to have been the unconformity
between the CRBG and Scotts Mills Formation.


STRUCTURAL GEOLOGY General
  The nature of flood basalts, such as the CRBG, makes them ideal
for studying structural deformation.  As fluids, although
rather viscous, they flowed into and across the lowest parts of the
topography so that their initial distributional patterns reveal the
locations of topographic features at the time of their emplacement.
Topographic highs greater than the flow thickness would not be covered,
forming a margin of the flow or a hole in the distributional pattern.
Topographic lows, such as a stream valley would be filled with an
anonymously thick deposit.  Since topography results from a combination
of structural deformation, erosion, volcanism, and sedimentary
deposition, the distributional patterns of the flood basalt flows, in
combination with a knowledge of the confining rocks, provides valuable
information about the geologic history to that time.  Also, these flood
basalt flows were very extensive, initially forming rather flat
surfaces that provide reference datum planes for post-emplacement
deformation.  The detailed mapping of CRBG units in the Scotts Mills,
Silverton, and Stayton NE quadrangles makes it possible to delineate
the topography existing prior to the emplacement of CRBG flows and also
the structural deformation occurring since the middle Miocene.
  The distribution and deformation of the CRBG flows in this area
reflect the more regional structural features.  In general, the
CRBG flows form a homocline that dips away from the axis of the Cascade
Range and towards the Willamette Valley.  The tabular CRBG unit plunges
beneath the Willamette Valley sedimentary fill at about the 250 ft
elevation and descends to a maximum depth of about 1300 ft below sea
level in the center of the valley.  Previous geologic studies have
identified three major structural features within our map area, the
northeast-trending Scotts Mills Anticline (Peck and others, 1964;
Miller and Orr, 1984; Werner, 1991), the northeast-trending Waldo Hills
range-front fault (Werner, 1991); and the Mount Angel Fault (Hampton,
1972; Beeson and others, 1985, 1989; Werner, 1991; Werner and others,
1992).  Our mapping provides new data on these features as well as
identifying many new structural features.
  The structural pattern and history as determined from the
mapping of these three quadrangles is subject to modification
as we continue mapping other quadrangles in this area.  We consider
this to be a progress report in a much longer study.  We are looking at
a fairly small part the regional pattern and will need to study the
larger area in order to better understand the nature and timing of
deformation in this area.

Scotts Mills Anticline
  Peck and others (1964) identified a northeast-trending
anticline that they mapped extending from the vicinity of
Victor Point School (sec. 36, T7S, R2W, Stayton NE quadrangle) to the
confluence of Trout Creek and the Molalla River (T6S, R3E).  This fold
was defined on the basis of bedding attitudes measured in marine tuff
and sandstone (Scotts Mills Formation) and the attitudes of flows and
beds within the Little Butte Volcanics.  Mapping by Orr and Miller
(1984, 1986) and Miller and Orr (1984) redefined the location and
extent of the southwestern portion of this fold.  Orr and Miller (1984,
1986) mapping depicted this fold dying out east of the Stayton NE
quadrangle (sec 33, T7S, R1E, Drake Crossing 7.5 minute quadrangle) and
redefined the trace of the fold axis across the southeast corner of the
Scotts Mills quadrangle (Miller and Orr, 1984).  These changes were
apparently based on 8 to 10 attitude measurements on beds within the
Scotts Mills Formation (Orr and Miller, 1984, 1986; Miller and Orr,
1984).  A cross-section through the Scotts Mills Anticline ( see
section A-A, Miller and Orr, 1984) depicts  this feature as a
relatively indistinct, low amplitude fold.
  In our mapping of the Scotts Mills quadrangle, we could not
find compelling field evidence for the existence of  the Scotts
Mills Anticline.  Attitude data measured on bedding planes within the
Scotts Mills Formation do not indicate the presence of a
northeast-trending anticlinal fold as depicted by either Miller and Orr
(1984) or Peck and others (1964).  Distribution and apparent attitude
of the CRBG units do not indicate the presence of a northeast-trending
anticlinal fold.  Instead, our mapping suggests that deformation
observed in the Scotts Mills Formation is likely related to block
faulting (both pre- and post-CRBG emplacement).  Windows through the
CRBG into older marine sediments of the Scotts Mills Formation and
Cascadian volcanic rocks of the Little Butte Volcanics are common in
this area indicating that they were topographic highs immediately prior
to, and during, the emplacement of CRBG flows.  This exposure pattern
of pre- CRBG rocks indicates that the paleotopography was irregular,
with enough topographic relief to contain up to 180 m of CRBG flows and
still not be totally covered by CRBG flows.  We think that a large
amount of the pre-CRBG topographic relief was produced by structural
deformation (faulting) and augmented by differential erosion along weak
rock zones.

Waldo Hills Range-Front Fault
  Werner (1991) inferred the presence of a major
northeast-trending fault, the Waldo Hills range-front fault,
that defined the margin of the Willamette Basin along the Waldo and
Silverton Hills.  His evidence for this major fault was the linearity
of the range front, distribution of geologic units (Hampton, 1972), and
interpretation of the geology and elevation of units in water wells
across this feature.  Although northeast-trending faults are fairly
common in the CRBG of this area, the northeast-trending topographic
lineament marking the edge of the valley on the three quadrangles we
mapped is probably not a major fault line. Surface mapping of CRBG
flows and interpretation of available water well drillers logs suggest
that the CRBG dips gently into the valley rather than being abruptly
down-faulted at this transition.

Northwest-Trending Faults
  The predominant structural features mapped in the CRBG in these
quadrangles are northwest-trending faults and the associated
cross-faults that typically have either north-south or northeast
trends.  We believe that many of the smaller cross-faults that we have
mapped in the CRBG could be largely confined to the CRBG slab, the
product of brittle deformation in a thin, rigid, slab (CRBG) over a
much less competent material (e.g., sedimentary rocks of the Scotts
Mills Formation).  Only the more continuous faults would project to
great crustal depths.
  The northwest-trending faults are the most continuous
structures identified within the map area, with two being of
particular significance - the Mount Angel fault zone (part of the Gales
Creek - Mount Angel structural zone) and the Waldo Hills structural
zone.

Mount Angel Fault Zone
  The Mount Angel fault was originally identified and mapped by
Hampton (1972) and subsequently considered as part of the
regional-scale, northwest-trending Gales Creek- Mt. Angel structural
zone (Beeson and others, 1985, 1989).  Werner (1991) conducted the
first detailed subsurface analysis of this feature in the  Mount Angel
- Woodburn area based on commercially available seismic-reflection data
and water well data.  His work confirmed that the Mount Angel fault
extended northwest of the town of Mount Angel and consists of  multiple
fault strands that form a positive flower structure and that these
faults offset Pliocene fluvial sediments.  Werners (1991) analysis also
found that the Mount Angel fault (zone) had a complex history, with
deformation related to dextral strike-slip and dip-slip movement.
Evaluation of focal mechanism solutions for recent earthquakes along
the Mount Angel fault also favors a dextral strike-slip solution
(Werner and others, 1992).  The damaging M=5.6 Scotts Mills earthquake
of 1993 was centered about 3 km east of Scotts Mills along the boundary
of the map area.  Focal mechanisms indicate oblique dextral-thrust
motion or a northeast dipping fault plane inferred to be the projection
of the Mt. Angel fault (Madin and others, 1993).
  Mapping of Mount Angel and interpretation of  water well
drillers logs from this area provide some additional insights
into the structural geology of this fault zone.  Our mapping (see
Silverton quadrangle and cross-section) indicates that  the structural
geology along this portion of the Gales Creek-Mount Angel structural
zone is more complex than previously realized (Werner and others,
1992).  Mount Angel fault, as depicted by Werner (1991) and Werner and
others (1992) does extend northwest from the Abiqua Creek valley along
the southwest side of Mount Angel.  The maximum apparent vertical
stratigraphic offset on this fault occurs in the  Mount Angel area
(>200 m) but decreases to < 30 m in the Abiqua Creek valley with an
opposite sense of throw (down to the northeast).  This decrease in
apparent offset and change in throw on the Mount Angel fault in the
Abiqua Creek valley (see Scotts Mills quadrangle) is probably related
to the numerous north-south- and northeast-trending faults (splays)
that intersect this fault zone; deformation has been distributed to
these splays significantly decreasing the magnitude of offset on the
Mount Angel fault.  These cross-trending faults, like most faults
within the CRBG, do not simply consist of a single fault plane, but
instead seem to consist of a series of parallel (anastomosing) faults.
These faults collectively define the fault zone, which can vary in
overall width (from tens to hundreds of meters).  Depending on the
intensity of deformation, CRBG flows between these fault strands can
range from nearly intact (intraflow structures readily recognizable and
undeformed) to high disrupted (shatter breccia).   On Davis Creek
(Scotts Mills quadrangle, NW 1/4 sec. 47, T6S, R1E) quarry operations
in the Winter Water flows (Grande Ronde Basalt) provide good exposures
of  fault strands related to both north- south- and northwest-trending
fault zones.  The apparent vertical stratigraphic offset on these
exposed fault strands is minor (< 3 to 5 m), with the width of the
individual faults (shatter breccia and gouge) typically less than 0.5
m.  The fault planes commonly display sets of both subvertical and
subhorizontal slickenside striae indicating that both dip-slip and
strike-slip movement(s) have occurred on these faults.
  No exposures of the Mount Angel fault are found within the map
area. However along the southeast projection of the Mount Angel
fault in upper Abiqua Creek (just above Abiqua Falls), we have found an
exposure of this fault zone (shatter breccia/gouge) that contains
several near- vertical slip surfaces on which subhorizontal slickenside
striae are found.
  The trace of the Mount Angel fault is inferred to be along the southwest 
side of Mount Angel (Hampton, 1972; Beeson and others, 1989; Werner, 1991).  
Our mapping indicates that Mount Angel is a fault-bounded block (horst) 
that dips to the northeast and plunges off along its long axis to both the 
northwest and southeast.  The Mount Angel horst is further segmented by 
northeast-trending cross-faults (see Silverton quadrangle).
  An apparently similar structure occurs along this same trend
approximately 1.5 km northwest of the Mount Angel horst.  This
uplifted block is separated from the Mount Angel horst by a
fault-bounded low (graben) where the City of Mount Angel is located
(see Silverton quadrangle). This structure is bounded on the southwest
side by the Mount Angel fault and on its southeast end by a
northeast-trending cross-fault.  No data were available to establish
whether faults were present along the northeast and northwest sides of
this structure.  Based on data from water well logs, the apparent
vertical stratigraphic offset on the CRBG across the Mount Angel fault
here is > 200 m.  This offset is of the same magnitude as that at the
Mount Angel horst, but the current topographic expression on this
structure is minimal.  South of Mount Angel, the Mount Angel fault and
another parallel fault form a northwest-trending graben (see Silverton
quadrangle and cross-section).  Subsurface data indicate that this
graben extends northwest from section 50 to at least Highway 214; water
wells in Walker Ditch (south and west of the City of Mount Angel)
suggest that this structure may continue northwestward to the Pudding
River.  The geomorphic expression of this feature and data from water
well drillers logs suggest that Abiqua Creek once followed this feature
to its confluence with the Pudding River northwest of the present-day
city of Mount Angel.  Abiqua Creek now turns west at section 50 and has
abandon this former channel.  Similar westward turn is exhibited by
Silver Creek.  The reason for these drainage changes are not known, but
could be caused by either tectonic events (e.g., deformation along
faults) or the cataclysmic Missoula Floods.
  CRBG distributional patterns indicate that the active  Mount
Angel fault existed during Middle Miocene time.  The Mt. Angel
fault (Gales Creek-Mount Angel structural zone) is one of the major
northwest-trending structural zones that acted as a topographic barrier
to the advance of certain CRBG flows.  Specifically, only the uppermost
Silver Falls flows appears to have advanced past this feature, while
the lower, abundantly phyric Silver Falls flow was stopped by this
structure.  The time interval between the emplacement of the last
Grande Ronde flow (Sentinel Bluffs Member) and the first Frenchman
Springs flow (abundantly phyric Silver Falls flow) is probably the
longest such interval, providing enough time for continuing deformation
along this structural zone to create a topographic obstacle for the
first Silver Falls flow.  The termination of this flow here indicates
that this northwest-trending fault zone was active at least by 15.5
m.y. ago.

Waldo Hills Structural Zone
  Prior to this study, no northwest-trending faults or folds had
been mapped in the southwest portion of the Stayton NE quadrangle 
(Orr and Miller, 1984; Werner, 1991).  The series of aligned
northwest-trending hills in the southwest corner of the Stayton NE
quadrangle was mapped by Orr and Miller (1984) as erosional remnants of
the Sardine Formation.  Our mapping has found that these hills are
CRBG-cored anticlinal folds that are related to a series of
through-going northwest- trending faults that we term the Waldo Hills
structural zone.
  The Waldo Hills structural zone consists of a series of
parallel, northwest-trending faults and elongate domical folds
that cross the southwest corner of the Stayton NE quadrangle.  This
structural zone as been traced for more than 20 km across portions of
the Stayton, Stayton NE, and Salem East 7.5 minute quadrangles, but it
total length is yet to be determined.  The width of this structural
zone is variable, ranging from 1 to >2.5 km.  The Waldo Hills
structural zone is parallel to the Gales Creek-Mount Angel structural
zone (Mount Angel fault zone) and the Portland Hills- Clackamas River
structural zone (Fig. 3); the dominant sense of movement on each of
these zones is probably dextral strike-slip movement (Beeson and
others, 1989; Werner, 1991; Blakely and others, 1995).
  The folds associated with this feature are typically elongate,
domical folds with their long axis either parallel, or subparallel, 
to the overall trend of the Waldo Hills structural zone.  The northeast 
limbs of these anticlines are steeper than their southwestern limbs, 
producing a pronounced asymmetric profile.  Two of these anticlines 
(sec. 78, T8S, R1W) have a northwest- trending fault along the base 
of their northeast limbs.  The overall geometry of the elongate, 
domical folds associated with the Waldo Hills structural zone
is very similar to domical folds associated with northwest-trending,
dextral wrench fault zones in the western Columbia Plateau region
(Anderson, 1987).    Another similarity is that several of the
northwest-trending faults within the Waldo Hills structural zone
display reversals of apparent dip-slip separation (scissoring) along
their strike.  Together, these features strongly suggest that the Waldo
Hills structural zone represents a dextral wrench fault zone.


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Figure 1.  Map showing the location of the Silverton, Scotts Mills, and
Stayton NE 7.5 minute quadrangle.

Figure 2. Generalized sketch map showing the distribution of the
Cascade volcanic arc and Columbia River Basalt Group in relation to
selected regional-scale tectonic features (modified from Beeson and
Tolan, 1990).  The westward-dipping Palouse Slope directed erupting
CRBG lava towards the Pasco Basin and Columbia Trans-Arc Lowland.  The
Columbia Trans-Arc Lowland was a low that transected the mid to late
Miocene Cascade Range and allowed CRBG flows to enter western Oregon
and Washington.  In western Oregon, CRBG flows encountered two major
northwest-trending dextral wrench fault systems, the Portland Hills -
Clackamas River structural zone (PHCR) and the Gales Creek - Mount
Angel structural zone (GCMA).  P = Portland, D = The Dalles, H = Mt.
Hood, A= Mt. Adams, SH = Mt. St. Helens.

Figure 3.  Sketch map showing the locations of major CRBG intracanyon
flow pathways western Oregon and Washington.  Modified from Beeson and
Tolan (1996).

Figure 4.  Stratigraphic nomenclature, age, and magnetic polarity for
Columbia River Basalt Group units (from Tolan and others, 1989).  Black
arrows indicate units that are present within the Silverton, Scotts
Mills, and Stayton NE 7.5 minute quadrangles.  N= normal magnetic
polarity, R = reversed magnetic polarity, T = transitional magnetic
polarity, E = excursional magnetic polarity.

Figure 5.  Maps showing the regional extent of Columbia River Basalt
Group unit, Scotts Mills, and Stayton NE 7.5 minute quadrangles.  A =
basalt of Sand Hollow, Frenchman Springs Member, Wanapum Basalt; B =
basalt Silver Falls, Frenchman Springs Member, Wanapum Basalt; C =
Sentinel Bluffs Member, Grande Ronde Basalt; D= Winter Water member,
Grande Ronde Basalt; E = Ortley Member, Grande Ronde Basalt; F= Umtanum
Member, Grande Ronde Basalt.  Note that Ortley and Umtanum Members are
shown as an undifferentiated unit (TgN2l) on the geologic map and
cross-section.  Distribution maps from Tolan and others (1989) and
Reidel and others (1989).

Table 1.  Chemical composition of Little Butte Volcanics and Columbia
River Baslat Group units
 Scotts Mills, Silverton, and Stayton NE 7.5 minute quadrangles.

Sheet 1.  Geologic map of the Silverton and Scotts Mills 7.5 minute
quadranges, Northwest Oregon.  Sheet 2. Geologic map of the Stayton NE
7.5 minute quadrangle, Northwest Oregeon.