Constraints on the Timing and Geometry of Kula-Farallon Ridge ...

Constraints on the Timing and Geometry of Kula-Farallon Ridge ...

Figure 3A. Age distribution
plot for sand sample 37A.


Joe D. Dragovich1, James H. MacDonald, Jr.2, Shannon
A. Mahan3, Megan L. Anderson4, Jr., S. Andrew DuFrane5,
Curtis J. Koger6, Daniel T. Smith7, Skyler P. Mavor8, and
Jennifer H. Saltonstall6
Our geologic mapping in concert with petrography,
geochronology, geochemistry, geophysics, and
stratigraphy of ten contiguous 7.5 quadrangles from North
Bend to Granite Falls (Fig. 1) indicate that most
Pleistocene nonglacial strata (PNS) have a distinct
Cascade Provenance (CP) fluvial origin. Our correlation of
this CP PNS with the Olympia beds (20-60 ka), Whidbey
Formation (75-130 ka), Hamm Creek unit (178-243 ka),
and pre-Hamm Creek PNS units are supported by 42 14C
and 44 OSL/IRSL ages (Table 1) and by stratigraphic
The CP PNS is distinctly compositionally and
stratigraphically similar to Holocene Snoqualmie,
Skykomish and Pilchuck River (SSPR) alluvium. Point
count petrography of CP PNS sands reveals a granitic
source containing mostly monocrystalline quartz,
plagioclase, K-spar, hornblende, pyroxene, mica and
granitic lithic grains (Table 2 and Fig. 2). The granitic CP
is largely ascribed to Quaternary erosion of the EoceneMiocene Snoqualmie, Index and Mount Pilchuck
batholiths in the upper SSPR basins and is supported by
detrital zircon age data for the PNS (Fig. 3). Higher Pb/Yb
ratios for the CP PNS and Holocene SSPR sediments
also distinguish glacial and CP sediments (Fig. 4). Most
CP PNS basin sediments (e.g. Monroe basin) have a
distinctive modest magnetic susceptibility due to the
inclusion of granite source oxides such as magnetite,
which we use to track basin axes and extent (Fig. 5).
Fault and fold geometry indicate local Quaternary
structural control of the SSPR basins by the Monroe,
Carpenter Creek, and southern Whidbey Island fault
(SWIF) zones. Local thick sequences of CP PNS were
deposited in actively subsiding tectonic basins such as
the Monroe synclinal basin north of the Monroe reverse
fault (Fig. 6). In the SWIF, the CP PNS were deposited in
small, strike-slip basins then inverted into active growth
folds such as the Tolt River anticline. Some extensional
PNS basins, such as the Explorer Falls basin, which is
bounded by the active Carpenter Creek reverse fault,
were later uplifted to form inverted basins (Fig. 6 and 7).
Neotectonism in the area is also suggested by widespread
liquefaction features in nonglacial Pleistocene sediments in
the study area (Fig. 8).

Regional CP PNS sand petrography reveals Pleistocene
SSPR alluviation into the lowlands as far W to NW as
central Whidbey Island and Lofall/Quilcene areas,
respectively. Although further work is warranted, the
apparent restriction of the CP PNS north of the Seattle
fault may reflect structurally controlled, Pleistocene SSPR
alluviation into the Seattle basin north of the Seattle uplift
(Fig. 1).

Figure 3B. Age
plot for the
younger age
peak for
Sample 37A.

Figure 3: Sand Detrital Zircon Ages
The Cascade provenance of the SP deposits (Fig. 6) is supported by a detrital zircon study of an Olympia
bed SP sand. We obtained and OSL age of 40 ka (Table 1) in the Monroe synclinal basin for this sediment
(site 37A on Fig. 6). A. Probability density plot with histogram shows the age distribution for all sample
zircons (N=158). B. Probability density plot with histogram shows the age distribution for the younger age
peak (3236 Ma) for sample 37A. The Tertiary and Cretaceous intrusive rocks to the east of the study area in
the Cascade Mountains (for example, the Grotto and Index batholiths) are predominantly intermediate and
locally contain true granite (Tabor and others, 1993). The detrital-zircon age populations, petrography, and
geochemistry all indicate that these intrusive rocks were a major source of the modern and ancient
Skykomish River alluvium. The detrital zircon spectra shown above shows a strong correlation with the age
and distribution of the intrusive rocks presently exposed in the Skykomish basin east of the study area.
Tertiary and Cretaceous intrusive rocks exposed over 66% of the Skykomish River basin area. Most
compelling are the large number of 32 to 36 Ma zircon ages, consistent with erosion of the widespread 33 to
34 Ma Index batholith in the Cascades directly to the east. PP nonglacial deposits also have a abundant
Tertiary to Cretaceous detrital zircon age populations but have more Western mlange belt detritus. (Data not
presented; see Dragovich and others, 2014a, 2015).

Figure 7

Figure 5


Figure 6: Regional Geologic Relationships
Simplified geologic map of the north-central part of the study area showing the Monroe syncline (MS), Monroe fault (MF), Explorer Falls basin (EFB), Cherry Creek fault zone
(CCFZ), Cherry Valley fault (CVF), Carnation fault (CF), and southern Whidbey Island fault zone. (Map extents located on Fig. 1.) The geochronology (age) sites shown on the
map are documented in Table 1. Structural, seismic and other evidence suggests the MS, MF, EFB, CF, SWIF and CCFZ are potentially active to active structures. For
example, the MS may be a synclinal basin related to reverse offset of the MF with the syncline folding or tilting Olympia bed SP deposits that are latest Pleistocene in age. The
CFZ and the EFB faults are locally associated with active seismicity with the Duvall earthquake centered on the CCFZ near the junction of the Carnation, Lake Joy, Sultan and
Monroe quadrangles. The Carpenter Creek fault in the EFB (Fig. 7 shown directly below) are associated with Pleistocene growth folding of PP deposits interpreted to be
associated with the active Carpenter Creek reverse fault. Neotectonism in the area is also suggested by widespread liquefaction features in nonglacial Pleistocene sediments in
the study area (Fig. 8).

Figure 6

Figure 5 map extent
Figure 6 map extent
Figure 7 map extent

Fall City


Figure 1: Regional Setting
Simplified regional tectonic map of the central Puget Lowland and Cascade Range foothills showing
the study area (red rectangles; North Bend quadrangle south of Fall City quadrangle not shown).
Structural features include in the north part of the study area include the Explorer Falls basin (EFB)
and Bosworth Lake Basin (BLB), Pilchuck River fault (PF). The EFB is illustrated in Figure 7 (green
rectangle). The PF and EFB might be related to rejuvenated Eocene extensional structures that
preserve Paleogene to Pleistocene basin sediments in the Pilchuck River valley. The broad Granite
Falls fault zone (GFFZ) bounds the Everett basin in the Granite Falls quadrangle. See Figure 6 (blue
rectangle) for more detailed map of the Monroe syncline (MS), Monroe fault (MF) and the Monroe
anticline (MA) as well as the Cherry Creek fault zone (CCFZ). The Tokul Creek fault zone (TCFZ) is
similar to the CCFZ; they are likely left-lateral conjugates of the southern Whidbey Island fault zone.
The Rattlesnake Mountain fault zone (RMFZ) is likely a southern continuation of the SWIF. The
Seattle fault merges with the SWIF/RMFZ. Note the apparent local structural control of the
Snoqualmie and Skykomish River valleys by the SWIF/RMFZ, MF, and MS.
Sample 33C plane light


Sample 33C
crossed polars

Sample 33C plane light
Granitic lithic

Figure 4: Sand Geochemistry
A large geochemical dataset for Quaternary sands (n=145) has been compiled from
our previously published mapping studies, covering ten quadrangles. The nonglacial
sand samples include ancient (Pleistocene) and modern (Holocene) alluvium and
various glacial deposits representing several sedimentary provenances (Table 2).
We are able to discriminate modern and ancient alluvial sands that have a local
provenance (LP) from samples that have a granitic Cascade provenance (SP) on the
basis of their generally higher Sc, V, and Ti (Fig. 4A and 4B). In addition, we can
distinguish glacial (NP) from nonglacial sediments using Pb/Yb ratios (Fig. 4C). The
higher Sc, V, and Ti for the LP most likely represents a strongly mafic volcanic source
area, mostly likely accreted Mesozoic and Cenozoic terranes (Fig. 4). The higher
SiO2, and low Ti and Sc of the SP is a result of its strongly felsic plutonic provenance
(Fig. 4A-B). The NP has low Pb/Yb ratios due to its strong mixing of numerous
accreted terranes from the north of the study area -- which do not have a strong
volcanic or plutonic source (Fig. 4C). The NP overlaps the LP and SP on Fig. 4A
and 4B due to mixing of various sources by outwash.

Sample 33C
crossed polars





Granitic lithic



Figure 2: Petrography
SP-provenance thin section sand sample 33C. SP sediments contain detrital grains of monocrystalline quartz
(MQ)(~20%), plagioclase (plag), K-spar (~8%), hornblende (hbl), pyroxene, and mica with some granitic lithic
clasts. The above microphotos of SP-provenance sand has the grus appearance typical of SP deposits
regionally, with much subangular to angular grains. Most important is the relative abundance of K-spar, which
is largely derived from Tertiary to Cretaceous granite, granodiorite, and tonalite. We obtained an OSL age 77
ka (Whidbey Formation) for this sand. (See 33 in Table 1 and Fig. 6). Glacial deposits are have more
polymictic (variable) lithic clast types, including variable high-grade metamorphic clasts. Glacial deposits also
have high polycrystalline/monocrystalline quartz ratio and less K-spar than in Cascade Range-sourced
nonglacial units. The Index batholith is an important source of detritus for ancient Skykomish River SP
deposits as suggested by the detrital zircon age information for SP sediments (Fig. 3). See Table 2 sand
provenance information for further details.

Acknowledgements/Coauthor Contact Information

This geologic map was funded in part by the U.S. Geological Survey (USGS) National Cooperative Geologic Mapping Program under award no.
G15AC00248. 1Associated Earth Sciences, Inc., 1552 Commerce Street, Suite 102, Tacoma, WA 98402, [email protected]; 2Marine &
Ecological Sciences, Florida Gulf Coast University, 10501 FGCU Blvd South, Ft. Myers, FL 33965; 3U.S. Geological Survey, Denver Federal Center;
Washington Geological Survey, Department of Natural Resources, 1111 Washington St. SE MS 47007, Olympia, WA 98504; 5Earth and
Atmospheric Sciences, University of Alberta, 1-26 Earth Science Building, Edmonton, AB T6G 2E3, Canada; 6Associated Earth Sciences, Inc., 911
5th Ave., Suite 100, Kirkland, WA 98033; 7King County Department of Natural Resources and Parks, Water and Land Resource Division;
Department of Geosciences, Colorado State University, 1401 Campus Delivery, Ft. Collins, CO 80523.

Figure 7: Basin Involvement
Schematic north-south cross section across the Pilchuck River anticline in the southernmost part of the map area. (See
Fig. 1 for location of EFB.) We interpret this anticline as a growth fold resulting from compression along the Carpenter
Creek fault (CCF). The Carpenter Creek earthquake cluster is likely related to active compression across the CCF as a
result of oblique reverse slip along this structure. We suspect that this growth folding occurred before deposition of the
Hamm Creek formation (not pictured above) but after deposition of pre-Hamm Creek nonglacial deposits (unit Qc ph) Unit
Qcph deposition occurred during the Early to Middle Pleistocene transtensional development of the Explorer Falls and
Bosworth Lake basins and deposition of thick Cascade provenance PP fluvial deposits (Table 2); later north-south
compression occurred during the Late Pleistocene. In the Granite Falls quadrangle, Whidbey and Hamm Creek PP strata
are exposed in the northernmost part of the EFB on the northern limb of the Pilchuck anticline, as well as in the newly
named Lake Bosworth basin shown on Figure 1.


Figure 5: Subsurface Distribution
Residual aeromagnetic map (reduced to pole) of the Monroe syncline region (See Fig. 1 and Fig 6
for map location and Monroe synclinal basin location, respectively). Data from Blakely and others
(1999). Heavy grey line encloses an area of a moderate magnetic high spatially correlated with the
Monroe synclinal basin. The likely source of this anomaly are the thick, moderately magnetically
susceptible, SP sediments including sediments of both the Olympia beds and Whidbey Formation in
the synclinal basin (Table 2). Because of its continuity and anomaly amplitude, this region is
distinctive from the stronger magnetic highs which are sourced from metagabbros (see gabbro) of the
western mlange belt. Superimposed fault and fold structures are from Mahan and others (under
review). Abbreviations: 124th St., 124th Street fault; CCFZ, Cherry Creek fault zone; JSFZ, Johnsons
Swamp fault zone; MA, Monroe anticline; MF, Monroe fault; MS, Monroe syncline; SRT, Sultan River
thrust; RMFZ, Rattlesnake Mountain fault zone; WCFZ, Woods Creek fault zone, WLF, Woods Lake

Figure 8:
Liquefied Olympia bed nonglacial sands
with overturned folds, flame structures,
and chaotic bedding. We obtained an
OSL age of 32.3 ka at this site in the
Lake Chaplain quadrangle (Table 1).

Ancient alluvium from several Pleistocene nonglacial intervals is
widespread in the lower Snoqualmie, Skykomish, and Pilchuck River
valleys. These Pleistocene alluvial deposits have a composition,
sedimentology, geochemistry and general stratigraphic architecture that
mirrors modern alluvium (Qm59-65Qp12-29 PF12-26). These sediments are
derived from an intermediate, continental arc and are predominately from
Tertiary granitic to granodioritic erosional sources in the Cascade
Mountains, such as the Index, Grotto, and Snoqualmie batholiths (Fig. 3
and 4)(Table 2). SP-provenance sediments typically have liquefaction
features, including contorted or chaotic bedding and (or) sand dikes (Fig.
8) suggesting past tectonism of these fluvial basin sediments during
earthquake events. The widespread nature of these features in ancient
alluvium, combined with the deformation (e.g. tilting and folding) of SP
and PP Pleistocene deposits in the Monroe Syncline, Explorer Falls basin
and the Tolt River anticline, suggest neotectonism of these sediments
locally. Similarly, the Snoqualmie and Skykomish River valleys appear to
be locally structurally controlled by faults such as the RMFZSWIF and
(or) Monroe fault and Monroe synclinal basin (Fig. 1).

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