Chuckanut Formation Research Articles

The First Discovery of a Fish Fossil (Phareodus sp.) from Paleogene Fluvial Deposits in Western Washington State, USA

The 2023 discovery of a fish fossil from lower Eocene strata of the Chuckanut Formation provides new insights into the paleoenvironment and paleoecology of one of the region’s most prolific fossil deposits. The detrital clastic fluvial and floodplain deposits of the Chuckanut Formation are not favorable for the preservation of fish, but the high quality of preservation of this specimen is evidence that some Chuckanut Formation sediments provide suitable depositional conditions for the preservation of skeletal remains. This information improves our understanding of the range of depositional environments within the Chuckanut Formation, and provides clues for searching for additional specimens. The discovery of this fossil has larger significance; the skeletal remains of fish are scarce in fluvial and floodplain deposits. Despite its incompleteness, dorsal fin and caudal fin ray anatomy suggest that the specimens represent the extinct genus Phareodus, an open-water carnivore that has previously only been reported in North America from the Green River and Bridger Formations in Wyoming and Utah, USA.

Invertebrate trace fossils from Paleogene fluvial strata in Western Washington, USA

Fluvial strata in the Paleogene Chuckanut Formation contain a wealth of trace fossils that include both vertebrate and invertebrate ichnofossils. Vertebrate traces have received careful scrutiny, but invertebrate traces have not previously been described. Trace fossils occur in two sediment types. Mudstones deposited in subaqueous environments include highly bioturbated layers that are dominated by horizontal meniscate burrows. Siltstone and fine-grained sandstone beds primarily contain vermiform trails on bedding surfaces, with sparse vertical traces. Some of these sandy beds also contain vertebrate tracks. Chuckanut Formation beds commonly show rapid facies changes in sediments that were deposited along a meandering river during a time of semitropical climate. Transitions from subaqueous to subaerial conditions may have been related to changing water levels caused by variations in seasonal precipitation or episodic flood events. Other possible factors include ponds created by abandonment of channels, overbank sediment s deposited by floods, and breaching of levees to form crevasse splay deposits. Trace fossil assemblages show local variations in the diverse depositional environments of the Chuckanut Formation. Many of the ichnofossil suites clearly represent Scoyenia Ichnofacies, but some occurrences may involve overprinting of Scoyenia and Mermia ichnofacies.

The Kanaka Creek fossil flora (Huntingdon Formation), British Columbia, Canada — paleoenvironment and evidence for Paleocene age using palynology and macroflora

Paleogene sediments of the Huntingdon Formation, a correlative to the Chuckanut Formation of neighbouring Washington State, USA, are exposed in the Greater Vancouver area, British Columbia, Canada. Palynology and plant macrofossils suggest the Kanaka Creek section is Paleocene rather than Eocene in age. Detrital zircon dating is less decisive, yet indicates the Kanaka rocks are no older than Maastrichtian. Analyses of plant macro- and micro-fossils suggest an early to middle Paleocene age for the Kanaka fossil flora. Paleocene indicators include macrofossils such as Platanus bella, Archeampelos, Hamamelites inequalis, and Ditaxocladus, and pollen taxa such as Paraalnipollenites, Triporopollenites mullensis, and Duplopollis. Paleogene taxa such as Woodwardia maxonii, Macclintockia, and Glyptostrobus dominate the flora. Fungal spores including the Late Cretaceous Pesavis parva and the Paleogene Pesavis tagluensis are notable age indicators. Physiognomy of 41 angiosperm leaf morphotypes from Kanaka Creek yields mean annual temperatures in the microthermal to lower mesothermal range (11.2 ± 4.3 to 14.6 ± 2.7 °C from leaf margin analysis; 14.8 ± 2.1 °C from Climate Leaf Analysis Multivariate Program), with mild winters (cold month mean temperature 3.9 ± 3.4 °C). Paleoclimate was cooler than the upper Paleocene and Eocene members of the Chuckanut Formation. Mean annual precipitation is estimated at ∼140 cm with large uncertainties. The Kanaka paleoflora is reconstructed as a mixed conifer–broadleaf forest, sharing common taxa with other western North American Paleocene floras and growing in a temperate moist climate. Kanaka Creek is a rare coastal Paleocene plant locality that provides new insights into coastal vegetation and climate prior to the Paleocene–Eocene Thermal Maximum.

Lower Eocene Footprints from Northwest Washington, USA. Part 1: Reptile Tracks

Lower Eocene fluvial strata in the Chuckanut Formation preserve abundant bird and mammal tracks. Reptile trace fossils include footprints from a small turtle (ichnogenus Chelonipus), and several Crocodylian trackways that consist of irregularly spaced footprints associated with linear tail drag marks. The latter trackways represent “punting” locomotion, where a submerged Crocodylian used intermittent substrate contacts to provide forward motion of their neutrally buoyant bodies. Two adjacent sandstone blocks preserve Crocodylian trace fossils that are named herein as a new ichnogenus and ichnospecies Anticusuchipes amnis. Two other Crocodylian trackways lack sufficient detail for ichnotaxonomic assignment.

New Paleogene paleoclimate analysis of western Washington using physiognomic characteristics from fossil leaves

The Chuckanut and Manastash formations were deposited in a lowland fluvial environment in western Washington during the Paleocene–Eocene. We provide a thorough paleotemperature analysis using the physiognomic characteristics from the fossil dicotyledonous leaves from these formations using two techniques, Leaf Margin Analysis (LMA) and Climate Leaf Analysis Multivariate Program (CLAMP). This work differs from previous analyses by 1) analyzing more localities and including the assessment of the never-before described Taneum Ridge Body and Manastash Main Body within the Manastash Formation and the Maple Falls Member of the Chuckanut Formation, and 2) using two different techniques to decipher mean annual temperatures through time. In this study, mean annual temperatures (MAT) were determined from 125 distinct morphotypes and are several degrees higher at each locality than previously reported. MAT values derived from LMA ranged from 17.0° to 28.8°C for the lowermost and 13.4° to 19.5°C for the uppermost Chuckanut Formation. The Manastash Formation had MATs ranging from 17.1°C to 29.1°C. Using CLAMP, paleotemperatures ranged from 15.3° to 19.4°C for the lowermost and 12.3° to 15.6°C for the uppermost Chuckanut Formation and 15.8 to 21.3°C for the Manastash Formation. These data are compared to other Eocene–Paleocene formations in the western US; the Chuckanut and Manastash formations record some of the warmest paleotemperatures in the Paleogene in the western US because of their low-elevation locations. These paleotemperatures may reflect the terrestrial signal of paleoclimate optima defined in marine rocks during the Paleocene–Eocene.

Evaluation of paleoclimatic conditions east and west of the southern Canadian Cordillera in the mid-late Paleocene using bulk organic δ13C records

Carbon isotope records from organic matter provide a key data set for constraining past fluctuations in the isotopic composition of atmospheric carbon dioxide (δ13Catm), hydrologic conditions, and plant community composition. Here we evaluate spatiotemporal shifts in these three conditions using δ13C values of bulk organic carbon (δ13Cbulk) from Paleocene strata in northwest Washington, U.S.A. and west-central Alberta, Canada. Our data set contains 352 analyses from the Chuckanut and Paskapoo formations. Mean δ13Cbulk values in the two areas are statistically indistinguishable (−24.5‰ and −24.4‰ (VPDB), respectively), and exhibit a wide range of values (~5–7‰). In both areas sequential vertical samples show large fluctuations in δ13Cbulk values, on average of ~1.0‰±1.0 (1σ). There are no distinct up-section trends or structured excursions in the ~850meter stratigraphic section in the Chuckanut Formation nor the ~125meter section in the Paskapoo Formation. Furthermore, δ13Cbulk values are independent of the weight percent organic carbon (%C) and lithology of the sample.The lack of a systematic relationship and correlation between δ13Cbulk and %C values indicates isotopic fractionation during post-mortem processes, such as microbial respiration, exerted minimal influence on the isotopic patterns. The independence of δ13Cbulk values from lithology suggests variability is unrelated to local spatial gradients in vegetation community and water availability across the paleo-landscape. However, the wide range and large fluctuations in δ13Cbulk values is similar to stochastic intra-site variability of plant δ13C values within modern settings. The absence of structured, up-section trends indicates minimal secular variation in the carbon isotopic composition of atmospheric CO2, rainfall, and plant community composition. Moreover, we show that a previous carbon isotope excursion ascribed to the Paleocene–Eocene Thermal Maximum in the Paskapoo Formation is actually a result of aliasing related to sampling density. Finally, the statistically indistinguishable mean δ13Cbulk values in the two regions imply similarly wet and humid climatic conditions when combined with sedimentologic and paleobotanical data. The records support early Paleogene circulation models that show strong orographic precipitation patterns in the Pacific Northwest associated with incipient extensional tectonics in the region and enhanced transport of moisture-laden polar air masses southward into the Canadian continental interior.

Giant Eocene bird footprints from Northwest Washington, USA

Abstract. Tridactyl bird footprints preserved in Lower Eocene sandstone of the Chuckanut Formation in Whatcom County, Washington, USA, were made by a species of giant ground bird that walked along the subtropical lowland riverbank. The morphology and age of the tracks suggest the track maker was Diatryma (? = Gastornis). Although these birds have long been considered to be predators or scavengers, the absence of raptor‐like claws supports earlier suggestions that they were herbivores. The Chuckanut tracks are herein named as Rivavipes giganteus ichnogenus and ichnospecies nov., inferred to belong to the extinct family Gastornithidae.

A re-examination of Cenozoic Polypodium in North America

The sterile holotype of Polypodium fertile MacGinitie was re-examined together with other fertile type specimens from the Miocene Weaverville Formation at Redding Creek (California, western USA). In its leaf morphology, venation and in situ spores Polypodium fertile matches the extant Polypodium vulgare complex. The spores belong to the verrucose type I (sensu Lloyd). In view of discrepancies between the original description and the real morphology of the sterile frond of ‘Polypodium’ alternatum Pabst from the Chuckanut Formation of northwestern Washington (Eocene), this fern must be excluded from the record of Polypodium.

Eocene Bird, Reptile, and Mammal Tracks from the Chuckanut Formation, Northwest Washington

Other| August 01, 2002 Eocene Bird, Reptile, and Mammal Tracks from the Chuckanut Formation, Northwest Washington GEORGE E. MUSTOE GEORGE E. MUSTOE 1Geology Department, Western Washington State University, Bellingham, WA 98225 Search for other works by this author on: GSW Google Scholar PALAIOS (2002) 17 (4): 403–413. https://doi.org/10.1669/0883-1351(2002)017<0403:EBRAMT>2.0.CO;2 Article history accepted: 22 Feb 2002 first online: 03 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation GEORGE E. MUSTOE; Eocene Bird, Reptile, and Mammal Tracks from the Chuckanut Formation, Northwest Washington. PALAIOS 2002;; 17 (4): 403–413. doi: https://doi.org/10.1669/0883-1351(2002)017<0403:EBRAMT>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyPALAIOS Search Advanced Search Abstract The diversity and abundance of bird and animal tracks preserved in Eocene strata of the Chuckanut Formation in Washington contrasts to the scarcity of body fossils. These ichnofossils were made by vertebrates that inhabited river margins, the only depositional environment favorable for track preservation. Three of the four localities described herein contain tracks from at least two different types of animals. Site SM-6 contains approximately 200 shallow circular plantigrade footprints, perhaps made by a type of archaic mammal of the Orders Pantodonta or Dinocerata. Site RU-1 yielded footprints from a small shorebird and tracks from an early equid or tapiroid. The same type of perissodactyl tracks were preserved at Site KC-1, along with a single webbed bird track, and trackways from a large heron-like bird and a turtle. Site SM-9.5 contained multiple bird tracks of a type not found at the other localities. The discovery of tracks only at Chuckanut Formation sites that expose large bedding planes indicates the importance of considering outcrop architecture during the search for vertebrate ichnofossils, and inspires the hope that similar fossils may eventually be found in correlative formations in the Pacific Northwest. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Apatite fission-track dating of the Cowichan fold and thrust system, southern Vancouver Island, British Columbia

The Cowichan fold and thrust system on southern Vancouver Island comprises a linked system of folds and thrust faults in crystalline and sedimentary rocks of the Wrangellia terrane and its sedimentary cover, the Upper Cretaceous Nanaimo Group, and Upper Paleocene to Lower Eocene sandstones of the Chuckanut Formation. Apatite fission-track analysis indicates that cooling below the closure temperature of fission-track retention in apatite (100 ± 20 °C) for both the Island Plutonic Suite from the Wrangellian basement and the Nanaimo Group occurred at ~ 50–40 Ma (Middle Eocene). Thrusting is interpreted to have occurred immediately prior to this time in the Middle Eocene, but after post-Cretaceous regional burial of the Turonian to Maastrichtian Nanaimo Group and deposition of Upper Paleocene and Lower Eocene sandstones, which are involved in the deformation. These timing constraints support the hypothesized temporal linkage between contraction and uplift of Wrangellia during the Middle Eocene and continued margin-normal contraction of the Pacific Rim terrane (± the Crescent terrane) at ~ 45 Ma.

Rock Mass Strength Assessment for Bedrock Landsliding

Application of a rock mass strength (RMS) classification scheme to estimate outcrop-scale strength discriminates stable hillslopes from sites of deep-seated bedrock landslides in the Chuckanut Formation, Washington. RMS values determined for joint surfaces and bedding planes provide quantitative measures of outcrop-scale strength encompassing the spatial variability, character, and frequency of discontinuities in a rock mass, features impossible to evaluate via conventional laboratory analysis. Strength quantification using bedding planes from 61 hillslopes, including 17 rockslides, reveals distinct populations of data for stable hillslopes and rockslides, with the latter exhibiting RMS values below 69. In contrast, RMS values obtained from joint surfaces in the study area do not correlate with hillslope stability. Analysis of the seven weighted variables in the RMS scheme indicates that bedding plane orientation relative to hillslope orientation, together with intact rock strength, serve as the primary controls on deep-seated rocksliding within the Chuckanut Formation. Rock mass strength estimates obtained by this approach characterize outcrop-scale material strength and identify those portions of a landscape most susceptible to bedrock landsliding.

High chromium contents in Tertiary coal deposits of northwestern Washington — A key to their depositional history

Chromium contents obtained from 20 coal and 5 associated rock samples collected from the basal part of the Eocene Chuckanut Formation, in Skagit and Whatcom counties, northwest Washington, range between 30 and 300 ppm (mean 120 ppm whole-coal basis). The lenticular coals, ranging in rank from subbituminous to anthracite, and with an ash content of 12–46%, crop out along the western flank of the Cascade Range. Results of X-ray diffraction analysis of low-temperature ash show that the mineral matter in the coal samples consists predominantly of quartz and clay (kaolinite, illite and chlorite group). However, accessory minerals, isolated from the coal samples and analyzed by X-ray diffraction, scanning electron microscope and optical methods, contain angular fragments and euhedral crystals of the spinel group (chromite, magnetite and trevorite ), kaolinite-serpentine group (antigorite and chrysotile), chlorite group, amphibole group and pyroxene group minerals (augite, diopside and enstatite), all of which are commonly enriched in chromium. Although associated primarily with the inorganic fraction of the coal, concentrations of chromium in the samples show no statistically significant correlation with ash content. Localized concentrations of chromium in the coal are the result of natural contamination from the alteration of detrital chromium-bearing mineral grains introduced into the peat-forming mires from nearby Jurassic ophiolite bodies. The coals formed in the early Eocene, in rapidly subsiding small basins that developed during the uplift and erosion of the pre-Tertiary ophiolite terrain. Scattered bodies of source rock, random distribution of chromium-bearing minerals within the coal and sample heterogeneity account for the variation in Cr contents of the samples.

Depositional history of the southeastern outcrop belt of the Chuckanut Formation: implications for the Darrington – Devil's Mountain and Straight Creek fault zones, Washington (U.S.A.)

The southeastern outcrop belt of the Eocene Chuckanut Formation contains the erosional remnants of a larger depositional system. In the study area, the Chuckanut Formation can be split into four units based upon differences in age, lithology, sedimentology, paleocurrents, and provenance relationships. The Coal Mountain unit (Early Eocene) represents a southwest-flowing fluvial system that shows no evidence for fault control of drainage. The overlying Higgins Mountain unit (early Middle Eocene) represents a northeast-flowing fluvial system east of the Devil's Mountain fault zone (DMFZ), with lithologies derived from western source areas. The Sperry Peak unit (early Middle Eocene) represents a fluvial system with a wide dispersion of paleocurrent azimuths and a possible mixture of sediment source areas. We believe the Sperry Peak unit was deposited in a fault-wedge graben at the junction of the DMFZ and Straight Creek fault zone (SCFZ), with sediment sources from both east and west. The Grade Creek unit (age unknown) is a fluvial unit found along the trace of the SCFZ with paleo-flow subparallel to the fault zone. The sedimentary evidence suggests that onset of Paleogene faulting on the DMFZ and SCFZ was at about 50–48 Ma, while movement on the Darrington fault zone may have been later (post-42 Ma).

Eocene bird tracks from the Chuckanut Formation, northwest Washington

Early Tertiary nonmarine sediments occur as a discontinuous series of outcrops that extends throughout large areas of northwest Washington and southwestern British Columbia. These fluvial and deltaic deposits contain abundant plant fossils, but faunal remains are rare. The recent discovery of an outcrop of Chuckanut Formation arkose containing nine fossil bird tracks provides the first clear evidence of terrestrial vertebrate life in the extensive coastal wetland zone that existed prior to the uplift of the Cascade Range.

Lithostratigraphy and deep-water setting of the upper Nanaimo Group (Upper Cretaceous), outer Gulf Islands of southwestern British Columbia

The Upper Cretaceous Nanaimo Group consists of extensive shaly units and lenticular bodies of sandstone and conglomerate deposited in neritic to bathyal marine paleoenvironments. Single sandstone and conglomerate lenses cannot be traced throughout the basin, but instead tend to concentrate at particular stratigraphic levels, defining crudely sheet-like composite units of coarse-grained rocks. Many of the lenticular coarse-grained units rest in major channels, as deep as about 550 m. Sand and gravel were generally transported to the west and northwest by sediment gravity flows through these channels into the basin centre. Recognition of widespread channeling is critical to the definition of formal lithostratigraphic units (four formations and two informal members are defined here for the upper Nanaimo Group).The top of the Nanaimo Group is probably truncated by a major unconformity with local relief of at least 2 km. The effect of the unconformity can only be seen by mapping the orientation of offshore ridges in the Strait of Georgia, which can be traced into the sandstones and conglomerates above the inferred unconformity on Tumbo and Cabbage islands. Large scale cross-bedding, unidirectional paleocurrents, and lack of evidence for marine conditions favour a subaerial, fluvial setting for deposition of these rocks, which correlate to the lithologically similar Paleogene Chuckanut Formation on the Sucia Islands.

Isotopic provenance of Paleogene sandstones from the accretionary core of the Olympic Mountains, Washington

Conventional modal sandstone data from Paleogene units of the Pacific Northwest are not precise enough to pinpoint source areas and constrain displacement histories of accreted sedimentary terranes. Isotopic provenance study, used in conjunction with traditional basin-analysis techniques, provides a powerful means of identifying source areas. Analysis of Rb-Sr data in both wholerock and detrital white micas of Paleogene sandstones from allochthonous units in the eastern core of the Olympic Mountains and coeval autochthonous sandstones from coastal Pacific Northwest shows that the Needles-Gray Wolf and Grand Valley lithic assemblages of the core came from the same source as sandstones of the Chuckanut Formation and Puget Group in northern Washington. The source of these units is isotopically distinct from the source for units farther south, such as the Tyee Formation in Oregon. Chemical compositions, conventional K-Ar age determinations, and oxygen- and hydrogen-isotope compositions of white micas support this conclusion. Similar analyses of sandstones from the Western Olympic lithic assemblage and the Yakutat terrane of southeastern Alaska suggest that these two units have a similar source, but that they differ slightly from sandstones of the eastern Olympic core and autochthonous Washington units. The overall composition of sandstones (lithic arkosic), and the very high initial-Sr values (>0.710), moderately high δ 18 O values (∼+9) and Mesozoic age for detrital white mica of Olympic core rocks and sandstone of the Yakutat terrane suggest a source in the high-rank metamorphic and plutonic rocks from the eastern part of the Omineca Crystalline Belt in southeastern British Columbia. Furthermore, rapid uplift of this source area during Eocene time is consistent with the depositional age of the Olympic rocks. Sediment derived from this source area was transported westward by major river systems that crossed the low-lying North Cascade Range and supplied the deposits of the autochthonous units of the northern Washington coast and the offshore equivalents before the latter were accreted to form the Olympic core. Limited data from the Yakutat terrane suggest that it lay offshore of southern British Columbia sometime during middle Eocene to early Oligocene time, consistent with paleomagnetic and some paleontologic interpretations, and subsequently migrated northward by plate motions.

Seismic investigation of the Coast Plutonic Complex – Insular Belt boundary beneath the Strait of Georgia

The Strait of Georgia, a topographic depression between Vancouver Island and the mainland of British Columbia, is considered to be the boundary between two tectonic provinces: the Coast Plutonic Complex on the east and the Insular Belt to the west. The allochthonous nature of the Insular Belt has been established, mainly on the basis of paleomagnetic measurements. Various tectonic models to explain the geological differences between the two provinces have been proposed. One of these suggests that the boundary is an old transform fault zone and is represented currently by a thrust fault along the eastern side of the Strait of Georgia. Other models propose that the Coast Plutonic Complex is a feature superimposed by tectonic and metamorphic events after the accretion of the Insular Belt. Such models do not require a major crustal discontinuity along the Strait of Georgia.In May 1982, a seismic refraction survey using a 32 L air gun and a radio telemetering sonobuoy system was carried out in the Strait of Georgia with the objective of investigating the nature of this boundary and determining the upper crustal structure. Three reversed profiles across the strait were shot; these are supplemented by several high-resolution reflection profiles from previous experiments. Two-dimensional models of the crustal structure across the strait have been constructed using a forward modelling ray trace and synthetic seismogram algorithm to match the travel times and amplitude characteristics of the data.Three basic layers or strata form the models, for which the maximum depth of reliability is 3 km. The first layer consists of unconsolidated sediments and Pleistocene glacial deposits, and the second represents Late Cretaceous – early Tertiary basin fill sediments that form the Nanaimo Group, the Burrard–Kitsilano formations, and the Chuckanut Formation. The third layer is likely the extension of the Coast Plutonic Complex beneath the strait, but the westerly limit of this unit is undetermined because of seismic properties similar to those of the Insular Belt volcanics. A local fault is located ~15 km northeast of Galiano Island on the west side of the strait. However, our study shows no evidence for a major fault along the strait. Thus those aspects of tectonic models that require the existence of a major transform or transcurrent fault boundary along the Strait of Georgia. may have to be revised.

Cyclic fluvial sedimentation in a rapidly subsiding basin, northwest Washington

The Eocene Chuckanut Formation of northwest Washington comprises as much as 6000 m of alluvial strata and is one of the thickest nonmarine sequences in North America. The Bellingham Bay Member forms approximately the lower half of the Chuckanut Formation; it is characterized by repetitive fining-upward cycles consisting of a lower coarse-grained and an upper fine-grained member. Two types of cycle have similar vertical profiles but can be distinguished by the nature of their coarse member. Coarse members of the first type are thick (> 2–3 m), are commonly multi-storey, have apparent sheet geometry and are interpreted as the deposits of meandering-river channels and point bars. Coarse members of the second type are thinner (< 2–3 m), have apparent ribbon geometry and are interpreted as floodplain (crevasse?) channel deposits. Down-basin changes in the Bellingham Bay Member include a major decrease in grain size and an increase in fine-member thickness and cycle-recurrence interval. These changes reflect decreased flow competence and increased stability of channel banks. Similar trends might be found in other ancient alluvial sequences and could be useful in studies of both basin geometry and coal exploration. Sediment-accumulation rates are a minimum of 50 cm 1000 yrs −1, implying rapid subsidence. This subsidence probably had a major control on the sedimentation style of the Bellingham Bay Member, which resembles computer-generated models of alluvial stratigraphy at high subsidence rates (Allen, 1978).

Stratigraphy, age, and paleogeography of the Eocene Chuckanut Formation, northwest Washington

The Eocene Chuckanut Formation of Washington's North Cascades comprises as much as 6000 m of alluvial strata and is one of the thickest nonmarine sequences in North America. It is exposed in several disconnected outcrop belts that are remnants of what was probably an extensive fluvial system in western Washington. In this study, seven stratigraphic members are defined in the main outcrop belt of the Chuckanut Formation near the town of Bellingham. Their ages, based on fission-track zircon dates, palynologic studies, and correlation by mapping, extend from the Early to the Late Eocene.Three types of fluvial systems contributed to the Chuckanut Formation and are distinguished on the basis of their lithology, sedimentology, and petrology. The first comprises fine-load meandering-river deposits of the Lower Eocene Bellingham Bay Member and the Middle Eocene Slide Member. Sandstones are arkosic and had their main source in rapidly uplifted, high-grade metamorphic terranes in eastern Washington. The second type comprises braided- and coarse-load meandering-river deposits of the Middle to Upper Eocene Padden Member. The Padden Member occurs only in the western part of the outcrop belt and was derived form the Coast Plutonic Complex of southern British Columbia to the north. Sandstones are arkosic but are richer in lithic fragments than sandstones of the underlying Bellingham Bay Member and interfingering Slide Member. The third fluvial-system type comprises conglomerate-rich braided-river and alluvial-fan deposits of the lower Middle Eocene Governors Point Member and the Middle to Upper (?) Eocene Maple Falls, Warnick, and Bald Mountain members. Sandstones from these units are both lithic and arkosic. These strata were locally derived from uplifts on the northern basin margin.The petrology of down-basin correlatives of the Chuckanut Formation on the northeast Olympic Peninsula is incompatible with derivation through the Chuckanut fluvial system. This petrologic contrast supports a model for large-scale Eocene dextral offsets in the continental margin of Washington.

Eocene Chuckanut Formation of Northwest Washington--Sedimentation in a Large Strike-Slip-Fault Controlled? Basin: ABSTRACT

The Eocene Chuckanut Formation of northwest Washington comprises as much as 6,000 m (20,000 ft) of alluvial strata and is one of the thickest nonmarine sequences in North America. It is exposed in several disconnected outcrop belts that are probably remnants of a regionally extensive fluvial system. Four distinct periods of sedimentation are represented in the main (50 × 20 km, 31 × 12 mi) Chuckanut outcrop belt near the town of Bellingham. These include: (1) early Eocene: rapid sedimentation in west-southwest-flowing fine-load meandering rivers; (2) early-middle Eocene: sedimentation in braided rivers draining northern fault blocks in the western part of the outcrop belt, synchronous with continued fine-load meandering-river sedimentation to the east; (3) middl Eocene: sedimentation in a south-flowing, coarse-load meandering river system in the western part of the outcrop belt, synchronous with continued sedimentation in west-flowing, fine-load meandering rivers of reduced size and competence to the east; and (4) middle to late(?) Eocene: sedimentation in alluvial fans and braided rivers in the eastern part of the outcrop belt draining uplifted pre-Tertiary basement north of the Boulder Creel fault. Following period 4, but still in the Eocene, the Chuckanut was first folded and then truncated by faulting. It is proposed that the Chuckanut basin formed in an extensional zone of right lateral shear between major strike-slip faults. Consistent with this interpretation are: (1) rapid sediment accumulation rates; (2) rapid facies changes; (3) an irregular basin margin characterized by dip-slip faults and intraformational unconformities; (4) deformation consistent with predicted structural patterns; (5) rapid changes between extensional and compressional tectonics; and (6) interbedded and intrusive relationships with extension-generated(?) volcanic rocks. The Chuckanut basin is considerably larger than most pull-apart basins generally associated with strike-slip faulting, yet shares many of the same attributes. Similar large basins might be found in other continental margins characterized by strike-slip faults. End_of_Article - Last_Page 491------------