Sedimentation and tectonics of the lower and middle Eocene Swauk Formation in eastern Swauk Basin, central Cascades, central Washington

The Swauk Basin lies between the Straight Creek and Entiat–Leavenworth fault systems in the central Cascades of Washington and is one of several early Tertiary strike-slip basins in the Pacific Northwest. In this paper, seven informal stratigraphic units are recognized in the lowest portion of the basin fill—the lower and middle Eocene Swauk Formation—in the eastern part of the basin. These units have a stratigraphic thickness of more than 4800 m and were deposited in alluvial fan, braided river, meandering river(?), lacustrine-deltaic, and lacustrine environments. Rapid facies changes and reversals in paleocurrent directions indicate numerous tectonically controlled drainage reorganizations. Sediments were mainly derived from crystalline rocks to the east, north, and west (?), and sediment accumulation rates were high (about 64 cm/1000 years). The Leavenworth fault has a complex history and, periodically, formed the eastern margin of Swauk Basin. During the final phases and shortly after deposition of the Swauk Formation, the vertical sense of slip on the Leavenworth fault reversed itself and the trace of the fault shifted westward, leading to formation of the Chiwaukum Graben. Following deposition, the Swauk Formation was deformed into west-northwest-trending folds and then intruded by north–northeast-trending dike swarms. Based on similarities with other well-documented strike-slip basins, we conclude that strike-slip faulting was the main control on the deposition and deformation of the Swauk Formation.

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

Canadian Journal of Earth Sciences ◽

10.1139/e84-010 ◽

1984 ◽

Vol 21 (1) ◽

pp. 92-106 ◽

Cited By ~ 37

Author(s):

Samuel Y. Johnson

Keyword(s):

Large Scale ◽

Middle Eocene ◽

Alluvial Fan ◽

Fluvial System ◽

Upper Eocene ◽

Meandering River ◽

The North ◽

River Deposits ◽

Metamorphic Terranes ◽

Eastern 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.

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River sediment flux and shelf sediment accumulation rates on the Pacific Northwest margin

Continental Shelf Research ◽

10.1016/j.csr.2004.10.001 ◽

2005 ◽

Vol 25 (3) ◽

pp. 311-332 ◽

Cited By ~ 63

Author(s):

R.A. Wheatcroft ◽

C.K. Sommerfield

Keyword(s):

Pacific Northwest ◽

River Sediment ◽

Sediment Accumulation ◽

Sediment Flux ◽

Accumulation Rates ◽

Shelf Sediment ◽

The Pacific

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Paleobiology of middle Eocene plant-insect associations from the Pacific Northwest: A preliminary report

Rocky Mountain Geology ◽

10.2113/gsrocky.37.1.31 ◽

2002 ◽

Vol 37 (1) ◽

pp. 31-59 ◽

Cited By ~ 32

Author(s):

C. C. Labandeira

Keyword(s):

Pacific Northwest ◽

Preliminary Report ◽

Middle Eocene ◽

The Pacific

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THE PALEOBOTANY AND PALEOECOLOGY OF THE EOCENE HERREN BEDS OF NORTH-CENTRAL OREGON, USA

Palaios ◽

10.2110/palo.2019.014 ◽

2019 ◽

Vol 34 (9) ◽

pp. 424-436 ◽

Cited By ~ 1

Author(s):

ANTHONY P. JIJINA ◽

ELLEN D. CURRANO ◽

KURT CONSTENIUS

Keyword(s):

Pacific Northwest ◽

Middle Eocene ◽

Plant Macrofossils ◽

Mean Annual Precipitation ◽

Future Research ◽

Mean Annual Temperature ◽

Leaf Margin ◽

North Central ◽

The Pacific ◽

Margin Analysis

ABSTRACT New collections of plant macrofossils and radiometric dates from the Herren beds of north-central Oregon provide the opportunity to document floral communities and calculate foliar-derived climate estimates from the warm early Eocene and the cooler middle Eocene. Plant macrofossils were collected from one fluvial site at East Birch Creek approximately 2 m below a 51.9 ± 0.9 Ma tuff. Collections were also made at two co-occurring fluvial sites at Arbuckle Mountain, whose ages are constrained to ca. 44.5–43.8 Ma based on a dated tuff from Willow Creek (44.5 ± 0.8 Ma) and reported ages for the overlying Clarno Formation. Floral findings show an almost complete vegetation overturn, with only two genera (Glyptostrobus and Allantodiopsis) appearing in both floras. Both floras are species poor, but the older East Birch Creek flora has higher richness and evenness than the younger Arbuckle Mountain flora. The four named genera at East Birch Creek are taxa found throughout Eocene North America; named genera at Arbuckle Mountain also include taxa restricted to the Pacific Northwest. Leaf margin analysis and leaf area analysis of the East Birch Creek community suggest a warmer and possibly wetter (mean annual temperature 23.4 ± 4.3 °C; mean annual precipitation 206 +89, -63 cm) climate than the Arbuckle Mountain flora (16.4 ± 4.2 °C; 165 +50, -71.4 cm). This research provides a framework for future research on Eocene floristic, environmental, and climatic trends of the Pacific Northwest.

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Eocene Ginkgo leaf fossils from the Pacific Northwest

Canadian Journal of Botany ◽

10.1139/b02-097 ◽

2002 ◽

Vol 80 (10) ◽

pp. 1078-1087 ◽

Cited By ~ 18

Author(s):

George E Mustoe

Keyword(s):

British Columbia ◽

Pacific Northwest ◽

Ginkgo Biloba ◽

Middle Eocene ◽

Washington State ◽

North Central ◽

Lacustrine Deposits ◽

South Central ◽

The Pacific ◽

Ginkgo Leaf

Middle Eocene lacustrine deposits in south-central British Columbia and north-central Washington state preserve two types of Ginkgo leaves. A morphotype characterized by deeply divided multiple lobes is herein described as Ginkgo dissecta sp.nov. Leaves that are either undivided or shallowly divided into bilobate symmetry are indistinguishable from foliage of extant Ginkgo biloba Linnaeus. These fossils contradict the widely held belief that only a single Ginkgo species, Ginkgo adiantoides (Unger) Heer, inhabited Cenozoic forests.Key words: British Columbia, Eocene, fossil, Ginkgo adiantoides, Ginkgo biloba, Ginkgo dissecta, McAbee, Republic, Tertiary, Washington.

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