T387 Geologic Mapping in the Pacific Northwest and Beyond: Recent Maps, Advances, and New Insights: In Memory of Russell C. Evarts and his Extensive Contributions to Pacific Northwest Geologic Mapping (Posters)

Wednesday, 25 October 2017
9:00 AM - 6:30 PM
Halls 4EF

GEOLOGIC MAP OF THE MALTBY 7.5’ QUADRANGLE, SNOHOMISH AND KING COUNTIES, WASHINGTON

Paper number: T387-4
Program: Technical Sessions
Time: 9:00 AM – 6:30 PM

Authors

Abstract

We present a new geologic map of the Maltby 7.5’ quadrangle at 1:24,000 scale. The densely populated quadrangle located 30km NE of Seattle, which has not been mapped in detail since the 1980s, spans the projected intersections of several active regional tectonic structures. We support the map with new field work, lidar, existing subsurface explorations, geochemistry, palynology, and U-Pb, luminescence, radiocarbon, and biostratigraphic chronology. We compare mapped geology with new isostatic gravity and existing aeromagnetic data for further observations of structural features in this complex area.

New radiocarbon and luminescence chronology constrains pre-Vashon stratigraphy of the Olympia non-glacial interval and may provide a limiting date for the end of the Possession Glaciation. New U-Pb zircon ages for both volcanic (~44 Ma) and sedimentary (~28 Ma) bedrock units are presented. Geochemical analyses of 18 samples of the volcanic rocks of Mount Persis range from andesite to dacite and most have adakite affinities. We compare these data with existing analyses of the volcanics and discuss a relation to the earliest stages of subduction-related magmatism as the modern Cascade arc initiated in the Eocene. We refine the stratigraphy of fossiliferous Tertiary sedimentary bedrock with constraints from U-Pb zircon chronology, as well as foraminiferal and molluscan biostratigraphy, and describe an Eocene-Oligocene transition from continental to mid-shelf environments with evidence for diffuse methane seep colonies. Biostratigraphy leads us to infer that most of the exposed sedimentary bedrock predates the Blakeley Formation type section, in contrast to correlations drawn by previous workers. A major bedrock anticline in the map area reflects Oligocene or later NNE-SSW compression and may be tied to a complex interaction of regional structures concealed by Quaternary cover. Detailed mapping is unable to further constrain the surficial trace of the active Southern Whidbey Island Fault zone or Monroe Fault, though geophysical lineaments indicate that strands of these structures extend through the quadrangle and merge with previously mapped faults to the southeast. Our new map concludes a decade-long campaign of detailed geologic mapping by the Washington Geological Survey in the eastern Puget Lowland.

Geological Society of America Abstracts with Programs. Vol. 49, No. 6

doi: 10.1130/abs/2017AM-304000

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