Magnetostratigraphy, paleomagnetic correlation, and deformation of Pleistocene deposits in the south central Puget Lowland, Washington

Abstract

[1] Paleomagnetic results from Pleistocene sedimentary deposits in the central Puget Lowland indicate that the region has experienced widespread deformation within the last 780 kyr. Three oriented samples were collected from unaltered fine-grained sediments mostly at sea level to determine the magnetostratigraphy at 83 sites. Of these, 47 have normal, 18 have reversed, and 18 have transitional (8 localities) polarities. Records of reversed-to normal-polarity transitions of the geomagnetic field were found in thick sections of silt near the eastern end of the Tacoma Narrows Bridge, and again at Wingehaven Park near the northern tip of Vashon Island. The transitional horizons, probably related to the Bruhnes-Matuyama reversal, apparently fall between previously dated Pleistocene sediments at the Puyallup Valley type section (all reversed-polarity) to the south and the Whidbey Island type section (all normal-polarity) to the north. The samples, in general, are of sufficient quality to record paleosecular variation (PSV) of the geomagnetic field, and a statistical technique is used to correlate horizons with significant agreement in their paleomagnetic directions. Our data are consistent with the broad structures of the Seattle uplift inferred at depth from seismic reflection, gravity, and aeromagnetic profiles, but the magnitude of vertical adjustments is greatly subdued in the Pleistocene deposits.

Active shortening of the Cascadia forearc and implications for seismic hazards of the Puget Lowland

“…The gravity gradients along the western and eastern segments are about 6 mgals/km and 2–3 mgals/km, respectively. Pratt et al [1997] and Hagstrum et al [2002] modeled the eastern portion of the gradient as a southwest dipping monocline in which dense basalt of the Eocene Crescent Formation rises to the northeast.…”

“…There are also a few places where aeromagnetic lows correspond to areas where other geophysical data suggest the Crescent Formation is within a few kilometers of the surface, such as in the central Seattle uplift north of Carr Inlet, and along the southeast flank of Maury Island (Figures 3 and 4). Hagstrum et al [2002] inferred that the aeromagnetic lows in these areas were caused by reversely magnetized Crescent Formation.…”

“…Predominantly marine basaltic rocks of the Eocene Crescent Formation form the basement below the southern Puget Lowland. These rocks tend to be dense and magnetic, and are considered to be responsible for the gravity and magnetic highs measured over the Seattle uplift [ Blakely et al , 2002; Hagstrum et al , 2002] (Figures 1 and 2). The Crescent Formation crops out in the northern part of the Seattle uplift [ Yount and Gower , 1991; Haeussler and Clark , 2000], in a rim surrounding the core of the Olympic Mountains [ Tabor and Cady , 1978], and on the south flank of the Tacoma basin in the Black Hills [ Walsh et al , 1987] (Figure 1).…”

“…(3) Only strata of the youngest glacial advance (Vashon) and the youngest part of the preceding Olympia interglacial period can be confidently dated with radiocarbon techniques. There are only a few dates or constraints on the ages of older Quaternary units [ Mahan et al , 2000; Hagstrum et al , 2002]. As a result, geologic maps in the region commonly group strata that is inferred to be older than the pre‐Olympia interglacial interval [e.g., Booth , 1991].…”

“…The zone of deformation imaged on Figure 13 between faults “3” and “4” can be traced with seismic reflection data across East Passage, projecting onland at Saltwater State Park (Figure 4) where there is considerable evidence for deformation in Quaternary deposits exposed along the coastal bluffs and in the wave‐cut bench. These strata yielded reversed magnetic polarities and are probably older than 780 ka [ Hagstrum et al , 2002]. In the southern part of the park, a 200‐m‐wide section of distinctive alluvial sand and silt strikes 95° and dips as steeply as 16° north.…”