STRATIGRAPHY AND SEISMIC STRATIGRAPHY

Tertiary rocks

Tertiary rock units imaged on seismic-reflection data in the vicinity of the Seattle fault include (1) mainly basaltic rocks of the lower Eocene Crescent Formation, (2) unnamed Eocene marine strata, (3) turbidite sandstone and siltstone of the upper Eocene to Oligocene Blakeley Formation, and (4) nonmarine sedimentary rocks of the Miocene Blakely Harbor Formation (Johnson et al., 1994). These rock units can be delineated on conventional industry data on the basis of seismic facies characteristics (Sangree and Widmier, 1977) and borehole data, as described in Johnson and others (1994, 1996) and Pratt and others (1997). On industry seismic data, the Tertiary-Quaternary contact is typically imaged as a strong, fairly continuous reflection separating higher amplitude, more continuous, commonly parallel reflections (Tertiary rocks) and lower amplitude, discontinuous, hummocky or irregular reflections (Quaternary strata). In uplifted areas, the Tertiary rocks are commonly folded and the contact is an angular unconformity. This angular unconformity passes laterally into a disconformity in the Seattle basin.

Pleistocene strata

Pleistocene deposits of the Puget Lowland comprise a stratigraphically complex basin fill of glacial and interglacial deposits that are locally as thick as 1,100 m (Yount et al., 1985; Jones, 1996). Easterbrook (1994a, b) described six distinct glacial drift units and there are significant time gaps in the Pleistocene stratigraphic record in which additional glaciations may have occurred (Fig. 3). Glacial and interglacial strata consist mainly of till, fluvial channel and floodplain deposits, and fine-grained lacustrine deposits. On seismic-reflection profiles, Pleistocene strata (excluding the latest Pleistocene) form a distinct seismic unit, bounded below by Tertiary rocks and above by typically flat-lying latest Pleistocene to Holocene deposits that commonly fill in erosional relief. On both conventional and high-resolution seismic-reflection data, Pleistocene strata display highly variable amplitudes, are discontinuous, and have parallel, divergent, and hummocky reflections. Internal truncation, onlap, and offlap of reflections are all common. Determining the age of the Pleistocene deposits imaged on seismic-reflection data in Puget Lowland waterways is problematic but important for understanding rates of deformation. No boreholes have penetrated the submerged section and multiple pulses of subglacial scour and subsequent filling (e.g., Booth, 1994) make correlation with adjacent dated units on land unreliable.

Latest Pleistocene and Holocene strata

On seismic-reflection data in Puget Sound and associated waterways, latest Pleistocene and Holocene deposits are accumulating in basins bounded by Pleistocene bathymetric highs. These basins are filled with flat lying Holocene deposits and show up clearly on digital elevation images (Fig. 4) as flat surfaces. The continuous, elongate, north-trending Holocene basin in central Puget Sound is here termed the central Puget Sound trough. In the Seattle area, this trough has a steep linear west flank and a more irregular east flank. Holocene basin fill typically yields variable-amplitude, parallel, and continuous reflections. Holocene sediment samples from Puget Sound are typically clay and silt (Wang, 1955).