Lake Washington, which forms the eastern boundary of Seattle, is located in a tectonically active area containing several strands of the Seattle fault. High-resolution seismic reflection profiling, sidescan swath imagery, and sediment coring were used to define deformation of Holocene lake sediments and the distribution, geometry, age, and causes of submarine landslides. Numerous large block slides, sediment slumps, and debris flows are present throughout the lake, and large landslides obscure the surficial structure of Seattle fault strands as they pass through the lake. In addition, most bays along the lake margin are the headwalls of large submarine slides. Submerged forests show evidence of deep-seated block failures that have exposed glacial sediments and Tertiary rocks. The massive submarine block slides, and retrogressive submarine slope failures were triggered most likely by large (m(b) > 7) earthquakes on the Seattle fault and/or large to great (m(b) > 8) temblors occurring elsewhere in Cascadia. Buried landslides suggest that submarine slope failures and mass wasting occurred more than once in the last 11,000 yr.Sediments in Lake Washington preserve a record of episodic deposition of turbidites possibly caused by seismically induced submarine landslides. Magnetic susceptibility profiles on 36 gravity cores show a characteristic series of magnetic peaks that can be traced throughout the lake. X-radiography and grain size analyses suggest that the magnetic peaks represent anomalous detrital layers that in some cases are turbidites. The areal extent and magnetic signatures of many of the deposits suggest multiple sources, which is consistent with numerous local landslides caused by large earthquakes. Radiocarbon dating and correlation of the downcore magnetic profiles establish a sediment record in which episodic sedimentary disturbances occurred seven times in the last 3500 yr. If these deposits are seismically induced turbidites (seismites), then large earthquakes have occurred about every 300-500 yr in the Puget Sound region.

Relatively unmetamorphosed Paleozoic miogeoclinal carbonate rocks in the Basin and Range of E Nevada, SW Nevada and adjacent California, and W Utah yield low-inclination magnetizations that reflect pervasive, regional remagnetization around the close of the Paleozoic. The rocks range in age from mid-Cambrian through Pennsylvanian and lie generally in a broad belt between the mid-Paleozoic Roberts Mountain Thrust and the late Cretaceous Sevier thrusts. Most of the magnetizations reside in magnetite, but at one site the magnetization is evidently carried by pyrrhotite. Preliminary rock-magnetic data suggest samples with magnetite-borne remanences have wasp-waisted hysteresis curves typical of remagnetized carbonates. The origin of the remagnetization is problematic and probably polygenetic: both the Permo-Triassic Sonoma orogeny and deformation associated with the Ancestral Rockies seem too spatially limited, but magnetite from smectite destruction seems difficult to reconcile with the great stratigraphic extent of late Paleozoic remagnetization unless combined with thermal resetting of the lowermost units. A number of sites also appear to have undergone some vertical-axis rotation, and the sense and magnitude of these rotations are grossly consistent with independent geologic evidence. However, the probably large age range of the low-inclination components complicates their use for resolving tectonic rotations. Younger, intermediate-stability components of magnetization, probably of Cretaceous or Cenozoic age, are also found in many sites and also probably have multiple origins. At sites farther W, the late Paleozoic component is not found, which probably reflects its destruction by later Mesozoic or Cenozoic heating. At sites farther E, on and near the Colorado Plateau, gray carbonates yield only Cenozoic magnetizations. Some reddish, oxidized carbonates there locally contain a hematite-borne magnetization of late Paleozoic age. However, it is probably related to the development of thick continental redbed sequences in overlying strata on the plateau rather than to the remagnetization process(es) in the miogeocline.

Coring at site ODP 1033B in Saanich Inlet recovered 59.4 m of mainly laminated olive-grey diatom ooze and an underlying 55.15 m of massive grey to olive-grey silty clay. Based on AMS radiocarbon dating, the boundary between the two units is between 11,000 and 13,800 calibrated years BP, and represents the Holocene-Pleistocene boundary. The lower unit represents glaciomarine deposition, whereas deposition of the upper unit began when the modern semi-restricted physiography of the fjord was established following glacial rebound and highly productive marine conditions were established. The glaciomarine clay is almost entirely terrigenous, whereas the diatom ooze contains 2–3 wt.% organic C and 20–40 wt.% biogenous silica; CaCO3 contributions are minor, but there are several peaks in carbonate abundance in the upper unit. The isotopic composition of organic C and total N suggests that organic matter in the glaciomarine clay is dominantly terrestrial (δ13Corganic<−25‰ and δ15Ntotal=ca. 3‰) and in the diatom oozes it is mainly marine (δ13Corganic>−22‰ and δ15Ntotal=ca. 10‰). The heavy δ15Ntotal values probably record a contribution of isotopically heavy nitrate to the surface waters of the inlet that is transported to British Columbia (BC) coastal waters from the eastern tropical Pacific by the California Undercurrent. Major and minor elemental data suggest that the composition of the terrigenous material and its grain-size has changed over the last 15 kyr, and there are marked enrichments in several redox-sensitive elements in the diatom oozes. Thus, Cu, Mn, Mo, Ni, Pb, V and Zn have higher concentrations in the upper unit; Br and I are also enriched because of their association with organic matter. Mn is enriched in the anoxic diatom oozes due to the presence of manganoan carbonate (Mn peaks generally corresponding with carbonate peaks) formed in the sediment when deep water renewal caused precipitation of Mn oxyhydroxides, which dissolved in the anoxic sediment and was precipitated as a diagenetic phase. The remaining metals are enriched because of their removal to the sediment as sulphides (Cu, Mo, Ni, Pb and Zn) or as particle-reactive reduced species (V). Cr enrichment is obscured by the presence of Fe-rich chlorite. The lag in the enrichment of Mo with respect to organic C in the sediments indicates that anoxia developed some time after marine production increased following the semi-isolation of the fjord.

The modern laminated coccolith marl (unit 1) of the Black Sea containing 2-6 wt% organic carbon overlies a finely laminated sapropel (unit 2) containing 5-20 wt% organic carbon. However, the accumulation rate of organic carbon in the sapropel is not significantly different from that in unit 1, In contrast, the accumulation rates of either CaCO3 or lithogenous matter (clay) are significantly lower in the sapropel compared with unit 1, Thus, the sapropel has very high organic carbon contents because of lesser dilution of the organic fraction by the other bulk components. Because the accumulation rate of organic carbon in the sapropel is similar to that in unit 1, which is not significantly different from accumulation rates in oxygenated environments in similar settings, the Black Sea sapropel is not necessarily characteristic of anoxic basins.

The abundance and isotopic composition of total and pyrite sulphur have been determined in a core in the central Black Sea. Pyrite varies in concentration from roughly 2 wt% in the sapropel (Unit 2), to 0.8 wt% in the modern horizon (Unit 1) and ca. 0.1 wt% in the lowermost horizons of Unit 3 (Lake Beds). An intermediate mud-flow horizon and the upper part of the lake beds have similar values to those in the modern sediments. The degree of pyritization (DOP) is lowest (<0.05) in the lower lake beds and highest (0.55) in Unit 1; the sapropel has values of ca. 0.48. δ34Spyrite varies from a minimum of −37‰ in the sapropel, to −33‰ in Unit 1, up to +15‰ in the upper lake beds, and 0‰ in the lower lake beds. The Unit 1 and sapropel values are similar to the δ34S values reported by Fry et al. (1991) of dissolved sulphide immediately below the water-column oxic-anoxic interface (−36 to −38‰), but significantly heavier than those in the deep waters of the basin (ca. −41‰. These results are interpreted as a reflection of the presence of four different types of pyrite in the sediments of the Black Sea: (1) pyrite (in Units 1 and 2) that is formed within the upper part of the water column immediately below the oxic-anoxic interface where δ34S of dissolved sulphide is −36‰; (2) pyrite in the mud-flow layer lying between Unit 1 and the sapropel (Unit 2), with a significantly heavier isotopic composition, that was formed around the margins of the basin under conditions that were probably more closed with respect to dissolved sulphate than those in which the pyrite in Units 1 and 2 were formed; (3) pyrite in the upper part of the Lake Beds (Unit 3) that was formed under closed-system conditions by reaction between excess reducible Fe remaining in these freshwater sediments and downward diffusing sulphate and/or sulphide from the overlying organic-rich sapropel; and (4) pyrite in the lower part of the Unit 3 representing a phase formed from the ambient sulphate in the freshwaters of the Late Pleistocene lake. The isotopic information supports earlier suggestions that pyrite in the modern sediments of the Black Sea forms mainly in the water column, with only minor amounts forming in the sediment, because of the severe Fe-limitation of the bottom sediments that accumulate 2,000 m below the main Fe and HS− reaction zone at the oxic-anoxic interface at shallow depths in the water column.

Large earthquakes can trigger slumping of the steep walls of lake basins and landslides in the drainage area, resulting in turbidite deposition in the lake and increased detrital flux from inlets. Holocene sediments in piston cores from Lake Washington contain a series of terrigenous layers that were episodically deposited in the lake. Sedimentological, geochemical, and paleomagnetic analyses on nine piston cores show that the detrital layers are temporally and areally correlatable, indicating basinwide disruptions. These layers are opaque on X‐radiographs, are coincident with magnetic susceptibility peaks, have abundant aluminosilicate minerals, are relatively coarse grained, and have low organic carbon and biogenic silica contents. The thicknesses and geographic distributions of the layers suggest that they are not due to floods or delta destabilization. Side scan swath imagery and subbottom profiling show that large slumps, subaqueous landslides, and debris flows are common along the margins of the lake. A detailed chronology, established from 21 radiocarbon ages on five cores, show that a prominent turbidite was deposited about 1000–1100 years ago. This turbidite apparently was triggered by a large earthquake that probably occurred on the Seattle fault. Other depositional events in the sediment record at 1500–1700, 2400–2500, and 2800–3200 years ago coincide with periods of landsliding that have been previously inferred from the dating of drowned trees in the lake. More than 30 depositional events have occurred in the last 12,000 years and 21 disturbances have occurred since the deposition of the Mazama ash about 7,600 years ago. If all the events are due to earthquakes, the Puget Sound region has been subject to major shaking every 300 to 400 years. The strong intensities needed to trigger subaqueous slides may not be generated by just local sources such as the Seattle fault but could also be caused by great subduction earthquakes occurring along the coast.

Holocene sediments in Lake Washington contain a series of turbidites that were episodically deposited throughout the lake. The magnetic signatures of these terrigenous layers are temporally and areally correlatable. Large earthquakes appear to have triggered slumping on the steep basin walls and landslides in the drainage area, resulting in turbidite deposition. One prominent turbidite appears to have been deposited about 1100 years ago as the result of a large earthquake. Downcore susceptibility patterns suggest that near-simultaneous slumping occurred in at least three separate locations, two of which now contain submerged forests. Several other large earthquakes may have occurred in the last 3000 years.

The ratio of sedimentary S to organic C has been used as a diagnostic criterion for recognizing anoxic environments in the sedimentary record. Here we examine the relationship between pyrite S and organic C in the modern (Unit 1) sediments of the Black Sea to re-evaluate this suggested relationship. In box cores from shallow oxic or near-oxic water, pyrite S contents, ranging from 0.1 and 2.5% by weight, do not correlate with organic C contents. In cores from the deep anoxic basin, pyrite S concentrations range between 1.0 and 2.5% and are linearly related to organic C contents with an intercept that is not significantly different from zero. The S/C ratio is around 0.31 in these sediments, which is somewhat lower than the ratio of 0.36 considered to be characteristic of "normal" marine oxic sediments. Thus, the Black Sea anoxic sediments appear to be depleted in S relative to organic C.The degree of pyritization (DOP) of the sediments may be a better measure of anoxic conditions of sedimentation. The DOP values measured here show that pyrite formation is Fe limited, the correlation between organic C and pyrite S probably being induced by a positive correlation between fine-grained sediment components (the original Fe source for pyrite formation in the sediments) and organic matter. Such Fe-limitation is probably characteristic of anoxic basins where pyrite formation can take place in the water column and Fe oxyhydroxides do not accumulate in the surface sediments. The results on the sample set examined here are at variance with earlier data and interpretations for the Black Sea. In particular, the suggested S enrichment of anoxic Black Sea sediments was not found because of Fe limitation.

THE Black Sea, the world's largest anoxic marine basin, is frequently used as a modern analogue for the formation of organic-rich sediments and carbonaceous rocks 1-3, on the widely held assumption that anoxic conditions promote the preferential preservation of organic matter in sediments. Data for testing this hypothesis have so far been equivocal 4-7, but here we use radiocarbon ages obtained using accelerator mass spectrometry for the organic fraction of recent Black Sea sediments to estimate the organic carbon accumulation rates. These range from 0.69 to 2.09 g C m-2 yr-1 and are significantly lower than earlier estimates based on varve counting 6. Depending on the value taken for the rate of primary production in the Black Sea 4,8, between 0.7 and 2.1% of the organic carbon is preserved in the bottom sediments. When compared with carbon accumulation rates in equivalent oxygenated environments 9, these results indicate that the modern Black Sea is not a site of anomalously high organic carbon accumulation. This suggests that anoxic conditions in the water column may not be a prerequisite for the preservation of organic matter in marine sediments, and that models of the origin of carbonaceous facies in the geological record may therefore need to be modified.