Joanne Bourgeois

Hummocky stratification: Significance of its variable bedding sequences

Hummocky cross-stratification is an important structure formed on the shoreface and shelf by waves. It characterizes a wave-dominated facies. Attention to its variability can reveal much about sedimentary history and paleogeography. Diagnostic traits are antiformal hummocks and synformal swales defined by randomly oriented, even lamination with dip angles and truncation angles of < 15°. Hummocky stratification forms primarily in silt to fine sand. Although size grading of individual laminae is not characteristic, concentrations of mica and plant detritus in the tops of many laminae indicate a shape sorting. Parting lineation is common. Hummocky beds vary in thickness from a few centimetres to 5 or 6 m; bed sets may be tens of metres thick. Hummocky stratification apparently is formed most commonly by redeposition below normal fair-weather wave base of fine sand delivered offshore by flooding rivers and scour of the shoreface or shoals by large waves. Deposition involves both fallout from suspension and lateral tractive flow due to wave oscillation. There is evidence that, under intense oscillatory flow, large waves drape sand over an irregular scoured surface and also mold sand into roughly circular, unoriented hummocks and swales. We postulate that these circumstances are analogous to the transition to upper flat-bed conditions in unidirectional flow. Hummocky stratification shows important variability. It occurs in both regressive (progradational) and transgressive strata in intervals a few centimetres to 175 m thick and may be interstratified with mudstone, sandstone, or conglomerate. Hummocky stratification commonly occurs in repetitive successions with the products of individual depositional events being clearest where mudstone separates hummocky beds. An idealized hummocky stratification sequence , which can serve a purpose similar to the Bouma sequence for graded beds, is as follows (bottom to top): first-order scoured base (± sole marks); characteristic hummocky zone with several second-order truncation surfaces separating individual undulating lamina sets; a zone of flat laminae ; a zone with well-oriented ripple cross-laminae and symmetrical ripple forms; all overlain by a more or less burrowed mud-stone or siltstone . This sequence reflects waning of storm waves followed by fair-weather sedimentation and burrowing. Variations from this idealized conceptual sequence involve omissions and/or expansions of one or more of the zones. The most common variant is amalgamation either by the stacking of successive hummocky zones or by intense bioturbation that obliterates original boundaries between depositional units. Other variations include units commencing with flat-lamination; units with predominant cross-lamination; and lenticular micro-hummocky lenses within shale. Combinations of relative sand supply, relative depth, tidal range, frequency, duration and magnitude of storms, and relative productivity for a burrowing benthos must account for such differences. Further documentation of variations in hummocky stratification should reveal important details about these factors.

A Tsunami Deposit at the Cretaceous-Tertiary Boundary in Texas

At sites near the Brazos River, Texas, an iridium anomaly and the paleontologic Cretaceous-Tertiary boundary directly overlie a sandstone bed in which coarse-grained sandstone with large clasts of mudstone and reworked carbonate nodules grades upward to wave ripple-laminated, very fine grained sandstone. This bed is the only sandstone bed in a sequence of uppermost Cretaceous to lowermost Paleocene mudstone that records about 1 million years of quiet water deposition in midshelf to outer shelf depths. Conditions for depositing such a sandstone layer at these depths are most consistent with the occurrence of a tsunami about 50 to 100 meters high. The most likely source for such a tsunami at the Cretaceous-Tertiary boundary is a bolidewater impact.

Summary of Coastal Geologic Evidence for Past Great Earthquakes at the Cascadia Subduction Zone

Earthquakes in the past few thousand years have left signs of land-level change, tsunamis, and shaking along the Pacific coast at the Cascadia subduction zone. Sudden lowering of land accounts for many of the buried marsh and forest soils at estuaries between southern British Columbia and northern California. Sand layers on some of these soils imply that tsunamis were triggered by some of the events that lowered the land. Liquefaction features show that inland shaking accompanied sudden coastal subsidence at the Washington-Oregon border about 300 years ago. The combined evidence for subsidence, tsunamis, and shaking shows that earthquakes of magnitude 8 or larger have occurred on the boundary between the overriding North America plate and the downgoing Juan de Fuca and Gorda plates. Intervals between the earthquakes are poorly known because of uncertainties about the number and ages of the earthquakes. Current estimates for individual intervals at specific coastal sites range from a few centuries to about one thousand years.

A millennial-scale record of Holocene tsunamis on the Kronotskiy Bay coast, Kamchatka, Russia

Abstract Deposits from as many as 50 large tsunamis during the last 7000 years are preserved on the Pacific coast of the Kamchatka Peninsula near the mouth of the Zhupanova River, southern Kronotskiy Bay. These deposits are dated and correlated using Holocene marker tephra layers. The combined, preserved record of tsunami deposits and of numerous marker tephras on Kamchatka offers an unprecedented opportunity to study tsunami frequency. For example, from the stratigraphy along southern Kronotskiy Bay, we estimate frequency of large tsunamis (>5 m runup). In the last 3000 years, the minimum frequency is about one large tsunami per 100 years, and the maximum about one large tsunami per 30 years; the latter frequency occurred from about 0 to 1000 A.D. This time interval corresponds to a period of increased seismicity and volcanic activity that appears to be recorded in many places on the Kamchatka Peninsula.

Tsunami field survey of the 1992 Nicaragua earthquake

An earthquake with surface magnitude (Ms ) 7.0 occurred 100 km off the Nicaraguan coast on September 2, 1992 (GMT). Despite its moderate size, this earthquake generated a sizable tsunami, which caused extensive damage along the coast of Nicaragua. In late September, about 170 people, mostly children, were listed dead or missing; 500 were listed injured; and over 13,000 were listed homeless, with more than 1500 homes destroyed. Damage was the most significant since the 1983 Japan Sea earthquake tsunami, which killed 100 people in Japan. The Flores (Indonesia) earthquake and tsunami of December 12, 1992, were more destructive than the Nicaragua or Japan Sea events.

Tsunami geomorphology: Erosion and deposition from the 15 November 2006 Kuril Island tsunami

The 15 November 2006 Kuril earthquake (Mw 8.1–8.4) and tsunami enabled us to collect a compelling data set of coastal geomorphic change in the Kuril Islands from ~3 months before to 9 (and 21) months after the tsunami. Our pre-tsunami and post-tsunami surveys of the islands, including four topographic profi les measured in 2006 and reoccupied in 2007, allow us the confi dence to attribute many changes to the tsunami, in spite of an absence of eyewitness accounts in the central islands. Areas with low runup, 15 m, underwent massive erosion that dramatically altered the coastline. Tsunami deposits roughly corresponded with the extent of tsunami runup and inundation. The amount of sediment eroded by the tsunami far outweighed the amount deposited on land in all cases studied. The tsunami was dominantly erosive in the Kuril Islands because the high-relief topography of the coastline accelerated tsunami outfl ow.

Holocene tsunamis in the southwestern Bering Sea, Russian Far East, and their tectonic implications

The Bering Sea coast of Kamchatka overlies a boundary between the proposed Okhotsk and Bering blocks, or (micro)plates, of the North American plate in the Russian Far East. A history of tsunamis along this coast for the past 4000 yr indicates that the zone north of the Kuril-Kamchatka subduction zone produces tsunamigenic earthquakes every few centuries. Such a record is consistent with convergence of the proposed Okhotsk and Bering blocks along the Bering Sea coast of Kamchatka. A tsunami deposit from the 1969 M w 7.7 Ozernoi earthquake helps us interpret older tsunami deposits. Newly studied tephra layers from Shiveluch volcano as well as previously established marker tephra layers in northern Kamchatka provide age control for historic and prehistoric tsunami deposits. Based on >50 measured sections along 14 shoreline profiles, tsunami-deposit frequencies in the southwestern Bering Sea are about five per thousand years for tsunamis generated north of the Kuril-Kamchatka trench.

Sandy signs of a tsunami's onshore depth and speed

Tsunamis rank among the most devastating and unpredictable natural hazards to affect coastal areas. Just 3 years ago, in December 2004, the Indian Ocean tsunami caused more than 225,000 deaths. Like many extreme events, however, destructive tsunamis strike rarely enough that written records span too little time to quantify tsunami hazard and risk. Tsunami deposits preserved in the geologic record have been used to extend the record of tsunami occurrence but not the magnitude of past events. To quantify tsunami hazard further, we asked the following question: Can ancient deposits also provide guidance on the expectable water depths and speeds for future tsunamis?

Geologic evidence of earthquakes at the Snohomish delta, Washington, in the past 1200 yr

Exposed channel banks along distributaries of the lower Snohomish delta in the Puget Lowland of Washington reveal evidence of at least three episodes of liquefaction, at least one event of abrupt subsidence, and at least one tsunami since ca. A.D. 800. The 45 measured stratigraphic sections consist mostly of 2‐4 m of olivegray, intertidal mud containing abundant marsh plant rhizomes. The most distinctive stratigraphic unit is a couplet comprising a 0.523-cm-thick, laminated, fining-upward, tsunami-laid sand bed overlain by 2210 cm of gray clay. We correlated the couplet, which is generally ;2 m below the modern marsh surface, across an ;20 km2 area. Sand dikes and sand-filled cracks to 1 m wide, which terminate upward at the couplet, and sand volcanoes preserved at the level of the sand bed record liquefaction at the same time as couplet deposition. Differences in the type and abundance of marsh plant rhizomes across the couplet horizon, as well as the gray clay layer, suggest that compaction during this liquefaction led to abrupt, local lowering of the marsh surface by as much as 50‐75 cm. Radiocarbon ages show that the tsunami and liquefaction date from ca. A.D. 800 to 980, similar to the age of a large earthquake on the Seattle fault, 50 km to the south. We have found evidence for at least two, and possibly as many as five, other earthquakes in the measured sections. At two or more stratigraphic levels above the couplet, sand dikes locally feed sand volcanoes. Radiocarbon ages and stratigraphic position gov. suggest that one set of these dikes formed ca. A.D. 910‐990; radiocarbon ages on a younger set indicate a limiting maximum age of A.D. 1400‐1640. We also interpret a sharp lithologic change, from olive-gray, rhizome-rich mud to grayer, rhizome-poor mud, ;1 m above the couplet, to indicate a second abrupt lowering of the marsh surface during an earthquake ca. A.D. 1040‐ 1400, but no conclusive liquefaction structures have been identified at this horizon. Two distinctive coarse-sand laminae, 30‐80 cm below the couplet, may record tsunamis older than A.D. 800. Thus, study shows that in the past ;1200 yr, this part of Washington’s Puget Lowland has been subjected to stronger ground shaking than in historic times, since ca. 1870.

Slip Distribution of the 1952 Kamchatka Great Earthquake Based on Near-Field Tsunami Deposits and Historical Records

We explore the magnitude and slip distribution of the 1952 Kamchatka earthquake (Mw 8.8-9.0) using constraints from the 1952 Kamchatka tsunami. Our new field data provide more comprehensive coverage of the near-field tsunami than had been available to date. We examine the effects of internal slip distribution within complex earthquake ruptures on near-field tsunami runup and evaluate some of the limitations of this approach. Our approach compares tsunami-deposit distribution with simulated runup from tsunamis generated by different configurations of seafloor deformation from hypothetical earthquakes resembling that of the 1952 Kamchatka earthquake. We identify areas of high slip because different distributions of seafloor deformation result in variations in tsunami runup in the near field. Mapped deposits and local observations of the 1952 Kamchatka tsunami indicate that near-field runup in central Kamchatka was consistently less than 10 m (averaging 6 m), while south Kamchatka to the northern Kuril Islands had more variability and higher average runup (8 m runup in South Kamchatka and 10 m runup in the northern Kuril Islands). Our simulations show that in order to produce the distribution of runup indicated by tsunami deposits and historical observations, the 1952 earthquake had regions of high slip off the coast of southern Kamchatka, and the location of high slip is shallower in the subduction zone than previously interpreted. Online Material: Sedimentary methodology, model inputs, and simulation results.