Walter A. Barnhardt

Late Quaternary relative sea-level change in the western Gulf of Maine: Evidence for a migrating glacial forebulge

New radiocarbon-dated cores obtained by Vibracorers in the western Gulf of Maine confirm that a short-lived, relative sea-level lowstand of ∼−55 m occurred at 11–10.5 ka. These cores and younger salt-marsh data also reveal that rates of transgression varied throughout the Holocene, probably due to local variations in glacial isostasy. The isostatic component is resolved by subtracting published approximations of eustatic sea level from our well-determined observations of local relative sea level. A large peak in the isostatic curve coincides with the lowstand and is interpreted as a forebulge 20–25 m in amplitude. Forebulge migration is estimated at 7–11 km/100 yr, based on the timing of lowstands across the region.

Giant sea-bed pockmarks: Evidence for gas escape from Belfast Bay, Maine

Circular depressions, or pockmarks, cover the sea floor in many estuarine regions of the western Gulf of Maine. In Belfast Bay, Maine, they are found in densities up to 160/km 2 , are up to 350 m in diameter and 35 m in relief, and are among the largest and deepest known. The pockmarks appear to form from the escape of biogenic natural gas and pore water and are far larger than features associated with thermogenic gas elsewhere. These pockmarks are thought to have formed (1) catastrophically during an earthquake, tsunami, or storm, or (2) slowly over thousands of years. Recent observations of bubble releases suggest continuing activity and a potential geologic hazard. The pockmarks involve a poorly documented coastal process of sediment redistribution and methane release, largely unrecognized in the rock record but widespread in middle- to high-latitude embayments.

Rapid resetting of an estuarine recorder of the 1964 Alaska earthquake

Tides and plants have already restored much of a landscape that the 1964 Alaska earthquake destroyed. At the head of a macrotidal estuary near Anchorage, in the vicinity of Portage, subsidence during the earthquake changed meadows, thickets, and spruce groves into barren tidal flats. Tidal-flat silt and sand soon buried the pre- earthquake landscape while filling intertidal space that the subsidence had made. The flats supported new meadows and thickets by 1973 and new spruce by 1980. Three new findings confirm that the flats aggraded rapidly and that their vegetation is maturing. (1) Most of the postearthquake deposits at Portage date from the first decade after the 1964 earthquake. Their thickness of 23 sites in a 0.5 km 2 area was 1.4 ± 0.2 m in 1973, 1.6 ± 0.2 m in 1991, and 1.6 ± 0.3 m in 1998. (2) Many of the deposits probably date from the first months after the earthquake. The deposits contain sedimentary couplets in which coarse silt or very fine sand is capped by fine or medium silt. About 100 such couplets make up the lowest 0.5 m or more of the postearthquake deposits in two outcrops. These couplets thicken and thin rhythmically, both as groups of 5–20 couplets and as pairs of successive couplets. Probably, the groups of thick couplets represent the highest tides, the groups of thin couplets represent some of the lesser high tides, and the pairs record inequality between twice-daily high tides. (3) In the 1980s and 1990s, thickets expanded and spruce multiplied. The vegetation now resembles the fossil assemblage rooted in the buried landscape from 1964. Had the 1964 Alaska earthquake been repeated a decade later, the two earthquakes would now be recorded by two superposed, buried landscapes near Portage. Much more than a decade is probably needed to reset similar recorders at mesotidal estuaries of the Cascadia subduction zone.

Grouted sediment slices show signs of earthquake shaking

Sand and mud from Washington State, sampled with Japanese methods for identifying structure in unconsolidated deposits, have provided new evidence for earthquakes over the past 2000 years at the Cascadia subduction zone. Each sample was collected as a vertical slice, 0.5 m wide and up to 8 m long, in sheetpile driven into wet sand and mud beneath a tidal bank of the Columbia River (Figures 1 and 2). Painted with flexible, hydrophilic grout, the slices yielded full-size peels that reveal bedding and its disruption (Figure 3). Evidence for liquefaction is common, even where it is absent at the ground surface. Especially common are sills that imply lateral escape of water. These findings may affect ground-motion estimates for plate-boundary earthquakes in the northwestern United States and Canada.

Geologic evidence for the incorporation of flood tidal deltas at Tavira Island, southern Portugal

ABSTRACT Tavira Island, the largest barrier island on Portugals Algarve coast, exhibits a broad backbarrier area that probably consists of relict flood tidal deltas that were incorporated when associated tidal inlets closed. The tidal deltas are preserved as lobate landforms extending into the marsh, and are bordered by linear, partially infilled depressions that resemble flood channels on modern tidal deltas in the region. The origin and evolution of the incorporated deltas and the location of former inlets were investigated using a dataset of 6.7 km of ground-penetrating radar (GPR) profiles and more than 30 cores. The data reveal a complex stratigraphic framework including a series of lateral facies changes that are interpreted relative to the incorporated tidal delta model and sedimentary environments as seen in the modern barrier chain.

Influence of Near-surface Stratigraphy on Coastal Landslides at Sleeping Bear Dunes National Lakeshore, Lake Michigan, USA

Abstract Lake-level change and landslides are primary controls on the development of coastal environments along the coast of northeastern Lake Michigan. The late Quaternary geology of Sleeping Bear Dunes National Lakeshore was examined with high-resolution seismic reflection profiles, ground-penetrating radar (GPR), and boreholes. Based on sequence-stratigraphic principles, this study recognizes ten stratigraphic units and three major unconformities that were formed by late Pleistocene glaciation and postglacial lake-level changes. Locally high sediment supply, and reworking by two regressions and a transgression have produced a complex stratigraphy that is prone to episodic failure. In 1995, a large landslide deposited approximately 1 million m3 of sediment on the lake floor. The highly deformed landslide deposits, up to 18 m thick, extend 3–4 km offshore and unconformably overlie well-stratified glacial and lacustrine sediment. The landslide-prone bluff is underlain by channel-fill deposits that are oriented nearly perpendicular to the shoreline. The paleochannels are at least 10 m deep and 400 m wide and probably represent stream incision during a lake-level lowstand about 10.3 ka B.P. The channels filled with sediment during the subsequent transgression and lake-level highstand, which climaxed about 4.5 ka B.P. As lake level fell from the highstand, the formation of beach ridges and sand dunes sealed off the channel and isolated a small inland lake (Glen Lake), which lies 5 m above the level of Lake Michigan and may be a source of piped groundwater. Our hypothesis is that the paleochannels act as conduits for pore water flow, and thereby locally reduce soil strength and promote slope failure.

Mapping southern Puget Sound delta fronts after 2001 earthquake

A moment magnitude 6.8 earthquake struck southern Puget Sound (Figure 1) on February 28, 2001, causing an estimated

Holocene Evolution of the Merrimack Embayment, Northern Massachusetts, Interpreted from Shallow Seismic Stratigraphy

0.7–

Pockmarks in Passamaquoddy Bay, New Brunswick, Canada

1.4 billion in damages to buildings and roadways in the region [Williams et al., 2001]. The earthquake source was 52 km deep, and the epicenter was located close to the Nisqually River delta in the same location as the epicenter of the magnitude 7.1 earthquake of 1949 (http://www.geophys.washington.edu/seis/pnsn/info_ general/). These deep earthquakes occurred in the eastward-dipping subducting slab of the Juan de Fuca plate and typically caused less damage than shallower, crustal events of the same magnitude. Details of the seismology and effects of the earthquake can be found at http://earthquake.usgs.gov/activity/latest/ eq_01_02_28.html.

NON-DESTRUCTIVE MEASUREMENT OF SOIL LIQUEFACTION DENSITY CHANGE BY CROSSHOLE RADAR TOMOGRAPHY, TREASURE ISLAND, CALIFORNIA

Recent multi-beam, backscatter, and bottom sediment data demonstrate that a large sand sheet was formed in the inner shelf by the reworking of the Merrimack River lowstand delta and braid plain (12 kya) during the Holocene transgression. Seismic data reveal the presence of widespread channel cut-and-fill structures landward of the delta suggesting that much of the sand sheet consists of braided stream deposits. These features map into several sets of cut-and-fill structures, indicating the avulsion of the primary river channels, which creates the lobes of the paleo-delta. Truncations of these, cut-and-fill structures suggest that the braid plain deposits were probably reworked during the Holocene transgression and may have contributed sand to developing barriers that presently border the Merrimack Embayment.