Geology topics

W. A. Barnhardt

Publications and source records attributed to W. A. Barnhardt.

10 recordsLinked to original sources

USGS2012

Holocene evolution of the merrimack embayment, northern massachusetts, interperted from shallow seismic stratigraphy

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.

Conference Paper↗

USGS2009

Coastal change along the shore of northeastern South Carolina— The South Carolina Coastal Erosion Study

The U.S. Geological Survey, in cooperation with the South Carolina Sea Grant Consortium, conducted a 7-year, multidisciplinary study of coastal erosion in northeastern South Carolina. Shoreline behavior along the coast of Long Bay is dictated by waves, tidal currents, and sediment supply that act within the overall constraints of the regional geologic setting. Beaches are thin ribbons of sand that sit on top of layered sedimentary rocks, which have been deeply eroded by rivers and coastal processes over millions of years. Offshore of the beaches, these sedimentary rocks are exposed as hardgrounds over large expanses of shallow seafloor and are locally overlain by a discontinuous veneer of sandy sediment generally less than 1 m thick. Rates of shoreline retreat largely depend on the geologic framework of the shoreface that is being excavated by ocean processes. Mainland-attached beaches have remained relatively stable, whereas barrier islands have experienced large shifts in shoreline position. In this sediment-limited region, erosion of the shoreface and inner shelf probably contributes a significant amount of new material to the beach system. Oceanographic studies and numerical modeling show that sediment transport varies along the coast, depending on the type and travel path of storms that impact Long Bay, but the long-term net transport direction is generally from north to south. Changes in storm activity that might accompany climate change, coupled with anticipated increases in sea-level rise, are expected to strongly affect low-lying, heavily developed areas of the coast.

South Carolina↗

USGS2006

Evolution of a Holocene delta driven by episodic sediment delivery and coseismic deformation, Puget Sound, Washington, USA

Episodic, large-volume pulses of volcaniclastic sediment and coseismic subsidence of the coast have influenced the development of a late Holocene delta at southern Puget Sound. Multibeam bathymetry, ground-penetrating radar (GPR) and vibracores were used to investigate the morphologic and stratigraphic evolution of the Nisqually River delta. Two fluvial–deltaic facies are recognized on the basis of GPR data and sedimentary characteristics in cores, which suggest partial emplacement from sediment-rich floods that originated on Mount Rainier. Facies S consists of stacked, sheet-like deposits of andesitic sand up to 4 m thick that are continuous across the entire width of the delta. Flat-lying, highly reflective surfaces separate the sand sheets and comprise important facies boundaries. Beds of massive, pumice- and charcoal-rich sand overlie one of the buried surfaces. Organic-rich material from that surface, beneath the massive sand, yielded a radiocarbon age that is time-correlative with a series of known eruptive events that generated lahars in the upper Nisqually River valley. Facies CF consists of linear sandbodies or palaeochannels incised into facies S on the lower delta plain. Radiocarbon ages of wood fragments in the sandy channel-fill deposits also correlate in time to lahar deposits in upstream areas. Intrusive, sand-filled dikes and sills indicate liquefaction caused by post-depositional ground shaking related to earthquakes. Continued progradation of the delta into Puget Sound is currently balanced by tidal-current reworking, which redistributes sediment into large fields of ebb- and flood-oriented bedforms.

Washington↗

USGS2006

Shallow-water pockmark formation in temperate estuaries: A consideration of origins in the western gulf of Maine with special focus on Belfast Bay

A systematic mapping program incorporating more than 5000 km of side scan sonar and seismic reflection tracklines in the western Gulf of Maine has identified more than 70 biogenic natural gas deposits, occupying 311 km 2 in nearshore muddy embayments. Many of these embayments also contain pockmark fields, with some exhibiting geologically active characteristics including the observance of plumes of escaping fluids and sediment. Pockmarks, hemispherically shaped depressions of various size and depths, formed through fluid escape of gas and/or pore water, are sometimes found within or outside gas fields, although many gas fields lack pockmarks altogether. Although the origin of the natural gas remains unclear, if coastal environments at times of lower sea level were similar to the present, numerous lake, wetland, valley fill and estuarine sources of organic-rich material may have formed on the inner shelf. If these deposits survived transgression and remain buried, they are potential gas sources. Intensive mapping of the Belfast Bay pockmark field in 1998 produced the first nearly continuous side scan sonar mosaic of a Gulf of Maine pockmark field with a corresponding 3-dimensional geological model generated from seismic data. Statistical analysis of pockmark geometry, gas deposit loci, and subsurface evidence for gas-enhanced reflectors suggest that gas migration from deeper lateral sources along permeable subsurface strata may be the mechanism for pockmark formation in areas lacking gas-curtain seismic reflections. The coarse-grained transgressive ravinement unconformity between Pleistocene glacial-marine mud and Holocene mud may act as a conduit for distributing methane to the field's margins. ?? 2005 Elsevier B.V. All rights reserved.

Marine Geology↗

USGS2006

Shelf stratigraphy and the influence of antecedent substrate on Holocene reef development, south Oahu, Hawaii

Paired analyses of drill cores and high-resolution seismic reflection data show that development of Holocene framework reefs on the Oahu (Hawaii) shelf is limited to settings of low wave energy and to the period 8000 to 3000 yr BP. A prominent bounding surface that is mapped across much of the Oahu shelf is an erosion surface cut into Marine Isotope Stages 5 and 7 limestones that show extensive loss of primary porosity, aragonite, and MgCO3 owing to meteoric and vadose-zone diagenesis. This acoustic reflector is found exposed at the surface where wave energy is high or in the shallow subsurface below Holocene reef and sand sheet deposits where energy is low. Ship-towed video along 30 km of the shelf reveals a steady decrease in limestone accumulation from offshore of Honolulu southeast to Koko Head where the seafloor is characterized by volcanic pavement and/or thin sand deposits. This may reflect the build-up of late Pleistocene volcanics associated with the Hanauma Bay eruption (30,000-7000 yr BP) that now comprise the substrate in depths shallow enough to limit reef accretion. The absence of significant Holocene reef build-up on the south Oahu shelf is consistent with observations from north-facing coasts that lack Holocene reefs, indicating that Holocene reef formation in Hawaii is complex and patchy.

Hawaii↗

USGS2005

Possible modes of coral-reef development at Molokai, Hawaii, inferred from seismic-reflection profiling

High-resolution, seismic-reflection data elucidate the late Quaternary development of the largest coral-reef complex in the main Hawaiian Islands. Six acoustic facies were identified from reflection characteristics and lithosome geometry. An extensive, buried platform with uniformly low relief was traced beneath fore-reef and marginal shelf environments. This highly reflective surface dips gently seaward to ???130 m depth and locally crops out on the seafloor. It probably represents a wave-cut platform or ancient reef flat. We propose alternative evolutionary models, in which sea-level changes have modulated the development of reef systems, to explain the observed stratigraphic relationships. The primary difference between the models is the origin of the underlying antecedent surface, which arguably could have formed during either regression/lowstand or subsequent transgression.

Hawai'i↗

USGS2004

Influence of near-surface stratigraphy on coastal landslides at Sleeping Bear Dunes National Lakeshore, Lake Michigan, USA

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.

Michigan↗

USGS2003

Sand volume and distribution on the paraglacial inner continental shelf of the northwestern Gulf of Maine

In an extensive program of side-scan sonar and seismic reflection profiling, bottom sampling and vibracoring, we have mapped the western Gulf of Maine between Canada and Massachusetts, from the shoreline to the 100 m isobath. The purpose of the program was, in part, to locate and evaluate sand resources on the inner shelf. Surficial sand occurs on only 7% of this formerly glaciated region, and most is located seaward of southern Maine's large beaches in Wells Embayment, Saco Bay, and off Cape Small. Sand deposits occur 1) at the lowstand position of sea level, between 50 and 60 m depth, 2) on parts of the inner shelf between 50 m and the shoreface, and 3) in the shoreface. A paleodelta of the region's largest river, the Kennebec, occurs off Cape Small. Elsewhere, the lowstand deposits are thinner (<3 m) and form a mantle over notches cut into glacial sediment. Inner shelf deposits are found as thin (<1 m) layers unconformably overlying glacial sediment. These are widespread in Wells and Cape Small, but are confined to rock valleys by high-relief bedrock in Saco Bay. In Wells Embayment, numerous moraines were eroded during lower stands of sea level, but still possess >5 m of relief on the inner shelf and contain large quantities of material. The shoreface contains the greatest concentration of sand in each of the regions. A wedge-shaped deposit of sand overlies estuarine muddy sands in each area and is inferred to have formed during a slowdown in the rate of sea-level rise between 7.5 and 9.5 ka. The volume of shoreface sand varies from less than 60 million cubic meters in Saco Bay to more than 300 million cubic meters off Cape Small, and is loosely correlated with the erosional state of adjacent beaches.

Journal of Coastal Research↗

USGS1998

Mapping the Gulf of Maine with side-scan sonar: A new bottom-type classification for complex seafloors

The bedrock-framed seafloor in the northwestern Gulf of Maine is characterized by extreme changes in bathymetric relief and covered with a wide variety of surficial materials. Traditional methods of mapping cannot accurately represent the great heterogeneity of such a glaciated region. A new mapping scheme for complex seafloors, based primarily on the interpretation of side-scan sonar imagery, utilizes four easily recognized units: rock, gravel, sand and mud. In many places, however, the seafloor exhibits a complicated mixture or extremely 'patchy' distribution of the four basic units, which are too small to map individually. Twelve composite units, each a two-component mixture of the basic units, were established to represent this patchiness at a small scale (1:100,000). Using a geographic information system, these and all other available data (seismic profiles, grab samples, submersible dives and cores) were referenced to a common geographic base, superimposed on bathymetric contours and then integrated into surficial geologic maps of the regional inner continental shelf. This digital representation of the seafloor comprises a multidimensional, interactive model complete with explicit attributes (depth, bottom type) that allow for detailed analysis of marine environments.

Journal of Coastal Research↗

USGS1997

Stratigraphic evolution of the inner continental shelf in response to late Quaternary relative sea-level change, northwestern Gulf of Maine

Accumulations of deltaic and littoral sediments on the inner continental shelf of Maine, Gulf of Maine, preserve a record of postglacial sea-level changes and shoreline migrations. The depositional response of coastal environments to a cycle of regression, lowstand, and transgression was examined with seismic-reflection profiles, vibracores, and radiocarbon dates collected from sediments at the mouths of the Kennebec and Penobscot Rivers. Sequence-stratigraphic analysis of these data reveals two distinctly different successions of late Quaternary deposits that represent end members in an evolutionary model for this glaciated coast. Seaward of the Kennebec River, coarse-grained shorelines with foreset beds occur at depths of 20–60 m and outline the lobate margin of the Kennebec River paleodelta, a complex, rock-framed accumulation of glaciomarine and deltaic sediments capped by estuarine and marine deposits. Sand derived from this system today supports large barrier spits and extensive salt marshes. In contrast, the mouth of the Penobscot River is characterized by thick deposits of glaciomarine mud overlain by marine mud of Holocene age, including gas-charged zones that have locally evolved into fields of pockmarks. The distinct lack of sand and gravel seaward of the Penobscot River and its abundance seaward of the Kennebec River probably reflect differences in sediment sources and the physiography of the two watersheds. The contrasting stratigraphic framework of these systems demonstrates the importance of understanding local and regional differences in sediment supply, sea-level change, bedrock structure, and exposure to waves and tides in order to model river-mouth deposition on glaciated coasts.

Gulf of Maine↗