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Geology topics

R.A. Morton

Publications and source records attributed to R.A. Morton.

At least 19 recordsLinked to original sources
USGS2013

Holocene sediment distribution on the inner continental shelf of northeastern South Carolina: implications for the regional sediment budget and long-term shoreline response

High-resolution geophysical and sediment sampling surveys were conducted offshore of the Grand Strand, South Carolina to define the shallow geologic framework of the inner shelf. Results are used to identify and map Holocene sediment deposits, infer sediment transport pathways, and discuss implications for the regional coastal sediment budget. The thickest deposits of Holocene sediment observed on the inner shelf form shoal complexes composed of moderately sorted fine sand, which are primarily located offshore of modern tidal inlets. These shoal deposits contain ~67 M m 3 of sediment, approximately 96% of Holocene sediment stored on the inner shelf. Due to the lack of any significant modern fluvial input of sand to the region, the Holocene deposits are likely derived from reworking of relict Pleistocene and older inner-shelf deposits during the Holocene marine transgression. The Holocene sediments are concentrated in the southern part of the study area, due to a combination of ancestral drainage patterns, a regional shift in sediment supply from the northeast to the southwest in the late Pleistocene, and proximity to modern inlet systems. Where sediment is limited, only small, low relief ridges have formed and Pleistocene and older deposits are exposed on the seafloor. The low-relief ridges are likely the result of a thin, mobile veneer of sediment being transported across an irregular, erosional surface formed during the last transgression. Sediment textural trends and seafloor morphology indicate a long-term net transport of sediment to the southwest. This is supported by oceanographic studies that suggest the long-term sediment transport direction is controlled by the frequency and intensity of storms that pass through the region, where low pressure systems yield net along-shore flow to the southwest and a weak onshore component. Current sediment budget estimates for the Grand Strand yield a deficit for the region. Volume calculations of Holocene deposits on the inner shelf suggest that there is sufficient sediment to balance the sediment budget and provide a source of sediment to the shoreline. Although the processes controlling cross-shelf sediment transport are not fully understood, in sediment-limited environments such as the Grand Strand, erosion of the inner shelf likely contributes significant sediment to the beach system.

South Carolina↗
USGS2012

USGS advances in integrated, high-resolution sea-floor mapping: inner continental shelf to estuaries

The U.S. Geological Survey (USGS) has been involved in geological mapping of the sea floor for the past thirty years. Early geophysical and acoustic mapping efforts using GLORIA (Geologic LOng Range Inclined ASDIC) a long-range sidescan-sonar system, provided broad-scale imagery of deep waters within the U.S. Exclusive Economic Zone (EEZ). In the early 1990's, research emphasis shifted from deep- to shallow-water environments to address pertinent coastal research and resource management issues. Use of shallow-water, high-resolution geophysical systems has enhanced our understanding of the processes shaping shallow marine environments. However, research within these shallow-water environments continues to present technological challenges.

Conference Paper↗
USGS2011

Recent storm and tsunami coarse-clast deposit characteristics, southeast Hawai'i

Deposits formed by extreme waves can be useful in elucidating the type and characteristics of the depositional event. The study area on the southeast coast of the island of Hawaiʻi is characterized by the presence of geologically young basalts of known age that are mantled by recent wave-derived sedimentary deposits. The area has been impacted by large swells, storms and tsunamis over the last century, and in combination with known substrate ages makes this an ideal location to study recent deposits produced by such events. Three distinct coarse-clast deposit assemblages can be recognized based on clast size, composition, angularity, orientation, packing, elevation and inland distance of the deposit. These deposits are characterized as one of three types. 1) Gravel fields of isolated clasts, primarily boulder-size material, and scattered pockets of concentrated sand and gravel in topographic lows. 2) Shore-parallel and cuspate ridges composed mostly of rounded basalt gravel and sand with small amounts of carbonate detritus. The ridges range in height from about 1 to 3 m and are 10s of m wide. 3) Cliff-top deposits of scattered angular and sub-angular clasts along sea cliffs that are generally greater than 5 m elevation. The gravel fields are primarily of tsunami origin from either the 1975 Kalapana event, or a combination of the 1975 tsunami, and 1868 tsunami or earlier events. The ridge deposits are presently active and sediment continues to be added during high wave events. The cliff-top deposits contain evidence of deposition by both tsunami and storm processes and require further investigation.

Hawai'i↗
USGS2011

Geological effects and implications of the 2010 tsunami along the central coast of Chile

Geological effects of the 2010 Chilean tsunami were quantified at five near-field sites along a 200 km segment of coast located between the two zones of predominant fault slip. Field measurements, including topography, flow depths, flow directions, scour depths, and deposit thicknesses, provide insights into the processes and morphological changes associated with tsunami inundation and return flow. The superposition of downed trees recorded multiple strong onshore and alongshore flows that arrived at different times and from different directions. The most likely explanation for the diverse directions and timing of coastal inundation combines (1) variable fault rupture and asymmetrical slip displacement of the seafloor away from the epicenter with (2) resonant amplification of coastal edge waves. Other possible contributing factors include local interaction of incoming flow and return flow and delayed wave reflection by the southern coast of Peru. Coastal embayments amplified the maximum inundation distances at two sites (2.4 and 2.6 km, respectively). Tsunami vertical erosion included scour and planation of the land surface, inundation scour around the bases of trees, and channel incision from return flow. Sheets and wedges of sand and gravel were deposited at all of the sites. Locally derived boulders up to 1 m in diameter were transported as much as 400 m inland and deposited as fields of dispersed clasts. The presence of lobate bedforms at one site indicates that at least some of the late-stage sediment transport was as bed load and not as suspended load. Most of the tsunami deposits were less than 25 cm thick. Exceptions were thick deposits near open-ocean river mouths where sediment supply was abundant. Human alterations of the land surface at most of the sites provided opportunities to examine some tsunami effects that otherwise would not have been possible, including flow histories, boulder dispersion, and vegetation controls on deposit thickness.

Central Coast↗
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↗
USGS2008

Hydrodynamic implications of textural trends in sand deposits of the 2004 tsunami in Sri Lanka

Field observations and sediment samples at a coastal-plain setting in southeastern Sri Lanka were used to document the erosional and depositional impacts of the 2004 Indian Ocean tsunami and to interpret the hydrodynamic processes that produced an extensive sand-sheet deposit. Tsunami deposit thicknesses ranged from 6 to 22??cm with thickness being controlled partly by antecedent topography. The deposit was composed of coarse to medium sand organized into plane-parallel laminae and a few laminasets. Vertical textural trends showed an overall but non-systematic upward fining and upward thinning of depositional units with an upward increase in heavy-mineral laminations at some locations. Repeated patterns in the vertical textural trends (upward fining, upward coarsening, uniform) were used to subdivide and correlate the deposit into five hydro-textural stratigraphic units. The depositional units were linked to hydrodynamic processes and upcurrent conditions, such as rates of sediment supply and composition of the sediment sources. Vertical changes in grain-size distributions recorded the depositional phases associated with flow acceleration, initial unsteady pulsating flow, relatively stable and uniform flow, flow deceleration, slack water, and return flow or flow redirection. Study results suggest that vertical textural trends from multiple cross-shore sections can be used to interpret complex tsunami flow histories, but at the location examined, interpretation of the lateral textural trends did not provide a basis for identifying the correct sediment transport pathways because flow near the landward boundary was multidirectional.

Sedimentary Geology↗
USGS2008

Coarse-clast ridge complexes of the Caribbean: A preliminary basis for distinguishing tsunami and storm-wave origins

Coastal gravel-ridge complexes deposited on islands in the Caribbean Sea are recorders of past extreme-wave events that could be associated with either tsunamis or hurricanes. The ridge complexes of Bonaire, Jamaica, Puerto Rico (Isla de Mona), and Guadeloupe consist of polymodal clasts ranging in size from sand to coarse boulders that are derived from the adjacent coral reefs or subjacent rock platforms. Ridge-complex morphologies and crest elevations are largely controlled by availability of sediments, clast sizes, and heights of wave runup. The ridge complexes are internally organized, display textural sorting and a broad range of ages including historical events. Some display seaward-dipping beds and ridge-and-swale topography, and some terminate in fans or steep avalanche slopes. Together, the morphologic, sedimentologic, lithostratigraphic, and chronostratigraphic evidence indicates that shore-parallet ridge complexes composed of gravel and sand that are tens of meters wide and several meters thick are primarily storm-constructed features that have accumulated for a few centuries or millennia as a result of multiple high-frequency intense-wave events. They are not entirely the result of one or a few tsunamis as recently reported. Tsunami deposition may account for some of the lateral ridge-complex accretion or boulder fields and isolated blocks that are associated with the ridge complexes. Copyright ?? 2008, SEPM (Society for Sedimentary Geology).

Journal of Sedimentary Research↗
USGS2006

Migration of the Pee Dee River system inferred from ancestral paleochannels underlying the South Carolina Grand Strand and Long Bay inner shelf

Several generations of the ancestral Pee Dee River system have been mapped beneath the South Carolina Grand Strand coastline and adjacent Long Bay inner shelf. Deep boreholes onshore and high-resolution seismic-reflection data offshore allow for reconstruction of these paleochannels, which formed during glacial lowstands, when the Pee Dee River system incised subaerially exposed coastal-plain and continental-shelf strata. Paleochannel groups, representing different generations of the system, decrease in age to the southwest, where the modern Pee Dee River merges with several coastal-plain tributaries at Winyah Bay, the southern terminus of Long Bay. Positions of the successive generational groups record a regional, southwestward migration of the river system that may have initiated during the late Pliocene. The migration was primarily driven by barrier-island deposition, resulting from the interaction of fluvial and shoreline processes during eustatic highstands. Structurally driven, subsurface paleotopography associated with the Mid-Carolina Platform High has also indirectly assisted in forcing this migration. These results provide a better understanding of the evolution of the region and help explain the lack of mobile sediment on the Long Bay inner shelf. Migration of the river system caused a profound change in sediment supply during the late Pleistocene. The abundant fluvial source that once fed sand-rich barrier islands was cut off and replaced with a limited source, supplied by erosion and reworking of former coastal deposits exposed at the shore and on the inner shelf.

South Carolina↗
USGS2006

Evidence of regional subsidence and associated interior wetland loss induced by hydrocarbon production, Gulf Coast region, USA

Analysis of remote images, elevation surveys, stratigraphic cross-sections, and hydrocarbon production data demonstrates that extensive areas of wetland loss in the northern Gulf Coast region of the United States were associated with large-volume fluid production from mature petroleum fields. Interior wetland losses at many sites in coastal Louisiana and Texas are attributed largely to accelerated land subsidence and fault reactivation induced by decreased reservoir pressures as a result of rapid or prolonged extraction of gas, oil, and associated brines. Evidence that moderately-deep hydrocarbon production has induced land-surface subsidence and reactivated faults that intersect the surface include: (1) close temporal and spatial correlation of fluid production with surficial changes including rapid subsidence of wetland sediments near producing fields, (2) measurable offsets of shallow strata across the zones of wetland loss, (3) large reductions in subsurface pressures where subsidence rates are high, (4) coincidence of orientation and direction of displacement between surface fault traces and faults that bound the reservoirs, and (5) accelerated subsidence rates near producing fields compared to subsidence rates in surrounding areas or compared to geological rates of subsidence. Based on historical trends, subsidence rates in the Gulf Coast region near producing fields most likely will decrease in the future because most petroleum fields are nearly depleted. Alternatively, continued extraction of conventional energy resources as well as potential production of alternative energy resources (geopressured-geothermal fluids) in the Gulf Coast region could increase subsidence and land losses and also contribute to inundation of areas of higher elevation. ?? Springer-Verlag 2006.

Environmental Geology↗
USGS2006

Geologic impacts of the 2004 Indian ocean tsunami on Indonesia, Sri Lanka, and the Maldives

The December 26, 2004 Indian Ocean tsunami was generated by a large submarine earthquake (magnitude ???9.1) with an epicenter located under the seafloor in the eastern Indian Ocean near northern Sumatra, Indonesia. The resulting tsunami was measured globally and had significant geologic impacts throughout the Indian Ocean basin. Observations of tsunami impacts, such as morphologic change, sedimentary deposits, and water-level measurements, are used to reconstruct tsunamogenic processes. Data from Sumatra, Sri Lanka, and the Maldives provide a synoptic view of tsunami characteristics from a wide range of coastal environments both near- and far-field from the tsunami origin. Impacts to the coast as a result of the tsunami varied depending upon the height of the wave at impact, orientation of the coast with regard to direction of wave approach, and local topography, bathymetry, geology, and vegetation cover. Tsunami deposits were observed in all the countries visited and can be generally characterized as relatively thin sheets (<80 cm), mostly of sand. ?? 2006 Gebru??der Borntraeger.

Conference Paper↗
USGS2005

Historical shoreline changes along the US Gulf of Mexico: A summary of recent shoreline comparisons and analyses

The US Geological Survey is systematically analyzing historical shoreline changes along open-ocean sandy shores of the United States. This National Assessment of Shoreline Change Project is developing standard repeatable methods for mapping and analyzing shoreline movement so that internally consistent updates can periodically be made to record coastal erosion and land loss along US shores. Recently, shoreline change maps and a report were published for states bordering the Gulf of Mexico. Long-term and short-term average rates of change were calculated by comparing three historical shorelines (1800s, 1930s, 1970s) with an operational mean high water shoreline derived from lidar (light detection and ranging) surveys (post-1998). The rates of change, statistical uncertainties, original shorelines, and complementary geographic information system layers, such as areas of beach nourishment, are available on an Internet Map Server (IMS). For the Gulf of Mexico region, rates of erosion are generally highest in Louisiana along barrier island and headland shores associated with the Mississippi delta. Erosion also is rapid along some barrier islands and headlands in Texas, whereas barrier islands in Mississippi are migrating laterally. Highest rates of erosion in Florida are generally localized around tidal inlets. The most stable Gulf beaches generally are along the west coast of Florida, where low wave energy and frequent beach nourishment minimize erosion. Some long beach segments in Texas have accreted as a result of net longshore drift convergence and around tidal inlets that have been stabilized by long jetties. Individuals and some communities have attempted to mitigate the effects of erosion by emplacement of coastal structures, but those efforts largely have been abandoned in favor of periodic beach nourishment.

Journal of Coastal Research↗
USGS2003

Causes of hot-spot wetland loss in the Mississippi delta plain

Field surveys and sediment cores were used to estimate marsh erosion and land subsidence at Madison Bay, a well-known wetland loss hot spot in coastal Louisiana. Former marshes of Madison Bay are under about 1 m of water. Nearly two-thirds of the permanent flooding was caused by rapid subsidence in the late 1960s, whereas the other third was caused by subsequent erosion. Subsidence rates near Madison Bay since the 1960s ( 20 mm/yr) are an order of magnitude greater than deltaic subsidence rates averaged for the past 400 4000 yr ( 2 mm/yr). The rapid acceleration and unexpected decline in wetland losses in the Mississippi delta plain are difficult to explain on the basis of most physical and biogeochemical processes. There are, however, close temporal and spatial correlations among regional wetland loss, high subsidence rates, and large-volume fluid production from nearby hydrocarbon fields. The decreased rates of wetland loss since the 1970s may be related to decreased rates of subsidence caused by significantly decreased rates of subsurface fluid withdrawal. Annual fluid production from the Lapeyrouse, Lirette, and Bay Baptiste fields that encompass Madison Bay accelerated in the 1960s, peaked about 1970, and then declined abruptly. Large decreases in pore pressure in the Lapeyrouse field have likely altered subsurface stresses and reactivated a major fault that coincides with the wetland loss hot spot. Therefore, wetland losses at Madison Bay can be closely linked to rapid subsidence and possible fault reactivation induced by long-term, large-volume hydrocarbon production.

Environmental Geosciences↗
USGS2002

Coastal geoindicators of environmental change in the humid tropics

The primary geoindicators appropriate for monitoring environmental changes in the humid tropics are transitory surface water levels, shoreline position, wetlands distribution, coral reefs, landforms, and sediment sequence and composition. Lateral zonations and temporal successions of vegetation also can be used as geoindicators of riverine and shoreline changes. All of these coastal geoindicators are sensitive to regional tectonic processes and anthropogenic alterations and they typically reflect significant changes in coastal conditions such as fluvial processes, coastal energy, water quality, relative sea level, and sediment supply. Where humid tropical coasts coincide with active tectonic margins, indicators of seismic activity are critical to understanding Coastal changes associated with co-seismic subsidence or uplift, tsunamis, and liquefaction of coastal sediments. Coastal landforms and sedimentary deposits that record late Quaternary environmental changes include perched fluvial and marine terraces, delta-plain morphologies, crevassesplay deposits, peats and other paleosols, beach ridges, mud capes, and mud volcanoes. Although these features and deposits typically reflect environmental changes spanning more than 100 years, they are relevant to modern processes, management of coastal lands and resources, and prediction of future conditions. In some regions of the humid tropics, large coastal areas are unaffected by hurricanes or typhoons. Nevertheless, these tropical coasts are vulnerable to other non-storm processes, such as El Nin??o events, tsunamis, and monsoons that increase water levels, and cause widespread flooding and beach erosion. The environmental and political significance of coastal geoindicators increases when they are integrated and applied to issues of human safety and health such as hazards mapping, risk assessment, and dispersion of contaminated sediments. However, to be relevant, those socio-environmental applications demand accurate predictions of future trends and rates of change.

Environmental Geology↗
USGS2002

A comparison of factors controlling sedimentation rates and wetland loss in fluvial-deltaic systems, Texas Gulf coast

Submergence of coastal marshes in areas where rates of relative sea-level rise exceed rates of marsh sedimentation, or vertical accretion, is a global problem that requires detailed examination of the principal processes that establish, maintain, and degrade these biologically productive environments. Using a simple 210Pb-dating model, we measured sedimentation rates in cores from the Trinity, Lavaca-Navidad, and Nueces bayhead fluvial-deltaic systems in Texas where more than 2000 ha of wetlands have been lost since the 1950s. Long-term average rates of fluvial-deltaic aggradation decrease southwestward from 0.514 ?? 0.008 cm year -1 in the Trinity, 0.328 ?? 0.022 cm year -1 in the Lavaca-Navidad, to 0.262 ?? 0.034 cm year -1 in the Nucces. The relative magnitudes of sedimentation and wetland loss correlate with several parameters that define the differing fluvial-deltaic settings, including size of coastal drainage basin, average annual rainfall, suspended sediment load, thickness of Holocene mud in the valley fill, and rates of relative sea-level rise. There is some evidence that upstream reservoirs have reduced wetland sedimentation rates, which are now about one-half the local rates of relative sea-level rise. The extant conditions indicate that fluvial-deltaic marshes in these valleys will continue to be lost as a result of submergence and erosion. ?? 2002 Elsevier Science B.V. All rights reserved.

Geomorphology↗