Ilya V. Buynevich

A 1500 yr record of North Atlantic storm activity based on optically dated relict beach scarps

Understanding of long-term dynamics of intense coastal storms is important for determining the frequency and impact of these events on sandy coasts. We use optically stimulated luminescence (OSL) dates on relict scarps within a prograded barrier sequence to reconstruct the chronology of large-magnitude erosional events in the western Gulf of Maine. OSL dates obtained on quartz-rich sediments immediately overlying relict scarps indicate severe beach erosion and retreat due to erosional events ca. 1550, 390, 290, and 150 cal yr B.P. Our data provide new evidence of increased storm activity (most likely frequency and/or intensity of extratropical storms) during the past 500 yr, which was preceded by a relatively calm period lasting ~1000 yr. The width of the coastal sequence preserved between successive paleoscarps shows strong correlation with the time interval elapsed between storms. Our fi ndings indicate that diagnostic geophysical and sedimentological signatures of severe erosional events offer new opportunities for assessing the impact and timing of major storms along sandy coasts.

New England tidal inlets with special reference to riverine-associated inlet systems

Abstract Tidal inlets along the glaciated coast of New England exhibit a diverse morphology due to widely different physical and geological settings and sediment abundance. Except for the glacial sediment coasts of Cape Cod, Nantucket Island, and Marthas Vineyard, most regions of New England are rocky, and barrier and tidal inlet development is related to isolated glacial and riverine sediment supplies. Inlet-connected bays and marshes encompass many different origins, including drowned river valleys, glacial lake discharge channels, indented bedrock and glacial sediment coasts, kettles, and groundwater sapping channels. Generally, inlet size is correlated to tidal range; the largest inlets (width >300 m) occur along mesotidal coasts, whereas most inlets along microtidal coasts are small (width In spite of their diversity, New England tidal inlets can be grouped into three broad categories based on their morphology, hydrographic regime, and sediment transport characteristics. These classes include the wave-dominated and mixed-energy tidal inlets, and riverine-associated tidal inlets. Wave-dominated inlets tend to occur in microtidal settings and are found primarily in southern New England. They contain flood-tidal deltas of variable morphology and poorly developed or absent ebb-tidal deltas. They have histories of migration, closure, and reopening during storms. Mixed-energy tidal inlets occur mostly along the mesotidal shorelines of central and northern New England. These inlets exhibit well-developed ebb- and flood-tidal deltas. They are backed by extensive marshes and tidal creeks and, in some cases, by a broad system of tidal flats. Riverine-associated tidal inlets coincide with major rivers that have significant freshwater discharge, especially during late winter and early spring. The throat cross-sections are large in comparison to other inlets in New England and are a function of their tidal prism. They contain well-developed subtidal ebb-tidal deltas and variable developed flood-tidal deltas. Ebb-oriented bedforms, grain size, and mineralogical analyses of channel sediments, and hydraulic data have been used to demonstrate that the estuaries are exporting sand to the nearshore, a process that has likely been ongoing since deglaciation. The mechanism for downstream movement of sand in the estuaries is moderate to large spring freshets and major floods when freshwater discharge supplants the saltwater tidal prism. During these periods, there are unidirectional seaward currents in the estuaries that may last for several days or weeks.

High-Resolution Subsurface (Gpr) Imaging and Sedimentology of Coastal Ponds, Maine, U.S.A.: Implications for Holocene Back-Barrier Evolution

Ground-penetrating radar (GPR) transects and sediment cores have been used to examine the basement morphology, stratig- raphy, and environmental history of maritime ponds along the pen- insular coast of Maine. Silver Lake, Lily Pond, and North Pond are shallow ( , 3 m) water bodies bordered by steep bedrock ridges in the north, east, and west, and sandy barriers to the south. The bedrock basins of the ponds are formed in metasedimentary rocks surrounded by resistant pegmatitic intrusions. A dense network of GPR traverses obtained over the ice-covered Silver Lake reveals a series of prominent wavy-parallel and basin-fill reflector geometries terminating against the bedrock or grading into the barrier sediments and interpreted as organic lake-bottom facies. The transparent units represent sand-rich horizons, mostly eolian in origin. Convex-up structures found both on the surface and within the basin-fill sequence are interpreted as pre- served parts of coastal dunes. The present study indicates that fresh- water conditions prevailed since at least 4.6 ka, with an initial sedi- mentation rate of 1.7 mm/yr. The position of this unit below the con- temporary sea level suggests presence of a welded barrier by that time. Radar profiles taken along the shores of Lily Pond, a small water body behind the Sand Dune Barrier, indicate a significantly larger areal extent of the pond in the past. A succession of organic deposits over- lying a Pleistocene glaciomarine unit indicates progressive inundation of the paleo-lagoon by rising sea level. Saltwater peat seaward of Lily Pond was buried by washover sands about 1.2 ka, and a narrow pond existed here prior to dredging and artificial infilling of its eastern part in the 1950s-60s. The organic and eolian units are absent in the North Pond, where sedimentary fill consists of glaciomarine clay overlain by marine sands. A proposed three-stage model of pond evolution along an embayed coastline consists of: (1) organic accumulation in an up- land depression during lower sea level; (2) predominantly washover or tidal deposition in a lagoon (Stage 2a) or blocked coastal pond (Stage 2b) during initial transgression, and (3) mainly eolian and organic de- position behind a prograded or aggraded barrier. Future accelerated rise in relative sea level and inadequate sediment supply will cause many back-barrier ponds to reenter Stage 2 of the proposed model.

Morphodynamics and Facies Architecture of Tidal Inlets and Tidal Deltas

Tidal inlets are highly dynamic systems marking positions along barrier coasts where dominant wave and longshore sand transport processes are juxtaposed with a tide-dominated regime in which onshore-offshore sand movement is manifested in the formation of flood- and ebb- tidal deltas. The morphodynamics of tidal inlets and distribution of their associated sand shoals are governed by the tidal prism, wave versus tidal energy, and the regional geological framework. Sand that is delivered to the inlet channel via longshore transport can be sequestered in the backbarrier, moved onto the ebb-tidal delta, or can bypass the inlet. Such bypassing is accomplished through wave and tidal processes and ultimately results in the landward migration and welding of large sand bar complexes to the downdrift shoreline. Tidal inlet-fill deposits typically exhibit a sharp basal contact with underlying units and consist of a fining-upward sequence in contrast to the generally coarsening-upward barrier lithosome. The preservation potential of inlet and associated tidal-delta deposits is high in regressive sequences, but relatively poor in transgressive systems due to the shallow nature of inlet-fill deposits compared to the base of the erosional wave- or tidal- ravinement surfaces. Exceptions occur in paleotidal inlet regions having large bay tidal prisms and deep inlet channels. Although tidal-inlet deposits have been reported in the rock record and may serve as important petroleum reservoirs, to date they are not readily recognized. High-resolution geophysical and sedimentological research of both active and relict inlets is providing a wealth of information necessary to improve the inlet facies models for ancient sedimentary sequences.

10.8 Morphodynamics of Barrier Systems: A Synthesis

The morphodynamics of open-ocean barrier systems (barrier islands, barrier spits, and mainland or headland beaches), synthesizing classic studies, current scientific knowledge, and future research directions regarding a number of barrier systems globally are reviewed. Within a coastal tectonic framework, the authors address: (1) Amero-trailing-edge coasts (USAs New England coast, mid-Atlantic Bight coast, North Carolina Outer Banks, Georgia Bight coast, and Florida Atlantic coast; Brazils Santa Catarina coast; German Bight coast; and southern and western Australian coasts); (2) marginal-sea coasts (USAs Florida Gulf Coast; Gulf Coast of Alabama, Mississippi, and Louisiana; Texas Gulf Coast; and eastern Australian coast); and (3) collision coasts (USAs Alaskan Pacific coast and New Zealand). Moreover, the chapter includes a glossary and robust current set of references.

Net ebb sediment transport in a rock-bound, mesotidal estuary during spring-freshet conditions: Kennebec River estuary, Maine

Hydrographic data—obtained concurrently along the 25 km longitudinal axis of the Kennebec River estuary during a 13 h semidiurnal tidal cycle of a spring freshet superimposed on near-perigean spring tides—revealed a strong ebb-current dominance along the length of the estuary. Ebb- current dominance is produced by riverine flow that supplants a substantial part of the flood-tidal prism. In addition, side-scan sonograms showed a suite of large bed forms (1–12 m height) with nearly ubiquitous ebb orientations veneering the estuary bottom. Embayment geometry, salinity, water temperature, discharge, current velocity, and bed-form data all suggest that ebb- velocity asymmetry, set up by seasonal changes in freshwater discharge superimposed on ebb-directed tides, is the most important control on net bed-load sediment transport within this high-latitude, rock- bound estuary. These data augment a model that shows that a freshwater discharge threshold exists for net seaward bed-load sediment transport. The results from this study can be used to refine existing conceptual sedimentologic and morphologic classifications of estuaries.

Coarse-Grained Sediment Transport in Northern New England Estuaries: A Synthesis

Although it is widely stated in the literature that estuarine river mouths are sediment sinks, northern New England estuaries are an exception to this model because they export coarse-grained sediment to the nearshore. The traditional view is that estuaries fill with sediment ranging from mud to gravel derived from fluvial or upland sources as well as from the inner continental shelf and adjacent shorelines. Fluvial sediments are deposited primarily in the inner and central portions of an estuary, although fluvial mud can be deposited in the outer estuary in some tide-dominated systems (e.g. sections of the Gironde River (France), Allen, 1991; Fly River (Papua New Guinea), Harris et al., 1993). The deposition of sediment in the inner and central portions of an estuary is due to the combined influences of a downstream decrease in the riverine current strength and clay flocculation produced by fresh and saltwater mixing. In estuaries having high sediment loads, fluidized mud can be an important component of estuarine sedimentation (Wells, 1983, 1995). Marine sediments enter the outer estuary due to residual, flood-oriented bottom currents and stronger flood than ebb tidal currents. This former flow pattern is caused by the seaward-flowing freshwater advecting the underlying saltwater producing a mass balance deficit of saltwater (Dyer, 1973).

Mud in the surf: Nature at work in a Brazilian bay

Massive discharge of mud from coastal rivers is a well-documented phenomenon. However, in areas with limited historical and instrumental records it is often difficult to assess the nature and history of the process. This article looks at Tijucas Bay, in southern Brazil (Figure 1a) (an area that was the landfall region in March 2004 for South Americas first recorded hurricane [Bossack, 2004]), to examine the time frame for extensive deposition of fluid muds in the nearshore (Figure 1b). The new geological data suggest that whereas recent human activities (e.g., massive sand mining) along the Tijucas River may be important in increasing the suspended sediment discharge, the shift to a mud-dominated regime was part of the natural evolution of this coastal plain.

Textural and compositional characterization of recent sediments along a paraglacial estuarine coastline, Maine, USA

Abstract Textural and mineralogical characteristics of bottom sediments collected from the lower Kennebec River estuary, Maine, and adjacent nearshore region were used to differentiate subtidal sedimentary environments. Five depositional environments, defined on the basis of bottom morphology, bathymetry, and dominant sedimentary processes—estuarine channel, channel margin, outer bar, shoreface, and offshore—possess distinct sedimentological characteristics. Trends in textural characteristics of these environments and those along the adjacent barrier beach include: (1) systematic decrease in mean grain size from the estuarine channel to the offshore facies; (2) similarity between the outer bar and estuarine channel sands; (3) increase in sediment sorting from the channel/margin and offshore to shoreface facies; and (4) fining of the low-tide terrace, beachface, and berm samples away from the bar-welding region. The fine-grained nature of the offshore facies is the result of deposition of suspended sediment delivered to the coast during large flood events, whereas the coarse fraction is contributed through the attrition of carbonate tests of the local macrofauna. Results of compositional analyses reveal: (1) a decrease in shell fragments from the estuarine channel (>50%) to the offshore (

Evolution of paraglacial coasts in response to changes in fluvial sediment supply

Abstract Paraglacial coastal systems are formed on or proximal to formerly ice-covered terrain from sediments with direct or indirect glacial origin. This review addresses the roles of tectonic controls, glacial advances and retreats, sea-level changes, and coastal processes in sediment production, delivery and redistribution along the paraglacial Gulf of Maine coast (USA and Canada). Coastal accumulation forms are compositionally heterogeneous and found primarily at the seaward edge of the Gulfs largest estuaries; their existence is directly attributable to the availability of glacial sediments derived from erosion of weathered plutons within coastal river basins. Multiple post-glacial sea-level fluctuations drove the redistribution of these sediments across the modern lowland and inner shelf. Central to the formation of barrier systems was the paraglacial sand maximum, a time-transgressive phase of relative sea-level fall and enhanced fluvial sand export c. 2000–4000 years following deglaciation. Vast quantities of sand and gravel were reworked landward during the subsequent transgression and combined with additional riverine sediments to form the modern barrier systems. Today, reduced fluvial sediment loads, anthropogenic modifications of barrier and river systems, and sea-level rise have combined to exacerbate long-term coastal erosion and may eventually force these barriers toward a state of rapid landward migration.