Sediments North Research Articles

Tectonic controls on the development of unconformities: The North Slope, Alaska

The geology of northern Alaska is defined by two complementary, synchronous tectonic events. Opening of the Canada basin breached the south facing Ellesmerian passive margin in the Late Jurassic, cutting off northerly derived sediment and initiating thermal subsidence of the north facing Beaufort passive margin. Uplift of the Brooks Range on thrust faults loaded the lithosphere, creating the Colville trough, a foreland basin more than 10 km deep, and providing the bulk of the sediments that filled it. The northerly advance of the Brooks Range caused the two independent loads to flexurally interfere. The Beaufort passive margin is separated from the foreland by the Barrow Arch, which is, in part, the flexural bulge due to the interaction of these two loading events [Nunn et al., 1987]. A record of this interaction is preserved in the sediments north and south of the Barrow Arch. We have used this stratigraphy, coupled with simple basin modeling, to consider the flexural and stratigraphic effects of the evolving thrust wedge and basin filling. Using a simple sedimentation model and loading on an elastic lithosphere, we have produced two unconformity‐bounded sequences without sea level change. The unconformities are produced as a consequence of sedimentation and two increments in the thrust load that cause the flexural bulge to translate laterally, rise, and fall. The generation of unconformities over a flexural bulge by mass redistribution on a purely elastic lithosphere calls into question the use of these unconformities in other yoked systems to document viscoelastic lithospheric rheologies. Even if the lithospheric strength decays significantly on geologic time scales, we may not observe this decay in stratigraphy if the basin filling time is a significant fraction of the relaxation time. The Beaufort passive margin and the thrust belt strike at an oblique angle to each other, causing the independent bulges to constructively interfere in the west and destructively interfere in the east. The advance of the thrust‐related bulge beyond the eastern Beaufort shelf has, by flattening the basal decollement, exerted significant control on the thrust wedge of the Brooks Range, allowing the thrust wedge tip to advance beyond the coastline.

Microbial mats in tidal channels at San Carlos, Baja California Sur, Mexico

Abstract Microbial communities of stratified phototrophic bacteria in laminated intertidal sediments north of Estuary El Puente, near San Carlos, Baja California Sur, Mexico,‐were studied. This study describes the macroscopic and microscopic characteristics of the mats, including their annual growth. The mats were located in and along meandering mangrove‐lined tidal channels. Their thickness ranged from 0.5 to 25 cm. Square‐meter areas of polygonal mats were detected in several ponds infiltrated by sea water. The principal microbial community of the upper surface of various morphotypes of microbial mats was identified as cyanobacteria belonging to the genera Microcoleus, Lyngbya, Phormidium, and Oscillatoria. Other cyanobacte‐rial genera such as Pseudanabaena, Spirulina, Synechococcus, and Gloeocapsa, as well as many unidentified diatoms, were also present but at lower population densities. The second inward reddish layers of the microbial mats contained similar cyano‐bacterial genera plus anoxygenic phot...

210Pb scavenging in the North Atlantic and North Pacific Oceans

The radionuclide 210Pb shows significant geographic variations in the extent of its removal from the open ocean water column. This “texture of scavenging” is defined by mapping: (1) the integrated deficiency of 210Pb in the water column, relative to its supply from the atmosphere and from in situ decay of dissolved 226Ra, and (2) inventories of excess 210Pb in deep-sea sediments. The ratio of 210Pb deficiency to its supply, termed the scavenging effectiveness, is ∼ 20% in the North Equatorial Pacific and ∼ 50% in the North Atlantic. This variation is related to the combined effects of uptake of 210Pb onto sinking particles and lateral transport of 210Pb to areas of more intense removal. Sediment inventories of excess 210Pb, normalized to the 210Pb deficiency in the overlying water column, permit evaluation of the relative importance of these effects. In the North Equatorial Pacific virtually all of the 210Pb removed from the water column is present in the underlying sediments but in the mid-latitude North Atlantic, the sediments comprise only about 50% of the 210Pb removed. The deficiencies of 210Pb in the mid-latitude North Atlantic sediments south of 50°N are qualitatively offset by surpluses in high-latitude sediments north of 50°N. Higher primary productivity and new production in the surface waters of the high-latitude North Atlantic and North Equatorial Pacific, relative to the oligotrophic central North Atlantic, may account for the greater fluxes of 210Pb to bottom sediments in those areas.

Petrology of a Basement core from the Champlin No. 1 International Paper Company Well, Jackson County, Mississippi: ABSTRACT

ABSTRACT In May of 1981, eleven feet (3.3 m) of granite from 18,678-89 (5,793-96 m) was cored on the Wiggins Anticline in the Champlin No. 1 International Paper Company well. Interpreted in the 1920s as a simple anticline and referred to locally as the Lucedale Ridge, subsequent drilling has delineated the Wiggins as a broad complex feature with the anticlinal axis of Cretaceous-Jurassic sediments north of the axis of a series of basement highs forming the Wiggins Anticline. In the context of plate tectonics, the Wiggins is significant in the understanding of the Ouachita-Appalachian tectonic junction and the origin of the Gulf of Mexico. Megascopically, the core consists of fine to coarse grained granite with irregular foliations. The foliation is produced by bands of biotite that are occasionaly interrupted with ptygmatic folds. Pink plagioclase, white orthoclase and clear to milky quartz phenocrysts are visible and some appear to have been weathered in situ and then subjected to intense regional metamorphic conditions producing selective anatexis. Preliminary interpretations by Smith et al. (1981), suggested the Wiggins represents an allochthonous block separated by rifting from the South American plate during the opening of the Gulf of Mexico. However, subsequent interpretations utilizing petrographic data and age dates of 272 M. Y. (Amoco No. 1 Cumbest) and 282 M. Y (Champlin I. P. C.) tend to indicate the Wiggins is a buried southwest continuation of the Appalachian Mountains. The potential for finding significant hydrocarbon reservoirs in Cretaceous reefs, dolomitized Smackover carbonate shelf sediments and/or Norphlet eolian sandstones on the flanks of the basement features will make exploration for both structural and stratigraphic traps increasingly attractive in this sparsely drilled area. End_of_Record - Last_Page 279-------

Sedimentary effects of interplay between the Kuroshio Extension and Pacific plate motion

Several apparent sediment drifts of Tertiary age occur in the central northwest Pacific, and some form east-trending bands of thick sediment north and south of the axis of a major near-surface current, the Kuroshio Extension. The intervening area of thin sediment is located directly below the present axis of the Kuroshio Extension. These spatial relationships suggest that the zone of condensed sediment section and the adjacent sediment drifts are causally related to the Kuroshio Extension. The condensed section probably results from the partial insulation of the sea floor from the normal pelagicrain due to the high-energy portion of the Kuroshio and from the erosion of the sea floor by benthic storms associated with the Kuroshio. The bordering sediment drifts may represent the preferential deposition of fine sediments transferred from the high-energy to the low-energy portion of the water column. The interplay between the Kuroshio Extension and the northwestward drift of the Pacific plate should result in predictable variations in the timing and locations of pronounced sedimentation-rate changes. Such variations will depend both on the intensity and spatial stability of the Kuroshio, as well as direction and rate of Pacific plate motion. If the Kuroshio has remained spatially stable and active for at least several million years, then sites presently north of the Kuroshio should exhibit two periods of increased sedimentation. These periods represent the times when the site came into the influence of the high-sedimentation zones, first south and later north, of the Kuroshio. There should also be a diachronous unconformity in areas north of the Kuroshio that separates the two periods of increased sedimentation and represents the effect of passage of the plate directly beneath the Kuroshio. We conclude that some regional unconformities and zones of anomalously high sediment accumulation both in the northwest Pacific and elsewhere may arise largely independent of regional variations in bottom-water flow induced by climate change.

Cacoxenite in Miocene Sediments of the Maryland Coastal Plain

AbstractCacoxenite having the composition (Al4.0Fe22.5O7.1(OH)14.3(PO4)17(H2O)23.7)·50.3H2O was identified in a bed of mature quartz sand in the Miocene Calvert Formation near Popes Creek, Maryland. This is the first reported occurrence of this mineral in Atlantic Coastal Plain sediments north of Florida. The cacoxenite occurs as silt-size to sand-size grains, both as irregularly shaped aggregates and as radiating arrays of delicate acicular crystals. The presence of discrete cores and overgrowths in some grains indicates at least two generations of crystal growth. Electron microprobe analyses reveal excess Si and Al (relative to the ideal composition), which is believed to reflect ultra-fine clay particles within the cacoxenite grains. Admixed clays probably served as a substrate for the formation of ferric oxyhydroxides, which were subsequently converted to cacoxenite through the addition of dissolved phosphorus.

The Clay Mineralogy of Calcitic Seat Earth in the Northern Everglades of Florida

ABSTRACT The interface between peat and underlying limestone bedrock in the Northern Florida Everglades is often occupied by an unlithified calcitic seat earth that is stratigraphically similar to coal-basin seat rock and contains a complex mineral assemblage composed of 14 different minerals. The distribution and characteristics of the clay-size fraction were used to indicate sediment-source areas, depositional environment, and the degree of alteration of the original mineral assemblage. X-ray-diffraction analysis of 42 samples showed that a significant lateral variation exists in the clay-mineral suite of seat earth with respect to location in the study area. The clay minerals palygorskite, sepiolite, and illite were found primarily towards the center of the study area and were derived from sediments north of the Everglades. Seat earth located along the basin flanks showed an increase in the abundance of mixed-layer smectite, kaolinite, and chlorite. The proposed source area for these minerals is from erosion of marginal marine-bar deposits that rim the basin. Underlying limestone bedrock erosion contributed to the detrital mineral assemblage but was not as important as source areas outside the basin. The study-area seat earth is interpreted to have been deposited in an alkaline environment that allowed much of the original detrital mineral assemblage to be preserved. The only exception to this was the occasional identification of well-crystallized, authigenic mixed-layer chlorite-smectite that occurred in samples located near the basin flanks, which were presumably deposited under more acidic conditions.

Authigenic dolomite in Bahamian peri-platform slope sediment

Authigenic, calcium-rich dolomite has been discovered in poorly consolidated lower Pliocene-upper Miocene carbonate slope sediments north of Little Bahama Bank. The dolomite was recovered from a 5.6-m-long piston core that penetrated a slide scar in 600 m of water. Below an unconformity at −105 cm, which extends from the late Pleistocene to about the Miocene/Pliocene boundary, X-ray diffraction data indicate only calcite (85%–99%) and dolomite (as much as 15%), and scanning electron microscopy observations indicate in situ precipitation of dolomite. Stable carbon-isotope data indicate a normal, open-marine source of carbon, and oxygen-isotope values are consistent with precipitation of both calcite and dolomite from deep, cold, open-marine waters. Trace-element data reveal that dolomitic sediments are depleted in Sr 2+ and Mg 2+ relative to nondolomitic upper Pleistocene oozes. Dolomitization occurred in a deep, open-marine diagenetic environment via “whole scale” dissolution of aragonite and magnesian calcite and reprecipitation of calcite and dolomite.

Rates of sediment accumulation and particle reworking based on radiochemical measurements from continental shelf deposits in the East China Sea

Radiochemical measurements of 234Th (t 1/2 = 24days), 137Cs (bomb-produced), and 210Pb (t 1/2 = 22y) have been used to characterize rates of mixing, deposition, and accumulation on 100-day and 100-y time scales in East China Sea sediments. In the inner-shelf mud deposit near the mouth of the Changjiang (Yangtze River), 234Th data indicate deposition rates as rapid as 4.4 cm month −1 on a 100-day time scale. 210Pb data indicate that on a 100-y time scale accumulation rates are an order of magnitude slower (typically 1 to 5 cm y −1) than the short-term deposition rates. Most of the sediment deposited near the mouth of the Changjiang on a 100-day time scale is transported southward along the dispersal system on a 100-y time scale, probably as a result of winter storms and a strong coastal current. 210Pb accumulation rates from the inner-shelf mud deposit indicate that approximately 40% of the sediment discharged by the Changjiang can be accounted for in the sediments north of 30°N. The offshore mud deposit in the East China Sea is associated with the Huanghe (Yellow River) dispersal system. The dominant process affecting radionuclide profiles within this deposit is particle mixing (not sediment accumulation). In this area the upper 5 cm of the seabed are intensely reworked ( D b = 26cm 2 y −1) relative to the zone between 5 and 25 cm ( D b = 2cm 2 y −1). The maximum accumulation rate in the offshore mud deposit is 0.3 cm y −1. Less than 2% of the sediment discharged by the Huanghe can be accounted for in the offshore mud deposit. The relative intensity of mixing and accumulation is different for the proximal deposits of the Changjiang (where accumulation dominates) relative to the distal deposits of the Huanghe (where mixing dominates). Near the Changjiang mouth radiographs show distinct horizontal stratification, and the value of G (mixing rate/accumulation rate) is near zero. In the off-shore mud deposit radiographs reveal nearly homogeneous sedimentary structure, and the G value is ⩾18.

Geomorphology, shallow structure, and geological hazards in the East China Sea

The inner shelf of the East China Sea is covered with a lens of fine-grained modern sediment derived from the adjacent Changjiang; modern sediments north of the Changjiang came from the Huanghe. Rapid accumulation results in a featureless seafloor, except where escaping biogenic gases cause vents. The modern mud thins towards the mid-shelf and sediment becomes increasingly sandy and locally rippled, indicative of its relict age and present-day mixing and transport. The outer shelf, particularly south of Cheju Island, is covered with fine-grained sediment that apparently reflects modern Huanghe-derived sedimentation. Shallow seismic profiles show that the Holocene sediment overlies an eroded late Pleistocene surface, dominated by a number of well-defined buried river channels formed (and filled) during the last low stand of sea level; some channels may be associated with recent faulting. These filled channels and depressions probably represent the single most important geological hazard for future emplacement of man-made structures (such as drilling platforms) on the seabed. Other potential hazards include methane gas vents on the inner shelf, possible vents of deeper gas on the mid-shelf, outcrops or near-surface expressions of the Fukien-Reinan Massif, and possibly migrating sand waves.

The provenance and crustal residence ages of British sediments in relation to palaeogeographic reconstructions

24 Sm Nd isotope analyses of fine-grained Phanerozoic and modern clastic sediments from Britain and Quebec are presented. In combination with published data, they have been used to calculate “crustal residence ages” ( t CR ) and to assess the provenance of the British sedimentary mass. Sediments now preserved on either side of the suture formed by the closure of the Iapetus Ocean ca. 400 Ma ago were derived from isotopically distinct source regions. Sediments north of the suture are characterised by 1.7 < t CR < 2.8 Ga, whereas those to the south exhibit a smaller range and an average value of 1.6 Ga. The northern and southern source regions were most probably Laurentia and Gondwana respectively. It seems likely that a third source, perhaps Baltica, provided the lower Palaeozoic Southern Uplands succession. Sediments deposited in southern Britain after the closure of Iapetus were derived mainly from the recycling of older sediments. The tectonic rearrangements which occurred during the Phanerozoic are not reflected in the Sm Nd isotopic structure of the southern British sedimentary mass, suggesting that the Caledonian and Hercynian orogenies, and even the Grenville orogeny, involved minimal accretion of new mantle-derived material into the British and adjacent continental crust. Sm Nd analyses of fine-grained clastic sediments provide a powerful sedimentological tool for elucidating palaeogeography, clastic source areas, sediment recycling and maturity, and some aspects of sediment transport.

The Paleogeothermal Conditions of the Swiss Molasse Basin: Implications for Hydrocarbon Potential

The paleogeothermal conditions in the Swiss Molasse basin (Tertiary foredeep sediments north of the Alps) have been investigated by means of coalification studies. These data suggest a generally low-temperature geothermal regime in the Molasse basin which apparently prevailed since the deposition of the sediments. In the light of these findings some general trends for the hydrocarbon potential can be specified; corresponding exploration targets are delineated.

The negative gravity field over the 85°E ridge

An isopach map made from seismic reflection and refraction data in the Bay of Bengal shows two prominent N‐S trending features in the basement topography. One is the northernmost portion of the Ninetyeast Ridge which is totally buried by sediments north of 10°N. The other buried ridge trends roughly N‐S for 1400 km at 85°E to the latitude of Sri Lanka and then curves toward the west. It has basement relief up to 6 km. Two free‐air gravity anomaly profiles across the region show a strong gravity low (∼−60 mGal) over the 85°E Ridge, while the Ninetyeast Ridge shows a gravity high. To understand the negative free‐air gravity anomaly over the 85°E Ridge, we model the lithosphere as a thin elastic plate and calculate its flexural and gravitational response to an uneven sediment load. A plausible formation history for a buried ridge consists of at least two major episodes. The first is the formation of the ridge on a lithosphere with a flexural rigidity of D1. At some later time the ridge is buried by an influx of sediments, the lithosphere is cooler, and the flexural rigidity has increased to D2. The character of the gravity field depends primarily upon the initial and final values of flexural rigidity. These D1 and D2 values are varied to obtain good agreement between the model and observed gravity anomalies. Best fitting models have a 180 times increase in flexural rigidity between ridge formation and sediment burial. An approximate relationship between flexural rigidity and crustal age shows that the 85°E Ridge was formed on relatively young lithosphere, 5–15 m.y. old and that it was buried when the lithosphere was 40–80 m.y. old.

Clay mineral dispersal patterns in the north Bering and Chukchi Seas

Characterization of the clay mineralogic constituents of the <2 μm e.s.d. (equivalent spherical diameter) size fraction of contemporary marine and fluvial sediments from the north Bering Sea—Chukchi Sea region and adjacent land areas has resulted in delineation of suites dominated by broadly defined “illitic” and “expandable group” components, with lesser amounts of chlorite and kaolinite. Distribution patterns elucidated indicate net northward transport of sediments relatively enriched in the “expandable group” with predominant terrigen ous source from the Yukon River system. This material is distributed across the study area in a medial zone along a general north—south trend, with peripherally decreasing relative amounts of this “expandable group” component, and concomitant relative increases in the “illitic” components. These patterns are consistent with known physical oceanographic and regional geologic relationships, in terms of sediment sources, transport processes, and depositional mechanisms. The foregoing suggest, further, that consideration of the stratigraphic relationships of these clay mineral component-types, as a function of time, in relevant marine sediments north and south of the present Bering Strait, should be informative with respect to regional Quaternary paleogeography, as related to global sea level fluctuations during glacial and interglacial episodes.

The anatomy of a Lower Palaeozoic accretionary forearc: the Southern Uplands of Scotland

Summary The Southern Uplands of Scotland consists of an accretionary prism which had a prolonged (over 45 Ma) development off the SE coast of ancient North America (Laurentia) during the late Ordovician and Silurian. To the north, a forearc basin occupied the Midland Valley. An arc massif of older metamorphic rocks in the Grampian Highlands, and capping calc-alkaline volcanics, supplied much of the sediment to the trench. In its early development (late Ordovician) the accretionary complex incorporated slivers of ocean-floor material as well as thick turbidites into a steep lower trench slope. Later, in the Silurian, a trench slope break emerged shedding sediment north into the Midland Valley forearc basin. By that time only turbidites and black shales were being accreted. The Southern Uplands is dominated by coherent strata, and, despite intense imbrication, is devoid of mélange. We conclude that the accretionary prism was the result of high sediment input to the trench and very slow, oblique, subduction of the Iapetus Ocean eastwards below Scotland. Décollement surfaces developed during accretion preferentially utilised a highly incompetent black shale layer near the base of the subducting sequence. Prior to the inception of accretion (in the Caradoc), we postulate sediment subduction, and possibly subduction erosion, during the Cambrian and early Ordovician.

Precambrian events at Sabaloka, near Khartoum, and their significance in the chronology of the basement complex of North-East Africa

A large inlier of basement complex rocks surrounded by Cretaceous sediments north of Khartoum comprises a grey gneiss assemblage intruded by late-tectonic adamellitic batholiths and a shallow-level granitic ring complex. The gneiss assemblage contains evidence that an early granulite facies metamorphism was followed by retrogression into the amphibolite facies and widespread migmatization. There are close affinities with basement rocks of Archaean age in northern Uganda. Amphibolite facies metasediments which overlie the grey gneiss to the north and northeast of the Sabaloka inlier probably equate with the Early and Middle Proterozoic cover sequences which in the east African sector of the Mozambique belt are strongly interfolded with thoroughly reworked Archaean basement. The adamellite batholiths are in places strongly discordant to their country rocks and may be coeval with the ‘batholithic granites’ of the Red Sea Hills, emplaced in late Proterozoic times, during the last stages of deformation in the Red Sea belt. The whole area was later affected by reheating and limited tectonism during the Pan-African event (550 ± 100 Ma) with consequent resetting of K-Ar apparent ages. Following a period of deep erosion, the Sabaloka ring complex and associated volcanic rocks were emplaced in mid-Palaeozoic times. It is suggested that the concepts embraced by the terms ‘Pan-African event’, ‘Red Sea belt cycle’ and ‘Mozambique belt cycle’ are best kept distinct from each other since their affects appear to differ in spatial and temporal distribution. One new analysis of a retrogressed and metasomatized amphibolite is presented.

Sediment transport model of the southeastern Mediterranean coast

The relationship between: (a) the gradual change of the shoreline orientation in the southeastern Mediterranean from east—west to north—south, (b) the longest fetch direction of the Mediterranean (east—west), and (c) the dominant source of sediments being the Nile Delta to the west, provides a unique opportunity for construction of a model of the longshore sediment transport patterns. The input for this model consisted of: (a) two depth grids totaling 33,795 depths, and (b) nine selected wave conditions. The nine selected wave conditions were based on an analysis of the existing wave observer data, fetch considerations and storm records. The input conditions were waves from the northwest and west with wave periods of 5, 9 and 11 sec, southwest waves of limited fetch with wave periods of 5 and 9 sec, and 13-sec waves from 282.5° azimuth, potentially the largest fetch (2,400 km) direction. The 13-sec waves are both the longest waves theoretically possible and also the longest waves observed. The results show that the rate of longshore transport is larger for short-period (5 sec) than for long-period (9, 11 sec) waves, for all wave approach directions, given the same input wave heights. This is because of the larger shoreline angles of the short-period waves, due to lesser wave refraction. The results of these computations also illustrate the presence of four distinct zones of longshore sediment transport (from southwest to northeast): 1. (1) Western Zone (El Arish to Rafah), where all wave approach directions cause transport to the northeast, and therefore net transport equals gross transport. 2. (2) Central Zone (Rafah to Haifa), which is characterized by converging longshore transport nodal points that shift along the coast in response to changes in wave direction and to a lesser extent, wave period (i.e., north of the nodal point longshore transport is directed to the south, and south of the nodal point transport is directed to the north). For waves from 282.5°, the nodal point is at Netanya and for 315° at Rafah, the southern border of the central zone. 3. (3) Haifa Bay, a 20-km shoreline segment having significantly lower wave energy than the other coastal zones for all wave conditions except 5-sec waves from the northwest which cause southern transport. Even for southwest 9-sec waves, the sediment transport in this area is essentially zero due to wave refraction, and this area may be considered a sediment sink. 4. (4) Northern Zone (Akko to Rosh Haniqra) which is subject to intense wave refraction over the nearshore Akhziv Canyon, resulting in great variably of local longshore transport for most wave conditions. This model of longshore sediment transport of the southeastern Mediterranean coast explains many observed shoreline phenomena. For example, the lack of quartz sediment north of Akko, derived from the Nile River and Kurkar cliffs to the south, appears to be due to Haifa Bay which acts as a sediment sink for northward moving sand. The large differences in sediment transport between the Bardawil Lagoon (Western Zone) and Gaza (Central Zone), as evidenced by the differing sand accumulations on the southern sides of coastal constructions in these two zones, is attributed to the increasing importance of waves from the northwest in causing southwestern transport along the coast northeast of Rafah. The lack of strong indicators of transport direction (such as sediment accumulations differentially on one side of the structures) along the coast of Tel-Aviv and to the north is due to the continuously changing directions of longshore transport with changing wave conditions in the Central Zone, and to intense refraction in the Northern Zone.

Airborne dust on the Arctic pack ice, its composition and fallout rate

Dust collected from snow samples on the Arctic pack ice approximately 500 km north of Alaska indicate lower fallout rates than previously reported for Arctic stations 1400 km to the east (3.3 and 14 μg/cm 2 yr, respectively). Either the lower frequency of southerly (off-shore) near surface winds at our sample sites off Alaska or the unknown influx of dust with upper level air masses could account for the difference in dust fallout. Irregardless, the airborne dust contribution to Arctic deep-sea sediments north of Alaska amounts to 1% or less. A consideration of the clay mineralogy and biogenous components of Arctic dust favors a global or distant source for most of the dust, especially the less than 2 μm fraction. No obvious industrial components were detected in the dust.

Sedimentation on the Western Delta-front of the Fraser River, British Columbia

The broad patterns of sedimentation and erosion on the western delta-front of the Fraser River are described on the basis of a detailed sedimentological survey and three successive bathymetric surveys.The surveyed portion of the delta-front can be subdivided into three sedimentary environments: salt marsh, main platform, and upper fore-slope. The salt marsh lies near high tide level and is a flat to hummocky, vegetated zone about 0.5 mi (1 km) wide, having sediments with a mean grain size finer than [Formula: see text] (0.063 mm). The main platform (which includes the tidal flats below the salt marsh) is the zone that slopes gradually for about 4 mi (6 km) from the salt marsh to the break in slope at approximately 30 ft (9 m) below lowest normal tide level. It is mantled mainly with [Formula: see text] to [Formula: see text] (0.35 mm to 0.125 mm) rippled sand. The upper fore-slope has here been designated as that portion of the delta-front extending from the main platform to the limit of the bathymetric survey (− 300 ft) (− 90 m). The gradient of the upper fore-slope is variable and attains a maximum inclination of approximately 12°. Upper fore-slope sediments are muddy at, and north of the main river channel, but consist mostly of sand south of there. The sharp transition between platform sand and muddy fore-slope sediment north of the main channel suggests that the − 30 ft (− 9 m) contour there denotes the maximum depth of vigorous wave and current action.Bathymetric surveys conducted in 1968 and April and August 1972 coupled with a calculation of the gross sediment budget of the western delta-front reveal that, as a consequence probably of local physical oceanography and confinement and dredging of the main channel, yearly freshet deposition is promoting the advance of the delta-front principally off the main channel and seems sufficient, at least, to maintain the stability of the upper fore-slope north of there. However, on the southernmost segment of the western delta-front, prevailing conditions have brought about the retreat of the upper fore-slope at least during the period 1968–1972.

Sediments of the Continental Margin off the Eastern United States

The character and distribution of surficial sediments on the continental shelf and slope between northern Maine and southern Florida have been denned on the basis of more than 6,000 bottom samples. Most shelf sediments were deposited in shallow water (probably littoral depths) during the last lower stand of sea level. Modern sediments are accumulating only in estuaries, in certain nearshore areas and on the continental slope. A large portion of the shelf sediment north of lat 41° N. was deposited by Pleistocene glaciers that covered the area. Sediments on the middle Atlantic shelf consist of predominantly arkosic to subarkosic fluvial sands. The inner shelf off the southeastern United States is covered by suborthoquartzitic fluvial sands, derived mostly from Piedmont rivers; carbonate-rich sands occur over much of the outer shelf and upper slope. Residual detritus, reworked from underlying mid-Tertiary formations, is an important sedimentary component on Georges Bank and Nantucket Shoals, Onslow Bay, the Florida-Hatteras slope, and the Blake Plateau. Although most sediments are not in compositional equilibrium with the present-day shelf environment, there is considerable evidence to suggest that many may be in at least partial textural equilibrium. Holocene reworking has removed most fine-grained sediment, leaving only coarse to medium sand. Some fine-grained fluvial sediment escapes the estuaries and near-shore during floods and storms, but this influx is not sufficient to offset the effect of winnowing by currents and waves. A significant portion of the modern near-shore sediment, in fact, may be derived from landward transport of fine-grained sediment from the central and outer shelf. Despite the evidence of active sediment movement on the shelf, however, the net lateral transport of sand and gravel appears to be limited. Because of the lack of present-day terrigenous sedimentation, a primary source of modern shelf sediment is calcareous skeletal material. If the present-day surface were preserved in the geologic record, much of it probably would be a carbonate-rich layer, containing altered and reworked skeletal material representing a variety of depositional environments.