Harley J. Knebel

Hudson River: Evidence for extensive migration on the exposed continental shelf during Pleistocene time

Analyses of seismic-reflection profiles collected off New Jersey reveal a large buried channel that splits from the topographic Hudson shelf valley beneath the inner shelf and extends southward for at least 80 km. The buried valley has a flat bottom, a width of 2 to 17 km, and a relief of 3 to 15 m, all features similar to those of the Hudson shelf valley. The buried valley apparently is an ancestral pathway of the Hudson River that has been filled with heterogeneous fluvial deposits and capped by an additional 10 to 30 m of sediments. Vibracores from over or near the ancestral valley show that interbedded marine sand and mud layers constitute the upper part of the sedimentary fill. Radiocarbon ages, geotechnical properties, and micro-paleontological analyses of the core sediments indicate that the valley was formed and filled sometime prior to 28,000 yr ago and then was exposed subaerially during at least one sea-level regression. These results are the first clear subbottom evidence that the ancestral Hudson River flowed south of the Hudson shelf valley on the exposed continental shelf during Pleistocene time.

Holocene evolution of an estuarine coast and tidal wetlands

Modern facies-distribution patterns, extensive core data, and chronostratigraphic cross sections provide a detailed history of Holocene inundation within the Delaware Bay estuary and sedimentation in adjacent coastal environments. Flooding of the estuary occurred with rising sea level as the shoreline retreated northwest along a path determined by the pre-transgression topography. Simultaneous migration of an estuarine turbidity maximum depocenter provided the bulk of fine sediments which form the coastal Holocene section of the estuary. Prior to 10 Ka, the ancestral bay was predominantly a tidal river, and the turbidity maximum depocenter was located southeast of the modern bay mouth. By 10 Ka, lowlands adjacent to the ancestral channel of the Delaware River were flooded, forming localized tidal wetlands, and the depocenter had initiated high rates of fine-grained sedimentation near the present bay mouth. At that time, coastal Holocene strata began to onlap the interfluve highlands. By 8 Ka, the fine-grained depocenter had migrated northwest along the main channel of the Delaware River, although the widened mouths of tributary valleys continued to be active sites of sediment accumulation. Following the passage of the fine-grained depocenter, coarse-grained sediments accumulated along the coast in response to increased wind-wave activity. During the middle Holocene, portions of the estuarine coast began to resemble modern geomorphology, and washover barrier sands and headland beach sandy gravels accumulated along the southwest shore. The late Holocene was characterized by erosional truncation and submergence of aggraded coastal lithofacies and by planation of remnant highland areas. Knowledge of the eroded Holocene section is fragmentary. At present, continued sea-level rise is accompanied by deposition of tidally transported muds in coastal environments and deposition of sandy sediments in some offshore regions. An unconformity marks the base of the developing open estuarine sequence of coarse clastic lithofacies and denotes the end of coastal accumulations. Modeling of coastal-lithofacies transitions identifies specific lithofacies complexes in the Holocene stratigraphic section which were influential in the evolution of the coast. Development of the Holocene section of the estuary coast involved both constructive, or aggradational, and destructive, or erosional, phases.

Seafloor environments in the Long Island Sound estuarine system

Abstract Four categories of modern seafloor sedimentary environments have been identified and mapped across the large, glaciated, topographically complex Long Island Sound estuary by means of an extensive regional set of sidescan sonographs, bottom samples, and video-camera observations and supplemental marine-geologic and modeled physical-oceanographic data. (1) Environments of erosion or nondeposition contain sediments which range from boulder fields to gravelly coarse-to-medium sands and appear on the sonographs either as patterns with isolated reflections (caused by outcrops of glacial drift and bedrock) or as patterns of strong backscatter (caused by coarse lag deposits). Areas of erosion or nondeposition were found across the rugged seafloor at the eastern entrance of the Sound and atop bathymetric highs and within constricted depressions in other parts of the basin. (2) Environments of bedload transport contain mostly coarse-to-fine sand with only small amounts of mud and are depicted by sonograph patterns of sand ribbons and sand waves. Areas of bedload transport were found primarily in the eastern Sound where bottom currents have sculptured the surface of a Holocene marine delta and are moving these sediments toward the WSW into the estuary. (3) Environments of sediment sorting and reworking comprise variable amounts of fine sand and mud and are characterized either by patterns of moderate backscatter or by patterns with patches of moderate-to-weak backscatter that reflect a combination of erosion and deposition. Areas of sediment sorting and reworking were found around the periphery of the zone of bedload transport in the eastern Sound and along the southern nearshore margin. They also are located atop low knolls, on the flanks of shoal complexes, and within segments of the axial depression in the western Sound. (4) Environments of deposition are blanketed by muds and muddy fine sands that produce patterns of uniformly weak backscatter. Depositional areas occupy broad areas of the basin floor in the western part of the Sound. The regional distribution of seafloor environments reflects fundamental differences in marine-geologic conditions between the eastern and western parts of the Sound. In the funnel-shaped eastern part, a gradient of strong tidal currents coupled with the net nontidal (estuarine) bottom drift produce a westward progression of environments ranging from erosion or nondeposition at the narrow entrance to the Sound, through an extensive area of bedload transport, to a peripheral zone of sediment sorting. In the generally broader western part of the Sound, a weak tidal-current regime combined with the production of particle aggregates by biologic or chemical processes, cause large areas of deposition that are locally interrupted by a patchy distribution of various other environments where the bottom currents are enhanced by and interact with the seafloor topography.

Delaware River: Evidence for its former extension to Wilmington Submarine Canyon

Seismic-reflection profiles indicate that during the Pleistocene the Delaware River flowed across the continental shelf east of Delaware Bay and emptied into Wilmington Submarine Canyon. The ancestral valley (width, 3 to 8 kilometers; relief, 10 to 30 meters) is buried, is not reflected in the surface topography, and probably predates the formation of the present canyon head

Sedimentary framework of Penobscot Bay, Maine

Abstract Analyses of seismic-reflection profiles, along with previously collected sediment samples and geologic information from surrounding coastal areas, outline the characteristics, distribution, and history of the strata that accumulated within Penobscot Bay, Maine, during the complex period of glaciation, crustal movement, and sea-level change since late Wisconsinan time. Sediments that overlie the rugged, glacially eroded surface of Paleozoic bedrock range in thickness from near zero to more than 50 m and consist of four distinct units. 1. (1) Massive to partly stratified, coarse-grained drift forms thin ( 2. (2) Well-stratified, fine-grained glaciomarine deposits are concentrated in bedrock depressions beneath the main passages of the bay. During the period of ice retreat and marine submergence, these sediments settled to the sea floor, draped the irregular underlying surface of bedrock or drift, and accumulated without disturbance by physical or biologic processes. 3. (3) Heterogeneous fluvial deposits fill ancestral channels of the Penobscot River beneath the head of the bay. The channels were incised during a −40 m postglacial low stand of sea level (due to crustal rebound) and later were filled as base level was increased during Holocene time. 4. (4) Muddy marine sediments, which are homogeneous to weakly stratified and rich in organic matter, blanket older deposits within bathymetric depressions in the middle and lower reaches of the bay and cover a pronounced, gently dipping, erosional unconformity in the upper reach. These sediments were deposited during the Holocene transgression as sea level approached its present position and the bay became deeper. Late Wisconsinan and Holocene sedimentation in Penobscot Bay has smoothed the sea floor, but it has not completely obscured the ice-sculptured bedrock topography.

The Georgia Embayment continental shelf: Stratigraphy of a submergence

Forty vibracores taken across the continental shelf and in proposed drilling lease areas of the southeast Georgia Embayment are used to document the unconsolidated shelf-sediment cover. The Holocene-Pleistocene sediment veneer is thin, generally less than 4 m thick. Lagoon sediments deposited during the last regression (shelf emergence) or the Holocene transgression (shelf submergence) occur in patches on the inner and central shelf. Because essentially only late Pleistocene and Holocene mollusk shells are present in the shelf-sediment cover, it is believed that most of the carbonate fraction was removed by subaerial leaching during low sea-level stands aided by mechanical abrasion and biological degradation during the regressive-transgressive cycle. During each transgression or submergence, the surficial sand sheet is recharged with a new biogenic carbonate fraction along with the addition of small amounts of clastic sediments derived from “overrun” estuaries and erosion of underlying Tertiary sediments. The stratigraphy based on the vibracores supports the concept of cross-shelf migration of the shore face—barrier island systems in response to rising sea level. Sedimentologic and paleontologic analyses also indicate that extensive in-place mixing of shelf sediments may have occurred, an important factor to consider in evaluation of the fate of particulate pollutants. The establishment of the time frame of such mixing should be given high priority in future studies.

Pockmarks in the floor of Penobscot, Bay, Maine

Hundreds of depressions (pockmarks) were found within a 40 square kilometer area of the sea floor near the head of Penobscot Bay, Maine. These roughly circular depressions range in diameter from 10 to 300 meters and extend as much as 30 meters below the surrounding sea floor. The pockmarks have formed in marine mud of Holocene age, which unconformably overlies glaciomarine deposits.The presence of shallow interstitial gas in the mud suggests that the pockmarks are related to the excipe of gas from the sediments, although other factors must be involved.

Late Wisconsinan-Holocene paleogeography of Delaware Bay; a large coastal plain estuary

Abstract Analyses of an extensive grid of seismic reflection profiles along with previously published core data and modern sedimentary environment information from surrounding coastal areas permit an outline of the paleogeography of the large Delaware Bay estuary during the last transgression of sea level. During late Wisconsinan times, the Delaware River system eroded a dendritic drainage pattern into the gravelly and muddy sands of Tertiary and younger age beneath the southern half of the lower bay area. This system included the trunk valley of the ancestral river and a large tributary valley formed by the convergence of secondary streams along the Delaware coast. The evolution of the estuary from this drainage system proceeded as follows: (1) When local relative sea level was at −50 m, the head of the tide reached the present bay-mouth area. (2) At −40 m (possibly 15,000–12,000 yrs ago), the trunk valley of the drainage system was a tidal river that extended more than 30 km up the bay, and a small contiguous inlet existed at the bay mouth. (3) At −30 m (approximately 11,000−10,000 yrs ago), the estuary comprised two narrow passages formed by the drowning of the main and tributary river valleys, and the bay-mouth inlet was 5–6 km wide. (4) At −20 m (between 8000 and 7000 yrs ago), the two passages of the estuary were joined, except for a series of small islands on top of a low intervening ridge, and the inlet channel was 11 km wide. (5) At −10 m (between 6000 and 5000 yrs ago), the estuary was nearly continuous and encompassed about 60% of the present lower bay area. Thin, coarse-grained fluvial deposits accumulated initially within the main channels of the former drainage system as base level was elevated by rising sea level. During the subsequent development of the estuary, clayey silts were deposited rapidly beneath the nontidal estuarine depocenter (turbidity maximum) as it migrated through the bay area, and organic muds accumulated in tidal wetlands that occupied the mouths of tributaries and small marginal embayments. As the fetch and tidal prism of the estuary increased, narrow barrier and headland beaches, composed of fine to coarse sands, were formed locally along the bay shorelines. In the later stages of development, sediment scour, reworking and transport became the dominant processes within the open estuary. Data from this study demonstrate the great temporal and spatial variability of sedimentary deposits within large drowned river-valley estuaries and outline a model that can be used to interpret ancient estuarine strata.

Holocene depocenter migration and sediment accumulation in Delaware Bay: a submerging marginal marine sedimentary basin☆

Abstract The Holocene transgression of the Delaware Bay estuary and adjacent Atlantic coast results from the combined effect of regional crustal subsidence and eustasy. Together, the estuary and ocean coast constitute a small sedimentary basin whose principal depocenter has migrated with the transgression. A millenial time series of isopach and paleogeographic reconstructions for the migrating depocenter outlines the basin-wide pattern of sediment distribution and accumulation. Upland sediments entering the basin through the estuarine turbidity maximum accumulate in tidal wetland or open water sedimentary environments. Wind-wave activity at the edge of the tidal wetlands erodes the aggraded Holocene section and builds migrating washover barriers. Along the Atlantic and estuary coasts of Delaware, the area of the upland environment decreases from 2.0 billion m 2 to 730 million m 2 during the transgression. The area of the tidal wetland environment increases from 140 million to 270 million m 2 , and due to the widening of the estuary the area of open water increases from 190 million to 1.21 billion m 2 . Gross uncorrected rates of sediment accumulation for the tidal wetlands decrease from 0.64 mm/yr at 6 ka to 0.48 mm/yr at 1 ka. In the open water environments uncorrected rates decrease from 0.50 mm/yr to 0.04 mm/yr over the same period. We also present data on total sediment volumes within the tidal wetland and open water environments at specific intervals during the Holocene.

Modern sedimentary environments on the Rhode Island inner shelf, off the eastern United States

Abstract Analyses of side-scan sonar records along with previously published bathymetric, textural and subbottom data reveal the sedimentary environments on the inner Continental Shelf south of Narragansett Bay, Rhode Island. The bottom topography in this area is characterized by a broad central depression bordered by shallow, irregular sea floor on the north and east and by a discontinuous, curvilinear ridge on the south and west. Four distinct environments were identified: 1. (1) Pre-Mesozoic coastal rocks are exposed on the sea floor at isolated locations near the shore ( waterdepths m ). These exposures have pronounced, irregular topographic relief and produce blotchy patterns on side-scan sonographs. 2. (2) Glacial moraine deposits form the discontinuous offshore ridge. These deposits have hummocky sea-floor relief, are covered by lag gravel and boudlers, and appear as predominantly black (strongly reflective) patterns on the side-scan records. 3. (3) Over most of the shallow, irregular bottom in the northeast, on the flanks of the morainal ridge, and atop bathymetric highs, the sea floor is characterized as a mosaic of light and dark patches and lineations. The dark (more reflective) zones are areas of coarse sands and megaripples ( wavelengths = 0.8–1.2 m that either have no detectable relief or are slightly depressed relative to surrounding (light) areas of finer-grained sands. 4. (4) Smooth beds that produce nearly featureless patterns on the sonographs occupy the broad central bathymetric depression as well as smaller depressions north and east of Block Island. Within the broad depression, sonographs having practically no shading indicate a central zone of modern sandy silt, whereas records having moderate tonality define a peripheral belt of silty sand. The sedimentary environments that are outlined range from erosional or non-depositional (bedrock, glacial moraine) to depositional (featureless beds), and include areas that may reflect a combination of erosional and depositional processes (textural patchiness). The distribution and characteristics of the environments reveal the general post-glacial sedimentary history of this area and provide a guide to future utilization of the shelf surface.