Daniel F. Belknap

Influence of antecedent geology on stratigraphic preservation potential and evolution of Delaware's barrier systems

Electric shaver of reciprocal vibration type having improved vibration member for converting rotary motion of motor into reciprocal vibrating motion of inner cutter is provided. The member comprises an integral body of a driving plate part to be coupled on one side to the inner cutter, leg parts respectively extended from each end in directions of the reciprocal vibrating motion of the plate part and flexible only in said directions, a mounting base part coupled to extended ends of the leg parts for mounting the member to stationary part in shaver housing, and an arm-shaped connecting part extended from the other side of the plate part for directly connecting it to eccentric cam of the motor at free end of the connecting part and having a flexibility in directions perpendicular to the reciprocal vibrating directions of the plate part. Components of the rotary motion of the eccentric cam directly transmitted to the connecting part of the vibration member in said perpendicular directions are absorbed by the flexibility of the connecting part, whereby the driving plate part is vibrated only in the reciprocal vibrating directions as supported by the leg parts being flexible only in such directions.

Giant sea-bed pockmarks: Evidence for gas escape from Belfast Bay, Maine

Circular depressions, or pockmarks, cover the sea floor in many estuarine regions of the western Gulf of Maine. In Belfast Bay, Maine, they are found in densities up to 160/km 2 , are up to 350 m in diameter and 35 m in relief, and are among the largest and deepest known. The pockmarks appear to form from the escape of biogenic natural gas and pore water and are far larger than features associated with thermogenic gas elsewhere. These pockmarks are thought to have formed (1) catastrophically during an earthquake, tsunami, or storm, or (2) slowly over thousands of years. Recent observations of bubble releases suggest continuing activity and a potential geologic hazard. The pockmarks involve a poorly documented coastal process of sediment redistribution and methane release, largely unrecognized in the rock record but widespread in middle- to high-latitude embayments.

Holocene Relative Sea-Level Changes and Coastal Stratigraphic Units on the Northwest Flank of the Baltimore Canyon Trough Geosyncline

ABSTRACT A new local curve of relative sea-level change for the Delaware coastal area is based on 88 radiocarbon dates. The curve is smooth with relatively narrow potential limits of variation in amplitude. It has a somewhat steeper slope than published eustatic sea-level curves and other local relative sea-level curves. Holocene radiocarbon isochrons in Delaware marsh sections are horizontal. Interpretations strongly support the concept that Holocene sea level rose slowly and continuously relative to the Delaware coast; the favored hypothesis is that eustatic rise of sea level was responsible. No seaward tilting is indicated for the Delaware segment of the Atlantic coastal plain or inner shelf. However, radiocarbon-died shallow-water samples of comparable age suggest a strong seaward tilt of the outer continental shelf. The outer shelf over the Baltimore Canyon trough geosyncline has subsided approximately 40 m in the last 10,000 years.

Rapid sea-level rise in the Gulf of Maine, USA, since AD 1800

Two sea-level records from salt marshes in coastal Maine are derived from foraminiferal analyses and AMS 14C, 210Pb, 137Cs and pollen chronology. Both records cover the period from AD 800 until the present and show corresponding patterns of sea-level change when corrected for trends which could accommodate millennial-scale isostatic adjustments. The records provide a detailed sea-level chronology for the last few centuries and thus link the instrumental (tide-gauge) record with the long-term geological record of sea-level change. Results show that sea level was relatively stable between AD 800 and 1300 and reached a lowstand around AD 1800, which was preceded by an oscillation in the eighteenth century. Since AD 1800, sea levels in the Gulf of Maine have risen by 0.3-0.4 m. The onset of this rise corresponds with regional climatic warming and could be interpreted as thermal expansion of the Gulf of Maine and North Atlantic sea surface. Sea-level rise possibly slowed temporarily during the mid-nineteenth century, but twentieth-century rates are unprecedented in the last millennium and correspond with hemispheric warming.

Halimeda bioherms along an open seaway: Miskito Channel, Nicaraguan Rise, SW Caribbean Sea

A recent research cruise to examine small, detached carbonate platforms situated on the Nicaraguan Rise in the SW Caribbean Sea has revealed the presence of numerous Halimeda bioherms. Based upon interpretations from seismic reflection data some exceed 140 m in relief. This is the first documented occurrence of these green-algal buildups in the Caribbean/Bahama Bank region. The Halimeda bioherms form a nearly continuous band that borders the margins of the Miskito Channel—a shallow, open, 125 km long seaway. This 220 m deep channel bisects the Miskito Bank which is a major carbonate shelf. In seismic profile the bioherms appear acoustically “soft” and reveal a local relief of 20–30 m. Tops of these features lie in about 40–50 m of water. Samples from dredge hauls are coarse, poorly cemented packstones/grainstones which are dominated by largely unbroken, disarticulated Halimeda segments set in a poorly sorted sandy matrix. Exposed surfaces were stained brown. Very little living material was brought up in the dredges. The significance of these bioherms and their full extent in the Caribbean are not understood. Undoubtedly, further study will provide important answers concerning their role in the geologic development of Caribbean carbonate platforms.

Patterns of sediment accumulation in the tidal marshes of Maine

One year’s measurements of surficial sedimentation rates (1986–1987) for 26 Maine marsh sites were made over marker horizons of brick dust. Observed sediment accumulation rates, from 0 to 13 mm yr−1, were compared with marsh morphology, local relative sea-level rise rate, mean tidal range, and ice rafting activity. Marshes with four different morphologies (back-barrier, fluvial, bluff-toe, and transitional) showed distinctly different sediment accumulation rates. In general, back-barrier marshes had the highest accumulation rates and blufftoe marshes had the lowest rates, with intermediate values for transitional and fluvial marshes. No causal relationship between modern marsh sediment accumulation rate and relative sea-level rise rate (from tide gauge records) was observed. Marsh accretionary balance (sediment accumulation rate minus relative sea-level rise rate) did not correlate with mean tidal range for this meso- to macro-tidal area. Estimates of ice-rafted debris on marsh sites ranged from 0% to >100% of measured surficial sedimentation rates, indicating that ice transport of sediment may make a significant contribution to surficial sedimentation on Maine salt marshes.

Modeling the contribution of M2 tidal amplification to the Holocene rise of mean high water in the Gulf of Maine and the Bay of Fundy

Abstract Increase in the M2 (principal lunar semi-diurnal) component of the tidal range is an important contributor to the postglacial rise of the high-tide level in the Gulf of Maine and the Bay of Fundy in addition to relative sea-level rise. The increase occurred as the shape of this semi-enclosed embayment changed with sea-level rise and its natural period approached resonance with the M2 tide. A three-dimensional tide model (3DENS) predicts the following M2 tidal ranges averaged for the Gulf of Maine/Bay of Fundy as a percentage of the present range: 54–59% at 7000 yrs B.P., 73% at 5000 yrs B.P., 78% at 4000 yrs B.P., 85% at 3000 yrs B.P., 94% at 2000 yrs B.P., and 98% at 1000 yrs B.P. With a predicted sea-level rise of 0.15–0.90 m by the year 2100, M2 tidal range may increase 0.3–1.9%.

Megabreccia shedding from modern, low-relief carbonate platforms, Nicaraguan Rise

Single-channel seismic reflection data from the margins of lowrelief (150-250 m, measured from edge of bank to basin) carbonate platforms on the northern Nicaraguan Rise reveal complex seismic intervals consisting of mounded, chaotic seismic facies interspersed with discontinuous, parallel/laminated seismic facies. We interpret that these intervals contain megabreccias (chaotic facies) and sandy turbidites (parallel/laminated facies). One megabreccia is exposed on the sea floor displaying an overall fan shape having individual blocks measuring nearly 300 m across and >110 m high. The source area consists of a scalloped embayment with a headwall scarp 180 m high. Reflections within the platform are sharply truncated by this escarpment. This single megabreccia is ∼120 m thick and extends ∼27 km along slope and ∼16 km out into the basin. Other megabreccias within the basin have individual blocks measuring >400 m across. Rocks from dredge hauls are a mixture of shallow- and deep-water facies. Shallow-water facies consist of mixed, skeletal grain-stones and Halimeda packstones. Deep-water facies are massive chalks, chalks with shallow-water skeletal grains, and chalk-block breccias. This indicates that the megabreccias formed as a result of bank-margin collapse, during which the ensuing debris flow eroded into slope and basin facies, mixing rock types together. We speculate that bank-margin-collapse events, resulting in megabreccia formation, may have been seismically triggered, and we emphasize that these large-scale, mass-wasting events occurred along margins of low-relief carbonate platforms.

Bedrock controls on barrier island development: West-central Florida coast

Abstract Fifty vibracores and 130 km of high-resolution seismic profiles were taken in the lagoons and nearshore area of northern Pinellas County, Florida, to determine the role of antecedent topography upon barrier island location, origin and development. East—west seismic profiles show a terrace with 1–1.5 m of relief located along the seaward trend of the barrier islands between −4 and −6 m MSL. This antecedent topography is composed of a relatively erosion resistant Miocene limestone and the terrace is usually present as an increase in slope. A contour map of the pre-Holocene surface shows that this subsurface feature is continuous along the entire study area and is mimicked by the opposing orientations of Honeymoon and Caladesi Islands. The correspondence of the modern barrier system to the antecedent topography indicates that: (1) the islands formed on the terrace; or that (2) landward migration of the barriers was stopped by grounding on this feature. Stratigraphic analyses of lagoonal and nearshore environments were conducted in order to establish the overall response of the northern Pinellas coastal system to the late Holocene sea-level rise. This analysis indicates a basically transgressive Holocene sequence with lagoonal sediments occurring seaward of the islands. Underlying the lagoonal deposits in all areas is a 1 m thick relict Pleistocene unit that has been flooded, reworked and vegetated by intertidal to subtidal organisms. A model of development for these barriers is proposed which incorporates an early transgressive history followed by a recent progradational phase. The progradational phase was probably initiated by the late Holocene slowdown in sea-level rise and an increased sediment supply as the barrier islands migrated over a relict sediment source. The antecedent topography thus served as a stabilization point for landward barrier migration and seaward progradation. The antecedent topographic control of barrier island location described by this model is dependent upon the coincident decline in the rate of sea-level rise and an increased sediment supply.

Morphology and stratigraphy of small barrier-lagoon systems in Maine

Abstract The coast of Maine contains over 200 individual barrier-lagoon systems, most quite small, with an aggregate length of nearly 100 km. Although they represent less than 5% of the tidally influenced coastline of Maine, they are widely distributed and occur in a variety of dynamic regimes and physiographic regions. Their morphology and backbarrier stratigraphy are different from better studied coastal plain systems, and provide important clues to the Holocene evolution of the Maine coast. In a study of geomorphic form and backbarrier stratigraphy, inlet processes and Holocene sea-level rise have been identified as the principal controls on coarse-grained barrier stratigraphy. Barriers in Maine are found in five distinct geomorphic forms, identified herein as: barrier spits, pocket barriers, double tombolos, cuspate barriers and looped barriers. The few long sandy beaches in southwestern Maine are mostly barrier spits. The remainder of the barrier types is composed primarily of gravel or mixed sand and gravel. The barriers protect a variety of backbarrier environments: fresh and brackish ponds, lagoons and fresh- and saltwater marshes. The barriers may or may not have inlets. Normal wave action, coarse-grain size and a deeply embayed coast result in barriers with steep, reflective profiles several meters above MHW. Occasional storm events completely wash over the barriers, building steep, lobate gravel fans along their landward margin. Few, if any, extensive storm layers are recognized as extending into the distal backbarrier environments, however. During sea-level rise and landward barrier retreat, this abrupt, storm-generated transition zone inters the backbarrier sediments. Statistical comparisons of barrier morphology, location and backbarrier environment type with backbarrier stratigraphy show that Holocene backbarrier stratigraphy is best predicted by the modern backbarrier environment type. This, in turn, is influenced most by the absence or presence, and long-term stability or instability of a tidal inlet. Geomorphic barrier form and location in coastal geomorphic compartments show little or no correlation with backbarrier stratigraphy. In contrast to previous classifications of barrier-lagoon systems based primarily on sandy, coastal plain examples, in Maine the shape or origin of the backbarrier system is relatively unimportant. The presence or absence of a tidal inlet is of paramount importance in shaping the Holocene stratigraphy of the backbarrier region.