Baltimore Canyon Research Articles

Sediment blanketing and the flexural strength of extended continental lithosphere

AbstractThe flexural rigidity of the oceanic lithosphere is strongly dependent on its temperature structure at the time of loading. It is commonly assumed that the depth to the 450°C isotherm defines the effective elastic thickness Te of the lithosphere. However, recent gravity studies across the Baltimore Canyon and Nova Scotian margins suggest that temperature may play a more complicated role in controlling the mechanical strength of extended continental lithosphere. For example, the flexural strength of the Baltimore Canyon margin (with sediment thicknesses of ≅ 15 km) appears to be controlled by the depth to the 150°C isotherm whereas the strength of the Nova Scotian margin (with sediment thicknesses cf ≅ 10 km) is controlled by the depth to the 250°C isotherm. The apparent correlation between sediment thickness and controlling isotherm suggests that sediment blanketing may play a role in modifying the flexural strength of extended continental lithosphere. This hypothesis was investigated by simulating the sedimentation history of a margin as a Gaussian function in which sedimentation peak and rate are determined by the mean and standard deviation of the function. The temperature structure of the lithosphere is continually modified as sediments are deposited on, and incorporated into the temperature structure of, the underlying lithosphere. Given a ‘starting’ value of Te defined by the degree of extension of the lithosphere, the modification of Te appears to be directly proportional to the sedimentation rate and cumulative sediment thickness, and inversely proportional to the time at which the sedimentation rate is a maximum. The first‐order consequence of sediment blanketing is to reduce the cooling rate of the lithosphere relative to cooling in the absence of sediments. At thermal equilibrium, the initial value of Te is reduced by the cumulative sediment thickness. Local isostatic conditions (i. e. Te≅ 0) can only be approached when the sedimentation rate is unrealistically high (> 1000 m/Myr) during the rift or early post‐rift phase of basin development. However, while these early loads may be locally compensated, any subsequent loads will be regionally compensated. Thus, it is unlikely that the low present‐day flexural strengths interpreted from the Baltimore Canyon and Nova Scotian passive continental margins are a consequence of sediment blanketing.

A paleoslope model of late Albian to early Turonian foraminifera of the western Atlantic Margin and North Atlantic basin

A paleoslope model of Albian to Turonian benthic foraminifera reveals seven successive biofacies which are associated with increasing paleodepth. The paleoslope model indicates that the biofacies occupied inner neritic to abyssal environments of deposition (10 m to > 3500 m water depth). Biofacies 1 and 2 characterize paleodepths of less than 60 m. The biofacies contain abundant species of Epistomina and Gavelinella and exhibit a rapid stratigraphic alternation in response to local sea level change, which considerably aids in the identification of depositional sequences. Species of Globorotalites, Eggerellina and Neobulimina characterize biofacies 3 which principally occurs between 60 m and 130 m paleodepth. Biofacies 3 is the only biofacies to occur in both the New Jersey coastal plain and the Baltimore Canyon Trough. Biofacies 4 is composed of species of Gavelinella, Gyroidinoides and Valvulineria and is abundant in outer neritic to middle bathyal paleodepths of 130 m to 600 m. It is largely replaced by biofacies 5 during times of maximum flooding during sea level rise. A diverse assemblage of species of Lingulogavelinella and Lenticulina characterize biofacies 5. Paleodepths of 600 m to greater than 3500 m contain abundant species of Osangularia, Pleurostomella, Praebulimina, Pseudopatellinella and Quadrimorphina which define biofacies 6 and 7. In addition, four biofacies of much more limited distribution are recognized. Biofacies 1 ★ characterizes shallow-water, open marine paleoenvironments of the North Atlantic and is composed of spirillinids and species of Epistomina. Turonian dysaerobic environments are indicated by biofacies DSA and DSB which are composed mostly of agglutinated species. Dysaerobic environments expanded rapidly from middle bathyal to outer neritic depths in the early Turonian on the western Atlantic Margin. Biofacies DAA represents disturbed benthic environments and is dominated by species of Psammosphaera and Reophax. Biostratigraphy and species composition of the studied biofacies are similar to previously described European and South Atlantic assemblages. Major speciation events occur among the genera Gavelinella, Lingulogavelinella and Valvulineria. Well defined neritic and bathyal biofacies formed during a rapid late Albian species diversification concomitant with the first major Cretaceous flooding of continental margins.

Sequence Stratigraphy Simulations of Carbonate, Clastics, and Mixed Basin Margins

Clastics, carbonates, and their mixtures have different depositional and post-depositional behavior that produces the different margin characteristics seen in seismic sequences. Carbonates undergo early cementation while maintaining higher angles of repose, while clays and sands accumulate at lower-angle slopes whose inclination is proportional to the grain size and post-depositional cohesive behavior. In higher energy regimes, waves or currents winnow less cohesive finer material that is transported downdip to from slope sediments rimming the basin. Simulations of mixed carbonate-clastic sediment accumulation, tectonism, and eustasy for settings in the Permian basin of west Texas and New Mexico show that sharp differentiation of clastics from carbonates is a product of higher angles of repose that carbonates maintain and the higher rates of clastic input at lowstands in sea level. In contrast, simulation of mixed grain-size margins like the Exmouth Plateau of Western Australia, the Baltimore Canyon, and the Gulf Coast Tertiary indicate that muds are winnowed preferentially from shelf-margin crests but accumulate on slopes, while sands accumulate on higher energy shelves. When they bypass at lowstands in sea level, they accumulate in the near slope basin but not on the slope. Simulation of pure carbonate systems like that of the Bahamian platform suggests more » that progradation is greatest in areas of low wave and current energy while backstepping and cliffed margins occur in high energy settings. The ability to accurately simulate mixed carbonate-clastic slopes is a key to development of exploration and production models of these systems. « less

Petroleum Geology of the Upper Jurassic and Lower Cretaceous, Baltimore Canyon Trough, Western North Atlantic Ocean (1)

Numerous hydrocarbon shows, including a noncommercial gas and gas-condensate accumulation, occur in the Baltimore Canyon Trough within sandstone units deposited in prograding coastal-plain and transitional-marine environments located updip of an Oxfordian/Kimmeridgian carbonate shelf edge. The coastal-plain and transitional-marine facies are overlain by a fine-grained deltaic complex dominated by delta-plain shales which collectively form a regionally extensive top seal unit. This deltaic complex prograded into a back-reef lagoon during aggradation of lower Kimmeridgian through Berriasian shelf-margin carbonates. Wells drilled seaward of the continental shelf edge (>1500 m water depth) tested large structural/stratigraphic closures along the downdip termination of the Upper Jurassic/Lower Cretaceous carbonate shelf edge but encountered no significant hydrocarbon shows. Reservoir rocks in these wells consist of (1) oolite grainstone, which was deposited within a shoal-water complex located at the Aptian shelf edge, and (2) coral-stromatoporoid grainstone and boundstone, which formed an aggraded shelf-margin complex located at the late Kimmeridgian through Berriasian shelf edge. Structural closures having reservoir and top seals are present in both updip and downdip trends. Hydrocarbon shows in wells along the shelf interior trend indicate the presence of mature source beds, at least locally. The absence of hydrocarbon shows in downdip carbonate reservoirs and around the Schlee Dome, however, suggests charge/migration mechanisms within the fetch areas of these objectives have failed. Failure of charge can be due to (1) absence of mature source rocks, (2) absence of migration pathways from source rocks to reservoirs, and/or (3) absence of top seals at the time of hydrocarbon migration. Continued development of play concepts in the Baltimore Canyon Trough, therefore, requires identification and mapping of potential source-rock intervals and construction of hydr carbon expulsion models to time hydrocarbon generation relative to trap formation.

Geology and Resource Potential of the Manteo Prospect

The Manteo exploration unit, located approximately 32 mi northeast of Cape Hatteras, North Carolina, is a potential East Coast elephant-size field. The unit consists of 21 leases that cost industry $296 million in bonuses since 1981. Mobil has estimated that the unit may contain as much as 5 tcf of natural gas, which would be the largest domestic find since Prudhoe Bay. The Manteo prospect, on the seaward margin of the Carolina Platform, lies between two major basins: the Baltimore Canyon Trough to the north and the Carolina Trough to the south. The prospect is within the western Atlantic carbonate trend, a Jurassic-Early Cretaceous reef and platform limestone complex that stretches from the Gulf of Mexico to Newfoundland. Seismic interpretation is complicated by distortions caused by abrupt changes of water depth map near the top of the Jurassic shows a large eat-northeast-trending structure about 30 mi long and 5 mi wide with structural closure of over 1,000 ft. Source rock analyses suggest that Upper Jurassic carbonate reservoirs would more likely contain gas than oil. The Minerals Management Service (MMS) estimates reservoir parameters from wells drilled on trend (Baltimore Canyon Trough) and recovery parameters from worldwide analogs. The 21-block unit wasmore » approved by the MMS in May of 1990. Since 1988, the MMS has worked with North Carolina, Federal and local agencies, and Mobil in preparation for wildcat exploration approval. The MMS has prepared an extensive environmental report for this exploration proposal.« less

Controls on Passive Margin Stratigraphy: Seismostratigraphic and Basin Modeling Evaluation of Georges Bank Basin

Seismostratigraphic interpretations constrained by well data were coupled with 2-D stratigraphic forward and thermo-subsidence models to predict stratigraphy and to better understand the roles of tectonics, eustasy, and sedimentation in controlling sequence development in the Georges Bank basin. Seventeen late Middle Jurassic-Cretaceous depositional sequences and 13 types of seismically defined depositional facies were recognized and correlated within the basin. Subsidence histories constrained by seismic correlations, a modified Exxon sea-level curve, and iteratively determined sediment infill parameters were the primary input for 2-D stratigraphic modeling analyses of three dip oriented seismic lines. The sea-level curve was the same as that used in earlier modeling studies of Baltimore Canyon. The integrated seismostratigraphic interpretations and stratigraphic modeling results predict similar aggradational Jurassic-Cretaceous mixed clastic/carbonate platforms. The extent and rate of shelf margin progradation can be attributed to focused infilling associated with an embayment or possible salt or basement tectonism. Relative sea-level changes controlled gross reciprocal clastic/carbonate depositional patterns. A series of time sequential cutaway block diagrams and facies maps will be shown illustrating the computer-modeled and seismically interpreted Jurassic-Cretaceous sequence stratigraphic evolution of the basin. The Georges Bank stratigraphic evolution will be compared to that of Baltimore Canyon. Even though the two basinsmore » had differing subsidence and sedimentation histories, the observed facies distribution and stratal geometries could be simulated with a single sea-level curve.« less

Hydrocarbon Potential of the Baltimore Canyon Trough

The Baltimore Canyon Trough, which is in the Mid-Atlantic Outer Continental Shelf (OCS) Planning Area, has a thick sediment accumulation and a large number of structural and stratigraphic traps. It is underlain by a seaward-thickening wedge of postrift sediments separated by an unconformity from older, more areally restricted rift sediments of Triassic and Early Jurassic age. Uplift and erosion landward of a hinge zone and regional subsidence seaward resulted in formation of the Baltimore Canyon Trough, an elongated northeast trending depression filled with over 49,000 ft (15,000 m) of sediments.

Seismic and Geologic Characteristics of Drowning Events on Carbonate Platforms(1)

Carbonate platform drowning events are recorded in Upper Jurassic-Lower Cretaceous carbonates of the Baltimore Canyon area, offshore United States East Coast, and lower Miocene carbonates of the Pearl River Mouth Basin, offshore People's Republic of China. Cursory examination of seismic data from both areas would indicate that the platforms fit the classic definition of a drowning unconformity. However, detailed lithologic and paleontologic data indicate that the depositional/erosional hiatuses vary widely in each area, from 0-25 m.y. in the Baltimore Canyon area, to a condensed section with no hiatus in the Pearl River Mouth Basin. The sedimentary sequence produced during drowning (the drowning sequence) in both areas is gradational with underlying shallow platform carbo ates and, in some places, overlying deeper marine limestones and shales. The generalized facies models proposed for drowning events suggest that they may appear as instantaneous or gradual changes in geologic and seismic data. However, high-resolution seismic data across the Baltimore Canyon and Pearl River Mouth basins show that some drowning sequences may be masked on conventionally processed seismic sections. Because of this resolution problem, a distinction should be made between seismically and geologically defined unconformity surfaces. The geologic characteristics of drowning events on carbonate platforms include a gradational lower (and sometimes upper) contact, chemical sedimentation (glauconite, phosphate), open-marine shelf sediments, and a variable hiatus at the upper boundary. Late-growth shelf margin reefs also can be a diagnostic feature of drowned platforms. Seismic characteristics include horizontal to sub-horizontal basinal marine onlap, basin-parallel reflector continuity above the carbonate sequence boundary, and downlap of later highstand units. Evidence from platforms adjacent to continental margins (Baltimore Canyon) indicates that drowning occurs in a step-wise fashion, advancing more or less symmetrically across the platform from shelf to nearshore environments. Data from isolated open-ocean atolls (Pearl River Mouth Basin) indicate that they respond differently to drowning, showing an asymmetric decrease in shallow platform environments as drowning occurs. Proper recognition by a geoscientist of the seismic and geologic characteristics of drowning events can lead to better reservoir and seal estimates, and the correct reconstruction of the depositional, tectonic, and eustatic histories of an area.

Stratigraphic Simulation of Sedimentary Basins: Concepts and Calibration (1)

A two-dimensional stratigraphic simulation program based on semiempirical algorithms has been successfully applied to clastic, carbonate, and mixed clastic/carbonate depositional regimes. The program may be used to predict reservoir distribution, to constrain interpretations of well and seismic data, to test exploration scenarios in frontier basins, and to evaluate the fundamental controls on basin stratigraphy. Applications to seismic and well-log data sets from Main Pass (U.S. Gulf Coast), offshore Sarawak (Malaysia), and the Baltimore Canyon (U.S. East Coast) demonstrate that the program simulates stratigraphy on a basin-wide scale as well as on the scale of individual prospects. The Main Pass section is an offlapping sequence of Neogene clastics. The model simulates 17 m.y. of geologic history at a 200,000-yr resolution and reproduces gross basin geometry, major sequence boundaries, depth and location of nearshore marine sands, intervals of sediment bypass into deep water, shelf margin positions, and paleobathymetry. Three Neogene carbonate buildups of central Luconia, offshore Sarawak (Malaysia), were simulated using a sea level history optimized to reproduce the carbonate growth anatomy. The model reproduced observed seismic reflection geometries and transgressive, aggradational, progradational, retrogradational, and subaerial exposure phases. The program has also simulated Jurassic-Pleistocene basin and stratigraphic sequence geometries and mixed clastic/carbonate facies distribution along a 300-km regional dip line of the Baltimore Canyon Trough. Modeled features of interest include a progradational Middle Jurassic carbonate margin punctuated by clastic deposition during sea level lowstands and a Late Jurassic-Early Cretaceous aggradational carbonate margin culminating in the development and subsequent drowning of isolated carbonate buildups. Simulated bathymetries, facies relations, and subaerial exposure surfaces for the Early Cretaceous aggradation margin are consistent with detailed petrographic, paleontologic, and seismic stratigraphic interpretations.

Seismic Properties and Modeling of an Early Aalenian Salt(?) Layer in Baltimore Canyon Trough, Offshore New Jersey: ABSTRACT

Seismic Properties and Modeling of an Early Aalenian Salt(?) Layer in Baltimore Canyon Trough, Offshore New Jersey: ABSTRACT

Early Cretaceous shelf-edge deltas of the Baltimore Canyon Trough: Principal sources for sediment gravity deposits of the northern Hatteras Basin

We present evidence that the principal sources for Early Cretaceous (Berriasian-Valanginian) gravity-flow deposits of the northern Hatteras Basin were three large shelf-edge deltas located along the outer margin of the Baltimore Canyon Trough, ∼ 100 km southeast of Cape Charles, Virginia, Ocean City, Maryland, and Long Branch, New Jersey. Sedimentary detritus from the central Appalachian highlands and the Maryland-Virginia coastal plain was transported across the Early Cretaceous continental shelf to form the Cape Charles and Ocean City deltas, whereas deposits of the Long Branch delta came chiefly from the Adirondack and New England highlands. Each delta supplied sediment gravity flows to large slope aprons and submarine-fan complexes on the Early Cretaceous continental slope and rise. The most conspicuous distributary of sediment on the Early Cretaceous continental rise extends 500 km basinward from the Ocean City delta, where its distal deposits were cored at Deep Sea Drilling Project Site 603.

Periodic resuspension in Baltimore canyon by focusing of internal waves

High concentrations of particulate matter are frequently reported in submarine canyons. This study demonstrates that in Baltimore submarine canyon, elevated concentrations result from periodic resuspension events. The sloping bottom and V‐shaped topography of submarine canyons could accelerate tidal flows in the canyon axis sufficiently to cause resuspension, which occurs predominantly at tidal frequencies. However, current meter and transmissometer data show that resuspension events are often associated with a borelike intrusion of cold water moving upslope near the canyon head; an event that is consistent with the focusing of internal wave energy toward the canyon head at tidal frequencies. Hydrographic conditions for the focusing of energy are most favorable in late winter and early spring when stratification in the canyon is diminished; direct observations reveal that the most pronounced resuspension events occur during those seasons. Bottom stresses generated in the canyon are sufficient to resuspend sediment along the axis at least between 200 and 800 m, with little resuspension occurring deeper along the axis, on the canyon walls, adjacent slope, or shelf. During a resuspension event, the sediment‐laden water is forced upcanyon, then sloshes back down the canyon until it reaches a layer of higher‐density water, detaches from the canyon floor, and moves seaward along a density surface. This is a means of transferring sediment and associated pollutants intercepted at the canyon head to deeper water, across the shelf‐slope front. Over geologic time, frequent resuspension and seaward transport may aid in the headward erosion of submarine canyons, or at least inhibit their infilling during interglacial times.

Upper cretaceous dinoflagellate biostratigraphy of the subsurface Atlantic coastal plain of New Jersey and Delaware, U.S.A.

Abstract The Upper Cretaceous rocks in five New Jersey wells (all conventional cores) can be informally subdivided using dinoflagellates. The seven zones and three subzones are, in ascending strati‐graphic order: the Trithyrodinium suspectum concurrent range zone (Cenomanian); Canningia sp. A abundance zone (latest Cenomanian to earliest Santonian); Chatangiella micracantha concurrent range zone (earliest Santonian to latest Santonian or earliest Campanian); Spinidinium mariae interval zone (latest Santonian or earliest Campanian to late early or early late Campanian); Palaeohystrichophora infusorioides interval range zone and its three subzones ‐ Chatangiella manumii subzone (late early Campanian to early late Campanian), Chatangiella porata subzone (early late Campanian to late late Campanian) and Diconodinium wilsonii subzone (late late Campanian to early Maastrichtian); Chatangiella? robusta interval zone (Maastrichtian) and Deflandrea galeata interval zone (middle or late to latest Maastrichtian to Early Tertiary). This dinoflagellate zonation is correlated with the standard planktonic foraminiferal zonation, thus allowing correlation with the standard European and North American sections. Based upon this integration of dinoflagellate and foraminiferal biostratigraphy, the unconformity separating the Raritan and Magothy formations appears to range from late Turonian through Coniacian. The analyses of two additional wells provide a means of evaluating the areal applicability of this New Jersey dinoflagellate zonation. One, the Delaware 904 well (also a conventional core) makes possible the correlation of Delaware lithostratigraphy with that of New Jersey. The second is the COST B‐2 well, located in the northern Baltimore Canyon Trough due east of Atlantic City. Analysis of selected samples from this well are supportive of a Campanian‐Eocene bounded unconformity as determined from foraminifers.

Petroleum geology of the mid-Atlantic continental margin, offshore Virginia

The Baltimore Canyon Trough, a major sedimentary basin on the Atlantic continental shelf, contains up to 18 km of Mesozoic and Cenozoic strata. The basin has been studied extensively by multichannel common depth point (CDP) seismic reflection profiles and has been tested by drilling for hydrocarbon resources in several places. The Mesozoic and Cenozoic strata contained in the basin were deposited in littoral to bathyal depositional settings and contain immature to marginally mature oil-prone and gas-prone kerogen. The more deeply buried strata of Early Mesozoic age are more likely to be thermally mature than are the younger strata with respect to hydrocarbon generation, but contain terrestrially derived coaly organic matter that would be prone to yield gas, rather than oil. An analysis of available CDP seismic reflection data has indicated that there are several potential hydrocarbon plays in the area offshore of Virginia. These include: (1) Lower Mesozoic synrift basins that appear similar to those exposed in the Appalachian Piedmont, (2) a stratigraphic updip pinchout of strata of Early Mesozoic age in the offshore region near the coast, (3) a deeply buried paleoshelf edge, where seismic reflectors dip sharply seaward; and (4) a Cretaceous/Jurassic shelf edge beneath the present continental rise. Of these, the synrift basins and Cretaceous/Jurassic shelf edge are considered to be the best targets for exploration.

Geologic framework of the offshore region adjacent to Delaware

Several multichannel, common depth point (CDP) seismic reflection profiles concentrated in the area of the entrance to Delaware Bay provide a tie between the known onshore geology of the Coastal Plain of Delaware and the offshore geology of the Baltimore Canyon Trough. The data provide a basis for understanding the geologic framework and petroleum resource potential of the area immediately offshore Delaware. Our research has focused on buried early Mesozoic rift basins and their geologic history. Assuming that the buried basins are analogous to the exposed Newark Supergroup basins of Late Triassic-Early Jurassic age, the most likely possibility for occurrence of hydrocarbon source beds in the area of the landward margin of the Baltimore Canyon Trough is presumed to be lacustrine, organic-rich shales probably present in the basins. Although buried basins mapped offshore Delaware are within reach of drilling, no holes have been drilled to date; therefore, direct knowledge of source, reservoir, and sealing beds is absent. Buried rift basins offshore Delaware show axial trends ranging from NW-SE to NNE-SSW. Seismic reflection profiles are too widely spaced to delineate basin boundaries accurately. Isopleths of two-way travel time representing basin fill suggest that, structurally, the basins are grabens and half-grabens. As shown on seismic reflection profiles, bounding faults of the basins intersect or merge with low-angle fault surfaces that cut the pre-Mesozoic basement. The rift basins appear to have formed by Mesozoic extension that resulted in reverse motion on reactivated basement thrust faults that originated from compressional tectonics during the Paleozoic. Computer-plotted structure contour maps derived from analysis of seismic reflection profiles provide information on the burial history of the rift basins. The postrift unconformity bevels the rift basins and, in the offshore area mapped, ranges from 2000 to 12,000 m below present sea level. The oldest postrift sediments that cover the more deeply buried rift basins are estimated to be of Middle Jurassic age (Bajocian-Bathonian), the probable time of opening of the Atlantic Ocean basin and onset of continental drift about 175–180 m.y. ago. By late Oxfordian-early Kimmeridgian time, the less deeply buried basins nearshore Delaware had been covered. A time-temperature index of maturity plot of one of the basins indicates that only dry gas would be present in reservoirs in synrift rocks buried by more than 6000 m of postrift sediments and in the oldest (Bathonian?-Callovian?) postrift rocks. Less deeply buried synrift rocks landward of the basin modeled might still be within the oil generation window.

Depositional sequences in the Cretaceous post-rift sediments on the New Jersey Atlantic margin

Deposition of the Cretaceous post-rift sediments of the New Jersey Atlantic margin was controlled by numerous cycles of sea-level change. The strata deposited within each cycle constitute the depositional sequence of the cycle. Two types of deposits are identified in the depositional sequences of the New Jersey coastal plain, transgressive deposits and highstand deposits. The transgressive deposits formed when sea level was rising at its greatest rate, which led to rapid inundation of the margin. The highstand deposits formed when sea-level rise slowed and began to fall. Unconformities developed in the coastal plain during lowstand of sea level at the end of a cycle and during the initial slow rise of sea level at the beginning of the next cycle. As a consequence, the depositional sequences are unconformity bounded and are separated by hiatuses of varying time dimensions. In the coastal plain the transgressive deposits are characterized by their fine-grained nature and by containing authigenic glauconite, often in abundance. They have sharp basal contacts where they unconformably overlie a lower sequence. The highstand deposits are sands and silts that grade from the underlying transgressive deposits. Nine depositional sequences are recognized in the Lower and Upper Cretaceous on the basis of these criteria. Each of these sequences is time constrained by biostratigraphic distribution of various microfossil groups (foraminifera, pollen and spores, dinoflagellates, and coccoliths). An additional four depositional sequences are suggested in the non-marine Lower Cretaceous on the basis of the interpretation of highstand deposits. Two major unconformities, five minor unconformities, and their accompanying hiatuses are recognized. A major Coniacian to early Santonian hiatus can be traced from the coastal plain to wells in the Baltimore Canyon Trough.

A record of Appalachian denudation in postrift Mesozoic and Cenozoic sedimentary deposits of the U.S. Middle Atlantic continental margin

The complex interplay between source-terrain uplift, basin subsidence, paleoclimatic shifts, and sea-level change, left an extensive sedimentary record in the contiguous offshore basins of the U.S. middle Atlantic margin (Salisbury Embayment, Baltimore Canyon Trough, and Hatteras Basin). Isopach maps of 23 postrift (Lower Jurassic to Quaternary) a allostratigraphic units, coupled with a revised stratigraphic framework, reveal that tectonism, by regulating sediment supply (accumulation rate), dominated the interplay of forcing mechanisms. Tectonic pulses are evidenced by abruptly accelerated sediment accumulation, marked latitudinal shifts in the location of depocenters, and regional changes in lithofacies. Relatively rapid tectonic subsidence during the Early and Middle Jurassic history of the basins may have enhanced sediment accumulation rates. Beginning in the Late Jurassic, however, subsidence rates decreased significantly, though occasional short pulses of subsidence may have effected relative sea-level rises. Sea-level change heavily influenced the distribution and redistribution of sediments one they reached the basins, and paleoclimate regulated the relative abundance of carbonates and evaporites in the basins. We conclude that source terrains of the central Appalachian Highlands were tectonically uplifted, intensely weathered, and rapidly eroded three times since the Late Triassic: (1) Early to Middle Jurassic (Aalenian to Callovian); (2) mid-Early Cretaceous (Barremian); and (3) Late Cenozoic (Middle Miocene). Intervals of tectonic quiescence following these three tectonic pulses provided conditions suitable for the formation of regional erosion surfaces, geomorphic features commonly reported to characterize the central Appalachian Highlands. This series of three, irregularly spaced, tectonic/quiescent cycles does not, however, match the traditional four-cycle concept of post-Triassic Appalachian “peneplanation”.

Neotectonism along the Atlantic passive continental margin: A review

An extensive body of geologic data including the modern state of stress, historical seismicity, surface and subsurface stratigraphy, numerical models of crustal deformation, surficial geomorphic systems, and historical precise leveling and tidal gauge records constrain the style and rate of neotectonic deformation for the Appalachians and Atlantic passive continental margin. There are two major styles of neotectonism in the eastern United States. The northeastern United States is dominated by isostatic uplift and northward migration of peripheral bulge collapse in response to deglaciation. This locally rapid, but decreasing rate of deformation is superimposed upon slower, long-term deformation along the Atlantic margin. Most of the long-term, continental margin deformation is attributed to lithospheric flexuring in response to sediment loading in sedimentary basins (especially the Baltimore Canyon Trough and Carolina Trough), isostatic deformation in response to continental denudation and water loading of the shelf, and stress from far-field plate tectonic sources. Significant deformational features include an uplift anomaly near Cape Fear, N.C.; northward and southward tilting of the Coastal Plain into the Salisbury and Southeast Georgia Embayments respectively; seaward tilting of the Coastal Plain/Piedmont, and a complex pattern of postglacial uplift and later subsidence in the northeast. Estimates of vertical crustal velocities for similar locations vary over several orders of magnitude. Measurement interval bias and systematic leveling errors may account for some of the discrepancies. Evidence for periodic deformation in the eastern United States in substantial and it is possible that historic data indicate a period of accelerated deformation along the Atlantic continental margin.

Baltimore Canyon as a modern conduit of sediment to the deep sea

An integrated study using moored current meters, transmissometers and sediment traps revealed that sediment in Baltimore Canyon was resuspended regularly when energy from internal tides was focused along the canyon axis. Such focusing occurred primarily during late winter and early spring. During resuspension events particle concentrations at 275 m in the canyon axis typically increased from a background level of 100 μg l −1 to over 1000 μg l −1 within a 5-min period and often exceeded 5000 μg l −1. Net transport of both water and sediment 5 m above bottom (mab) at 275 m in the canyon axis was consistently downcanyon, while 5 mab at 600 m in the canyon axis net transport was upcanyon, implying convergence between the two sites. The distribution of suspended sediment was determined using a CTD/transmissometer during hydrographic transects along the canyon axis and across the slope adjacent to either side of the canyon. Resuspension on the shelf, canyon walls, and along the slope was minimal, but in the canyon interleaving tongues of turbid water were always present between 200 and 800 m. Although concentrations of resuspended sediment were high, they were not sufficient to generate downcanyon flows. Turbid plumes moved seaward along density surfaces at the convergence between 275 and 600 m and other regions of the canyon. The passage of atmospheric storms and Gulf Stream rings may have changed the pattern of currents of resuspension in portions of the canyon sometimes, but not always. Fluxes of particles collected in sediment traps along the canyon axis increased exponentially toward the buttom, but the size of particles collected in traps decreased with distance from the canyon head, suggesting that resuspended sediment was advected, but depleted in larger, denser particles away from the canyon. An exponential increase with depth in trap fluxes at 1000 m, despite fairly uniform concentrations of suspended particles measured by transmissometers, suggests that many particles are advected as large aggregates (marine snow) that are not detected quantitatively by transmissometers. Currents outside the canyon walls quickly dilute and dissipate sediment moving out of the canyon. Subamrine canyons can act as conduits of sediment to the deep ocean not only by channeling turbidity currents, but also by focusing internal tides that resuspended sediment, after which it is advected to deeper water.

Gravity anomalies, crustal structure and flexure of the lithosphere at the Baltimore Canyon Trough

Seismic and gravity anomaly data have been used to examine the long-term mechanical properties of the lithosphere beneath the Baltimore Canyon Trough, a 15 km deep sedimentary basin in the rifted U.S. Atlantic margin. Seismic data constrain the crustal structure at the time of rifting as well as the amount of sedimentation and erosion that has occurred. The gravity effect of the initial crustal structure, sedimentary loading and erosional unloading have been computed and compared to the observed free-air gravity anomaly. The best fitting model is one in which the elastic thickness of the lithosphere ( T e) changes at the East Coast magnetic anomaly. Landward of the anomaly, T e is significantly smaller than previously reported values for continental or oceanic lithosphere of the same thermal age. Seaward of the anomaly, however, T e follows the depth to the 450°C oceanic isotherm which also describes the response of oceanic lithosphere to seamount and oceanic island loads. These results are interpreted as indicating that the shelf part of the trough is underlain by continental lithosphere which, following rifting, did not acquire any significant long-term strength.