Baltimore Canyon Trough Research Articles

Deep structure of the US East Coast passive margin from large aperture seismic experiments (LASE)

Multiple ship multichannel seismic measurements in the Baltimore Canyon Trough reveal a deep crustal layer with p-wave velocity of 7.2 km s −1. It apparently continues into oceanic layer 3 from beneath the inner shelf where it presumably underlies the continental basement. The layer may be plutonically solidified mantle melt. Its continuity from continental shelf to deep ocean basin may reflect a continuous progression between plutonic emplacement into the continental crust and plutonic construction of the lower oceanic crust. The deep magmatic expression of late stage continental rifting and early seafloor spreading may be very similar and blur the structural expression of the continent-ocean boundary.

Continental Margin Mapping Project, U.S. Atlantic Margin: A Synthesis of Geologic and Geophysical Data and Interpretations: ABSTRACT

Syntheses of geologic and geophysical studies on the US Atlantic continental margin are being compiled in a series of maps and digital data bases to facilitate use of extensive information acquired by the US Geological Survey (USGS) on the US continental margins. The maps, which are at a scale of 1:1,000,000, are on an Albers Conic Equal Area Projection, with standard parallels at 29.5/sup 0/N and 45.5/sup 0/N, and are compatible with the US National Atlas and the USGS geologic and tectonic maps of the US. For this project, the Atlantic margin has been divided into four regions: Georges Bank basin and Gulf of Maine, Baltimore Canyon Trough, Carolina Trough, and Blake Plateau basin. Each region extends from the seaward edge of the US Exclusive Economic Zone to the coastal plain onlap (fall line). Primary data sets for this series were derived from multichannel seismic reflection profiles and magnetic and gravity surveys, as well as from drill-hole and bottom-sample data. Individual maps in the series include topography and bathymetry, tectonic features, isopachs of various sedimentary units, depth contours of basement and various sedimentary surfaces, paleoenvironment, magnetic anomaly contours, gravity-anomaly contours, earthquake epicenters, drill-hole locations, seismic-line navigation, and offshore leases. Amore » summary of the geologic, geophysical, and tectonic features compiled in this map series will be used to discuss the geologic evolution of the Baltimore Canyon Trough.« less

Depositional Sequences of Upper Continental Rise Seaward of Baltimore Canyon Trough: ABSTRACT

Depositional Sequences of Upper Continental Rise Seaward of Baltimore Canyon Trough: ABSTRACT

Attenuation changes due to diagenesis in marine sediments

Cenozoic marine sediments were drilled and cored by the Deep Sea Drilling Project at Site 613 in the Baltimore Canyon Trough, and four‐channel sonic waveforms were recorded from 115 to 585 m below sea floor. Compressional wave velocities and spectra were calculated from the sonic waveforms. In situ porosity was estimated and compared to core measurements. As expected, velocity increased with depth as porosity decreased. However, the energy loss between near and far receivers increased and the peak crosspower frequency decreased with depth, suggesting that attenuation increases over the same interval. The large changes in energy and frequency cannot be explained solely by changes in the elastic propeties of the sediments. We model the observed compressional wave spectra using measured values of velocity and density. Reasonable fits to the observed spectra are obtained using either a constant Q (CO) or nonconstant Q model. For the CO model, O decreases from 100 to 20 over this depth interval. Diagenesis obs...

Middle and Upper Jurassic Depositional Environments at Outer Shelf and Slope of Baltimore Canyon Trough

New CDP data acquired in the Baltimore Canyon Trough during project LASE made it possible to map a continuous Jurassic sedimentary sequence from the continental margin to the abyssal plain without interruption by basement structures. Intense carbonate sedimentation is inferred at the outer shelf during the Middle and Late Jurassic. Carbonate sedimentation probably started during the Middle Jurassic with a platform that prograded seaward with the development of ramps. By the Late Jurassic, a major reef complex had developed at the outer continental shelf. The onset of reef growth can be tentatively dated as 138 Ma by using the J1 reflector dated by the Deep Sea Drilling Project. A well-developed reef-talus deposit can be identified overlying the interface that generates th J1 reflector. A detailed analysis of semblance-derived interval velocities in the reef-talus sequence indicates a compressional velocity of 4.3-4.5 km/sec (14,100-14,800 ft/sec) for that interval, which was part of a major barrier reef along the United States eastern margin. After the reef formed, the deep oceanic basin was mostly starved from shelf-derived sediments until the reef died and was buried by clastic sediments. By correlation of our seismic data and COST well information, we infer that in the Baltimore Canyon Trough this reef had terminated by about the end of the Jurassic Period.

Geologic Evaluation of Leasing on Atlantic Outer Continental Shelf: ABSTRACT

To date, 9,160 tracts, totaling 51.7 million ac, have been offered for lease in the Atlantic. Of the 506 tracts receiving bids, 410--totaling 2.3 million ac--were leased for a total bonus of $2.8 billion. There have been four Mid-Atlantic sales, three South Atlantic sales, and one North Atlantic sale, with one reoffering sale in the Mid-and South Atlantic. Resource potential for proposed North Atlantic sales centers around the Georges Bank basin and the Upper Jurassic shelf edge (reef trend). Associated with the reef trend are a series of back-reef anticlines, faults, and pinch-outs. Cross sections indicate post-rift depocenters affected by block faulting and salt movement. Eight exploratory wells, all dry, and two continental offshore stratigraphic test (COST) wells have been drilled in the area. Thirty-two exploratory wells have been drilled in the Mid-Atlantic in the Baltimore Canyon Trough. Targets have included an intrusive dome, fault blocks, deep-seated diapirs, and the Jurassic shelf-edge reef trend. Two COST wells were drilled, one encountering a show of gas. Five of the exploratory wells tested hydrocarbons, the others were dry. The Carolina Trough appears to offer the best resource potential in the South Atlantic even though it has yet to be drilled. It has sufficient sediment thickness, a regional salt bed producing a number of diapirs on the seaward edge of the basin, and growth faults associated with the salt flow that may provide other traps. The Blake Plateau offers some stratigraphic trap possibilities and may contain thermally mature sediments. However, no wells have been drilled in the basin. The Southeast Georgia embayment consists of a thin sedimentary section of mainly Cretaceous continental clastics and Paleozoic metasedimentary basement rocks. Six exploratory wells, all dry, and one COST well have been drilled in this basin. End_of_Article - Last_Page 1435------------

DSDP Site 603: First deep (>1000-m) penetration of the continental rise along the passive margin of eastern North America

Drilling at Deep Sea Drilling Project Site 603 has provided the first deep (>1000-m) penetration of strata beneath the continental rise off the Atlantic margin of North America. Nearly continuously cored through 1585 m of section down to Berriasian pelagic limestones, the site 435 km (270 mi) east of Cape Hatteras intersected an extensive Lower Cretaceous deep-sea fan complex, which provides new information on the petroleum potential of the continental rise. Hauterivian to early Aptian in age, this 208-m interval of interbedded limestones, sand, and black shale turbidites begs the existence of any post-Valanginian reefs along the Baltimore Canyon Trough. Less extensive terrigenous turbidites were encountered higher in the section up to the Cretaceous/Tertiary boundary, which is marked by a current-laminated sand rich in dark spherules. Pelagic early Paleogene clays are disconformably overlain by Miocene pelagic mud. Turbiditic silts and clays began to accumulate rapidly at this site during the middle Miocene, leading to deposition of muddy contourites that formed the Lower Continental Rise Hills of the Hatteras Outer Ridge as sand turbidites were ponded concurrently on its landward side. The section at Site 603 confirms the concept that eustatic and other large-scale events subdivide Earth history into distinct chapters allowing the correlation of deep-sea seismic sequence boundaries with continental shelf and margin unconformities.

Great Stone Dome--Great Disappointment?: ABSTRACT

Baltimore Canyon Trough's Great Stone dome is one of the largest, most prominent subsurface structural features in the Atlantic offshore. The dome, with nearly 140,000 ac under closure and more than 11,000 ft of structural relief, was formed by the early Late Cretaceous intrusion of a large mafic igneous body. The uplifting resulted in a major Barremian unconformity across the dome along with minor higher unconformities. The dome was the site of intense bidding during 1976 when $651 million was bid for 37 tracts including $107.8 million for one crestal tract. Seven wells were subsequently drilled on the dome to total depths from 12,000 to 17,449 ft. The wells penetrated Cretaceous and Jurassic sandstones with porosities ranging up to 30%, but none encountered pay or significant shows. Lack of hydrocarbons in the section tested appears to be a result of low organic matter content of shales. Thermal maturation profiles show source rocks changing abruptly from immature to overmature across the Barremian unconformity. If hydrocarbons were generated (probably gas), lack of adequate seals at or above the unconformity prevented entrapment in crestal parts of the structure. The seven wells mainly tested structural traps on the dome. Additional prospects remain to be tested, such as possible flank fault traps and stratigraphic traps formed by sands pinching out around the dome's flanks where mature source rocks are likely to occur. End_of_Article - Last_Page 1433------------

New COCORP profiling in the southeastern United States. Part II: Brunswick and east coast magnetic anomalies, opening of the north-central Atlantic Ocean

New COCORP profiles on the coastal plain of Georgia and northern Florida support the hypothesis that the Brunswick anomaly marks a late Paleozoic suture. They do not support the alternate view that this anomaly is caused by a Mesozoic rift basin. The trend of the Brunswick anomaly relative to the Appalachian gravity gradient indicates that in westernmost Georgia and adjacent Alabama, African basement (Suwannee terrane) is in proximity to autochthonous North American basement (Grenville). Farther east one or more Paleozoic accreted terranes intervene between the North American and African sides of the orogen. Offshore, the Brunswick anomaly closely parallels the east coast magnetic anomaly. This relationship implies that the east coast magnetic anomaly marks not only the present continental/oceanic crustal transition but also the northward continuation of the late Paleozoic suture between North America and Africa. Transitional crust beneath the Carolina Trough, Baltimore Canyon trough, and correlative parts of the African Atlantic margin is thus likely to have formed within a preexisting suture zone. The dramatic change in character of the U.S. Atlantic margin southward from the Carolina Trough to the Blake Plateau probably reflects the fact that two different types of prerift crust are juxtaposed in this region.

Oil and Gas Developments in Atlantic Coastal Plain and Outer Continental Shelf in 1984

Exploratory drilling on the Atlantic outer continental shelf remained at about the same level as in 1983. Two wells were spudded in the Baltimore Canyon Trough. Three were completed and announced as dry holes, one in a world record water depth of 6,952 ft (2,119 m). Onshore in the Atlantic coastal plain, a dry hole was drilled in Colleton County, South Carolina, and another well, drilled in 1982 in Lee County, North Carolina, was plugged and abandoned as a dry hole. North Atlantic Lease Sale 82, which was to be held in 2 parts, was cancelled in late 1984. The International Court of Justice determined the United States-Canada boundary line in the North Atlantic. Seismic data acquisition decreased 41% below the 1983 level to 7,223 line-mi (11,625 line-km).

Mesozoic Biostratigraphic Framework of U.S. Atlantic Outer Continental Shelf: ABSTRACT

The geologic age of the first marine sediments deposited in the North Atlantic basin is of basic importance in reconstructing the basin's early geologic history. Thus, although the oldest dated marine sediments penetrated on the U.S. North Atlantic margin by DSDP wells are of Callovian age (Blake-Bahama basin), both the Georges Bank and Baltimore Canyon basins include much older Jurassic strata of Bajocian-Bathonian age, as well as an Upper Triassic section. Microplankton provide the basis for a Mesozoic biostratigraphic framework of three basins where drilling has occurred on the U.S. Atlantic outer continental shelf. Studies of closely sampled subsea sections from multiple wells drilled on the Georges Bank basin, the Baltimore Canyon Trough, and the Southeast Georgia em ayment, make dating possible by standard age and subage equivalents ranging from Middle Jurassic (Bajocian) to Maestrichtian (Late Cretaceous). This age sequence is documented by dinoflagellate range tops recorded in the offshore Canadian Grand Banks and Scotian Shelf, as well as by reference to ammonite-controlled type localities in northwestern Europe. The age of this sequence is supported from the Callovian to the top of the Maestrichtian by calcareous nannofossils. Additional biostratigraphic markers include a few benthonic and planktonic foraminifers and spores and pollen of age-guide value. The post-Middle Jurassic section in the Baltimore Canyon Trough averages over twice the thickness of the equivalent section in the Georges Bank basin, so that the base of the Middle Jurassic mar ne section has not been penetrated in the Mid-Atlantic. Georges Bank also has an Upper Triassic section. End_of_Article - Last_Page 1435------------

Shear wave velocity and attenuation in ocean bottom sediments from acoustic log waveforms

In boreholes where formation shear wave velocity is lower than borehole fluid velocity neither refracted shear waves nor pseudo‐Rayleigh waves can propagate. When frequency response of the sonde does not extend to low frequencies (e.g. 2 kHz) Stoneley waves are not excited. In such cases refracted P and leaking PL modes become dominant phases on a full waveform acoustic log. The P wave velocity can be determined from the first arrivals. Then, using synthetic seismograms and a waveform matching technique, formation shear wave velocity and attenuation can be estimated. This method is demonstrated using data from DSDP Site 612 in the Baltimore Canyon Trough area of the Atlantic margin.

Neogene stratigraphy of the submerged U.S. Atlantic margin

Thirty boreholes and several hundred sea-floor samples provide a detailed but incomplete record of Neogene strata and depositional environments along the submerged part of the United States Atlantic margin. Three major sedimentary basins, the Blake Plateau Basin, the Baltimore Canyon Trough, and the Georges Bank Basin, contain Neogene sedimentary prisms as thick as 1200 m, comprising bathyal marine to nonmarine strata. Middle Miocene rocks compose the most widely represented and thickest unit (> 600 m in the Baltimore Canyon Trough). Calcareous microfossils provide excellent biostratigraphic and paleoenvironmental indicators except in some sparsely fossilferous shallow-water intervals and where shallow-water diatomaceous assemblages predominate. However, in outer sublittoral assemblages of the Blake Plateau Basin and Baltimore Canyon Trough, the co-occurrence of abundant diatoms, radiolarians, and planktic foraminifera provide an excellent opportunity for intercalibration of biozones. Preliminary correlations between chronostratigraphic and paleoenvironmental interpretations indicate a close association of depositional sequences with sea-level fluctuations, but some local exception persist.

A paleoslope model of Campanian to lower Maestrichtian foraminifera in the North American basin and adjacent continental margin

Campanian to early Maestrichtian benthic foraminiferal assemblages of the New Jersey and Delaware coastal plain and the Baltimore Canyon Trough contain distinct bathymetric biofacies each occurring in adjacent settings to one another in a profile downdip toward the North American basin. The assemblages are interpreted as representing inner shelf to uppermost middle bathyal environments of deposition (10 to 500 m water depth). A unique biofacies which consists of predominantly arenaceous foraminifera in a lithology of dark lignitic shales is present in restricted intervals within the Baltimore Canyon region. This biofacies probably developed under oxygen deficient conditions in middle shelf to upper bathyal paleodepths. Within the North American basin, benthic foraminifers from Campanian to lower Maestrichtian sections at various DSDP sites indicate that deposition took place in middle bathyal to abyssal environments (500 m to more than 3000 m paleodepth). A complete paleoslope profile (shelf to abyssal) is identified within the North American basin and adjacent continental margin. The foraminiferal composition of this paleoslope is as follows: Gavelinella nelsoni, G. pinguis, Lenticulina pseudosecans, Cibicides harperi and Pullenia americana characterize shore-face to inner shelf regions of 10 to 50 m water depth. Species that show maximum development in middle shelf environments (50 to 100 m water depth) include Clavulina trilatera, Hoeglundina supracretacea and nodosariids. In the outer shelf, 100–200 m water depth, Praebulimina carseyae, Gavelinella compressa, G. spissocostata, Coryphostoma plaitum and Pullenia jarvisi predominate. The upper bathyal environment of 200–500 m water depth is characterized by species such as Arenobulimina subspherica, Dorothia oxycona, Verneuilina dickinsonensis, n. sp., Stensioina exculpta gracilis and Gyroidinoides globosus. Pullenia cretacea, Gavelinella ammonoides, G. whitei, G. intermedia, Praebulimina reussi, P. cushmani, Pleurostomella subnodosa and Stilostomella pseudoscripta are species common to the middle bathyal environment of 500 to 1500 m water depth. Species that characterize lower bathyal environments (1500 to 2500 m water depth) include Aragonia velascoensis, Reussella szajnochae, Osangularia utaturensis, Clavulina gabonica, Tritaxia aspera and Spiroplectammina dentata. Many of the middle and lower bathyal indicator species extend into the abyssal realm (2500 m water depth) where they are associated with more abundant species such as Aragonia ouezzanensis, Reussella pseudospinulosa, Hyperammina elongata, Rhizammina indivisa and Saccammina complanata. The resulting paleoslope model allows a more critical evaluation of the role played by benthic foraminiferal species in shelf, slope and abyssal environments. It also enhances the accuracy of estimating paleobathymetry especially in shelf regions of the Campanian to early Maestrichtian.

Resin rodlets in shale and coal (Lower Cretaceous), Baltimore Canyon Trough

Rodlets, occurring in shale and coal (uppermost Berriasian to middle Aptian, Lower Cretaceous), were identified from drill cuttings taken from depths between 9330 ft (2844 m) and 11, 460 ft (3493 m) in the Texaco et al., Federal Block 598, No. 2 well, in the Baltimore Canyon Trough. Under the binocular microscope, most of the rodlets appear black, but a few are reddish brown, or brownish and translucent on thin edges. They range in diameter from about 0.4 to 1.7 mm and are commonly flattened. The rodlets break with a conchoidal fracture, and some show an apparent cellular cast on their longitudinal surfaces. When polished and viewed in reflected light, the rodlets appear dark gray and have an average random reflectance of less than 0.1% whereas mean maximum reflectances are 0.48–0.55% for vitrinite in the associated shale and coal. These vitrinite reflectances indicate either subbituminous A or high-volatile C bituminous coal. The rodlets fluoresce dull gray yellow to dull yellow. The scanning electron microscope (SEM) and light microscope reveal the presence of swirl-like features in the rodlet interiors. Minerals associated with the rodlets occur as sand-size grains attached to the outer surface, as finely disseminated interior grains, and as fracture fillings. Electron microprobe and SEM-energy-dispersive X-ray (EDX) anlayses indicate that the minerals are dominantly clays (probably illite and chlorite) and iron disulfide; calcium carbonate, silicon dioxide, potassium aluminum silicate (feldspar), titanium dioxide, zinc sulfide, and iron sulfate minerals have been also identified. The rodlets were analyzed directly for C, H, N, O, and total S and are interpreted as true resins on the basis of C and H contents that range from 75.6 to 80.3 and from 7.4 to 8.7 wt. % (dry, ash-free basis), respectively. Elemental and infrared data support a composition similar to that of resinite from bituminous coal. Elements determined to be organically associated in the rodlets include S (0.2–0.5 wt.%), Cl (0.03–0.1 wt.%), and Si (0.05–0.08 wt.%). The ash content of the resin rodlets ranges from 4 to 24 wt.% and averages 12 wt.%. Total sulfur contents range from 1.7 to 3.6 wt.%. Resins of fossil plants are known to have little or no sulfur and ash; therefore, these data and the presence of minerals in fractures indicate that most of the sulfur and mineral matter were introduced into the resin partly or wholly after the time of brittle fracture of the resin. The probable source of the resin rodlets is fossil pinaceous conifer cones, which are known to have resin canals as much as 2400 μm in diameter.

Stratigraphic Framework and Petroleum Potential of Northeastern Baltimore Canyon Trough, Mid-Atlantic Outer Continental Shelf

Geologic data from 29 wells in the Baltimore Canyon Trough were released to the public as of 1982, and provide sufficient information for establishing Upper Jurassic through Cenozoic rock-stratigraphic units. Most of the units are lithologically similar to formations on the Scotian Shelf and are, therefore, considered in this study to be the Scotian Shelf homotaxial equivalents. The oldest rocks penetrated by exploratory wells in the Baltimore Canyon Trough are of Late Jurassic age and are correlative to the Scotian Shelf Mohawk, Mic Mac, and Abenaki Formations. The Mohawk(?) sandstone and Mic Mac shale equivalents in the Baltimore Canyon Trough represent lower delta plain to predominantly prodelta environments, and the Abenaki-equivalent limestone represents a shelf-margin carbonate buildup. Delta progradation continued during the Early Cretaceous, and lower delta-plain and delta-front sand and shale, equivalent to the Scotian Shelf Missisauga Formation, were deposited. A destructional phase of the delta is represented by the Naskapi equivalent, a calcareous shale. Inundation by the sea was incomplete, however, and channel and distributary-mouth-bar sands (Logan Canyon equivalent) were deposited. A final marine transgression began during the Late Cretaceous and resulted in the deposition of mudstone, which is considered equivalent to the Sco ian Shelf Dawson Canyon Formation. Marine conditions prevailed through much of the remainder of the Cretaceous and Cenozoic in both the Scotian Shelf and Baltimore Canyon Trough areas. Lithologic similarities between Cenozoic units of the Scotian Shelf and Baltimore Canyon Trough are less significant. Although both basins exhibit an upward increase in sand content, the Scotian Shelf Cenozoic strata are more sandy than those in the Baltimore Canyon Trough. In ascending order, the Baltimore Canyon Trough Cenozoic section consists of the uppermost Dawson Canyon equivalent, a chalk unit, a mudstone unit, an unconsolidated sand unit, and an uppermost clay unit. Stratigraphic traps may be present in the Baltimore Canyon Trough Upper Jurassic and Lower Cretaceous deltaic sandstones and shelf-margin carbonates. The deltaic units contain channel and distributary-mouth-bar sandstones, which may be potential reservoirs. Seals may be formed by lateral and vertical contacts against the prodelta, interdistributary bay, and marine shales. The Abenaki stratigraphic-reef trend provides another potential target. Source rocks, reservoir rocks, and traps may be associated with the trend, but its potential can only be defined by future drilling.

Exploration History and Future Prospects of United States Atlantic Margin: ABSTRACT

The Atlantic Margin of the United States comprises four large basins that are, from north to south, Georges Bank basin, Baltimore Canyon Trough, Carolina Trough, and Blake Plateau basin (which includes the Georgia Embayment and Bahama Platform). Most exploratory drilling has focused on structures in the postrift basin depocenters, with eight Georges Bank wildcats having been drilled into structures formed by block faulting and salt movement. Target zones were Upper and Middle Jurassic sandstones and carbonates. All were dry holes with some minor gas shows. Twenty-nine exploratory wells have been drilled into four types of structures in Baltimore Canyon Trough: an intrusive dome, deep-seated diapirs, fault blocks, and most recently a Lower Cretaceous-Upper Jurassic shelf-e ge reef. Some hydrocarbons were associated with a deep-seated diapir structure, but the other wells were dry. Wells in the Southeast Georgia Embayment penetrated Lower Cretaceous-Upper Jurassic continental clastics and shallow Paleozoic basement rocks. All wells were dry. No exploratory wells have been drilled in the Carolina Trough or Blake Plateau. Recent leasing in Georges Bank has been delayed because of litigation. In the Baltimore Canyon Trough, most recent leasing appears targeted at a prominent Lower Cretaceous-Upper Jurassic paleo-shelf edge, where it is hoped that synrift deposits rich in organic matter source porous shelf-edge carbonates and overlying clastics. Shell Oil, the largest leaseholder, is drilling a series of wells to test this structure. Between the Baltimore Canyon and Carolina Troughs, several leases have been taken over a large anticline possibly cored by salt. Another block of leases has been taken in the Carolina Trough immediately south of Cape Fear over a large grabenlike structure. Other, smaller groups of blocks appear to have lesser structures as targets. The Blake Plateau as yet has no active leas s. End_of_Article - Last_Page 1207------------

Upper Cretaceous and Cenozoic Sedimentary Sequence, Baltimore Canyon Trough, United States Atlantic Margin: ABSTRACT

The extent and character of Upper Cretaceous and Cenozoic rock units in the Baltimore Canyon Trough are revealed by geologic data from 29 exploratory wells. These data, released to the public in 1982, have been used previously in the description of regional rock-stratigraphic units in the trough. Now, a more detailed interpretation and correlation of electric logs and mud logs from these wells and a petrographic examination of thin sections have revealed the composition and lateral distribution of these units. Four detailed stratigraphic cross sections were constructed to show rock-unit correlations among 19 exploratory wells based on lithology and electric logs. Thin-section photomicrographs of drill cuttings and conventional cores document the mineralogic composition of these units. The Upper Cretaceous section is composed principally of gray to brown, calcareous mudstone and is 1,555-3,220 ft (475-980 m) thick. One major shaly, glauconitic quartzarenite within the mudstone is present at most of the well sites. An Eocene carbonate unit, 100-1,100 ft (30-335 m) thick, overlies the mudstone. This unit grades from fossiliferous wackestones to calcareous shale. Oligocene to Miocene calcareous mudstone overlies the Eocene carbonate. White to gray, calcareous clay, shale, and siltstone are common constituents, and limestone, dolomite, and sand are minor constituents. Miocene to Pliocene unconsolidated fine to very coarse-grained sand overlies the mudstone. An uppermost unit of predominantly gray clay is present in the eastern part of the basin. End_of_Article - Last_Page 1923------------

Upper Jurassic and Lower Cretaceous Sedimentary Sequence, Baltimore Canyon Trough, United States Atlantic Margin: ABSTRACT

The extent and character of Upper Jurassic and Lower Cretaceous rock units in the Baltimore Trough are revealed by geologic data from 29 exploratory wells. These data, released to the public in 1982, have been used previously to define regional rock-stratigraphic units. In this study, four detailed stratigraphic cross sections were constructed to show rock-unit correlations based on lithology and electric logs. Thin-section photo-micrographs document the mineralogic composition of these units. The stratigraphic terminology used herein is informally adopted from the nomenclature used on the Scotian Shelf. Most of the Upper Jurassic section consists primarily of Mic Mac gray shale and siltstone with minor amounts of very fine to medium-grained quartzarenite, red-brown shale, and lignite. This interval also End_Page 1923------------------------------ contains some anomalously thick sandstones and siltstones, which are generally limited to the north and have been tentatively assigned to the Mohawk(?) unit. These sandstones are mostly medium to coarse-grained, calcite-cemented quartzarenites. Upper Jurassic Abenaki limestone as much as 2,210 ft (675 m) thick was penetrated by most of the eastern wells. The limestone is mostly wackestone to grainstone, with varying amounts of oolites and fossils. Thick-bedded sandstones characterize the Lower Cretaceous Mississauga unit. These sandstones are mostly fine to medium-grained, calcite-cemented quartzarenites. The overlying Naskapi unit consists of calcareous shale. Thick sandstone beds dominate the uppermost Logan Canyon unit, which consists mostly of fine to coarse-grained, cal ite-cemented quartz arenite. End_of_Article - Last_Page 1924------------

Tectonic Development of Baltimore Canyon Trough: ABSTRACT

New well data and a new gravity model across the southern end of Baltimore Canyon Trough provide a more complete history of the basin's tectonic evolution and deep crustal structure than was previously known. The basin, which formed during the separation of North America from Africa, narrows and shallows along strike, as basement depth decreases from about 18 km (59,000 ft) in the north near New York to about 4-6 km (13,123-19,685 ft) in the south near Cape Hatteras. Previous analysis of the Continental Offshore Stratigraphic Test (COST) B2 and B3 wells using backstripping techniques showed a seaward increase in the amount of stretching during the basin's formation. The new biostratigraphic and paleoenvironmental interpretations are from the USGS Island Beach well 1 jus landward of the hinge zone in the basin. This well, along with the COST B2 and B3 data, provides a sampling of the sedimentary sections overlying continental, transitional (rift-stage), and oceanic crust. The subsidence histories derived from these data give a cross-sectional view of the basin's evolution. A gravity model of the southern end of the basin, along USGS multi-channel seismic line 28, primarily analyzes a 60-mgal shelfedge anomaly. This anomaly reflects the change in bathymetry and more important a change toward the continent in underlying crustal thickness from typical oceanic to thinned continental crust. The crustal thinning is compared to the broad thinning zone to the north. Well-defined rift structures on the landward edge of the basin are modeled as rift grabens near the hinge zone. End_of_Article - Last_Page 533------------