United States Atlantic Margin Research Articles

Molecular indicators of methane metabolisms at cold seeps along the United States Atlantic Margin

Anaerobic oxidation of methane (AOM) and the environmental conditions supporting AOM on continental margins is an essential component to global methane budgets. Diagnostic lipid biomarkers and their compound specific isotope analysis preserved in authigenic carbonates at cold seeps can serve as “fingerprints” to archaeal−bacterial consortia involved in AOM. However, despite the discovery of several hundreds of seeps along the United States Atlantic Margin (USAM), there are relatively few biomarker investigations of cold seep carbonates along this passive margin. A lipid biomarker, carbon isotope, and DNA marker gene study was therefore undertaken to determine the microbial origins of authigenic carbonates from two USAM seeps, Norfolk and the Baltimore Canyon seep fields. Results from this study capture a distinct archaeal lipid signature from putative methanotrophic archaea, including archaeol (I), sn-2-hydroxyarchaeol, 2,6,10,15,19-pentamethylicosane (PMI), and crocetane. The 13C-depleted AOM-related archaeal lipid samples (i.e., archaeol: −91.6‰, sn-2-hydroxyarchaeol: −129.2‰, PMI −92.8‰, and crocetane: −70.9‰) confirm the dominance of methane assimilation and isotope fractionation during AOM. These results are consistent with the detection of archaeal anaerobic methanotrophs (ANMEs) based on 16S rRNA gene sequencing. The Norfolk authigenic carbonate contained ANME-1a, -1b, 2a-2b, and 2c whereas only the ANME-2 clade was detected at Baltimore and present as the subclusters 2a-2b and -2b. The ANME-2d clade may also be present, particularly at the Baltimore seep site, given the high abundance of Candidatus Methanoperedens nitroreducens detected in the mcrA gene sequencing. The presence of terminally branched fatty acids, antesio- and iso-C15:0 components, as well as C16:1ω7 with δ13C values as low as −107.6‰, are indicative of sulfate-reducing bacteria (SRB) at the Norfolk seep site and supports syntrophy of SRB with methane-oxidizing archaea. In contrast, nitrate-driven AOM in syntrophy M. nitroreducens at the Baltimore seep site is consistent with elevated fatty acid δ13C values and lack of Deltaproteobacteria at the Baltimore seep site. Taken together, the range in lipid composition, distribution, and carbon isotopic composition observed at the Norfolk and Baltimore seep sites suggests AOM is performed by multiple archaea instead of a single species and may be paired with either or both nitrate- and sulfate-reduction. Given the heterogeneous nature of cold seep ecosystems, this study fills a critical spatial gap in our knowledge of AOM activity at two seep sites along a passive margin.

Evidence for extensive methane venting on the southeastern U.S. Atlantic margin

We present the first evidence for widespread seabed methane venting along the southeastern United States Atlantic margin beyond the well-known Blake Ridge diapir seep. Recent ship- and autonomous underwater vehicle (AUV)–collected data resolve multiple water-column anomalies (>1000 m height) and extensive new chemosynthetic seep communities at the Blake Ridge and Cape Fear diapirs. These results indicate that multiple, highly localized fluid conduits punctuate the areally extensive Blake Ridge gas hydrate province, and enable the delivery of significant amounts of methane to the water column. Thus, there appears to be an abundance of seabed fluid flux not previously ascribed to the Atlantic margin of the United States.

Trends in late Maastrichtian calcareous nannofossil distribution patterns, western North Atlantic margin

First and last occurrences of several Maastrichtian calcareous nannofossil species are shown to be diachronous across paleodepth and paleoenvironment using the graphic correlation method. Calcareous nannofossil assemblages examined from eleven cores from a deep- to shallow-water transect along the eastern United States Atlantic margin document that the first occurrence of Micula murus (Martini 1961) Bukry 1973 is diachronous, appearing 2.0 million years earlier in open ocean sites than in shallow marine sites. The first occurrence (FO) of Lithraphidites kennethii Perch-Nielsen 1984 is also nonsynchronous, appearing in the deep ocean before its FO in neritic waters. The last occurrence (LO) of L. praequadratus Roth 1978 is diachronous across paleodepth, going locally extinct first in deeper water. The LO of Watznaueria bybelliae Self-Trail 1999 is also diachronous, going locally extinct first in shallow-water settings. Ceratolithoides amplector Burnett 1997, C. pricei Burnett 1997, C. self-trailiae Burnett 1997, C. ultimus Burnett 1997, Cribrocorona gallica (Stradner 1963) Perch-Nielsen 1973, Micula praemurus (Bukry 1973) Stradner and Steinmetz 1984, Pseudomicula quadratus Perch-Nielsen et al. 1978, and Semihololithus spp. are present consistently in common to frequent abundances in ODP holes 1050C and 1052E on the Blake Nose, but they are rare or absent from neritic sections in Coastal Plain cores. It is apparent that these species flourished in an open ocean setting, suggesting that differences in assemblage abundance and diversity between deep ocean and nearshore areas were controlled by paleoceanographic factors. These species are not used for biostratigraphy, but may be useful indicators of open ocean conditions. The line of correlation (LOC) for nine Coastal Plain cores clearly defines the Cretaceous-Tertiary (K/T) boundary unconformity at the top of the Maastrichtian section (Peedee Formation) and the Campanian-Maastrichtian (C/M) unconformity at the base of the Maastrichtian section (Peedee/Donoho Creek formational contact). The K/T boundary unconformity is undulatory in nature; updip Maastrichtian sections have been stripped to a greater depth than the downdip sections. The uppermost Campanian, all of the lowermost Maastrichtian, and the basal upper Maastrichtian sediments are missing from the study area.

Trends in late Maastrichtian calcareous nannofossil distribution patterns, Western North Atlantic margin

First and last occurrences of several Maastrichtian calcareous nannofossil species are shown to be diachronous across paleodepth and paleoenvironment using the graphic correlation method. Calcareous nannofossil assemblages examined from eleven cores from a deep- to shallow-water transect along the eastern United States Atlantic margin document that the first occurrence of Micula murus (Martini 1961) Bukry 1973 is diachronous, appearing 2.0 million years earlier in open ocean sites than in shallow marine sites. The first occurrence (FO) of Lithraphidites kennethii Perch-Nielsen 1984 is also nonsynchronous, appearing in the deep ocean before its FO in neritic waters. The last occurrence (LO) of L. praequadratus Roth 1978 is diachronous across paleodepth, going locally extinct first in deeper water. The LO of Watznaueria bybelliae Self-Trail 1999 is also diachronous, going locally extinct first in shallow-water settings. Ceratolithoides amplector Burnett 1997, C. pricei Burnett 1997, C. self-trailiae Burnett 1997, C. ultimus Burnett 1997, Cribrocorona gallica (Stradner 1963) Perch-Nielsen 1973, Micula praemurus (Bukry 1973) Stradner and Steinmetz 1984, Pseudomicula quadratus Perch-Nielsen et al. 1978, and Semihololithus spp. are present consistently in common to frequent abundances in ODP holes 1050C and 1052E on the Blake Nose, but they are rare or absent from neritic sections in Coastal Plain cores. It is apparent that these species flourished in an open ocean setting, suggesting that differences in assemblage abundance and diversity between deep ocean and nearshore areas were controlled by paleoceanographic factors. These species are not used for biostratigraphy, but may be useful indicators of open ocean conditions. The line of correlation (LOC) for nine Coastal Plain cores clearly defines the Cretaceous-Tertiary (K/T) boundary unconformity at the top of the Maastrichtian section (Peedee Formation) and the Campanian-Maastrichtian (C/M) unconformity at the base of the Maastrichtian section (Peedee/Donoho Creek formational contact). The K/T boundary unconformity is undulatory in nature; updip Maastrichtian sections have been stripped to a greater depth than the downdip sections. The uppermost Campanian, all of the lowermost Maastrichtian, and the basal upper Maastrichtian sediments are missing from the study area.

Plate reorganization: a cause of rapid late Neogene subsidence and sedimentation around the North Atlantic?

Subsidence analysis of wells in the central North Sea and Labrador-Grand Banks and oft the West Greenland, Scotian shelf and United States Atlantic margin shows distinct quantitative stratigraphic correlation patterns of circum North Atlantic sites. A significant departure from the overall decrease in subsidence for the Pliocene occurs in many wells, when the rate is found to have increased one or more orders of magnitude from Oligocence/Miocene rates. Wells were selected along transects from shore to basin to find if relative basin position is influenced by differential basin subsidence. Although stratigraphic resolution is not detailed, more basinward sites experienced up to four times larger subsidence rates in the late Neogene than in the Oligocene/Miocene, with a peak in the Pliocene. Wells at the basin edge experienced much less subsidence or showed uplift. The observations are consistent with a rapid change of intraplate stress at the cause for this observed transition in Neogene subsidence. Major reorganizations of spreading direction and rate occurred during the Pliocene along the entire Atlantic spreading system, possibly in conjunction with more global changes in plate motions. We propose that the associated changes in intraplate stress caused the excess margin subsidence. Relative uplift along basin edges is consistent with this mechanism of relative movement and may explain apparent eustatic changes in sea level.

Marine Geology of Sohm Basin, Canadian Atlantic Margin

Multichannel seismic reflection, seismic refraction, and magnetic anomaly data are used to interpret the Mesozoic sea-floor spreading history and subsequent sedimentary evolution of the Sohm basin southeast of Nova Scotia. Observed M-series (Mesozoic) magnetic anomalies show that oceanic crust in the Sohm basin was generated at very slow rates of sea-floor spreading (5-15 mm/yr half rates). The slow spreading created very rough crustal topography, but the crustal thickness and seismic-velocity structure are typical of normal oceanic basement. Basement depths corrected for sediment loading fall below the predicted age/depth curve in the anomaly M25-M4 interval. These depth anomalies and an associated regional negative geoid anomaly suggest that the Sohm basin is underlai by asthenosphere denser and possibly colder than normal. Correlation of seismic reflection profiles in the basin to regional seismic stratigraphy, together with age constraints from reflection pinch-outs on dated oceanic crust, provides a chronology for interpreting the seismic sequences. The Jurassic to Lower Cretaceous sequences are similar in thickness and acoustic character to sequences observed along the United States Atlantic margin, suggesting that the New England Seamount Chain had little effect on sediment distribution patterns within the early western North Atlantic basin. Upper Cretaceous sediments in the Sohm basin are thin, probably because seaward transport of sediment was restricted by eustatic sea level rise and by a sedimentary ridge uplifted because of salt tectonics along the central and upper continental rise. Progradati nal sedimentary wedges and erosional unconformities characterize the Tertiary and Quaternary sedimentary record along the lower continental rise. The prograding sequences generally correlate to sea level lowstands. Repeated episodes of erosion by abyssal currents are marked by seismic unconformities Horizon Au (lower Oligocene), LM (lower Miocene), MM (middle Miocene), and L (upper Pliocene). The deep currents also controlled local patterns of deposition and modified cross-slope sediment transport and accumulation patterns. Thick supra-Horizon L sediments cover the Sohm basin and reflect rapid sediment influx caused by glacio-eustatic fluctuations in sea level during the Quaternary.

High-Resolution Seismic Data of Atlantic Margin Basement: ABSTRACT

Reflection seismic data for petroleum exploration have improved resolution of the rift structure of the United States Atlantic margin. Previously conjectural features are not clearly seen and additional features have been recognized. Our best data are from the Long Island-George's Bank area. Generally, the geologic elements visible, looking landward, are: (1) a belt of block-faulted oceanic crust that merges into a wedge of synrift sediments, (2) a basement ridge 10-15 km wide with maximum relief of 1.5 km, at a depth generally below 9 km, under the East Coast Magnetic Anomaly (ECMA), (3) a very wide zone of rising basement, structurally bland except for a major hinge near the updip edge of the ECMA, (4) an updip zone of complicated rift-stage faults with basinward-dippin listric faults, which visibly sole at ~14 km, and common listric counter-regional faults, and (5) a major unconformity, the base of the drift sequence, with rift-stage structures extensively pene-planed. Graben fill is 2-stage and includes earlier highly rotated beds separated by an unconformity from mildly deformed sediments. Downdip, the major unconformity at the base of the drift sequence becomes a strong, continuous reflector that downlaps the landward edge of oceanic crust. End_of_Article - Last_Page 271------------

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.

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------------

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------------

Chronostratigraphic Sequences Beneath Northeastern United States Continental Margin: ABSTRACT

Chronostratigraphic sequences of wide areal extent beneath the United States Atlantic margin north of Cape Hatteras have been delineated by: (1) examination of four COST (Continental Offshore Stratigraphic Test) wells, 1 exploratory well (Shell, Blk. 272, No. 1, OCS A 0096), 14 shallow AMCOR (Atlanctic Margin Coring) core holes, and eight ASP (Atlantic Slope Project) core holes; (2) analysis of multichannel seismic reflection profiles collected along several thousand kilometers of track lines; and (3) comparison of these interpretations to the Canadian offshore chronostratigraphy. In the Baltimore Canyon Trough and Georges Bank basin, seismically identified regional unconformities are associated with Lower Jurassic, Middle Jurassic, Upper Jurassic-Lower Cretaceous, upper auterivian, Cenomanian, Turonian-Coniacian, Upper Cretaceous-lower Paleocene, Oligocene, and upper Miocene-Pliocene rocks. The Tertiary unconformities are identified best in the Baltimore Canyon Trough where the thickness of Tertiary rocks exceeds 1,400 m. There, seismic profiles near the COST B-3 well reveal several probable unconformities of Miocene to Pliocene age that are not documented in the wells. Across the Georges Bank basin, subtle Jurassic unconformities appear to be present on seismic records within a thick, poorly dated section of interbedded limestone, dolomite, and anhydrite. Microfossil records from the deep wells and shallow core holes reveal seven unconformities of regional extent, forming chronostratigraphic gaps in the early to middle Cenomanian, late Turonian-early Coniacian, late Maestrichtian-early Paleocene, late Eocene-early Oligocene, early Miocene, late Miocene-early Pliocene, and late Pliocene-early Pleistocene. These gaps in the fossil record are comparable with the gaps found in wells in the Scotian basin, and correspond to unconformities inferred from the seismic profiles. The chronostratigraphic sequences bounded by these unconformities help to better define the curve of coastal onlap during the Cretaceous and to support the major trends in sea level change during the late Mesozoic and Cenozoic noted by Vail, Pitman, and others. End_of_Article - Last_Page 973------------

Correlation and zonation of miocene strata along the atlantic margin of North America using diatoms and silicoflagellates

Six Atlantic Miocene siliceous microfossil zones are proposed based on onshore and offshore samples from the United States Atlantic Margin. Diatoms and silicoflagellates are used to establish the zones. These zones are from oldest to youngest: 1. Zone I Actinoptychus heliopelta Concurrent Range Zone - Early Miocene 2. Zone II Delphineis ovata Partial Range Zone - late Early to early Middle Miocene 3. Zone III Delphineis ovata/Delphineis penelliptica Concurrent Range Zone - early Middle Miocene 4. Zone IV Delphineis penelliptica Partial Range Zone - Middle Miocene 5. Zone V Delphineis penelliptica/Coscinodiscus plicatus Concurrent Range Zone - Middle Miocene 6. Zone VI Coscinodiscus plicatus Partial Range Zone - Middle Miocene The six zones are easily traced along the Southern and Middle Atlantic Seaboard, but the older three are found for the most part between Cape Hatteras and New Jersey. There is some suggestion of sea-level change during Zone IV. Using rare planktonic diatoms that are index species from other regions and the zonal markers established in this study, correlation can be made with the Standard Foraminiferal Zones, the North Pacific Diatom Zones and with DSDP core 391A in the Blake-Bahama Basin.