Younger Sequence Research Articles

Silurian marginal marine sedimentation and the anatomy of the marine-Old Red Sandstone transition in Pembrokeshire, SW Wales

Abstract Deposition of the Silurian (Wenlock) siliciclastic Gray Sandstone Group of southwest Pembrokeshire took place within littoral environments close to the palaeogeographical shelf-margin of the Welsh Basin. Sedimentation described a northerly, basinward progradation across an earlier Late Ordovician? to Aeronian rift basin. Changes in relative sea level (rsl) had profound effects on depositional environments, and five depositional sequences are recognized. During highstands of rsl, the area was influenced by wave-dominated, shallow-marine conditions. During lowstands of rsl, shelf incision and sediment bypass occurred. Associated valley fills vary in nature from high-sinuosity estuarine channels, tidal flats and tidally-influenced, high-sinuosity fluvial channels. The last of these predominate within the youngest sequence, with abundant subaerial emergence indicators heralding the onset of true continental deposition, and the conformable transition into the overlying Old Red Sandstone Red Cliff Formation within the Marloes Peninsula. A renewal of tectonic activity ensued within the Lower Old Red Sandstone, with pebbly low-sinuosity alluvium of the Albion Sands Formation, and fanglomerates of the Lindsway Bay Formation refecting reactivation of earlier rift-margin faults, probably within a transtensional tectonic regime.

Factors controlling the Cenozoic sequence development in the eastern parts of the North Sea

The causal relationship between the Cenozoic sequence development in the southeastern North Sea Basin and sea-level changes, climatic fluctuations and tectonic events is unravelled by relating variations in the relative sea level and base level, based on interpretations of seismic surveys, to published δ 18 O variations and eustatic changes. The latter curve is based on the Earth's orbital forcing, and here informally termed as the GSI curve. The analysis shows that the Cenozoic sequence development in the southeastern North Sea was influenced by climatically and tectonically induced sea-level changes. The major Cenozoic sequence stratigraphic boundaries (lower order) are highly influenced by tectonic events, e.g. uplift of Fennoscandia and increased subsidence rates in the basin centre. Reactivation of Mesozoic fault zones controlled the deposition of minor sand bodies transported to the centre of the basin during the Late Palaeocene by mass flows. The location of an Oligocene mound structure, which constitutes part of a sequence, is controlled by the overall palaeotopography of the basin and local fault-related depressions. Correlation between (i) the ages of our sequences and the δ 18 O variations in the Oligocene succession, and (ii) the GSI curve and the base-level fluctuations of the late Miocene and younger sequences, show that the generation of the higher order sequence boundaries were driven by glacio-eustatic sea-level changes. A climatic control of the sequence formation due to glacio-eustatic sea level changes is therefore suggested for the Oligocene and Pliocene sequences, and probably also for the Upper Miocene sequences.

Extension et sédimentation au paléozoïque terminal et au Mésozoïque dans le bassin de Majunga (Nord-Ouest de Madagascar)

The Majunga Basin is located in the northwestern part of Madagascar with a N45–60°E trending axis. It was filled by almost exclusively continental Karoo Supergroup sediments, which are Permian to Early Jurassic in age, and by younger sequences, mainly marine, that were deposited from the Middle Jurassic to the present. The Karoo Basin geometry is deduced from the analysis of seismic sections. A central northeast trending horst is flanked by two sub-basins. Deposition of the Karoo sequences was controlled by these northeast trending faults. On the contrary, the Middle Jurassic to present sequences witness only a slight tilting of the basement towards the northwest. The development of the Majunga Basin includes, therefore, two successive stages. In the synrift episode, from Permian to Early Jurassic times, the sedimentation was syntectonic, controlled by synsedimentary faulting and the creation of a horst and graben extensive pattern. The postrift episode started during the Middle Jurassic. These two stages of the Majunga Basin development correspond to the geodynamic evolution recorded elsewhere in this part of the Gondwana.

An alternative stratigraphic scheme for the Sarawak Basin

The present stratigraphic scheme for the Tertiary of Sarawak (Ho, K.F., 1978. Stratigraphic framework for oil exploration in Sarawak. Bulletin of the Geological Society of Malaysia, 10, 1–14), which subdivides the entire sedimentary succession into eight sedimentary cycles, is based on the genetic sedimentary cycle concept. Each cycle starts with a transgressive basal part, followed by a regressive unit, which is in turn overlain by the basal transgressive unit of the next cycle. The limitation of the present scheme is, however, in its general applicability; for instance, when one tries to identify the equivalents of marine surfaces within non-marine sediments. This is critical for some areas in Sarawak where the sediments were deposited predominantly within lower coastal plain to upper coastal plain environments, normally barren of foraminifera. Among other inadequacies of the scheme is the lack of basin-wide association between the cycle boundaries and seismic reflectors. In practice cycle boundaries, picked on the composite well logs, often do not agree with the seismic pick. The scheme is, however, well accepted and continues to be used. The alternative stratigraphic scheme for the Sarawak Basin, which will be discussed in this paper, is based on sequence stratigraphic concepts, with the utilisation of unconformity or its correlative conformity as the stratigraphic boundary. This scheme has been generated as the result of a programme of basin-wide seismic mapping of the Sarawak Basin. Seven regional unconformities within the Tertiary sedimentary sequences have been mapped. The sedimentary units between the unconformities can be recognised as ‘Sequences’. The oldest unconformity is that between the basement (Belaga Formation) and the overlying Tertiary sediments, mainly of Late Oligocene age. Since the sediments in the basin are mainly of Tertiary age, the oldest unit of the succession is referred to as the Tertiary One Sequence (T1 S). The next younger sequence is called Tertiary Two Sequence (T2 S) and so on. The proposed scheme, which is based on a more conventional and multi-disciplinary approach, can be understood and appreciated, not only by palaeontologists, but by all members of exploration and production teams. The scheme should be able to facilitate prediction of stratigraphic levels, not only after the well has been drilled, but also prior to drilling.

Facies analysis and sequence stratigraphy of a Miocene warm-temperate carbonate ramp, Montagna della Maiella, Italy

Abstract The uppermost Oligocene/Lower Miocene to Upper Miocene ramp carbonates from Montagna della Maiella (Italy) form a supersequence bounded by deeply incised truncation surfaces. This supersequence is subdivided into four sequences. Each sequence is composed of skeletal limestones in its lower part and marly limestones in its upper part. The lower parts of the sequences are foramol limestones, which suggest deposition in the warm-temperate climate zone. Changes in climate, oceanography and relative sea level combined to control sedimentation in the four sequences. In the lower parts of the two older sequences, the skeletal sands built dunes, suggesting high-energy conditions. The dominant skeletal grains in the oldest sequence are larger foraminifers and in the next sequence they are bryozoans; this change reflects cooling around the time of the Aquitanian/Burdigalian boundary. In the lower parts of the two younger sequences, of Middle and Late Miocene age, sediment sheets with red-algal–bryozoan oncoids suggest deposition under calmer conditions. Transgressive and highstand systems tracts are recognized in all sequences; a shelf margin systems tract may be exposed in the second oldest sequence. In contrast to the situation that exists when warm-water carbonates are deposited, sedimentation of the foramol limestones on this isolated ramp was unable to balance accommodation during sea-level rise; this led to hemipelagic sedimentation during sea-level highstands. Conglomerates resulted from reworking along flooding surfaces.

The Southeast Greenland glaciated margin: 3D stratal architecture of shelf and deep sea

Abstract A seismic stratigraphic study of the Upper Neogene and Pleistocene shelf and deep-sea deposits of Southeast Greenland is presented. The depositional environment was controlled by glacial, turbidity current, and contour current processes. Below the mid- and outer shelf a Tertiary sedimentary wedge, as much as 1.7 km thick, is resting on a seaward-dipping volcanic acoustic basement. A lower marine part is separated by a regional unconformity, SB-A, from two seismic sequences of early Pliocene to Pleistocene prograding clinoforms. SB-A is surmised to represent a seaward-dipping palaeo-relief surface, accentuated by modest submarine erosion related to a sea-level lowering. The overlying two Sequences x and y reflect shelf-edge progradation in front of grounded ice during periods of maximum glaciation. The topset beds of the youngest sequence, possibly of Pleistocene age, are characterized by six or more sub-horizontal, landward-dipping internal unconformities, taken to reflect individual glacial cycles of grounded ice erosion. The present-day shelf is overdeepened, landward dipping, and traversed by troughs. These features are characteristic of a shelf sculptured by grounded ice. The position of the transverse troughs, reflecting the positions of former icestreams, seems to have been more or less permanent since the late Neogene. The shelf progradation has been higher in front of the troughs than in front of adjacent banks, resulting in the development of trough-mouth fans (TMF). The strata continue seawards below the shelf break across a crustal monoclinal flexure to the deep sea. Three seismic sequences have been identified within the late Miocene and Pleistocene deep-sea sedimentary succession on the continental rise. They are bounded by unconformities of mid-Pliocene, early Pliocene, and (?) late Miocene ages. The oldest sequence is interpreted as basin floor fans, probably linked to a sea-level lowstand. The two youngest sequences were deposited from interplay of contour and turbidity currents during glacial periods, and they form large sediment ridges on the sea floor, oriented almost perpendicular to the continental rise, nucleated on the older sequence. Inter-ridge channel-levee complexes recognized within the youngest sequence were deposited from turbidity currents. The major turbidity channels are prolongations of the transverse shelf trough-trough-mouth fan systems. A moat zone, at the base of the continental slope, reflects the position of strong modern contour currents. Appreciable contourite deposition did not commence before the early Pliocene, and was probably linked to glaciation of the shelf and onset or intensification of the Labrador Sea Water deep current.

Sequence Stratigraphy of Pliocene and Pleistocene Turbidite Systems, Northern Green Canyon and Ewing Bank (Offshore Louisiana), Northern Gulf of Mexico

Neogene turbidite systems are major reservoirs in the northern deep Gulf of Mexico. Few publications have described the stratigraphic variations or the three-dimensional geometries of these turbidite systems in detail; hence, an understanding of the stratigraphic characteristics of the producing sands is important for deep-water exploration in the Gulf of Mexico and similar basins worldwide. This study focuses on the northern Green Canyon and central Ewing Bank protraction (lease) areas, where the Pliocene-Pleistocene turbidite systems were mapped using an integrated exploration database. Interpretation of 10,000 km of two-dimensional seismic, 185 well logs, and biostratigraphy from 180 wells allowed us to define the regional sequence stratigraphic framework for this area and potential areas for future exploration. A complex Pliocene-Pleistocene geologic evolution of the area is indicated by the seismic and geologic facies, depositional rates, nature of turbidite systems, and sand content. Significant sand deposits (basin-floor fans) were deposited in most sequences and directly overlie sequence boundaries. Salt tectonics and faulting greatly influenced the loci of these fans’ deposition. Large, thick fans fill entire salt-withdrawal minibasins at the base of the Pliocene sequences. In the Pleistocene sequences, where the rate of salt withdrawal was less, smaller and thinner fans were deposited downdip of faults and adjacent to shallow salt bodies. Channel systems, interbedded with overbank shales, constitute most of the sediments in the sequences. The older sequences contain more channels and sandier channel fills than the younger sequences. Analysis of all sequences indicates a complex depositional history where significant sands were deposited where abrupt decreases in bathymetric gradient are associated with salt tectonics or faulting.

Seismic Stratigraphy and Geological Evolution of the Cenozoic, Cool-Water Eucla Platform, Great Australian Bight

Seismic images of the continental margin in the western Great Australian Bight reveal the internal anatomy of a long-lived Cenozoic cool-water carbonate shelf. The Cenozoic succession is divisible into seven seismic sequences that reflect four depositional phases. Phase A (Paleocene-middle Eocene) is a progradational siliciclastic wedge deposited in a depositional sag, and represents lowstand and transgressive sedimentation. Phase B (middle Eocene- earliest middle Miocene) contains cool-water ramp carbonates with biogenic mounds (Eocene- Oligocene), overlain by a warm-water, flat-topped platform rimmed by the early middle Miocene(?) Barrier Reef. Coeval deep-water carbonate deposition formed a multi-lobed sediment apron. This phase was terminated by gentle uplift and tilting throughout southern Australia in the late middle Miocene. Phase C (late Miocene-early Pliocene) represents cool-water lowstand wedge and ramp deposition, and contains numerous biogenic mounds in the youngest sequence. This phase is terminated by an unconformity attributed to marine erosion. Phase D (Pliocene-Quaternary) is a thick succession of cool-water carbonates with spectacular clinoform ramp geometry that forms most of the modern outer shelf, and contains large deep-water biogenic mounds. This platform, the first large cool-water carbonate shelf imaged by high-quality seismic reflection data, demonstrates the interaction between regional tectonic and local and global paleo-oceanographic processes. As Australia drifted northward during the Cenozoic, the Great Australian Bight moved from high to middle latitudes, and the regional oceanographic regime remained cool water largely because of coeval development of the Antarctic Circumpolar Current in the evolving Southern Ocean. Short episodes of warm-water deposition probably reflect incursions of a proto-Leeuwin current from the Indian Ocean, whereas growth of the Miocene coral-algal Little Barrier Reef resulted from a short-term global increase in sea-surface temperatures. This platform, dominated by stacked carbonate ramps, is most similar to the West Florida Shelf, but contains many more biogenic mounds. The western Great Australian Bight carbonate platform is an excellent modern analog for the mesoscale structure of cool-water platforms in the older geologic record.

Sequence Stratigraphy and Depositional History of the Eastern Mississippi Fan (Pleistocene), Northeastern Deep Gulf of Mexico

The eastern Mississippi Fan is a moderate-size, mud-dominated, Pleistocene submarine fan in the northeastern deep Gulf of Mexico. Analysis of 6900 km of multifold seismic data identified eight discrete depositional sequences interpreted to be coeval to the younger sequences in the western Mississippi Fan. All sequences consist of channel-levee deposits and slides. Channel-fill deposits are characterized by high-amplitude subparallel reflections, and levee-overbank deposits are characterized by interbedded subparallel to hummocky and mounded reflections. All sequences are affected by a series of volumetrically and areally large submarine slides, that are characterized by hummocky to chaotic reflections. Individual slides are up to 5000 km2 in area. The channel-levee systems within six of the sequences are derived from sediment sources located northwest in the Mississippi Canyon lease area. The channel-levee systems within the remaining two sequences are downfan continuations of systems in the western Mississippi Fan. These changing positions of the channels presumably reflect changes in the position of the shallow-marine depocenter that fed the fan throughout the Pleistocene. The eastern Mississippi Fan can serve as an exploration analog for mud-dominated turbidite systems with similar seismic facies and geometries. The fan has four potential reservoir facies: channel-fill sediments with sinuous to linear trends, thin-bedded sands in levee-overbank sediments, sheet sands deposited at the terminus of channels, and one possible basin-floor fan. Channel valley width/thickness ratio values range from 2.9 to 13/1 upfan to 2.9 to 8.8/1 downfan.

Sequence Stratigraphic Analysis of Plio-Pleistocene Turbidite Systems, Northern Green Canyon and Central Ewing Bank Areas, Northern Gulf of Mexico : ABSTRACT

ABSTRACT Plio-Pleistocene turbidite systems were mapped in the northern Green Canyon and central Ewing Bank protraction areas of the Gulf of Mexico. Ten depositional sequences were identified in the 5.5 Ma and younger intervals using 2560 km 2-D seismic profiles, biostratigraphic data from 34 wells and well logs from 39 wells. The Plio-Pleistocene geologic evolution of the area is divided into four geologic intervals. Each phase differs in its seismic and geologic facies, depositional rates, nature of turbidite systems, sand content, amount and type of slides and presence of erosional features. Three turbidite elements, basin-floor fans, channel-levee systems and overbank sediments, constitute most of the Plio-Pleistocene succession. Large unchannelized flows became volumetrically significant in the younger sequences. Slides represent only a small portion of the sequences. Basin-floor fans (sheet sands) were deposited overlying sequence boundaries. Salt tectonics greatly influenced the development and the location of these fans. These sheet sands are relatively thin (100-150 m), and consist of 15-50 m thick sand layers. The areal extent of the basin-floor fans varies between 20 and 210 km2. The size and thickness of the fans gradually decrease in the younger sequences. The basin-floor fans are reservoirs in some fields in the area, and they also have the highest reservoir potential. The channel-levee systems are composed of up to 25 m thick channel-fill sands, which may coalesce into 60-90 m thick intervals forming stacked channel-levee systems. The number and sand content of the channel systems gradually decrease upward through the interpreted sequences. Coalesced channel-fill sands form the reservoirs in several fields of the area.

The late Archaean and early Proterozoic: what happened next? (extended abstract)

One of the most interesting processes of the Archaean and early Proterozoic is the development of continental crust, from the generation of greenstone belts to the gradual amalgamation, rifting and regrouping of continental fragments. The geological history of Western Australia is an excellent illustration of this process. This abstract gives a brief and simplified overview of the main events. Western Australia consists of two Archaean blocks, the Pilbara and Yilgarn Cratons, which have completely different histories (Figure 1). The Pilbara Craton is discussed in detail by White et al., Nijman and Zegers et al. in this issue and consists of half a dozen oval to round granitoid batholiths surrounded by greenstone belts and small sedimentary basins. This apparently simple structure was built up during three main cycles of greenstone formation and sedimentation, each associated with intrusion of granitoid rocks within the batholiths (Figure 2). The relatively simple-looking round and oval shape of the batholiths is therefore misleading, since each batholith is composed of several intrusions of different age; their present shape is largely due to the development of shear zones between the surrounding rocks and the batholiths. Although younger sequences occur dominantly in the west of the Pilbara, and older ones in the east, there are no indications that the Pilbara is made up of terranes of different age. Cratonisation of the Pilbara took place around 2.77 Ga, followed by deposition of a platform sequence of the Mt. Bruce supergroup (Fortescue and Hamersley Basins) between 2.77 and 2.4 Ga (Figures 1, 2). The Yilgarn Craton has a completely different history. This craton has a structural ‘grain’ of elongate greenstone belts and granitoid batholiths trending NNW-SSE. Granitoid rocks occupy up to 70% of the present erosion surface. Contrary to the Pilbara, the Yilgarn Craton consists of up to a dozen crustal fragFigure 1. Schematic map of Western Australia showing the main tectonic units and basins, as discussed in the text and shown in Figure 2. E-Gf. – eastern Goldfield terrane, LG – Lake Grace terrane, Bo. – Boddington terrane, Ba. – Balingup terrane, Bi. – Biranup Complex, No. – Nornalup Complex. Area shown measures 1600 km from north to south.

Seismic stratigraphy of McMurdo Sound, Antarctica: implications for glacially influenced early Cenozoic eustatic change?

Analyses by Barrett et al. (1989) of the stratigraphic sequences of the CIROS-1 drill core provided evidence of late Paleogene and early Neogene glacial advances and retreats that appear to correlate with the global eustatic curve of Haq et al. (1987). During Leg 2 of the Polar Duke 90 (PD90) cruise in the Ross Sea approximately 650 km of highresolution seismic data were collected in McMurdo Sound with the main objective of facilitating regional correlation of stratigraphic events observed in the cores of the CIROS-1, DVDP-15, and MSSTS-1 drilling programs, and therefore providing an opportunity to test hypotheses on linkages between Cenozoic eustatic and Antarctic ice volume fluctuations. Twenty unconformity-bound, seismic-stratigraphic sequences (labeled from top A to T) were identified in the McMurdo Sound PD90 data base. However, seismic data from the shallow shelf on which CIROS-1 was drilled show erosional surfaces at many levels, indicating a condensed section in this area. It is difficult to distinguish individual lithological units identified by Barrett et al. (1989) in the seismic data, but seismic units O, P and Q correspond to the upper Oligocene interval where they recognized 3 glacioeustatic events. These three sequences, totaling around 300 m in thickness, can be traced over a distance of almost 100 km. However, they lack the seismic characteristics of glacial facies identified by Anderson and Bartek (1992) and thus the significance of waxing and waning events suggested for upper Oligocene strata in CIROS-1 remains equivocal. The younger sequences in the McMurdo Sound data base are largely Miocene (E-N), the oldest of these forming the upper 70 m in CIROS-1. These sequences can be traced 300 km north into the center of the Victoria Land basin, where they have seismic features characteristic of glacial facies. This suggests that ice sheets of continental scale (and hence large enough to affect eustasy) waxed and waned across the Ross Sea continental shelf at least 10 times during the Miocene. The data presented here for the Oligocene do not preclude similar ice sheet behavior, but are insufficient to test the hypothesis properly.

Stratigraphic Analysis of Eolian Interactions with Marine and Fluvial Deposits, Middle Jurassic Page Sandstone and Carmel Formation, Colorado Plateau, U.S.A.

ABSTRACT The eolian Page Sandstone (Middle Jurassic) in south-central Utah and adjacent Arizona consists of multiple mostly eolian and sequence-bounding unconformities (super surfaces). The super surfaces are a powerful correlation tool that provide the basis for a detailed regional stratigraphic analysis of the Page Sandstone and coeval parts of the marine and coastal-plain Carmel Formation. Some Page Sandstone super surfaces correlate with sharp lithologic breaks in the Carmel Formation that are interpreted as marine flooding surfaces. Others correlate with fluvial surfaces in the Carmel Formation. The lower part of the Page-Carmel interval encompasses seven onlapping accumulation bounded by marine flooding surfaces. The uppermost two represent the Middle Jurassic marine high-water mark. These seven units comprise marine transgressive and marine highstand system tracts. Each interval is composed of progradational eolian deposits over marine deposits. Major tectonic and sedimentologic changes affected accumulation 8 and 9. The two intervals are continental in origin, fluvial to the west and eolian to the east. A major slightly angular regional unconformity marks the top of accumulation 9. The unconformity reflects regional tilting, up to the northeast, and ensuing erosion of older units in the Page Sandstone. Accumulation 10, the top of the s udy interval, displays complexly juxtaposed continental and marine facies that buried the unconformity as regional subsidence continued. The broad patterns described above can be correlated with thick marine carbonate and minor siliciclastic deposits of the Twin Creek Limestone 400 km north of the study area. The resulting depositional patterns define three broad depositional events. The oldest of these sequences defines a regional marine transgression and highstand. The second sequence reflects strong siliciclastic influx and slight regional tilting that resulted in widespread continental deposition. Regional tilting and erosion closed out this event. The youngest sequence indicates a partial return to marine transgressive conditions but was complicated by regional uplift and tilting related to are tectonism. This study demonstrates that a stratigraphic framework can be applied to sparsely fossiliferous eolian, sabkha, and restricted marine deposits in a marine-coastal setting. Using detailed sedimentologic and stratigraphic studies, tectonic, eustatic, and possibly climatic signals can be differentiated within the complex, cyclic facies patterns displayed in the Page Sandstone and Carmel Formation.

Foraminiferal sequence biostratigraphy and seismic stratigraphy of a tectonically active margin; the Yakataga Formation, northeastern Gulf of Alaska

Upper Miocene through Recent glacial-marine deposits of the Yakataga Formation, which underlie the northern Gulf of Alaska continental shelf, are the focus of an integrated study to understand the depositional and climatic history of the upper Cenozoic Gulf of Alaska. Biostratigraphic, lithologie, and multichannel seismic data were integrated in a sequence stratigraphic analysis of the Yakataga continental margin. A paleontologically based approach to sequence stratigraphy, known as sequence biostratigraphy, was used to define eight seismic stratigraphic sequences on this tectonically active, glaciated continental margin. Foraminifera provide information on the age, paleoenvironments, and paleoclimates of strata within these eight sequences. Lithologic evidence is used to infer the presence and influence of tidewater glaciers.The three oldest sequences, arbitrarily labeled Brown, Red, and Orange (early Pliocene, mid-Pliocene, late Pliocene, respectively), were deposited in mainly outer neritic through upper bathyal environments. Planktic foraminiferal assemblages show that sea surface temperatures during deposition of these sequences were mainly cool temperate in the Gulf of Alaska at this time, yet benthic assemblages indicate cold bottom waters and dropstones indicate glacial-marine sedimentation during the late Pliocene.The five youngest sequences (labeled Yellow, Green, Aqua, Blue, Violet), which are Pleistocene to Recent in age, were deposited in deeper environments on average (upper bathyal to middle bathyal) than the older sequences. Glacial-marine sedimentation was common during deposition of these sequences (abundant dropstones) and foraminiferal assemblages indicate both cold sea surface and bottom water temperatures.

Chronology of Avalonian events on Presqu'île du Cap Miquelon (Le Cap), Saint-Pierre et Miquelon (France)

U/Pb geochronology and field studies on Presqu'île du Cap Miquelon demonstrate that amphibolite-grade metamorphism, ductile deformation, and subsequent plutonism in this part of the Appalachian Avalon Zone are of Proterozoic age. Clear, crosscutting relationships between a dated granite dyke (part of the Cap Blanc granite) and foliated, amphibolite-grade metasedimentary and metaigneous rocks provide a younger age limit of [Formula: see text] Ma for tectonism. Proterozoic titanite and hornblende ages indicate that no significant younger thermal overprint occurred such as those recorded in nearby southern Newfoundland. Deformation and metamorphism of the country rocks and intrusion of the Cap Blanc granite are at least 30 Ma older than the onset of deposition of the youngest Proterozoic volcano-sedimentary successions in adjacent insular Newfoundland. Possible equivalents of these younger sequences occur near Cap Blanc, and contain what may be interpreted as clasts of granite, dated at 620 ± 3 Ma. The Miquelon metamorphic sequence is unique with respect to the southern Newfoundland Appalachians in representing a significant area of Proterozoic migmatite, not observably affected by tectonothermal events after 580 Ma.

Sequence Stratigraphy of Pleistocene Sediments, Northwestern Green Canyon, Gulf of Mexico: ABSTRACT

ABSTRACT A sequence stratigraphic study of Pleistocene sediments in the northwestern portion of the Green Canyon Area has identified five depositional sequences: 1.1-0.8, 0.8-0.7, 0.7-0.5, 0.5-0.4, and 0.4-0.3 Ma. Detailed seismic and geologic facies maps were constructed for each of these sequences. The data base for this study comprises 1420 km (882 miles) of multifold seismic, petrophysical information from 20 wells and 23 biostratigraphic reports. The area is characterized by several large mini-basins, separated by complex salt bodies and minor faults. The depositional settings are structurally controlled by these salt bodies and normal faults. Fields/discoveries in the area are in Green Canyon 18, and in the Mahogany sub-salt discovery (Ship Shoal 349). Paleoecology indicates that the sequences were deposited in water depths ranging from lower slope to middle shelf. Bathyal turbidite systems represent most of sediments. Prominent basin-floor fan deposits are present only in sequences 1.1-0.8 Ma and 0.8-0.7 Ma in the southeastern portion of the area. All of the sequences consist primarily of channel-levee systems and overbank deposits (slope fan facies). Abrupt facies changes within these deposits are common. Well logs indicate that most of the area is mud-dominated, with sands primarily occurring in channel-fill facies. Extensive slides are also present in these slope fan facies. In the northern part of the area, the younger sequences (0.7 Ma and younger) also contain sediments deposited in prodelta/delta environments (prograding complex/HST). Relatively thick TST are present and stacked along the downthrown sides of faults.

Seismic stratigraphy of Lake Huron – Georgian Bay and postglacial lake level history

Five seismic reflectors in the basinal sedimentary section have been identified as representing erosional surfaces in shoaler regions of the lake basins and can be traced throughout most of the study area. The reflectors representing these erosional surfaces, and their conformable equivalents in the deep lake basins, are caused by intervals of coarser grain size in the sediments and are thought to represent lowering of lake levels prior to, and following, the Main Algonquin highstand (about 10.2 – 11.0 ka). These reflectors are used to define seismic sequences in the postglacial sedimentary fill of the basins. Two additional erosional surfaces, which closely follow the initial fall of the Main Algonquin lake level, are most clearly seen in thicker sections found in the northern part of the basins. The two oldest (pre-Main Algonquin) sequences appear to onlap highs and fill in lows within the basins, whereas the younger sequences (associated with the Main Algonquin through Mattawa times) tend to drape the topography and show little difference in thickness between the lows and their adjacent highs. The most recent sedimentary sequence represents deposition occurring since about 7.5 ka. It laps out against topographic highs and leaves older sequences exposed at the lake floor in many areas. The most pronounced erosional surfaces are associated with the Light Green and the Light Blue sequence boundaries. The Light Green sequence has a pair of coarser layers at its top, which are dated at 9.6 – 9.8 ka and 9.05 – 9.2 ka. The coarser interval at the top of the Light Blue sequence is dated at 7.45 – 7.80 ka. These layers are thought to mark maximum lake level lowstands and to represent the times of Ottawa–Marquette to latest Mattawa lake level fluctuation.

Sequence Stratigraphy of Late Pliocene and Pleistocene Sediments of Northwestern Green Canyon Area/Western Ewing Bank, Northern Gulf of Mexico: ABSTRACT

Northwestern Green Canyon and Western Ewing Bank lease areas are characterized by complex faulting and salt deformation affecting the late Pliocene-Pleistocene sediments. The sequence stratigraphy has been studied using 1300 km of multifold seismic data, and 40 wells with biostratigraphy data (12 with high-resolution analysis). Fossil abundance and diversity curves were used to recognize condensed sections. Eight depositional sequences have been recognized (2.4, 1.9, 1.4, 0.8, 0.7, 0.5, and 0.4 Ma). Maximum thickness of these sediments is 6 km. Paleobathymetry indicates that sequences were deposited primarily in bathyal water depths. Most of the sediments are in the lowstand systems tracts and consist of basin-floor fans, slope fans, and prograding complexes. Thick blocky sand packages (basin-floor fan) are present in two major sequences (1.4 and 1.1 Ma) and represent potential reservoirs in the area. Transgressive and highstand systems tracts are fairly thin across the area and are thicker only in the younger sequences (< 0.5 Ma). The syndepositional structures play an important role in controlling the geometry and distribution of the depositional units, as well as creating structural highs for petroleum entrapment. Existing discoveries in the area include Green Canyon Blocks 6, 52, 184, and 228, and are associated primarily withmore » amplitude anomalies on the flanks of salt structures and/or faults.« less

Seismic Facies and Variations in Channel Evolution, Eastern Mississippi Fan (Pleistocene), Northeastern Deep Gulf of Mexico: ABSTRACT

ABSTRACT The Eastern Mississippi Fan is a moderate size, Pleistocene submarine fan in the deep northeastern Gulf of Mexico. Analysis of 4,290 mi (6,900 km) of multifold seismic data identified nine discrete depositional sequences coeval to sequences 7-11 and 13-16 previously described in the Western Mississippi Fan. Each of the eight youngest sequences consists of a channel-levee system; five of the channel-levee systems are derived from submarine canyons located in the Viosca Knoll and Mississippi Canyon lease areas. The three remaining channel-levee systems are downfan continuations of systems previously described in the Western Mississippi Fan. Typical evolution of a channel-levee system consists of an erosional surface at the base, overlain by hummocky to mounded reflections (interpreted as disorganized slides and debris flows), which in turn are overlain by aggradational channel-levee deposition. Each channel-levee system includes one to several channel valleys. Their physical characteristics are described using four parameters: aggradation, lateral migration, sinuosity, and bifurcation. Values for these parameters differ both between and within individual channel-levee systems. Maximum aggradation values for channel valleys range from 980 to 2,125 ft (300 to 650 m), whereas lateral migration values for channel valleys range from 0 to 2.0 mi (0 to 3.0 km). Channel valley sinuosities range from low to high, and the number of downfan channel bifurcations ranges from 0 to 3. A single channel-levee system, sequence 9, is described in detail to illustrate the variabilities in channel-levee evolution.

Contourites in the Fram Strait

Multichannel seismic reflection profiles from the Fram Strait show many features that can be attributed to bottom-current influence on the sediments, by comparison with well-known contourite sequences elsewhere. A greater than 2 km thick contourite drift with a smooth internal reflection pattern is observed northwest of Svalbard. The currents here have probably been controlled by local topography. We interpret sequences with abundant diffractions in the broader basins to be current-related, probably deposited during periods of intensified circulation. The continental slopes on the Svalbard side have fairly continuous sequences, whereas the younger sequences on the Greenland lower slope onlaps the lowermost one. This can be attributed to differences in current regime, sediment supply, or margin subsidence. The contourites of the lower slope in the northeastern part of the strait (about 1000 m water depth) can be explained by long-term influence of the West Spitsbergen Current, but current-influenced patterns down to 3000 m water depth are not compatible with the weak deep-water circulation observed at present. The oldest contourites west of Svalbard are estimated to be of Late Miocene to Pliocene age (between 10 and 3 Ma), based on correlation to the DSDP Site 344 and inferred basement ages near the present plate boundary. Hiatuses may be present beneath the contourites. The onset of currents may be related to opening of the gateway between Greenland and Svalbard, to climatic cooling, and to palaeogeographic changes outside the Fram Strait. Contourite deposition has continued through the late Neogene and Quaternary, but vertical changes in the pattern suggest that current intensities have decreased through time.