Regional Seismic Stratigraphy Research Articles

A record of Eocene (Stronsay Group) sedimentation in BGS borehole 99/3, offshore NW Britain: Implications for early post-break-up development of the Faroe–Shetland Basin

Synopsis A punctuated Eocene succession has been recovered in British Geological Survey borehole 99/3 from the Faroe–Shetland Basin. The borehole was drilled close to the crest of the Judd Anticline and penetrated 110.5 m into the post-break-up Stronsay Group. The borehole proved 23.8 m of Ypresian–earliest Lutetian paralic to shallow-marine deposits, unconformably overlain by 43.85 m of mid-Lutetian lower shoreface to shallow-marine shelf deposits, in turn unconformably overlain by 42.85 m of late Bartonian–Priabonian mass-flow (slope apron) and basinal deposits. At the borehole site, the Ypresian–earliest Lutetian sediments were subaerially exposed in early Lutetian times; these deposits were subsequently transgressed by the mid-Lutetian shelf – itself eroded in the late Lutetian with reworked material transported northwards and re-deposited in the Mid-Eocene basin-floor fan complex in Quadrants 213 and 214. The eroded mid-Lutetian shelf was subsequently downlapped and buried beneath prograding late Bartonian–Priabonian slope apron-to-basinal sediments as the borehole site subsided. Integrating the borehole data with the regional seismic-stratigraphy indicates three major intervals of regression – early/mid-Lutetian, late Lutetian and late Bartonian – which, it is suggested, coincide predominantly with tectonic activity, particularly compressional deformation, possibly linking the early post-break-up development of the Faroe–Shetland Basin to the evolution of the adjacent Norwegian Basin.

Switching of a paleo-ice stream in northwest Svalbard

Ice streams are the fast-flowing zones of ice sheets that can discharge a large flux of ice. The glaciated western Svalbard margin consists of several cross-shelf troughs which are the former ice stream drainage pathways during the Pliocene–Pleistocene glaciations. From an integrated analysis of high-resolution multibeam swath-bathymetric data and several high-resolution two-dimensional reflection seismic profiles across the western and northwestern Svalbard margin we infer the ice stream flow directions and the deposition centres of glacial debris that the ice streams deposited on the outer margin. Our results show that the northwestern margin of Svalbard experienced a switching of a major ice stream. Based on correlation with the regional seismic stratigraphy as well as the results from ODP 911 on Yermak Plateau and ODP 986 farther south on the western margin of Spitsbergen, off Van Mijenfjord, we find that first a northwestward flowing ice stream developed during initial northern hemispheric cooling (starting ∼2.8–2.6 Ma). A switch in ice stream flow direction to the present-day Kongsfjorden cross-shelf trough took place during a glaciation at ∼1.5 Ma or probably later during an intensive major glaciation phase known as the ‘Mid-Pleistocene Revolution’ starting at ∼1.0 Ma. The seismic and bathymetric data suggest that the switch was abrupt rather than gradual and we attribute it to the reaching of a tipping point when growth of the Svalbard ice sheet had reached a critical thickness and the ice sheet could overcome a topographic barrier.

Cyclic sediment waves on western slope of the Caspian Sea as possible indicators of main transgressive/regressive events

New high-resolution seismic reflection profiles collected on the western slope of the Caspian Sea during cruises of RV “Rift” in 2004, 2006 and 2007 allowed the identification of regional sedimentary processes formed its present morphologic and structural image. The upper, gentle part of the slope, from the shelf edge to 400 m water depth, was formed in the Pleistocene–Holocene. Its architecture is dominated by very prominent wavy features, sedimentary wave bedforms, which were formed mostly by density gravitational (turbidity) streams downslope during lowstands of the Caspian Sea, i.e. during the regressive intervals, and affected by bottom currents partly. This specific sedimentary process was slowed during transgressive phases. Downsection, several distinct generations of the sediment waves are distinguished within the upper sedimentary sequence. Sampling by gravity corer characterized the uppermost undulation only. Therefore, several deeper generations of the sediment waves distinguished within the upper sedimentary sequence were compared with the available drilling results from boreholes in the shallow-water northern Caspian Sea shelf, by means of extrapolation and correlation of horizons along the available seismic profiles. Three strong wavy reflectors on the western slope were identified on the shelf as unconformities, marking the Atelian, Enotaevian, and Mangyshlakan regressions. Lacking submarine boreholes, a rough estimation for general regional seismic-stratigraphy of the upper sedimentary section on the western slope of the Caspian Sea is proposed from the stratigraphic identification of the observed sediment waves.

Regional seismic stratigraphy and controls on the Quaternary evolution of the Cape Hatteras region of the Atlantic passive margin, USA

Seismic and core data, combined with amino acid racemization and strontium-isotope age data, enable the definition of the Quaternary stratigraphic framework and recognition of geologic controls on the development of the modern coastal system of North Carolina, U.S.A. Seven regionally continuous high amplitude reflections are defined which bound six seismic stratigraphic units consisting of multiple regionally discontinuous depositional sequences and parasequence sets, and enable an understanding of the evolution of this margin. Data reveal the progressive eastward progradation and aggradation of the Quaternary shelf. The early Pleistocene inner shelf occurs at a depth of ca. 20–40 m beneath the western part of the modern estuarine system (Pamlico Sound). A mid- to outer shelf lowstand terrace (also early Pleistocene) with shelf sand ridge deposits comprising parasequence sets within a transgressive systems tract, occurs at a deeper level (ca. 45–70 m) beneath the modern barrier island system (the Outer Banks) and northern Pamlico Sound. Seismic and foraminiferal paleoenvironmental data from cores indicate the occurrence of lowstand strandplain shoreline deposits on the early to middle Pleistocene shelf. Middle to late Pleistocene deposits occur above a prominent unconformity and marine flooding surface that truncates underlying units, and contain numerous filled fluvial valleys that are incised into the early and middle Pleistocene deposits. The stratigraphic framework suggests margin progradation and aggradation modified by an increase in the magnitude of sea-level fluctuations during the middle to late Pleistocene, expressed as falling stage, lowstand, transgressive and highstand systems tracts. Thick stratigraphic sequences occur within the middle Pleistocene section, suggesting the occurrence of high capacity fluvial point sources debouching into the area from the west and north. Furthermore, the antecedent topography plays a significant role in the evolution of the geomorphology and stratigraphy of this marginal system.

Jurassic tectono-sedimentary evolution of the Northern Lusitanian Basin (offshore Portugal)

The Lusitanian Basin is one of a series of rift-related basins developed during the rifting that led to the break-up of Iberia and Canada. The strong influence of halokinesis on the depositional evolution of its northern sector provides an analogue for the development of other evaporite-prone rift basins on Atlantic-type passive margins. In this paper, the pre-Valanginian evolution of the Northern Lusitanian Basin is documented based on regional seismic stratigraphy, isochron and time-structure maps tied with sonic and stratigraphic data from 10 exploration wells. The pre-salt structure of the study area is marked by two sets of faults: (i) a N–S to NE–SW striking set active throughout the Jurassic period and (ii) a NW–SE striking set active during the Late Jurassic. The latter defines five Late Jurassic sub-basins northeast of a basin-margin structure, the Marinha-Grande Fault. The development of half-graben basins in the Northern Lusitanian Basin during the Jurassic was overprinted by halokinesis over the Marinha-Grande Fault and other major marginal basement faults. Halokinesis climaxed during the Oxfordian–Kimmeridgian rifting with the formation of a 10 km long salt ridge over the Marinha-Grande Fault. The ridge limited the southward progradation of fluvial/deltaic units derived from hanging-wall drainage systems and restricted the syn-rift sediments to a 15 km long×5 km wide bowl-shaped depocentre located 10 km west of the present coastline. In order to better characterise the Jurassic tectono-sedimentary evolution of the Northern Lusitanian Basin, the interpreted seismic data is compared with physical models for extensional forced folds above active normal faults and with stratigraphic information from well and outcrop.

Aspects of sedimentary basin evolution assessed through tectonic subsidence analysis. Example: northern Gulf of Thailand

Tectonic subsidence and subsidence rate analyses were conducted using a forward burial technique for the Cenozoic sediments of the northern Gulf of Thailand, a region presently bounded and intersected by major strike-slip fault systems. Basins represented by the seven wells studied are the Thon Buri, Hua Hin, Chumphon, Kra, and Pattani basins. The total observed subsidence was stratigraphically calibrated using well biostratigraphy and/or regional seismic stratigraphy. Tectonic subsidence was subsequently determined assuming local Airy isostasy by correcting decompacted sediments for sediment loading and variations in paleowater depths. Statistical comparison of the observed tectonic subsidence profile versus the theoretical thermal subsidence profile reveals zero-intercept times of incipient thermal-rifting and furthermore helps differentiate times of thermal subsidence from episodes of fault-controlled mechanical subsidence. Differences in tectonic subsidence, tectonic subsidence rates, and in the zero-intercept times of thermal rifting imply the Paleogene thermal associated rifting of the northern Gulf of Thailand was neither restricted spatially nor universally synchronous among the basins, but instead both spatially and time transgressive. Although coupled thermal-mechanical subsidence played a major role in the evolution for most of these basins, in some basins, e.g. the Thon Buri and northern Kra basins, subsidence was not thermally initiated. Instead, these basins experienced relatively slow-paced sediment loaded subsidence until a sudden fault-associated acceleration commenced in the Pliocene. Additional evidence for temporal and spatial changes in local strain is demonstrated by observed asynchronous episodes of “see-saw” subsidence-uplift of the basin floors. For example, while the northern Hua Hin Basin experienced Miocene-Pliocene alternations of subsidence and uplift, portions of the Pattani Basin to the southeast underwent periods of subsidence and uplift which differed in magnitude and phase. In comparison to the tectonic subsidence rates compiled for basin classes worldwide, these Gulf of Thailand basins exhibit time-transient characteristics of cratonic, rift, and wrench style basins. The opening of the northern Gulf of Thailand, basin formation, and subsequent basin evolution have varied both in time and space. Therefore, in terms of hydrocarbon potential, it would be both inappropriate and inaccurate to suggest the thermal histories and subsequent hydrocarbon maturities of these wrench-associated basins to be equivalent. Though tectonically related, each basin has had its own geohistory.

Undercrusting by serpentinite beneath rifted margins

THE west Galicia passive margin (Spain) is bounded by a continuous belt of serpentinized peridotite. Here we present the results of petrological studies of the peridotite, 39Ar–40Ar dating of high-temperature ductile shearing of a dioritic dyke crossing the peridotite and regional seismic stratigraphy which demonstrate that the uplift, ductile shearing and pervasive serpentinization of mantle rocks occurred at early Cretaceous time, during the rifting stage of the margin. The serpentinite layer resting on fresh peridotite is probably several kilometres thick. At present seismic data and gravity data cannot distinguish serpentinite from crustal material, but our results from the Galicia margin indicate that the seismic crust beneath rifted margin may include mantle material transformed into serpentinite by syn-rift hydrothermal activity. In some cases, this undercrusting process might explain the young age and mobility of the Moho beneath stretched continental basement.

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.