Finn Surlyk

Terrestrial and marine extinction at the Triassic-Jurassic boundary synchronized with major carbon-cycle perturbation: A link to initiation of massive volcanism?

Mass extinction at the Triassic-Jurassic (Tr-J) boundary occurred about the same time (200 Ma) as one of the largest volcanic eruptive events known, that which characterized the Central Atlantic magmatic province. Organic carbon isotope data from the UK and Greenland demonstrate that changes in flora and fauna from terrestrial and marine environments occurred synchronously with a light carbon isotope excursion, and that this happened earlier than the Tr-J boundary marked by ammonites in the UK. The results also point toward synchronicity between extinctions and eruption of the first Central Atlantic magmatic province lavas, suggesting a causal link between loss of taxa and the very earliest eruptive phases. The initial isotopic excursion potentially provides a widely correlatable marker for the base of the Jurassic. A temporary return to heavier values followed, but relatively light carbon dominated the shallow oceanic and atmospheric reservoirs for at least 600 k.y.

Sea-level change and facies development across potential Triassic-Jurassic boundary horizons, SW Britain

The Late Triassic to Early Jurassic aged succession of SW Britain (the Penarth and lower Lias Groups) comprises mudstone, sandstone and limestone strata deposited in a variety of marine to non-marine environments. Faunal and floral characteristics of these successions have led to the proposal that one location in SW England, St Audries Bay, should serve as the Global Stratotype Section and Point (GSSP) for the base of the Hettangian Stage and, thus, for the Triassic–Jurassic (Tr–J) boundary. The sections of SW Britain have also been used previously to infer sea-level change history and relate this to potential kill mechanisms associated with the Tr–J boundary mass extinction. Chemostratigraphic, biofacies and lithofacies data are used here to suggest alternative models of sea-level change in relation to possible Tr–J boundary horizons in the sections of SW Britain. A sea-level lowstand surface of erosion is inferred to occur within the Cotham Member of the Lilstock Formation, a unit deposited in an environment that was often subaerially exposed. In contrast to previous interpretations, the top surface of the overlying Langport Member (here inferred to be deposited on a carbonate ramp of depositional or tectonic origin) represents a drowning event of at least regional extent. All horizons regarded as plausible levels at which to place the Tr–J boundary based on fossil distributions lie within strata deposited during relative sea-level rise. However, it is doubtful whether the higher horizons proposed to mark the boundary faithfully record times of true biotic change on a global scale and, additionally, there is no positive evidence that sea-level fall had any relation to the genesis of proposed Tr–J marker horizons. It is unlikely that sea-level fall played a significant role in the Tr–J boundary extinctions in either a local or a global context.

Macroecological responses of terrestrial vegetation to climatic and atmospheric change across the Triassic/Jurassic boundary in East Greenland

Abstract The magnitude and pace of terrestrial plant extinction and macroecological change associated with the Triassic/Jurassic (Tr/J) mass extinction boundary have not been quantified using paleoecological data. However, tracking the diversity and ecology of primary producers provides an ideal surrogate with which to explore patterns of ecosystem stability, collapse, and recovery and to explicitly test for gradual versus catastrophic causal mechanisms of extinction. We present an analysis of the vegetation dynamics in the Jameson Land Basin, East Greenland, spanning the Tr/J extinction event, from a census collected paleoecological data set of 4303 fossil leaf specimens, in an attempt to better constrain our understanding of the causes and consequences of the fourth greatest extinction event in earth history. Our analyses reveal (1) regional turnover of ecological dominants between Triassic and Jurassic plant communities, (2) marked structural changes in the vegetation as reflected by potential loss of a mid-canopy habit, and (3) decline in generic-level richness and evenness and change in ecological composition prior to the Tr/J boundary; all of these findings argue against a single catastrophic causal mechanism, such as a meteorite impact for Tr/J extinctions. We identify various key ecological and biological traits that increased extinction risk at the Tr/J boundary and corroborate predictions of meta-population theory or plant ecophysiological models. These include ecological rarity, complex reproductive biology, and large leaf size. Recovery in terms of generic-level richness was quite rapid following Tr/J extinctions; however, species-level turnover in earliest Jurassic plant communities remained an order of magnitude higher than observed for the Triassic. We hypothesize, on the basis of evidence for geographically extensive macrofossil and palynological turnover across the entire Jameson Land Basin, that the nature and magnitude of paleoecological changes recorded in this study reflect wider vegetation change across the whole region. How exactly these changes in dominance patterns of plant primary production affected the entire ecosystem remains an important avenue of future research.

Timing, style and sedimentary evolution of Late Palaeozoic-Mesozoic extensional basins of East Greenland

Abstract The late Palaeozoic-Mesozoic sedimentary basins of East Greenland record a complex series of events which eventually led to the successful opening of the Norwegian-Greenland Sea in the late Palaeocene. Major tectonic events and regional sea-level changes caused by plate movements and reorganizations, overprinted by the effects of local tectonics and associated sea-level changes are reflected in thick, unconformity bounded sequences. Caledonian crustal shortening and thickening culminated in the Silurian, and was succeeded by extensional collapse, vulcanism and intrusion of post-tectonic granites in the Devonian. Large, transtensional pull-apart basins were formed and became filled with continental red beds. Basin margins underwent repeated episodes of thrusting. A change to rifting took place in the latest Devonian. Rapid subsidence and continental deposition punctuated by episodes of igneous activity and contractional deformation continued from the Devonian until early Permian times for about 120 Ma leaving a record of alluvial fan and flood plain siliciclastics alternating with rhyolitic and alkaline basaltic intrusions and extrusions. Later basin subsidence reflected thermal cooling and contraction following the prolonged late Palaeozoic period of crustal thinning. Important phases of block faulting occurred at several intervals during Mesozoic times climaxing in the Volgian-Valanginian interval. Cenozoic basin inversions resulted in dramatic decoupled uplift and tilting of large blocks. The degree of Devonian extensional collapse, the position and orientation of Caledonian thrust planes, and the localization of mid- and late Palaeozoic transtensional strike-slip basins exerted a profound control on localization and style of the compartmentalized Mesozoic rift. The importance of pre-Mesozoic history, timing of Mesozoic tectonic events and the interplay of tectonism, and rates of subsidence, sea-level change and sediment influx have close parallels in the North Sea area. Examples from the exposed Mesozoic succession of East Greenland may thus serve as excellent predictive guides for subsurface geology around the British Isles.

Numerical Paleoceanographic Study of the Early Jurassic Transcontinental Laurasian Seaway

The forces governing marine circulation of a meridional transcontinental seaway is explored with the Princeton Ocean Model. The Jurassic Laurasian Seaway, which connected the low-latitude Tethys Ocean with the Arctic Sea is modeled quantitatively. The global ocean is found to have a profound influence on seaway dynamics. A north-south density difference and hence sea level difference of the global ocean was probably the main factor in forcing the seaway flow. When the Tethys waters were the denser water, the net seaway flow was southward, and conversely, it was northward for denser Arctic waters. Marine bioprovincial boundaries and sediment data indicate that the seaway probably was dominated by Boreal faunal groups and reduced salinities several times in the Jurassic. The model results suggest that this can be explained by southward flowing seaway currents, which may have been related to an oceanic thermohaline circulation where no northern high-latitude deep convection occurred.

Epifaunal Zonation on an Upper Cretaceous Rocky Coast

In Cenomanian time, the deeply weathered Precambrian bedrock of southern Sweden was transgressed by a shallow sea, resulting in the formation of an archipelago with low islands and peninsulas during early Campanian time. On one of these islands, a high-diversity fauna characteristic of a rocky coast ecosystem is found comprising nearly all levels of the food pyramid from phytoplankton and zooplankton to reptiles. The fauna is dominated by bivalves, especially oysters, and includes the northernmost Upper Cretaceous reef corals and rudists. Three distinct epifaunal zones exist: the lowest zone is characterized by large numbers of serpulids and occupies the underside of the boulders; the next higher zone is found on the vertical sides of the boulders and is characterized by oysters and a large, vertically orientated craniacean brachiopod; and the highest zone occupies the upper part of the vertical sides and the rounded upper surfaces of the boulders and is characterized by spondylid bivalves. All species normally had a clumped distribution. The zonation is interpreted as a result of the combined effect of negative phototropism (serpulids), protection against sedimentation (craniaceans), and tolerance of strong turbulence, occasional exposure, and limited competition by other species for space (spondylids).

Slope and Deep Shelf Gully Sandstones, Upper Jurassic, East Greenland

The Upper Jurassic Hareelv Formation of Jameson Land, East Greenland, occurs in an area of 60 × 75 km. It consists of 200-500 m of black shale with thick, closely spaced sandstone bodies. The sandstones fill deep, steep-walled gullies and elongate scours, or form more regular, laterally extensive, parallel-sided, but erosive gully mouth or lobe deposits. Both types of sandstone bodies occur completely juxtaposed, and systematic vertical or lateral trends in bed thickness or grain size have not been observed. The sands were deposited in a deep-water shelf basin by high-density turbidity currents traveling from basin-margin source areas. A northeastern source area was represented by a short-lived, rapidly prograding delta, whereas the main, northwestern source area was a shallow, sandy barrier occurring along the length of a major north-northeast-striking fault-controlled slope. Voluminous turbidity currents probably were triggered by earthquakes in the fault zone. The resulting slope and basinal sand bodies are up to 50 m thick and hundreds of meters wide, and may be more than 5 km long in a downcurrent direction. They occur in a thick, rich oil-prone source rock. Thus, they may form potential stratigraphic reservoirs and help drain the source rock. The Hareelv Formation shows important similarities to the ramp facies model for delta-fed sand-rich turbidite systems; however, the formation is mainly tectonically controlled and independent of eustatic sea level changes. The Hareelv Formation may serve as a model for an unusual type of stratigraphic hydr carbon reservoir, and at least one North Sea oil field seems to have formed in an analogous setting.

Sequence Stratigraphy of the Jurassic-Lowermost Cretaceous of East Greenland (1)

The Jurassic-lowermost Cretaceous succession of East Greenland was deposited in a seaway formed over a series of old extensional basins between Greenland and Norway. The succession is interpreted within a sequence stratigraphic framework with the main emphasis on the Middle and Upper Jurassic. The vertical and lateral dimensions of the stratigraphic units are measured in kilometers and hundreds of kilometers, respectively. The interpretation is comparable to seismic-scale sequence stratigraphy, and the results can be compared directly to those derived from conventional reflection-seismic studies of subsurface successions. Sequence boundaries, and thus sequences, are defined differently by various research groups. In contrast, systems tracts representing linkages of deposi ional systems are considered the basic building blocks in both genetic and sequence stratigraphy. In the present study, systems tracts are recognized as the unit of highest rank within the concept of sequence stratigraphy. Ten major systems tracts are recognized. A Pliensbachian-Toarcian transgressive systems tract consisting of a slightly retrogradational parasequence set is overlain by a thin uppermost Toarcian-Aalenian(?) highstand systems tract (290-420 m in total thickness). The 140-700 m thick upper Bajocian-middle Callovian interval is represented by a basal aggradational to slightly retrogradational parasequence set interpreted as a shelf margin systems tract transitional to a transgressive systems tract. This interval is overlain by a strongly onlapping retrogradational set form d under rapid sea level rise and high sediment input, and interpreted as a transgressive systems tract. The upper Callovian-middle Oxfordian forms a composite progradational parasequence set that downlaps onto the top of the transgressive systems tract and represents a highstand systems tract. The transitional strata between the two systems tracts are highly condensed distally and contain the maximum flooding surface. Major regional deepening began in the late Oxfordian, and a thick succession of shales and turbiditic gully sandstones was deposited across the whole region. This succession represents another transgressive systems tract formed during a rapid sea level rise reaching highstand in the early Volgian when a sandy highstand systems tract prograded into the basin. The middle Volg an-Valanginian interval was characterized by rotational block faulting in northern East Greenland, in contrast to southern East Greenland, which continued its regular subsidence. A succession of a lowstand or shelf margin systems tract, a transgressive systems tract, and a highstand systems tract is recognized in both regions. This similarity may allow separation of sea level and tectonic signals in the two contemporaneous successions. The sequence stratigraphic analysis forms the basis for a coherent genetic model for the Jurassic-lowermost Cretaceous succession. The model may prove to be of value in interpreting deeply buried correlative hydrocarbon reservoirs in the northern North Sea and the Norwegian shelf.

The Cretaceous–Palaeogene boundary at Stevns Klint, Denmark: inversion tectonics or sea-floor topography?

The Cretaceous–Palaeogene boundary interval is exposed over 12 km in the coastal cliff, Stevns Klint, Denmark. An important lowermost Danian hardground has been interpreted as an originally horizontal marine abrasion surface. Its present elevation varies, from a few metres below, to about 35 m above sea level. This relief has traditionally been considered as resulting from late or post-Danian Laramide folding. New seismic profiles offshore Stevns Klint show, however, that the Base-Chalk reflector is not folded, is remarkably planar and has a gentle northward dip. Thus, the folding hypothesis cannot be upheld. Seismic stratigraphic analysis of the Chalk Group necessitates a fundamental revision of general ideas of chalk deposition. A highly irregular sea-floor topography was formed at many levels, and includes broad valleys, ridges, channels, drifts and mounds. A system of major WNW–ESE-oriented valleys and ridges can be traced into the succession exposed in Stevns Klint and further inland where it corresponds to the relief of the Cretaceous–Palaeogene boundary. The marked topographic elements of the chalk sea floor are elongate, with a WNW orientation roughly parallel to the axis of the Danish Basin and to the Sorgenfrei–Tornquist Zone forming the NE border of the basin. The sea-floor relief undoubtedly reflects the influence of strong contour-parallel bottom currents. The Cretaceous–Palaeogene boundary succession at Stevns Klint was thus developed on an underlying sea-floor topographic relief of about 40 m. Recognition of the highly irregular, current-influenced topography of the late Cretaceous sea floor stands in marked contrast to the conventional picture of quiet pelagic deposition of the chalk.

Forced regressions in a large wave- and storm-dominated anoxic lake, Rhaetian-Sinemurian Kap Stewart Formation, East Greenland

During Rhaetian-Sinemurian time a large wave- and storm-dominated lake was situated in the Jameson Land basin, East Greenland. Lake deposits consist of alternating black unfossiliferous mudstones and sheet sandstones. Anoxic conditions dominated at the lake bottom during deposition of the muds, and the water column was probably stratified. The sandstones were deposited by progradation of wave- and storm-dominated deltas in a water depth of less than 15 m. Sequence-stratigraphic interpretation suggests that the mudstones were deposited in periods of rising and very high stands of lake level, whereas progradation of the deltaic sheet sandstones took place during forced regressions caused by significant falls. The lake thus underwent a large number of fairly high amplitude changes in level, probably caused by climatic fluctuations. The high-order cycles can be grouped into several long-period cycles that show the same number of major fluctuations as published eustatic sea-level curves. This similarity suggests a causal link between eustasy and long-period variations in the lake. The Kap Stewart Formation represents one of the few ancient examples of a large wave- and storm-dominated lake, and it is probably the first documented case of abundant well-developed lacustrine forced regressions.