Jon R. Ineson

Stratigraphy of the Rotliegend Group in the Danish part of the Northern Permian Basin, North Sea

A stratigraphic study of the Rotliegend offshore Denmark has resulted in a coherent lithostratigraphic framework and a better understanding of the mid-Permian evolution of the eastern Northern Permian Basin. The Rotliegend Group is subdivided into the Karl Formation composed of syn-rift volcanics and volcaniclastic sediments, and the younger post-rift Auk Formation dominated by aeolian and fluvial sandstones. Radiometric age data based on K–Ar dating of plagioclase from the Karl Formation volcanics, combined with previously published age data suggests that volcanism took place during two separate events, 276–281 Ma and 261–269 Ma. Both events post-date the Lower Rotliegend volcanism (288–300 Ma) in the Southern Permian Basin, suggesting that the Northern Permian Basin was formed during a later tectonic event. The main volcanic event in the Northern Permian Basin was broadly coeval with volcanic activity in the Altmark area of NE Germany, reflecting simultaneous rifting and onset of Upper Rotliegend 2 sedimentation in the Northern and Southern Permian Basins.

A cool temperate climate on the Antarctic Peninsula through the latest Cretaceous to early Paleogene

Constraining past fl uctuations in global temperatures is central to our understanding of the Earth’s climatic evolution. Marine proxies dominate records of past temperature reconstructions, whereas our understanding of continental climate is relatively poor, particularly in high-latitude areas such as Antarctica. The recently developed MBT/CBT (methylation index of branched tetraethers/ cyclization ratio of branched tetraethers) paleothermometer offers an opportunity to quantify ancient continental climates at temporal resolutions typically not afforded by terrestrial macrofl oral proxies. Here, we have extended the application of the MBT/CBT proxy into the Cretaceous by presenting paleotemperatures through an expanded sedimentary succession from Seymour Island, Antarctica, spanning the latest Maastrichtian and Paleocene. Our data indicate the existence of a relatively stable, persistently cool temperate climate on the Antarctic Peninsula across the Cretaceous-Paleogene boundary. These new data help elucidate the climatic evolution of Antarctica across one of the Earth’s most pronounced biotic reorganizations at the Cretaceous-Paleogene boundary, prior to major icesheet development in the late Paleogene. Our work emphasizes the likely existence of temporal and/or spatial heterogeneities in climate of the southern high latitudes during the early Paleogene.

Marine volcaniclastics of the Hidden Lake Formation (Coniacian) of James Ross Island, Antarctica: an enigmatic element in the history of a back-arc basin

Abstract The Coniacian Hidden Lake Formation of James Ross Island, Antarctica is a 300–400 m-thick succession of marine volcaniclastic conglomerates, sandstones and mudstones. It occurs at a point of transition in the evolution of the James Ross Basin, as it is underlain by deep-marine strata and overlain by shallow-marine strata. The succession reflects the two main factors controlling the deposition of the formation: (1) the influx of large quantities of volcaniclastic sediment; and (2) a pronounced inversion event in the early Coniacian heralding the cessation of transpressive tectonic activity in the James Ross Basin. The succession is dominated by a range of sediment density-flow deposits, which, combined with the limited faunas and the lack of wave-induced structures, suggest deposition in a relatively deep-marine environment below storm-wave base. Three main facies associations are recorded representing base-of-slope, fan-delta and basin-floor depositional environments. The volcaniclastic fan-delta association is dominated by fresh pyroclastic detritus and was deposited in response to volcanic eruptions on the adjacent arc. Thick beds of parallel-stratified sandstone record deposition from sustained, concentrated sediment density flows. The conditions immediately following pyroclastic eruptions lend themselves to the deposition of such deposits, as vegetation cover is destroyed and large amounts of poorly consolidated sediment are available for reworking. An enigmatic feature of the succession is the presence of units of cross-bedded sandstones thought to be of tidal origin that are locally abundant and are intimately interbedded with sediment density-flow deposits. The occurrence of tidal sediments in a substorm-wave base setting is explained by appealing to partial basin inversion during the final phases of strike-slip tectonic activity in the basin creating an irregular basin floor that focused and amplified tidal currents. The infilling of this basin topography by sediment and waning intrabasinal tectonism during the Coniacian resulted in the progressive elimination of this basin-floor topography and the onset of shallow-marine shelf sedimentation.

The early origin of the Antarctic Marine Fauna and its evolutionary implications

The extensive Late Cretaceous – Early Paleogene sedimentary succession of Seymour Island, N.E. Antarctic Peninsula offers an unparalleled opportunity to examine the evolutionary origins of a modern polar marine fauna. Some 38 modern Southern Ocean molluscan genera (26 gastropods and 12 bivalves), representing approximately 18% of the total modern benthic molluscan fauna, can now be traced back through at least part of this sequence. As noted elsewhere in the world, the balance of the molluscan fauna changes sharply across the Cretaceous – Paleogene (K/Pg) boundary, with gastropods subsequently becoming more diverse than bivalves. A major reason for this is a significant radiation of the Neogastropoda, which today forms one of the most diverse clades in the sea. Buccinoidea is the dominant neogastropod superfamily in both the Paleocene Sobral Formation (SF) (56% of neogastropod genera) and Early - Middle Eocene La Meseta Formation (LMF) (47%), with the Conoidea (25%) being prominent for the first time in the latter. This radiation of Neogastropoda is linked to a significant pulse of global warming that reached at least 65°S, and terminates abruptly in the upper LMF in an extinction event that most likely heralds the onset of global cooling. It is also possible that the marked Early Paleogene expansion of neogastropods in Antarctica is in part due to a global increase in rates of origination following the K/Pg mass extinction event. The radiation of this and other clades at ∼65°S indicates that Antarctica was not necessarily an evolutionary refugium, or sink, in the Early – Middle Eocene. Evolutionary source – sink dynamics may have been significantly different between the Paleogene greenhouse and Neogene icehouse worlds.

Geochemistry of the Cambrian Sirius Passet Lagerstätte, Northern Greenland

The lower Cambrian Sirius Passet Lagerstatte (∼518 Ma) consists of mudstones about 8 m thick located in the Franklinian Basin of North Greenland. We analyzed major and trace elements plus the S, C, Cu, Fe, Zn, and Mo isotope compositions. Factor analysis allowed the lithology of the mudstone to be broken down into variable proportions of two inputs, a dry felsic component and a hydrous mafic component (smectite or chlorite). Zircons U-Pb ages indicate multiple sources, the local Proterozoic basement of Northern Greenland (1250–2400 Ma) and Pan-African felsic magmas (620–650 Ma) from across the Iapetus ocean. Diagenesis involved the reduction of Fe, S, and Mo from seawater and pyritization. The Sirius Passet Lagerstatte formed in oxygen-starved muds inhibiting degradation of organic matter underneath a well-ventilated water column. The chemistry of the samples, their very fine grain size, their apparent lack of graded bedding, and the age of zircons suggest that the Lagerstatte may represent wind-blown dust deposited on the continental slope.

Characterization and zonation of a marly chalk reservoir: the Lower Cretaceous Valdemar Field of the Danish Central Graben

In the Valdemar Field of the Danish Central Graben, production is from the Upper Hauterivian–Aptian succession (Tuxen and Sola formations) which comprises interlayered pelagic/hemipelagic chalks, marly chalks and marlstones. Based on core data, the reservoir chalks (6–10% insoluble residue (IR)) and marly chalks (10–30% IR) possess porosities in the 20–48% range and matrix permeabilities of 0.1–4 mD. The porosity (ϕ) of these chalks correlates negatively with the IR (particularly clay) content. A permeability cut-off of 0.1 mD was defined for reservoir studies, corresponding to a ϕ of 20–30% and an IR of c. 35%. A detailed reservoir zonation, based on integration of core and petrophysical data constrained within a sequence stratigraphic framework, illustrates the stratigraphical compartmentalization of the field. The distinctive nature of this heterogeneous argillaceous chalk reservoir is illustrated by comparison with Maastrichtian–Danian chalks of the Central Graben. For a given porosity, the matrix permeability of the Valdemar Field chalks is a factor of ten lower than that of an equivalent Maastrichtian chalk, the irreducible water saturation is a factor of ten higher and the capillary entry pressure of the Lower Cretaceous reservoir is 2–3 times that of the younger chalks.

Carbonate megabreccias in a sequence stratigraphic context; evidence from the Cambrian of North Greenland

Abstract In carbonate sequence stratigraphy, carbonate megabreccias have acquired particular significance, being deemed characteristic of the lowstand systems tract (LST) or the forced regressive systems tract (FRST). Large-scale mass-wastage can, however, result from factors other than sea-level change and it is rare that the sequence stratigraphic significance of megabreccias can be rigorously tested. In the Cambrian of North Greenland, erection of a robust sequence stratigraphic framework is facilitated by extensive fjord-wall exposures of the platform to deep shelf transect and by a well-developed carbonate-siliciclastic reciprocal sedimentation pattern within off-platform strata. On the basis of this independent framework, megabreccias are represented locally within the LST and the high-stand systems tract (HST), but occur systematically above the HST. These HST-capping megabreccias are composite sheets tens of metres thick that extend up to 50 km distally and flank the platform for up to 400 km along strike. They comprise debris derived from the highstand platform margin and slope and are directly overlain by mixed carbonate-siliciclastic sediments of the succeeding LST. The HST-capping megabreccias are assigned to the FRST; they record extensive failure of the platform margin and upper slope during relative fall of sea-level and prior to the onset of lowstand sedimentation. Although the LST megabreccias are compositionally distinctive, the sole example of an intra-HST megabreccia differs from those of the FRST only in terms of areal extent. In the absence of an independent framework, therefore, the sequence stratigraphic affinities of megabreccias may be ambiguous.

Carbonate slope aprons in the Cambrian of North Greenland: geometry, stratal patterns and facies

Spectacular fjord wall exposures in central North Greenland allow direct observation of the relationship between the evolution of a Cambrian shallow-water platform and the accumulation of a mixed carbonate-siliciclastic succession in the deeper waters of the adjacent outer shelf. This contribution illustrates the characteristic features of extensive carbonate slope apron deposits that accumulated during relative highstands of sea level. Such line-sourced deep-water systems are characteristic of carbonate settings and contrast with the point-sourced systems that dominate in deep-water siliciclastic settings.

The Valdemar Field, Danish Central Graben: field compartmentalization and regional prospectivity of the Lower Cretaceous chalk play

Despite representing a widespread play in the Central North Sea, production from Lower Cretaceous chalks is currently confined to the Valdemar Field in the Danish Central Graben. The field comprises a heterogeneous reservoir succession, less than 100m thick, consisting of hemipelagic chalks, marly chalks, and marlstones of Late Hauterivian–Early Aptian age. Although the field has in-place reserves in the order of 115 × 10 6 m 3 (725 × 10 6 BBL), the recovery from this complex reservoir was initially estimated to be only approximately 1%, primarily due to low permeability. The argillaceous chalks of the Lower Cretaceous reservoir are highly faulted and fractured, overpressured and undercompacted, giving rise to a complex distribution of hydrocarbons. Due to the lithological heterogeneity of the succession, internal stratigraphic barriers are common and result in stratigraphic compartmentalization. In addition, clay smearing in fault zones has created structural barriers and the development of structural compartments, as reflected by spatial differences in oil saturations, oil types and maturity, formation pressure and porosity distribution. Analysis of the reservoir properties and structural development of the Valdemar Field has provided data that can be extrapolated to the remainder of the Danish Central Graben, indicating that the Lower Cretaceous is most prospective in the central and southern Danish Central Graben.