Chalk palaeoenvironments and depositional model, Valhall-Hod fields, southern Norwegian North Sea

Abstract

Palaeoenvironmental analysis in the North Sea chalk is hampered by the prevalence of widespread allochthonous units, intervals of massive reworking of microfossils and an often monotonous lithology of massive chalks of uncertain depositional history. Most difficult of all is devising precise regional biostratigraphic correlations in a complex section with large, but very subtly expressed, disconformities and condensed zones. Facies change quickly both laterally and vertically. Without very precise regional correlations, it is impossible to formulate any detailed palaeoenvironmental model. Through a multidisciplinary palaeontological approach, employing graphic correlation methodology and a composite standard database, a highly accurate and detailed regional chronostratigraphy was generated. Biofacies recognized within the section were then scaled in absolute time, allowing for more reliable recognition of facies trends at any one time horizon. Biofacies were formulated based upon recovered benthic foraminifera, planktic foraminifera, calcispheres, siliceous microfossils and macrofossil debris. Biofacies mapping for several higher-order sequences recognized in the Upper Cretaceous chalks has allowed recognition of four major periods of deposition: (1) early Turonian-early Santonian high pelagic productivity and rapid deposition of chalks over a relatively continuous ridge from the Valhall to Hod fields; (2) early Campanian-earliest Maastrichtian deep-water autochthonous chalk deposition off-structure with structural crests mantled by submarine hardgrounds; (3) very heterogeneous autochthonous and allochthonous middle Maastrichtian chalk deposition controlled by local grabens formed from crestal collapse; and (4) widespread sediment flows and redeposition of sediments with crestal shoaling and winnowing in the latest Maastrichtian culminating in crestal erosion of Cretaceous chalks in the early Danian.

Characterization of dense zones within the Danian chalks of the Ekofisk Field, Norwegian North Sea

“…This correlation is confirmed by core facies, log trends and biostratigraphic analysis (D'Heur 1986;Bailey et al 1999). By contrast, in the Eldfisk and Valhall fields, this interval is absent or poorly developed (Sikora et al 1999;Hampton et al 2010). In the Ekofisk Field, the AI interval corresponding to the EE unit shows an overall regular thickness (~15-25 m; Fig.…”

“…3) (nomenclature after Bramwell et al 1999). The Ekofisk Trough acted as a catchment area capturing most of the gravity flows that originated from the surrounding structural highs and the eastern graben margin (Bramwell et al 1999;Sikora et al 1999).…”

Microfossil diachronism in southern Norwegian North Sea chalks: Valhall and Hod fields

“…Marginotruncana marginata is extremely abundant in the upper Turonian-Coniacian portion of the Hod Formation. Two distinct pelagic microfossil assemblages are recovered from the Hod Formation, indicative of two oceanographic water masses (Sikora et al 1999): (1) a deeper-water assemblage dominated by keeled planktic species (such as M. marginata) and diverse radiolarians, and most frequently found off-structure; and (2) a shallower-water pelagic assemblage dominated by non-keeled planktic species (mainly Whiteinella spp.) and low-diversity radiolarians, which was prevalent along the Valhall-Hod anticline.…”

“…However, the central crestal area of the Valhall anticline was marked by very shallowwater environments throughout much of the Maastrichtian and, except for discreet flooding events, planktic foraminifera were absent (Sikora et al 1999). Off-structure, the Tor Formation is often characterized by thick, stacked sequences of distal mudflows and turbidites containing wellsorted, tiny foraminifera of the genera Globigerinelloides and Heterohelix.…”

“…at Indiana University Libraries on July 11, 2015 http://sp.lyellcollection.org/ Downloaded from is characteristic of carbonaceous, pyritic, often laminated chalks signifying low-oxygen palaeoenvironments or, at the very least, depositional environments with a high organic influx. Such palaeoenvironments are highly discontinuous, both geographically and stratigraphically, and tied to local tectonics and palaeoceanographic conditions (Sikora et al 1999). In basinal facies S. granulata polonica does not meet the requirements of a good biostratigraphic index.…”