Sedimentology and sequence stratigraphy of Middle Jurassic deposits. Danish and Norwegian Central Graben. Ph.D. thesis

A sequence stratigraphic framework is established for the Jurassic of the Danish Central Graben based primarily on petrophysical log data, core sedimentology and biostratigraphic data from about 50 wells. Regional seismic lines are used to assist in the correlation of some wells and in the construction of isopach maps.

In the Lower Jurassic (Hettangian-Pliensbachian) succession five sequences have been identified. The Middle Jurassic is subdivided into four sequences that together span the latest Aalenian or earliest Bajocian to the Callovian. In the Upper Jurassic, better well coverage permits greater stratigraphic resolution, and 11 sequences are identified and mapped.

On the basis of the sequence stratigraphic correlation and the construction of isopach maps for individual sequences, the basin 'history of the Danish Central Graben in the Jurassic can be subdivided into seven discrete phases:

(1) Shallow marine and offshore sediments were deposited in a pre-rift basin extending from the North Sea to the Fennoscandian Border Zone (Hettangian-Pliensbachian).
(2) Uplift and erosion took place in association with the Toarcian-Aalenian North Sea doming event. A major hiatus represents this phase in the study area.
(3) Terrestrial and marginal marine sediments were deposited during a rift initiation stage (latest Aalenian or earliest Bajocian-Late Callovian)
(4) Early Oxfordian-Early Kimmeridgian transgression took place during rift climax. The sedimentary environment changed from coastal plain and marginal marine to fully marine.
(5) Regression and renewed transgression took place in the Late Kimmeridgian in association with a cessation of subsidence during a structural rearrangement. Shallow to marginal marine sandstones were deposited above an erosion surface of regional extent.
(6) Deep water mudstones were deposited in a composite graben with high subsidence rates during rift climax (latest Late Kimmeridgian-middle Middle Volgian)
(7) Deposition of organic-rich mudstones and turbidite sandstones took place during the late Middle Volgian-Ryazanian. The main basin became more shallow and symmetric and experienced a decreasing rate of subsidence.

A relative sea-level curve is constructed for the Middle-Late Jurassic. It shows good similarity to earlier published, eustatic (global) and relative (North Atlantic area) sea-level curves in the Oxfordian-Early Kimmeridgian, but differs in the Late Kimmeridgian-Middle Volgian interval probably due to the high rate of subsidence in the study area.