| dc.description.abstract | Abstract
This thesis is written as a part of the Cand. Scient. degree at the Institute for Geosciences at the University of Oslo.
Traditionally, age determinations of late Cretaceous chalks in the Central Graben in the North Sea are based on calcareous microfossils. So far, late Cretaceous dinoflagellate cysts have received little attention from the industry. Phillips Petroleum Company with Lucy Costa changed this in 1985, with their study of several wells on the Ekofisk Field resulting in an almost complete sequence of dinoflagellate cysts from the late Albian to Pliocene. Ongoing studies by Robert Williams (NPD) and Eric Monteil (IKU) show that in sections that are hard to date and correlate due to massive reworking of older material, using calcareous microfossils is difficult. But the dinoflagellate cysts are mainly in situ and provide a better alternative for biostratigraphical studies.
The main object of this thesis is to define a new, high resolution biostratigraphy for the base Ekofisk Formation (informally known as The Ekofisk Tight Zone ) based on dinoflagellate cysts, and a new high resolution biozonation will be proposed. The paleoenvironment during deposition will then be discussed, and an attempt to place the biostratigraphical results in a sequence stratigraphical framework will be made.
The 26 samples studied come from well 2/7-14, situed in the southern part of the Ekofisk Field. The samples where provided by the Norwegian Petroleum Directorate.
The lithological and biostratigraphical results, together with the gamma ray log, indicate that the studied interval was deposited in the lower Danian. It presumeably represents the lower part of the Ekofisk Formation informally named The Ekofisk Tight Zone . The Cretaceous/Tertiary boundary is not represented within the interval.
This study proposes a new biozonation for the lower Danian based on dinoflagellate cysts. The interval was generally difficult to relate to existing zonations, mainly because of poorer sample spacing, and/or the use of ditch cuttings. This proposal is based 26 cored samples in an 18 meter interval, providing a very high resolution zonation that should be recognizable in other wells, whether the samples are ditch cuttings or cores.
Chalk consists almost entirely of fine grained calcite derived mostly from planctonic nanno- and microfossils. The sedimentary structures found here suggest calm, hemi pelagic deposition beneath the storm wave base. Trace fossils are found throughout the interval, indicating low rates of deposition in a deep sea environment. The relationship between the trace fossils identidfied, implies that deposition has taken place on the transition between shallower and deeper water depth; outer neritic to oceanic.
The palynological results, together with the gamma ray log and diversity curve, suggest that the lower part of the studied interval was deposited during a trangressive systems tract. At 10650, signs of a maximum flooding surface (e.g. flooding zone) are present. From 10639 to 10623, the results indicate a highstand systems tract, and the top part of the interval probably represents the beginning of a lowstand systems tract. The sequence stratigraphical interpretation supports the biostratigraphical findings; the lower biozone (C) is recognized by the oceanic indicator Thalassiphora pelagica, while the uppermost zone (A) is characterized by the neritic species Cometodinium sp. B.
The chalk in the studied interval alternates between clean chalk and marls. This alternation is characteristic of a pelagic sediment termed periodites, which are undeniably autochtone. The sedimentary structures showed no signs of redeposition, and the presence of trace fossils is a strong indicator of autochtone deposition since they can´t be transported. This support the assumption that the interval has not been reworked.
Based on these results, the conclusion must be that the studied interval represents the lower part of the Ekofisk Formation informally named The Ekofisk Tight Zone . It is deposited as a shallowing upwards interval, going from deposition in deeper sea in the lower part, to shallower deposition in the upper part. The interval has probably not been reworked. | nor |