Rhaetic-Jurassic-Lower Cretaceous Sediments from Deep Wells in North Jylland, Denmark

RHAETIC-JURASSIC-LOWER CRETACEOUS SEDIMENTS FROM DEEP WELLS IN NORTH JYLLAND. DENMARK G U N N A R LARSEN

Geological Survey of Deninurk, Hellerup (Denmark )

INTRODUCTION

The knowledge of the Rhaetic-Jurassic-Lower Cretaceous formations of Denmark (except for the island of Bornholni) is of rather recent date; up to about 25 years ago the Senonian White Chalk was the oldest known bed in this part of the country. The extension of knowledge of older formations since that time is due to explorations by the Danish American Prospecting Co., which went on until the year 1959 when the company gave up the concession. The exploration has proved that the Upper Cretaceous overlies widely extended and thick beds of Lower Cretaceous, Jurassic, Triassic and Permian age. The sequence is not uniformly developed all over the country. The most complete sequence is found in the Danish Emhayment (Fig. 1). a trough which towards the northeast borders on the Fennoscandian Border Zone. Towards the southwest the Ringkrabing-Fyn High partly separates the trough from the North German Basin. The geological and geophysical material is now in the possession of the Geological Survey of Denmark. In 1962 this institution cominenced a n extensive examination of the material, led by professor Th. Sorgenfrei. The following presents the first results of the sedimentological part of this examination. DEEP WELLS 1N NORTH JYLLAND

In North Jylland a great number of boreholes have been completed. A few have penetrated deeply into or through Jurassic and Rhaetic beds. Of these deeper holes the following four will,be treated in some detail: Skagen no.2, Frederikshavn City no. I , Berrglum no. 1 and Haldager no. 1 (Fig. I). Part of the data of some of the holes have previously been published (GREGERSEN and SORGENFREI, 1951; N 0 R V A N G . 1957). LITHOLOGICAL PROPERTIES

The lithological properties of the sequences have been classified from studies of Samples and Schlumberger logs.

228

G. LARSEN

Fig. 1.

Limitation of the sequence

In North Jylland the Rhaetic, Jurassic, and part of the Cretaceous consist of a series of light coloured sand and dark clay beds which lithologically can be clearly distinguished from the older and younger formations. The underlying formation is preRhaetic Keuper developed as reddish claystones and arkoses. The overlying Upper Cretaceous sediments consist of whitish to grayish limestones and marls. The lithological boundary: highly calcareous deposits over clay and sand sediments, is not identical with the chronostratigraphical boundary between Upper and Lower Cretaceous. According to provisional determinations it seems to correspond approximately to the transition Turonian-Cenomanian. Hence the Cenomanian has been treated together with the Lower Cretaceous during the sedimentological studies. Subdivision of the sequence

The clay of this Rhaetic to approximately Cenomanian series is usually pyritic and

229

MESOZOIC SEDIMENTS FROM DEEP WELLS I N DENMARK

micaceous; it is often silty, and transitions to argillaceous silt occur frequently. In certain horizons in the Cretaceous and Upper Jurassic the sediments have a greenish tinge due to the content of glauconite. Marine fossils, e.g., Mollusca and Foraminifera, occur especially in argillaceous facies. Small amounts of lignite have been noted in some of the sand beds. The distribution of sand and clay facies is so characteristic that by comparing the four well-sections it is natural to subdivide the sequence in three cycles of sedimentation, each containing a lower, mostly sandy and an upper, mostly argillaceous member. This subdivision is shown in Table I. It is t o be noted that the dating of the members is provisional. The cycles are not uniformly developed in all the wells; e.g., there are variations in thickness. This is shown in the schematic northeast-southwest cross section through the wells, Fig.2. In the Border Zone the total thickness is considerably less than in the Embayment. However, the sand phase of the third cycle has its greatest thickness in the Border Zone; at Skagen the argillaceous member of this cycle seems to be missing entirely. As mentioned in the table, the argillaceous phase of cycle 2 is rather sandy in the upper part. This is especially so in the wells at Skagen and Frederikshavn where a rather gradual transition to cycle 3 is actually seen. In these wells the boundary between cycles 2 and 3 has been placed where glauconitic fine grained sand is overlain by light coloured non-glauconitic sand.

sw

NE HALDAGER 1

k-L

"

EERGLUM 1

DANISH

EMBAVHENT

FREDERIKSHAVN CITY 1

G.01 Surv d Oenm 1962

SKAGEN 2

'BE

Gunnor Loram

230

G. LARSEN

TABLE I SUBDIVISION OF THE WELL-SECTION I N CYCLES ~~~

Cycle riirniher

3

~

~~~~~

Appr0.r. age

Grrirrnl litholq:y

clay, dark, in part glauconitic; with silty and sandy beds; fossils ~~~

~~

~~

~

~~~~~~

Cenoiiianian and

~

sand, light; siiiall occurrences of lignite

Lower Cretaceous

clay, dark, in part glauconitic; with silty and sandy beds especially in the top part; fossils

Malm

~

-~

~~~

-

sand, light, with clay beds and thin lignite seams -.

~

~~

~

~~~~~

~~~

Dogger ~

~

~~

~

clay, dark, silty, with fossils

Lias

sand, light, with few clay beds and some lignite

Rhaetic

Within each of the larger sedimentary units sandy and argillaceous beds are interchanging; thus every cycle may be divided into a number of sub-cycles.

PETROGRAPHIC FEATURES

In Frederikshavn City no. 1 about 75 % of the sequence here treated has been cored. A total of 300 samples was selected from the cores for analysis: i.e., chemical deter-

mination of carbonate content and petrographic exmanination of the grain-size fraction 75-250 p. The Frederikshavn City no.1 log is shown in Fig.3, which present an extract of about 1/5 of all examined samples. The difference between the later Cretaceous deposits and the sediments dealt with here appears clearly in the carbonate curve. Besides, this curve shows that within the Rhaetic to Cretaceous sequence an alternation occurs between calcareous and non-calcareous units. Furthermore it is seen that as a rule calcareous deposits are also glauconitic, but the maximum value of glauconite coincides only in a few cases with the carbonate maximum. In addition, it appears that the highest concentration of glauconite is found in the Lower Cretaceous-Cenomanian and the lowest concentration in the Rhaetic-Lias cycle. Intervals without glauconite often contain plant remains. The curve showing the quartz-feldspar ratio correlates rather well with the division in cycles. In the upper two cycles the coarse sandy members have very high values of the quartz-feldspar ratio, while the argillaceous members have much lower values. There is, however, a pronounced difference between the second and the third cycle in that the argillaceous phase of the latter is on the whole more feldspatic than the corresponding phase in cycle 2. The lowermost cycle differs from the two others by the rather high content of feldspar in the sandy beds at the base.

23 1

MESOZOLC SEDIMENTS F R O M DEEP WELLS IN D E N M A R K

~~

FREDERIKSHAVN CITY 1 Lithology

1 Corbonotecontent

~

C

,-

RHAETIC-JURASSIC

0

c

1

in t o t a l sample

I

Mineral

-

LOWER CRETACEOUS SEDIMENTS

content

in

fractlon

75-250~.

j

*

U

I

U Q

' ' 1 - --

500 + light san d

b 000-

dork

light sand

WO --PRE-RHAETIC I KEUPER

jEOLOGlCAL SURVEY OF D E N M A R K 1962

GUNNAR LARSEN I

Fig.3.

232

G. LARSEN

All through the sequence examined, the same types of heavy minerals are found. However, with respect to the frequency of the different minerals rather large variations occur. From Fig.3 it is seen, that the correlation between the frequency distribution of the heavy minerals and the lithological cycle subdivision is rather good. The most outstanding features of this correlation may be summarized as follows: The association of the first cycle is rich in garnet; in cycle 2 stable minerals are quite dominating; and in cycle 3 epidote is the characteristic mineral, except in sections with low content of feldspar, where stable minerals again predominate. To this may be added that a few analyses of pre-Rhaetic formations seem to show that the clastic mineral association of the pre-Rhaetic Keuper is of almost the same composition as that of the Rhaetic-Lias cycle. Remarks on some of the minerals

Up to about half of the glauconite grains in the upper glauconitic horizon contain remains of organisms or prove to be casts of organisms, among which radiolarians (Fig.4) are rather abundantly represented. Similar organic structures have not been noted in the older glauconitic horizons. In coarse sands some of the quartz grains consist of orthoquartzite fragments, i.e., well rounded quartz grains surrounded by secondary overgrowths of quartz; the outer contours of the grains are often subrounded.

Fig.4. Grains of glauconite, one with inclusion of a radiolarian.

MESOZOIC SEDIMENTS FROM DEEP WELLS IN DENMARK

233

Fig.5. Grain of tourmaline with secondary overgrowth.

Similar secondary overgrowths are found in some of the tourmaline grains (Fig.5). Evidently the well rounded core represents a clastic sand grain. The mantle is probably authigenically formed in a sedimentary stage older than the present sediments. The outer contours of the grains generally show mechanical wear. Such “two-generation” tourmaline has been found throughout the sequence.

INTERPRETATION

On account of the above mentioned sedimentary properties some preliminary interpretations of the geological conditions during deposition of the sequence will be given. Environment of deposition

During Rhaetic to earlier Cretaceous times marine and non-marine conditions have evidently alternated in the present part of the field of accumulation. Environments of marine type, as indicated by the content of glauconite and marine fossils, have mainly existed during the deposition of the argillaceous formations; but also some of the sandy beds may be counted as marine. However, the main part of the sands seems to be predominantly non-marine according to the presence of lignite and the lack of the above mentioned marine evidences. As the non-marine formations have great thick-

234

C . LARSEN

ness and wide extension it seems obvious to classify them as deltaic formations. Cnnsequeiitly each of the three cycles apparently comprises a lion-marine deltaic stage succeeded by a mainly marine stage. Of the three transgressions, in the Lias, the Malm and the Lower Cretaceous respectively, the first has probably been the most extensive, as marine sediments from this period are found all over the area, mainly as argillaceous deposits. In the Malm the sea likewise has covered the whole area, but the considerable content of fine grained sand and silt, especially in the Fennoscandian Border Zone seeins to indicate a closer position of the coast during this period than during the Lias. The Lower Cretaceous transgression has been the least extensive in this area; marine deposits have not been found at Skagen. Furthermore the distribution of carbonate, glauconite and plant remains in the Frederikshavn profile reflects oscillations in the extension or’ the sea during the Lower Cretaceous. The appearance of radiolarians in the top of the sequence possibly marks the beginning of the more highly marine conditions which characterized the area during the following Upper Cretaceous time.

Origin oj‘the clastic inaterial Evidently the clastic minerals originate from older sediments and from metamorphic complexes. The orthoquartzite fragments and the “two-generation” tourmaline are referred to the first group, but a considerable part of the other stable heavy minerals and quartz grains may probably be referred to this group as well. Epidote and a good deal of the feldspar grains must be regarded as representatives of metamorphic rocks. Representatives of the two types of source material are found in all parts of the sequence, but the mutual frequency varies from cycle to cycle. The fact that the mineral association of Rhaetic-Lias fairly well corresponds to that of the pre-RhaeticKeuper may lead to the conclusion that they have had almost the same type of source material, or possibly that the Rhaetic-Lias material in part consists of re-deposited Keuper. The Dogger-Malm cycle has apparently had another type of initial material. Probably this consisted of older “mature” sediments. In the Lower Cretaceous, metamorphic complexes have constituted the principal part of the source area, which was probably situated in Fennoscandia. The fact that the transition from one mineral association to another does not occur gradually but rather abruptly near the boundary between the cycles, may indicate that “sudden” changes of the denudation area have taken place at these times; such changes have probably been caused by tectonic movements. This again may lead to the idea that the cyclic development of the whole discussed sequence in North Jylland has a background of tectonic nature.

SUMMARY

This paper presents some results of lithological investigations of the Rhaetic-Jurassic-

MESOZOIC SEDIMENTS FROM DEEP WELLS IN DENMARK

235

Lower Cretaceous sequence in four deep holes in North Jylland, together with petrographic data from one of the well sections (Frederikshavn City no. l ) . According to these investigations the sequence may naturally be subdivided into three sedimmtary cycles, each comprising a lower more sandy and an upper more argillaceous nieniber. Apparently the sandy members are mainly non-marine, probably of deltaic origin, while the argillaceous phases seem to represent mainly marine environments. The clastic material is not of the same type in the three cycles. The lower cycle (RhaeticLias) is rich in garnet; in cycle 2 (Dogger-Malm) stable minerals are dominating and in cycle 3 (Lower Cretaceous and Cenomanian) epidote is the most characteristic mineral. Because of these differences in clastic niaterial it is assumed that the cyclical development of the sequence is caused by tectonic movements.

REFERENCES

GRECERSEN, A. and SORGENFREI, TH.,1951. Efterforskningsarbejdet i Danmarks dybere undcrgrund. Medd. Dritisk Ceol. Foren., 12 : 141-151. NBRVANG, A,, 1957. The Foraminifera of the Lias Series in Jutland, Denmark. Medd. Dairsk Gcol. Foreti.. 13 : 275414.