Depositional environments of coal and associated siliciclastic sediments in the Lower and Middle Jurassic of Denmark. The Øresund -5, -7, -13, -15 and -18 wells

Authors

  • Henrik Ingermann Petersen DGU

DOI:

https://doi.org/10.34194/seriea.v33.7053

Keywords:

Øresund wells, Lower and Middle Jurassic, coal, Lignite, peat, swamps, peat bogs, elastic sediments, organic geochemistry

Abstract

Five cored wells located in the Fennoscandian Border Zone in the Øresund area, Denmark, encountered Lower or Middle Jurassic coal-bearing strata; the coal seams are Lignite to Sub-bituminous A/High Vol. Bituminous C in rank. A number of shallowing-upward units are recognized in the five wells. Each unit is capped by a coal seam. Correlation of these shallowing-upward units between wells is difficult on the basis of available biostratigraphy and log data. Seven of the coal seams result from establishment of peatforming conditions due to infilling of freshwater lakes, whereas the last two of the coal seams result from peat accumulation on top of restricted brackish lagoon or bay sediments. However, only one of the latter two seams accumulated in an environment influenced by saline water. Hence, the investigated coals represent almost entirely freshwater peat-forming environments. Three main types of environments are defined: 1) Type 1 is a sparsely vegetated open water swamp; it is represented by a limnic facies. The deposit is typically a carbonaceous claystone with a high content of allochthonous organic matter; 2) Type 2 is a densely vegetated rheotrophic, nutrient-rich and anoxic swamp; it is represented by a limnotelmatic to telmatic facies. The coal has a very high content of humified organic matter; 3) Type 3, subdivided into the types 3a and 3b, is the driest environment of the three types. Type 3a is a desiccated ombrotrophic raised bog represented by a terrestrial facies. It is strongly influenced by a fluctuating watertable. The coals contain a high content of inertinite that generally shows a low reflectance. Type 3b is a mesotrophic to ombrotrophic domed bog; the environment alternates between dry oxidizing conditions and wet conditions. It is represented by a telmatic to terrestrial facies. In general, the three types of environments form ecosystems characterized by the groundwater influence, nutrient supply, and vegetation. Successions representing the hydrological evolution towards drier conditions due to vertical peat accretion are recognized in some of the seams. The vegetation was small-statured and consisted of a prominent herbaceous type of flora, shrub-like plants, smaller arboreous plants and, to a lesser extent, larger plants.

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Published

1994-09-30

How to Cite

Petersen, H. I. (1994). Depositional environments of coal and associated siliciclastic sediments in the Lower and Middle Jurassic of Denmark. The Øresund -5, -7, -13, -15 and -18 wells. Danmarks Geologiske Undersøgelse Serie A, 33, 1–55. https://doi.org/10.34194/seriea.v33.7053