Abstract
In situ weathering of the Mesoproterozoic, hornblende- and biotite-rich Rønne granite at
Nygård kaolin pit provides a well preserved, Jurassic to Early Cretaceous saprolite section on
Bornholm. Fresh Rønne granite could be sampled in the approximately 800 m distant granite
quarry at Klippeløkken. This study comprises petrographical and mineralogical thin section-,
XRD- and SEM- analysis to distinguish the mode of primary mineral dissolution and secondary
clay mineral precipitation in the weathering profile. These analysis are based on sedimentary
logging, regular interval sampling, gamma ray- and fracture/joint measurements in five
weathering profiles at the inactive Nygård kaolin pit. The weathering classification of saprolitic
lithology has been conducted according five facies, from fresh rock to highly weathered, friable
soil. The weathering sequence has been interpreted based on the mineralogical and petrographical
analysis. Specific clay mineral abundance of kaolinite>>>chlorite>>smectite>illite
studied in the weathering profile have been used to interpret the depositional environment of
the succession.
Three main generations of weathering could be determined accordingly, 1. subaerial
weathering, where original plagioclase feldspar, hornblende and biotite was dissolved and
illite, smectite, illite/smectite mixed-layers and kaolinite clay minerals were formed during
Early- to Late Jurassic. 2. Mineral formations during burial of the profile, are associated
with the precipitation of carbonate concretions, foremost siderite, and authigenic chlorite in
?Late Jurassic to Early Cretaceous. 3. Post-burial mineral reactions are mostly associated with
oxidation conditions during seasonal changing groundwater percolation and the precipitation
of hematite. Re-exposure and erosion of the overlying Rabekke Formation is suggested of Late
Cretaceous to Early Cenozoic time. Transpressional tectonics have led to fault reactivation and
obduction of the southeastern margin of Bornholm.
Corestone associations in the weathering section are well preserved and fracture/joint
measurements revealed comparable strike directions with the adjacent granite quarry at
Klippeløkken, suggesting only little to none tectonic impact between the two study localities.
Tectonic stability, relatively low relief and prevailing humid and tropical climatic conditions
can be addressed as driving parameters for the argillization of thick kaolinite deposits during
the Mid Mesozoic on Bornholm.
The acquired results of this master thesis are used as comparison with other localities. An
example of similar paleo-chemical weathering can be found at Ivö Klack (southern Sweden)
also constituted by thick kaolinite deposits in the upper profile. In contrary, a recent weathering
profile formed under more temperate arid conditions in NW Portugal displays arenization and
kaolinite is replaced by gibbsite.
As part of the Utsira Project, a collaboration of the University of Oslo and Lundin
Petroleum AS, it was the main focus of this study to provide lateral information of onshore
weathering profiles, that may be used as comparison to marine weathering successions found
in two drill cores from the Johan Svedrups field and the Edvard Grieg field in the North Sea.