Abstract
The quartz sand of the Lower Cretaceous Robbedale Formation and lowermost part of the Jydegard Formation in the Arnager-Sose fault block of Bornholm has been investigated with respect to mineralogy, grain-size, grain rounding and grain shape. Analyses of both light and heavy minerals have been carried out for 18 samples from different localities and facies. The purpose of the investigation was to try to recognize the possible types of source rocks and provenance areas. The Precambrian basement rocks of Bornholm are not the main sources of the sand, especially not the heavy minerals, but parts of the sand may have originated from the basement. The Palaeozoic sandstones and siltstones have delivered only a small amount of material. Parts of the older Mesozoic sediments of the Bornholm Group and Homandshald Member may have been redeposited in the Lower Cretaceous as they contain the same kinds of heavy minerals as the Robbedale and Jydegard Formations, although in differing amounts. It seems very possible, however, that most of the Mesozoic sediments of Bornholm have had a source area outside Bornholm, and this source area has been nearly the same during the whole span of time. The Fenno-Scandian Shield seems to be the most obvious provenance area, but eastern and southern areas are possibilities too. It is concluded that most of the Lower Cretaceous sands are first deposition cycle sediments of both local and distant origin combined with minor amounts of polycyclic sediments of mainly local origin.
Highlights
The Lower Cretaceous Robbedale and Jydegaard Formations on Bornholm comprise sand deposits with a very high quartz content which can be classified as quartz arenites (Pettijohn, Potter & Siever, 1972)
A part of the quartz might originate from the kaolin, pegmatites or granites as the content of polycrystalline quartz points to a first cycle deposition of material from this kind of rocks
The rounding of the grains might be in situ rounding at weathering in kaolinized profiles (Moss, 1966; Crook, 1968) or it might have happened in the foreshore and shallow shoreface at continuous transport, deposition and erosion (Balazs & Klein, 1972; Gravesen, 1982a)
Summary
The Lower Cretaceous Robbedale and Jydegaard Formations on Bornholm (fig. 1) comprise sand deposits with a very high quartz content which can be classified as quartz arenites (Pettijohn, Potter & Siever, 1972). The Lower Cretaceous Robbedale and Jydegaard Formations on Bornholm Gravesen et al (1982) subdivided the Berriasian Robbedale Formation into the 0sterborg and Langbjerg Members and the Late Berriasian-Valanginian Jydegaard Formation into the Tornh0j and R0dbjerg Members. The present paper describes the petrographical characters of the white, brown, yellow and reddish non-calcareous sands of the 0sterborg and Langbjerg Members and the lowermost five metres ofthe Tornh0j Member in the Arnager-Sose fault block The members are mainly deposited in shallow nearshote marine and lagoonal areas (Gravesen, 1982a; Noe-Nygaard & Surlyk, in prep.) The paper deals with the description and discussion of the results of textural analyses and of analyses of the light and heavy minerals. The provenance area and source rocks of the sand are proposed
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