Tracking the sources of marine sediments on the Norwegian Sea continental shelf by using geochemical properties of detrital quartz

Abstract

Provenance studies reconstruct the erosion and transport history of sediments based on character and composition of deposited sedimentological records with implications for uplift and subsidence of the investigated area. This Master thesis presents a new combination of in situ analytical methods for qualitative provenance studies exemplified on siliciclastic sediments from three offshore exploration wells in the Miocene Molo Formation on the Norwegian Sea continental shelf (wells 6610/3-1, 6610/7-1, 6510/2-1). The first method applied was scanning electron microscope backscattered electron and cathodoluminescence (CL) imaging. The second method was high-resolution trace element analysis by means of laser ablation combined with inductively coupled plasma mass spectrometry. The study focused on Al, Ti, Li, and Ge, which are among the most common trace elements in quartz and indicative for their crystallization environment. Based on CL and trace element properties, individual quartz grains were characterized and seven generic types of plutonic and metamorphic quartz grains were distinguished. These are indicative of different magmatic/metamorphic histories. The geochemical properties were used for a comparison with quartz grains in rocks sampled from Precambrian basement at saprolitic outcrops on Hadseløya and Hamarøy as well as with weathered bedrock samples from the Froan Basin (6408/12-U-01), and Ribban Basin (6814/4-U-1). An overlap of the quartz types within particular wells and among the three sampled wells implies common provenance and hence common sediment catchment areas for a depositional area comprising 130 km horizontally and 500 m vertically. The subtypes show various local basement provenance signatures and the classification implies a main provenance of granitic plutonic rocks. The properties of the quartz grains sampled offshore do not correspond to those from quartz in saprolitic sand and mangerite thin sections sampled on Hadseløya and Hamarøy. This implies that the locations did not act as sediment source for the Molo Formation but rather for the overlying Plio-Pleistocene Naust Formation. Best match between source and sink material was found among deeply weathered bedrock sampled from the Froan Basin and the Molo Formation, suggesting an alongshore south to north-eastern sedi-ment transport direction.