Distinguishing between primary and reworked volcaniclastic deposits in the reconstruction of continental rift evolution.

Failed continental rifts are diverse intra-continental sedimentary basins that hold records of major Earth-history events. They are representative of huge tectonic forces involved in the break-up and reorganisation of continental masses and are typically accompanied by the deposition of large volumes of volcanic and sedimentary material. A significant hurdle in determining the geological history of these ancient rift zones is the recognition of, and distinction between primary volcanic material and partial reworking of unconsolidated volcanics as dominantly volcaniclastic sediments of alluvial origin. This is a fundamental distinction when interpreting geological histories and depositional settings, the difference between continued volcanism and relative quiescence. The Permian Oslo Rift is a well-studied example of a failed continental rift, nevertheless, there is much still to be investigated, especially regarding the explosive volcanism and re-sedimentation of volcaniclastic material in the later stages of the rift. The Oslo rift hosted up to 18 large calderas, formed in the middle to late stages of rift activity, yet despite the significance of this phase in the Oslo Rift these deposits have not yet been the focus of any detailed study. Significant post-rift erosion (estimated at 1-3km) has removed the youngest deposits and much of what remains is hidden by soil and undergrowth cover. The calderas are now dominated by larvikite, syenite, and granite batholiths, with lava and pyroclastic deposits in varied amounts. There is one known locality at the north-eastern edge of Oslo city, the Alnsjø field, that contains a volcaniclastic and sedimentary succession that has a partial record of the caldera volcanism phase, thought to have been preserved by subsidence within a caldera centre (the Nittedal caldera). The succession comprises aphyric to porphyritic lavas, ignimbrites, tuffs, breccias, conglomerates, sandstones, and mudstones, with faulting and a moderate degree of contact metamorphism overprinting obscuring geological reconstruction and interpretation. Although the majority of deposits are dominated by volcanic clasts and grains, many have neither a clear volcanic nor sedimentary origin. Initial results show that the succession comprises, among other things, what is likely the youngest basalt flow preserved in the rift, early explosive felsic volcanism , a thick lacustrine mudstone reflecting an extended period of quiescence, and renewed explosive activity with the transition phase clearly preserved in the upper section of the mudstone. To clarify the uncertain origins of these deposits we combine standard geological field methods with thin section analyses (microscope, scanning electron microscope (SEM), and electron microprobe), micro-computed-tomography (µCT) scanning, and geochemical analyses. Initial results suggest that the examination of matrix components may be a way to differentiate between primary and secondary origins for deposits in the Alnsjø field. Visualisation of 3D textures from µCT scans provides a more accurate understanding of paleo-flow directions, quantitative measurements of clast properties (rounding, orientation, dimensions, clast-clast relationships etc), and can be used to place the high resolution SEM analyses of thin sections (2D slice) in a 3D context. Volcanic activity was more significant at this time than previously thought as indicated by the predominantly volcanic deposits that have previously been considered alluvial.