SCANIA R MATHIESEN TRANSPORT !FULL!
Zircon U-Pb dating has proven useful for the provenance analysis in this study, where a large dataset was obtained from wells across the region. Zircon is a heavy mineral that occurs in small amounts in most sandstones. Zircon grains are physically and chemically robust [17] so they can survive multiple episodes of deposition and reworking while still preserving the radiometric age of the crystalline rock in which it formed during an igneous or high temperature metamorphic event and from which it later eroded. This ability makes U-Pb dating of detrital zircon a unique provenance tool that is employed to trace the zircon grains in sedimentary rocks back to their original source area. Thus, zircon age information can give important insights into sediment transport routes, sediment distribution in the basin, and thereby also the depositional environments and paleogeography. There has been reported partial open system behavior in the U-Pb decay scheme for zircon and ancient Pb-loss at relatively low temperatures [18,19]. However, the high closure temperature of Pb in U-Pb zircon geochronology (>800 C) allows all high temperature metamorphic or magmatic events in the region to be recorded.
SCANIA R MATHIESEN TRANSPORT
The primary aim of the study is to contribute to the understanding of the governing processes for the reservoir properties of the Gassum Formation by analyzing evidence for source terranes. The formation contains many excellent reservoir sandstone units, but their distribution and quality are strongly dependent on the mineralogical composition and maturity in addition to the burial history. Previous studies have shown that the mineral assemblages were controlling the type of diagenetic alterations and resulting reservoir quality [5,20]. It is thus mandatory for reliable pre-drill predictions of reservoir properties that the source areas and the sediment transport routes are mapped to facilitate area specific predictions. By this means, we aim to illustrate the value of applying detrital zircon geochronology in source to sink analyses including paleogeographic reconstructions of basin development and changes in its hinterland and how it affects the detrital mineralogical composition.
The zircon grains were ablated for 30 s in an air-tight helium-flushed chamber using a focused laser beam with a diameter of 25 or 30 micrometres (µm), a repetition rate of 10 Hz, and an output energy density of c. 10 J/cm2. The liberated material was transported through inert Tygon tubing by the helium carrier gas to the mass spectrometer for isotopic determination. To minimize instrumental drift, a standard-sample-standard analysis protocol was followed, bracketing the zircon analyses by measurement of the zircon standard GJ-1 [41]. For quality control, secondary zircon standards were used, i.e., Plešovice [42] and for the newer analyses also Harvard 91500 [43,44], both yielding an average age accuracy and precision (2σ) within 3% deviation.
In the northwestern part of the Gassum Formation, the highly varying proportion of the individual zircon age populations indicates a local provenance where limited homogenization of the provenance signal has occurred along the short sediment transport pathways (Figure 5A). The opposite is found in the southeastern part of the formation as evident by the uniform age distributions, implying intensive homogenization and mixing of sediment from several source areas (Figure 5B). This trend is well illustrated in Figure 9 where the homogeneity of the SE groups is evident, in the sense that the distribution of ages and relative abundance of age populations are comparable between the samples. The individual samples in the NW groups show larger variation in their age distributions so they are less comparable showing that less mixing of sediment supplies from the different source areas has occurred.
Most of the sediment in the northwestern part of the Gassum Formation was only transported a short distance from its source area, the Telemarkia Terrane in southernmost Norway, which is in accordance with the low mineralogical maturity presumably reflecting short fluvial transport (Figure 8). The plagioclase content is halved in the NW Upper group compared to the NW Lower group (Table 1). This is in accordance with the longer sediment transport route from the Caledonian Orogen in central southern Norway interpreted as the primary source of the upper part of the formation in this area, which could have caused more breakdown of feldspars and in particular of plagioclase than in the lower part of the formation, since plagioclase is more sensitive to weathering than K-feldspar [60]. The sediment was probably transported in large meandering rivers, with temporary storage along the route allowing time for intensive chemical weathering in the humid climate. The increased content of metamorphic rock fragments and muscovite in the NW Upper group compared to the NW Lower group also shows that different types of rocks were supplying the sediment since the Caledonian nappes comprise primarily metasedimentary rocks. This change may be related to the general backstepping of the basin edge due to the marine drowning that culminated with the deposition of the Fjerritslev Formation [24], or it could be related to a changed topography of the source area. However, in Skagerrak, sediment was continuously supplied from the Telemarkia Terrane to the Gassum Formation so a change in provenance did not occur closest to this source area.
Most of the sediment in the southeastern part of the Gassum Formation was transported a long distance from the Caledonian Orogen, and some Sveconorwegian sediment has been added to the detritus in the fluvial system when passing through this area. Especially from the Telemarkia Terrane, whereas zircon dates corresponding to the broad spectrum of ages present in the Idefjorden Terrane and the Eastern Segment are not prominent in the Gassum Formation (Figure 5B and Figure 6). The southeastern part of the Gassum Formation is positioned further away from the Caledonian Orogen than the northwestern part (Figure 2), which is part of the explanation for the lower feldspar content in the southeastern part (Table 1). The sediment supplied from the Variscan Orogen to the southeastern part of the Gassum Formation was also transported a long distance before deposition. Intensive weathering during temporary storage along the transport route is a more likely cause than repeated reworking of the high mineralogical maturity present in the southeast since there are no major variations in feldspar abundances between fluvial and shoreface sandstones [5]. Reworking of older sedimentary rocks from the south has also contributed to the high mineralogical maturity as seen by the presence of rounded quartz overgrowths enclosing red coatings (Figure 7), which are not seen in the northeastern part of the Gassum Formation. The composition of the sediment source rocks is also of importance, where smaller proportions of feldspars are characteristic of sediments sourced from the south.
The Bunter Sandstone Formation present in the southern part of the Danish area contains Variscan-aged zircon grains, which were transported across the North German Basin from the Variscan massifs by aeolian processes in this arid Early Triassic climate [61]. However, the Bunter Sandstone Formation contains other age populations that are not prominent in the southeastern part of the Gassum Formation, so reworking the Bunter Sandstone Formation cannot explain the entire zircon age distribution. The very prominent c. 1.65 Ga zircon age population found in all samples from the southeastern part of the Gassum Formation is not discernable in the Bunter Sandstone Formation since erosion of the Caledonian Orogen was not yet a prominent sediment source to the Danish area in the Early Triassic.
The sediment supplied to the Gassum Formation cannot have been reworked from Fennoscandia-derived older sedimentary rocks such as Cambrian and Devonian since the zircon age populations do not match [39,62,63]. Reworking of the German Triassic Buntsandstein Group deposited north of the Variscan massifs is a likely source of the Variscan-aged zircon grains found in the Gassum Formation, possibly combined with sediment eroded from the highs at the time. The zircon ages in the Buntsandstein comprise first-cycle Carboniferous magmatic zircon grains besides Ediacaran to Paleozoic zircon grains reworked from metasediments and Gondwana-derived zircon grains with ages of c. 2 Ga [62]. These are comparable to age populations that are present in the SE Lower and Upper groups, but are absent in the NW Lower and Upper groups (Figure 6). A further indication of sediment supply from reworking of the Buntsandstein Group comprises Fe-oxide/hydroxide coatings on grains overgrown by quartz, rounded during transport (Figure 7). Such grains are found in the southeastern part of the Gassum Formation and proves that these grains were supplied from the reworking of sediments deposited during an arid climate such as the Buntsandstein.
A contour map of the Variscan input to the Gassum Formation is produced based on a qualitative estimation of the relative content found in each sample (Figure 10). All 12 samples from the eight southernmost sampled wells have high Variscan input relative to the other samples. Thus, the Variscan zircon grains must have been supplied to the Danish area from the south or southeast, which is in agreement with their source area in the Variscan Orogen present in Central Europe. The Variscan input decreases to a medium to low relative amount in the samples from northern Sjælland and Kattegat. In Jylland, Variscan input is only distinct in the upper sample from the Horsens-1 well, so it appears that the deposition of Variscan sediment spread to a wider area extending further towards the west during the deposition of the upper part of the Gassum Formation. Variscan input has not been found in the samples from the 11 remaining wells, which are present farthest to the northwest with the Voldum-1 well being the most southeasterly. Based on the distribution of Variscan zircon grains in the Gassum Formation, it seems likely that sediment was transported from the southeast across the Skurup High and that sediment was supplied from the south simultaneously (Figure 10). 041b061a72
