One step or two: using the trace element geochemistry and U-Pb age of detrital apatite to evaluate a direct versus a foreland basin inversion model for sand dispersal from the Uralian Orogen to the Barents Shelf

A thick siliciclastic succession was deposited onto the Barents Shelf via prograding deltaic complexes during the Triassic. Seismic and provenance studies indicate that the sediment originated from the Uralian Orogen and was routed from the southeast to the exploration area on the southwest Barents Shelf. Shoreface and channel sand bodies are potential reservoirs but exploration activity targeting such units has been met with limited success partially because of lower than expected reservoir quality. The fundamental control on the highly variable reservoir properties is attributed to its provenance from the Uralian Orogen, which delivered compositionally and texturally immature sand that is prone to porosity loss during burial and diagenesis. First delivery of Uralian sourced sand onto the Barents Shelf was during the Induan. The Uralian orogen occurred from the earliest Permian to the Triassic, with thick siliciclastic successions of Uralian-derived flysch and mollasse deposited throughout the Permian including within the Korotaikha and Kara basins in Arctic Russia. Uralian foreland-basin inversion and subsequent sediment recycling is presented as one possible explanation for the source of Uralian-sourced sand onto the Barents Shelf during the Early Triassic. Apatite crystallises as an accessory phase in most metamorphic and igneous rocks, irrespective of their composition. The protolith within which apatite grew is indicated by its trace element geochemistry and its U-Pb ages record a time of cooling after its crystallisation. In combination, it is thus a powerful provenance indicator for orogenic belts. Given its relatively labile nature and susceptibility to acidic weathering, it may also be well placed to evaluate sedimentary reworking.

In order to test the basin inversion model, combined trace element geochemistry and U-Pb geochronology was completed on detrital apatite from a suite of Permian sandstones from the Kara Basin , Pai-Khoi, NW Russia, along with separate trace element geochemistry and U-Pb geochronology have been gathered from the Permo-Triassic succession of the Korotaikha Basin, Pai-Khoi, NW Russia. These data were collected from a sedimentary system that was trending axially along the Subpolar to Polar Urals foreland basin. These data are compared to Triassic sandstones from the wellbore materials from the southwest Barents Shelf, and from Triassic outcrop from Svalbard. Additionally, in the Pai Khoi region, Permian samples in the Kara Basin, where it has been possible to tie the geochronology to the geochemistry, are sourced from mafic and I-type igneous rocks which cooled between 250-380 Ma. It has not been possible to tie the apatite geochemistry of Triassic apatite grains to their U-Pb ages, however Triassic grains are sourced predominantly from mafic and I-type igneous rocks and low- and medium-grade metamorphic rocks. Samples from the Barents Shelf can be separated into three sand types: Lower Uralian, Upper Uralian and Uraloid. The Uraloid sand type has the highest proportion of grains <250 Ma alongside dominant age groups 250-380 Ma and 380-500 Ma, the grains have a varied composition corresponding to metamorphic and mafic igneous lithologies. Apatite grains found in samples classed as Lower Uralian are dominantly aged 250-380 Ma and have a composition consistent with mafic and I-type igneous rocks. Upper Uralian samples resemble the Lower Uralian pattern but have a higher number of younger (<250 Ma) grains and a greater proportion of grains which correspond to Low-medium-grade metamorphic, ultramafic and high-grade metamorphic source rocks.

The dissimilarity between the composition of apatite from the Russian Arctic foreland basin samples and Uraloid samples indicates that they have a different source. There is a similarity between Upper Uralian apatite composition and that of the Permian Pai-Khoi samples and the Upper Uralian samples. This is consistent with the basin inversion model; where inverted basins are a source for reworked Urals sediment which was subsequently delivered to the SW Barents Shelf. Reservoir properties are expected to be better for the reworked sands than those directly sourced from the orogen.