First record of carbonates with spherulites and cone-in-cone structures from the Precambrian of Arctic Norway, and their palaeoenvironmental significance

Abstract

We report for the first time carbonates from the upper Ediacaran sedimentary succession of Finnmark, Arctic Norway. Carbonates occur as calcareous siliciclastic beds, lenses, and concretions, some with calcite spherulites and cone-in-cone (CIC) calcite, in a mudrock to fine-grained sandstone succession from approximately 3 m to 26 m above the base of the 2 nd cycle of the Manndrapselva Member of the Stáhpogieddi Formation (Vestertana Group). They occur c. 40 m below the Ediacaran-Cambrian boundary, which is well defined by trace fossils. Thin-section petrography and scanning micro X-ray fluorescence elemental mapping reveal a layered composition of the calcareous sedimentary rocks. In some of those, welldeveloped nested cones of CIC calcite form the outer layer. Thin clay coatings outline individual cones. The inner layers are composed of (1) carbonate with calcite spherulites (grainstone) and (2) thinly laminated fine-grained calcareous siliciclastics (mudstone and wackestone) indicated by elevated concentrations of Al, Si, Fe, and Ti. The inner siliciclastic layers contain framboidal pyrite and probably organic matter. Formation of calcite spherulites took place probably at the sediment-water interface either in a coastal littoral environment or in situ in the sublittoral zone under high alkaline conditions whereas CIC calcite formed during burial diagenesis and clearly in pre-Caledonian time before metamorphism and cleavage formation. This new record of carbonates with calcite spherulites and CIC structures from the Ediacaran of Arctic Norway adds to their rare occurrences in the geological record.

U–Pb dating of calcite in ancient carbonates for age estimates of syn- to post-depositional processes: a case study from the upper Ediacaran strata of Finnmark, Arctic Norway

“…The calcite spherulites are interpreted as a primary feature, forming in the sedimentary environment (Meinhold et al 2019a). Within the large uncertainty (563 ± 70 Ma), the obtained age overlaps with the estimated timing of sedimentation (late Ediacaran; c. 545 Ma) based on body and trace fossil assemblages (Högström et al 2013;McIlroy & Brasier, 2017;Jensen et al 2018b) (Fig.…”

“…The age of the undeformed CIC calcite is interpreted as the age of CIC calcite formation during a period of fluid overpressure as the sediments were buried. The age fits well with age estimates based on the required overburden to obtain the fluid overpressure needed to form CIC structures (see discussion in Meinhold et al 2019a). In the case of the undeformed CIC calcite, the robust isochron (in terms of MSWD) implies that the isotopic system has remained a closed system (no loss or gain of U or Pb).…”

“…The maximum metamorphic overprint is given as low epizonal and reached around 300°C (see Meinhold et al 2019b). If we assume that the CIC calcite from the top part of the concretion was originally undeformed and formed contemporaneously with the CIC calcite from the bottom part of the concretion (see Meinhold et al 2019a), the data suggest that the original age of CIC calcite from the top part of the concretion has been reset during the Caledonian metamorphic overprint. Volume diffusion of Pb at temperatures of 250-300°C is a possibility, based on the experimental study of Pb diffusion in calcite (Cherniak, 1997), but mobility may also have been enhanced by grain deformation.…”

“…Högström et al 2013;Jensen et al 2018a, b). The upper Ediacaran succession comprises siliciclastic sedimentary rocks with recently described carbonates, some with calcite spherulites and cone-in-cone (CIC) calcite (Meinhold et al 2019a). Although the age of the carbonate-bearing part of the succession is well established as late Ediacaran based on fossils, the timing of the formation of the various types of calcite is poorly constrained.…”

“…1b). Meinhold et al (2019a) described carbonates within the second cycle of the Manndrapselva Member, which otherwise largely comprises alternating thin layers of silt-and mudstone and minor sandstone. Some of the sandstone beds show wave-formed ripple marks.…”