Sedimentological setting and biogeochemistry of "sandberries" in the Carboniferous Ebbadalen Formation (Spitsbergen, Norway) – A potential Mars analogue?

Distinct spherular diagenetic facies observed in the Carboniferous Ebbadalen Formation (Spitsbergen, Norway) are of interest with respect to understanding the processes creating similar morphologies (“blueberries”)... [ view full abstract ]

Distinct spherular diagenetic facies observed in the Carboniferous Ebbadalen Formation (Spitsbergen, Norway) are of interest with respect to understanding the processes creating similar morphologies (“blueberries”) observed on Mars by NASA’s Opportunity rover. Sandstone diagenesis requires time and chemical interaction between host rock minerals and/or fluids, with or without bacterial catalysis in transient temperature and pressure regimes. The spherules in Spitsbergen appear as stages of diagenetically altered patches, some which may indicate biogeochemical interaction. The Ebbadalen valley is an excellent Mars analogous site which has previously served as type locality to various NASA/ESA expeditions that tested instruments and mobile roving equipment for future life detection missions to Mars.

The studied Ebbaelva Member comprises a 22 m succession of siliciclastic mud, silt, and sandstones, interbedded with 10-20 cm beds of evaporites formed in a semi-arid to arid environment. Within layers of fluvial cross stratified sandstones, 0.5 – 3.0 cm diameter concretions stand out in positive relief, or occur as differently coloured circles within a matrix of light grey, gypsum-cemented quartzarenites. Fine grained, chlorite and glauconite rich sands, containing no gypsum or concretions overlie these fluvial units, and are interpreted to represent marine flooding events. Completely disintegrated and weathered sands comprise the uppermost section. Dolomite-rich patches may indicate bacterial activity in some parts. Diagenetically altered areas containing cemented (concretions) or un-cemented spherules, here termed sandberries, were observed in loose blocks scattered within the succession. These sandberries are inferred to represent an intermediate stage of rock disintegration, within solid and cemented host rock. The sandberries are unevenly distributed throughout the host rock irrespective of bounding surfaces. They appear as loose sand filling intact, spheroidal gypsum-encrusted cavities.

Ongoing mineralogical and biological analysis supports a working hypothesis that red patches in fluvial sandstone and slight red colouring inside the sandberries are remains of Fe-rich precursors. In an arid environment with high evaporation rates and a fluctuating groundwater table, redox conditions may vary. Initial concretions may have formed chemically as grain-enclosing precipitates. Subsequent dissolution and mass removal of pore-filling cement forming sandberry morphologies would allow for the observed inward growth of pure gypsum rinds. Preliminary results suggest that bacterial activity may have played a role in some of the progressive stages of sandstone cementation and subsequent disintegration of cement. However, the occurrence of concretions appears to be controlled by depositional rates, evaporation and groundwater flux in a Mars-similar chemical environment.