SEMIDIURNAL RHYTHMITES, INTERTIDAL MICROBIAL MAT DETRITUS, PYRITE BEDS AND THICK FLUID-MUD LAYERS: DYNAMIC SUBTIDAL NEARSHORE DEPOSITION OF IRON SULFIDE-RICH CARBONACEOUS MUDSTONES–SILTSTONES OF THE PALEOPROTEROZOIC TALVIVAARA FORMATION (2.1–2.0 GA), FINLAND

The Paleoproterozoic Talvivaara (TV) fm (2.1–2.0 Ga) in Sotkamo, Finland, contains metalliferous C-S-Fe –rich sedimentary units, locally metamorphically updated to a mineable Ni-Zn-Cu-Co –ore (2053 Mt; Ni 4.5 Mt, Zn 10.3 Mt). The prevailing genetic model includes hydrothermally enriched rift-bottom seawater in a stratified restricted basin and metal retention in sulfidic “black shales”, plus additional hydrothermal percolation. However, new observations from drill core DDKS-010 (Kolmisoppi) support tidally resedimented origin for the TV fm. Low-deformation section 476.40–390 m has the best-preserved original stratigraphy of TV fm known so far, showing the gradation from underlying Hakonen (HK) fm to TV fm. Tightly folded TV fm is in fault contact at 168 m with the overthrusted Kuikkalampi (KU) fm.

Gradation from the detrital biotite-rich subarkosic muddy silts–fine sands of HK fm to TV fm is transgressive; Ni-Zn-Cu levels follow the increase of black, clayey sapropel mud component. The transitional TV fm up to 454 m is dominated by variably rhythmic packages of horizontally laminated to bedded gray silt–sand / black mud (graded to coupleted). Semidiurnal paired ordinate–subordinate laminae and decrementally fining spring–neap ebb-tide laminasets are prominent. A 20-cm-thick tidal rhythmite displaying full fortnight-cycle occurs at 456 m, and thin section 455.55 still retains plasmic fabric. Sample 446.90 typifies a thick homogenous fluid-mud layer.

Rhythmic-laminated sandy packages continue to interlude the TV muds, appearing to record periodicities from monthly to pluriannual lunar cycles and seasonal changes in paleotidal range. In 443.19, coarse quartz-sand laminae contain “soft” mud-clots and several types of organic detritus; rounded POC-grains resemble pure aggregates of coccoidal cyanobacteria cells, and flakey microbial mat fragments vary from silt grain-binding to non-binding, all likely sourced from nearby intertidal flat. Traits of daily to seasonal variations appear also in massive to laminated–bedded pyrite-rich muds in stratigraphically higher part of TV fm (412–390 m). Thinly- to microlaminated, partly biomat-like regular pyritic shales and black shales (s.s.) of lithologically distinctive KU fm may represent coeval distal deep-water facies, or later sedimentation.