INVESTIGATING A STURTIAN CAP-CARBONATE SEQUENCE, SCOUT MOUNTAIN MEMBER, POCATELLO FORMATION, IDAHO

Cap carbonates and greater cap sequences have implications for understanding the Earth system's recovery from low-latitude glaciations. Most detailed data sets are from Marinoan cap sequences; few detailed studies have focused on the Sturtian cap record. The Scout Mountain Member of the Pocatello Fm. is an exceptional unit to study a Sturtian cap sequence; it is well exposed along strike for >50 km and geochronologically bounded by absolute ages. The diamictite and cap carbonate have maximum depositional ages between 689-675 Ma (Dehler and Link, 2009), and can be no younger than a 667+/-5 Ma tuff ~70 m upsection (Fanning and Link, 2004).

The Scout Mountain Member contains a mixed siliciclastic-dolomite sequence (~60 m thick) that sits sharply on the underlying upper glacial diamictite. The lower 14 m of the sequence shows m-scale vertical alternations (m-scale) and abrupt lateral changes (m-100 m) between three main facies. The cap-dolostone facies is a tan-pink, thinly bedded, planar to low-angle laminated peloidal dolomicrite; C-isotope values (n=3) are ~-3 per mil (Smith et al., 1994). It becomes rhythmically bedded with quartz-rich rippled and HCS sandstone upsection. The dolomite-chip breccia facies is clast-supported with a sandstone matrix and is thickly bedded in complex geometries. Clasts are typically tabular (<1 m in length), composed of the cap-dolostone facies, show syndepositional deformation, and exhibit bedding-parallel and edgewise fabrics; some clasts form breccia hummocks. The sandstone facies is arkosic, medium to thickly bedded, massive to trough crossbedded, and contains load, flute, and groove casts, dish structures, parting lineations, and dolomite intraclasts (<3 cm dia.). The rest of the sequence is dominated by the sandstone facies and rare dolomite breccia, and is succeeded by dark argillite and carbonate of the overlying sequence.

Facies associations suggests sub-wave base (TST) to shoreface (HST) deposition on a mixed siliciclastic-carbonate shelf, with the cap-associated facies at the base recording punctuated intense storm flows. These storm deposits are in about the same stratigraphic level and frequency (n=1) as the storm-generated giant wave ripples found in Marinoan cap carbonates. Overall, this ~680 Ma Sturtian cap sequence is akin to Marinoan cap sequences.