ANOTHER POSSIBLE CAP CARBONATE IN THE NEOPROTEROZOIC POCATELLO FORMATION, SOUTHEASTERN IDAHO

The Neoproterozoic Pocatello Formation includes, from base to top, the Bannock Volcanic Member, the Scout Mountain Member, and the Upper Member. The Scout Mountain Member contains iron-rich turbidites, cobble conglomerate, and diamictite units capped by a thin, finely laminated dolostone unit that is overlain by ~100 meters of upward fining siliciclastic sediments. The thin dolostone records d¹³C values –3.1 to –5.0‰ PDB (Smith et al, 1994). The co-occurrence of diamictite capped by finely laminated, d13C depleted dolostone is common in Neoproterozoic successions worldwide and has been related to low-latitude glacial episodes (“snowball Earth”).

The southeastern Idaho succession is traditionally interpreted to contain two glacial episodes. The Pocatello Formation is interpreted to record a single episode of “Sturtian” glaciation (ca. 750-700 Ma) (Link, 1987; Christie-Blick et al, 1988). Incised valleys of the overlying Caddy Canyon Quartzite (~2000 meters above) are interpreted to represent sea-level drawdown associated with “Marinoan” glaciation (ca. 600 Ma).

A ~26 meter thick, finely laminated carbonate unit caps the Scout Mountain Member. It contains cm-scale crystal fans that appear to have nucleated on the seafloor and records negative carbon isotopic values between –2.9 and –6.9‰ V-PDB. Many cap carbonates in other Neoproterozoic glacial successions contain similar unusual seafloor precipitates. The unit may represent another (second Sturtian?) cap carbonate (or cap-like carbonate) in the absence of a demonstrably glaciogenic underlying unit. Thus, the Neoproterozoic glacial history and stratigraphy of the Pocatello area may be more complex than previously thought.