11 P3: Proterozoic Paleogeography and Paleoclimate

Wednesday, 8 May 2002
8:00 AM - 12:00 PM

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

Paper number: 11-10
Program: SYMPOSIA SESSIONS
Time: 10:45 AM – 11:05 AM

Authors

Abstract

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.