THE IRON FORMATION TO PHOSPHORITE OCEANOGRAPHIC TRANSITION: A DIACHRONOUS EVENT ALONG THE NUNA CONTINENTAL MARGIN AS RECORDED IN THE ~1.8 BILLION YEAR OLD BARAGA GROUP, MICHIGAN, AND FERRIMAN GROUP, LABRADOR

The Paleoproterozoic Baraga and Ferriman groups in northern Michigan and Labrador, respectively, are successions of marine clastic, iron formation and phosphatic sedimentary rocks that accumulated along the Nuna continental margin during a major ocean restructuring event at ~1.84 Ga. This event is interpreted to have been driven by the initial oxygenation of the atmosphere between 2.3 and 2.0 Ga and marks a dramatic change in oceanographic conditions that lasted for more than a billion years. It records a change in seawater chemistry that terminated global iron formation deposition and led to the first occurrence of widespread phosphorite accumulation in Earth history.

The well preserved, relatively undeformed character of the Baraga and Ferriman groups provide a unique opportunity to investigate the nature of Paleoproterozoic ocean reorganization and its influence on the accumulation of iron formation and phosphorite in a sequence stratigraphic context. Major sequence boundaries and a volcaniclastic unit in the Baraga Group interpreted as an ejecta layer associated with the Sudbury impact event at ~1.85 Ga provide key stratigraphic markers. Relatively shallow marine to deltaic organic matter-rich facies dominate the Baraga Group and include a phosphate-rich interval marked by multiple silicified, iron-carbonate subaerial exposure surfaces. In the Labrador Trough, phosphatic sediments are interpreted to have also accumulated within organic-rich nearshore environments that became episodically emergent during minor fluctuations in relative sea level. Such shallow water phosphorite accumulation contrasts many Phanerozoic depositional systems where phosphatic sediments are interpreted to have formed in an array of middle and distal shelf environments. Ongoing lithogeochemical analysis of the Sudbury ejecta layer seeks to provide a “fingerprint” for use as a regional chronostratigraphic marker in other Paleoproterozoic sedimentary successions, thus providing a basis on which to understand the global significance and timing of Paleoproterozoic ocean reorganization.