『Abstract
The Palaeoproterozoic Baraga Group (ca. 1850±1 Ma) of northern
Michigan is a 〜1200 m thick sedimentary succession of marine clastic,
iron formation, chert, and phosphatic sedimentary rocks that accumulated
at the peak of the world's first major phosphogenetic episode.
Lithofacies stacking patterns are interpreted to record the flooding
of the Nuna continental margin during two sea-level cycles. The
base of the first sequence is marked by a transgressive lag on
Archean basement that is transitional into a highstand accumulation
of sandstones deposited in peritidal environments. The bottom
of the second sequence is characterized by a chert and carbonate
unit with numerous subaerial exposure surface deposited in intertidal
to supratidal environments. This chert grades upward into subtidal
deposits composed of interbedded organic-rich mudstone and sandstone.
The highstand and falling stage systems tracts are composed of
progradational deltaic deposits. Sequences were framed using the
newly discovered Sudbury impact ejecta horizon as a datum. Its
emplacement approximately coincides with unrelated environmental
changes that increased the delivery of river-borne sediments to
the Nuna margin.
Iron bearing minerals ankerite, siderite, and pyrite show that
ferrous iron was present in peritidal to deep marine environments.
Ankerite dominates nearshore settings where pyrite is absent;
pyrite occurs in deeper, progradational deltaic deposits where
sulfide was produced by bacterial sulfate reduction. Phosphorite
is restricted to shallow-water sediments of the first sequence
and the lowstand of the second sequence. Precipitation is interpreted
to be the result of Fe-redox pumping just below the sediment-water
interface where photosynthetically-produced, nearshore oxygen
oases impinged on the seafloor. Such shallow-water phosphorite
accumulation is in stark contrast to many Phanerozoic depositional
systems in which phosphogenesis occurs across the shelf. This
difference likely reflects the dissimilarity in the oxygenation
state of the seafloor. In the Precambrian, Fe-redox pumping and
thus, phosphogenesis, was restricted to shallow-water settings
with a suboxic seafloor. In the Phanerozoic, phosphorite forms
in the full spectrum of shelf environments because the entire
seafloor is generally well oxygenated. The concentration of bioavailable
P in neritic environments during the Proterozoic may have played
a major role in the development of benthic microbial ecosystems
and evolving eukaryotes.
Keywords: Precambrian; Phosphorites; Michigan; Baraga Group; Sedimentology;
Oceanography』
1. Introduction
2. Geologic setting
3. Baraga Group stratigraphy
4. Methods
5. Depositional framework and sequence stratigraphy
5.1. Sequence 1
5.1.1. Transgressive systems tract
5.1.2. Highstand systems tract
5.1.3. Falling stage systems tract
5.2. Sequence 2
5.2.1. Lowstand systems tract
5.2.2. Transgressive systems tract
5.2.3. Highstand systems tract
6. Baraga Group phosphorite
6.1. Pristine phosphorite
6.2. Reworked phosphorite
7. Iron minerals and Animikie Basin paleoceanography
7.1. Ankerite and siderite
7.3. Pyrite
8. Phosphorite cycling, phosphogenesis and phosphorite
9. The Precambrian P cycle, Baraga Group, and the sulfidic ocean
model
10. Conclusions
Acknowledgements
References