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Tidal deposits in the early Proterozoic basin of the Lake Superior region—The Palms and the Pokegama Formations: Evidence for subtidal-shelf deposition of Superior-type banded iron-formation

Richard W. Ojakangas
Richard W. Ojakangas
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January 01, 1983

The Palms Formation in Wisconsin and Michigan and the correlative Pokegama Quartzite in Minnesota are interpreted as tidal deposits formed along the margins of the early Proterozoic Animikie basin.

The well-exposed Palms Formation, which extends for 85 km along the Gogebic range, is 150 m thick and can be divided into three units on the basis of rock types and bedding styles: (1) a thin lower unit of thin-bedded argillaceous rocks that unconformably overlies a low-relief surface of Archean granite and greenstone and older Proterozoic sedimentary units; (2) a thick middle unit of thin alternating beds of argillite, siltstone, and sandstone that vary considerably in texture and composition. Bedding types include parallel, wavy, cross, and flaser lamination, and a variety of sedimentary structures are present; (3) an upper unit of thicker beds of parallel and cross-bedded sandstone.

A total of 199 cross-bedding measurements and 52 measurements of other paleocurrent indicators from the Palms Formation yields a crude bimodal-bipolar distribution, with a broad primary mode to the west and a weaker mode to the east. The sandstones are mineralogically mature; most of the framework grains are rounded quartz grains. The sandstones of the middle unit have an abundant sericitic matrix, whereas those of the upper unit are texturally more mature.

The Pokegama Quartzite is exposed at only a few places along the 130 km long Mesabi range near the northwestern margin of the Animikie basin. However, the entire formation can be viewed in two drill cores in which it is 50 m and 26 m thick. Sedimentary sequences and the mineralogical attributes are similar to the Palms. The paleocurrent plot (N = 38) is crudely bimodal-bipolar with primary modes to the north and south.

By utilization of Walther’s Law of Succession of Facies and comparisons with modern environments, it is postulable that both formations were deposited under transgressive tidal conditions. In this model, the lower (shaly) facies was deposited in a low-energy domain of the upper (shoreward) tidal flat; the middle facies (shale-siltstone-sandstone) was deposited on a middle tidal flat under alternating low- and high-energy conditions; and the upper facies (sandstone) was deposited in a lower tidal flat or subtidal high-energy environment.

The Palms and Pokegama formations pass upward into the Ironwood and Biwabik Iron Formations, respectively. Again using Walther’s Law, it can be postulated that the iron-formations were deposited on a shelf located seaward from the subtidal sandstone facies. The “cherty” (coarser-grained, thicker-bedded, iron oxide-chert) facies was deposited in shallower water than was the “slaty” (finer-grained, thinner-bedded, iron silicate-iron carbonate) facies.

The tidal-subtidal facies model developed here provides an independent approach in evaluating the environment of deposition of one kind of Superior-type banded iron-formation. The model is primarily based upon the siliciclastic lithologies associated with iron-formation rather than upon the iron-formation itself.

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GSA Memoirs

Early Proterozoic Geology of the Great Lakes Region

L. G. Medaris, Jr.
L. G. Medaris, Jr.
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Geological Society of America
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Publication date:
January 01, 1983




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