Plate tectonics - Precambrian supracrustal sequences

Kent Condie (who worked on Precambrian tectonics extensively) identified 4 types of these:

There is nothing distinctively Precambrian about these since Paleozoic platform cover strata (supracrustals) are very common. However, they are significant in that they mark the time of stable craton initiation. Could some of these also be passive margin sequences?

Examples?

Baraboo, Sioux and other Lake Superior quartzites:

View of Baraboo quartzites exposed at Devils State Park in Wisconsin. The cliffs in the foreground and background are comprised mostly of one mineral quartz. Quartz arenites (sandstones made out of mostly quartz) occur in the Phanerozoic, but not in such thick sequences. Was there something different about Precambrian sedimentary systems that allowed such substantial volumes of quartz arenite to be produced?

Towards the top of the quartzites in the Baraboo region there are also pyrophyllitic phyllites that are interbedded (and which because of their more incompetent nature show a host of very interesting outcrop scale structural features such as the cleavage shown here). Pyrophyllite is a very aluminous mineral, which suggests someting about the composition of the mud protolith for these layers, which in turn may indicate that weathering was severe.

Snowy Mountain Precambrian quartzites: these are 2.0 to 2.4 billion years old quartz arenites.

These cliffs that comprise the Snowy Range are all comprised of tilted whitish to light grey quartzites,

Huronian sequence:


This is a striated flat outcrop of the Gowganda tillite. Note its massive character, the angular clasts, and the poorly sorted character. Many of the clasts are granitic, derived from the underlying basement rocks. In this photo then, there is evidence of glaciation from over 2 billion years and then 10,000 years ago. Recent work suggests this was a global event.


This is a image of dropstones in finer grained silts and muds associated with the Gowganda tillites within the Huronian sequence in Canada. The interpretation is that these were dropped from icebergs floating in lakes or seas.


This is a photo of a clean quartz arenite (sandstone dominated by quartz) that occurs in the upper part of the Huronian sequence. Such quartz rich sands are usually thought to have formed from polycyclic erosion-deposition on a stable craton platform (such as some of the interior Cambrian sands of Wisconsin).



Course materials for Plate Tectonics, GEOL 3700, University of Nebraska at Omaha. Instructor: H. D. Maher Jr., copyright. This material may be used for non-profit educational purposes with appropriate attribution of authorship. Otherwise please contact author.