The Precambrian-aged Chocolay Group in the Marquette area of Michigan's Upper Peninsula consists of the following units, all metamorphosed:


---------------------------------------- unconformity

              Wewe Slate


            Kona Dolomite


        Mesnard Quartzite

---------------------------------------- paleosol?

Enchantment Lake Formation

---------------------------------------- unconformity


The Chocolay is the lowest of three groups in the Paleoproterozoic-aged Marquette Range Supergroup.  It is overlain by quartzites, slates, and iron formation of the Menominee Group & is underlain by Neoarchean basement rocks.  Isotopic dating has constrained the age of the Chocolay Group to between 2.20 and 2.29 billion years (mid-Paleoproterozoic).





The Enchantment Lake Formation occurs at the base of the Chocolay Group.  It consists of a variety of lithologies, including glaciogenic conglomerates, slates, arkoses, and quartzites.  Regional correlations indicate that it is equivalent to the Gowganda tillites of Ontario.  The glacial conglomerates of the Gowganda & Enchantment Lake are classic evidence for a regional or even widespread Paleoproterozoic Ice Age.





The Mesnard Quartzite overlies the Enchantment Lake, possibly separated from it by an unconformity.  The Mesnard is dominated by quartzites (metaquartzites) and is considered to represent shallow-water marine deposition.

Here are a couple samples of Mesnard from a roadcut along Rt. 41 south of Marquette, UP of Michigan, USA (see map).


Mesnard Quartzite (8.6 cm across at its widest)


Mesnard Quartzite (5.6 cm across at its widest)





Above the Mesnard Quartzite is the Kona Dolomite, a pretty well-known unit in the Marquette, Michigan area.  The unit was named after the Kona Hills to the southwest of Marquette.  The color variation and susceptibility to polish makes Kona rocks quite popular with rockhounds and lapidarists.  Published studies on the Kona indicate that it is moderately heterolithic - common lithologies include dolostone, silicified dolostone, stromatolitic dolostone, quartzite, argillite, and slaty dolostone.  The Kona has been tilted and metamorphosed during multiple Precambrian orogenic events - its rocks are better termed metadolostones.


Stratigraphy: Kona Dolostone, Chocolay Group, lower Marquette Range Supergroup, Paleoproterozoic, ~2.20-2.29 billion years.


Kona Dolomite outcrop - roadcut on northern side of Co. Rd. 480, Ragged Hills, a little west of gravel pits, SW of Marquette, UP of Michigan, USA (see map).



Kona Dolomite outcrop with steeply tilted bedding - roadcut along Co. Rd. 553 at hilltop just northeast of Pelissier Lake, south of Marquette, UP of Michigan, USA (see map).


Kona Dolomite at Co. Rd. 480 outcrop.  Lens cap for scale.


Kona Dolomite - steeply tilted & faulted beds of slaty metadolostone (lens cap for scale).  Marquette South outcrop - roadcut on the southwestern side of Rt. 28/Rt. 41, S of Marquette, UP of Michigan, USA (see map).



Purple quartzite (6.8 cm across at its widest) from the Kona Dolomite, Marquette South outcrop.


Kona Dolomite - stromatolitic metadolostone with “cryptalgal laminations” at Marquette South outcrop.  Guidebook for scale.  These stromatolites are layered structures built up by extensive mats of cyanobacteria.


Stromatolitic metadolostone (lapidary grade, cut & polished slice; 15.2 cm across) from the Kona Dolomite.  Specimen owned by Tabatha Beaver & Matthew Hartman.


Kona Dolomite - nice stromatolite mound in silicified metadolostone at Co. Rd. 480 outcrop.


Gypsum Casts (small dark slits) in metadolostone at Co. Rd. 480 outcrop.  These gypsum casts preserve very well the original monoclinic crystal shapes (see closeups below).  Gypsum only crystallizes in dolomite muds if the sediments were part of an evaporitic supratidal setting.  The original gypsum has long since dissolved away.


Gypsum Casts in metadolostone (Kona Dolomite, Paleoproterozoic, 2.20-2.29 billion years).  Rock is 6.8 cm across.


Gypsum Casts with well preserved monoclinic crystal shapes in metadolostone (Kona Dolomite, Paleoproterozoic, 2.20-2.29 billion years).  Field of view ~2.2 cm across.



Based on the dolostone lithology, the common stromatolitic bedding, and the presence of gypsum casts, it's clear that much of the Kona Dolomite was deposited in shallow-water marine to intertidal to supratidal environments.





The Wewe Slate sits at the top of the Chocolay Group and appears to represent the deepest water portion of the Mesnard-Kona-Wewe transgressive succession.


Wewe Slate sample (7.4 cm across).  The original mudshale's horizontal bedding is well-preserved.  The front surface here is a slaty cleavage plane (foliation plane).  Locality: unpaved road near gravel pits in the Ragged Hills, due W of Pelissier Lake, SW of Marquette, UP of Michigan, USA (see map).



Some info. from:


Gair & Thaden (1968) - Geology of the Marquette and Sands Quadrangles, Marquette County, Michigan.  U.S. Geological Survey Professional Paper 397.  77 pp.  7 fold-out pls.  1 fold-out table.


Cambray (2004) - The Evolution of a Paleoproterozoic Plate Margin, Northern Michigan.  Michigan State University.  45 pp.



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