CHOCOLAY GROUP
The Precambrian-aged
Chocolay Group in the Marquette area of Michigan's Upper Peninsula consists of
the following units, all metamorphosed:
---------------------------------------- unconformity
Wewe Slate
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Kona Dolomite
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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).
ENCHANTMENT LAKE
FORMATION
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.
MESNARD QUARTZITE
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)
KONA DOLOMITE
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.
WEWE SLATE
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.