MENOMINEE GROUP
The Menominee Group
unconformably overlies the 2.20-2.29 billion year Chocolay Group and is
unconformably overlain by metamorphosed siliciclastics of the Baraga
Group. The Menominee succession in the Marquette-Negaunee-Ishpeming area
of Michigan principally consists of conglomerates, quartzites, slates, and
various lithologies of iron formation. The units are:
---------------------------------------- unconformity
Negaunee Iron-Formation
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Siamo Slate
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Ajibik Quartzite
---------------------------------------- unconformity
Published ages of Menominee
rocks vary quite a bit. Traditionally, the upper Menominee (the Negaunee
Iron-Formation) is dated to 2.11 billion years. Recent isotopic dating of
inferred correlatives of upper Menominee rocks suggests that the group is only
about 1.8 to 1.9 billion years.
AJIBIK QUARTZITE
The Ajibik Quartzite
is the lower unit of the Menominee Group. Its lower contact is an
unconformity. The dominant Ajibik lithologies include quartzite
(metaquartzite), some arkosic quartzite, and some metagraywackes. The
basal portions of the Ajibik are conglomeratic (with clasts of slate,
quartzite, chert, & vein quartz), and have been referred to as the Seaman
Conglomerate. The original sandstones of the Ajibik were very likely
shallow-water marine.
Ajibik Quartzite - outcrop along southern
side of Co. Rd. 480, just E of Mud Lake, western side of the Ragged Hills, SW
of Marquette, UP of Michigan, USA (see
map).
Ajibik Quartzite (4.5 cm across at its
widest) - nice white quartzite (metaquartzite) from Co. Rd. 480 outcrop shown
above. The original quartz sand grains are very well defined on freshly
broken surfaces (not so well defined on weathered surfaces).
Ajibik Quartzite - here we've got a metagraywacke
(4.4 cm across at its widest) from the Co. Rd. 480 outcrop. The original
sand grains & clastic texture of this rock are still readily visible.
The grayish component is mud & clay. "Graywacke" is a term
often loosely used to refer to dirty sandstone (with lots of mud & clay in
the matrix). A metamorphosed graywacke is thus a metagraywacke.
SIAMO SLATE
Overlying the Ajibik
Quartzite is the Siamo Slate. It consists of fine-grained siliciclastics
and interbedded fine-grained & coarser-grained siliciclastics.
Geologists have interpreted Siamo rocks as turbidites - underwater
sediment slide deposits in a moderately deep, low-energy setting.
Siamo Slate - outcrop along northern
side of Co. Rd. 480, just WNW of Mud Lake, N of the Kona Hills, SW of
Marquette, UP of Michigan, USA (see
map).
Siamo Slate (9.6 cm across) with
well-preserved horizontal bedding at high angle to slaty cleavage. From
Co. Rd. 480 outcrop shown above.
NEGAUNEE
IRON-FORMATION
Jasper Knob (9 pics)
Champion Specularites (3
pics)
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.