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


              Siamo Slate


          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.





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.





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.





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.



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