Author Institution: Department of Geology, Wester Reserve University, Cleveland, Ohio and The Evanston Exploration Corporation, Evanston, Illinoi

Beckman, Walter A. Jr.

Cain, J. Allan

The Ohio Journal of Science

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No. 1 NEW TERRESTRIAL ISOPODS 57PRELIMINARY REPORT ON THE PRECAMBRIAN GEOLOGYOP THE ATHELSTANE AREA, NORTHEASTERN WISCONSIN1J. ALLAN CAIN AND WALTER A. BECKMAN, JR.Department of Geology, Western Reserve University, Cleveland, Ohio andThe Evanston Exploration Corporation, Evanston, IllinoisIn a recent issue of this Journal, the general geology of part of northeasternWisconsin was reviewed (Cain, 1963). It was pointed out that, while a large partof the Precambrian granitic and metamorphic complex has been differentiatedinto mappable units, an area of some 200 square miles in the central part of theregion remained unmapped (Cain, 1963, fig. 2).Field work completed during the summer of 1962 has provided preliminaryinformation on this area. A detailed description of the petrography, structuralpetrology, and petrogenesis of the rocks will not be completed until the authorsand Dr. J. J. Mancuso have synthesized the results of their several investigationsin northeastern Wisconsin. In the interim, it seems appropriate, in view of theprevious gap in basic mapping and the problem of correlation between the HighFalls Granite (Mancuso, 1960) and Amberg Granite (Cain, 1961), that the newresults be made available as a preliminary report.DESCRIPTION OF THE AREAFigure 1 indicates that outcrops are quite abundant but not uniformly dis-tributed throughout the area. The southeastern and northwestern parts, par-ticularly, are blanketed by thick glacial deposits so that only rarely is bedrockexposed.Only two major units are distinguished within the area: the Quinnesec Forma-tion and the younger Amberg Granite (fig. 1).Quinnesec FormationThis unit forms the northern limit of the Amberg mass, is the oldest unit recog-nized in northeastern Wisconsin, and is presumed to be equivalent to the WaupeeContribution No. 10, Department of Geology, Western Reserve University.THE OHIO JOURNAL OF SCIENCE 64(1): 57, January, 1964.58 J. ALLAN CAIN AND WALTER A. BECKMAN, JR. Vol. 64Volcanics described by Mancuso (1960). In addition to the main belt of QuinnesecFormation, small areas occur within the granite mass near Amberg and MiddleInlet.Rocks of the formation are typically "greenstones," including pyroclastics,sediments, and lava flows. The northern strip of Quinnescec Formation consistsof metabasalt at the extreme west, metarhyolite in the central part, and metabasaltwith local amphibole schist (presumably meta-sediments and pyroclastics) in theeastern part.In the northern exposures (fig. 1), rhyolite occurs closer to the granite thanany other rock type in the formation, but no actual contacts were observed. Therhyolite is strongly sheared, both macroscopically and microscopically, and pheno-FIGURE 1. Precambrian geology of the Athelstane area.crysts of quartz and feldspar show well-developed mortar structure. In addition,veinlets and p<prphyroblasts of potash feldspar are common close to the granite.It seems clear tjhat the rhyolite has been affected by the emplacement of the graniteand is older tljian the Amberg mass. The extent of this rhyolite unit and therocks immediately north of it were described by Wadsworth (1962).The small knass of Quinnesec Formation near Amberg consists mainly ofmetabasalt with a few outcrops of amphibole schist at the eastern margin (U. S.Highway 141).; In the northeastern part of this remnant, the greenstone showssharp, linear contacts with granite, and mineralogical changes are minor. North-west of Middle Inlet, at an abandoned molybdenum mine, a small wedge ofNo. 1 ATHELSTANE PRECAMBRIAN GEOLOGY 59schistose greenstone is present in the granite, but no other outcrops of QuinnesecFormation were found in that area.Apart from the very local rhyolite unit, the Quinnesec Formation in the presentarea is essentially as described by Prinz (1958) and Cain (1961) from exposuresto the north.Amberg GraniteAlthough this is mapped as one unit, figure 1 shows that varieties characterizedby grey feldspars and pink feldspars, respectively, are present in definite geo-graphical areas. Most of the Amberg mass is characterized by pink, or fleshcolored, Carlsbad-twinned potash feldspar, plagioclase, quartz, biotite, and horn-blende in hypidiomorphic-granular texture. Microcline-perthite is the typicalpotash feldspar and often contains subhedral plagioclase and biotite. Plagioclaseis sodic andesine which is rarely twinned and shows alteration to sericite flakes.Commonly, small plagioclase laths enclose microcline crystals in a rapakivi-liketexture. Quartz grains are usually surrounded by a fine-grained aggregate ofquartz suggestive of mortar structure. Biotite and hornblende are present asragged grains showing alteration to iron oxides.Typically, the grey variety is finer-grained and contains more mafic mineralsthan the pink one. Further, it commonly contains schlieren, small angularxenoliths of Quinnesec Formation, biotite gneiss and quartz diorite, and exhibitsgneissic structure. The gneissic rock is restricted to the margins of the granitemass and, farther from the contacts, is intruded by the pink variety. While muchof the gneissic grey granite is merely a border phase of the main Amberg mass, itis very likely that a more complicated picture will emerge after detailed study.For example, it is conceivable that one or both of the gneissic units mapped asMacauley Granite (Mancuso, 1960) and biotite gneiss (Cain, 1961) to the southand north, respectively, are present in this area. Also, several quarries nearAmberg are situated in a grey granite which has hypidiomorphic-granular texture.Buckley (1898: 142-155) described the granites from three large quarries atAmberg and one at Athelstane. He separated the granites into fine-grained greyand coarse-grained red types at Amberg, and a coarse-grained grey variety atAthelstane. We believe that pink seems a more appropriate term than red, whilethe "grey" granite at Athelstane is perhaps better described as flesh colored(Buckley, 1898, plate 20) and is included in the pink variety (fig. 1).The pink granite exhibits obvious variations in grain size, color, and biotite/hornblende ratio, and it is hoped that an areal variability study (now in progress)will provide significant information on the composition of this extensive unit. Anew isotope geochemistry laboratory at Western Reserve University (underDr. P. O. Banks) will, it is hoped, shortly provide the geochronologic informationessential to the elucidation of the geologic history of the area.CONCLUSIONSThe generally unaltered and undeformed appearance of the main Ambergmass suggests (in the absence of conclusive field relationships or radiometric dates)that the unit is younger than other granitic units to the north (Cain, 1961). Asyet, the relative ages of the several grey and pink varieties of Amberg Granitehave not been established completely. Since outcrops of pink granite can betraced from the High Falls reservoir to Amberg (fig. 1), it seems clear that theAmberg Granite and High Falls Granite are part of the same mass. Similar pinkrapakivi granites, in which mortar structure is apparently ubiquitous, have beenmapped by the authors along the Wolf River, 30 miles southwest of the presentarea, and, farther west near Wausau, by Weidman (1907) and others. Detailedpetrologic and geochronologic work will be necessary, however, before these60 J. ALLAN CAIN AND WALTER A. BECKMAN, JR. Vol. 64widely-separated exposures can be assigned to a "Wisconsin batholith" (Van Hiseand Leith, 1911: 344).REFERENCESBuckley, E. R. 1898. On the building and ornamental stones of Wisconsin. Wis. Geol. andNat. Hist. Surv. Bull. 4. 544 p.Cain, J. A. 1961. Geology of the Pembine area, Marinette Co., Wisconsin. Wis. Geol. andNat. Hist. Surv. open-file rept. 105 p.. 1963. Some problems of the Precambrian geology of northeastern Wisconsin: A review.Ohio J. Sci. 63(1): 7-14.Mancuso, J. J. 1960. The stratigraphy and structure of the McCaslin District, Wisconsin.Ph.D. Thesis, Michigan State University.Prinz, W. 1958. The geology of part of the Menominee District, Michigan and Wisconsin.Ph.D. Thesis, Yale University.Van Hise, C. R., and C. K. Leith. 1911. The geology of the Lake Superior Region. U. S.Geol. Survey Mon. 52. 641 p.Wadsworth, W. B. 1962. Petrogenesis of a quartz diorite pluton near Pembine, Wisconsin.M. S. Thesis, Northwestern University.Weidman, S. 1907. The geology of north central Wisconsin. Wis. Geol. Nat. Hist. Surv.Bull. 15(4). 697 p.

Preliminary Report on the Precambrian Geology of the Athelstane Area, Northeastern Wisconsin

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Preliminary Report on the Precambrian Geology of the Athelstane Area, Northeastern Wisconsin

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