Geologic units in Marathon county, Wisconsin

Wolf River batholith; Wolf River Granite (Middle Proterozoic) at surface, covers 12 % of this area

Red, coarse-grained rapakivi granite consisting of large (1-3 cm) ovoid alkali feldspar sporadically mantled by plagioclase, interstitial plagioclase, quartz, biotite, hornblende, and ilmenite

Alkali feldspar granite of 1835 Ma age group (Early Proterozoic) at surface, covers 12 % of this area

Red to pink, medium-grained leucocratic alkali feldspar granite; contains miarolitic cavities; biotite is altered to opaque oxide minerals; contains fluorite locally. Called red granite previously (Sims, 1990). Exposed in both Pembine-Wausau and Marshfield terranes.

Granodiorite-tonalite (Early Proterozoic) at surface, covers 12 % of this area

Gray, medium-grained intrusive rocks, including intrusion breccias. Exposed in central Wisconsin. Zircon ages range from 1837 to 1847 Ma

Felsic metavolcanic rocks of 1835- to 1845-Ma age group (Early Proterozoic) at surface, covers 10 % of this area

Rhyolite to dacite and, locally, andesite tuff, breccia, and minor sedimentary rocks, including conglomerate. Exposed in central Wisconsin.

Mafic metavolcanic rocks (Early Proterozoic) at surface, covers 9 % of this area

Dominantly tholeiitic basalt and basaltic andesite flows and tuffs; associated with sheet dikes, massive and layered metagabbro, and ultramafic rocks. In northeastern Wisconsin, rocks have been named the Quinnesec Formation.

Gneiss and amphibolite (Early Proterozoic) at surface, covers 6 % of this area

Interlayered quartzofeldpathic gneiss and amphibolite between Athens shear zone and Owen fault, central Wisconsin

Porphyritic granite (Early Proterozoic) at surface, covers 5 % of this area

Granite containing plagioclase, microcline, and quartz phenocrysts in a fine-grained granophyric matrix. Probably subvolcanic intrusions related to volcanic rocks of 1835- to 1845-Ma age group

Wausau Pluton; Granite near Nine Mile Swamp (Middle Proterozoic) at surface, covers 5 % of this area

Orange to pink, coarse-grained massive granite composed of alkali feldspar sporadically mantled by plagioclase, quartz, and biotite; contains miarolitic cavities

Fault rocks (Archean to Early Proterozoic) at surface, covers 4 % of this area

Mylonitic rocks of uncertain lithology in Eau Pleine shear zone

Wausau Pluton; Quartz syenite (Middle Proterozoic) at surface, covers 4 % of this area

Contains flow-oriented xenoliths of biotite schist and sillimanite-bearing quartzite

Granite near Cherokee (Early Proterozoic) at surface, covers 3 % of this area

Pink, coarse-grained granite consisting of microcline microperthite and oligoclase phenocrysts in a fine-grained matrix of quartz, potassium feldspar, and oligoclase. Hornblende and (or) biotite occur in clots

Wolf River batholith; Red River Granite (Middle Proterozoic) at surface, covers 3 % of this area

Contains alkali feldspar phenocrysts (0.5-2 cm), subordinate plagioclase and quartz, and rare clusters of biotite with or without hornblende

Cambrian, undivided (Cambrian) at surface, covers 3 % of this area

Sandstone with some dolomite and shale, undivided; includes Trempealeau, Tunnel City, and Elk Mound Formations

Wolf River Batholith; Anorthosite (Middle Proterozoic) at surface, covers 2 % of this area

Gray, coarse-grained (1-20 cm) plagioclase-rich (An45-53) rock containing interstitial orthopyroxene, clinopyroxene, Fe-Ti oxides, and apatite.

Milladore Volcanic Complex (Early Proterozoic) at surface, covers 2 % of this area

Succession of poorly exposed interlayered metavolcanic and metasedimentary rocks. Lithic units described in Sims (1990)

Gabbro (Early Proterozoic) at surface, covers 2 % of this area

Massive to layered intrusive gabbroic rocks; weakly metamorphosed and deformed. Exposed in both Pembine-Wausau and Marshfield terranes

Foliated tonalite (Early Proterozoic) at surface, covers 1 % of this area

Medium-gray, mottled, medium-grained, equigranular tonalite and granodiorite. Intrudes rocks of the Milladore Volcanic Complex.

Wausau Pluton; Granite near Big Eau Pleine Reservoir (Middle Proterozoic) at surface, covers 1 % of this area

Pink to red, medium- to fine-grained granite containing miarolitic cavities

Stettin pluton (Middle Proterozoic) at surface, covers 1 % of this area

Pyroxene syenite, amphibole syenite, tabular syenite, and nepheline syenite. Age approximately 1520 Ma (Sood and others, 1980).

Rhyolite at and near Cary Mound and near Brokaw (Early Proterozoic) at surface, covers 1 % of this area

Flow-banded rhyolite, welded tuff, volcanic conglomerate, and volcanogenic sedimentary rocks. Exposed in both Pembine-Wausau and Marshfield terranes.

Wausau Pluton; Aplite (Middle Proterozoic) at surface, covers 0.9 % of this area

Pale-orange to pink, fine-grained porphyritic aplite

Granite gneiss (Early Proterozoic) at surface, covers 0.8 % of this area

Medium-grained granodiorite gneiss in northwestern Marathon County.

Granitic rocks, undivided (Early Proterozoic) at surface, covers < 0.1 % of this area

Gray, weakly foliated to massive granite in poorly exposed areas.

Mafic to felsic metavolcanic rocks (Early Proterozoic) at surface, covers < 0.1 % of this area

Volcanic rocks in areas of sparse geologic data and nondefinitive magnetic and gravity data