Geology of a Lower Proterozoic Volcaniclastic Sequence near Wausau, Marathon County, Wisconsin

Abstract

A sequence of Lower Proterozoic (1859 m.y.) volcanic and volcaniclastic rocks is exposed in the vicinity of Brokaw, north of Wausau, in Marathon County, Wisconsin. These rocks were mapped in detail and sampled for petrographic study. This sequence has been metamorphosed to lower greenschist facies and is only mildly deformed, dipping to the west at 10-30°. Based on rock type, the study area can be divided into three segments. The easternmost and westernmost segments are poorly exposed; they consist mainly of intermediate to felsic lava flows and pyroclastic rocks. Basaltic rocks are only minor components of these two segments. The central segment contains the best exposures, and consists mainly of sedimentary and pyroclastic rocks. The stratigraphy of this segment, from oldest to youngest, consists of red sandstones and pebble conglomerates, thinly bedded siltstones and tuffs, pyroclastic deposits and greenish-black sandstones and conglomerates. The greenish-black and red sandstones and conglomerates are interpreted as fluvial sediments deposited by a braided river system. Paleocurrent study of the greenish-black sandstones indicates the sediment source was to the E-SE. The red sandstones are composed mainly of felsic volcanic rock fragments and quartz with only small amounts of plagioclase; plutonic rock fragments are very minor components. The greenish-black sandstones contain more felsic to intermediate volcanic rock fragments and roughly equal amounts of quartz and plagioclase; plutonic rock fragments and quartzite grains represent 1-3% of these sediments. The modes of the two sandstone units, when plotted on QFL and QmFLt diagrams (after Dickinson and Suczek, 1979) indicate the sediment was derived from a magmatic arc-type setting. An increase in the percentage of plutonic and quartzite grains in the greenish-black sandstones, compared to the red sandstones, suggests an increased depth of erosion and the input of material from outside the magmatic arc. The thinly bedded siltstones and tuffs represent lacustrine sediments associated with the fluvial system. Three pyroclastic units are present in the southern half of the central segment, and are apparently unconformably overlain by the greenish-black sandstones. These pyroclastic units overlie the thinly bedded siltstone and tuff unit and include; a unit varying from lithic-rich at its base to crystal-rich near its top, a block-and-ash flow, and a welded dacite tuff. The association of fluvial and lacustrine deposits and the dominance of ash and lapilli tuffs, with only minor pyroclastic breccias, suggests this sequence represents an intermediate-source facies (Fisher and Schmincke, 1984) within the volcanic field. The intermediate-source facias is comparable to the dispersal facies of Dickinson (1974) and is found at distances > 5 km from the central vent. The depositional setting for this sequence of rocks is interpreted as a small restricted basin within a continental margin magmatic arc (intra-arc basin). These rocks were deposited following the main phase of deformation associated with the Penokean Orogeny.