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The AAPG/Datapages Combined Publications Database

AAPG Bulletin


Volume: 63 (1979)

Issue: 3. (March)

First Page: 486

Last Page: 486

Title: Chocolay Group--Early Proterozoic Cratonic Sequence: ABSTRACT

Author(s): David K. Larue

Article Type: Meeting abstract


The Chocolay Group of the southern Lake Superior region (upper Michigan, Wisconsin, and Minnesota), a stable-shelf assemblage bounded by unconformities, resembles Phanerozoic cratonic sequences. The Chocolay Group consists of a lenticular basal conglomerate overlain successively by quartzite (0 to 700 m), dolomite (30 to 800 m), and locally slate; it is capped by a regional unconformity. This sequence, although generally thicker, is similar to cratonic sequences (e.g., Sauk) in that both become finer upward, contain texturally mature sediments, are areally extensive, and are contained by unconformities.

New data on the Chocolay Group show interesting divergences from a quiescent stable-shelf model. Paleocurrents from cross-beds in quartzites show strong unimodality (^sfgr = 20 to 60°) atypical of stable platforms. Current directions from ripple marks and cross-beds crudely parallel later Precambrian troughs. Locally there are apparent reversals in the fining-upward trend of the Chocolay Group. Granite and basalt pebbles and abundant feldspars are present locally in dolomitic formations, but are absent in the underlying quartzites. The presence of irregular topography and the occurrence of uplifting locally are suggested by data which include thinning of quartzite members, variations in modal percent of detrital feldspar, and intraformational unconformities in the dolomites.

Based on large stratigraphic thickness, variations in sedimentary texture and mineralogy, and unimodality of paleocurrents, analogy to a simplistic stable craton interior is rejected. Rather, the influence of pericratonic tectonic conditions, including fault-bounded troughs, is suggested.

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