ABSTRACT

Shoreline deposits have been recognized rarely in pure quartz arenites that were deposited before the advent of land vegetation. The Late Cambrian Munising Formation of northern Michigan is an exception. Both conglomerate and shale are relatively more common here than in most similar cratonic sandstones, apparently because of an important fluvial influence. A basal conglomerate member is interpreted as a low-sinuosity (braided) fluvial deposit. The overlying Chapel Rock Member has unfossiliferous, large-scale cross bedded sandstones with minor but widespread conglomerate interpreted as braided fan-delta distributary deposits and associated horizontally-stratified sandstone with wave ripples, polygonally-cracked shales, and trace fossils, interpreted as interdistributary deposits. Both of these latter facies are punctuated by unsorted, unstratified channelized conglomerate and intraclast breccia, which are interpreted as river-flood avulsion deposits. Although fluvially-dominated, the depositional system also had wave and probable tidal influences as well as local eolian reworking of fluvial sands. Thus a shoreline setting adjacent to highlands centered over the northern Michigan-Wisconsin border is indicated.

The overlying Miners Castle Member is interpreted as transgressive marine deposits formed as nearby highlands became mostly submerged. The lower strata are sandstones with medium-scale trough cross bedding; thin flaser-like shale drapes accumulated along many cross laminae and over rippled beds. These are interpreted as prodeltaic. The upper strata have little shale but much bioturbation and some skeletal fossils, and are interpreted as more distal shelf deposits.

The rather unusual association in the Munising Formation of conglomerate and shale with cross-bedded quartz arenite, the particular distribution of trace and skeletal fossils, and of mud crocks and wave ripples, allow an environmental reconstruction that may provide valuable analogies for other, hitherto-unrecognized, pre-vegetation shoreline deposits.