Precambrian tidalites from the Baraboo Quartzite Wisconsin, U.S.A.

Abstract The Baraboo Quartzite is a 1.7 billion year old, almost pure meta-quartzarenite unit that is 1500 m thick and that crops out in a doubly-plunging syncline in south-central Wisconsin, U.S.A. It is one of several similar quartzites that are exposed throughout the upper part of the Midwestern United States. Two general depositional environments are represented by this well-sorted, quartzarenite: a lower braided stream complex characterized by wedge sets of cross-strata with pebble conglomerate at the base and small cut and fill structures, and an upper, tidally-influenced environment characterized by small-scale planar–tabular cross-strata separated by reactivation surfaces and scattered tidal bedding sequences. The Baraboo Quartzite represents one of the oldest tidally influenced rock units known but has no direct modern analog.