Supermature quartz arenites of the Whiterock Member, deposited in the Early Proterozoic, intracratonic Hurwitz Basin, have a maximum thickness of 400 m and a minimum areal extent of 100,000 km 2 . The unit is unusual in that it is virtually devoid of other rock types and displays sedimentary structures that consist almost entirely of short-spaced ripple marks and parallel stratification, both formed by oscillatory flow. The ripples have spacings that are consistently small (average < 5 cm) for the entire thickness and lateral extent of the unit. Application of empirical and analytical methods yields average water depths in the range 2 cm to 2 m (maximum 5.6 m). Apparently, shallow water was maintained over a Large area for a long period of time, yet evidence of tidal and evaporitic conditions are lacking. We interpret that sedimentation was in a large, hydrographically open, tide-free, fresh-water lake, or series of lakes, that occupied a low-relief continental depression in a wet, possibly equatorial, climate where continuously high rainfall and well-developed outlets combined to prevent raised salinities.

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