Heavy Mineral Shadows, a New Sedimentary Structure Formed Under Upper-Flow-Regime Conditions: Its Directional and Hydraulic Significance

Abstract

ABSTRACT Distinct heavy mineral accumulations, named here as heavy mineral shadows, were observed on a current, lineated bedding plane in the quartzose Whirlpool Sandstone (Silurian) near Georgetown, Ontario. The shadows are irregular in form but show one consistent feature: in plan, along lines parallel to the current lineation, one side of a shadow is defined by an abrupt boundary between the heavy mineral-rich (dark) area and adjacent quartz-rich (light) area, while the other side is characterized by a gradual transition into the light area. Comparison with paleocurrent directions determined from nearby sedimentary structures and with grain imbrication a few millimeters below the bedding plane on which they occur, indicates that the paleoflow was from the quartz-rich areas towards th sharply defined side of the heavy mineral shadows. A set of flume experiments was undertaken with a mixture of 97% (by weight) quartz sand and 3% chromite sand as the bed material. Stream power during each run was within the range of upper-flow regime plane-bed stability. With increasing stream power, the chromite sand behaved as follows: (1) heavy mineral shadows were stable, identical in form to those described above moving, intermittently beneath a streaky, faster-moving quartz-sand bed load; (2) shadows were completely destroyed, and the chromite sand formed a diffuse layer beneath the quartz sand in transport; and (3) chromite sand was concentrated across the crests of very low standing-wave bed forms with wavelengths of 0.4 m and amplitudes as small as 1.5 min. Although the data available is too limited to allow anything more than speculation on their origin, the recognition of heavy mineral shadows provides a new tool for paleocurrent studies. Flume experiments also suggest that its presence on bedding planes may have paleohydraulic implications.