BRAIDED-STREAM/EOLIAN ENVIRONMENT OF PROTEROZOIC HINCKLEY SANDSTONE, KEWEENAWAN RIFT, EAST-CENTRAL MINNESOTA

The Proterozoic Hinckley Sandstone, a late-stage Keweenawan rift-fill quartz arenite, was previously interpreted as a shallow lacustrine deposit. We re-interpret this unit as a braided-stream/eolian deposit based on evidence from eight measured partial sections in Pine Co., MN. Three facies are identified. 1) Trough cross-stratified sandstones are predominantly medium-grained, planar beds with multiple troughs up to 2 m. wide and 20 cm. thick, locally containing rounded lithic or angular siltstone pebbles. Trough and cross-strata orientations indicate a SE paleocurrent, and are interpreted to represent sandy braided streams that flowed across the rift axis. 2) Planar cross-stratified sandstones are finer-grained, with predominantly simple sets up to 2 m. thick that typically display mm.-scale laminated tangential foreset/toesets, in places alternating with cm.-scale straight foresets. Locally, basal foresets, toesets and bounding surfaces display climbing adhesion-ripple structures or low ripple-index (r.i.<10) ripple-forms; cross-strata consistently dip NE. This facies represents eolian dunes that migrated along the rift axis across dry, damp or flooded stream margins and interfluves. 3) Planar stratified sandstones comprise complexly interbedded sequences (up to 2 m. thick) of crinkly/mottled strata (including climbing adhesion-ripple structures), preserved ripple-forms (symmetrical/asymmetrical, r.i. <10) and mm-scale planar laminae with rare thin (<20 cm.) single sets of cross-strata. These were deposited in interdune or stream-margin environments alternating from dry to damp to wet conditions. Soft-sediment deformation is of four types: 1) low-amplitude m.-scale convolute beds, 2) dewatering structures, 3) small (mm.- to cm.-scale) extensional faults, and 4) a flame structure with an overlying structureless zone. Many of these structures can be explained by migration of eolian dunes over saturated sediment.

Limited exposure prevents detailed interpretation of fluvial architecture. However, lateral facies changes over a few hundred meters result from stream downcutting into eolian and interdune strata, typical of a shifting braided stream system. No evidence of aridity was discovered; dunes formed easily on the margins of sandy streams in the vegetation-free Proterozoic landscape.