Abstract

Eolian sandstones occur interbedded with alluvial-fan and braided-river red beds in the Upper Triassic Wolfville Formation and at the base of the Lower Jurassic McCoy Brook Formation in Nova Scotia. Extensive semiaridity and aridity in the subtropical rift valley favored deposition of eolian sandstones, caliche paleosols, alluvial-fan conglomerates, braided-river alluvium, and playa mudstones. In postdrift configuration, paleowinds blew to the southwest, west, and northwest down the valley, reflecting the direction of trade winds at 25 degrees N paleolatitude. A three-step hierarchy of surfaces truncates cross-beds. First-order bedding planes of low relief extend for at least hundreds of meters, truncating all eolian structures. These surfaces were sculptured by desert flash floods and then deflated to ventifact-bearing bimodal regs. Second-order surfaces bound cross-bed sets and are mostly concave-up and dip downwind, recording shifts in migration direction of individual dunes or dunes superimposed on larger dune structures. Third-order surfaces within cross-bed sets formed on the lee sides of dunes due to fluctuations in wind direction and strength. Barchans initially developed on the alluvial-fan and braided-river alluvium, later coalescing into barchanoid transverse ridges, as evidenced by the internal geometry of the eolian sandstones.

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