Origins of late- Pleistocene coastal dune sheets, Magdalena and Guerrero Negro, from continental shelf low-stand supply (70-20 ka), under conditions of southeast littoral- and eolian-sand transport, in Baja California Sur, Mexico

摘要

Shallow morpho-stratigraphic sections (n = 11) in each of two large coastal dune sheets including the Magdalena (7000 km(2)) and Guerrero Negro (8000 km(2)) dune sheets, from the Pacific Ocean side of Baja California Sur, Mexico, have been analyzed for dune deposit age. The shallow morphostratigraphic sections (similar to 2-10 m depth) include 11 new TL and C-14 ages, and paleosol chronosequences, that differentiate cemented late Pleistocene dune deposits (20.7 +/- 2.1 to 99.8 +/- 9.4 ka) from uncemented Holocene dune deposits (0.7 +/- 0.05 to at least 3.2 +/- 0.3 ka). Large linear dune ridges (5-10 m in height) in the dune sheet interiors trend southeast and are generally of late Pleistocene age (similar to 70-201ka). The late Pleistocene dune deposits reflect eolian transport of marine sand across the emerged continental shelf (30-50 km southeast distance) from low-stand paleo-shorelines (-100 +/- 25 m elevation), which were locally oriented nearly orthogonal to modeled deep-water wave directions (similar to 300 TN). During the Holocene marine transgression, onshore and alongshore wave transport delivered remobilized shelf sand deposits to the nearshore areas of the large dune sheets, building extensive barrier islands and sand spits. Submerged back-barrier lagoons generally precluded marine sand supply to dune sheet interiors in middle to late Holocene time, though exceptions occur along some ocean and lagoon shorelines. Reactivation of the late Pleistocene dune deposits in the dune sheet interiors lead to generally thin (13 m thickness), but widespread, covers of Holocene dune deposits (0.41 +/- 0.05 to 10.5 +/- 1.6 ka). Mechanical drilling will be required to penetrate indurated subsoil caliche layers to reach basal Pleistocene dune deposits. (C) 2017 Elsevier B.V. All rights reserved.