Coastal Eolian Sand-Ramp Development Related to Paleo-Sea-Level Changes during the Latest Pleistocene and Holocene (21–0 ka) in San Miguel Island, California, U.S.A.

Abstract

Coastal eolian sand ramps (5-130 m elevation) on the northern slope (windward) side of the small San Miguel Island (13 km in WE length) range in age from late Pleistocene to modern time, though a major hiatus in sand-ramp growth occurred during the early Holocene marine transgression (16-9 ka). The Holocene sand ramps (1-5 m measured thicknesses) currently lack large dune forms, thereby representing deflated erosional remnants, locally covering thicker late Pleistocene sand-ramp deposits. The ramp sand was initially supplied from the adjacent island-shelf platform, extending about 20 km north of the present coastline. The sand-ramp deposits and interbedded loess soils were 14 C dated using 112 samples from 32 archaeological sites and other geologic sections. Latest Pleistocene sand ramps (66-18 ka) were derived from across-shelf eolian sand transport during marine low stands. Shoreward wave transport supplied remobilized late Pleistocene sand from the inner shelf to Holocene beaches, where dominant NW winds supplied sand to the sand ramps. The onset dates of the sand-ramp deposition in San Miguel are 7.2 6 1.5 ka (sample n ¼ 14). The internal strata dates in the vertically accreting sand ramps are 3.4 6 1.7 ka (n ¼ 34). The sand ramps in San Miguel show widescale termination of sand supply in the latest Holocene time. The sand-ramp top dates or burial dates are 1.7 6 0.9 ka (n ¼ 28). The latest Holocene sand ramps are truncated along most of the island's northern coastline, indicating recent losses of nearshore sand reserves to onshore, alongshore, and, possibly, offshore sand sinks. The truncated sand ramps in San Miguel Island and in other sand-depleted marine coastlines provide warnings about future beach erosion and/or shoreline retreat from accelerated sea-level rise accompanying predicted global warming.

Origins of The Santa Maria And Vandenberg Coastal Dune Sheets (~100-0 ka) Under Changing Sea Levels, Shoreline Orientations and Wave Directions: Long-Term Records of Coastal Sand Supply in South-Central California, USA

“…Surplus beach sand supplies onshore eolian transport to develop coastal dunes during the slowing phase of marine transgression, as reported for South East Australia (Thom, 1984;Short, 1987) and many other coastlines around the world. The near high-stand conditions of latest-Holocene time (3-0 ka) could have been associated with the reduction or termination of transgressive sand supply in the Santa Maria and Vandenberg dune sheet areas, as found for the small San Miguel Island (Peterson et al 2017b). However, the near high-stand sea levels would have been associated with relatively stable shoreline conditions for longshore littoral transport in the nearshore and inner-shelf of the continental margin between Point Sur and Point Conception (Figure 1).…”

“…Submarine canyons (bold lines) intercept the outer-shelf offshore of Point Arguello and Point Conception. Mean deep-water wave directions (arrows) for winter months (DJF) and summer months (JJA) for years 2014-2015 were averaged from an offshore wave buoy (Figure 1) as summarized in Peterson et al (2017b).…”

“…Part B: Relative sea level (RSL) rise for the Holocene marine transgression, as reported for the central west coast of North America (solid line) and modeled for the south-central California coastal region (solid squares). Figure is redrafted from Peterson et al (2017b).…”

“…The weathered Holocene dune (wHD) deposits in the Point Purisima vicinity (site S26) reach a maximum thickness of 17 m (Figure 12). The landward extents of the weathered Holocene dunes decrease to about 0.5 km distance near the south end of the Vandenberg dune sheet, but reach maximum thicknesses of 54 m and 42 m, respectively, in steep eolian sand ramps (Peterson et al, 2017b) at sites V43 and V46 (Figure 13). …”

“…However, latest-Holocene shoreline retreat has undercut and over-steepened the middle-Holocene sand ramps, leading to the narrow (~0.5 km width) strip of truncated eolian sand ramps or 'remnant perched dunes' (Peterson et al, 2017b) along the bluff edge (Figure 11). Dune form ages are from corresponding morphostratigraphic sections (V), including unweathered Holocene dune deposits (uHD), weathered Holocene dune deposits (wHD), and cemented Pleistocene dune deposits (PD).…”