Abstract
Two large coastal dune sheets, including the Santa Maria dune sheet and Vandenberg dune sheet, have been analyzed for late-Quaternary distributions, ages and volumes of dune sand deposition. Six new thermoluminescence (TL) ages establish the age range of dune sand deposition from >106±21 ka to <4.1 ka in the study area. Seven late-Pleistocene TL and 14C dated mid-depth samples (≤30 m depth subsurface), yield a mean of 33 ka for the late-Pleistocene dune deposits. Both TL and 14C dated Holocene dune deposits establish a transition from weathered middle-Holocene dune deposits to unweathered latest-Holocene dune deposits after 4 ka. Marine sand supply to the large dune sheets occurred by 1) cross-shelf eolian transport during late-Pleistocene marine low-stands (70–13 ka), 2) shoreward wave transport during slowing of the Holocene marine transgression (9–5 ka), and 3) longshore littoral transport during the latest-Holocene marine high-sand (3.5–0 ka). Measured and dated dune deposit sections (n=66, ranging from 2 to 60 m depth) demonstrate substantial differences in preserved sand volumes between the two adjacent dune sheets, Santa Maria (~ 2,300x106 m3) and Vandenberg (~430x106 m3). Asymmetric distributions of dune deposit volumes between and within the dune sheets show that long-term sand supply was locally controlled by paleo-shoreline orientations relative to corresponding deep-water wave propagation directions (260–290° TN) from the North Pacific Low Pressure Area. Recently declining sand supplies and/or -trapping efficiencies in the dune sheet littoral subcells led to ongoing shoreline retreat (≥ 200 m) and under-cutting of late- Holocene eolian sand ramps at the south ends of the Santa Maria and Vandenberg dune sheets. The termination of transgressive cross-shelf sand supply and locally variable longshore retention of littoral sand confirm previously reported framework models of regional coastal sand supply. Such models help to identify shorelines that are most susceptible to future beach erosion from predicted sea level rise following ongoing global warming.
Highlights
The Santa Maria and Vandenberg dune sheets represent two key localities within the larger Central West Coast of North America region (Figure 1) that merit study for paleo-sea level and paleo-wave climate forcing of coastal sand supply in late Quaternary time
Nearing the conclusion of these regional dune sheet studies, it was discovered that the Santa Maria-Vandenberg study area provided exceptional resolution of the relations between paleo-shoreline orientations, paleo-wave climate, and longshore sand transport mechanisms of coastal sand supply, as outlined below
The Santa Maria and Vandenberg dune sheets are generally located within Quaternary basins, which are bounded by uplifted fault blocks at Point San Luis, Point Sal, and Point Arguello (Lettis and Hanson, 1992)
Summary
The Santa Maria and Vandenberg dune sheets represent two key localities within the larger Central West Coast of North America region (Figure 1) that merit study for paleo-sea level and paleo-wave climate forcing of coastal sand supply in late Quaternary time. Other coastal dune sheets within the larger study region, extending ~ 1000 km both north and south of the Santa Maria-Vandenberg study area (Cooper, 1958; 1967), originated from shelf depocenters that fed landward eolian sand transport during late-Pleistocene marine low-stands (Peterson et al, 2007; 2015, 2017a). The Santa Maria and Vandenberg dune sheets are generally located within Quaternary basins, which are bounded by uplifted fault blocks at Point San Luis, Point Sal, and Point Arguello (Lettis and Hanson, 1992). The Santa Maria dune sheet is split into the north and south sides of the wide Santa Maria River Valley (Figure 2), which was deeply incised to at least 30 m depth (- 30 m elevation MSL) during the last marine low-stand, and back-filled by alluvial deposits during the Holocene marine transgression (11–0 ka) (Knott and Eley, 2006)
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