Complex coastal change in response to autogenic basin infilling: An example from a sub‐tropical Holocene strandplain

Abstract

Thick bay-fill sequences that often culminate in strandplain development serve as important sedimentary archives of land-ocean interaction, although distinguishing between internal and external forcings is an ongoing challenge. This study employs sediment cores, ground-penetrating radar surveys, radiocarbon dates, palaeogeographic reconstructions and hydrodynamic modelling to explore the role of autogenic processes -notably a reduction in wave energy in response to coastal embayment infilling -in coastal evolution and shoreline morphodynamics. Following a regional 2 to 4 m highstand at ca 5Á8 ka, the 75 km 2 Tijucas Strandplain in southern Brazil built from fluvial sediments deposited into a semi-enclosed bay. Holocene regressive deposits are underlain by fluvial sands and a Pleistocene transgressiveregressive sequence, and backed by a highstand barrier-island. The strandplain is immediately underlain by 5 to 16 m of seaward-thickening, fluvially derived, Holocene-age, basin-fill mud. Several trends are observed from the landward (oldest) to the seaward (youngest) sections of the strandplain: (i) the upper shoreface and foreshore become finer and thinner and shift from sand-dominated to mud-dominated; (ii) beachface slopes decrease from >11°t o ca 7°; and (iii) progradation rates increase from 0Á4 to 1Á8 m yr À1 . Hydrodynamic modelling demonstrates a correlation between progressive shoaling of Tijucas Bay driven by sea-level fall and sediment infilling and a decrease in onshore wave-energy transport from 18 to 4 kW m À1 . The combination of allogenic (sediment supply, falling relative sea-level and geology) and autogenic (decrease in wave energy due to bay shoaling) processes drove the development of a regressive system with characteristics that are rare, if not unique, in the Holocene and rock records.

Reconstructing Coastal Sediment Budgets From Beach‐ and Foredune‐Ridge Morphology: A Coupled Field and Modeling Approach

“…A similar deceleration in shoreline growth at Pedro Beach, Australia, may be linked to changes in accommodation (Oliver et al, ). A possible concern is that the rates of strandplain progradation (0.4–1.8 m/year; Bristow & Pucillo, ; Brooke et al, ; Hein et al, ) are generally lower than those observed in our barrier island study sites. However, as we demonstrate through application of our model to Parramore Island, the model can be used at sites undergoing slower progradation (i.e., 0.16 m/year; Q S = 1.6 m/year), particularly if corresponding foredune fluxes are also low (i.e., Q D = 0.7 m/year).…”

“…The planar, near‐horizontal transgressive surface upon which Parramore Island prograded makes it an ideal site to record externally mediated changes in shoreface sediment fluxes, as accommodation effects such as growth into an offshore‐deepening basin (e.g., Bristow & Pucillo, ) or into an infilling bay (e.g., Hein et al, ) are minimal. Hence, the rate of progradation of this barrier island should reflect only the rate of sea level rise and net (longshore and cross‐shore inputs minus long‐term erosion) external sediment fluxes.…”

“…Coastal ridge and swale systems, composed predominantly of relict wave‐built beach ridges and/or aeolian foredune ridges, are found in association with progradational shorelines around the world (Tamura, ). The morphology of these elongate, shore‐parallel to subparallel features preserve paleoenvironmental signatures which have been used to infer changes in shoreline position (Mason, ), coastal sediment delivery rates and textures (Bristow & Pucillo, ; Hein et al, ), relative sea level (Billy et al, ; Hede et al, ; Long et al, ), and storm frequency (Buynevich et al, ; Costas et al, ). Subsequently, ridge proxies could be useful in predicting future changes to the coastal zone in response to autogenic forcings, climate change, and anthropogenic interventions.…”

Technical note: Optimizing the utility of combined GPR, OSL, and LiDAR (GOaL) to extract paleoenvironmental records and decipher shoreline evolution

“…Nielsen et al, 2017;Rodriguez and Meyer, 2006;van Heteren et al, 2000), (3) quantify sediment budgets (Bristow and Pucillo, 2006;Dougherty et al, 2015;van Heteren et al, 1996), and (4) decipher coastal 30 evolution (e.g. Barboza et al, 2009;Costas and FitzGerald, 2011;Hein et al, 2016). Combining GPR, OSL, and LiDAR (GOaL) on certain systems offers the possibility to determine a history of storms, sea level, sediment supply, and their impact on shoreline evolution.…”

“…Because GPR is sensitive to subtle changes in the subsurface, the record must be ground-truthed using cores, augers, or outcrops, in order to verify 10 barrier facies and boundaries (e.g. Costas and FitzGerald, 2011;Hein et al, 2013;Hein et al, 2016). Additionally, topographic profiles of the present-day beach and sediment samples from each facies should be collected, preferably capturing both storm and swell geometry and sedimentology.…”