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Holocene Progradation and Retrogradation of the Central Texas Coast Regulated by Alongshore and Cross-Shore Sediment Flux Variability

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Holocene Progradation and Retrogradation of the Central Texas Coast Regulated by Alongshore and Cross-Shore Sediment Flux Variability Received: 4 May 2020 | Revised: 8 October 2020 | Accepted: 15 October 2020 DOI: 10.1002/dep2.130 ORIGINAL RESEARCH ARTICLE Holocene progradation and retrogradation of the Central Texas Coast regulated by alongshore and cross-shore sediment flux variability Christopher I. Odezulu1 | Travis Swanson2 | John B. Anderson1 1Department of Earth, Environmental and Planetary Science, Rice University, Abstract Houston, TX, USA Fifteen transects of sediment cores located off the central Texas coast between 2Department of Geology and Geography, Matagorda Peninsula and North Padre Island were investigated to examine the off- Georgia Southern University, Statesboro, shore record of Holocene evolution of the central Texas coast. The transects extend GA, USA from near the modern shoreline to beyond the toe of the shoreface. Lithology, grain *Correspondence size and fossil content were used to identify upper shoreface, lower shoreface, ebb- Christopher I. Odezulu, Department tidal delta and marine mud lithofacies. Interpretations of these core transects show of Earth, Environmental and Planetary Science, Rice University, Houston, TX, a general stratigraphic pattern across the study area that indicates three major epi- USA. sodes of shoreface displacement. First, there was an episode of shoreface prograda- Email: [email protected] tion that extended up to 5 km seaward. Second, an episode of landward shoreline Funding information displacement is indicated by 3–4 km of marine mud onlap. Third, the marine muds Rice University Shell Center for Sustainability. are overlain by shoreface sands, which indicates another episode of shoreface pro- gradation of up to 5 km seaward. Radiocarbon ages constrain the onset of the first episode of progradation to ca 6.5 ka, ending at ca 5.0 ka when the rate of sea-level rise slowed from an average rate of 1.6–0.5 mm/yr. Results from sediment budget calculations and sediment transport modelling based on reasonable estimates of an ancient shoreline shape and wave climate indicate that the first progradation was a result of sand supplied from erosion of the offshore Colorado and Rio Grande deltas. The transgressive phase occurred between ca 4.9 ka and ca 1.6 ka and coincided with a major expansion of the Texas Mud Blanket, which resulted in burial of offshore sand sources and the shoreface being inundated with mud. The second, more recent episode of shoreface progradation began ca 500 years ago with a maximum rate of ca 6 m/yr. This most recent change signals a healing phase of coastal evolution from the late Holocene transgressive event. Currently, the shoreline along the central Texas coast is retreating landward at an average rate of 0.30 m/yr, indicating that the second progradation event has ended. KEYWORDS Central Texas, cross-shore sediment flux, longshore sediment flux, progradation, retrogradation, sea level, Texas Mud Blanket This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited. © 2020 The Authors. The Depositional Record published by John Wiley & Sons Ltd on behalf of International Association of Sedimentologists. Depositional Rec. 2021;7:77–92. wileyonlinelibrary.com/journal/dep2 | 77 78 | ODEZULU ET AL. 1 | INTRODUCTION Rodriguez et al. (2001) conducted research on sediment cores from the shoreface and inner continental shelf off Barrier islands and peninsulas dominate the Texas Coast. the central Texas coast (Figure 2) and recognised different These coastal barriers are known to have had very different stages of shoreface growth and retreat, but lacked radio- evolutionary histories during the Holocene (Anderson et al., carbon age constraints on the timing of these changes. The 2014; Rodriguez et al., 2001). The main factors that influ- results from a more detailed analysis of the cores studied enced coastal change over this time interval include vari- by Rodriguez et al. (2001) are presented here, including a ability in antecedent topography, relative sea-level change, revised facies and stratigraphic analysis that relies heavily variable sediment supply, changes in surface current patterns, on grain size as an indicator of depositional environment wave climate and storm history. (Figure 3). In addition, radiocarbon ages provide constraints The central Texas Coast occupies a shelf embayment on key stratigraphic surfaces (Table 1). There are also the re- between the ancestral Colorado and Rio Grande deltas sults from more recent studies of central Texas coastal bar- (Figure 1). The Holocene evolution of coastal barriers situ- riers (Anderson et al., 2014; Garrison et al., 2010; Simms ated off the central Texas coast has been a focus of numerous et al., 2006) and by Weight et al. (2011), who conducted a investigators (Anderson et al., 2014; Garrison et al., 2010; detailed investigation into the evolution of the Texas Mud McGowen et al., 1977; Morton, 1977; Shideler, 1986; Simms Blanket (TMB), to examine the causes of coastal change in et al., 2006; Wilkinson, 1975; Wilkinson & Basse, 1978; the study area. Wilkinson & McGowen, 1977). These studies focussed on onshore records, which do not allow observations of shore- face progradation and retrogradation and where pristine ma- 2 | STUDY AREA terial needed for radiocarbon dating is limited. The offshore study area covers about 12,000 km2 (shoreface to inner shelf) and extends from Matagorda Island south- wards to North Padre Island (Figure 2). The continental shelf is an embayment characterised by a relatively steep, ramp-like profile that is bounded to the north by the ancestral Colorado Delta and to the south by the ancestral Rio Grande Delta (Figure 1). Shoreface profiles vary across the study area in response to changes in the antecedent topography and associated changes in the thickness of Holocene shoreface deposits (Figure 4). The central Texas coast is a wave-dominated, micro-tidal environment. Tides are diurnal or mixed and have an average range of less than 60 cm (Morton et al., 2004). South-easterly winds are prevalent most of the year, with prevailing long- shore currents towards the west in the northern sector and towards the north along the north-south oriented south Texas coast. Thus, despite the limited sand delivery from central Texas rivers, sand supply by longshore transport is relatively high for the central Texas coast compared to the upper Texas coast and south Texas coast (Curray, 1960; Lohse, 1955; McGowen et al., 1977). Texas has one of the longest aerial photographic records of historical shoreline change in the United States, dating FIGURE 1 Map of the study area (dashed boxed region). back to the late 1930s in some areas. For more than four de- Ingleside palaeo-shoreline is drawn as a thick red line. Depth along cades, these data have been used to analyse historical shore- the continental shelf is contoured at a 20 m interval as indicated by line change (Gibeaut et al., 2006; Morton, 1977; Paine et al., blue contour lines. Also shown are the locations of the ancestral 2012; 2016). These results show that 84% of Texas coastal Colorado and Rio Grande deltas (mapped as dark grey regions; from Snow, 1998; van Heijst et al., 2001 and Banfield & Anderson, 2004, barriers are currently experiencing shoreline retreat, whereas respectively) and the Texas Mud Blanket (mapped as light grey region; 16% are experiencing shoreline advance (Paine et al., 2016). from Weight et al., 2011). Seismic line 36 is drawn as a green line The most recent reviews of these data show that shoreline along the Holocene Rio Grande Delta. Note that only the northern change varies spatially and temporally along the coast, with portion (US portion) of the Rio Grande Delta has been mapped. shoreline landward retreat rates ranging from 0.5 to 5.5 m/ ODEZULU ET AL. | 79 FIGURE 2 Locations of sediment core 97°0 PC 96°0 transects used for this study. Aransas Pass and Pass Cavallo are annotated by labels AP TR 1 and PC, respectively 2 Texas Matagorda Pen. 3 4 5 6 Matagorda Is. 28°0 7 28°0 8 9 10 San JoseIs. Gulfof Mexico AP 11 12 sediment core core transect 13 MustangIs. 01020km 14 N. PadreIs. TR 15 96°0 97°0 yr and averaging ca 1.6 m/yr along the upper Texas coast, et al., 2012; Simms et al., 2007; 2013). Variations in subsid- 0.30 m/yr in central Texas and ca 1.9 m/yr in south Texas ence are primarily related to differences in the thickness and (Gibeaut et al., 2006; Paine et al., 2012). Comparisons of degree of compaction of Holocene sediments, which ranges these rates to the pre-historic record of coastal change have from ca 1 to 50 m along the coast; thicker accumulations are shown that current rates along the upper and south Texas associated with incised valleys (Anderson et al., 2014). coast are unprecedented, a result of diminished sand sup- ply, accelerated sea-level rise and anthropogenic influences (Anderson et al., 2014; Odezulu et al., 2018; Wallace & 2.1 | Early Evolution of the Central Anderson, 2013). Texas Coast Historic rates of relative sea-level rise, based on tide gauge records, vary along the Texas coast from 6.4 mm/yr at east During the MIS (marine isotope stage) 5e highstand, the Galveston Island to 2.1 mm/yr at Port Mansfield on South Texas shoreline was situated approximately 30 km land- Padre Island (NOAA Tides & Currents, 2012). These differ- ward of the modern shoreline along the upper Texas coast, ences are due to variations in glacial isostatic-adjustment, local but about half that distance from the modern central Texas steric changes, local oceanographic influences and subsidence shoreline (Ingleside palaeo-shoreline, shown as a thick red (Kolker et al., 2011; Morton et al., 2006; Paine, 1993; Paine line in Figure 1; Otvos & Howat, 1996; Simms et al., 2013).
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