Rates of Shoreline Progradation During the Last 1,700 Years at Beachmere, Southeastern Queensland, Australia, Based on Optical Ages of Beach Ridges

The morphology and chronostratigraphy of seven beach ridges at Beachmere, southeastern Queensland, Australia, provide a record of changes in shoreline accretion and relative sea level over the last 1,700 years. Optical dating of beds of pebbly sand that form the inner 7th ridge reveals that 1,700 -140 yrs ago relative sea level at the site was approximately 1 m higher than at present. During the subsequent 600 yrs, the coast prograded at ~ 0.16 m yr-1 until ridge 6 was emplaced 1,140 ?80 yrs ago; this increased to a rate of ~0.40 m yr-1 as ridges 5 (optical age: 860 ?70 yrs) to 1 (140 - 50 yrs) were emplaced. A relatively wide (~200 m) intertidal to supratidal flat between ridges 4 (690 - 60 yrs) and 3 (190 ?40 yrs) marks a change from well-defined and regularly spaced ridges, 7 - 4 (~ 95 m apart), to lower amplitude closely spaced ridges, 3 ? 1 (< 33 m apart). Ridges 3 ? 1 have Optical ages that overlap within their 1 ? uncertainty and probably represent a rapid phase of shoreline accretion related to an increase in sediment supply associated with European settlement. The optical age of ridge 1 (140 ? 50 yrs), which sits immediately behind the modern beach, provides a maximum age for the onset of the modern erosional regime at the site.

澳大利亚昆士兰州东南部比奇米尔（Beachmere）的7条海滩脊（beach ridges）的形态学与年代地层学（chronostratigraphy）特征，记录了过去1700年以来海岸线加积作用与相对海平面的变化过程。对构成第7条内侧脊的砾质砂层开展的光释光测年（Optical dating）结果显示，1700~140年前该区域的相对海平面较现今高出约1米。在随后的600年间，海岸以约0.16米/年的速率向海进积，直至第6条脊形成于1140±80年前；在第5条脊（光释光年龄：860±70年）至第1条脊（140~50年前）形成期间，这一进积速率提升至约0.40米/年。第4条脊（690~60年前）与第3条脊（190±40年前）之间存在一片相对宽阔（约200米）的潮间至潮上带平原，这标志着海岸地貌从间距约95米、轮廓清晰且规则分布的7~4号脊，转变为振幅更低、间距更窄的3~1号脊（间距小于33米）。3~1号脊的光释光年龄在其1倍标准偏差误差范围内相互重叠，它们大概率代表了与欧洲殖民时期沉积物供给增加相关的快速海岸线加积阶段。紧邻现代海滩后方的第1号脊的光释光年龄为140±50年，这为该区域现代侵蚀过程的起始时间提供了最大年代约束。