Provenance and sediment characteristics of contemporary gravel deposits at Sellicks Beach, eastern shore of Gulf St Vincent, South Australia

Abstract

Sellicks Beach, located on the eastern shore of Gulf St Vincent, South Australia, is subject to wave-dominated processes and northward longshore transport. During winter, when wave energy is typically vigorous, gravel deposits are exposed across most of the beach, and three step-like berms are well developed. Sand is restricted to a narrow strip that is exposed only at low tide. In contrast, during summer, when wave energy is generally moderate to low, much of the gravel is covered by a thin veneer of sand and only the high berm, on the landward edge of the beach, remains as an obvious feature. Steeply dipping Neoproterozoic to Cambrian strata that outcrop strongly across Sellicks Hill are the original source rocks for the beach gravel; distinctive sedimentary textures, structures and fossils in the cobble-size clasts can be confidently matched with those of the provenance rocks. Much of the sediment entered the modern beach environment as a consequence of coastal erosion of transitional alluvial fan sediments. The oldest alluvial fan sediments are of late Pliocene to earliest Pleistocene age. Mount Terrible Gully provides a conduit for the input of fluvial sediment at the mouth of Cactus Canyon, where clasts as large as boulders accumulate across the beach. Sellicks Beach gravels are subject to longshore transport northwards. Relatively softer clasts, such as those derived from the Heatherdale Shale, are rare beyond Cactus Canyon. In contrast, quartzite clasts are more abundant towards the north. This lithological differentiation is attributed to preferential survivorship of clasts that are physically harder and chemically less reactive. The change in the shapes of clasts northwards, from predominately shingle-like ‘very platy’ and ‘very bladed’ at Cactus Canyon, to more ‘compact’ towards the boat ramp, is in accord with the more massive fabric of the surviving quartzite clasts. At Sellicks Beach, preservation of uplifted, coarse gravels, with entire and comminuted marine molluscan shells, of last interglacial age, provides evidence of neotectonism. At the landward margin of the beach, imbricated gravels in which pore spaces have been infilled with mud, and which show no evidence of modern coastal erosion, may provide evidence of continuing uplift during the recent Holocene. The geological setting, geomorphic framework and modern sedimentary regime at Sellicks Beach combine to provide an exceptionally useful outdoor laboratory for education in field geology.

位于南澳大利亚St. Vincent海湾的东海岸的Sellicks海滩，受波浪为主的作用和北向沿岸搬运的影响。在冬季，当波浪能特别强烈的时候，碎石沉积物出露在大部分海滩上，三个阶梯状护堤都发育很好。沙子被限制在一个狭窄的地带，只有在退潮时露出。与此相反，在夏季，波浪能量一般中至低度，很多砾石由薄层沙覆盖着，在沙滩陆向边缘只有高护堤是一个明显的标志。陡倾的新元古代到寒武纪地层露头在整个Sellicks山都有出现，是海滩砾石的源岩; 鹅卵石般碎屑的独特沉积​​结构、构造和化石与源岩完全一致。大部分进入现代海滨环境的沉积物是过渡性冲积扇沉积物经海岸侵蚀的结果。最古老的冲积扇沉积物的年龄是上新世末至更新世早期。Mount Terrible沟壑提供了一个渠道，使得河流冲积物在Cactus峡谷口流入，在那里大如巨石的碎屑堆积在整个沙滩。 Sellicks海滩砾石沿岸向北输移。相对较软的碎屑，如那些来源于Heatherdale页岩碎屑，在Cactus峡谷之外罕见。相比之下，石英岩碎屑往北更丰富。这岩性差异是由于碎屑更易保存，物理学上它较硬，化学上它反应性较低。往北碎屑形状的变化，从Cactus峡谷主要是木瓦般的“高度片状”和“高度刃状’至两侧的更“致密”，符合保存下来的石英岩碎屑的大块组构。在 Sellicks海滩，末次间冰期的含有完整和碎裂软体动物外壳的上隆粗砾石的保存，为新构造运动学提供了证据。在沙滩的近陆边缘，叠瓦状砾石的孔隙已经被泥土填实，并没有显示现代海岸剥蚀的证据，可能提供了在全新世近期持续抬升的证据。Sellicks海滩的地质环境、地貌框架和现代沉积区综合在一起，可以为野外地质教学提供异常实用的户外实验室。

KEY WORDS:

• coastal processes
• beach morphology
• gravel clasts
• alluvial fan
• Quaternary
• Cambrian
• education

ACKNOWLEDGEMENTS

The authors gratefully acknowledge the useful comments of Tony Belperio, Barry Cooper, Victor Gostin and an anonymous reviewer. We are particularly indebted to Bob Bourman for his helpful comments throughout the preparation of the manuscript and especially for his advice concerning the stratigraphy of the alluvial fan sediments.

Supplemental data

Table 1 Numerical data for selected clasts at Site #4, Cactus Canyon, Sellicks Beach. GPS 35°20′29.64″S, 138°26′16.41″E.

Table 2 Numerical data for selected clasts at Site #3, Sellicks Beach. GPS 35°20′19.56″S, 138°26′21.34″E.

Table 3 Numerical data for selected clasts at Site #2, Sellicks Beach. GPS 35°20′10.33″S, 138°26′26.14″E.

Table 4 Numerical data for selected clasts at Site #1, Boat ramp, Sellicks Beach. GPS 35°19′48.63″S, 138°26′29.98″E.