渤海湾西岸贝壳堤堆积与海陆相互作用*

渤海湾西岸淤泥质海岸已见两类砂体,滨外坝先形成于水下,而后随着海岸进积和在海洋动力作用下,逐渐出露于潮间带和陆地上; 另一种普遍出现的是具沿岸堤意义的贝壳堤,而且是在海岸进入弱侵蚀期即形成。研究发现,在海岸进积和海-陆相对稳定条件下,海岸线附近中潮坪-高潮坪受向岸风和波浪作用也可形成雏形贝壳堤砂体。目前研究区海岸上的贝壳堆积有3类,即:风暴潮堆积的沿岸堤型贝壳堤,主要堆积在中潮坪-高潮坪-滨海低平原上; 潮沟~潮汐河道型"贝壳堤",为潮汐河口湾顶或潮道外侧风暴贝壳堆积(自潮汐通道水下堆积延续到低地); 以及沿海低平原在大潮或风暴潮天气背景下出现的无序贝壳堆积。贝壳堤堆积既有风暴潮大浪瞬时、高能幕式堆积,又有大浪、向岸风综合作用下堆积的可能,贝壳测年及贝壳堤层序表明其形成是复杂的再堆积过程,不宜解释为海平面变化。在三角洲不均衡推进过程中,有可能形成不规则的海岸线,向岸风浪对不同地貌岸段的作用,可导致形成并非与现代海岸线平行的贝壳堤。贝壳堤术语的随意使用,已经造成海岸线变迁研究的简单化。严格使用沿岸堤的贝壳堤概念,可进一步划分三角洲叶瓣变迁,但是事件堆积必须剔除。将贝壳堤的研究与三角洲顶面泥炭测年相结合,可能更精确重塑海岸形成过程。

SHELLY RIDGE ACCUMULATION AND SEA-LAND INTERACTION ON THE WEST COAST OF THE BOHAI BAY

On the muddy west coast of the Bohai Bay, two kinds of sandy bodies have been discovered. The first is the offshore bar formed in underwater area initially, which afterward under marine dynamic effect gradually accumulated on the inter-tidal zone and on the land with shoreline progradation. The other is the widespread shelly ridge (chenier). The initial shelly ridge is formed when the coast enters into its weak erosion phase. The present paper reports that when the shoreline progressing and sea-land interaction are stable, under the effect of wind- and wave-landward, the sandy body of the initial shelly ridge could be formed near the shoreline. Shelly accumulation on the coast in the studied area includes three types: The shelly ridge on the front margin of the abandoned delta lobe; the storm shelly accumulation on the head of tidal river gulf and/or the outer side of tidal channel (the shell could accumulate from the underwater area of channel to the low land continuously); irregular shelly accumulation on the low delta plain under the setting of spring tide or storm weathers. The shelly ridge includes the instantaneous/episodic high energy accumulation and that under the comprehensive activity of spring and wind-landward. The shelly ridge could not be used to interprete sea-level change because the shell dating and the system of beds in shelly ridge have shown that the formation process of shelly ridge is a complex re-accumulation. In the highly disturbed progressing process of the delta, the irregular shoreline could be formed. The effect of wind- and wave-landward for different topographic shoreline parts could thus cause the formation of shelly ridge which is unparallel to modern shoreline. Terminology of shelly ridge has been used at random, causing the study of shoreline change a simplified one. To use rigorous concept of shelly ridge, the deltaic lobe change could be precisely subdivided when event deposits have been excluded. When the study of shelly ridge is combined with the dating of delta top peat, the formation process of muddy coast could be precisely reconstructed.