渤海湾障壁岛-潟湖型成陆过程及对今后海岸带可持续发展的启示--纪念天津地质调查中心第四纪地质调查与研究转入海岸带方向40年

Barrier-island-and-lagoon characterized land formation in the Bohai Bay and its enlightenment to sustainability of coastal development

以1980年代中期为界,天津地质调查中心的第四纪地质学研究,经历了前后两个阶段。前一阶段是以第四纪冰川与地层学为主,后一阶段则逐渐转为海岸带地学调查与研究。本文简要回顾了历经全室三代人的不懈努力,在第二阶段的泥质海岸带地层学、年代学、古生物学和地球物理学等领域以及在服务海岸带经济社会发展方面取得的成果。接着,通过对渤海湾中全新世以来海退过程中的“障壁岛-潟湖成因”假说的论证,介绍一个从地学基础研究向战略性应用研究“转化”的具体案例。基于对海面变化、海面标志点的地层学内涵、地貌学和年代学的研究,获得以下认识:(1)消除现代人为抽取地下水引起的下沉后,渤海湾的校正相对海平面变化带位于全球冰融水等效海面变化带内、或比后者稍高,说明区域性均衡掀升可以抵消局地新构造和沉积自压实二者共同造成的下沉,使得近7 ka以来研究区海退过程中的陆地地表长期处于潮间带海水的影响之下,这成为该地区形成“障壁岛-潟湖体系”的前提条件。(2)牡蛎礁平原的岭地与贝壳堤平原的泥质堤相同,它们可被称为“泥质堤”,与贝壳堤同属从潮间带发育起来的障壁岛型古海岸线。贝壳堤-泥质堤与堤后潟湖(及潟湖型洼地),共同构成了渤海湾“障壁岛-潟湖体系”。在前期研究的基础上,再次勾勒并订正了贝壳堤-泥质堤古海岸线、潟湖(及潟湖型洼地)的时空分布。(3)最大海侵边界与具明显凸起地貌特征的最老古海岸线(最老贝壳堤-泥质堤)之间的宽达~30~100 km、距今~7~4.7(3.5)ka的低地,应属于第一期“古潟湖类洼地”;此后,随着不断向海推进的、逐渐年轻的多道贝壳堤-泥质堤障壁岛岸线的形成,在它们向陆一侧相应出现逐渐年轻的多期潟湖洼地环境。这种周期性的反复,成为渤海湾“沧海变桑田”的基本特征。(4)尽管这个过程被近两个世纪以来的人类活动极大地掩盖,但面对21世纪海面上升以及局地环境现状(保有高程偏低、地面下沉、潜水位与含水层顶板反倾等),本文预测障壁岛-潟湖特征将重新凸显:例如围海造陆区块-沿海堤坝共同构成的人工岸线向陆一侧的面积广大的原始潟湖地貌景观的湿地特征将可能进一步加强。虽然当前的“海岸带绿色屏障带”布局契合本文揭示的障壁岛-潟湖自然过程,但仍应从地质历史发展的角度,予以重新审视,以使中长尺度的规划具有更为科学、坚实的基础。

The research on Quaternary geology in Tianjin Center has been experienced two stages divided approximately in the middle of 1980s.The first phase was mainly focused on the Quaternary glaciostratigraphical study,while the second had being gradually oriented to the coastal geological investigations and study as well.This paper reviews succinctly the profound achievements in the field of fundamental researches,including litho-,bio-and chronostratigrahies and applied geology serving for the coastal ecosocietal development.Then,taking the local marine regressive processes as a case study,this paper re-emphasizes the hypothesis of‘Barrier-island-lagoon cause’for the land formation of Bohai Bay.Based on multi-disciplinary approaches on sea-level change,stratigraphic significance of the existing sea-level indicators,microgeomophology and chronology,we found:(1)The RMSLc(corrected relative mean sea level)belt is to fall into the global ESL(ice-volume equivalent sea level)belt or even slightly higher than the latter after eliminating the local subsidence caused by the groundwater withdrawal.It suggests that the regional isostatic uplift can fully offset the local subsidence given by both neotectonics and self-compaction.As a result,a configuration of the palaeo altitudes of being formed land following marine regression since the last~7 ka have been basically under the simultaneous tidal water influences and,thus,it provides a precondition for the‘Berrier-island-Lagoon System’of the study area.(2)The muddy mounds,found in the Chenier Plain,are equivalent to the muddy mounds of the Oyster Reef Plain and can be characterized as the‘muddy cheniers’.Both shelly and muddy cheniers altogether form a number of the palaeo barrier-shaped shorelines.Based on our previous study,this paper re-depicts a temporo-spatial distribution of the regional palaeoshorelines under such a new concept of the shelly-muddy cheniers.(3)A coastal wetland,~30 km wide apart in between the maximum marine transgression boundary and the oldest morphologically-remarkable shoreline(a linkage of the Chenier V and the Muddy Mound Shizhuang-Dongjituo-Mengzhuang),with time span~7-4.5 ka,should be attributed to the‘Palaeo Lagoon I’.Afterwards,following periodically formed barrier shorelines,new lagoons may disconsecutively occur behind the each barrier shoreline simultaneously and/or even obviously later(The younger high waters may even farther pour into those previously older lagoons).This cyclicity is thus a basic characteristic of the area and is an essential feature named Canghaisangtian,i.e.,seas change into mulberry fields.(4)Although such processes have been greatly impacted by human activities during the past two centuries,facing to the predicted coming rise of sea level this century and potential environmental deterioration(ground subsidence,landward-reversed groundwater table,etc.),this paper forecasts the Barrier-island-Lagoon environment will reappear obviously again in the coming future.Fortunately,the geographical layout of‘Green Coastal Defence Belt’perfectly tallies with the aforementioned cyclicity and the youngest lagoonal area.Nevertheless,it is necessary to re-examine closely this urban plan under the geologic evolution point of view and better to further aggrandize its strategic position in order to meet the mid-and long time scale coastal sustainability.