广东达濠半岛近代高海滩岩的研究

广东达濠半岛现已发现距今三千多年的3个时期的海滩岩残留体,对它的认识有海滩岩与风成砂丘岩两种观点,笔者确定是海滩岩。本文从层面构造的流水线理、障碍物痕迹、波痕、鸟类足迹,岩层的韵律,胶结物的文石,微体古生物动物群,以及氧碳同位素的测定等多方面研究,认为该地海滩岩为热带、广盐度、与外海相通的滨海沉积物。 达濠海滩岩不是高海面的遗迹,而是海岸地壳抬升的证据。在构造格架上,位于北西西向强(震)隆起带上,上地幔隆起达2km。

A STUDY ON THE RECENT BEACHROCKS IN THE DAHAO ISLAND OF GUANGDONG

The' highevel beachrock residues of three different stages, which may be .dated back to 3000dd years B. P., have been found in the Dahao Island near the Shafrtou City. There are two viewpoints on the origin of these beachrock residues, beachrock and aeolian rock. Through careful stu-. dies, the authors have defined them as beachrocks. The sedimentary environment for the beachrocks is considered in the major aspects related to sedimentary rocks. Besides the Phenomena-that are common in the interti-dal zone (such as two-directional beach laminations, predominant macro-clasts, considerable finerained clasts and dispersed gravels, subangular shape, moderate sorting, etc. ) , there are also some phenomena that are seldomly seen in sedimentary rocks and are rarely seen in beachrocks,gjlch as trace fossils of sea birds and their microstructures, obstacle, marks and parting Hneations on the horizontal bedding. The high-level beachrocks are dated back to 3100?0 years B. P. for the elevation of 8?0m, 2750?5, 2650?0 and 2550?0 years B. P. for the elevation of 20?6m, and 1720 + 90 years B. P. for the elevationof 70?4 m. The main reason of why they have been mistaken as aeqlian dune rocks is that they lack aragonite cements. It is argued from different angles that they are littoral sediments that have been uplifted to the present elevations. The most reliable evidence are the characteristic cementing materials and the microfossil taphocoenoces. In the cementing materials there still remain the aragonite mud, the typical aragonite needles and the rarelyeen aragonite assemblages that are changing to calcite in appearance. The results of the electron probe analysis indicate that the aragonite commonly contains a small amount of MgO with an average molecular percentage of 2.5% and partially contains (for example, the beachrock at elevatiori of 52m) Na2O with an average molecular percentage -of 0.32%. This indicates that the beachrocks were originally formed in the Mg++ and !Na+ich sea water environment. The microfossil assemblages are characterized by euryhaline littoral and beachy Ammonia annecteus and tropical neritic Pseudorotalia schroeteriana et al. and also include stenohaline marine species such as Aitrila miii Ishizaki and Pseudorotalia gaimardii, even include stenohaline deeper species such as Poroeponider cribropandus, indicating that the beachrocks were formed in the tropical littoral water connected with the open sea. The existence of typical beach Balanus and the results of the carbon and oxygen isotope analysis of Balanus have also demonstrated that the beachrocks are products of tropical coasts. The comprehensive analysis with the tectonic data indicates that the active surface faults are in NWW direction and the upper mantle at depth has been uplifted for about 2km etc., which are confirm with the existence of the highevel beachrocks. The study area is one of the four NWW strongly-plifted zones (the Hengchun Island-hantou zone) on both sides of the Taiwan Strait. Therefore,the highevel beachrocks are the manifestations of the activities of the strongly-plifted zones and are the evidence showing that the area is most active along the continental coast of China. ft