Sediments of the Dry Tortugas, south Florida, USA: Facies distribution on a ramp-like isolated carbonate platform
Sediments of the Dry Tortugas, south Florida, USA: Facies distribution on a ramp-like isolated carbonate platform
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DOI：
10.1016/j.sedgeo.2017.02.004
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发表时间：
2017-04-15
影响因子：
2.8
通讯作者：
Hudson, J. Harold
中科院分区：
文献类型：
--
作者：
Gischler, Eberhard;Isaack, Anja;Hudson, J. Harold
通过对干托尔图加斯碳酸盐岩台地现代表层沉积物结构和组成的定量分析，圈定了4个沉积相。包括(1)软体动物-水母纹岩，(2)软体动物-水母纹岩，(3)珊瑚-水母纹岩，(4)珊瑚纹岩。尽管托图加斯地台的边缘河道深而宽，缺乏连续的破面边缘礁，具有开阔的旋回特征，但相的分布并非随机，呈现出等深分带。此外，褪色似乎覆盖了宽带，而不是形成马赛克。软体动物-哈利梅达类尾岩发生在约10-18米深的受保护台地内部区域。软体动物群-颗粒岩产于靠近河道的更开阔的台地内部环境和水深14-25米的更深的外礁区域。在浅滩的边缘（1-11米深）发现了珊瑚-水母颗粒岩，在这些浅滩的较深（3-16米）的向海边缘发现了珊瑚颗粒岩。然而，在约5 ~ 17 m的中间深度存在等深的相重叠。这一限制影响了对化石记录的解释，因为水深的变化通常被认为反映在相变化中，例如层序地层学。与20世纪30年代、60年代和70年代在该地区进行的沉积物研究相比，珊瑚碎片减少，珊瑚藻和软体动物壳碎片增加。这些观察结果可能是环境变化的结果，如珊瑚在温度事件、疾病和大型藻类增加（由于棘刺类Diadema的生态灭绝）期间的下降和死亡。研究结果表明，环境变化对珊瑚礁沉积物组成的影响需要进行更长期的研究。干Tortugas表层沉积物与邻近的佛罗里达礁带沉积物相比，由较低比例的水母板块和软体动物壳碎片和较高比例的珊瑚藻碎片组成，这可能是由于平台形态的差异以及波浪和水流的影响。(C) 2017 Elsevier B.V.版权所有
Four sedimentary fades may be delineated based on quantitative analysis of texture and composition of modern surface sediments on the Dry Tortugas carbonate platform. These include (1) mollusk-Halimeda wackestone, (2) mollusk packstone-to-grainstone, (3) coralgal-Halimeda grainstone, and (4) coralgal grainstone. Even though the Tortugas platform is characterized by an open circulation due to deep, broad marginal channels and a lack of a continuous surface-breaking marginal reef, facies are not distributed at random and show bathymetrical zonation. Also, fades appear to cover wide belts rather than forming a mosaic. Mollusk-Halimeda wackestone occurs in protected platform interior areas ca. 10-18 m deep. Mollusk packstone-to-grainstone occurs in more open platform interior settings adjacent to channels and in deeper outer reef areas of 14-25 m water depth. Coralgal-Halimeda grainstone is found on shallow marginal shoals (1-11 m deep), and coralgal grainstone on the somewhat deeper (3-16 m), seaward edges of these shoals. However, there is bathymetrical overlap of facies in intermediate depths of ca. 5-17 m. This limitation has implications for the interpretation of the fossil record, because changes in water depth are commonly thought to be reflected in facies changes, e.g., in sequence stratigraphy. Comparison with previous sediment studies of the 1930s, 1960s, and 1970s in the area exhibit a decrease in coral fragments and increases in coralline algal and mollusk shell fragments. These observations might be a result of environmental changes such as coral decline and die-outs during temperature events, disease, and the increase in macroalgae (due to the ecological extinction of the echinoid Diadema). The results suggest that more long-term studies are needed that further explore the influence of environmental change on reef sediment composition. Dry Tortugas surface sediments consist of lower portions of Halimeda plates and mollusk shell fragments and higher percentages of coralline algal fragments as compared to sediments of the adjacent Florida Reef Tract, possibly as a consequence of differences in platform morphology and exposure to waves and currents. (C) 2017 Elsevier B.V. All rights reserved.