弗拉斯晚期桂林组上段旋回性和双孔层孔虫堆积的关系

桂林地区泥盆纪弗拉斯晚期的沉积物以含有大量细枝状的双孔层孔虫为特征,并表现出明显的旋回性。本文研究认为,双孔层孔虫最适宜在潮下带弱动荡环境中生长。完全闭塞,滞静的泻湖不利于双孔层孔虫的生长,只能是其堆积的场所。

Relationships Between Cyclicity and Amphipora Accumulation in the Late Frasnian Limestone near Guilin

The small outcrop, a part of Guilin Formation, is about 30m thick and represents a deposits from subtidal lagoon to tidal flat in the carbonate platform. Its fossils are characterized by Amphipora sp. (or Paramphipora sp.), Tenticospirifer and the fossils of its equivalent beds are marked by Ozarkodina poster, Icrious alternants alternatus, 1. a. helmsi , indicating a late Frasnian age. Microfacies analysis revealed that limestone bearing Amphipora sp. in the upper member of Guilin Formation forms the cyclic deposits. These cycles are mainly composed of three microfacies. 1. Gastropod wackestone 2. Twing-like Amphipora and spherical stromatoporoid packstone 3. Spherical, hemispherical and irregular stromatoporoid packstone. Biofacies characteristics of the outcrop show three main fossil assemblages in each cycle: 1. Gastropod fossil assemblage 2. Amphipora, gastropod and brachiopod fossil assemblage 3. Spherical, hemispherical and irregular stromatoporoid assemblage. According to the observation in the field and under the microscope, Amphipora taphocoenosis in the study outcrop exhibit many allochthonos characters. The conclusion is that the subtide with slightly fluctuated water is best suited for Amphipora growth, and the completely restricted and euxinic lagoon is not beneficial to their growth and only fit to be accumulated instead.