Presenter: Tzu-Hsuan Chiu
Date: 2017/02/16
Abstract
Studies of biostratigraphy have been demonstrated to have stratigraphic significance, including the dating, environmental description, and correlation of stratum and sequences. Examples of lithology, fossil content, trace features of shellbeds are presented from Plio-Pleistocene successions. In this study, common styles of shellbeds can be recognized that formed under conditions of marine onlap, backlap, downlap and toplap. Onlap and toplap shellbeds contain low-diversity macrobenthic associations, which are often abraded or fragmented. Backlap shellbeds, which are equivalent to the condensed section formed at the maximum transgression, are characterized by dominance of epifaunal macrobenthos, preserved in a slightly cemented, glauconitic muddy matrix. In contrast, the shell density and species diversity of downlap shellbed associations are rather low. Variations in the stratigraphic distribution of shellbed types are reflected in symmetrical (inner shelf ) and asymmetrical (outer shelf ) sequence architectures, palaeodepositional settings. Symmetrical sequences have roughly the same thickness of transgressive systems tracts (TST) and highstand systems tracts (HST), and have well segregated shellbeds that were formed during marine onlap and backlap. Asymmetrical cycles have very thin TSTs and much thicker HSTs, and are characterized by the amalgamation of condensed onlap and backlap shellbeds. On the other hand, they focused on the Glossifungites trace assemblages, which can be observed in semi-consolidated substrate. Three distinctive biological associations can be recognized in modern firm substrates. These are the Polydora, Petricola and Upogebia association. Generally, subtidal and lower- to middle-intertidal firmgrounds are colonized by crustaceans (the Upogebia Association) or the bivalve Petricola (the Petricola Association), and the upper intertidal is dominated by the Polydora Association. All three assemblages produce characteristic ichnocoenoeses that are similar to ichnofacies recognized in the extensive Pleistocene record.
Reference
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