Abstract:

Regarding to the opinion of the impact origin of Taihu lake based on the discovery of the iron-rich siderite ejecta
and iron-poor ejecta, we restudied similar samples by way of mineralogy, geochemistry and chronology. Our data show that

those samples found in Taihu Lake are in fact siderite and limonite concretions which formed in Quaternary mud layer during
diagenesis. Supporting evidence come from that a large number of aquatic plant debris, opal phytolith of plants, bacteria and
a few animal shell fragments were found in those concretions. After high temperature burning, siderite completely changed
into hematite, indicating that the high temperature phase of siderite mineral does not exist. The siderite 14C dating shows that
the various samples formed at different times. δ13C data of siderite indicates a source of inorganic carbon in fresh water, not a carbonate direct source in the Taihu area. The so-called iron-poor ejecta are actually diagenetic calcium concretions found in loess layers. Those evidences showed that the weird-shaped concretions found in the Taihu Lake are diagenetic concretion, which are not impact ejecta.

Key words: Taihu, impact ejecta, ferruginous concretion, calcareous concretions, _¹⁴C dating