Abstract: The paper mainly focuses on the original upright occurrence of the elongated (rod-shaped) concretions in one specific silt layer, distribution, morphology, internal colloidal structure and mineral assemblages of the iron-rich elongated concretions in Taihu Lake area. The main purpose of the paper is to discuss the formation mechanisms of the iron-rich elongated concretions. In horizontal,
the rod-shaped concretions are widely distributed in one specific silty layer in Taihu Lake. Vertically, the rod-shaped concretions are found at different heights, from the bottom of Tai lake to the farm fields on the hillside, and in the grooves near the foothills. Fresh rodshaped concretions only can be seen in the landmark silty layer with a rigid gray for the outer surface and the color of the natural broken surface is gray. The original occurrence of the rod-shaped concretions is upright inserted in the landmark silty layer, with the head down and the tail upward, and the heads of elongated concretions have hemispherical shape and curved threads lines. While, another kind of concretions found in the disturbed layer have short columns with smooth outer surface due to moving wearing, and the color appears grayish brown or yellowish brown due to weathering. The length of the rod-shaped concretions range from 1 to 80 cm, some is short and some is long. Most of short ones are broken sections of the long one. The diameters of the elongated concretions range from half cm to 2 cm. Most of rod-shaped concretions have weathering crusts with a thickness about 1 to 2 mm. The outer surface of the rod-shaped concretions has unique spiral patterns. Major inclusions in the concretions are angular sharp-edged quartz grains with poor roundness and poor separation surrounding by fine matrix of cement of the concretions. Minors inclusions are rock debris or dusts aggregates. The main cements are siderite euhedral crystals with size of several micrometers. In addition, many iron-rich spherulites adhere to outer surface of the concretions, and part of elongated concretions consisting with iron-rich spherules. The porosity of the elongated concretions is less than 20%, and they are much denser than most concretions formed in surface depth. The original upright occurrence of the rod-shaped concretions indicate that it is not the residue product after leaching the underlying hard loess layer. Also, the upright occurrence infers that the age of the elongated concretions are not older than the age of the specific silty layer which is dated as ~7 ka B. P. There are two possible origins of iron-rich concretions. One is lake deposition hypothesis, that concretions grow and form in the porous space of mudlayer

in lake bottom；the other is airburst fallout hypothesis, that elongated concretions form in airburst plume and fall down and insert
into lake mud layer. We think airburst fallout hypothesis is more reasonable, because the fallout materials of airburst plume can  explain the unique morphology and internal characteristics of the elongated concretions.

Key words: Taihu Lake, siderite, elongated concretions, impact, airburst