论华南栖霞组中的菊花石

REVIEWS ON CHRYSANTHEMUM STONES FROM THE QIXIA FORMATION IN SOUTH CHINA

摘要:

本文中的菊花石指华南栖霞组中由柱状晶体放射状排列而成的菊花状晶簇。它最早发现于湖南浏阳永和镇栖霞组上部(方邺森等人,1988),之后又见于广西来宾县城郊栖霞组上部(沙庆安等人,1990),它们被认为是天青石或天青石假象。笔者(1991)在湖北省黄石地区栖霞组上部发现该类假象,并认为其原生矿物是六水碳钙石。虽然它们分别被解释成不同矿物,但无论是外部形态还是内部成分、结构构造均具极大的相似性。对该矿物集合体的不同解释将对栖霞组沉积环境的认识产生重要影响。本文通过与六水碳钙石假象的对比,讨论该矿物集合体的成因及其环境意义。不妥之处请批评指正。

Abstract:

Chrysanthemum stones recognized in the Qixia Formation in South China are known as the chrysanthemum-shaped druses which consist of the radially-arranged columnar crystals. They occur usually as calcite pseudomorphs in the upper part of the Qixia Formation(Artinskian)in Liuyang of Hunan, Laibin of Guangxi and Huangshi of Hubei. Despite its different locaties, the following similarities may be seen.(1)They occur almost in the same horizon in the Qixia Formation and have only been confined to the formation so far;(2)They are identical in form and dimensions;(3)They are composed dominantly of calcite, with small amounts of chalcedony. Calcite generally shows generation structure and chalcedony occurs as the filling material in the voids. Although the mineral celestite has been observed in some samples from Liuyang in Hunan, it seems that the celestite is the product resulted from replacement during diagenesis rather than a kind of original mineral, and (4)they commonly enclose micro-fossils and pierce the bedding plane. The chrysanthemum stones in the study area were interpreted as celestite or celestite pseudomorphs by the previous workers. The author contend, however, that they should be pseodumouphs after ikaite. This explanation is strongly supported by the geological settings during the Qixiaian including cold climates and cold upwelling currents and by sedimentary characteristics of the Qixia Formation such as the presence of foramol skeletal grain assemblages and the absence of non-skeletal grains and organic reefs. It Is concluded that the chrysanthemum stones from the study area may be pseudomorphs after ikaite and the Qixia Formation might have resulted from cool- to cold-water deposition.