湘西北江坪地区志留系风暴沉积特征及意义

Silurian storm deposits in Jiangping area,northwestern Hunan Province:Characteristics and geological significances

湘西北江坪地区志留系小河坝组发育了大量与风暴作用有关的沉积构造,如渠模、风暴丘状体、波状侵蚀面、介壳层、递变层理和振荡波痕等。在垂向厚度不足5,m的地层中连续发育了3期垂向结构特征不尽相同的风暴岩序列:Ⅰ.具波状侵蚀面的风暴丘状体序列;Ⅱ.具渠模构造的递变层理段+块状层理段:Ⅲ.具波状侵蚀面的递变层理段+水平层理段。上述风暴沉积都具有原地或近原地特点。该套风暴沉积的发现进一步证实了早志留世小河坝期湘西北地区的古地理环境为古海水深度相对较小、接近于正常浪基面的临滨环境。

A great quantity of storm-related sedimentary structures, such as gutter cast, storm domal bodies, wavy erosion surface, lumachelle, graded bedding and wave ripple, were discovered in the Silurian Xiaoheba Formation in Jiangping area, northwestern Hunan Province. Three sequences with different vertical stacking patterns were deposited in the formation, whose vertical thickness is less than 5 m. Sequence Ⅰis the storm domal bodies with wavy erosion surfaces. Sequence Ⅱ is the graded bedding and massive bedding interval with gutter cast. Sequence Ⅲ is the graded bedding and horizontal bedding interval with wavy erosion surfaces. All the storm deposits are in situ or of proximal deposition. The occurrence of storm deposits indicates that during deposition of the Early Silurian Xiaoheba Formation, the palaeogeographic environment of northwestern Hunan Province was characterized by a shoreface setting with a relatively lower water depth.