唐古拉山中侏罗统缅甸光牡蛎(Liostrea birmanica REED)壳层的事件沉积成因及其环境意义

ORIGIN OF LIOSTREA BIRMANICA (OYSTER) SHELLBEDS IN MIDDLE JURASSIC OF THE TANGGULA AREAAND THEIR SIGNIFICANCE INPALEO-ENVIRONMENTAL ANALYSIS

唐古拉山地区中侏罗统产出丰富的牡蛎类化石,特别是缅甸光牡蛎Liostrea bir—manica Reed分布更为广泛,而且多以介壳层形式产出。在唐古拉山的冬曲,窝布茸曲及青藏公路108道班等地剖面上,栖息于软基底的表生底栖双壳类Liostrea birmanica形成近原地埋葬的介壳层堆积;内生的Ceratomya动物群则多散布于沉积物中,形成原地埋葬的化石群．它们和以丛藻迹Chondrites为主体的遗迹化石群依次重复出现,构成一个和风暴沉积事件相关的底栖动物群替代序列。Liostrea birmanica介壳层是风暴高峰期的侵蚀和再沉积所形成的,风暴期以后水下沉积间断所形成的轻度固化的软基底则迅速地被潜穴生物占领(丛藻迹Chondrites ich—nosp．和根珊瑚迹Rhizocorallium ichnoSp．);而以深挖穴滤食类型为主的Ceralomya组合则是好天气时的原地埋葬。在大多数情况下,它们指示着介于正常浪基面以下,风暴浪基面以上的低沉积速率浅水碳酸岩台地沉积环境。

In the Tanggula area of southern Qinghai province, the Middle Jurassic (mainly the Upper Bajocian to Lower Bathonian) yields a large number of oyster specimens, especially Liostrea birmanica Reed 1936. which is distributed widely in this.area and , in most cases, is accumulated densely to form shell beds. The fossiliferous carbonate deposits in which Liostrea birmanica occurs abundantly are commonly 0. 5 ~ 2. 0 m in thickness, and alternated with argillaceous and sandy silts. The fauna consists of infaunal Ceratomya ,Pleuromya and PholaJomya, scattered throughout the sediments; Liostrea birmanica, concentrated into shell beds and , in turn ichnofauna, domi nated by Chondrites. These faunal assemblages frequently display a cyclic replacement. Based on the sedimentary characters together with the biostratinomic evidence, the follow ing conclusions have been made in this paper; 1. Liostrea birmanica is a secondary soft-bottom recliner as recognized by Seilacher in 1984. This species is thought to have been well-adapted to various water depths, ranging from fair weather wave-base to storm wave-base. 2.The Liostrea birmanica shells were concentrated by storms in most cases.The Liostrea birmanica monospecific beds are distal in origin, while the Liostrea birmanica pau-cispecific beds, characterized by amalgamation,are proximal in origin. 3.During late Bojocian to early Bathonian, the shelf sediments at Dongqu Zha-worongqu and the 108 Highway Maintenance Squad in the Tanggula area are interpreted in terms of transgressive/regressive cycles . The transgressive period may be represented bythe tempestites of Liostrea birmanica shell beds, which record an upward transition from proximal to distal storm shell beds, i.e. progressive deepening. Liostrea birmanica is an opportunist species well - adapting to soft substrata of the carbonate platform with a very low rate of sedimetation during the transgression. 4. The fossil sequence, consisting of epibenthic Liostrea birmanica assemblage, infaunal Ceratomya assemblage and ichnofauna, cannot be taken as a true ecological replacement of the communities, but taphonomic accumulations of the benthic fauna resulted from the episodic storm deposits.