侏罗纪菊石形态──特提斯喜马拉雅海的深度标志

侏罗纪菊石类化石的形态特征是其生存环境水深的良好标志，而通过对菊石壳体结构（体管和隔壁沟）强度的裂限水深（ｉｍｐｌｏｓｉｏｎｄｅｐｔｈｌｉｍｉｔ）计算可获得半定量的水深数据。对西藏南部的聂拉木－古错地区的中保罗世阿连期至晚侏罗世提唐期沉积环境的分析，共识别出１３个自水下２０ｍ到水下５５０ｍ的陆棚至陆坡的不同水深环境。中侏罗世阿连－早巴柔期时，该区内的海水深度表明基本限于浅水碎屑－碳酸岩台地型的内陆棚环境，从晚巴柔至早提唐期间，沉积环境颇不稳定，互相转换于外陆棚和陆坡之间，尤以位于陆棚和陆坡转折端、水深１５０—３００ｍ的中等水深环境延续较长时间。相对下降较快的地块出现浊积底栖相沉积。而在晚提唐期则表现为相对稳定的外陆坡沉积环境。整个侏罗纪期间的特提斯喜马拉雅海最大水深大概不会超过５５０ｍ。菊石类所提供的年代格架及古生态研究所得到的水深数据，可以比较合理地解释本区中、上侏罗统所以有巨大的沉积厚度和复杂的沉积相组合，在很大程度上是源于特提斯被动大陆边缘发育的同沉积断裂为特征的张性犁式断层系和地垒－地堑地质构造的叠加作用。

JURASSIC AMMONITE MORPHOTYPES AS WATER-DEPTH INDICATOR OF TETHYS-HIMALAYA SEA

Middle and Late Jurassic depositional environments in the Nyalam--Gucuo Area,South Tibet, are reconstructed in terms of comprehensive paleoecological analysis,especially the study of ammonite morphotypes and implosion depth limits,with 13 water--depthzones recognized from the Jurassic sediments, ranging from 20m to 550m at submarinedepth along continental shelf and slope environments. However, a transitional environment from outer shelf to slope, ranging from 150m to 300m at water--depth, dominatedmost of the times from late Bajocian to early Tithonian in this area. From Aalenian to early Bajocian sedimentary factes have shown distinctions of shore siliciclastic and carbonateplatform deposits,and from late Bajocian to early Tithonian the factes have exhibited alternative changes of outer shelf with slope environments, or vice versa. It is noted that thedepositional environment of more than 550m at water depth is considered to have occurredmerely after early Tithonian in the Tethys--Himalaya Sea of South Tibet. In the light ofammonite biostratigraphic approach, a pattern of extensional faulting is established in theregion,where the nature of a passive--margin has been shown. Since the sedimentation wascontrolled by activities of listric normal faulting and horst--and--graben structure, paleoecological environments on bordering blocks are thought to have been characterized with agreat disparity in water depth over a limited area. As a result, turbidite--benthos factestook place on blocks which descended more quickly, and became deeper than borderingblocks. Because of the strong tectonic overprint for the periods, the great thickness of theJurassic sediments can be interpreted with this pattern. It is suggested that a major challenge is to separate the tectonic, eustatic and regional sea level signals in studying sea levelchanges of the passive--margin region.