香港九龙破火山的地层结构与划分讨论

Stratigraphic Structure and Subdivision of the Kowloon Caldera in Hong Kong

香港岛—九龙半岛地区以往火山地层的划分主要依据岩性特征,基本不考虑火山构造和火山岩相的鉴别,由于陆相火山岩区特有的岩性岩相时空上的复杂多变,导致地层组越划越多,不能合理进行区域地层对比,更无法通过火山地层划分恢复古火山活动过程。笔者等在该地区开展了野外路线地质调查和火山构造—火山岩相研究,鉴别出九龙复活破火山机构,在此基础上确定火山地层主要由同一期普林尼式喷发形成的冷却单元构成,有类似的地层结构,结合年龄资料和区域地质对比,它们基本上可归入同一地层组;同时,分析了陆相火山地层的划分方法,指出岩石地层单位划分不能作为主要方法;冷却单元或流动单元才是合理划分陆相火山地层的基本单位。

Volcanic strata in the Hong Kong Island—Kowloon area were subdivided according to their lithologic and petrochemical features, regardless of volcanic structure and petrofacies. As a result, due to specifically lithologic and petrofacies variations in time and space of continental volcanic rocks, the stratigraphic formations were subdivided to be more and more, and could not be used rationally to compare regional volcanic sequence and to probe into paleo-volcanic process. The authors carried out field route surveys and studed volcanic structure—petrofacies in the Hong Kong Island and Kowloon area, and identified the Kowloon Revived Caldera. On this basis, we demonstrate that volcanic strata within this caldera consist mainly of the same cooling unit formed during a Plinian eruption, and have similar stratigraphic structure. Combined with age data and regional geologic comparison, they are inferred rationally to classify into the same formation. It is also proposed that cooling units or flow units, other than lithologic unit, are rational basic unit to subdivision of continental volcanic strata.