Abstract:

The typical section of the Fossil Hill Formation is at Fossil Hill between the Great Wall Station and President Frei Station in Fildes Peninsula, King George Island.The formation chiefly consists of sedimentary volcanic breccia and tuffites,intercalated with normal sedimentary rocks and two thin beds of inferior coal at the upper part.It can be divided into two cycles of sedimentation.The sedimentary volcanic breccia and tuffites in cycle Ⅰ are composed of volcanic detritus, plagioclase crystallinoclats and matrix of fine volcanic ash which reach to more than 90%.The sedimentary characteristics reflect a rapid deposition from volcaniclastic debris flow to low density flow.The coarse tuffites of Beds 1 and 2 were obviously zeolitized by laumontite and analcite. The fine volcanic ash of the mudstone like tuffite intercalations had been changed into the regularly mixed layer mineral of chlorite and montmorillonite before the zeolitization,because the mixed layer mineral was also often replaced by zeolite.In the fine tuffites of Bed 2 there are a number of albite by X ray diffraction analysis. It is inferred that Na + of albite and analcite might take its source at laumontitization of andesine. Cycle Ⅱ consists of sedimentary volcanic breccia and tufittes intercalated with greywacke as well as thin coal Beds. It reflects that normal sedimentary materials entered the basin.The mudstone like tuffites of Beds 3 and 4 are characterized by strong smectitization and poor zeolitization. Plagioclase of these rocks mostly is andesine and (or) labradorite. The composition and content of trace elements of the tuffites correspond with underlying andesite of the Agate Beach Formation. The movement and concentration of some elements in the Beds 2 and 3 are referred to the process of thermal fluid. Low content of B(<1×10 -6 ) in the section indicates that the sedimentary environment is not related to sea water. Excepting graded rhythmite, various structures are developed in Bed 2 of cycle I, such as small zeolitized “concretions” on the Bedding surface and the “window lattice like” fractures resulted from tectonic pressed stress after diagenesis which were described respectively as “rain print” and “mud crack” in the previous literature. The remain mottled structure produced by zeolitization is commonly visible in Bed 2. On the basis of petrologic and palaeontologic characteristics,the Fossil Hill formation of the Fossil Hill section probably represents a fluvio lacustrine depositional environment. It was in near shore or bay and under the warm and moist climate condition. The sedimentation was affected by both volcanic action and seasonal flood.