ORIGIN OF INORGANIC CHERT IN LACUSTRINE ENVIRONMENT – INSIGHTS FROM THE K-PG PING CHAU FORMATION OF HONG KONG

Several saline lakes were present in the northern margin of South China Sea in its incipient stage of rifting during the Late Cretaceous to early Paleogene. These lakes are characterized by the presence of organic-rich shales, evaporites, biogenic sediments as well as stromatolites. The Ping Chau formation comprises a rhythmic sequence of dolomites, and biogenic and clastic sediments that were deposited in an alkaline lake located in the present-day Mirs Bay in northeastern Hong Kong. Stromatolite and algal structures are common in the sediments. The presence of mud cracks and rain prints in the formation reveals a semi-arid paleoenvironment with ephemeral water influxes. Gypsum, glauberite and other minerals indicative of a sabkha condition are found to have been replaced by zeolite, and in turn by aegirine. The formation also contains a unique, 0.8-m thick, non-biogenic, bedded chert with smooth, planar bedding surfaces. Internally, the bed consists of a lower section of completely silicified rock with lamination and stromatolite structures, and an upper section of massive chert. The sole occurrence of the bed and the well-defined bedding surfaces argue against a volcanic or pedogenic origin. The basin-wide presence of the bed does not support it being a sinter layer deposited in hot springs either. We postulate that the chert bed was formed by direct chemical precipitation of amorphous silica resulting from a sudden decrease in the pH value of the alkaline lake. This singular event was probably triggered by an influx of hydrothermal fluids or a global acid rain. The former hypothesis is supported by the presence of aegirine that indicates an elevated temperature (around 130-200^oC) event experienced by the sediments and the occurrence of multiple hydrothermal bodies in the same region. Alternatively, the global acid rain triggered by the terminal Cretaceous impact event may also plausibly explain the sudden drop in the pH value. (That would make it the K-Pg Boundary). While the origin of the chert bed remains enigmatic, the study has yielded insights into the formation of inorganic chert in lacustrine environments. The rhythmic bedding of the Ping Chau formation as well as several inorganic cherty layers observed in the area can be similarly explained by a change in acidity of the alkaline lakes during their deposition.