INTERBEDDED FOSSIL-BEARING BEDS AND TUFF HORIZONS IN SECTORS OF THE SIBUMASU TERRANE CALIBRATE PARTS OF TRILOBITE-BASED BIOZONATION OF THE LATEST CAMBRIAN AND EARLIEST ORDOVICIAN IN EQUATORIAL GONDWANA

Sandstone from widespread parts of the globe show that the late Cambrian was a time of peak production of detrital zircons, indicating extensive felsic igneous activity. Ironically, the late Cambrian remains one of the geochronologically most poorly calibrated intervals of Phanerozoic time. The felsic activity was concentrated about the equatorial margin of Gondwana. Its record is poorly preserved due to uplift at the time in the Kurgiakh orogeny, and the complex later history of the region related to the opening and closing of multiple branches of Tethys during the construction of the eastern Asia craton. However, tuff layers interbedded with fossil-bearing sandstone occur at many horizons in Cambrian rocks that were deposited in outboard parts of equatorial Gondwana and that became the Sibumasu terrane with the opening of the Meso-Teyths in the late Paleozoic. Ongoing work in association with IGCP project 668 in Ko Tarutao in the southern part of Sibumasu, and in the south and north parts of Shan State in Myanmar, record (1) the deposition of a considerable thickness of sandstone around 490 Ma during the middle of Cambrian Stage 10, (2) a possible hiatus or condensed interval across the Cambrian–Ordovician boundary, and (3) further deposition of fine-grained siliciclastic sediment in the early Tremadocian, around 484 Ma. Trilobites interbedded with the tuff horizons permit calibration of biozones in North China and other areas beyond, where absolute dates themselves are unavailable.