STRATIGRAPHIC AND MICROFACIES ANALYSIS OF THE KAILI FORMATION AT BALANG VILLAGE, GUIZHOU, CHINA

The Kaili Formation of South China is a succession of mudstones and fine-gained carbonates that spans the proposed Cambrian Series 2-Series 3 boundary. A section near Balang Village has been proposed as a candidate Global Stratotype Section and Point (GSSP). At this locality, the Kaili Formation contains well-preserved fossils throughout its ~200 m thickness, including an abundant and diverse Burgess Shale-type biota of early middle Cambrian (Series 3) age. A composite section of the complete Kaili Formation was studied in detail to address its suitability as a candidate GSSP and to examine the depositional setting of the Burgess Shale-type biota. The section was measured at the cm-scale and sampled at 1 m intervals throughout the complete thickness of the unit, except where covered. Microfacies analysis of 138 samples was conducted in the laboratory, using SEM microscopy, polished slabs, X-radiography, thin sections, and acetate peels. Analyses confirm that depositional processes within the Kaili Formation were consistent throughout the formation, and that event-driven deposition was maintained across its complete thickness; no evidence for condensation is present. The signal of global transgression in the boundary interval is manifest as a slight thinning of millimeter-scale event-deposited lamina from 50-55 m above the base of the formation, around the proposed GSSP boundary at 52.8 m. The entire Kaili Formation was deposited in a distal ramp setting below storm wave base. No silt-sized or coarser clastic particles are present, and no evidence of cross-bedding, scour, or graded bedding occurs in any interval of the formation. The majority of its thickness, including the entirety of the boundary interval, is comprised of mm-laminated calcareous claystones that exhibit randomly oriented clay microfabrics characteristic of deposition from turbid suspension by sediment-gravity flows.