Stratigraphic and microfacies analysis of the Kaili Formation, a candidate GSSP for the Cambrian Series 2–Series 3 boundary

Abstract

The Kaili Formation of South China is a ~ 200–300 m thick succession of fine-grained siliciclastic sediments with minor carbonates that spans the proposed Cambrian Series 2–Series 3 boundary interval, formerly the Early-Middle Cambrian boundary. It was deposited at low latitude on a slope lying between the Yangtze carbonate platform to the northwest and deep-water outer shelf facies of the Jiangnan Basin to the southeast. Because the Kaili Formation contains abundant and well-preserved fossils throughout, most importantly oryctocephalid trilobites that have a broad geographic distribution, and because it appears to be less condensed than other contemporaneous sections found on other paleocontinents, a section near Balang Village, Jianhe County, Guizhou Province, has been proposed as a candidate Global Stratotype Section and Point (GSSP) for the Cambrian Series 2–3 boundary. However, because the Kaili Formation is mudstone-dominated, it is not possible to determine from outcrop study whether a change in depositional regime or a condensed interval may be present around the proposed boundary. Because the proposed boundary interval coincides with the maximum flooding stage of a global transgression, it is possible that subtle, yet significant facies changes may occur within the mudstones that comprise the proposed boundary interval, rendering its utility as a potential GSSP questionable. Here, we present an analysis of a composite section of the complete Kaili Formation exposed along two ridges and in a road cut near Balang Village. The section was measured at the cm-scale and sampled every 1 m throughout the complete thickness of the unit, except where covered. Microfacies analysis of 138 samples, using thin sections, polished slabs, acetate peels, X-radiographs and scanning electron microscopy was conducted in the laboratory. These analyses confirm that depositional processes within the Kaili Formation were consistent and unshifting throughout complete thickness of the formation. Event-driven deposition was maintained across the Kaili Formation with no evidence for condensation present, even at the 5–20 μm scale. The signal of the global transgression in the boundary interval is manifest as a slight thinning of individual millimeter-scale event-deposited lamina from 50 to 55 m above the base of the formation, around the proposed GSSP boundary at 52.8 m. The entire Kaili Formation appears to have been deposited below storm wave base, as no silt-sized or coarser clastic particles are present, and no evidence of cross-bedding or graded bedding occurs within any of the facies present within it. The great majority of its thickness, including the entirety of the proposed 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.