LATEST SANDBIAN–EARLIEST KATIAN (LATE ORDOVICIAN) "GUTTENBERG" POSITIVE δ13C EXCURSION (GICE) IN ASIA SPANS CA. 1.2 MYR (3 LONG-ECCENTRICITY CYCLES)

The boundary interval between the Sandbian and Katian stages of the Late Ordovician is characterized by a major positive excursion in carbon isotopes. This isotope excursion may be a more reliable way to correlate the Sandbian/Katian boundary on a global scale than the semi-provincial biostratigraphic events. It was originally named the "Guttenberg" isotope carbon excursion (GICE) after its discovery in the Guttenberg Limestone Member of Decorah Formation in Iowa of basal Katian, but that record is both condensed (ca. 5 m) and truncated at the base by a hiatus. Elsewhere in North America, the duration of the GICE is constrained by dated volcanic-ash beds to span less than ca. 400 kyr.

In contrast, the major sustained positive excursion of 2‰within the Qilang Formation of northeast Tarim Basin in northeast China that is considered equivalent to the GICE had a broad peak spanning nearly 100 meters. In addition to enhancing the carbon-isotope records, we applied cyclostratigraphy analysis to the detailed natural-gamma logging of two sections (Dawangou section, which is the auxiliary GSSP for the Sandbian Stage, and Sishichang section) located ca. 25 km apart. This outer shelf facies exhibits cycles of ca. 30 meters in the average proportions of carbonate to marl, which we interpret as a sedimentary record of long-eccentricity cycles. Superimposed on these large-scale cycles are 1.5-m oscillations that are interpreted as a record of precession. The astronomical-tuned sedimentary stratigraphy of both sections yielded identical durations of 1.2 Myr for the broad GICE excursion.

The broad GICE plateau in these sections exhibits three secondary oscillations in δ¹³C that are in-phase with the long-eccentricity cycles, indicating that regional δ¹³C values were slightly elevated during decreased relative carbonate influx. The coeval δ¹⁸O oscillations imply that these secondary peaks occurred during the relatively warmer phase of the long-eccentricity cycle. The broad GICE excursion subsides into an additional ca. 400-kyr interval of medium-elevated (ca. 1.2‰) values before returning to pre-GICE background levels of δ¹³C. We suggest that North American GICE excursion is equivalent to only one of the three secondary oscillations within the broader "Asian GICE".