Carbon chemostratigraphy of the Cambrian-Ordovician transition in a midlatitude mixed platform, Montagne Noire, France

The Cambrian Drumian Carbon Isotope Excursion (DICE) and the Steptoean Positive Carbon Isotope Excursion (SPICE) have been so far reported in subtropical platforms: the DICE event is a large negative excursion in δ13Ccarb record that nearly coincides with the beginning of the Drumian (Cambrian Series 3), whereas the SPICE event is a large positive shift in δ13Ccarb identified close to the base of the Paibian (Furongian Series). The chemostratigraphic excursions have been recognized in carbonate-dominated platforms, and their application to mid- and high-latitude, siliciclastic-dominated platforms has been problematic. This paper offers, for the first time, a high-resolution stratigraphic analysis of this time span in a temperate-water platform (Montagne Noire, France) that recorded episodes of carbonate productivity in nearshore environments. Two sections were analyzed in detail: a complete Cambrian Series 3–Tremadocian succession, devoid of carbonate interbeds and paleogeographically located in the distal part of the platform (Pardailhan nappe), and a Furongian-Tremadocian succession, which has limestone interbeds and represents the proximal part of the platform (Minervois nappe).

The lower part of the La Gardie Formation in the Pardailhan nappe displays background δ13Corg values of −22‰ punctuated by a large negative, middle Languedocian (regional substage) shift, peaking at −24.5‰. This negative excursion is similar to the DICE event reported close to the base of the Drumian, although the latter is exclusively based on δ13Ccarb values. Stable isotopes of δ13Ccarb and δ13Corg from both sections also indicate a common chemostratigraphic shift at the onset of the Furongian. In nearshore deposits of the Minervois nappe, the δ13Ccarb background values for the interbedded limestone strata of the Val d'Homs Formation gradually increase from −3‰ to −1‰ and are punctuated by a sharp increase in δ13Ccarb values to >3.0‰. By contrast, in basinal shales of the La Gardie Formation in the Pardailhan nappe, the δ13Corg background values decrease from −22‰ to −27.1‰. This trend is directly controlled by total organic carbon (TOC) contents and is interpreted to have been initiated by changes in the degree of biodegradation of organic matter, sulfate reduction, and methanogenesis before or soon after burial. Despite this overprinting and the very low degree of metamorphism reached by the shales, the δ13Corg background trend is also punctuated by a sharp positive shift in δ13Corg values to >4.0‰, similar to the SPICE event. In addition, the asymmetric shape of the SPICE excursion in the Montagne Noire shows evidence for the HERB event, a late Furongian negative carbon isotope excursion not yet accepted as a worldwide chemostratigraphic anomaly.

The recognition of both chemostratigraphic shifts in mixed and clayey strata opens new possibilities of chemostratigraphic correlation in mid- and high-latitude platforms, where carbonate factories did not widely develop, and strata only contain endemic fauna. This implication has a major consequence because the Global Stratotype Section and Point (GSSP) of the Drumian and Paibian stages occurs near the base of both excursions.

Field of study: Geology