"BURGESS BIOTAS" AND EPISODIC SLOPE AND EPEIRIC SEA DYSAEROBIA IN THE LATE PRECAMBRIAN--PALEOZOIC

The exquisiteness and seeming frequency Burgess Shale-type preservation have been considered peculiar to the Cambrian. However, Burgess-type faunas are better regarded as occurring in two intervals of the Cambrian associated with intensified mid-water dysaerobia and incursion of dysoxic waters onto carbonate platforms. The earliest Cambrian Burgess-type biotas (Chen Jiang, Sirius Passet, Poleta, lower Kinzers) occur only after the appearance of trilobite-dominated faunas halfway through the period. They roughly correlate with late Early Cambrian intensification and thickening of a mid-water dysaerobic water mass on the east Laurentian slope (Brown’s Pond dysaerobic interval [DI]) and incursion of dysoxic waters in Siberia (Sinsk event). The second interval of Cambrian Burgess-type preservation followed re-establishment of dysoxic slope waters after the oxic facies of the latest Early Cambrian Hawke Bay regression. Terminal Early Cambrian–Middle Cambrian Burgess-type biotas (Burgess, Spence, Marjum, Parker) appeared with repeated onlaps of dysoxic slope water during a short interval (ca. 5–10 m.y.) early in the Hatch Hill DI. (latest Early Cambrian–early Tremadocian). Burgess-type preservation required tropical continents and restricted oceanic circulation that allowed dysoxic slope waters to thicken with climatic maxima/sea-level rise until they onlapped the platform and the lower parts of aggrading, metazoan-rich carbonate build-ups. Episodically dysoxic epeiric seas, rather than Monterey events or rise of poorly oxygenated, deep waters associated with a thermocline, can explain Cambrian through Devonian (Hunsruck) occurrences of exceptional preservation in unrestricted marine facies. These conditions likely extended into the latest Proterozoic, where phosphatized metazoan embryos and metaphyte preservation in the Dayangcha Formation suggest the role of dysoxic epeiric sea water in accumulation and preservation of organic materials.