THE EDIACARAN TO EARLY CAMBRIAN IN MONGOLIA: STRATIGRAPHY, GEOCHEMISTRY AND PALEONTOLOGY OF A TECTONICALLY REACTIVED BASIN

Ediacaran and early Cambrian carbonates, clastics, and phosphorites in the Zavkhan (Dzabkhan) Basin, Govi-Altay, Mongolia were rapidly deposited in a tectonically reactivated foredeep basin as the Dzabkhan terrane subducted beneath the Khantayshir-Dariv arc. Here we present integrated stratigraphy, δ¹³C chemostratigraphy, and paleontology for < 1600 m of Ediacaran through early Cambrian strata, and discuss problems in trying to identify the Precambrian-Cambrian boundary in Mongolian strata.

The Zuuny (Zunne) Arts Formation overlies the Cryogenian Ulaanbulag (Ulaan Bulagyn) Formation, a dolomite capped by Boxonia grumulosa. Phosphatic shales underlying several debris flows and dark, thinly-bedded limestones mark the base of the Zuuny Arts Formation. The formation is < 200 m thick and shallows up to a trough cross-bedded ooid grainstone. A light grey ooid grainstone at the top of the Zuuny Arts Formation is capped by phosphatic shales, marking the boundary between the Zuuny Arts Formation and the Bayangol Formation. Here, we subdivide the < 1000 m of Bayangol Formation into members to fascillitate mapping, a facies model, and basinal correlations. The lower ~300 m of the Bayangol Formation has mixed siliciclastics, calcimicrobial reefs, thrombolites, and limestone grainstones. The upper ~700 m is bedded to turbiditic sands, silts, and conglomerates with abundant trace fossils. There are frequent shallowing up parasequences capped by calcimicrobial patch reefs. In the top ~300 m, there are frequent channelized calcarenite, oncoid, and ooid beds. Overlying the Bayangol Formation is the Khairkhan Formation, the flysch in this collisional basin. It is a mix of silts, sands, and conglomerates with pebble to boulder sized clasts. The Salaany Gol Formation is a grey to pink archaeocyathid-rich limestone that is usually faulted on both sides.