EARLY CRETACEOUS METHANE SEEPAGE SYSTEM AND ASSOCIATED CARBONATES, BIOTA AND GEOCHEMISTRY, SVERDRUP BASIN, ELLEF RINGNES ISLAND, NUNAVUT

Over 130 carbonate deposits, interpreted as having formed at methane seepage sites in the Sverdrup Basin have been discovered on Ellef Ringnes Island, Canadian Arctic. The deposits, up to 2.7 metres tall and 60 metres wide, are found within the lower member of the Lower Cretaceous Christopher Formation. The carbonates have complex and heterogeneous structures typical of seep carbonates, including banded botryoidal and clotted textures as well as void filling sparite. Stable carbon isotopes show highly ¹³C-depleted values, as low as δ¹³C_VPDB = -53‰, indicative of authigenic carbonate precipitation via the anaerobic oxidation of biogenic methane. The seep carbonates preserve abundant fauna including multiple species of bivalves, worm tubes, ammonites and gastropods. Methane seepage is estimated to have lasted ~500,000 years and was brought about by a tensional stress regime as well as salt diapirism related faulting.

The seep deposits are composed of six major carbonate phases, two of which are closely associated with one another, high magnesian botryoidal calcite and clotted yellow calcite, together known as banded calcite. Banded calcite is composed of layers (micron to mm scale) of botryoidal calcite and yellow calcite that alternate with one another, forming deposits up to 20 cm thick. The two cement phases are texturally very different from one another, one being composed of clear sweeping botryoids of high magnesian calcite and the other, anhedral clotted iron rich yellow calcite with abundant pyrite and organic debris. It is proposed here that the textural, as well as geochemical differences between the two phases are attributed to these cements precipitating under different redox condition, as well as the hypothesized presence of preserved biofilms in clotted yellow calcite.