Stratigraphy, Sedimentary Structures, and Textures of the Late Neoproterozoic Doushantuo Cap Carbonate in South China

Abstract

The 3-to 5-m-thick Doushantuo cap carbonate in south China overlies the glaciogenic Nantuo Formation (ca. 635 Ma) and consists of laterally persistent, thinly laminated and normally graded dolomite and limestone indicative of relatively deep-water deposition, most likely below storm wave base. The basal portion of this carbonate contains a distinctive suite of closely associated tepee-like structures, stromatactis-like cavities, layer-parallel sheet cracks, and cemented breccias. The cores of tepees are composed of stacked cavities lined by cements and brecciated host dolomicrite. Onlap by laminated sediment indicates synsedimentary disruption of bedding that resulted in a positive seafloor expression. Cavities and sheet cracks contain internal sediments, and they are lined by originally aragonitic isopachous botryoidal cements with acicular radiating needles, now replaced by dolomite and silica. Pyrite and barite are common, and calcite is locally retained as a primary mineral. These features share morphological and petrographic attributes with modern and ancient methane seeps in which methane gas and fluids provide both a force for physical disruption from buoyancy and a source of alkalinity for significant cementation. The presence of d 13 C values as low as 241% in well preserved limestone crusts and cements within and immediately above the tepee-like structures provides unequivocal evidence for methane influence, and the widespread distribution of identical sedimentary structures and paragenetic cement sequences across the entire basin at the same basal cap carbonate level is consistent with gas hydrate destabilization and the development of methane seeps as a result of postglacial warming of the ocean. Considering the broad distribution of similar features at the same stratigraphic level in other cap carbonates globally, we suggest that the late Neoproterozoic postglacial methane release may have influenced the oceanic oxygen level as well as contributed to postglacial warming via the greenhouse effects of methane.

Evolution of Neoproterozoic Wonoka–Shuram Anomaly-aged carbonates: Evidence from clumped isotope paleothermometry

“…9; Clayton et al, 1966;Allan and Matthews, 1982;Egeberg and Aagaard, 1989;Clark and Fritz, 1997). The temperatures recorded in Death Valley and South China carbonates lie well within those that would have been reached at maximum burial, as constrained by the thickness of overlying strata: in both Death Valley and South China ∼4-7 km of rock overlies the excursion facies (Stewart, 1966;Jiang et al, 2006), implying possible maximum burial temperatures of up to ∼200 • C (assuming a geothermal gradient of 25 • C/km). In northwestern Mexico, the overlying strata thickness has not been quantified beyond the Cambrian, and therefore maximum burial depths cannot be independently assessed.…”

Pulsed oxidation and biological evolution in the Ediacaran Doushantuo Formation

“…At the Jiulongwan section [supporting information (SI) Fig. 3], member I represents an Ϸ5-m-thick cap dolostone overlying the Nantuo glacial diamictite and contains a suite of enigmatic sedimentary structures and textures (12,13). Member II is characterized by Ϸ70 m of alternating organic-rich shale and dolostone beds with abundant pea-sized chert nodules.…”

Neoproterozoic glaciation in the Earth System

“…However, in almost all basins where cap carbonates are preserved, local exposures show an assemblage of peculiar sedimentary structures, including tepee-like antiforms (Fig. 2h-j) of several different morphologies and origins (Aitken 1991b;James et al 2001;Allen & Hoffman 2005;Gammon et al 2005;Jiang et al 2006), aragonite fans (James et al 2001;, parallel vertical tube-like structures (Cloud et al 1974;Hegenberger 1993;Corsetti & Grotzinger 2005), sheet cracks (Kennedy 1996;Jiang et al 2006), and baritic and iron oxide crystal fans indicative of condensation at the top of the unit (Walter & Bauld 1983;Kennedy 1996). There is often a shift from microcrystalline, fabric-retentive dolomite to calcite.…”