Hematite U-Pb Dating Of Snowball Earth Meltwater Events
The Snowball Earth hypothesis predicts global ice cover; however, previous descriptions of Cryogenian (720-635 Ma) glacial deposits are limited to continental margins and shallow marine basins.
The Tavakaiv (Tava) sandstone injectites and ridges in Colorado, USA, preserve a rare terrestrial record of Cryogenian low-latitude glaciation. Injectites, ridges, and chemically weathered crystalline rock display features characteristic of fluidization and pervasive deformation in a subglacial environment due to glacial loading, fluid overpressure, and repeated sand injection during meltwater events.
Photographs of sedimentary features and structures found in Tava dikes. (A) Polycrystalline quartz and quartzofeldspathic granules within structureless medium sand matrix of 2nd-generation, light-colored injectite. (B) Diamictite with non-touching clasts of granite (angular) and quartz (subrounded) in purple-grey unsorted sandy matrix. (C) Margin of metre-wide dike marked by a concentration of matrix-supported rounded quartz pebbles. (D) Fluted margin of a light-colored injectite. (E) Dark, hematite-cemented, non-sorted fine- to coarse-grained sand injectite. Location for A, C, and D is Manitou Springs; for B is Cheyenne Cañon, and for E is Williams Fork. Tava study locations are indicated in Fig. S2. — PNAS
In situ hematite U-Pb geochronology on hematite-quartz veins, which crosscut and are cut by Tava dikes, constrain sand injection at ~690-660 Ma. We attribute early Tava sand injection episodes to basal melting associated with rifting and geothermal heating, and later injections to meltwater generation during ~661 Ma Sturtian deglaciation.
A modern analog is provided by the Ross Embayment of Antarctica, where rift-related faults border sediment-filled basins, overpressurized fluids circulate in confined aquifers below ice, and extensive preglacial topography is preserved.
Field evidence and geochronology in Colorado further highlight that deep chemical weathering of Proterozoic bedrock and denudation associated with the Great Unconformity predate Cryogenian injection of fluidized sand, consistent with limited glacial erosion.
Geological map showing distribution of Tava sandstone ridges (red) along the southern Ute Pass fault (bold black line). Locations labeled are Ch, Cheyenne Cañon; Cr, Crystola; and MS, Manitou Springs. Geological formations (pink, Proterozoic; grey, Phanerozoic) and subsidiary faults (thin lines) are simplified from Morgan et al. (2003) and Keller et al. (2005). Hillshade digital elevation is from National Elevation Database (NED) 30m DEM. (https://www.usgs.gov/programs/national-geospatial-program, U.S. Geological Survey, EROS Data Center)
Hematite U-Pb dating of Snowball Earth meltwater events, PNAS
Astrobiology
