CRYOGENIAN SANDSTONES IN COLORADO: A NEW TERRESTRIAL RECORD FOR LAURENTIA (RODINIA) REVEALED THROUGH DETRITAL ZIRCON PROVENANCE ANALYSIS

A longstanding problem in the geology of Colorado, USA, has been the origin and age of mature quartz sandstone hosted by Mesoproterozoic plutono-metamorphic rocks. The sandstone unit, informally named Tava sandstone, consists of structureless moderately fine to fine grained quartz sandstone supporting dispersed, non-touching granule-to-cobble-sized rounded to subrounded quartz clasts, with some angular feldspar and lithics. It forms dikes and sills, that are granite-hosted in most instances, and large fault-bounded, internally-faulted bodies that attain volumes of millions of cubic meters. Rare instances of dikes and sills within a Tava sandstone host are found, and one locality there is continuity of injectites from a Tava host into a crystalline host. The distinctive sedimentological characteristics and geological context suggest liquefaction and remobilization of mature quartz sediment, some portion of which was injected into parent material and wall rock. In some instances it is clear that the sediment injected along preexisting faults ; nearly all sites also display a brittle deformation overprint.

Eleven detrital zircon (DZ) samples from seven sites in the Colorado Front Range and one site in the Williams Creek Range were analyzed, with acquisition of 100 to 235 zircons per sample. All contain a dominant broad age group of 0.97 to 1.33 Ga, indicative of derivation of DZs from the distant Grenville orogen, and a majority also display narrow, defined age peaks at circa 1.09 Ma, 1.45 and 1.68 Ga. These Tava sandstone shares these DZ age characteristics in common, and shows a high probability of statistical correlation, with siliciclastic units of Neoproterozoic age in the western USA. DZ data for Phanerozoic mature sandstones are dissimilar. On this basis, we conclude that Tava sandstone represents a vestige of the 800 – 660 Ma intracratonic sediment distribution system, that its age of origin is Neoproterozoic, and that Tava offers a new repository of paleoclimatic and paleogeographic information for Laurentia. The spatial association of Tava sandstone with NNW-oriented faults, considered together with the unusual physical conditions required for liquefaction and formation of injectites, point to a role for seismicity as a trigger for liquefaction/remobilization of sand.