Origin of thick, first-cycle quartz arenite successions: Evidence from the 1.7 Ga Ortega Group, northern New Mexico

Abstract

The 1700-Ma-old Ortega Group in northern New Mexico represents a > 1000-m-thick succession of metamorphosed quartz arenites and subordinate pelites. Published ages of detrital zircon grains from the Ortega Group indicate that the sediments were derived mainly from 1760-1700-Ma-old volcano-plutonic basement rocks and not from the Archean Wyoming Province to the north. Evidence for recycling within the Ortega Group also is lacking. Consequently, the Ortega Group quartz arenites are considered to be first-cycle sediments. The compositional and textural maturity of these sandstones is attributed to intense chemical and mechanical weathering. Abnormally high Al contents of pelites within the Ortega Group imply intense chemical weathering in the provenance similar to present-day low-relief tropical regions. Quartz arenites in the Ortega Group were deposited exclusively within a shallow-marine shelf environment influenced by tide, storm and wave processes. Prolonged sediment transport, especially on the tide-dominated inner shelf, in conjunction with inferred gradual subsidence optimized mechanical destruction of labile grains and rounding of quartz grains. Although the maturity of sediments in the Ortega Group can be accounted for, the mode of origin of > 850 m of quartz arenites remains enigmatic. Thick quartz arenite sequences of global distribution in the Phanerozoic coincide with first-order eustatic sea-level rises. World-wide occurrences of 1700-Ma-old, thick quartz arenite shelf successions suggest that the Ortega Group may have accumulated in response to a similar major eustatic sea-level rise.