SURFACE PROCESSES ON THE EARLY PRECAMBRIAN EARTH – A RETROSPECTIVE

Analysis of Early Precambrian (>1.8 Ga) sedimentary rocks reveals the non-uniformitarian nature of “chemical/biochemical” and the uniformitarian nature of physical sedimentary processes. Evidence for trapping and binding of clastic sediment by cyanobacteria is rare. Fibrous, radiating carbonate crystals, interpreted by others as pseudomorphs after aragonite, are common in the pre-2.0 Ga record and provide evidence for abiotic precipitation of carbonate favored by oversaturation of seawater. Evidence for sediment cohesion may favor local biostabilization of sediment in intertidal-supratidal settings. Precipitation of halite in the absence of gypsum supports a high HCO₃ content of the ambient ocean. Archean iron-formations are interpreted as analogs of Holocene pelagic sediments that accumulated in environments starved of terrigenous influx.

Recognizable physical processes include debris flows on high-gradient alluvial fans, mid-channel bar formation in low-sinuosity braided rivers, wind ripple and dune migration in eolian settings, tidal and wave reworking in shallow-marine environments, and deposition from turbulent flows. Maximum water depths of sedimentation are difficult to constrain from the preserved early Precambrian sedimentary record. Archean siliciclastic turbidites do not necessarily record abyssal depths. The presence of hummocky cross-stratification in association with some turbidite beds suggests that maximum depths did not exceed tens of meters. The lack of bank stabilization in the absence of land plants promoted the formation of braid deltas rather than river deltas at the land-ocean interface. Quantitative analysis of tidal facies reveals the presence of semi-diurnal, diurnal and fortnightly periodicities and constrains tidal current velocities as no greater than on modern Earth. Precambrian siliciclastic sandstones typically are compositionally mature consisting primarily of quartz +/- chert whereas mudstones are enriched in aluminum relative to average shales. Relatively intense weathering is implied and this may be related to the ambient CO₂-rich atmosphere. Biostabilization on early Earth is recognized in the form of microbially induced sedimentary structures (MISS) from terrestrial and shallow-marine facies.