THE PROTEROZOIC PEDIMENT-SEDIMENT SYSTEM

In the absence of land plants, broad pediments were stable landforms that bevelled Proterozoic continents. A flux of clastic sediment moved across a Proterozoic pediment in a thin layer, transported by braided streams or in sheet floods. The Proterozoic pediment-sediment system is represented by extremely flat regional unconformities beneath locally preserved, commonly supermature, braid-plain or sheet-flood sandstones. The age of the sediment could be difficult to assign; the thin, unlithified mass might have been in flux for hundreds of millions of years. Sediment transport could be transcontinental, as shown by detrital minerals provenance. Special circumstances, such as tectonic subsidence, however, could trap, bury, and remove sediment from the flux. Once buried, sediment could lithify to define a stratigraphic age. Continental rifting destabilized Proterozoic pediments by lowering base level and funnelling runoff along rift axes to create large rivers, which were not favored otherwise in the Proterozoic landscape. Sediment flux captured from pediments would lead to rapid point-source deposition in rift deltas and alluvial fans. Deep rifts might first fill with lakes, with clastic deltas on the upstream side and clean carbonate shoals on the downstream side. After a rift-lake shoaled back up to the pediment surface, sediment loading and thermal decay could continue to trap sediment from the pediment flux for tens of millions of years. The result could be a typical Proterozoic cratonic section, with tabular red sandstone, siltstone, and subordinate conglomerate and carbonate beds. The thickness and longevity of the section would depend on the depth and crustal structure of the rift.