GEODYNAMIC SIGNIFICANCE OF SUPERSOLS IN THE STRATIGRAPHIC RECORD

Successions of extraordinarily mature, thick paleosols punctuate stratigraphic records in many foreland regions adjacent to fold-thrust belts. Covering areas on the order of 100,000 km² and time-spans of 10-20 Myr, these “supersols” record periods of intense, widespread stratigraphic condensation. Some supersols are time-equivalent to major disconformities. Supersols may include any type of paleosol and parent material, suggesting primarily geodynamic, rather than climatic, controls. Examples are presented from the Himalayan, Andean, and North American Cordilleran foreland basin systems. In the Himalaya, oolitic ironstones and a several-m-thick Oxisol, or a regional disconformity representing most of the Oligocene, are present from western Pakistan to eastern Nepal. The Oxisol is developed on top of Eocene shallow-marine strata containing the first evidence for erosional unroofing of the Himalaya and is overlain by a 4-6 km thick, lower Miocene-Quaternary foredeep sequence. In the Cordilleran foreland basin, massive stacked Calcisols, representing roughly 20 Myr of the Early Cretaceous, are present over an area of >500,000 km² in Montana, Wyoming, Idaho, and Utah. Below this supersol is the upper Jurassic Morrison Formation, which contains the first clear-cut evidence of westerly thrust-belt provenance, and above it sits the classic Cordilleran foredeep succession of Aptian-Santonian age. The central Andean foreland basin in Bolivia and Argentina contains a supersol of mainly Eocene age comprising Calcisols with subordinate Histosols. Above this supersol sits a several-km-thick upward coarsening foredeep sequence. Simple flexural modeling predicts that supersols in foreland settings result from stratigraphic condensation associated with migration through the foreland region of the flexural forebulge. Preservation potential is dictated by sediment supply rate and far-field dynamic subsidence. With an estimate of plausible forebulge width, the temporal duration of a supersol may be used to calculate the phase velocity of the flexural wave, which in turn can be used to deconvolve the rate of thrust belt shortening.