Abstract

Portions of the Andes Mountains of South America close to sub-base-level depositional sites currently yield a wide variety of clastic material in suites ranging from dominant clay with sand to coarse boulder-clay composites, which indicates that present concepts, associating source-area tectonic intensity as expressed through relief with control of clastic size, require revision. Relief and source-deposit distance do not alone determine depositional clastic-size range in the Andes, because, of four source areas examined, all are extremely high, and three are very close to depositional sites. The element controlling clastic size of sediments is capable of producing differing results where relief and distance of transport remain relatively constant and country rock suites are varied. Evidence presented indicates that the governing element is climate.

Humid weathering, as a dominautly chemical process, generates fine clastic material which is protected from fluvial or eolian erosion by vegetation and forms soil. Arid weathering, as a dominantly mechanical process, generates a high per cent of coarse alluvium which tends to accumulate within arid regions due to eolian removal of fine fractions and absence of perennial runoff.

Evidence presented shows that climate shifts can bring erosional and weathering processes of a given climate into contact with features previously developed by basically different processes. Regolith in a source and sediments yielded therefrom can reflect a single and prevailing climatic genesis (humid, Cordillera de la Costa, Venezuela; arid, Crodillera Occidental and coastal desert, Peru) or multiclimatic genesis where weathering and erosion have failed to eliminate inherited climatic effects (arid-humid transitions, Cordillera Oriental, Peru and Cordillera Occidental, Ecuador).

Clastic size being yielded by the four Andes regions correlates directly with vegetal cover, regolith, nature and/or competency of agents of transportation, and, therefore, climate. Sediments moved following a climate change tend to be weathering mixtures of a coarseness reflecting inherited regolith and progressing weathering and erosion.

The delay between the generation of a quantity of weathered material and its eventual delivery to a deposit by agents of subsequent climate(s) is designated sedimentary lag. Because movement of essentially all clastic material to sub-base-level deposits is characterized by sedimentary lag, the principle is important in tectonic-sedimentary relations and marine-nonmarine correlation.

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