On pediments and associated gold-bearing placers
Abstract Generated abstract
This paper examines pediments and their relationship to gold-bearing placer deposits in the Martayga region of the Altai-Sayan folded area. Using geomorphological observations, it argues that many landforms previously interpreted as ancient or abandoned valleys are instead elements of denudational planation surfaces, with loose deposits dominated by eluvium, deluvium, and colluvium rather than alluvium. The author contends that there has been no major drainage reorganization since at least the Neogene, and that the form and position of certain placers can indicate the direction of sediment movement and the location of primary gold sources. The paper recommends adjusting placer prospecting layouts in broad flattened valleys, especially by aligning exploration lines longitudinally except where true terrace features are present.
Full Text
On Pediments and Associated Gold-Bearing Placers
S. F. Dubinkin (SNIIGGiMS)
One of the characteristic features of the relief of the Martayga region, as well as of certain other regions of the Altai–Sayan folded area, is the presence of planation surfaces (pediments). These planation surfaces should be regarded as surfaces formed mainly by denudational processes. Only an insignificant part of them arose as a result of accumulation.
In the relief forms of pediments, as a rule, the geological structure of the substratum is clearly reflected; this substratum is usually overlain by thin clastic deposits in the form of eluvium and deluvium. The latter, passing into colluvium, usually attains considerable thickness (the first tens of meters). Alluvium plays a modest role in the formation of pediments.
In the Mariinsk taiga one can observe the following elements of pediments: pedestals (narrow terrace-like ledges), pediment passes, monadnocks, residual rocks, etc.
Pediment passes are often regarded as open valleys (or parts of abandoned valleys). On this basis, some researchers arrive at unfounded conclusions about a major reorganization of the drainage network, which naturally cannot but lead to erroneous forecasts for gold placers.
A geomorphological study of the relief of the Martayga region makes it possible to assert that, beginning with the Neogene—and perhaps even earlier—there has been no significant reorganization of the drainage network here. What many researchers take to be ancient valleys (open valleys and valleys with broad flattened bottoms) are separate elements of planation surfaces, i.e., pediments. The alluvial deposits of broad “ancient” valleys with a flattened floor (similar to the valley of the Talanovaya River) are represented by local rocks. In the “open valleys,” the loose deposits are represented by deluvium and colluvium, which overlie thick structural eluvium. From the configuration and position of the “spoon-shaped” placers that cut across the convex bottoms of the “ancient” valleys, one can trace the directions of movement of the loose material during the formation of the planation surface. The heads of such placers point to the areas where the primary sources of gold are located.
Proceeding from the above, it is recommended that, within the flattened floors of broad valleys, prospecting and exploration lines be laid out not across the valley but along it; the transverse arrangement of prospecting and exploration lines should be retained only within the lowered middle part of the valley, where floodplain and above-floodplain terraces are clearly expressed.