AboutDr Thomas Bell Expertise I can answer questions regarding surface earth processes and the chemical transformations that sediments and rocks undergo with burial. I can also answer questions regarding deep time, the evolution of the elements, and the last 4.5 billion years of earth history. I specialize in metallic ore forming processes, the major geologic time periods when they were produced and what they tell us about the evolution of our planet. Learn more about my professional interests at Stratamodel.com.
Experience I am a professional consulting geologist with a background in the petroleum, mining, environmental, and geotechnical industries with over 25 years of experience.
Education/Credentials Ph.D., Geology, University of California at Berkeley, 1984
M.A., Geology, University of California at Berkeley, 1980
B.S., Geology, San Jose State University, 1978
Expert: Dr Thomas Bell Date: 7/8/2008 Subject: braided and meandering rivers
Question significance of braided and meandering river channels to mineral exploration
Answer Ann,
It depends what mineral you are looking for.
Meandering streams develop on river floodplains of modest slope and sediment load. The sediment in meandering stream courses also has sufficient fine grained material to create cohesive floodplain deposits that are relatively resistant to erosion. Braided channels are found on floodplains where the sediment load is high and where the gradient is higher than that for a meandering stream. Coarse, non-cohesive sediment contributes to the unstable banks. These conditions are common in areas undergoing active glaciation. Anastomosing channels develop where the sediment load is also high and flow is intermittent, usually in arid or strongly seasonal climates. Though similar and hard to distinguish in the geologic record, braided and anastomosing channels tell a far different story about the paleoenvironment. Each develops under different conditions and when we find one type of channel or another, a geologist can infer what those conditions were.
Two main mineral deposit types form in rivers or buried channels by either mechanical or chemical processes. Mechanical concentration of dense mineral grains like gold, gemstones, and tin are found in some braid plains. Meandering rivers are not as efficient at sorting and concentrating heavy minerals. The sediment recycling of braided streams is far more effective.
When stream channels are buried, they become the plumbing system for ground water. A network of channels composed of coarse well sorted sediment can transmit most of the water in an aquifer focusing the flow along preferred pathways. The complex interplay of oxygen content and salinity play a role in concentrating dissolved elements whose solubility may be a function of the redox potential (amount of dissolved oxygen)or salinity of the groundwater. Uranium, vanadium, copper, and other metals are concentrated by subsurface chemical processes.
In general, braided and anastomosing channels are far more favorable for all types of mineral deposits than meandering channels due to their permeability and the relatively high stream power when they are formed.
