Question
Good day, I have a question regarding Pleistocene to Holocene river deposits. Commonly along the Agua Fria and the Salt River channels, and about 20-50 ft below the surface elevation, exists cross-bedded, poorly sorted cobbles and pebbles with a distinct even black coating. The black coating is not a weathering rind since it does not penetrate the surface. It also seems to coat the entire cobble with no preference to orientation. I have a few interpretations such as preserved algae (but if true, I would expect a preferred orientation of the coating to be on the "top" of the cobble), desert varnish during a dried abandoned river bed event (but again - orientation is a problem), ground water contamination (but I might expect more of a rind in the cobbles and that elevation should be more consistently black), or an organic coating from decayed fauna or flora? It seems that the latter of the options is most likely.
Since I've noticed this at a distinct depth below surface and according to this source; http://azgs.az.gov/arizona_geology/summer09/article_sanpedro.html
"...the presence of orange or black coated pebbles and cobbles on Pleistocene-age surfaces is equally diagnostic". Are there any research papers available that further explain the diagenesis of black-coated cobbles of Pliocene to Holocene age?
Let me know if you'd like to see some pictures and I'll send them over. Thanks, Cameron
Answer
Cameron, My guess is that the black gravel coatings are mainly manganese and iron oxide concentrations. These compounds give desert varnish its brown or black color, but can also be deposited by groundwater. There are some nice examples in our Dome Rock Mtns SW quadrangle map (Jon Spencer and I led an AGS fieldtrip to this area in 11/15), where manganese and iron coatings exist on basal Bouse carbonate but also in gravel layers in old Colorado River deposits (Bullhead Alluvium) and tributary gravel deposits.
Contributed by Phil Pearthree, Arizona State Geologist; Photos by Brian Gootee, AZGS Research Scientist
