c89a5c28_mical_Sediments_and_Landscapes__1
[0] Lithobionts and organic remains [2] The occurrence is a prerequisite of this model. [3] The occurrence of less than a percent of organic matter is readily explained by adventitious organics incorporated into varnish. [4] The occurrence of organic matter is predicted by silica binding processes. [1] 
The occurrence of typically less than a percent of organic matter is readily explained by purely adventitious incorporation as Mn enhancement occurs.
Rock Varnish Chapter 8 in Geochemical Sediments and Landscapes
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