44 Subsurface Geologic Methods 



to result in a peculiar type of silicification of the included fossils. The 

 shells in the limestone matrix are wholly calcareous, while those in the 

 overlying residuum are wholly silicified. On the surface of the shells 

 occur many of the so-called "beekite rings." These are small doughnut- 

 like circlets of bluish-gray to white, opaque to translucent quartz. Find- 

 ing of silicifed shell fragments with beekite rings indicates that an ero- 

 sional zone has been entered and marks an unconformity in a series of 

 limestones even if there is no lithologic evidence. 



Color Contrasts: A sharp contrast above and below an unconformity 

 in the colors of the sediments often marks a disconformable contact where 

 there is little other evidence. A bright red, yellow, or purple below the 

 unconformity indicates possible weathering and oxidation of iron — or 

 manganese-bearing minerals. 



Chemical Sediments. Concentrations of chemical sediments on un- 

 conformity surfaces may be indicative of erosional breaks or cessation 

 of sedimentation. 



Manganese: Many manganese deposits are found associated with 

 the basal strata overlying unconformities. Some of these concentrations 

 are due to accumulation of residual materials on a weathered surface. 

 The oxides of manganese are very stable, and accumulations of them are 

 present on deeply eroded surfaces. Other accumulations of manganese 

 may be found at the base level of deposition during times of little or no 

 deposition of clastic sediments. 



Phosphates: Phosphate nodules are indicative of cessation or ex- 

 treme slowness of deposition. The source of phosphorus may be shells 

 and other organic material. Marine currents working over a base level 

 of deposition may remove parts of the sediments, concentrating the phos- 

 phates. Limestones or limy muds are particularly susceptible; the car- 

 bonates are removed, and the phosphates are concentrated. 



Glauconite: Glauconite deposits are formed by diagenetic processes 

 and require periods of nondeposition or very slow deposition for their 

 formation. A concentration of glauconite indicates but does not prove 

 unconformity. 



Caliche: Caliche deposits, if identifiable as such in the geologic 

 column, are indicative of unconformities. Caliche is formed by the 

 evaporation of carbonate-bearing capillary waters that come to the surface 

 in semi arid regions. These deposits may be a yard or more in thick- 

 ness and conform to the surface. They form on uplands as well as low- 

 lands. On submergence, these deposits may be buried beneath a blanket 

 of sediments and, if identified, mark an unconformity. 



Pyrite: Pyrite may be concentrated at or near surfaces of discon- 

 formity in marine sediments. Precipitation of iron sulphide apparently 

 takes place along ocean bottoms where profiles of deposition have been 

 established. Pyrite crystals may be disseminated through carbonaceous 

 shales and organic limestones where no unconformity exists and are, 



