MODELS OF ROCK-VARNISH FORMATION CONSTRAINED BY HIGH-RESOLUTION TRANSMISSION ELECTRON-MICROSCOPY

Nanometre-scale transmission electron microscope imagery of manganifer ous rock varnishes from Death Valley, California, Peru, Antarctica, an d Hawaii confirms prior infrared mineralogy studies. The building bloc ks of rock varnish are clay minerals that are cemented to the rock by oxyhydroxides of manganese and iron. Rock varnish is layered on the sc ale of nanometres, with the basic structure defined by the subparallel alignment of detrital clay minerals. Although only a few examples of bacteria were found, possible cell-wall encrustations are ubiquitous a nd aligned with the clay minerals. Mn-Fe oxides appear to be mobilized from bacterial casts and then reprecipitate on clay minerals that wea ther into monolayers. These observations have implications for varnish dating and palaeoenvironmental techniques: K-Ar and uranium-series da ting of rack-varnish oxides can only yield minimum ages; however, this small spatial scale of mobilization would not alter the signal from m icron-scale microchemical laminations.