Microbial Matter and Diagenesis in Organic-Rich Mudrock: An SEM and LA-ICP-MS Study

Abstract

Numerous beds of lacustrine and marine organic-rich mudrock, including those from the Green River Fm. of Utah and Monterey Fm. of California, are anomalously rich in calcium fluorapatite (CFA) and toxic trace metals. SEM records algal mats, phosphatized vermiform (coprolitic?) to globular (coccoid microbe?) structures, and planktonic shell debris associated with extensive bedded intervals, and localized concretions, of phosphate. Particularly of note in the lacustrine oil shale is the interpretation of early diagenetic dolomite followed by CFA and later calcite, with mercury trapped in the blocky CFA cement. LA-ICP-MS analysis of over 50 elements at ∼50 micron spacing further indicates a complex zonation of lanthanide and actinide enrichment across the margins of porous microcrystalline CFA intervals - likely the result of variable diffusion of these heavy elements from surrounding porewaters and their selective replacement of Ca in the CFA lattice.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016