Phosphogenesis and Organic-Carbon Preservation in the Miocene Monterey Formation at Naples and Goleta Beach, California

Föllmi, Karl B.; Laurent, David; de Kaenel, Eric
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The Miocene Monterey Formation at Naples and Goleta Beach, west of Santa Barbara, California, is largely composed of organic-rich mudstone interstratified with phosphatic laminae. The presence of erosional surfaces, angular unconformities, and reworked clasts and nodules suggests that bottom-current activity and gravity-flow deposition have been instrumental in sediment accumulation. The phosphatic laminae were precipitated at a very early stage of diagenesis, during periods of non-sedimentation. They formed less permeable sedimentary lids and may as such have contributed to enhanced organic-carbon (OC) preservation. Between 13 and 10.6 Ma, the thus formed phosphatic laminae were frequently subjected to subsequent sediment winnowing and reworking, resulting in the formation of condensed phosphatic beds. Calculated P:C molar ratios suggest that (1) the measured section is highly enriched in phosphorus (P) relative to OC; (2) regeneration of organic P from organic-matter decomposition occurred (in contrast to what was described in Föllmi et al., (2005); and (3) the sources of P were both internal (organic matter decomposition) and external, likely upwelled bottom water rich in dissolved inorganic P. In spite of good preservation conditions and correspondingly high TOC contents, the overall OC accumulation rates are moderate in comparison to those of actual high productivity areas, which is mainly due to the episodic character of depositional processes and the intervening long periods of non-deposition and sediment reworking.

AAPG Search and Discovery Article #90162©2013 Pacific Section AAPG, SPE and SEPM Joint Technical Conference, Monterey, California, April 19-25, 2013