Phosphogenesis and organic-carbon preservation in the Miocene Monterey Formation at Naples Beach, California—The Monterey hypothesis revisited

Abstract

The middle part of the Miocene Monterey Formation at Naples Beach, west of Santa Barbara, California, is predominantly composed of organic-rich mudstone interstratified with phosphatic laminae. Minor lithologies include volcanic ash, dolomite, porcelanite and chert, and condensed phosphatic beds. Sediments dated as 14.3–13.5 Ma have average total organic carbon (TOC) values around 8.5 wt%, and organic carbon (OC) accumulation rates are around 565 mg/cm2/k.y. Sediments dated as 13.5–13 Ma are characterized by average TOC values of 12.6 wt% and OC accumulation rates of around 1130 mg/cm2/ k.y. The interval between 13 and 10.6 Ma is marked by condensation; average TOC values are around 8.6 wt%, and OC accumulation rates diminished to around 55 mg/cm2/k.y. The last interval studied is dated as 10.6–9.4 Ma, and average TOC values are around 6 wt%, whereas OC accumulation rates rose again to 320 mg/cm2/k.y. 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 nonsedimentation. They formed less permeable sedimentary lids and may as such have contributed to enhanced 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 was negligible; and (3) the source of P was external, likely upwelled bottom water rich in 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 nondeposition and sediment reworking. They preclude this section, and by extrapolation, the Monterey Formation in general from being an important OC sink during the middle Miocene. Alternatively, large OC sinks were probably created on the continent (lignite deposits) and in sedimentary depocenters, which received increasing amounts of detrital sediments due to a combination of climate change, spreading of grasslands, and the increasing importance of mountain chains such as the Himalaya. The associated high nutrient fluxes may have been involved in the backstepping and drowning of carbonate platforms and in the generation of widespread phosphat-rich deposits during the late early and early middle Miocene.