and organic-carbon preservation in the Miocene Monterey
Formation at Naples Beach, California—The Monterey
Karl B. Föllmi† Christophe Badertscher,
Eric de Kaenel,Peter Stille,Cédric M. John‡
Thierry Adatte Philipp Steinmann
Department of Earth Sciences, University of California,
Santa Cruz, California 95064, USA.
The middle part of the Miocene Monterey
Formation at Naples Beach, west of Santa Barbara,
California, is predominantly composed of organic-rich
mudstone interstratifi ed 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-fl
ow 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 thusformed 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 fl uxes may
have been involved in the backstepping and drowning
of carbonate platforms and in the generation of widespread
phosphate-rich deposits during the late early and
early middle Miocene.
Keywords: Monterey Formation, Naples
Beach, organic carbon preservation, phosphogenesis,
Monterey hypothesis, Miocene.,