precision stratigraphy associates
PRECISION STRATIGRAPHY ASSOCIATES

Upper Albian OAE 1d event in the Chihuahua Trough, New Mexico, U.S.A.

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Oceanic anoxic events are clues to ocean processes and are correlation datums. In North America only
OAE 1a and 2 are well documented. Based on a low-resolution sampling program, a multi-proxy
geochemical approach constrained by a biostratigraphic framework was utilized to identify OAE 1d in
the upper part of the upper Albian Mesilla Valley Formation near El Paso, Texas. Chronostratigraphic and
biostratigraphic evidence indicate that the OAE 1d event in the Mesilla Valley section is located in the
lower part of the upper AlbianeCenomanian Ovoidinium verrucosum zone, which correlates with the
uppermost Albian Parathalmanninella appenninica and Stoliczkaia dispar zones. The chronostratigraphic
age of the geochemical event in the Mesilla Valley Formation is uppermost Albian (97.39e97.30 Ma).
The classic geochemical signatures for OAEs are enriched total organic carbon (TOC) concentrations
and coupled positive d13C excursions. OAE 1d at this location records TOC values ranging from 0.25 to
0.69 wt.% throughout the Mesilla Valley Formation, where TOC increases during the OAE (21.0e40.0 m)
to more than 0.40 wt.%. Interestingly, the organic matter in the Mesilla Valley is dominantly type III,
which indicates a pervasive terrigenous source. Although marine organic matter is abundant from the
base into the middle of the proposed OAE interval, it is progressively replaced by terrestrial material
above the OAE section during progradation. The d13Corganic values record a positive d13C shift of þ1.6&
from 26.41 to 24.80& across the stratigraphic interval from 21.0 to 40.0 m, which correlates with
OAE 1d.
Mn and Fe geochemistry suggest the depositional conditions of the Mesilla Valley Formation were
dominated by anoxic and possibly Fe-rich bottom waters, specifically during the time period associated
with the OAE 1d event. This interpretation is supported by the presence of Fe enrichment recorded by
FeTotal/Al and FeHighly Reactive/FeT with the lack of Fepyrite/FeHighly Reactive associated with Mn depletion.

Read entire text here →  Scott etal 2013 CretRes OAE1d_r

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