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High-Resolution
Carbon Isotope Stratigraphy: Separating Global from
Local Effects on a Jurassic Carbonate Ramp of the
Pigott, Kulwadee L.,
Michael H. Engel, Richard P. Philp,
The sequence stratigraphy of the Upper
Jurassic (Oxfordian) Smackover Formation was
investigated by chemostratigraphic analysis in the
Conecuh and the Manila Embayments of southwest
The &delta13Ccarb record
from cores suggests that isotopic sequence stratigraphic
signals can be preserved regardless of the depositional environment and diagenetic differences. Four short-term cycles,
indistinguishable because of local effects, could be differentiated. Chemostratigraphy indicates the lowest sequence boundary
was not concordant with a lithostratigraphic boundary
between the Norphlet and the Smackover Formations.
During a relative sea level lowstand which was
associated with a disconformable boundary, local and
global effects caused a negative &delta13Ccarb excursion.
In the ensuing relative transgression and sea level highstand,
low, or no siliciclastic contamination increased
carbonate sedimentation and resulted in heavier &delta13Ccarb
values. While lateral paleoenvironment changes
during the Smackover sequence were recorded by lithological
and elemental variations, the sequence stratigraphy
of the Smackover can be vertically revealed exclusively by applying a &delta13 Ccarb
approach. The interpretation is supported by a global correlation with
other Oxfordian sequences and to the third-order eustatic cycles.
Carbon isotope chemostratigraphy is a powerful tool for helping decipher
variable effects in carbonate sequence analysis, providing correlative insight
into the timing of intrabasinal depositional events.
Carbonate ramp platforms such as the Smackover, owing to rapid lateral changes
with small changes in vertical sea level, provide ideal proxies for testing
hypotheses of global versus local sea level effects upon carbonate development.