SEMINAR: MGG Student Seminar Tuesday 12-1 CIMAS Conference Room

[apiggot@rsmas.miami.edu]

Third Floor CIMAS Conference Room
Tuesday, February 16
12-1pm

Rosely Marcal and Yan Jiang


"Petrographic Comparison between Pleistocenic and Albian Shoals"
by
Rosely Marcal

Geological survey researches commonly use the Pleistocenic oolitic shoals
of the Bahamas to create sedimentological and diagenetic models for the
oil industry. But, some considerations should be taken in using this
example of meteoric diagenesis as an actualistic model for ancient
limestone diagenesis. The geologist needs to keep in mind some factors
that directly affect the meteoric and marine diagenesis: (1) the original
mineralogy of the grains (aragonite, high-magnesium calcite, and
low-magnesium calcite), (2) changes in the chemistry of the
ocean-atmosphere over time, (3) climate, and (4) time of subaerial
exposition.
Pleistocene and Albian shoals yields the following differences a) The
Pleistocene grains were formed in an ocean favorable to precipitate
aragonite and high-magnesium while Albian rocks were formed in an ocean
favorable to precipitate calcite, b) Pleistocene rocks were formed in an
Ice House Episode while Albian rocks were formed in a Green House Episode,
and c) the climate was warm and arid in the Albian age while it was warm
and humid in the Pleistocene Epoch in the Bahamas.
The similarities are that both of the shoals were deposited on tropical
and subtropical land masses. The paragenetic sequence of the two sites is
remarkable similar. Both shoals show facies related diagenetic changes.
Fine-grained laminations are more cemented than the coarse ones.
The most notable difference is in the degree of diagenetic overprint, in
particular the amount of freshwater dissolution and cementation, which is
mainly related to the original mineralogy of the grains. The small amount
of meteoric cementation indicates that freshwater lens was either inert or
did not have enough time to produce visible cement at Albian rocks.







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