Lower Miocene Foraminiferal Biofacies and Paleoenvironments in the Lower Magdalena Basin, Colombia: Implications for Sequence Stratigraphy
Martha Beltran, Martin B. Lagoe
The lower Miocene sequence stratigraphy of the lower Magdalena Basin,
Colombia is being studied with a combination of seismic, wireline log and
micropaleontological data. A key element of this analysis
is the determination
of paleoenvironments through an integrated study of foraminiferal biofacies,
lithology and electric log signatures.
Paleoenvironmental
trends are illustrated
with a well section located southeast of Astrea, Colombia. Three formations are
penetrated by the well: San Antonio Formation (0-1788 ft), interbedded
fossiliferous sandstones and siltstones, dominantly siltstone in its lower part;
Porquero Formation (1788-2576 ft.), mainly mudstone throughout; and Cienaga De
Oro Formation (2576-5140 ft.), complexly interbedded sandstone, siltstone and
mudstone, wi h rare limestone in its uppermost part. Good to excellent
foraminiferal faunas are obtained from the San Antonio and Porquero Formations,
samples from the Cienaga De Oro Formation contain mostly down-hole contamination
but some in-situ fauna may be present in some samples. Planktic foraminfera and
palynomorphs indicate that the section studied is early Miocene in age.
Paleoenvironmental
interpretations are based on total foraminiferal abundance,
planktic foraminiferal abundance and benthic foraminiferal biofacies. Biofacies
trends are defined using a combination of cluster and ordination analyses. The
Cienaga De Oro Formation represents a shallow, deltaic paleoenvironment based on
possible in-situ neritic foraminifera and palynomorphs. A rapid deepening occurs
near the top of the formati n where neritic sands and mudstones are overlain by
mudstones containing rich, lower middle bathyal foraminiferal foraminiferal
faunas. A deepening of 1000 meters or more occurs over a section less than 40
meters thick. Biofacies within the Porquero Formation are characterized by
Uvigerina peregrina, Bolivina marginata, Cibicidoides spp., Rectuvigerina
transversa, Melonis pompilioides and diverse agglutinated species. Biofacies
within the San Antonio Formation reflect a progressive shoaling of the basin to
neritic faunas at the toy of the penetrated section, characterized by
Amphistegina lessoni, Ammonia beccarii, Hanzawaia concentrica and
Quinqueloculina spp. An abrupt shoaling occurs between 990 and 1200 ft. in the
well (upper middle bathyal to neritic) and is a candidate for a deposit onal
sequence boundary. Within the neritic section at the top of the well, a
deepening event (upper bathyal water depths) at 300-330 ft. represents a
flooding event. This
paleoenvironmental
history is shown to be useful in
calibrating seismic geometries within the basin and in distinguishing tectonic
from eustatic controls on deposition.
AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995