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