--> ABSTRACT: Variations in Alluvial Style of Tertiary Units in Response to Tectonism, Las Monas Area, Middle Magdalena Valley, Colombia, by D. W. Jordan and C. T. Siemers; #91022 (1989)

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Variations in Alluvial Style of Tertiary Units in Response to Tectonism, Las Monas Area, Middle Magdalena Valley, Colombia

D. W. Jordan, C. T. Siemers

Detailed sedimentologic and petrographic analyses of Tertiary alluvial sandstone outcrops within and east of producing oil fields in the Las Monas area in Colombia, South America, indicate that depositional style changed from fluvial-deltaic to braided streams atop alluvial fans to high-sinuosity meandering streams in response to uplifts in the surrounding areas.

The Paleocene Lisama Formation contains sandstones with erosional bases, large trough cross-bedding changing upward to ripple cross-lamination, and interbedded carbonaceous siltstones and claystones (distributary channels). The Eocene La Paz Formation in the perimeter area consists of thick, amalgamated, lenticular to sheetlike sandstones and conglomerates interbedded with rare root-mottled claystones (braided streams). The lower part of the Eocene Esmeraldas Formation in the same area contains amalgamated, trough cross-bedded, coarse-grained sandstones interbedded with thicker claystones (braided stream), and the upper part has thinner lenticular sandstones with lateral accretion surfaces encased in thick carbonaceous claystones (low to high-sinuousity meandering streams). The Oligoc ne Mugrosa Formation contains large to small-scale, trough cross-bedded sandstones having erosional bases and numerous, thick claystone lenses, suggesting deposition solely by high-sinuosity streams.

Diverse paleocurrent trends in the Tertiary formations in the perimeter area demonstrate that streams flowed northeast and northwest. Streams in the oil field had easterly and southerly components.

Source areas contributing sediment were different and reflected uplifts to the west and south of the Las Monas area. Petrographic composition of sandstones that have easterly and southerly paleocurrent trends in the field area contain more feldspar and less polycrystalline strained quartz than sandstones having a northerly trend in the perimeter area. Sandstones in the field area represent an unroofing of a western granitic terrain, possibly in the ancestral Central Cordillera.

AAPG Search and Discovery Article #91022©1989 AAPG Annual Convention, April 23-26, 1989, San Antonio, Texas.