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Facies Variations, Source Rock Potential, and
Sea Level Changes in Cretaceous
Black Shales of the Quebrada Ocal, Upper Magdalena Valley, Colombia1
Ute Mann and Ruediger Stein2
A 290-m-thick middle Cretaceous
black shale sequence in the upper Magdalena Valley, a present-day intramontane
basin located between the Central and Eastern cordilleras of Colombia,
was investigated with organic-geochemical and microscopic analyses. As
a result of the investigation, we were able to (1) differentiate four organic
facies types, (2) estimate their source rock potential, and (3) integrate
these facies into a sequence stratigraphic framework.
The four organic facies types were type C,
BC, B, and D. Type C contains a distinct terrigenous organic matter component
in lowstand or highstand deposits. Organic facies type BC is characterized
by an increase and a better preservation of marine organic matter. BC belongs
to the lower part of the transgressive systems tract. Sediments of organic
facies type B have the highest amount of marine organic matter due to excellent
preservation under anoxic conditions. The absence of bioturbation and the
enrichment of trace metals are further implications for deposition under
anoxic conditions. Facies type B is found in the upper part of the transgressive
systems tract and contains the best petroleum source rock potential. Facies
B occurrence coincides with sea level highstand and correlates especially
with a maximum flooding in northern South America during the Turonian.
Organic facies type D is also related to highstand
1997. The American Association of Petroleum Geologists. All rights reserved.
received July 11, 1994; revised manuscript received July 15, 1996; final
acceptance November 13, 1996.
Wegener Institute for Polar and Marine Research, Columbusstrasse, 27568
the colleagues from the Ingeominas (Bogotá) we would like to thank
especially L. Vergara Streinesberger for cooperation in the field and data
discussion. For technical assistance, we sincerely thank R. Stax and C.
Schubert (Alfred Wegener Institute, Bremerhaven). Also, we are grateful
to P. Cepek and K. Prössl for their contributions in identifying nannoplankton
respectively. Financial support by the Deutsche Forschungsgemeinschaft
(Grant No. STE 412/4) is gratefully acknowledged.
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