Evolution of a fluvial-dominated delta during the Oligocene of the Colombian Caribbean: Sedimentological and ichnological signatures in well-cores

Abstract

Basin analysis from Colombian Caribbean is particularly important given the interest in finding hydrocarbon reservoirs, but their complex geological evolution, and the frequent lateral and vertical variation of facies difficult a conclusive characterization, highlights the need for detailed sedimentological and ichnological studies. The study succession corresponds to an interval of a well core drilled in the south of the Sinu-San Jacinto Basin (Colombian Caribbean), with 1069 ft (similar to 326 m) thick of an Oligocene siliciclastic succession, interpreted in general terms, as deposited in a deltaic system. The integrated sedimentological/ichnological analysis allows the differentiation of dominant facies, with predominant lithologies such as conglomerates, sandstones, mudrocks, bioclastic sediments, as well as coal beds. The ichnological assemblage is low in abundance and moderately diverse, composed by Conichnus, Cylindrichnus, Dactyloidites, Macaronichnus, Ophiomorpha, Phycosiphon, Skolithos, Taenidium, Teichichnus, and Thalassinoides, as well as rhizoliths.

The complexity of the sedimentary system is reflected in its evolution throughout the Oligocene. A type succession with coarsening-upward trend was identified and it is repeated through the succession studied. It presents a general trend from bioclastic sediments (bioclastic conglomerates, sandstones and mudrocks) that pass into horizontal lamination and massive mudrocks occasionally bioturbated by Phycosiphon, and interbedded by mudrocks and sandstones with lenticular bedding, and the occurrence of Teichichnus. Above, bioturbated muddy sandstones with Ophiomorpha, Taenidium, Thalassinoides, and rarely Teichichnus, muddy sandstones with planar cross-lamination, and horizontal lamination sandstones with Dactyloidites, Ophiomorpha, Skolithos, and Thalassinoides are registered. Transition to carbonaceous mudrocks with Teichichnus, coal medium beds, and fine-to coarse-grained sandstones sometimes with Macaronichnus and/or Ophiomorpha is observed. Towards the top, are observed mudrocks with rhizoliths. This succession is interrupted by massive and horizontal lamination sandstones with low bioturbation index generated by the ichnological assemblage and/or by the exclusive occurrence of Ophiomorpha and/or Taenidium. Massive sandstones with erosive bases, asymmetrical ripples, and high content of organic debris are occasionally recorded. This succession reflects a progradational trend similar to those of fluvial-dominated deltaic sequences.

Detailed analysis revealed that even the fluvial processes were dominant in the deltaic system; however, local tidal and wave influence is recorded. Moreover, integration of sedimentological and ichnological information allows characterizing the evolution of the different sub-environments of the deltaic system, as prodelta bay, distal delta front, proximal delta front, distributary channels, mouth bars, and lower delta plain, and this is essential for areas of economic interest.