Ichnological analysis of the Upper Miocene in the ANH-Tumaco-1-ST-P well: assessing paleoenvironmental conditions at the Tumaco Basin, in the Colombian Pacific

Abstract

Tumaco is a frontier basin located on the SW Colombian Pacific coast. It is composed of a thick siliciclastic sequence up to reach 10,000 m-thick. In recent years, the National Hydrocarbon Agency-ANH has promoted new exploration wells in order to understand the sedimentary dynamic and its relationship with petroleum systems. One of them, the ANH-Tumaco-1-ST-P well has ∼3000 m (12,000 feet). We carried out sedimentological, geochemical, and micropaleontological detailed analyses with special attention to the ichnology on a 55 m-cored interval (from 1695.3 to 1640.4 m = 5563–5382 ft) in order to assess paleoenvironmental conditions. Beds are composed of green and gray mudrocks interbedded with lithic sandstones and fine-grained tuffs. Calcareous microfossil assemblages defined by the recovery of Uvigerina carapitana, Uvigerina laviculata, Uvigerina pigmaea, Globigerina woodi, Globigerionoides obliquus, Discoaster bellus gr., Catinaster coalitus, Reticulofenestra pseudoumbilicus and Sphenolithus abies indicated a Tortonian age, between CN6/CN7 biozones. Six sedimentary facies were identified: (1, 2) massive and laminated mudrocks, (3, 4) massive and normal-graded sandstones, (5) heterolithic beds, and in some cases (6) sandstones with soft-deformation structures. These rocks were accumulated in a shallowing platform-prodelta environment with continuous volcanic influence. Ichnotaxonomic analysis, conducted for the first time in the Colombian Pacific, allowed the identification of eighteen ichnogenera: Alcyonidiopsis, Asterosoma, Chondrites, Conichnus, Cylindrichnus, Diplocraterion, Ophiomorpha, Palaeophycus, Phycosiphon, Planolites, Rhyzocorallium, Schaubcylindrichnus, Scolicia, Siphonichnus, Taeinidum, Teichichnus, Thalassinoides, and Zoophycos. The ichnological association belongs to the archetypal Cruziana ichnofacies and its “distal” expression. By integrating lithofacies and ichnological results, two segments have been distinguished: 1) the lower one (1695.3–1677.1 m) dominated by massive and laminated mudrocks and 1 to 6 ichnofabric indexes and, 2) the upper segment (1677.1–1640.4 m) characterized by the increase of the sandstone facies, and ichnofabric indexes from 1 to 5. Along these segments, four ichnofabrics have been identified: 1) Phycosiphon-Planolites, 2) Planolites-Thalassinoides, 3) Zoophycos-Chondrites, and 4) Phycosiphon-Thalassinoides, reflecting stable sedimentary conditions with low-moderate energy, low-moderate sedimentation rates, and dominant aerobic facies. Minor variations in these parameters (sedimentation rate, energy conditions, substrate consistency, oxygenation and organic matter), however, induced changes in the macrobenthic tracemaker community, and consequently in the registered trace fossil assemblages.