Abstract
The top of the Querales Formation is well exposed at the Quebrada Corralito section, 17 m thick, in northern Venezuela. The section, dominated by siliciclastic accumulations of fine-grained sediments, preserves one cycle of transgressive–regressive phases of the deltaic environments facing the marine platform. An X-ray analysis revealed kaolinite as the main clay mineral. The age ranges from late Early Miocene to early Middle Miocene—zones N8 to N9, Late Burdigalian to Langhian. The section shows low biodiversity, including eleven macroinvertebrate taxa and two ichnotaxa; at its base, it contains mostly plant debris and amber from the continent, thus revealing a continental source near the littoral zone. Its middle portion preserves macroinvertebrates from the littoral zone, but also including the cephalopod Aturia sp., transported by a hydraulic flow to an area of greater depth; these macroinvertebrates are absent from overlying sections, at a time in which the basin was flooded most. The X-ray diffraction on six rock samples revealed mainly the presence of quartz with calcite, subordinated plagioclase and traces of potassium feldspar. Other minerals that compose this stratigraphic section are pyrite, dolomite and siderite, but they are disseminated in a siltstone. The section is dominated by mudstone sediments. The succession is kaolinite in the most pure fraction, and in lesser proportion smectite and illite; the stratified illite–smectite, chlorite and chlorite–smectite are present in low quantity. The mineralogical components of a molluscan valve were also examined, showing abundant pyrite, and moderate amounts of siderite and dolomite. In a part of the section, we recorded a rich assemblage of calcareous microfossils with foraminifers, nannofossils and marine ostracods. In the foraminiferal assemblage, 99 % correspond to planktonic forms. Most macroinvertebrates from benthonic assemblages suggest that they may have inhabited a near-shore marine environment, and sporadically experienced storm transport to deeper, shelf waters.
Highlights
The Falcon Basin in northwestern Venezuela is an area of importance for the oil industry (e.g. Gonzalez de Juana 1937, 1938; Wheeler 1963; Gonzalez de Juana et al 1980; Dıaz de Gamero 1989) and is known for a sedimentary sequence extending for most of the Neogene that preserves a diverse extinct fauna of evolutionary and biogeographic significance (e.g. Jung 1965; Sanchez-Villagra et al 2010; Smith et al 2010)
Its middle portion preserves macroinvertebrates from the littoral zone, and including the cephalopod Aturia sp., transported by a hydraulic flow to an area of greater depth; these macroinvertebrates are absent from overlying sections, at a time in which the basin was flooded most
These beds show massive to laminated levels with macroinvertebrates, microfossils and plant debris, with a layer containing small nodules of amber (Fig. 2)
Summary
The Falcon Basin in northwestern Venezuela is an area of importance for the oil industry (e.g. Gonzalez de Juana 1937, 1938; Wheeler 1963; Gonzalez de Juana et al 1980; Dıaz de Gamero 1989) and is known for a sedimentary sequence extending for most of the Neogene that preserves a diverse extinct fauna of evolutionary and biogeographic significance (e.g. Jung 1965; Sanchez-Villagra et al 2010; Smith et al 2010). The study of marine microfossils and macroinvertebrates in this area is potentially relevant to address several issues, including changes of marine communities in the neotropics (Hendy 2013), the evolution of the Caribbean Sea (Sanchez-Villagra et al 2010), and changes of the fluvial systems in the northern portion of the continent (e.g. Dıaz de Gamero 1996; Johnson et al 2009; Aguilera et al 2013). The Querales Formation (Hodson 1926), with the type locality in the southwest of Sabaneta, represents a geological unit from the ‘‘surco de Urumaco’’ which consists mainly of large accumulations of laminated black clay– shales and silty shales, interbedded siltstone and to a lesser extent, by thin layers of fine sandstones (Dıaz de Gamero 1989; Hambalek et al 1994). We present a detailed palaeontological, sedimentological, and biostratigraphical study from the uppermost Querales Formation
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