Urumaco and Venezuelan Paleontology: The Fossil Record of the Northern Neotropics

8 Stratigraphy and Sedimentary Environments of Miocene Shallow to Marginal Marine Deposits in the Urumaco Trough, Falc

153 Stratigraphy and Sedimentary Environments of Miocene Shallow to Marginal Marine Deposits in the Urumaco Trough, Falcón Basin, Western Venezuela Luis I. Quiroz and Carlos A. Jaramillo Falcón state in northwestern Venezuela lies in the complex interaction zone between the Caribbean and South American plates and several discrete tectonic blocks, including Maracaibo and Bonaire (Audemar 2001). The Paleogene collision of the Caribbean and South American plates generated the Falcón basin as a small, narrow structural depression trending nearly east-west in an intra-arc rifting setting (fig. 8.1). The basin evolved as a pull-apart basin generated by transpression during the Oligocene and Early Miocene, which resulted in narrow, northwest-trending depressions (Macellari 1995; Gorney et al. 2007). From the Middle Miocene to Recent, the Falcón basin was inverted and sedimentation was restricted exclusively to the northern part of Falcón state (Audemar 2001). 8 Fig. 8.1. Location of the study site in the Falcón state, western Venezuela. Quiroz and Jaramillo 154 The Urumaco Trough is one of these northwest-trending depressions , extending onland from the Mitare River in the east, to the Zazárida River in the west, and it divides the western margin of the Paraguaná Península with the Gulf of Venezuela (fig. 8.1). Miocene sedimentation in the Urumaco Trough was characterized by a thick sequence of more than 9000 m of shallow to marginal marine deposits, controlled tectonically during deposition. In this study, we address the lithostratigraphy and the sedimentary environments of the Miocene in the Urumaco Trough, based on fieldwork and published data, and discuss the paleoenvironmental evolution in relation to the tectonic history of the Falcón basin. The Cenozoic history of northern Venezuela is influenced by the diachronous emplacement of allochtonous metamorphic rock of the Lara Nappes, related to the collision of the Caribbean and South American plates. The convergence began during the Paleocene in the Maracaibo basin (Escalona and Mann 2006) and progressed eastward, reaching the eastern Venezuela basin during the Late Miocene (Pindell and Barrett 1990). The eastward movement of the Caribbean plate relative to South America has generated a >100 km wide active transpressional zone with associated topography along the Falcón coastal plain (Audemar 2001). This transpressional zone boundary extends southward in the Mérida Andes, where strain is partitioned between the strike-slip Boconó fault system, located along the axis of the mountain chain, and the fold-andthrust belts along the foothills (Audemar 2001). The most conspicuous tectonic feature in Falcón state is the rightlateral strike-slip Oca fault system, which has a documented displacement of at least 20 km (Tschanz et al. 1974). This fault system, which extends for more than 650 km in an east-west direction, forms the northern limit of the Sierra Nevada de Santa Marta and Sierra de Perijá in Colombia and cuts through the inlet of Lake Maracaibo, the coastal plain of northwestern Falcón state, and the mountain ranges in the center, extending to the eastern coast of the state (fig. 8.1). The Falcón basin in northwestern Venezuela outcrops for more than 36,000 km2 , covering most of Falcón state and the adjoining parts of the states of Zulia, Lara, and Yaracuy (Wheeler 1963; Audemar 2001). According to Wheeler (1963), the opening of the basin began during the Late Eocene, as a small, narrow structural depression trending nearly east-west in an intra-arc rifting setting (Gorney et al. 2007). During the Oligocene and Early Miocene, a pull-apart zone was developed as the Caribbean plate continued to move eastward relative to South America, forming northwest-trending depressions in the Falcón Geological Setting The Falcón Basin Stratigraphy and Sedimentary Environments 155 basin, the Urumaco Trough, and east of La Vela, separating more stable highs as the Dabajuro platform and the Paraguaná Península (Macellari 1995; Gorney et al. 2007). Intra-arc rifting in the Falcón basin continued to be active throughout this Oligocene–Early Miocene phase, but with reduced intensity (Gorney et al. 2007). During this time, more than 6000 m of Oligocene to Early Miocene sediments were accumulated in relatively deep marine environments in most of the area of the Falcón basin, and in shallow to marginal marine environments toward the west, south, and north-central margins, including reef limestones toward the north and south-central area (Wheeler 1963). During the Middle...