Upper Eocene Limestones, Associated Sequence Boundary, and Proposed Eocene Tectonics in Eastern Venezuela

The Tinajitas Limestone and Peñas Blancas Limestones are thin bioclastic carbonate units of Eocene age within the foreland fold and thrust belt of eastern Venezuela. They represent the only limestone deposits in the otherwise siliciclastic Cenozoic succession of the eastern Venezuelan shelf. The geometry of the carbonate units suggests a coastparallel, bar-like form, the particle population (larger benthic forams, glauconite, and algal balls) indicates shallow marine provenance, and there is a regional unconformity with major lacuna landward from the location of the limestone units. We interpret these carbonates as the capping deposit of a coast-parallel ridge on the continental shelf that rose and sank within Eocene time. We propose this transient ridge was an elastic forebulge that migrated landward across the shelf and craton hinge of the South American continent due to a brief episode of Eocene subduction of northern South American lithosphere. Such subduction may have been due to closure between the North and South America plates. The bulge had an amplitude of at least 800 m and migrated landward at least 50 100 km. This distance suggests the minimum closure between the plates at the subduction zone. Supporting evidence comes from facies suites of the passive margin wedge, now juxtaposed in the thrust and fold belt. The most seaward facies suite contains no evident Eocene lacuna or limestone; these facies were at too great a depth for the passing bulge to affect sedimentation. In the shallower intermediate facies, which contains the TinajitasPeñas Blancas Limestones, the passing ridge brought the seabed to depths of carbonate production, but not to shoal. In the most landward facies of the passive margin wedge and on the platform, the ridge caused emergence and erosion, producing the Eocene unconformity of large coast-parallel extent. The resubsidence of the margin in the Oligocene is attributed to passage of a foreland basin, which trailed the forebulge. INTRODUCTION Among the debates that have characterized sequence stratigraphy through its history, and that continue to this day, is the question of controls on relative sea level change and the development of regional unconformities. The concept of eustasy (Vail et al., 1977; Haq et al., 1986) as the predominant driver of relative sea level change certainly had great initial appeal, but it has been repeatedly challenged and refined (Sloss, 1991; Miall, 1991, 1992). Contributing to this effort have been a number of studies focused on the recognition of changes in accommodation due to causes other than eustasy, such as tectonics and climate-induced sediment supply (e.g., Williams and Dobb, 1991; de Boer and Smith, 1994). Of particular relevance to this study are the conclusions of Cloetingh et al. (1985, 1989), Peper (1994), and others who have proposed that tectonic stresses may cause regional tilting and/or flexure of the lithosphere on a scale sufficient to produce stratigraphic expressions of relative sea level change. In this paper we interpret the development of an isolated Eocene carbonate platform and associated regional