Agglutinated foraminifera from a section of the Chipaque Formation (Cenomanian-Campanian) in th iorope Eastern Cordillera of Colombia are reported. The foraminiferal assemblage is dominated by species of the genera Ammobaculites and Haplophragmoides. The stratigraphic ranges of Ammobaculites colombianus, Haplophragmoides walteri, and Haplophragmoides rugosus are extended from a previously known Coniacian-Maastrichtian to a Cenomanian-Maastrichtian distribution, according to their appearance in the Chipaque Formation. Similarly, the range of Saccamina globosa (Cenomanian-Turonian) is extended to the Maastrichtian. The predominance of agglutinated foraminifera in the section is ascribed to undersaturation of calcium carbonate in the water column over the inner to outer shelf, due to the influx of terrigenous material. The array of parasequences composed of mudstone-sandstone couplets indicate landward and basinward coastline migrations that correspond to third order depositional sequences. INTRODUCTION AND GEOLOGICAL SETITING The paleoecological and chronological value of agglutinated foraminifera has increased in recent years in view of the worldwide attention they have received. Agglutinated foraminiferal assemblages of bathyal and abyssal habitats are particularly well known (e.g., Kuhnt et al. 1989; Kaminski et al. 1988; Kuhnt 1990; Gooday 1990), but there have been few paleoecological studies of neritic settings with predominantly agglutinated microfauna. Present assemblages of this type of microfauna described on shelf environments (e.g., Zheng and Fu 1990) may prove useful for accurate paleobathymetrical appraisals of ancient deposits. Although still subject to major changes in their stratigraphic ranges, the biostratigraphic use of agglutinated foraminifera in Upper Cretaceous rock successions has been stressed in various papers (Morgiel and Olszewska 1981; Kaminski et al. 1988, Kuhnt et al. 1989; Bolli et al. 1994, and references therein). Agglutinated foraminifera from the Upper Cretaceous of northwestern South America have been seldom studied. In comparison, more attention has been devoted to the planktonic and calcareous benthonic microfauna, illustrated in early works that concentrated mainly on systematics (e.g., Cushman and Hedberg 1930, 1941). More recent contributions have widened the knowledge of foraminifera as age indicators and paleobathymetric proxies in the Cretaceous of Colombia (Martinez 1989, 1995; Tchegliakova 1993; Vergara 1994). Even though species of agglutinated foraminifera in Colombia are rare, there is a growing need to evaluate their usefulness as biochronostratigraphic tools or as elements in paleoenvironmental appraisals. In spite of intense exploratory work and huge hydrocarbon discoveries in the foothills of the Eastern Cordillera (e.g., Cusiana and Cupiaga oil fields), very little is known about the foraminiferal content of strata along the eastern flank of the Cordillera and of the Llanos foothills. Except for a recent contribution by Tchegliakova (1995), the taxonomic aspects of many agglutinated lineages recorded in Colombia are yet unknown. A single species (Ammobaculites colombianus) was postulated by Petters (1955) as an index fossil. Yet, biozonations based upon benthonic species are considered of limited chronologic value in Colombia (Martinez 1989), and much work is needed to define precisely the ranges of most of the benthonic species. In this paper we intend to draw attention to the ubiquitous presence of agglutinated foraminifera in a section of the Chipaque Formation from the eastern flank of the Eastern Cordillera. Biostratigraphic aspects related to the microfaunal occurrence as well as paleoenvironmental and sequence stratigraphical considerations are discussed. The geological setting of the present study area is believed to correspond to a supracontinental rift basin that originated in the early Mesozoic (e.g., Macia et al. 1985). During the Late Cretaceous, this setting became an epicontinental sea that extended regionally around the Guyana shield towards the west. Macellari and De Vries (1987) recognized strips of coastal, neritic and pelagic domains that developed roughly in a NE-SW trend during the Turonian to Santonian (text-figure 1). The section of the Chipaque Formation is located close to the boundary between the coastal and neritic domains depicted by Macellari and De Vries. The large extensional faults that delimit the basin reactivated during the Cenozoic, resulting in phases of episodic uplift in the present Eastern Cordillera that continued until the early Quaternary (van der Hammen 1961). LOCATION AND METHODS The stratigraphic section of El Crucero is located in the province of Boyaca along the highway connecting Sogamoso to Yopal at the Llanos foothills, close to Lake Tota (text-figure 2). The section was described previously in an unpublished thesis by Alzate and Bueno (1994). Regional mapping of the area, carried out by Ingeominas and published as Sheet 192 (Ulloa et al. 1973), shows that the measured section occurs on the eastern limb of the El Crucero anticline (text-figure 2). Approximately 480m of the Chipaque Formation that outcrop in this section together with the main lithological characteristics are shown in text-figures 5 and 6 (see text-figure 4 for captions). micropaleontology, vol. 43, no. 2, pp. 185-201, text-figures 1-6, plates 1-2, 1997 185 This content downloaded from 157.55.39.181 on Thu, 29 Sep 2016 06:25:23 UTC All use subject to http://about.jstor.org/terms Luis Vergara et al: Agglutinated foraminifera and sequence stratigraphy from the Chipaque Formation, Colombia, South America TEXT-FIGURE 1 Paleogeography of eastern Colombia and western Venezuela during the Late Cretaceous (Turonian and Santonian). Modified after Macellari and De Vries (1987). Box shows location of text-figure 2. The base of the unit is not exposed and the measured sections include the basal part of the overlying Arenisca Dura Formation. Folding and faulting are of minor importance and the distortion of the stratigraphical succession could be estimated in certain cases. For instance, at m 240, nearly 20m of section are likely repeated by thrusting. The section of the Chipaque Formation was sampled for foraminifera, palynology and organic geochemistry. Fresh rock exposures with no signs of weathering facilitated the sampling. Samples were prepared for foraminifera by soaking them in Quaterary-O prior to conventional washing with a 0.063mm mesh sieve. Due to the little amount of individuals recovered no quantitative treatment was approached. The palynological study was undertaken by Bioss Ltda., whereas geochemical analyses were performed at Ecopetrol-ICP (Bucaramanga). Stratigraphic succession of the Chipaque Formation Text-figure 3 shows the nomenclatural equivalencies of the Chipaque Formation based on prior publications of the unit along the eastern flank of the Eastern Cordillera. The name Chipaque was introduced by Hubach (in Kehrer 1933), and later described in Hubach (1958). The Chipaque Formation in the study area lies conformably on the Une Formation (Aptian-Cenomanian) and is conformably overlain by the Arenisca Dura Formation (Campanian) of the Guadalupe Group. This is the concept of the Chipaque Formation (sensu Renzoni 1962) that we use herein. In the Llanos foothills, the Chipaque Formation has similar stratigraphic relations, but lies beneath the sandstones of the Palmichal Group (Campanian-Paleocene). In the Cocuy area in the northern part of the Eastern Cordillera, the Chipaque Formation is overlain by the La Luna Formation of Santonian to Campanian age (Etayo-