Palynostratigraphy and lithostratigraphy of Upper Cretaceous and Paleogene outcrop sections, Mérida Andes (Maracaibo Basin), Western Venezuela

Highlights

• Outcrops of Late Cretaceous to Oligocene age along the Mérida Andes of Western Venezuela are studied palynologically.

• Depositional environments include open marine, upper shoreface, and floodplain.

• Significant gaps in time resulting from non-deposition or major erosion are noted on a regional and local scale.

• Temporal ranges vary for key lithostratigraphic units of Paleogene strata in Colombia and Western Venezuela.

Abstract

Detailed palynologic and lithostratigraphic studies of outcrop sections on the western flank of the Mérida Andes in Western Venezuela were undertaken to establish a framework for interpreting and correlating regional and local unconformities from the outcrop to subsurface data. The Upper Cretaceous to lower Paleogene section of the Maracaibo Basin consists of laterally continuous rock units attributed to the Mito Juan and Barco/Catatumbo formations, which are characterized by marine sediments deposited relatively close to shore. The continuity of plant and dinoflagellate species across the Cretaceous-Paleogene boundary in this area may have potential significance to K/Pg studies elsewhere. The lower to upper Paleogene Los Cuervos, Mirador, and Carbonera formations were predominantly deposited in non-marine environments, though palynologic and stratigraphic data from each unit show the variable influence of marine and lacustrine conditions. A new chronostratigraphic framework for the study area (the “Venezuelan Composite Standard”) was developed based on first and last appearances of spores, pollen, and dinoflagellates, and three pollen acme events and calibrated to the latest time scale. Graphic correlation plots of the most complete outcrop sections at Río Lobaterita, Río Lora, and Río Chama indicate the presence of significant hiatuses in time, due to erosion and non-deposition (or a combination of the two) reflecting the complex structural and depositional history of the southeastern Maracaibo Basin. A regionally significant upper Paleocene to lower Eocene unconformity, as well as several local and regional intra middle Eocene unconformities appear to have been caused by a drop in base level (sea level plus structural uplift) forming incised valleys over much of the study area. Unconformities are also present in the upper middle Eocene to Oligocene and represent ravinement surfaces caused by transgressive marine reworking of pre-existing units. These surfaces are often marked by the presence of dinoflagellates within the upper Eocene Carbonera Formation. Depositional environments during much of the Upper Cretaceous to Paleogene succession vary from fluvial-floodplain to marginal marine, with newly noted periods of widespread lacustrine deposition in all geologic units. Temporal differences between similar Paleogene lithostratigraphic units in northeastern Colombia and southwestern Venezuela are likely due to the influence and timing of structural events on regional and local depositional environments.

Introduction

The Upper Cretaceous to Paleogene stratigraphic succession of the Maracaibo Basin of Western Venezuela has been the subject of much study for over 75 years (e.g., Notestein et al., 1944). The discovery of substantial hydrocarbon reserves within Eocene strata of the basin led to the establishment of a now well-documented lithostratigraphic nomenclature (see Gonzalez de Juana et al., 1980), and past studies have shown that key Paleocene and Eocene units can be correlated from outcrop to the subsurface with little difficulty (Erlich et al., 1997). Within this context, our understanding of the chronostratigraphy within the southeastern Maracaibo Basin study area (Fig. 1), as based on palynostratigraphy, has advanced and continues to be refined as new work and new stratigraphic sections are studied (e.g. Kuyl et al., 1955; Pardo-Trujillo and Roche, 2009).

This study builds on previous work, utilizing the existing lithostratigraphic framework while emphasizing a more quantitative chronostratigraphic approach to the palynological zonation of Upper Cretaceous through Paleogene strata. In this regard, the objectives of this study are twofold:

• To firmly establish the temporal extent of known lithostratigraphic units, and,

• To document the palynological succession and evolution of depositional environments of key Upper Cretaceous and Paleogene units.

The use of palynology in petroleum related investigations in northern South America began in the 1930's, gained some momentum in the 1940's, and eventually during the 1950's developed as an essential tool in the study of sedimentary basins. One of the earliest publications illustrating the potential of palynology in correlation of subsurface sections in Western Venezuela was that of Kuyl et al. (1955). Their objective was to correlate some of the major sand bodies of Eocene age in an area that included Lake Maracaibo, the Sierra Perijá (west of the lake), and the west side of the Mérida Andes, and their results were illustrated with a series of cross-sections and correlation charts. Of relevance to this study, Kuyl et al. (1955) noted that when compared to established stratigraphic nomenclature, as defined by Notestein et al. (1944) and Sutton (1946), the Catatumbo, Barco, and most of the Los Cuervos formations were of Paleocene age (Fig. 2). In addition, they questioned the original correlation (Sutton, 1946) of Mirador Formation in the south lake area to the Misoa Formation in the subsurface of the lake.

Since the work of Kuyl et al. (1955) many palynological studies have been conducted in Cretaceous and Paleogene rocks in Colombia and Venezuela and other countries in northern South America. In their classic work, Germeraad et al. (1968), endeavored to relate the age of their palynofloral zones to the foraminiferal record, thereby calibrating ages of non-marine palynomorphs to the marine record. Over the past 50 years there has been a large volume of work published and many zonal schemes have been defined for northern South American Cretaceous and Tertiary sediments (summarized by Jaramillo et al., 2011). Additional relevant work includes Lorente (1986), Muller et al. (1987), Colmenares (1988), Colmenares and Terán (1990, 1993), and Caitlin et al. (1994), Helenes et al. (1998), Rull (1997a, 1997b; 1998a, 1998b; 1999; 2000; 2002), and Yepes (2001) for Venezuela; Pardo-Trujillo and Roche (2009), and De la Parra (2009) for Colombia and Venezuela; Van der Hammen, 1957, Van der Hammen, 1957, Van der Hammen, 1958, van der Hammen and Mutis (1966), Gonzalez (1967), Duenas (1980), Sarmiento (1994), Guerrero and Sarmiento (1996), Jaramillo and Dilcher (2001), Pardo-Trujillo and Jaramillo (2002), Pardo-Trujillo et al. (2003), Jaramillo and Rueda (2004), Jaramillo et al. (2005, 2007; 2009; 2011), Rodríguez-Forero et al. (2012), Garzon et al. (2012), and Pardo-Trujillo and Jaramillo (2014) for Colombia; Van der Hammen and Wijmstra (1964) and Leidelmeyer (1966) for Guyana; Wijmstra (1969) for Suriname; and Regali et al. (1974a, 1974b), and Herngreen (1975) for Brazil. Relevant works from Africa include van Hoeken-Klinkenburg (1966) and Salard-Cheboldaeff (1990), the latter describing an intertropical palynostratigraphy for the Cretaceous to late Quaternary.

The Maracaibo Basin is bordered on the east by the Mérida Andes and on the west by the Sierra Perijá (Fig. 1). Upper Cretaceous strata within the basin were generally deposited in a passive margin setting, while Tertiary strata were deposited within a foreland basin setting, especially from the late Paleocene to the early Miocene. Interactions between the Caribbean and South American plates produced progressive foreland basin subsidence from west to east through time, as well as late Eocene-early Oligocene uplift and erosion in the northeastern part of the basin (Lugo and Mann, 1995; Erlich et al., 1997; Pindell et al., 1998, 2005; Villamil, 1999; Duerto et al., 2006; Escalona and Mann, 2006; Mann et al., 2006; Pindell and Kennan, 2009; Ayala et al., 2012).

This robust structural evolution produced significant thickness, facies, and stratigraphic variations within Eocene-Oligocene strata, with accompanying unconformities along the south to north depositional strike of the Mérida Andes. Exhumation of Cretaceous to lower Pliocene rocks in the southeastern Maracaibo Basin study area began along the Mérida Andes in the early Miocene, and continued through the Holocene (Giegengack, 1984; Macellari, 1984; Arminio and Allen, 1990; Audemard and Audemard, 2002; Castillo and Mann, 2006; Duerto et al., 2006).

Upper Campanian and Maastrichtian siliciclastic rocks of the Maracaibo Basin were deposited within a tectonically shallowing basin, evolving from deltaic and pro-delta environments to upper shoreface and non-marine depositional environments (Lugo and Mann, 1995; Parnaud et al., 1995; Erlich et al., 1999, 2003, and references therein; Pardo-Trujillo and Roche, 2009). Upper shoreface to non-marine sandstones and shales of the Maastrichtian Mito Juan Formation were buried by lower Paleocene shallow shoreface limestones and shales of the Marcelina Formation in the western Maracaibo Basin (Gonzalez de Juana et al., 1980; Cabrera, 1995; Lugo and Mann, 1995; Parnaud et al., 1995; Dirección de Geología, 1997; Pindell et al., 1998; Villamil, 1999; Pardo-Trujillo and Roche, 2009; Ayala et al., 2012), and upper shoreface to non-marine sandstones and shales of the Catatumbo and Barco formations in the southeastern Maracaibo Basin (Notestein et al., 1944; Gonzalez de Juana et al., 1980; Colmenares and Terán, 1990, 1993; Lugo and Mann, 1995; Parnaud et al., 1995; Dirección de Geología, 1997; Pindell et al., 1998; Villamil, 1999; Pardo-Trujillo and Roche, 2009; Ayala et al., 2012). These were in turn overlain by upper Paleocene fluvio-deltaic, non-marine sandstones and shales of the Los Cuervos Formation, and fluvial to inner shoreface sandstones of the lower to middle Eocene Mirador and Misoa formations (especially in the southwestern to northeastern part of the basin, Fig. 2; Lugo and Mann, 1995; Parnaud et al., 1995; Erlich et al., 1997; Pindell et al., 1998; Pardo-Trujillo and Roche, 2009; Ayala et al., 2012). Middle to upper shoreface shales of the middle to upper Oligocene León Formation were deposited in the eastern Maracaibo Basin following significant early Oligocene basin inversion in the northeastern part of the basin.

In this study, three well-exposed Upper Cretaceous (Maastrichtian) to lower Miocene sections within the Mérida Andes study area (Fig. 1) were measured on a meter and sub-meter basis and logged with a handheld gamma ray scintillometer to produce a pseudo-gamma log for each section. Key units and lithologies were sampled for palynology. Outcrop logs were then normalized to clearly show local variations in lithology and used for correlation purposes between outcrop sections (Erlich et al., 1997). Outcrops studied here were previously studied by Boesi et al. (1988), Márquez and Mederos (1989), Azpiritxaga and Casas (1989), Arminio and Allen (1990), and Toro et al. (1994). Pocknall et al. (1997, 2001) documented palynofloral data across the Cretaceous-Paleogene transition zone (K/Pg boundary) at Río Lora.

Subsequent studies by Yepes (2001) at Río Lora focused on the Upper Maastrichtian and Danian section and described the dinoflagellate assemblages of the Mito Juan and Barco/Catatumbo formations. De la Parra (2009) compared Colombian data to Río Lora in a broad study of the palynological changes across the K/Pg boundary in Colombia. Jaramillo et al. (2006) incorporated data from Río Lora to support the concept of warming across the Paleocene-Eocene Boundary, and most recently, Jaramillo et al. (2011) incorporated data from the Paleogene section to support a palynological zonation for the Cenozoic of the Llanos and Llanos Foothills of Colombia.

Exposures at each outcrop locality (Fig. 1) varied from short to continuous sections along rivers and road cuts, in which only a few meters to more than 600 m of section could be measured and sampled. Palynological analyses were run on selected samples from all sections. In general, fine grained sediments were sampled and processed at the former Amoco Production Company West Little York Laboratory, Houston, Texas, using standard processing techniques outlined by Wood et al. (1996). Palynomorph identifications were aided by a multitude of publications (mentioned previously) and through access to a morphological database for Northern South America (Jaramillo and Rueda, 2019).

None of the zonations previously mentioned were used to date the sections in this study but were instead dated using the graphic correlation method (Shaw, 1964; Edwards, 1989; Carney and Pierce, 1995). Graphic correlation uses a composite standard and assigns ages that have been calibrated to the marine record. To facilitate a chronostratigraphic evaluation of the sections discussed in this paper, a composite standard was built based on palynostratigraphic events published by Jaramillo et al. (2011). Thirty-five (35) first appearance datums (FADs), 23 last appearance datums (LADs), and 3 acme events (Table 1) were included in the “Venezuelan Composite Standard”. Key events are shown on the graphic correlation plots as abbreviations, and their ages have been calibrated to the ICC 2019 v5 (see Cohen et al., 2013). These are used to define a “best fit” line of correlation (LOC) for each section. A sloped line indicates deposition and a flat line indicates a gap in time (hiatus/unconformity). The duration of each hiatus is calculated by extrapolation to the time scale on the x-axis of the graphic correlation plots. Many previous studies have used graphic correlation (e.g., Rodríguez-Forero et al., 2012; Pardo-Trujillo et al., 2003) in Colombian Paleogene rocks but to our knowledge there are none for Venezuela.