Introduction

The purpose of this section is to document the stratigraphic distributions of smaller benthic and planktic foraminifera in the Early Cretaceous of eastern Venezuela and their correlation with a planktic foraminiferal zonal scheme. The complexity of the Early Cretaceous in this area as regards lithologies, stratigraphic subdivisions and lateral facies changes has led to differing interpretations by the numerous investigators particularly from the point of view of correlation.

Most of the studies were carried out as oil company projects, some using internal company formational names. Some of these results were later published. One of them is Rod & Maync's (1954) paper ‘Revision of Lower Cretaceous stratigraphy of Venezuela’. In the part on eastern Venezuela the authors proposed a number of new lithologic and stratigraphic units and gave lists of foraminifera and ammonites occurring in the different formations. They also proposed a zonal scheme based on foraminifera which will be discussed in more detail below.

Of the published larger scale investigations only in one (Guillaume, Bolli & Beckmann, 1972) was the occurrence and distribution of benthic and planktic foraminifera followed in detail through all lithologic units investigated (Fig. 66) and tied into a predominantly planktic foraminiferal zonal scheme (Fig. 68). This publication is a shortened and revised version of Guillaume's (1961) oil company report. Before considering the foraminiferal results of this publication and report, a brief overview on the Early Cretaceous formations of eastern Venezuela and their faunal content is given below.

Geologic-stratigraphic background

This section deals with the Middle Cretaceous (Late Albian) to Early Eocene benthic foraminifera of Central and South Trinidad. This time interval includes, from bottom to top, the Gautier, Naparima Hill, Guayaguayare and Lizard Springs formations, as defined in the Stratigraphic Lexicon of Trinidad (Kugler 1956). Their position within the stratigraphic framework of the southern Caribbean area is shown on the regional correlation charts (Fig. 2 and Fig. 3). This complex of four formations consists of a marine sequence of calcareous to argillaceous rocks which has a compound thickness of approximately 1500 metres; its actual thickness, however, may vary considerably due to local or regional hiatuses and/or tectonic disturbances. It lies above a Lower Cretaceous group of formations, of which the youngest is the Maridale Formation (Late Aptian-Early Albian) and is, in turn, overlain by the Navet Formation (mostly of Middle to Late Eocene age). The underlying and overlying formations form the subject of the preceding and the following chapter, respectively, of this volume.

The rocks which nowadays represent the Gautier, Naparima Hill, Guayaguayare and Lizard Springs formations have for a long time been known from relatively small and isolated surface outcrops such as those described by Wall & Sawkins (1860), Waring (1926), Liddle (1928; 2nd edition 1946), Jarvis (1929) and Skelton (1929). These authors have reported various megafossils, usually indicative of a Cretaceous age. Gradually, attention was also focussed on micro-fossils, usually foraminifera, particularly since the pioneer papers on the Lizard Springs area by Cush-man & Jarvis (1928, 1932).

Introduction

The Falcon Basin (Fig. 70) is one of the classical regions where both Oligocene and Miocene benthic and planktic foraminifera and their correlation with established planktic zonal schemes have been studied. The rich and well-preserved Miocene foraminiferal faunas, in particular those of the eastern part of the basin, have since the 1920s attracted the attention of micropaleontologists and stratigraphers. Their work was initiated and supported mainly by interests of the oil industry.

Following earlier investigations, mostly unpublished, it was in particular the monographic studies by Renz (1948) and Blow (1959) that provided the base for a correlation of the distribution of the benthic and planktic foraminifera in the continuous Miocene sections of southeastern Falcon.

The most recent investigations on foraminifera in Falcon are those published by Diaz de Gamero (1985a, b) from the northeastern part of the basin, an area to the North of that previously studied by Renz (1948) and Blow (1959).

The benthic and planktic foraminifera in the central part of the Falcon Basin, ranging in age from Middle Oligocene to Early Miocene were studied by Diaz de Gamero (1977). Here conditions are not nearly as favourable as in the northeastern part of the basin. Despite these drawbacks Diaz de Gamero was able to obtain results comparable to those of eastern Falcon, though on a reduced scale.

The above quoted investigations by Renz (1948), Blow, (1959) and Diaz de Gamero (1977a) complemented by unpublished observations by one of the present authors (HMB) form the base for this chapter.

Introduction

The selection for this section of taxa and their illustrations for the interval uppermost Early Eocene to Middle Miocene is largely from the three publications by Cushman & Renz (1947b, 1948) and Cushman & Stainforth (1945). In 1943,1944, and 1945 H. H. Renz and R. M. Stainforth, both at the time paleontologists with Trinidad Leaseholds Ltd in Pointe-à-Pierre, Trinidad, West Indies, issued detailed company reports on the occurrence and distribution of Trinidad foraminifera from Early Cretaceous to Recent. Based on the foraminifera the formations were either subdivided by zonal schemes as in the San Fernando and Cipero formations, or into a combination of members/lentils and zones, as in the Ste. Croix and Brasso formations. The Eocene Navet Formation was subdivided by Cushman & Renz (1948) into seven marl members.

These reports contain detailed maps and sections of the type localities for each of the zones, members and lentils, data on lithology, age, thickness and strati-graphic relations. For each of the units the foraminiferal content was listed and documented in assemblage slides arranged by genera and species, applying a number system to the latter. These detailed reports by H. H. Renz and R. M. Stainforth formed the base for their joint publications with J. A. Cushman.

The foraminiferal fauna of the Cipero Formation published by Cushman & Stainforth (1945) was at the time regarded as wholly belonging to the Oligocene. It was subdivided from bottom to top into the three zones Globigerina concinna (now Globigerina ciperoensis ciperoensis) Zone, Globigerinatella insueta Zone and Globorotalia fohsi Zone.

Review of foraminiferal investigations

There are two distinct areas in eastern Venezuela where Oligocene, Miocene and younger sediments with benthic and planktic foraminifera occur:

(a) The coastal area from Cabo Blanco in the west to the peninsula of Araya, the islands of Cubagua and Margarita in the east. No Oligocene and almost no Early Miocene occurs in this area. As virtually all sediments are of Late Miocene, Pliocene and younger age they are not considered in this compilation.

(b) The southern foothills of the Serrania del Interior and, mainly in the subsurface, extending to the south into the Llanos (Fig. 69). In this area the formations carrying benthic and planktic foraminifera are of Oligocene and Miocene age.

Planktic and benthic foraminifera have been studied from both areas and the planktic foraminiferal zonal scheme applied widely. Bermudez & Stainforth (1975) provided a detailed chronological overview on the introduction and application of the Tertiary planktic foraminiferal zonal scheme in Venezuela which was largely adopted from that previously developed in neighbouring Trinidad. Bermudez & Farias (1977) in a complementary paper, presented in more detail the individual Cenozoic to Recent planktic foraminiferal zones, with illustrations of the zonal markers.

In contrast to Falcon and Trinidad little published information is available in these two areas on the stratigraphic correlation of benthic foraminifera with the planktic foraminiferal zonal scheme, the prime objective of this compilation. It is therefore only possible to summarize the available data for the above quoted two areas in eastern Venezuela.

Introduction

Trinidad has a long history of investigation of fossil foraminifera. Following the classical ‘Report on the geology of Trinidad’ by Wall & Sawkins (1860) who drew attention to the presence of fossils ‘which might render important services to geological science’, it was Guppy (1863,1873) who first pointed out the presence of Tertiary foraminifera in Trinidad. He was also the first to illustrate some of them in 1894.

Applications of benthic foraminifera to stratigraphy were initiated about 1917 by W. F. Penny and P. W. Jarvis at the demand of Trinidad's expanding oil industry. In 1929 Cushman & Jarvis published a number of new benthic and planktic species from the Tertiary of Trinidad while, shortly before, Nuttall (1928) published a more comprehensive study on Oligo-Miocene benthic foraminifera from the Napa-rima region in southern Trinidad. This was also the first study on Trinidad foraminifera where the distribution of the species comprising a total of 144 taxa was plotted on a chart.

Following these pioneer studies which demonstrated the usefulness of the benthic species (the very frequent planktic forms were at the time almost completely ignored) to solve stratigraphic problems, oil companies began to establish their own micropaleon-tological laboratories. This increased the level of activity, and the initiative of some dedicated company paleontologists resulted in a number of detailed predominantly systematic studies on the Tertiary foraminifera, again still concentrating on benthic forms. Publications include those by Cushman & Stainforth (1945) and Cushman & Renz (1946, I947a,b, 1948) which span the interval Paleocene to Middle Miocene.

Introduction

Bartenstein, Bettenstaedt & Bolli and Bartenstein & Bolli described and illustrated in five papers (1957-1986) the benthic and some planktic foraminifera from the Early Cretaceous Barremian to Early Albian of Trinidad. They distinguished 78 benthic genera, 185 species and subspecies of which 13 were described as new. The idea of these joint publications with Bartenstein and Bettenstaedt dates back to a suggestion by Bolli (1950) which was guided by the following considerations:

Compared with those from western Europe, where Early Cretaceous foraminifera are well developed and can be studied in successive sedimentary sequences including boreholes, those from the Americas were at the time less well known. The study of the European forms began in the nineteenth century with authors such as Roemer, Reuss, Berthelin and Chapman. These early studies were supplemented in the Northwest German Early Cretaceous from 1932 to 1942 by the more stratigraphically orientated investigations of Eichenberg, Hecht and Wicher. This period was followed by modern revisions by authors such as Albers, Bartenstein, Bettenstaedt and Brand.

In their publication ‘Stratigraphic correlation of Upper Cretaceous and Lower Cretaceous in the Tethys and Boreal by the aid of microfossils (Germany)’ Bettenstaedt & Wicher (1955) for the first time pointed out the worldwide occurrence of numerous Cretaceous benthic foraminifera with apparently the same stratigraphic distribution as in the boreal and tethyan realms. This was based on a comparison of the North German faunas with those of other European countries, Israel, Egypt, Morocco, USA, Mexico and Trinidad.

Benthic foraminifera have been used for stratigraphic purposes almost since they began to be studied systematically. Already around the middle of the nineteenth Century, A. E. Reuss realised the stratigraphic value of benthic foraminifera. However, the real breakthrough in the application of foraminifera to biostratigraphy – at the time almost exclusively benthic foraminifera – began with the requirements of the oil industry for dating well sections and comparing them with the strata seen on surface. One of the first areas to use the tool was the Gulf Coast region of the United States and Mexico, above all by the great pioneer in promoting the stratigraphic application of foraminifera, Joseph A. Cushman. His textbook and that by J. J. Galloway were the first modern comprehensive references on the subject. Other significant contributors in this area were Helen J. Plummer, M. P. White, W. L. F. Nuttall and R. W. Barker. The use of benthic foraminifera in stratigraphy spread rapidly, mainly through Cushman and his co-authors, to the Caribbean (mainly Trinidad), then to California (R. M. Kleinpell, V. S. Mallory) and Venezuela (A. Senn). By the middle nineteen-thirties, foraminifera – still almost exclusively benthic forms – were the major microfossil group used worldwide in the micropaleontological laboratories of the oil industry.

Cretaceous and Tertiary planktic foraminifera, that had previously been largely ignored, gradually began to attract more attention from about the mid-nineteen- thirties but this was mainly in sediments where benthic forms were poorly represented. Early studies included those on the Globotruncanids of the Alpine-Mediterranean area followed by investigations on the Tertiary planktics, first in the North Caucasus region but then mainly in Trinidad.

Introduction

The Maracaibo Basin (Fig. 82) is the major oil producing area of Venezuela. Extensive micropaleontological studies by the oil industry have, based primarily on foraminifera and palynomorphs, greatly contributed towards the understanding and interpretation of the stratigraphy, correlations and changing environmental conditions of the Cretaceous and Tertiary sediments of the area. Much of this information has remained unpublished. Exceptions to this are the palynological investigations by Kuyl et al. (1955) and Fuenmayor's (1989) ‘Manual on the foraminifera of the Maracaibo Basin’. In addition there exist publications by Barbeito et al. (1985) and Pittelli Viapiana (1989) on the paleoenvironmental significance of Eocene benthic foraminifera in the Maracaibo Basin.

Barbeito et al. (1985) distinguish in the Mara- Maracaibo area (Fig. 82) 14 associations mainly based on benthic foraminifera, each characterizing a certain paleoenvironment, primarily water depth but also on variations in salinity. The range is from shoreline (alluvial) to bathyal. One of the 14 associations is characterized by a strong presence of Globigerina spp. amongst a dwarfed calcareous benthic fauna. On their table 2, intervals of the 14 associations are plotted against the 10 investigated well sections within the area Mara Maracaibo.

Pittelli Viapiana (1989) distinguishes five paleobathymetric foraminiferal assemblages through the Early Eocene to late Middle Eocene of the eastern and northwestern parts of the Maracaibo Basin based on well and surface evidence. They range from lower alluvial, deltaic, nearshore, inner mid-neritic to outer neritic/upper bathyal.

Fuenmayor's (1989) manual, together with the ‘Lexico Estratigrafico de Venezuela’ (1970 edition) forms the base for the following outlines. It draws extensively from Maraven's micropaleontological data.

Introduction

Studies on the geology of Barbados go back to the last century with the first account of the Oceanic Formation sediments being given by Jukes-Browne & Harrison (1892). They recognized that what they called the basal Globigerina marls had been deposited at a water depth of 900 m to 1800 m while the radiolarian earths were evidence of a depth from 3600 m to 5400 m. These were unusual findings as it was thought at that time to be highly unlikely that such deep sea sediments would be found on land.

The first modern comprehensive study of the sediments grouped as the Oceanic Formation of Barbados was carried out by Alfred Senn at the time that he was also producing a map of the pre-Pleistocene rocks of the island. His publications (Senn 1940, 1947) gave field information on the outcrops of the Oceanic Formation but no detail of the foraminiferal faunas, though these were used by him for dating. Beckmann (1953) used Senn's samples for a detailed analysis of the foraminiferal fauna and this has been the basis on which the ranges of the benthics have been plotted here (Fig. 86).

The earlier studies were hampered by a lack of understanding of the nature of accretionary prisms and their relationship to subduction zones at plate boundaries. The new generation of work undertaken by R. C. Speed and his co-workers (Speed 1981,1988; Torrini 1988) has provided a sound framework in which to place rock sequences, the dating of which by the help of microfossils has, in return, been essential for the understanding of the structural complexities.