Cotillon e, Eric Jaut6e e ... Fig.l. Turonian paleogeographical setting of northwest-South. America (simplified .... matrix made of clay minerals and organic matter;.
Palaeogeography, Palaeoclimatology, Palaeoecology, 81 (1991): 313-321
313
Elsevier Science Publishers B.V., Amsterdam
Cretaceous black shales of Venezuelan Andes: preliminary results on stratigraphy and paleoenvironmental interpretations Nicolas-Pierre Tribovillard a't, Jean-Franqois Stephan b, H616ne Manivit c, Yves Reyre d, Pierre Cotillon e, Eric Jaut6e e aUniversit6 de Genkve, Dkpartement de Gkologie et Palkontologie, 13, rue des Marafchers, 1211 Genkve 4, Switzerland bUniversitb de Nice, Sophia Antipolis, Institut de G6odynamique, URA 1279 CNRS, 06034 Nice cedex, France CBRGM, B.P. 6009, 45006 Orleans cedex, France dUniversitb d'Avignon, Laboratoire de Gbologie, 84000 Avignon, France eUniversitk Claude-Bernard-Lyon I, Centre des Sciences de la Terre and URA 11 CNRS, 69622 Villeurbanne cedex, France (Received J a n u a r y 30, 1990; revised version accepted April 19, 1990)
ABSTRACT Tribovillard, N.-P,, Stephan, J.-F., Manivit, H., Reyre, Y., Cotillon, P. and Jaut6e, E., 1991. Cretaceous black shales of Venezuelan Andes: preliminary results on stratigraphy and paleoenvironmental interpretations. Palaeogeogr., Palaeoclimatol., Palaeoecol., 81: 313-321. This paper deals mainly with the Late Cretaceous La Luna Formation on the Venezuelan Andean Platform. The studied area (the so-called Transversale de Barquisimeto) is located at the contact zone between the Andes of Merida and allochthonous slabs thrusted from the Caribbean Domain and is crosscut by a major active transform fault (the Bocono Fault), which is part of the contact zone between the Caribbean and South-American plates. The La Luna Formation is made of dark, organic-rich, laminated limestones and shales and, though always appearing with a black shale facies, is different in the areas located in the north or in the south of the Bocono Fault. We provide here some new information about the biostratigraphic frame of the La Luna Formation and of the overlying Colon Formation. In the studied area, the depositional environment of the La Luna Formation was certainly rather shallow (near storm wave base) and stratified. During deposition of La Luna Formation, dysaerobic to anoxic conditions prevailed, which may be linked with a regional anoxic event, perhaps related to seasonal upwelling along the northern coast of South America during Late Cretaceous time.
Introduction The La Luna Formation is a major geological unit in the foreland of the southern Caribbean Ridge. This unit of black shales, dated as Cenomanian-Santonian, extends across a large part of northwest Venezuela, from the Sierra de Perija to the M6rida Andes and to Colombia via the Maracaibo Basin in the subsurface (Fig. 1). With its eastern equivalent, the Rio Querecual Formation, the La Luna Formation is a probable ~Present address: Universit6 Paris-Sud, Laboratoire de G6ochimie des Roches S6dimentaires, Bfitiment 504, F-91405 Orsay Cedex, France. 0031-0182/91/$03.50
source rock for the major part of Venezuelan oil. The main area of study is located at the northeastern end of the Cordillera of Merida, where the Andean autochthon clips axially below the Caribbean Allochthon (the so-called Transversale de Barquisimeto; Aubouin, 1975). The Cordillera of Merida is crosscut by a major active transform fault (the Bocono Fault), which is part of the contact zone between the Caribbean and SouthAmerican plates. The horizontal displacement of the fault has been estimated to be 80-100 km long (St6phan, 1985). This major fault divides the area of study in two domains. The facies of the La Luna Formation appear different in each of the domains (Fig.2).
© 1991 - - Elsevier Science Publishers B.V.
314
N.-P. TRIBOVILLARD ET AL.
I
__1
Blackshale, thinly bedded limestone& thick chert intervals
Sandstone ~
Sandstone, shale & porcelanite
~
~
lackshale, thinly bedded ~ limestone& chert
Chert,ophiolites& shale Semi-emergentarea
Fig.l. Turonian paleogeographical setting of northwest-South America (simplified from Marcellari and De Vries, 1987), with the location of the Transversale de Barquisimeto. The La Luna Formation sediments were deposited in the two horizontally hatched areas. We present here the first results from a study of the La Luna Formation and of the depositional environment of the Cretaceous black shales of northern South America.
Geological outline of the Venezuelan Andes During Jurassic time, the present Venezuelan Andean domain was a succession of horsts and grabens. Continental red molassic sediments accumulated in the grabens (Schubert et al., 1979; Gonzalez de Juana et al., 1980). During Early Cretaceous time, an epicontinental sea covered the area, as indicated by proximal clastic-dominated deposits. During Latest Albian and Cenomanian times, the transgression rapidly extended towards the craton (Gonzalez de Juana et al., 1980); black shales of the La Luna Formation were deposited in a pelagic, dysaerobic environment (Tribovillard
Fig.2. Outcrops of the La Luna Formation in the Transversale de Barquisimeto located at the contact between the Venezuelan Andes and thrust slabs coming from the Caribbean Domain (modified from St6phan, 1985). The location of two reference sections is indicated, the San Pedro section and the Altamira section are representative of the La Luna Formation in the northern part and in the southern part of the Transversale de Barquisimeto, respectively. and St6phan, 1989), until the Early Senonian. This transgression was followed by a return to normally oxygenated conditions and the deposition of pelagic fine-grained shales. A major regression occurred at the time of the Cretaceous-Cenozoic boundary; a thick succession of land-derived sediments was deposited from Paleocene to Middle Eocene time before the emplacement of Caribbean tectonic slabs (St6phan, 1977, 1985). Andean structural development and uplift started during Miocene and are still under way.
The La Luna Formation in the Transversale de Barquisimeto
Lithology North of the Bocono Fault The La Luna Formation overlies the Maraca Formation of A p t i a n - A l b i a n age. The Maraca Formation consists of platform deposits and marks
315
CRETACEOUS BLACK SHALES OF VENEZUELAN ANDES: PRELIMINARY RESULTS
the transition from Lower Cretaceous neritic deposits to the pelagic sediments of the La Luna Formation. The La Luna Formation (thickness: ca. 200 m) is Cenomanian-Santonian in age and consists of dark, laminated black shales which may be divided into three members (Fig.3). Ellipsoidal carbonate concretions occur throughout the formation. The lower La Aguada Member (60 m thick) consists of alternating dark-grey, laminated limestones and shales. Limestone beds are 20-60 cm SAN
PEDRO
thick; shale layers are thinner. In the lowermost part of the member, only shale levels are laminated and some scarce, low-angle cross-lamination is visible. Ellipsoidal carbonate concretions are common with a mean long axis of 20-30 cm. From base to top, sedimentological structures reflect a marked decrease in depositional energy. The middle Chejende Member (60-70 m thick) is a shaly laminated sequence which contains a few limestone beds. Shales as well as limestone are dark and very micaceous. Carbonate concretions increase upward
& TRUJILLO
SECTIONS Z
,
7 I I
I
l
I
i
B
m
B
e
D
m
i
l
I
ALTAMIRA
( I
I
~
m
m
m
--
02
--
04
01
03
=
i
~
NT
It.
~
immllmmlmui m
7 --
I
i
i
SECTION
:
05
i --
06
--
07
--
08
-u
g
_
AM
--
02
--
04
01
H
i
(.3 I-
