Asociación Venezolana de Geología, Minería y Petróleo. Boletín Informativo, vol. 12, no. 8, August 1969.
(EL CONCEPTO DE DILATACION DEL PISO DEL MAR APLICADO A VENEZUELA)
Par R. M. Stainforth
The concept of seafloor spreading appears to provide a more satisfactory explanation of the geologic history of Venezuela than the geosynclinal concept which has usually been invoked. A model is described in which an expanding convection cell formed in the Caribbean during the Cretaceous and accounts for the generally south‑directed orogeny which persisted into the Early Tertiary in northern Venezuela. The violent Mid‑Eocene orogeny, which particularly affected the hitherto stable western sector, is explained by conversion of the ovate outline of the Caribbean cell into an east-moving lobe of Pacific crust. Post‑Eocene events represent response to this drastic change in the pattern of crustal flow. The veracity of the model is subject to test by searching for mirror‑image replicas of the Venezuelan events in the northern Caribbean region.
El concepto do la dilatación del fondo del mar ofrece una explicación do la historia geológica de Venezuela ms satisfactoria que el concepto de geosinclinal anteriormente postulado. Se propone un modelo que consiste en una celda de convección f’ormada en el Caribe durante el Cretáceo y que, al expandirse, motivó la orogénesis dirigida principalmente hacia el Sur, que persistió hasta el Terciario Inferior en el Norte de Venezuela. La violonte orogénesis del Eoceno medio, que afectó en particular a la hasta entonces estable región occidental, se explica por la conversión repentina de la celda ovalada de convección en un lóbulo de la corteza del Océano Pacífico qua se movía hacia el Este. Los eventos del Post-Eoceno representan los ajustes al cambio drástico en el sistema de flujo de la corteza. Se podría probar la veracidad del modelo buscando en el Norte del Caribe la continuidad de las imágenes de eventos geológicos similares a aquellos que ocurrieron contemporáneamente en Venezuela.
Paleozoic formations are too sparsely distributed in Venezuela to provide a basis for a coherent historical account of that era, but the Mesozoic and Tertiary record is complete enough to offer a fairly clear picture of advances and retreats of the sea and of associated phases of orogeny and epeirogeny. The succession of events has generally been interpreted in terms of geosynclinal theory, and the eastern region in particular has been regarded as close to a textbook example. The Guiana Shield has been accepted as the central craton, shelving off into the deep ocean in Early Mesozoic time, then developing a mobile borderland which gradually grew and advanced towards the craton until mobility ceased in Late Tertiary time, leaving a static basin between the craton and a highly folded mountain range. The terms “Eastern Venezuela Basin” and “Eastern Venezuela (or Orinoco) Geosyncline” have been used to some extent interchangeably.
Although the geosynclinal theory has served the purpose of unifying the markedly different sequences of events recorded in different sectors of the country, it has undeniable defects. In particular it fails to supply a rational explanation for such features of recorded geologic history as the following:
a) formation of deep, narrow troughs in the Early Mesozoic and their infill by great thicknesses of mainly non‑marine deposits;
b) the presence of remarkably long rectilinear faults with variable orientation relative to the supposed geosynclinal grain of the country;
c) the anomalous status of the Maracaibo Platform, which remained a rigid block until mid‑Tertiary time; and the related sigmoid bend of the supposed geosyncline, through the Netherlands Antilles and Guajira, then south into Colombia;
d) the late emergence of the Andes and Perijá ranges and their horst‑like character, completely different from the compressive style of the coastal ranges to the east;
e) the scale of both vertical and horizontal movements necessary to explain the Villa de Cura allochthon.
In recent years the concept of continental drift motivated by seafloor spreading has become prominent in the geologic literature and has intrigued geologists by providing a plausible and unifying explanation of many known but seemingly disconnected geologic phenomena on a global scale. The gross relationships of Africa and North and South America imply that the Caribbean islands and Central America, as known today, are segmented remnants of a land mass which in Paleozoic time linked the two American continents.
In the following pages this concept is applied to the Venezuelan region. A model is developed which seems to account nicely for the known geologic history and to supply a logical explanation of the various features, in part mentioned above which were anomalous under the theory of geosynclinal evolution. The writer would emphasize that this is a broad‑brush approach which leaves many details still to be explained. The evidence used is almost entirely derived from Venezuela and immediately adjacent areas. Reference is made to some more distant areas in explanation of the supposed regional setting but such remarks and sketches are highly generalized.
Attention is largely confined to horizontal movements and to the vertical movements associated with lineaments of up‑ or down‑flowing crust Undoubtedly the more widespread vertical movements of epeirogenic types which resulted in major marine transgressions and regressions, were interlinked with the crustal movements described and can be attributed in broad terms to varying buoyancy (possibly a function of temperature) with more localized adjustments of isostatic character.
The continental masses of Africa‑Europe and of North and South America were united, but were about to separate and start drifting to east and west respectively, carried in conveyor‑belt fashion by new oceanic crust welling up along what is now the Mid‑Atlantic Ridge
The two halves of America moved divergently and segmentation resulted along “pull‑apart” rifts: these were partly filled with thick continental detritus, partly invaded by the sea: the blocks which now form Central America and the Antilles became completely detached from the two main continental masses.
The formation in the Early Mesozoic of deep elongate trenches, from 3,000 to over 8,000 meters deep, has been a puzzling feature in the geology of Western Venezuela. They are particularly well shown by isopachous maps of the Basal Cretaceous Rio Negro Formation, but the underlying red beds of the Triassic‑Jurassic La Quinta Formation also appear to be thickest along the same lineaments. These areas of thick Mesozoic deposits have been termed the Machiques, Uribante and Lara troughs and are notably aligned along the axes of the present‑day Andes and Perijá mountains.
On the Guajira Peninsula a great thickness of marine Jurassic to Early Cretaceous beds occurs along a narrow belt (“Guajira Trough”) outside of which they are unknown. The lineament of metamorphic rocks which form the Araya and Paria peninsulas and the Northern Range of Trinidad includes a vast thickness of Triassic(?)-Jurassic‑Lower Cretaceous marine beds which have no equivalents in the Guiana Shield province to the south.
On the Guiana Shield itself in Guyana a fill of 6,000 to 11,000 feet of the Late Jurassic‑Early Cretaceous Takutu Formation has been described along a lineament fault-bounded by ancient Precambrian rocks, interpreted as a rift valley.
The theory of continental drift offers an explanation of these formerly puzzling features by actually forecasting their incidence in areas of divergently flowing crust. North and South America have diverged about 1,000 miles along a NW‑SE line since drifting began. Self‑evidently the rifts formed at or near the edges of continental blocks (e.g.. Guajira, Paria‑Trinidad) would be invaded by the sea more rapidly than those in the interior (e.g. Uribante, Machiques).
The American blocks had by now drifted so far west that their leading edges reached the down‑flowing crustal suture of the Pacific region. Impingement of west-moving blocks against east‑flowing crust set up the initial compressive forces which resulted in growth of island arcs and eventually the Cordilleran systems. These events were mostly remote from Venezuela.
Compressive stresses were accentuated along the western margin of the Americas. However, these do not explain how a roughly E‑W‑aligned orogenic belt developed in the vicinity of the present Venezuelan coastline during the Upper Cretaceous. Seemingly there was uplift to the north associated with volcanic activity (e.g. Campanian granodiorites in Aruba, diorites of the Garrapata Formation) and a trough to the south which filled with turbiditic beds and gravity slides (Bahia Honda Group of the Guajira, Washikemba Formation of Aruba, Garrapata Formation of north central Venezuela). Emplacement of the Villa de Cura allochthon was a notable element of the orogeny.
It is suggested that a subsidiary convection cell had developed in the Caribbean region at this time. The reason for its appearance could be simply the attenuation of the upper crustal layers caused by divergent drift of the two halves of America. This stretching of the cool upper layer would permit hotter fluid magma to break through from below, initiating a vortex‑ring type of flowage. It is conceived that such a system, once initiated, would tend to increase in size by heating the contiguous crust and absorbing it as it became ductile.
In Paleocene to Lower Eocene time a deep marine trough extended westward from Trinidad to Lara and then apparently swung to the northwest. (This swing is based on extrapolation of trends, as no Paleocene beds are preserved in the Guajira region.) The trough was bordered to the north by an unstable uplift which was the source of extensive flysch and wildflysch deposits (Chaudiere, Guárico, Matatere formations) and gigantic slump blocks in the trough. The whole region to the south and southwest remained a stable province of shallow marine to continental deposition (Guasare, Orocué and other formations).
This Early Tertiary pattern, which is well documented, resembles the Upper Cretaceous picture as interpreted from piecemeal evidence, but the trough axis and unstable uplift have migrated to the south. Conjecturally the explanation is expansion of the Caribbean convection cell, the southern margin of which by now extended appreciably underneath the continental block.
The Mid‑Eocene was marked by a phase of violent orogeny, remarkable both for its wide extent—far beyond the immediate environs of Venezuela—and for the rapidity with which it occurred. In several sectors beds of Middle Eocene age were involved in the upheaval, yet the initial beds of the ensuing marine transgression are dated late Middle Eocene in places.
The main elements of this Mid‑Eocene orogeny in Venezuela were: (a) the Lower Tertiary trough from Trinidad to Lara was uplifted and deformed; (b) the hitherto stable shelf was downwarped, creating a new east‑west basinal axis farther south; (c) this basinal axis now swung to the southwest (Barinas Basin) instead of the northwest; (d) the long‑stable Maracaibo Platform was extensively deformed by tilting and block‑faulting; (e) there was volcanic activity on the Guajira and probably in Paraguaná (Santa Ana Gabbro) and Falcón; (f) intense crushing
of formations as young as Upper Cretaceous on the Guajira seems more probably related to this orogeny than to the Early Tertiary movements. In all the uplifted areas, rapid and deep erosion followed the orogeny.
The suggested explanation of these cataclysmic events is the sudden conversion of the ovate Caribbean convection cell into an east‑pointing lobe of the Pacific crust. This change would require a sudden surge of crustal energy through the Maracaibo Basin and would result in the low axis swinging from a northwest to a southwest trend, both of which are prominent among the observed features of the orogeny.
Two possible reasons offer themselves for the sudden change in the crustal flow pattern. One is that the North and South American blocks were now “anchored” at the N‑S downflow suture, hence attenuation of the upper crustal layers was no longer operative, upwelling of fluid magma was inhibited, and the Caribbean convection cell ceased to exist. In such case its outline would make it a ready victim for integration into the east‑flowing Pacific crustal sheet. The second is that the Caribbean convection cell expanded, vortex‑ring fashion, until its western edge impinged on the N‑S downflow suture, thus triggering an abrupt change in the regional pattern of crustal flow.
Especially in Western Venezuela, the Mid‑Eocene orogeny was followed by a set of events of quite different character from those recorded earlier. This fact in itself tends to confirm that a dramatic change in crustal conditions had occurred.
A few of the noteworthy happenings were: (a) the uplift of the Perijá and Andes mountains in the form of gigantic horsts, quite distinct from the compressive structures of the coastal ranges; (b) development in the Guajira of graben‑like rifts at right‑angles to the older structural grain, and their infill by marine Upper Tertiary beds; (c) deposition of a thick marine Upper Tertiary sequence in the Falcon Basin and its later folding along well defined ENE‑WSW axes; (d) spasmodic southward shift of the depositional axis of the Eastern Venezuela Basin, accompanied by compressive southward expansion of its north flank; (a) appearance at the end of the Tertiary of a set of NW‑SE right‑lateral wrench faults in northeastern Venezuela and Trinidad.
It appears that these, and related events which need not be discussed in detail here, all find a ready explanation in the pattern of crustal flow as modified by the Mid-Eocene orogeny, as explained below:
(a) The crust below the triangular area of the Maracaibo Basin would be under compression by opposition of the Pacific and Atlantic sheets. The compressive stress could‑be relieved by upward movement of crust into weak zones within the continental block. Such zones were provided by the Early Mesozoic rifts already described. Consequently it appears entirely logical that the Perijá and Venezuelan Andes should rise vertically along these lineaments. Figure 7 depicts the writer’s views on their history. Worth note is that, where the old rifts were not under compression (Guajira, Takutu), mountain ranges have not risen along them.
(b) The distribution of Upper Tertiary sediments on the Guajira is extraordinary, firstly for its orientation at right angles to the old structural axes and secondly for the abrupt changes in thickness in the “Castilletes Basin” and on the structurally high blocks to north and south. This configuration suggests a graben, but by analogy the adjacent Serranías de Jarara and Macuire would be horst blocks separated by grabens only a few kilometers wide. The mechanics of horst-and‑graben formation on such a scale are difficult to visualize, and a much simpler explanation is that these are rifts comparable in type to the Early Mesozoic features already discussed. Granted this basic concept, it could follow that Aruba, Curacao, Bonaire and smaller islands to the east are blocks of a former “Guajira-Paraguan” landmass, which in succession became detached and drifted eastward. The postulated crustal flow is compatible with such a process and the pattern suggests, in fact, that the Oca Fault could mark the line of detachment of the moving elements from the fixed Maracaibo block.
(c) According to the model, Falcón would be situated between east‑moving crust to the north and the fixed buttress of the Lara‑Yaracuy-Carabobo mountains to the southeast. The postulated eastward drift of the Guajira‑Paraguan blocks would impose an oblique compression on the Tertiary sediments in Falcón, and development of concertina-like folds could be anticipated.
(d) and (e) Radial growth of the Caribbean lobe would account for both southward migration of the crustal low defining the axis of the Eastern Venezuela Basin and the oblique shears denoted by the Guárico, Urica, Los Bajos and other sub‑parallel faults in the northeastern region.
Certain additional points deserve mention as arising from the model here presented, if its gross elements be accepted.
Firstly, the status of some of the major faults of Venezuela may be clarified. There has been a tendency in the literature to treat them all as wrench faults because of their length and rectilinearity, but according to the model they have appreciably different histories. Some specific cases are:
The Boconó Fault. This lineament can most logically be regarded as the scar of a pull‑apart rift (the Uribante‑Lara Trough) formed in Early Mesozoic time. It has been modified appreciably because it provided a line of weakness in subsequent phases of movement, such as the Andean uplift. Following the Mid‑Eocene orogeny the Boconó Fault has been in a zone of oblique sheer and some right‑lateral shift may have occurred. The seismicity of the fault may support this concept, but the model suggests that horizontal movement is a late and minor element in its history.
The Tigre Fault of the Perijá could be analogous to the Boconó Fault.
The Oca Fault is in a province which according to the model (and to existing paleogeographic reconstructions) was stable until Mid‑Eocene time. After the Mid‑Eocene orogeny its lineament was intermediate between a rigid block to the south and free‑flowing crust to the north. It therefore seems plausible that the fault marks the line of detachment of the Guajira‑Paraguan blocks from the main continental block. Thus it may be dated as post‑Mid‑Eocene and a measure of the right-lateral displacement is given by the amount of compressive foreshortening in the Tertiaries of the Falcón Basin: this is not a great amount.
The El Pilar Fault. This major fault differs from the others in that, according to the model, immediately after the Mid‑Eocene orogeny its line lay within a zone of direct east-west shear between crustal sheets moving in opposite directions. Hence it is feasible, at least mechanically, that large horizontal displacement has occurred, as has been postulated by certain authors. The Araya‑Paria‑North Trinidad lineament might have originated as far to the west as Falcón, with the Sebastián Fault remaining as a scar of the movement.
Published discussions of the presumed wrench faults of Venezuela have paid insufficient attention to their chronologic aspects. In the case of the El Pilar Fault, according to the model horizontal movement could not start before Upper Eocene time and (if it did take place) was completed before Pliocene time, approximately (because recent geophysical studies appear to demonstrate offset of the Sebastian‑El Pilar lineament by the Urica Fault). In round figures this calls for up to 300 kilometers of movement within 30‑million years, or an average rate of up to one centimeter per year. Published estimates of rates of crustal drift are in the range of 0.5 to 2.5 centimeters per year, so the conjectured movement is plausible.
A second field of interest is that, if the model is basically correct, certain events must have occurred in parts of the Caribbean region distant from Venezuela. This opens up possibilities for verifying, modifying or rejecting the model by means of studies in the areas concerned. Specific cases are as follows:
(a) The Greater Antilles, parts of Central America, and northern South America were presumably united until drift started in Early Mesozoic time (Figs. 1, 2). At present it would be guesswork to designate the sutures along which the dissociated blocks were formerly joined or to specify which blocks were formerly contiguous. However, by analogy with the old rifts within the South American block, it seems likely that long and roughly rectilinear coastlines are the most likely sectors to represent the ancient sutures. There is scope for matching of the pre-Mesozoic sequences in such areas and attempting to find “fingerprints” which would assist in restoring this ancient jigsaw puzzle.
(b) The conjecture of a Caribbean convection cell developing in Mid‑Cretaceous to Early Tertiary time is based solely on Venezuelan evidence (Figs. 3, 4). However, if the basic idea is sound the cell must have had an ovate outline and in the Greater Antilles, for instance, appearance of a depositional trough with an unstable rim to the south might be anticipated (the mirror-image of the Venezuelan pattern). Evidence to this effect would be significant. It must be noted that the effects in Venezuela are likely to be more complex because of impingement of the (presumed) convection cell against a continental mass.
(c) The most drastic effect of the Mid‑Eocene orogeny (Fig. 5) was a sweep of crustal energy through a triangular area extending some 400 miles from north to south and pivoting about an area some 250 miles to the east. The upheaval was typified by general uplift and some volcanic activity to the north and by erratic uplift, tilting and block‑faulting to the south. According to the model there should be a mirror‑image of this orogeny somewhere in the northwestern Caribbean region.
(d) The mechanical feasibility of right‑lateral movement along the Sebastián‑El Pilar fault zone has been mentioned. Even though the validity of the model does not stand or fall on the reality of such movement, it is an intriguing topic. One way to test it would be a critical comparison of the metamorphic sequences at intervals along the Araya‑Paria‑Trinidad lineament and along the coast from Carabobo eastward to Miranda, to see how good a match could be established.
The writer regrets that circumstances prevent him from providing a detailed bibliography with annotations. The account of the Mesozoic‑Tertiary history of Venezuela herein accords with those in well-known texts (e.g. Sutton, 1946; J.B. Miller et al., 1958, 1963; H.H. Renz et al., 1958, 1963) though a different motivation is proposed. Theses of the Princeton Caribbean Research Project have contributed notably to understanding the complex geology of the Guajira Peninsula and the coastal mountains: many of these have been published in the Boletín de Geología. The main structural elements to which reference is made are well depicted on the geologic-tectonic map prepared for the First Venezuelan Petroleum Congress in 1962. There is a voluminous literature on the modern concepts of continental drift: as good a summary as any is that of P.M. Hurley in “Scientific American”, vol. 218, No. 4, p. 53‑64 (1968)—though it is already out of date in certain respects.
This final personal note is inserted to express my appreciation to several colleagues whose sympathetic reception of the ideas here expressed seemed to justify publishing them as a trial balloon. Will it stay aloft or will it be shot down? My special thanks go to Phillip Wolcott, Frank De Joia and Virgil Winkler of Creole, Alirio Bellizzia of the Ministerio de Minas e Hidrocarburos, and Sebastian Bell, now at the University of Alberta. Also I gratefully acknowledge the stimulus of discussions with Peter Temple of Esso Production Research Company on general and specific applications of the theory of continental drift.