A SYMPOSIUM
Extracted from the
Journal of Paleontology
Vol. 23, No. 2, March 1949
with the kind permission of SEPM Society for Sedimentary Geology, February 2003
ABSTRACT‑The Hannatoma fauna is an assemblage of marine and brackish‑water mollusks present in Tertiary strata at several widespread points in northwestern South America. The distinctiveness and wide geographic range of the fauna combine to make it important in stratigraphic correlation, but its value in this respect has been impaired by difference of opinion as to whether its age is middle Oligocene, as originally claimed, or tipper Eocene. The seven papers in this symposium comprise the response to a circular requesting current opinions as to the age of the Hannatoma fauna. Different aspects of the age‑relationships of associated mollusks and Foraminifera in Venezuela, Colombia, Ecuador and Peru are discussed. The authors conclude that the Hannatoma fauna occurs mainly in upper Eocene but locally in Oligocene beds.
THE Hannatoma fauna is an assemblage of marine and brackish‑water mollusks known from several Tertiary formations in northwest South America. In its first detailed description by A. A. Olsson (1931)[2] correlation was made with the fauna of the Middle Oligocene Antigua formation. This age assessment has usually been accepted without comment (e.g. Davies 1934, Notestein et al. 1944, Sutton 1946) despite the modification of Olsson's Peruvian time‑scale by Wiedey and Frizzell (1939), but in more than one instance an anomaly has arisen because beds considered Middle Oligocene on the basis of the Hannatoma fauna must be dated as Upper Eocene on alternative evidence.
Such anomalies are undesirable and this one is especially troublesome because the Hannatoma fauna is the sole age‑criterion of certain Tertiary formations. It would seem that either the Hannatoma fauna must be a facies‑controlled assemblage ranging from late Eocene into the Oligocene, or else its assessment as Middle Oligocene in Peru must be out of accord with established biochronology in the Caribbean region. To test current opinion R. M. Stainforth undertook to circularize geologists and paleontologists familiar with the problem, requesting from them objective statements on the age of the fauna in areas which they had studied. The seven statements which follow comprise the response to this canvass. Based on age‑relationships partly of mollusks and partly of Foraminifera in Venezuela, Colombia, Ecuador and Peru, the following conclusions are reached:
(1) In the Venezuela‑Colombia borderland known records of the Hannatoma fauna are all from Upper Eocene beds.
(2) In the northern Peru‑southern Ecuador region some occurrences are in Eocene, others in Oligocene beds.
(3) The presence of the main elements of the Hannatoma fauna signifies a brackish or semi‑brackish facies and is not an accurate guide to geologic age.
As indicated in the text, the field and laboratory data discussed in this symposium are drawn mainly from the geological files of certain oil companies operating in South America. Thanks are due to these companies for permitting publication of these data.
R. M. STAINFORTH
Tropical Oil Co., Bogotá[3]
The age of the Hannatoma fauna in the Zapotal sands can be based either on long-range correlation of a minor fraction of the mollusc fauna or on a closely integrated foraminiferal zonation applicable to the whole Caribbean region. The writer considers the latter basis very much more satisfactory, and therefore strongly favours the Upper Eocene age determination deduced from it. It may prove that the Hannatoma fauna is facies‑controlled and likely to occur in semi‑brackish facies of both Eocene and Oligocene ages, but at least it is considered to be of proven Upper Eocene age in Ecuador by the standards currently accepted in the Caribbean region.
The mollusc assemblage commonly known as the Hannatoma fauna was described by A. A. Olsson (1931) conjointly from the Mancora sands of Peru and the Zapotal sands of Ecuador. The presence of Ampullinopsis spenceri Cooke and Hemisinus sp. aff. H. antiguensis Cooke was taken as evidence of correlation with the Middle Oligocene Antigua formation. In the field, the Zapotal sands were considered to be up-faulted with respect to contiguous Mambra (now termed Dos Bocas or Rodeo) shales, equivalent to the Upper Oligocene Heath shales of Peru.
Some ten years after Olsson published his opinions, the International Ecuadorean Petroleum Co. undertook detailed geological exploration in coastal Ecuador. Numerous wells were drilled in the Progreso Basin and foraminiferal zonation was applied to their correlation. It was discovered that between the Zapotal sands and the Upper Oligocene Rodeo shales there exists an extensive series, 3000 to 4000 feet thick, of shallow marine, brackish and terrestrial deposits. The marine shales in this section carry rich foraminiferal faunas with species such as Bulimina jacksonensis, Hastigerinella eocenica and (locally) Hantkenina alabamensis, which on a regional scale are regarded as Upper
Eocene markers. Furthermore, within a few hundred feet of the top of this sequence there is a minor change in the faunas; the rigidly Eocene markers disappear, Bulimina sculptilis replaces B. jacksonensis, and the relative frequency of various Globigerina species changes. In a regional sense this faunal break corresponds to the Eo‑Oligocene boundary. At the surface the Zapotal sands are in fault contact not with the Upper Oligocene Rodeo shales, as was once thought, but with the Lower Oligocene shales of the section just mentioned. This fault is one of the major graben‑faults of the Progreso Basin, and its throw is sufficient to warrant direct correlation of the surface and subsurface sands which carry the Hannatoma fauna.
These Foraminifera‑based findings, with their implication that the Hannatoma fauna occurs well below the top of the Eocene, flatly contradict Olsson's claim that this mollusc assemblage is Middle Oligocene. In the writer's opinion the foraminiferal evidence is much the more reliable. The Tertiary strata of coastal Ecuador carry rich microfaunas which have been studied in great detail at surface and in well sections. The Upper Eocene deposits vary in facies, but pelagic Foraminifera and Radiolaria can be used to confirm correlation across facies‑province boundaries, thus giving a zonal scheme applicable along the whole extent of the coastal belt. On this basis the shallow marine to brackish shales overlying the Zapotal sands in the southwest are correlated unequivocably with the neritic Zapallo shales of the northwest. The Zapallo in turn carries a rich Hantkenina‑bearing foraminiferal fauna comparable in detail with Upper Eocene faunas from western Colombia, the Tranquilla shale of Panama, the Amoura shale of Costa Rica, the Chapapote formation of Mexico, various Californian shales, the Gulf Coast Jacksonian, the San Fernando (Mount Moriah) silt of Trinidad, the Paují shale of Venezuela, etc. (for references see Stainforth, 1948). The Upper Eocene (Jackson) age of these units is agreed upon without notable exception by specialists in both megafossils and microfossils.
In the face of this convincing evidence of Upper Eocene age, the molluscs were determined as Middle Oligocene on the strength of only two species out of a list of nearly a hundred, the rest being described as new or indeterminate at the time the age assessment was made. Subsequent studies of the Zapotal molluscs by J. G. Marks and A. A. Olsson for International Ecuadorean Petroleum Co. have still further weakened the case for a Middle Oligocene age. The specific determination of Ampullinopsis spenceri was modified and in manuscript it was finally referred to a subspecies with affinities to the Vicksburg A. mississippiensis Conrad. The determination of Hemisinus sp. aff. H. antiguensis Cooke was left in this tentative form and never confirmed. Marks referred certain mollusc assemblages from wells to pre‑Zapotal zones because of admixture of species previously only known in the Peruvian Eocene. In structural sections subsequently prepared by R. L. Milner these so‑called “pre‑Zapotal” faunas are seen to be no older, and in part definitely younger, than typical Zapotal assemblages with Ampullinopsis spenceri, etc. In other words the Zapotal mollusc faunas are locally intermingled with recognized Eocene forms.
In his original discussions Olsson drew support for an Oligocene age of the Zapotal sands from their inferential correlation with the Ancon Point and San Mateo sands on the coast. These latter carry cool‑water marine molluscs of the Acila‑Thyasira‑Pleurophopsis suite, commonly regarded as Oligocene markers on the basis of studies in the California region. In Ecuador and Peru, however, these molluscs have been found within the life range of such Eocene markers as Tubulostium and Hantkenina.
The Zapotal sands and overlying beds mark the end of a cycle of sedimentary in fill. The ensuing Rodeo shales are of marine origin indicative of subsidence and the commencement of a new cycle. In a single well in the centre of the Progreso Basin the basal Rodeo shales carried the late Middle to early Upper Oligocene 'Manta' fauna of marine Foraminifera. Around the margin of the basin there are remnants of Miogypsina‑bearing reef‑sands marking the initial phase of subsidence in the younger Oligocene. In northwest Ecuador a parallel subsidence is recognizable and its start is marked by strong marginal unconformities which dwindle away into the basin sediments. Miogypsina reefs occur at the base of the unconformable sediments. The time of the initial subsidence in the northwest fell between the extinction of Globigerina cf. concinna Reuss and G. dissimilis Cushman and Bermudez, i.e. at mid‑Middle Oligocene. Assuming that subsidence was contemporaneous in the south, it is difficult to reconcile the presubsidence accumulation of up to 4000 feet of sediments above the Zapotal sands with the claim that these sands are Middle Oligocene.
The foregoing data are drawn, by permission from various reports in the geological files of International Ecuadorean Petroleum Co. Much credit is due for detailed studies of megafossils to J. G. Marks and A. A. Olsson and of microfossils to F. V. Stevenson and H. E. Thalmann, all previously on the company's paleontological staff.