Reprinted from the Asociación Venezolana de Geología, Minería y Petróleo Boletín Informativo, Nov 1964.
R. M. Stainforth
An unusually large number of members attended the monthly meeting on November 19, when the luncheon was followed by an open discussion of the age of the Roraima Formation.
Mr. George Fournier led off with a description of the discovery of pollen in Roraima “shales” collected by Mr. G. C. K. Dunsterville at Cerro Venamo. Subsequently samples submitted by the British Guiana Geological Survey, from the Kamarang and Kopinang areas, have yielded similar plant remains. The utmost care has been taken to eliminate any possibility of superficial contamination, but even the small nucleus broken out of a large solid block yielded plant microfossils. Some of them are suggestive of Cretaceous, others of Miocene age. The suite has been compared with the pollen of 150 modern plants of the Gran Sabana and Roraima Plateau, but they are distinct. Grains and tissue fragments up to 150 microns in size are present, and are considered too big to have been carried by meteoric waters percolating through cracks.
Mr. Fournier did not challenge the Precambrian radiometric dating of certain sills which cut the Roraima Formation, but suggested that the formation has accumulated gradually through the whole geologic time, and that careful correlations would show that the polleniferous beds belonged to a higher level. He cited an intrusion of established Triassic age which cuts the basement rocks.
Dr. Gordon Young spoke next on the regional setting of the Roraima Formation. By means of regional cross sections from the North Venezuelan coastline to the Amazon Basin, he showed that beds of Roraima type could plausibly be fitted into the paleogeography of several periods. Even age-equivalence, in part, to the Freites Formation would not be a preposterous suggestion. Records indicate that, in a general sense, the Roraima is more indurated and heavily intruded in its lower parts, less so in its higher beds. On this basis a multiple age for the Roraima was suggested.
Mr. Myles Bowen showed photomicrographs, some in color, of the pollen-bearing “shales”, and demonstrated that they are really hornfelses. The groundmass is fine-textured micas (mainly sericite) in optical continuity, defining an incipient schistosity oblique to the original bedding. Intense search for plant microfossils in the thin sections was unsuccessful. Conspicuous on the slides were angular phenocrysts of magnetite and porphyroblasts identified as probably andalusite (chiastolite) and cordierite. This suite indicates baking of the original shales to temperatures of 400-800°C. Harker has suggested that porphyroblasts form around nuclei of carbonaceous matter, and chiastolite typically absorbs carbonaceous matter into its crystal lattice. Fine opaque matter in the groundmass could be graphite, though admittedly distinction from magnetite is next to impossible. Mr. Bowen considered the preservation of pollen in these metamorphosed rocks to be highly unlikely. He therefore favored an external origin, though he could not explain how such contamination took place,
On opening the meeting to general discussion, several speakers raised the question of how resistant pollen might be to anaerobic baking. It seems that no experiments have been conducted, hence it cannot yet be taken for granted that primary pollen may not occur in hornfelsized shales.
Dr. Dallmus supported Dr. Young’s concept of multiple age for the Roraima, and cited known sequences of beds elsewhere in the Brazilian and Colombian Guayana regions, ranging in age from Paleozoic to Tertiary, and cut by flat-lying sills. In contrast Dr. Stainforth claimed that the remarkably horizontal attitude of the Roraima Formation and its sills permitted long-range correlation, and he did not doubt that the hornfelses from Cerro Venamo, Kamarang and Kopinang were correlative and were all altered by intrusion of the underlying Kamarang-Kopinang sill. This sill is dated Precambrian at several localities, and recently the same age was determined for a still higher sill. Thus there is a direct conflict between the radiometric and the palynologic dating.
Other speakers included Dr. Salvador, Dr. and Dra. Rivero, Dr. Gonzalez de Juana, Dr. Krause and Messrs. Key, Kiser and Lamb. No unanimous decision was reached. Some participants stressed the apparent physical impossibility of pollen entering the hornfelses from outside: others emphasized the unlikelihood of finding well-preserved indigenous pollen in hornfelses and, further, the conflict with radiometric dating of this part of the Roraima Formation as Precambrian.
In conclusion Dr. Young proposed formation of a study-group to organize more definite research into this intriguing problem. Mr. Fournier volunteered to make palynologic studies of material sent to him, even though on the point of leaving Venezuela. Forthcoming studies of paleomagnetism of the Roraima Formation by Prof. Hargreaves of Princeton University were mentioned as an important independent approach to deciding between Precambrian and a younger age.
[AVGMP Boletín Informativo 1966, v. 9, p. 173-176. Reprint from Nature, Vol. 210, No. 5033, pp. 292-294, April 16, 1966. Occurrence of Pollen and Spores in the Roraima Formation of Venezuela and British Guiana.]