POLLEN AND SPORES — LEOPOLD AND SCOTT 321 



broad-leafed genera that now grow exclusively in Asia and also 

 woody types that are now temperate in their distribution. Late 

 Tertiary pollen assemblages show progressively more modern tem- 

 perate floras in successively younger sediments. Pleistocene sedi- 

 ments in the west contain an essentially modern flora that underwent 

 north-south or altitudinal migrations during the several climatic 

 changes of that interval. 



Generalized floristic trends, such as those outlined above, can be 

 safely used within a limited region, like the Great Basin, as a broad 

 standard with which to evaluate assemblages of completely unknown 

 age from the same area. This type of dating requires identification 

 to modern family or genus, where possible, of the dominant fossil 

 pollen forms. 



A striking example of applied palynology has been described by 

 pollen workers employed in petroleum geology studies in the Mari- 

 caibo Basin, Venezuela (Kuyl et al., 1955). By extraction of organic 

 residues from cores as long as 3,000 feet that included sediments of 

 Cretaceous and Paleocene (early Tertiary) age, these workers ob- 

 tained characteristic fossil pollen assemblages that could be traced 

 laterally from well to well for total distances of as much as 100 miles. 

 Pollen zones marked by changes in relative numbers or qualitative 

 composition of the assemblages in the long vertical sections were the 

 basis for a subdivision of underground sediments that could not be 

 successfully delimited by other means. By means of identical floral 

 successions revealed by pollen, four facies provinces in the Tertiary 

 of western Venezuela were correlated. 



The lower parts of these cores are composed of shales deposited 

 in a marine environment, as indicated by remains of marine algae 

 and Foraminifera. The sediments that unconformably overlie these 

 obviously marine (Cretaceous) beds are mostly nonmarine coal beds, 

 sandstones, and fresh-water shales. A pollen zone boundary that 

 was fundamental in the oil-geologic interpretation of the basin struc- 

 ture forms a nearly horizontal stratigraphic line that transects both 

 the rough plane of contact between the marine and nonmarine beds 

 and the irregularities of the textural sediment zones. The most 

 reliable pollen zone boundaries were based on fossil pollen and spore 

 types that showed a similar vertical succession over a very wide area ; 

 these types were assumed to reflect the regional vegetation changes. 



Because of the apparent usefulness of stratigraphic correlation 

 by means of fossil pollen and spores, many of the large oil companies 

 throughout the world now have installed research laboratories 

 equipped for the study of these microf ossils in sediments pertinent to 

 petroleum-geologic problems. 



