The determination of time horizons is difficult because of the comparatively slight amount of information collected to date; because many palynomorph-bearing sediments are devoid of other fossils which may permit accurate dating; and because many lithologic strata are known to transgress time.

Complications to accurate dating involve recognition of following facts.

1.There are no finite boundaries based on evolution of plants alone. Unless an unconformity is present all horizons involve more or less gradual change.

2. Sediments may be carried into a basin from different directions, resulting in a mixing of suites of palynomorphs from dissimilar floral provinces.

3. River transport may carry a suite of fossils different from those by air transport from the same floral province as well as from a different province.

4. Circulation (coriolus or other currents) can reorient and partially re-distribute fossils after they have arrived within a basin of deposition.

At the present time, the general boundaries (worldwide) such as the Mio-Oligocene, Pennsylvanian-Permian, and Mississippian-Devonian are recognized. Absolute determination has yet to be made, in most instances. Such boundaries are largely theoretical and have to be established and extended as each new fossil province is studied. However, time lines, within an individual depositional basin, can be established palynologically, with reliability and confidence. If palynomorphs are present in sediments of a basin, time-stratigraphic determinations can be made and correlations established.

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