TIME-STRATIGRAPHIC CLASSIFICATION OF SEDIMENTARY ROCKS

Abstract

There are many kinds of stratigraphic units—rock-stratigraphic units, bio-stratigraphic units, and stratigraphic units based on mineral character, electrical character, seismic properties, environment of deposition, etc. None of these are intrinsically time -stratigraphic units though each may have time significance to a lesser or greater degree. Radioactivity may eventually furnish a method for truly measuring time in sedimentary rocks, but at present paleontologic (bio-stratigraphic) evidence is the most useful means we have. It should be recognized, however, that paleontologic evidence of time in rocks is always imperfect , that it may in certain cases be exceeded in value by other types of stratigraphic evidence, and that true time-stratigraphic units exist for us only as the ideal concepts of material stratigraphic units comprising the rocks formed during a given time interval, bounded only by time surfaces and independent of lithologic, paleontologic, or other facies. Because of the more or less orderly evolutionary sequence of life forms, the present large-scale geochronologic time-stratigraphic divisions ( systems and series ) can generally be recognized paleontologically with rough accuracy throughout the world, although it is doubtful that their division points are marked by “natural breaks” in the fossil record. However, it is useful and desirable to apply the time-stratigraphic concept also to units of lesser rank where the purpose is perhaps not so much geochronology as the mere determination of synchrony. In the case of these smaller units ( stages ) we are approaching a limit to the degree of resolution possible in the use of evolutionary sequence of fossils, and at this scale the percentage error in the carrying of time-stratigraphic divisions by means of fossils has increased so much that this method is not infrequently exceeded in value by evidence from other stratigraphic features—lithology, mineralogy, electrical character, chemical composition, relation to diastrophism, relation to climate, etc. Since the application of time-stratigraphic concepts to the sediments of brief time spans is nevertheless extremely useful and important to us, we are justified in employing any or all available lines of evidence for dividing and extending such minor time-stratigraphic units and we should not restrict criteria to any one group of fossils or any one facies of fossils, nor yet to fossils in general. Even when the bases for stage divisions are restricted to fossils, we inevitably have an overlap of these units when considered on a world-wide or even a regional scale, since we cannot reasonably expect correspondence between time lines in one area based on one group or one facies of fossils and those of another area based on other groups or other facies of fossils or those of a third area with no fossils at all. Since we thus already have overlapping stages, there is no objection to the use of other than paleontologic bases for stage division on the grounds that this would result in overlapping time-stratigraphic divisions. On the contrary it would seem to be a healthy procedure to allow initial minor time-stratigraphic units to be based on any feature characterizing any well-defined part of a sedimentary section whose time equivalent it might be useful to recognize (either practically or theoretically) in another section, regardless of whether the feature were paleontologic, lithologic, mineralogic, or of still different type. Obviously the number of stages should desirably be maintained at a minimum compatible with maximum service to stratigraphy. However, if stages are adequately defined and correctly used as purely time-stratigraphic units, it does not so much matter how many are made or on what they are based—only those which are of service will survive. Although all of the useful minor time-stratigraphic units would not necessarily have chronologic time significance there should be nothing to prevent the eventual establishment of definite chronologic Stages forming aliquot parts of the standard Series if, as, and when adequate bases for them are found. Since it has been proposed that the terms System and Series be capitalized when used in a geochronologic sense, the formal geochronologic Stage might be conveniently distinguished by a capital S from the stage used merely in the sense of any minor time-stratigraphic unit.