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Time Scale Divisions & Geology 101



I'd hate to say this, but restructuring a geologic division based on
the animals or the possible advant of the fugal terrestrial
succession instead of the clearest geological division, to suit the
idea that a certain succession must occur in a certain division
instead of another, makes a very illogical argument. Geological
divisions are based on geology, not the fossil record.

OK, let's see...where to start...how 'bout Geology 101?

No offense to Jaime (or anyone else), for whom I have a great deal of respect, but this is flatly, patently FALSE. Geological divisions are based ENTIRELY on the fossil record (at least for the Phanerozoic). With a few minor exceptions (and again, like BIFs, most of those are Proterozoic, not Phanerozoic), uniformitarianism really does rule the geologic record: the same processes that are occurring today occurred in the past. Thus, rocks, regardless of the time of their origin, have similar or identical origins. Rivers continue to deposit sediment, and have done so as far back as there's been land. Volcanoes continue to erupt and spew ash and lava onto the surface. Tectonics continues to push granitic intrusives from deep within the Earth to the surface. Etc., etc., etc., ad nauseum. These things happened in the Cambrian, in the Permian, in the Jurassic, the Miocene, and today. There is no essential different to the deposits formed in any one of those time periods that _in_and_of_itself_ can be used to tell when the deposits were formed. To do that, you need something that is indigenous to that time period _and_that_time_period_alone_. These, of course, are fossils.

Long before radioactivity was recognized, the divisions of the time scale were set down, based solely on the occurrences of particular fossils or fossil assemblages. Most of the familiar Phanerozoic divisions, (Cambrian, Ordovician, Silurian, ... Cretaceous) were delineated by geoscientists recognizing that particular taxa and assemblages had limited stratigraphic ranges globally. If one found those fossils, one knew with which other rocks one could correlate, and know (in a relative sense) where one was in prehistory. Sure, one couldn't say "These rocks were deposited 150 million years ago," but one _could_ (and did) say "These rocks were deposited in the Jurassic." Divisions of the time scale are based on _fossils_.

The advent of absolute dating techniques did _not_ (and I feel compelled to, very emphatically, emphasize _NOT_) shift the standard away from fossils and on to non-fossil materials as the basis for time scale divisions. What these techniques did (and continue to do) is _fine_tune_ the time scale. Where previously geoscientists could only recognize that there was a marked change in the fossilized contents of rocks that they would call, for example, Silurian and Devonian, they could _additionally_ now say that that change occurred 417 million years ago. But, the definition of the Silurian-Devonian boundary _was_ not, and _is_ not, defined by any marker that can be dated to 417 million years ago! It was, and is, defined by the disappearance (last appearance datum) of various taxa and/or the first appearance datum of others. Only new data of the occurrence of the defining fossil taxa can shift the boundary. Thankfully, these taxa are not such vastly broad groups as "land plants" (so Ken and Tom are perfectly correct in that there is no way that pushing the origin of land plants back to 700 million years ago would push any time scale division boundary back with it)!

A key issue for Mesozoic paleontologists, and one that has been bandied about on this list innumerable times in the past, is the K/T boundary. The Cretaceous-Tertiary boundary was, and is, as are almost all Phanerozoic boundaries, based on the disappearance of some taxa and the appearance of others. It _is_ not, and never _has_ been, based on the appearance of a layer of soot, iridium-rich clay, coal, tektites, shocked quartz, or any other geological marker. Nor it is based on any igneous material that can be dated to 65 million years ago. In fact, that initial date of 65 million years (which has fluctuated rather greatly over even the last 50 years) was based on a radiometric assessment of rocks _previously_ pinpointed as containing the K/T boundary _based_on_fossils_. The absolute dating techniques were used to provide an absolute age for an _already_identified_ time boundary. Yes, the actual numbers have shifted over time, due in part to refinement of various radiometric techniques and in part due to refinement of the fossil-based boundary with new discoveries and better correlations, but at no time has anything geologic (radiometric or otherwise) overshadowed the fossil record as the definition of the K/T boundary. Absolute techniques only aid in the _resolution_ of the fossil-based time boundaries; they do not replace it.

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Jerry D. Harris
Dept of Earth & Environmental Science
University of Pennsylvania
240 S 33rd St
Philadelphia PA  19104-6316
Phone: (215) 573-8373
Fax: (215) 898-0964
E-mail: jdharris@sas.upenn.edu
and     dinogami@hotmail.com
http://www.sas.upenn.edu/~jdharris

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