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Group & Formations vs. Formations & Members



I'm just starting to get a handle on that myself and am in early stages
of massive reference collection. As I understand it, there is considerable
disagreement between Spencer Lucas and, well, almost everyone. Lucas has
raised Chinle to a Group, and included the Dockum in it, a move strongly
opposed by Thomas Lehman here at Tech.

As I understand it, the classification system is intended to reflect our understanding of the depositional system "genetics." The traditional "Chinle" and "Dockum" Formations have the same sedimentary source and are contemporaneous within stratigraphic resolution -- they may be deposited in separate but adjacent basins, but are otherwise more or less identical. Hence, we include them together in one, all-encompassing "genetic" unit -- the group, which is the Chinle Group. Within that, there are regional differences -- a mass of sediment at one end of the group may not be contiguous with a coeval one at the other end, so there can be separate formations in different portions of the basin. A series of such strata at one end may be considered a "subgroup," "genetic" unto itself but still related to all the surrounding group. One formation in such a subgroup may be more widespread, however, and occur stratigraphically over unconnected strata in different portions of the basin -- this helps hold the "Group" concept together.


Of course, one can explain this system by just having everything in one big formation (instead of a group), and subdivide it into members. Nothing magical happens to the sediments involved because of this -- there is no physical change to the rocks if they are a "group," a "formation," or a "horizon." Unlike biological nomenclature, the rules of stratigraphic nomenclature are not hard and fast (which is a shame, because the system has been grossly abused in the past). The "bible" of stratigraphic nomenclature is:

Salvador, A. 1994. International Stratigraphic Guide: a Guide to Stratigraphic Classification, Terminaolog, and Procedure. Trondheim and Boulder: International Union of Geological Sciences and the Geological Society of America, 214 pp. (distributed by GSA)

Specifically, it states (p. 35), "A _group_ is the formal lithostratigraphic unit next in rank above a formation. THe term is applied most commonly to a sequence of two or more contiguous or associated formations with significant and diagnostic lithologic properties in common...Formations need not be aggregated into groups...It is considered preferable to establish the unit as a formation and allow future workers to subdivide it, establish constituent formations, and change the rank of the original formation to group while keeping the same geographic component of the name. Thickness of a sequence is not a valid reason for defining a unit as a group rather than a formation.
The aggregation of formations into groups provides a useful means of simplifying and generalizing stratigraphic classification for those who may not need...the often complex detail of formational subdivision in certain regions or certain intervals. Groups are useful in small-scale mapping and in regional stratigraphic analysis.
The component formations of a group are not necessarily everywhere the same...The wedging-out of a component formation or formations may justify the reduction of a group to formation rank, retaining the same name."


As above, some parts of the Chinle can be seen to pinch out, while an overlying unit continues and is much more expansive. This fits the criterion of the above, and it's much easier for me, at any rate, to view it as a self-contained "group" in which some members pinch out laterally than to perceive it as less-related, not self-contained formations, some of which pinch out and some of which don't. However, the system is arbitrary -- for example, a similar debate rages about the placement of some present members of the Morrison Formation which occur only in southwestern Colorado and southeastern Utah with respect to the much more widespread Brushy Basin member of the same formation. (Morrison stratigraphy has a whole 'nother set of problems...) In comparison with the Chinle, however, we may examine the Newark Supergroup of the eastern U.S. -- again, we have a series of depositional basins with more or less common sources of sediment, common fossil faunas and floras (partly even comparable to the Chinle), but as far as I can tell, there is little argument about calling it a "supergroup" with constituent groups and formations, instead of just naming a bunch of separate formations in each basin -- this is because we have a much clearer perception of the overall depositional mechanics, and the evolution of the eastern U.S., when we examine all the basins together, rather than separately.

In my perception, a lot of the disagreement between researchers in the Chinle vs. Dockum arena appear to be more political than geologic -- most of the people who oppose including traditional Dockum sediments in with traditional Chinle rocks tend to be Texans; those who wish to include it tend to be New Mexicans. The problem is compounded by the fact that, thanks to the Llano Estecado and some other units, the Triassic cannot just be walked from west Texas into eastern New Mexico (and even into Arizona) to test the actual continuity of units -- we have to rely on less direct evidence, such as fossil correlation, which is, of course, eminently testable. At any rate, we all know that the best research is done with the presentation of evidence, not the bluster of politics...don't we? ;-D


~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Jerry D. Harris Dept of Earth & Environmental Science University of Pennsylvania 240 S 33rd St Philadelphia PA 19104-6316 Phone: (215) 898-5630 Fax: (215) 898-0964 E-mail: jdharris@sas.upenn.edu and dinogami@hotmail.com

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