Group & Formations vs. Formations & Members

["Jerry D. Harris" <dinogami@hotmail.com>]

>     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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