STRATIGRAPHY TERMINOLOGY: GROUPS, FORMATIONS, MEMBERS

The other day I was out looking at a great outcrop of the Fountain Formation near the town of Manitou Springs.  A”stranger” pulled in a parking area and wandered over to the road cut and struck up a conversation.  Essentially he told me that his work brought him past the outcrop on most days and he always wondered why the rocks were pink and red.  Well, to an old geology instructor like me, that question was an invitation to “educate” the stranger about the Ancestral Rockies.  After a nice visit and some wild hand waving by me, the “stranger” wanted to know if the formation was related to Fountain Creek or the town of Fountain.  This was a very intuitive question and gave me a good opening into the discussion of geologic formations and type localities.  Alas, he could only stick around to get the short answer---yes, it is related to Fountain Creek.  I then gave him the address of this Blog and told him an expanded answer would be forthcoming. 

As I have discussed before, geology is a very terminology-oriented discipline.  And, unfortunately, many of the terms used in discussing “everyday” geology are new and somewhat confusing to the reader and listener.  That is one reason why I try to fully explain most of the terms used in the Blog—I want the readers able to use geological terms in an everyday language when discussing rocks and minerals and fossils. 

Fortunately, the internet is a handy reference for getting information on most geological terms---as long as you are able to sort out the plethora of misinformation.  A good “hard copy” desk reference book is the Dictionary of Geological Terms by Robert Bates and the American Geological Institute (I keep a well used copy on my desk) and available at many booksellers.  WWW.Geotech.org has a much shorter on-line version of common geological terms that is easy to use. 

In addition to a somewhat complex terminology, there are numerous rules and regulations concerning the correct usage of these terms in the published literature; the “geologist’s bible” used by most writers is Suggestions to Authors of the Reports of the United States Geological Survey and available online at www.nwrc.usgs.gov/lib/lib_sta.htm.  Two of the most useful sections of this book are entitled “Suggestions To Expression” and “Choosing The Right Word”.  For example, this book tells me that the word “outcrop” is a noun while “crop out” is the verb.  So, the Lyons Sandstone in Garden of the Gods crops out and forms fins (the outcrops).  It may seem insignificant and trivial but correct terminology is an important element of a well constructed paper.

Another invaluable source of information for the geologist is the North American Stratigraphic Code (http://ngmdb.usgs.gov/Info/NACSN/Code2/code2.html) which presents information for “classifying and naming stratigraphic and related units”.  he Code is directly related to the “stranger’s” question about the Fountain Formation, a stratigraphic (layered sedimentary rocks) unit. 

A “formation” is the fundamental rock unit used to describe the sedimentary geology of a region.  It is a unit that may be identified by its: 1) position in the stratigraphic (rock) column; 2) mapability on the earth’s surface; and 3) mineral/rock characteristics.  The name of a formation is compound, that is, it contains two parts—a binomial name.  The first part is a geographic name from a location where the unit was first studied and named (such as Fountain for exposures along the Creek).  The second part is either the term “Formation” for exposures containing a variety of rock types, or a lithologic term (rock type) where exposures are of the same rock type, such as the Mancos Shale.  Each formation should have a designated type locality and a type section (published in a juried journal) so that later workers understand exactly the thoughts of the original workers. 

Thus far it all sounds so simple, if only it was!  Many of the formations named in the 1800’s and early 1900’s do not have precise type localities let alone a type section.  For example, the Fountain Formation was named (Cross, 1894) for “typical development on Fountain Creek below Manitou Springs and at head of Fountain Creek, El Paso Co, CO”.  So, we know about where the type locality is located but we have no type section.  The Graneros Shale, named by G. K. Gilbert in 1896 when studying rocks east of Pueblo, also does not have a type section nor a type locality.  However, workers at the U. S. Geological Survey have designated a “principle reference section” in lieu of a type section and allowed by the Code.  Another interesting occurrence is with the Eagle Valley Evaporite Member of the Minturn Formation first named in 1958 from Eagle County, CO.  It was redefined and elevated to Eagle Valley Evaporite in 1962, was changed to the Eagle Valley Formation in 1968, and then back to the Eagle Valley Evaporite in 1971, its current usage (I think).  Perhaps beauty (or the name of a formation) is in the eye of the beholder!

At times geologists find it advantageous to subdivide formations into formal members in order to highlight rock units of special interest.  A formation need not be subdivided into members although some will be completely divided while others will have only certain parts named.  For example, the Niobrara Formation exposed along the Front Range is divided into the lower Fort Hays Limestone and the upper Smoky Hill Chalk members.  As with formations, the unit has a compound name with a geographic “first” name and the term member or a rock term as the ‘second” name. At other times, geologists may find it advantageous define informal members-- the Colorado Geological Survey maps and divides the Upper Cretaceous Laramie Formation at Colorado Springs into the upper member, the middle sandstone member, and the lower member.  One can distinguish between formal members and informal members by noting capitalization (or lower case) of the initial letter of each term.

Members may be further subdivided into beds with naming rules similar to formations and members.  Most beds that I am familiar with have an economic significance, such as some of the beds in the Green River Formation (Eocene of northwestern Colorado).

And finally, two or more formations may be combined into a group with the compound name consisting of a geographic term and the word Group.  For example, the Benton Group consists of the Carlile Shale, Greenhorn Limestone, and Graneros Shale.  Groups are commonly employed on large scale maps. 

It also might be of interest to examine type localities of other formations  exposed near Colorado Springs: 1) Dawson Formation [1912] no type locality but named for Dawson Butte near Castle Rock, CO;   2) Laramie Formation [1888]---no type locality, “exposed along Front Range”; 3) Fox Hills Sandstone [1862]---no type locality, named for Fox Hills near Fort Pierre, SD; 4) Pierre Shale [1862]---no type locality, named for Fort Pierre, SD; 5) Niobrara Formation [1862]---no type locality, named for exposures near mouth of Niobrara River, NE; 6) Carlile Shale [1896]---no type locality but named for Carlile Station and Carlile Springs 21 west of Pueblo, CO; 7) Greenhorn Limestone [1896]---no type locality, named for Greenhorn Creek and Greenhorn Station, CO; 8) Graneros Shale [1896]---no type locality and no name derivation but USGS has named a Principal Reference Section in CO; 9) Dakota Sandstone [1862]---no type locality, named for Dakota, NE but type designated by Nebraska Geological Survey; 10) Purgatoire Formation [1912]---no type locality, named for Purgatoire Canyon, CO; 11) Morrison Formation [1896]---no type locality, named for Morrison, CO; Colorado geologists have designated a type section; 12) Lykins Formation [1905]---no type locality, named for Lykins Gulch, Boulder, CO; 13) Lyons Sandstone [1905]---no type locality, named for  Lyons, CO. 

REFERENCES CITED

 Cross, C.W., 1894, Description of the Pikes Peak sheet [Colorado]:

 U.S. Geological Survey Geologic Atlas of the United States,

 Pikes Peak folio, no. 7, 5 p.

Gilbert, G.K., 1896, The underground water of the Arkansas Valley 

   in eastern Colorado, IN Walcott, C.D., Seventeenth annual 

   report of the United States Geological Survey to the Secretary 

   of the Interior, 1895-1896; Part II: U.S. Geological Survey 

   Annual Report, 17, pt. 2, p. 551-601.