Thanks, Bill, for reposting the stratigraphic column by Grotzinger et al. constructed from the efforts at Eagle and Endurance craters. It is a good place for us to start our discussions of the Victoria exposures.
This is also a good place to do a little nomenclatural housekeeping. I tend to be very exacting with my students on these topics, but get a little sloppy myself, especially in an informal situation like internet discussions or email.
When communicating ideas about rocks or sediments, especially layered rocks or sediments, it is always good practice to keep observations and interpretations separate, especially in how we name and classify them. This is expressed in the concept of lithofacies
in which we classify and name a sediment or rock based upon the fundamental properties of composition, texture, sedimentary structures, form, association, and fossils (if present). An example of a lithofacies would be a "medium-grained, cross-bedded sandstone". On the other hand, the depositional environment of a package of ancient sediments or sedimentary rocks is an interpretation based on analyses of these fundamental properties and the changes between, and association with, units above, below, and laterally adjacent. We sometimes mix these two concepts into a hybrid "depositional facies" like an "eolian sandstone". The word "evaporite" also fits this situation. The lithofacies is probably something like a "thinly-laminated to ripple cross-laminated, tightly cemented, recrystallized, magnesium sulfate sandstone". I'm not about to type that every time, so I lazily fall back on evaporite.
There is one more way to classify and name layered rocks, and that is formal stratigraphic nomenclature. These would be the formal names of supergroups, groups, formations, and members. There are two publications that deal with the details of naming these units that codify how it is done, the North American Stratigraphic Code
and the International Stratigraphic Guide
. Inherent in both are the concepts that depositional environment AND age should have no bearing on the delineation and naming of formal stratigraphic terms. The preferred root of the name is typically a geographic location where the unit was first described, or where it is exceptionally exposed.
So back to the figure from Grotzinger et al....they term the units as parts of the "Burns formation". By the fact that they do not capitalize the word "formation", I am guessing that they are not trying to erect a formal stratigraphic nomenclature here. In fact, I'm not sure if the Code or the Guide have any extrerrestrial impact at all (although I do not see why this should matter). However, they do erect several subformational units, the Lower, Middle, and Upper units, which they (confusingly) do capitalize, implying that they are "formal" stratigraphic units. The other parts of the diagram, especially the sections labled "Primary Facies" and the text to the right of the column have the evil mix of observation and interpretation that I was discussing above. The "Primary Facies" are grouped by their interpretation of paleohydrology (Dry, Dry to Damp, and Damp to Wet), but followed by both terms of environmental interpretation and physical sedimentologic description (i.e. eolian sandsheet interdune and ripple cross-stratified). The text to the right of the column groups the units by depositional environment and diagenetic features, and the smaller font supporting text has physical sedimentological lithofacies descriptions, environmental interpretations, and even speculative interpretations. It's a bit of a mess...if this manuscript was sent to me to review (hint, hint to any editors or PI's reading...), I would have recommended that the observations/data be convincingly separate from the interpretations in this diagram (and in the main text, by the way...).
Now, this is not to take anything away from this paper...it's a great summary of some landmark research that has implications far beyond Endurance crater, and in fact, along with other information coming out of the MER program, it has changed some of our basic ideas regarding the history of surfical conditions on Mars.
However, it does show that even the Big Guns/Chosen Ones get a little sloppy sometimes, too...and mea culpa, I will try to strive to set a better example, even in the informal setting of UMSF!