Depositional Environments
Walther's Law of the Succession of Facies
“The various deposits of the same facies-area and similarly the sum of the rocks of different facies-areas
are formed beside each other in space, though in a cross-section we see them lying on top of each other …
it is a basic statement of far-reaching significance that only those facies and facies-areas can be superimposed primarily which can be observed beside each other at the present time” (Translation by Middleton,
1973)
Walther's Law is probably the most important statement in stratigraphy. Without it, we
would be forced to interpret sedimentary environments solely from features found in
that environment, such as lithology, fossils, and sedimentary structures. In many cases,
these features are not diagnostic of a single environment. Walther's Law allows us to link
successive beds in a section. It is this linking of beds that allows us to determine sedimentary environments by the context of sedimentary indicators.
Walther's Law works only in conformable intervals of rock. At unconformities, environments are vertically superimposed even thought they were not originally adjacent to
one another. In Walther's phraseology, these environments would be superimposed secondarily. Misapplications of Walther's Law abound in which authors ignored or were unaware of unconformities. To be fair, many unconformities are subtle and can be difficult
to detect.
In this laboratory, you will use Walther's Law and evidence of sedimentological processes to interpret several depositional environments. Your facies interpretations are important, because they will be the basis of your interpretations in subsequent labs. For example, in the next lab, you will correlate measured sections in a stratigraphic cross-section,
and intelligent correlations must be based on facies.
In making an environmental interpretation, you should consider several things:
1) Think about the meaning of specific features, such as grain size, sedimentary structures, or rock color, and what they mean in terms of sedimentological and diagenetic
processes. For example, bidirectional paleocurrent directions, paired clay drapes, and reactivation surfaces may indicate tidal processes. Once you have interpreted the processes
indicated by all the features in the facies description, think about what sedimentary environments would have those processes.
2) Remember Walther's Law - the sequence of observable features formed in a set of
vertically superimposed environments. Do the environments you've interpreted for each
environment make sense if those environments were originally laterally adjacent? If you
have environments that are difficult to pin down, does their vertical association with other sedimentary environments help in their interpretation? For example, you may have a
mudstone facies lacking any diagnostic features, but based on its Waltherian relationships
with another facies, you may be able to determine the environment of deposition from
the context of that facies.
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Depositional Environments
3) Look for obvious trends: coarsening-up, fining-up, changes in fossils or trace fossils,
changes in sedimentary structures, changes in bed thickness, etc. Compare these to various facies models. Some environments are characterized by coarsening-upwards, others
by fining-upwards, and these are often a quick way to narrow the list of candidate environments.
4) Look for sharp breaks that punctuate these trends. These breaks may indicate erosional or non-depositional surfaces that bound Waltherian packages, which will help you define what facies to interpret together.
5) Try to develop several hypotheses for environment at the same time. Compare what
features you would expect for each facies model to what is actually in the section. Try to
falsify (that is, disprove) your hypotheses until only one facies model fits. This approach
of simultaneously testing several hypotheses and attempting to falsify them is valuable in
all scientific endeavors and is called the Method of Multiple Working Hypotheses. If you
falsify all of your hypotheses, go back and see if any of them could be modified somewhat to accommodate your data. Alternatively, see if a new model can be developed.
6) Once you've decided on an interpretation, go back to the facies description and try to
understand how particular sedimentary structures may have been formed within that environment. Is everything consistent? Does anything not make sense? Think about other
data that could be collected to test your facies interpretations. The best facies interpretations are revealed by this process of proposing a hypothesis and re-evaluating and refining it.
7) Use your class notes and textbook extensively to interpret features with which you are
not familiar. You will use your lecture notes and text extensively for this. You may also
use outside sources, but if you do, you must cite them in your final report. Not citing
them constitutes plagiarism and will be handled as cheating. The Treatise on Invertebrate
Paleontology (Volume W) has pictures of most of the trace fossils described here, so
that you can see their overall morphology for interpreting ichnofacies. Our library has
several good volumes on trace fossils; check the library catalog. It also has many good
references on sedimentary structures and depositional environments. Most of the good
references are not available electronically. If you simply Google for something, you are
greatly limiting your options. Head to the library; it will show in your final report.
8) Citations are typically handled as follows. Let's say you found a 2007 paper by Gene
Simmons describing a particular trace fossil. You would state what she found, then cite
the paper in parentheses: “Teichichnus is characteristic of infernally hot sedimentary environments (Simmons 2007).” You should not make the author the subject of the sentence; do not write “Simmons (2007) said that Teichichnus is characteristic of infernally
hot sedimentary environments.” You should include a reference list at the end of your
facies interpretations with each of your references cited in Journal of Sedimentary Research format. Here's an example of a journal article, a book section, and a book:
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Depositional Environments
WALKER, K.R., SHANMUGAM, G., and RUPPEL, S.C., 1983, A model for carbonate
to terrigenous clastic sequences: Geological Society of America Bulletin, v. 94, p.
700-712.
WALKER, R.G., 1984, General introduction: Facies, facies sequences and facies models,
in Walker, R.G., ed., Facies Models: Toronto, Geological Association of Canada, p. 1-10.
WILSON, J.L., 1975, Carbonate facies in geologic history: Berlin, Springer-Verlag, 471 p.
Follow this format precisely and look at any reference list in a Journal of Sedimentary
Research article for more details. If you are uncertain how to format a reference, ask us.
Last, webpages are not scholarly resources. Do not use them and do not cite them. That
includes Wikipedia. It is fine to use them as a start of your research, and they can lead
you to legitimate, peer-reviewed works.
What to do
For this laboratory exercise, you should prepare a one-page table for each of the five facies (do not interpret facies FC for now). In the left column, list the features that are reported, and in the right column, list the corresponding interpretation of that feature in
terms of sedimentary process. For each facies, include a 3-5 sentence paragraph indicating what environment that facies was deposited in, briefly explaining your logic, and explaining why other similar environments (if any) were rejected. The topic sentence of
each paragraph should give the interpreted environment for the facies.
In some cases, you may find data that indicate the orientation of shoreline (e.g., N-S) or
the direction towards seaward (e.g, E). Include such interpretations and the data supporting them.
If you use external sources, cite them in your tables where you use them, and include a
single reference list for all of your sources at the end of your lab.
Facies Descriptions
Facies FM
The lower part of Facies FM consists of dark shale and silty mudstone, with burrows of
Helminthopsis, most obvious in sideritized beds. This strata pass gradationally upwards
into very thinly bedded dark gray (brown where weathered) silty mudstone with very thin
to thin beds of very fine to fine-grained sandstone. Organic matter is abundant. Common trace fossils include Planolites and Chondrites. Sandstone beds have sharp bases and
wave-rippled tops; they commonly display planar lamination and wave-ripple lamination.
Facies FM grades upward into Facies FH2. Facies FM can be over 15 meters thick, and it
thickens to the southeast.
Facies FH1
Facies FH1 has a slightly erosional basal contact, usually with less than a meter of relief.
Mud rip-up clasts are common just above this basal contact and are up to a few centimeSedimentary Geology
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Depositional Environments
ters in diameter. Many of these clasts are sideritized. These lags also contain silicified
wood bearing Teredolites as well as large bones. Facies FH1 contains dipping beds of alternating fine-grained to coarse-grained sandstone and very thin to thin beds of mud;
overall, this facies is fining-upward. Large-scale tabular and trough cross-stratification are
common and well-developed, and sets of this range from a half-meter to three meters
thick. Thinner sets 10-20 cm thick are interspersed within these larger sets. Mud drapes
primarily occur on the lower portion of foresets and on bottomsets, and sand beds thin
downward along these large-scale tangential foresets. Cross-bed sets show a repeated
alternation of thicker and thinner sand laminae, with sand laminae separated by clay laminae. Reactivation surfaces are present within these large-scale sets. Large slump blocks
are locally present, and bedding within these blocks may be contorted. Oyster shells are
locally present, sometimes in dense accumulations. Some beds have dense concentrations
of simple, straight, vertical burrows. This facies is not present to the southeast.
Facies FH2
Facies FH2 has alternating beds of very fine to fine-grained sandstone and silty mudstone and shale. The shale and silty mudstone are dark gray and contain abundant organic matter as finely ground carbonaceous plant matter. The shale and siltstone are variably
bioturbated, often with abundant Chondrites. Beds of sandstone have sharp, erosional
bases with prod marks and rare gutter casts. Sandstone beds are typically graded, and
they contain planar lamination and hummocky cross-stratification. Sandstone beds thicken upwards, and the percentage and thickness of mudstone beds decreases upwards; as a
result, this facies is overall coarsening-upwards. Soft-sediment deformation features such
as ball-and-pillow structure are present. Facies FH2 is variably calcite-cemented, with
local sideritization of some beds. Facies FH2 grades upward into Facies FS1.
Facies FS1
Facies FS1 coarsens upward. The base of the facies consists of nearly all very fine
grained to fine grained sandstone with very thin beds of mudstone. The middle and top
of the facies is entirely sandstone. Sandstone beds have sharp, erosional basal surfaces,
and are often overlain by a thin conglomerate of shale rip-up clasts. Sandstone beds near
the base of the unit contain planar lamination and hummocky cross-stratification, as well
as soft-sediment deformation features. In the middle of the unit, the sandstone beds
display swaley cross-stratification and large-scale trough cross-stratification. Near the top,
the sandstone beds contain planar laminae that are variably horizontal to gently dipping.
A diverse suite of trace fossils is present in the lower and middle portions of the facies,
including Conichnus, Macaronichnus, and Ophiomorpha, Rosselia, Skolithos, Teichichnus, Asterosoma, and Rhizocorallium. In the upper portion, only Macaronichnus is common. Many trace
fossils are sideritized, especially Rosselia and Ophiomorpha. In many outcrops, the laminae
at top of the facies are obscure, and this interval contains common downward-branching
coal-filled roots and rootlets.
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Depositional Environments
Facies FS2
Facies FS2 consists of coarse-grained to very coarse-grained sandstone. The base of FS2
is highly erosional, with over ten meters of relief, and this basal contact is overlain by a
lag of quartz gravel, mudstone intraclasts, silicified wood, and rare bone. Dune trough
and tabular cross-bedding is pervasive. The internal stratigraphy of Facies FS2 is complex, with numerous intersecting erosional surfaces. Facies FS2 is extensive to the
northwest, but absent to the southeast.
Facies FC
Densely to loosely packed shells with a matrix of very fine-grained sandstone finegrained sandstone. Shells include a diverse mix of bivalves (cucullaeids, trigoniids,
Panopea, Pectinella, and Goniomya), rare gastropods, and rare ammonoids. Shark teeth and
rare bone are present. Glauconite peloids comprise up to 15% of the sand matrix.
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