One of my favorite Blog
sites, written by master storyteller and botanist Hollis, operates out of Laramie,
Wyoming, home of the University of Wyoming.
Although I did not attend graduate school in Laramie, the University employed
a fantastic “old-time” vertebrate paleontologist by the name of Paul McGrew. Dr. McGrew sort of took me under his wing and mentored me through the
dissertation process involving Eocene mammals.
I spent several days with him exploring for mammals out in southwestern
Wyoming and later returning to Laramie seeking his guidance and help with
identification of my collection. I have
always held a special place in my heart for his kindness in accepting and helping
a neophyte paleontologist.
But the Blog that I
mentioned, In the Company of Plants and
Rocks (www.plantsandrocks.blogspot.com),
had a September 1, 2016 posting about stromatolites found in the Medicine Bow
Mountains of southeastern Wyoming. Cyanobacteria, formally known as blue-green
algae, were some of the few organisms inhabiting warm marine waters along the
coast line of a proto North America in the Precambrian ~ 2 Ga. These warm waters were depositing calcium
carbonate but also receiving an influx of fine-grained clastic sediments. As the living Cyanobacteria mats, often
growing in the inter-tidal zone, were covered with sediments the organisms reacted
by growing upwards and these mats then developed into domelike structures. Today these structures, known as
stromatolites, are preserved in a dolomite (originally limestone) and beds of
phyllite and argillite (the original fine-grained sediments subjected to low
grade metamorphism). Readers should
check out the Blog posting for some great photographs.
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| Cluster of small stromatolites (top view) collected from Green River Formation, Wyoming. Width FOV ~9 cm. |
The Plants and Rocks
post reminded me of a stromatolite collected long ago from rocks of the Green
River Formation (probably the Laney Member) exposed in southwestern
Wyoming. The interesting “thing” about
these small stromatolites is that the Green River Formation was deposited by
processes in a large, intermontane basin occupied by a fresh water lake, Lake
Gosiute. The stromatolite “maker” was probably a lacustrine alga of uncertain
taxonomic affinity but named Chlorellopsis
coloniata (Khattak, 2016). Like the Medicine Bow stromatolites, the mats of
the Cynobacteria/alga trapped sediments from the water and domal structures
were created. Recent work by Frantz and
others (2014) has shown that Green River stromatolites may be used to determine
changes in lake volume, water temperature, water depth, and location of
shorelines. Although the Green River
stromatolites were originally composed of calcareous minerals, today the
calcite has been completely replaced by silica (chert).
Last summer (and in
previous summers) I had the opportunity to visit the “Driftless Area” around La
Crosse, Wisconsin. This section of
southern Wisconsin along the Mississippi River seemly escaped glaciation by the
widespread Quaternary glaciers that otherwise affected the northern quarter of
the “lower 48”. Without the cover of
glacial drift (debris left behind), rocks of Cambrian and Ordovician age are
well exposed in the Driftless Area. There
are minor outcrops of Precambrian rocks near Black River Falls, but otherwise
the basement rocks are in the subsurface.
Wherever Cambrian rocks are found in the stable heartland of North America,
geologists call it the Craton, the initial rocks are always sandstone. Sometime around 600 Ma marine waters begin transgressing
onto the Craton leaving behind beach and near-shore sands that later became
sandstones. These were overlain by
offshore sands, and in some localities, by deeper water muds. By around ~500 Ma the shallow sea had reached
Wisconsin and a nice section of Cambrian sandstones are well exposed,
especially along I-90 leaving La Crosse and heading west across Minnesota. But, it is interesting to note that the
latest Cambrian sandstones indicate the marine waters were regressing from the Craton
near La Crosse. In fact, the shallow
marine waters did leave the Craton and an unconformity (missing time in the
rock record) exists between the Cambrian sandstones and the overlying Ordovician
carbonate rocks.
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| Cambrian sandstone exposed along I-90 near La Crosse, Wisconsin. Note the numerous vertical burrows of a worm-like animal named Skolithos. |
Ordovician marine
waters came in with a vengeance, or at least with a rush, and covered much of
the North American continent with warm, shallow, carbonate-depositing seas full
of soft-bodied marine life. Evidently the
composition of the early Ordovician marine water did not encourage construction
of animal shells. However, the
limestones (now turned to dolomite) are full of trace fossils such as tracks
and borrows of these soft-bodied animals.
In addition, stromatolites are common in these dolomites and probably
represent Cyanobacteria trapping particles of sediments.
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| Quarry in Prairie du Chien dolomite near La Crescent, Minnesota. |
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| Cyanobacteria/alga mats and stromatolites preserved in sandy dolomite, Prairie du Chien group. |
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| The Worms Crawl In, The Worms Crawl Out, Into your stomach, And out your mouth. |
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| Building stone quarried from Prairie du Chien Group now used as a door step in La Crosse. Not that slab is covered by burrows and tracks of soft-bodied animals. |
The earliest Ordovician
rocks in the La Crosse area are termed the Prairie du Chien Group. Rocks of this group are commonly very hard
dolomite and “hold up” the bluffs so common along the upper stretches of the
Mississippi River. Quarries are common
along the River and therefore building stones often expose numerous geological
features, especially trace fossils. Other
common features in the Prairie du Chien are concretions, or partial concretions,
of silica (chert), and vugs containing calcite crystals. As I noted, most of the Prairied du Chien is
composed of dolomite (magnesium carbonate); however, primary dolomite rarely
forms today and so most carbonate petrologists believe the original limestone “turned
into’ dolomite by a process known as dolomitization. Fully understanding this process where some
of the calcium ions are replaced by magnesium ions is somewhat above my pay
grade.
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| Bluffs along the Mississippi River near La Crosse with Prairie du Chien caprock. The bluffs at La Crosse are 600-700 feet above the River. |
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| Small calcite crystals exposed in a vug of dolomitic limestone of the Prairie du Chien group. |
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| Chert nodule in dolomitic limestone. |
REFERENCES CITED
Frantz, C.M., F.A.
Corsetti, V.A. Petryshyn, M. Wagner and A. Tripati, 2014, Stromatolites as fine
records of terrestrial environmental conditions; examples from the Eocene Green
River Formation (Wyoming) (abst. PP34A-03): American Geophysical Union December
2014 Abstracts.
Khattak, O., 2016,
Calcimicrobes/Cyanobacteria (Blue-green Algae): www.geologylearn.blogspot.com.