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.
|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.
|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.
|Cyanobacteria/alga mats and stromatolites preserved in sandy dolomite, Prairie du Chien group.|
|The Worms Crawl In,|
The Worms Crawl Out,
Into your stomach,
And out your mouth.
|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.
|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.|
|Small calcite crystals exposed in a vug of dolomitic limestone of the Prairie du Chien group.|
|Chert nodule in dolomitic limestone.|
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.