Occurrence Formation: The St. Lawrence Formation

VI. Cambrian
When the Cambrian started, there was an epicontinental sea that covered the Southeastern portion of what is today Minnesota (Ojakangas and Matsch 1982). Streams were flowing into the sea off the land in western and northern Minnesota carrying sand to the seashore. This is why there was a lot of sandstone deposited during the Cambrian. Figure 11 shows a strat column for the Cambrian rocks found in Minnesota.
The sand that was deposited came from the erosion of the igneous and metamorphic rocks that were formed during the volcanism and orogenies from earlier in history. Ojakangas and Matsch state that the weathering and erosion has been occurring since the end of the Algoman mountain building event 2.8 billion years ago. The reason why mainly sand was

deposited is because quartz is more resistant to weather and erosion compared to most other minerals.
The Mt. Simon Sandstone is the lowest unit, and is a white, gray, yellow, or pink, medium-grained quartzose sandstone (Ojakangas and Matsch 1982). Crossbeds are common and coupled with well rounding and sorting of the sandstone, which indicate a high energy marine environment of deposition with strong waves and currents. Thin green or red shale interbeds present in the Mt. Simon indicate periods of quieter water (Ojakangas and Matsch 1982). Ojakangas and Matsch also state that the Mt. Simon is as much as 100 meters thick.
The Eau Claire Formation overlies the Mt. Simon. The Eau Claire Formation includes many different rock types including red shale, gray-green shale, fine grained quartz sandstone, and fine grained glauconitic quartz sandstone (Ojakangas and Matsch 1982). The Eau Claire Formation has burrows present which indicate deposition in a relatively deep and low energy environment or a shallow water environment. Ojakangas and Matsch state that the Eau Claire Formation is about 60 meters thick.
The Galesville Sandstone overlies the Eau Claire Formation. The Galesville Sandstone is a coarser, white to gray sandstone that contains some glauconite (Ojakangas and Matsch 1982). The glauconite would turn the sandstone a green color. The Galesville Sandstone is more mature compared to the Eau Claire and indicates a higher energy near shore environment. The Galesville Sandstone is as much as 30 meters thick (Ojakangas and Matsch 1982)
The Ironton Sandstone overlies the Galesville sandstone. The Ironton Sandstone is a quartz sandstone just like all the other sandstones that sit below it. The Ironton Sandstone contains a lot of fine grained silty material in addition to quartz. The formation was deposited in a lower energy environment than the coarser Galesville Sandstone (Ojakangas and Matsch 1982).
The Franconia Formation overlies the Ironton Sandstone is as much as 30 to 60 meters thick (Ojakangas and Matsch 1982). The Franconia Formation is commonly characterized by abundant glauconite, which makes it green in color (Ojakangas and Matsch 1982).
Glauconite forms on the sea floor in oxygen-poor waters where the rate of sedimentation is very slow. This makes the glauconite a good indicator of a marine environment. The parent material is usually micalike clay minerals that have the same platy crystalline structure as the glauconite but that do not contain the potassium and iron of the glauconite (Ojakangas and Matsch 1982).
The distribution of rock types and their variation in thickness suggest that the Hollandale Embayment came into existence during the Franconia whereas deposition of the preceding units was not controlled by the embayment (Ojakangas and Matsch 1982). The Hollandale Embayment is just a shallow depression. The Franconia Formation, St. Lawrence Formation and Jordan Sandstone were deposited during the Hollandale Embayment.
The St.

Lawrence Formation is 20 meters thick, and is unique in that it is the first major carbonate unit in the Paleozoic strat column of Minnesota. A limestone unit would signify the dominance of chemical and biochemical precipitation of calcium carbonate out of the seawater, a lack of sand and mud reaching that part of the basin of deposition, and probably a depositional site far from shore. Most of the Paleozoic carbonates were originally composed of calcite but were altered by the replacement of calcium ions by magnesium to form the mineral dolomite. The St. Lawrence Formation is not clean but contains clay, silt, sand, and glauconite, indicating fluctuating conditions.
The Jordan Sandstone is 25 to 35 meters thick, and variable in sandstone type. The Jordan Sandstone is either a white or yellow, fine to coarse grained quartz sandstone with well-rounded and well sorted grains. There are burrows present at the middle and bottom of the Jordan Sandstone and the coarser grain size is at the top. There is some hummocky cross stratification inter mixed with the burrows at the bottom of the sandstone, and there is trough cross bedding present in the middle of the sandstone (Runkel

1994).
When I look at all of these features present in the Jordan Sandstone, I determined that the Jordan Sandstone was deposited during a regression. The areas of the Jordan that have the hummocky cross stratification and the trough cross bedding were deposited in an offshore marine environment, and the top of the Jordan Sandstone that has the larger grain size was deposited closer to shore in perhaps a beach. Because the parts of the Jordan Sandstone that do have the hummocky cross stratification and trough cross bedding are at the bottom of the Jordan Sandstone, and the part that doesn’t have the hummocks or the trough cross bedding but does have the larger grain size is at the top, then the Jordan Sandstone was deposited during a regression. All of the rocks in the Cambrian were deposited during an epeiric sea.
VII. Ordovician
The end of the Cambrian marks the end of dominate sand deposition in Minnesota. The previously low lying landmasses that had stayed above sea level during the Cambrian had been eroded further and were largely overwhelmed during the Ordovician (Ojakangas and Matsch 1982). Therefore, quartz sand was no longer available in great quantities, and carbonate deposition became the dominant rock forming process (Ojakangas and Matsch 1982). Figure 11 shows a strat column of all these carbonate rocks that were deposited during the Ordovician.
The Oneota Dolomite sits at the bottom of the strat column. The Oneota Dolomite is 50 meters thick, is tan to gray in color, and is a fine to medium grained dolomite. The dolomite signifies the original precipitation of calcite that was later dolomitized by magnesium-rich brines (Ojakangas and Matsch 1982). Since the Oneota Dolomite was deposited in a deep marine environment below storm weather wave base, and the top of the Jordan Sandstone was deposited in a near shore environment, then there is an unconformity between the Jordan Sandstone and the Oneota Dolomite.
The Shakopee Formation consist of a 20 meter thick well-rounded and well-sorted quartz sandstone unit called the New Richmond Member and an upper, 70 meter thick Shakopee Dolostone (Ojakangas and Matsch 1982). There are many stromatolites and oolites present in the dolomite, despite the obliteration of original textures and fossils that commonly result from dolomitization (Ojakangas and Matsch 1982). If you have the Oneota Dolomite and then you have the New Richmond Sandstone on top of the Oneota Dolomite, and the Shakopee Dolostone on top of the New Richmond Sandstone, then there is an unconformity between the Oneota Dolomite and the New Richmond Sandstone along with another unconformity between the New Richmond Sandstone and the Shakopee Dolostone.
The St. Peter Sandstone is a white to light yellow, medium grained quartz sandstone that is as much as 50 meters thick (Ojakangas and Matsch 1982). The St. Peter sandstone is a well-rounded, well-sorted, pure quartz sandstone, it is about 99.44% pure quartz (Ojakangas and Matsch 1982). The excellent sorting, rounding, and purity suggest a source from an older sandstone like the Jordan Sandstone. The St. Peter Sandstone represents the last period of quartz sand deposition in Minnesota during the Paleozoic.
The Glenwood Formation is a 5.5 meter thick, gray-green shaly unit with a sandy base (Ojakangas and Matsch 1982). It was deposited in a deep water, low energy environment. The Glenwood overlies the Saint Peter Sandstone, and underlies the Platteville Formation. This marks a time of sea level rise during the Ordovician.
The Platteville Formation is a 9 meter thick limestone. It is one of the most fossiliferous limestones in Minnesota, and the fossils found in the Platteville are brachiopods, cephalopods, gastropods, bryozoan, crinoids, and trilobites (Ojakangas and Matsch 1982). The upper part is mostly limestone, whereas the lower part contains more interbedded shaly units. I think the interbedded shaly units could actually be part of the Glenwood formation. The Platteville Formation was deposited on a shallow marine bank of widespread carbonate (Ojakangas and Matsch 1982). The Platteville Formation along with the Glenwood Formation was deposited during the Tippecanoe sequence.
The Decorah Shale is a gray-green shale with thin limestone beds, and is as much as 24 meters thick (Ojakangas and Matsch 1982). Fossils are super abundant in the Decorah Shale. Ojakangas and Match even state that the Decorah Shale has more abundant fossils than the underlying Platteville Formation.
The Galena Formation is a 70 meter thick carbonate unit consisting of dolomite, limestone, and shale (Ojakangas and Matsch 1982). Fossils are common and varied when they were not obilerated by dolomitization. The Stewartville member of the formation, which is the top of the formation, has a very different faunal assemblage than the cummingsville member (lowest member); this has been interpreted to mean a change from normal marine waters to hypersaline water (Ojakangas and Matsch 1982).
The Dubuque Formation is a 10 meter thick gray-green to yellow-gray to gray unit of crinoidal limestone and shale. The fossils are those common to shallow marine environments. The Dubuque Formation is sometimes listed as part of the Galena Formation.
The Maquoketa Formation is less shaly than the Dubuque, containing limestone and dolostone (Ojakangas and Matsch 1982). The upper part contains more sand than seen since the St. Peter deposition, suggesting another minor pulse of uplift of the Transcontinental Arch (Ojakangas and Matsch 1982).
The Ordovician in Minnesota was quite boring considering it was under an epeiric sea. The sea was the only place any activity was going on because there was all those species that are now fossilized were are living in the sea.