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PALEOECOLOGY of the CAMBRIAN and ORDOVICIAN STRATA of MINNESOTA Gerald F

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PALEOECOLOGY of the CAMBRIAN and ORDOVICIAN STRATA of MINNESOTA Gerald F PALEOECOLOGY OF THE CAMBRIAN AND ORDOVICIAN STRATA OF MINNESOTA Gerald F. Webers Minnesota was the site of at least three marine trans­ The thicknesses and to ome ex tent th e character of th e gressions during Cambrian and Ordovician times. The Ordovician fo rm ati ons were affected by growing intrabas in­ epeiric sea was generall y confin ed to th e southern and al fl ex ures such as th e Twin C ity bas in and the Roche ter­ southeastern parts of th e state, with th e shoreline trending Red Wing anticline. southwestward from an area north of th e Twin Cities to the southwestern corner of the state. Sediments accumulated in a shall ow depression, which rapidly shoaled to the north, ST. CROIXAN SERIES between the northeast-trending Transcontinent al Arch in Cambrian sedimentary rocks in Minnesota were de­ Minnesota and the northwest-trending Wisconsin Arch (fig­ posited during two major transg ressive-regress ive cycl es. ure VI- 22). This sedimentary bas in has been call ed th e Hol­ During the first cycle, th e Dresbachi an sequence, including landale embayment of the Ancestral Fo rest C it y basi n the Mt. Simon Sandstone, th e Eau Claire Form ati on, and (A ustin, 1970b). Although the position of the shoreline dur­ the G alesv ill e Sandstone, was depo ited ; during th e second, ing Late Cambrian time undoubtedl y va ri ed, isopach maps the Franconi an-Trempealeauan sequence, including th e of the Upper Cambrian strata (Berg and oth ers, 1956; Os­ I ronton Sandstone, the Franconia and St. Lawrence for­ trom, 1964; Austin, this volume) suggest that the max imum mati ons, and the Jordan Sandstone, was laid down (fi gure transg ressive shoreline was roughl y parall el to the present VI-23). boundary of the Paleozoic rocks. earl y all the sedimentary rocks consist of cl astic ma­ Probabl y, the sea remain ed in Minnesota within the teri al, with fo ur principal lithotopes being repeated many H oll andale embayment continuously from Late Cambrian tim es (modi fied aft er Berg and oth ers, 1956; Austin, this through Earl y Ordovician tim e, and then retreated. The volume) . These consist of a (I ) coarse-grained lithotope of sea returned during the Middle and Late Ordovician, and cross-bedded, fin e- to coarse-grained sandstone, (2) fin e at this time also covered extensive parts of northwestern cl asti c Iithotope of th in-bedded, fi ne-grain ed to shaly sand­ Minnesota, and possibly the entire state, as the seas en­ stone and siltstone, (3) greensand lithotope of fine-grained, croached on either side of the Transcontinental Arch (fi g­ moderately glauconitic worm-burrowed sandstone, and (4) ure VI-22). sandy dolomite lithotope. A S~e m bl Qq e L()(ol RQ.Dqe Forma.hon Membtr LllholOlff Zo ne Zo ne SUNSET POINT · g Souklelio­ JORDAN VAN OSER ; Col v.nello WILLISTON ~ SOUklO - Upper - ~ 51 LAWRENCE Olk elocephotus BA SIN i5lol ycolpus °j:~~f~I~!~ PrOSQu kl o PlychosplS­ FRANCONIA PraSQuklo Plychospis IRONTON Aphelosp's GALESVILLE :z [AU CLAI RE !! ~ ~ f-----F=====-t--"-"---'i- cr '" MT SI MON Cedono Figure VI-22. Regional setting of Paleozoic rocks in south­ Figure VI-23. Croixan Series in southeastern Minnesota eastern and northwestern Minnesota. (modified from Austin, 1969). 474 PALEOZOIC AND MESOZOIC The area inundated by the transgressing sea was one of cies have been described from the Crepicephalus Assem­ low relief. A series of low bluffs composed of Upper Pre­ blage Zone. cambrian basalt formed islands near Taylors Falls, along The Galesville Sandstone, representing the regressive the northeastern shoreline of the Hollandale embayment. phase of the Dresbachian sequence, is a white, cross­ These islands were not completely inundated until Fran­ bedded, medium-grained quartzose sandstone similar to the conian time. Islands also were present in the Baraboo region Mt. Simon, except that it has somewhat less shale, is finer of Wisconsin, on the flanks of the Wisconsin Arch. grained, and generally is better sorted. The sandstone at the Fossil remains generally are not abundant in rocks of base of the Galesville is moderately well-sorted and be­ Late Cambrian age. Bottom communities consist mostly of comes well-sorted at the top (Austin, 1970b, and this vol­ trilobites, inarticulate brachiopods, and burrowing soft­ ume). The Galesville and Mt. Simon Sandstones represent bod ied organ isms that presumably were annelids. I narticu­ similar depositional environments. Fossils are sparse and late brachiopods are most abundant numerically, but are trilobites predominate. Most of the Galesville is in the reprcsented by only a few species. Trilobites show great di­ Aphelaspis Assemblage Zone although Aphelaspis itself has versity of form and are abundant with in certain beds. How­ not been reported in Minnesota. The top of the Aphelaspis ever, even so-called fossiliferous formations commonly Assemblage Zone is probably identical to the top of the show gaps of tens of feet where fossils are rare or absent. Galesville that is marked by an unconformity; the DlInder­ Examination of a section of Upper Cambrian sedimentary bergia Assemblage Zone, described from a continuously­ rocks, which shows both columnar sections and fossil zones evolving fauna in western United States (Palmer. 1965; (Berg and others, 1956, figure 5), indicates that the fossil Lochman and Wilson, 1958) is absent in IVlinnesota. The zones are rare and comprise about one bed per forty feet unconformity at the base of the Galesville Sandstone in of strata. Wisconsin is absent in ivlinnesota. THE DRESBACHIAN SEQUENCE FRANCONIAN-TREMPEALEAUAN SEQUENCE The shallow epicontinental seas that flooded Minnesota The second Upper Cambrian transgressive-regressive in Late Cambrian time initially deposited the IVI t. Simon cycle was not greatly different from the first. A shallow Sandstone over mostly Upper Precambrian clastic sedi­ epicontinental sea again transgressed an area of low relief. mentary rocks. The Mt. Simon typically is composed of Again, islands of Upper Precambrian basalt were present white or gray, medium-grained, cross-bedded quartzose at Taylors Falls although they eventually were inundated sandstone. Inarticulate brach iopods are the most common during this interval. The transgressive Ironton Sandstone, fossils, and their macerated remains in the coarser. cross­ however. records a lower energy environment than that of bedded units indicate a nearshore, high-energy marine en­ its older analog. the Mt. Simon Sandstone of the Dres­ vironment of normal salinity. Scattered interbeds of fine­ bach ian sequence. The Ironton is not as well sorted and the grained sandstone and shale are present locally. and indicate proportion of silt is significantly higher. The Ironton is a a quieter offshore environment. The transgressive-regressive white, medium-grained. well- to poorly-sorted quartzose coarse-grained sandstone lithotopes in the St. Croixan Series sandstone that has a significant amount of admixed silt. The appear to have rather restricted bottom communities. Tri­ fauna is dominated by trilobites of the E/vinia Assemblage lobite communities representing the Cedllria Assemblage Zone. but is neither diverse nor abundant. Apparently, life Zone':' are present but not abundant. and are incompletely was sparse in the nearshore phase of sedimentation repre­ known. sented by the Ironton. In the Taylors Falls area, coarse The Eau Claire Formation marks the maximum trans­ conglomerates of I ronton age have yielded an unusual mol­ gression of the Dresbachian sequence and includes several luscan fauna that includes monoplacophoran species. These clastic rock types. The most atypical of these is a red, fossils are found in sandstone pockets among coarse basalt silty, fine-grained sandstone and red shale that is found near boulders (as large as two feet in diameter), and lived in an the western border of the Hollandale embayment. The rela­ intertidal environment at the shoreline. I ncluded among tively silty units are commonly worm-burrowed, and the the monoplacophorans are hypseloconids. which are high­ entire section may represent normal marine offshore con­ coned, septate. and probably representative of the group ditions where sedimentation was sufficiently rapid to pre­ from which cephalopods evolved. vent reduction of the hematitic pigment. The bulk of the The Franconia Formation was deposited in a wide Eau Claire Formation consists of fine- to medium-grained variety of sedimentary environments that varied from shal­ quartzose sandstone with interbeds of green shale and glau­ low littoral to offshore marine. The Birkmose, the lower­ conitic, fine- to medium-grained sandstone. The Eau Claire most member. is a glauconitic, worm-burrowed, fine-grained is the most fossiliferous unit of the Dresbachian sequence, sandstone representing a widespread offshore lithotope. It and contains inarticulate brachiopods and worm burrows is characterized by trilobites of the ElvilJill Assemblage as well as a diversity of trilobites. The Eau Claire is charac­ Zone. The Mazomanie, Reno, and Tomah Members repre­ terized by the Crepicephailis Assemblage Zone, although sent progressively basinward rocks that are laterally equiva­ the zone actually continues into the bottom of the Gales­ lent facies rather than vertical divisions, although the Reno ville Sandstone. Trilobites are invariably disarticulated and overlies the Tomah where the two are found together in the crowd the bedding surfaces at many intervals. Over 30 spe- same stratigraphic section (Austin. this volume). The Mazo­ manie Member. a thin- or cross-bedded, dolomitic. fine­ ,', Fossil Zones are shown in Figure VI-23. to coarse-grained quartzose sandstone, is present in the CH. VI! GEOLOGY OF MINNESOTA 475 northern part of the Ho ll andale embayment, a nd represents Group Formol lon Member Macrofos sil s a shall ow, perhaps sublittoral environment. T he Mazomani e z interfingers with and repl aces both the Reno and T omah w Molluscan WI L LOW RIVER fa una Member in a shoreward d irection.
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