BULLETIN OF THE GEOLOGICAL SOCIETY OF AMERICA VOL. 66. PP. 857-882. 9 FIGS.. 1 PL. SEPTEMBER 1964 FRANCONIA FORMATION OF MINNESOTA AND WISCONSIN By ROBERT R. BERG ABSTRACT The Upper Cambrian Franconia formation in southeast Minnesota and west-central Wisconsin consists of glauconitic, quartzose sandstones that average 175 feet in thick- ness. Previous subdivision of the Franconia resulted in faunal zones to which geographic member names were given. These zones are not rock units and cannot properly be called members. In this paper, members are based on rock type. They are, in ascending order, the Wood- hill member—medium- to coarse-grained sandstone; the Birkmose member—fine-grained, glauconitic sandstone; the Tomah member—sandstone and shale; and the Reno member —glauconitic sandstone. A fifth unit, the Mazomanie member—thin-bedded or cross- bedded sandstone, forms a nonglauconitic facies that interfingers with and replaces the Reno member. Faunal zones are independent of the lithic units. The Woodhill member was deposited in the transgressing Franconia sea. Birkmose greensands formed in shallow waters far from shore, while Tomah sand and shale and Mazomanie thin-bedded sand were deposited nearer shore. Later, Reno greensands formed offshore, Mazomanie thin-bedded sand was deposited shoreward, and cross-bedded Ma- zomanie sand accumulated nearest the strand line. CONTENTS TEXT Page Measured sections 877 Page General statement 877 Introduction 857 Taylors Falls 877 Franconia problem 857 Hudson 877 Previous studies 858 Arkansaw 878 Proposed nomenclature 859 Franklin 878 Acknowledgments 859 Maynard Pass 879 Regional stratigraphy 860 Goodenough Hill 880 General character 860 Lone Rock 880 Woodhill member 861 References cited 881 Birkmose member 862 Tomah member 863 ILLUSTRATIONS Reno member 864 Mazomanie member 866 Figure Page Paleontography 867 1. Comparison of attempts to subdivide the Local stratigraphy 869 Franconia formation 858 2. Index map 860 St. Croix Valley 869 3. Conaspis zone thickness and facies 868 Dunn County 870 4. St. Croix Valley cross section 872 Mississippi Valley 871 5. Dunn County cross section 872 Buffalo River Valley 871 6. Mississippi Valley cross section 872 Houston to Adams counties 871 7. Houston to Adams counties cross section.... 873 Baraboo area 874 8. Buffalo Valley cross section 873 Wisconsin River Valley 874 9. Wisconsin Valley cross section 873 Paleogeographic speculations 875 Facing Significance of lithic types 875 Plate page Depositional history 876 1. Franconia outcrops. 864 INTRODUCTION tinent-wide extent and to a lithostratigraphic unit of local importance in the Upper Missis- Franconia Problem sippi Valley. Studies of the Franconia during The term Franconia has been applied to an the past 50 years have been concerned chiefly Upper Cambrian biostratigraphic unit of con- with the faunally defined unit, now called the 857 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/65/9/857/3431659/i0016-7606-65-9-857.pdf by guest on 25 September 2021 858 R. R. BERG—FRANCONIA FORMATION Franconian Stage (Howell et al., 1944). The In Wisconsin, Franconia was applied to the Franconia as a rock unit has been neglected, entire sequence of glauconitic sandstones be- and the subdivisions of the formation now in tween the Dresbach formation below and the use in Minnesota and Wisconsin are faunal St. Lawrence dolomite above. Such was the TWENHOFEL and ULRICH, 1924 TWENHOFEL, RAASCH, THIS PAPER THWAITES, 1919 and THWAITES, 1935 Rock Units Rock and Faunal Units Faunal Units Rock Units BAD AXE MEMBER UPPER MAZOMANIE GREENSAND Dikelocephalus FORMATION postrectus fauna RENO !> YELLOW SANDSTONE Prosaukia fauna HUDSON X^MAZOMANIE MEMBER \. MEMBER Ptychaspis-Prosaukia LOWER fauna MEMBER J> GREENSAND FRANCONIA FORMATION GOODENOUGH MICACEOUS MEMBER TOMAH ^v SHALE (subdivisions as Conaspis MEMBER Twenhofel and fauna Thwaites, 1919) BASAL BIRKMOSE BEDS MEMBER IRONTON MEMBER DRESBACH IRONTON MEMBER Camaraspis WOODHILL WORMSTONES Camaraspis fauna fauna MEMBER FIGURE 1.—COMPARISON OP ATTEMPTS TO SUBDIVIDE THE FRANCONIA FORMATION units that fail to describe the nature and dis- usage of Twenhofel and Thwaites (1919, p. tribution of sedimentary rock types. The 623) derived from Ulrich's earlier tentative Franconia problem can be solved independently classification published by Walcott (1914, p. of faunal study, and only a knowledge of rock 354). This constituted a redefinition of the types can lead to an understanding of deposi- three formations involved. tional history. The present study was made Twenhofel and Thwaites (1919, p. 616) first during 1949 and 1950 in an attempt to dis- subdivided the Franconia in their study of the criminate lithic units within the Franconia Tomah-Sparta area of Monroe County, Wis- formation and to trace these units throughout consin; they described lithic units but did not the outcrop area. name them. This and the most important sub- sequent classifications are compared in Figure Previous Studies 1. Ulrich introduced two names: Iron ton Berkey (1897, p. 373) defined the formation member for the basal Camaraspis-besn'mg sand- from outcrops at the village of Franconia just stone of the Franconia (Ulrich, 1924, p. 93- south of Taylors Falls on the St. Croix River. 94), and Mazomanie formation for sandstone Berkey's Franconia consisted of 100 feet of in central Wisconsin containing the Prosaukia fine-grained sandstone containing fossils later fauna (Ulrich, 1920, p. 73-75). Although both assigned to the Conaspis fauna, and he re- these units were defined on rock characters, stricted the term Dresbach to underlying Ulrich relied on fossils for their identification, greensands and shales. Subsequent usage of and he believed that the Mazomanie sandstone Franconia in Minnesota followed the original was younger than the Franconia. The present definition, and glauconitic sandstone overlying study concludes that the Mazomanie sandstone the Conaspis beds elsewhere in the State were represents a nonglauconitic fades that inter- assigned to the St. Lawrence formation. fingers with Franconia greensands. Trowbridge Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/65/9/857/3431659/i0016-7606-65-9-857.pdf by guest on 25 September 2021 INTRODUCTION 859 and Atwater (1934, p. 48) obversely suggested characters. A faunal zone may pass from a this possibility when they disagreed with highly glauconitic sandstone to a nonglauco- Ulrich and stated that the Mazomanie is the nitic sandstone within a short distance. Further- same age as the Franconia. more, two faunas may be found in the same Twenhofel, Raasch, and Thwaites (1935) type of sandstone in one outcrop, necessitating denned faunal units of the Franconia in Wis- complete fossil collections to draw a "member" consin, and the zones received geographic names boundary. Such "members" then consist of and were called members. The Wisconsin no- not one, but two or more rock types, and they menclature was firmly established by the may be identical in their variability of lithic "Conference Classification" published for the characters. Ninth Annual Field Conference of the Kansas The present subdivision of the Franconia Geological Society (Trowbridge, 1935). This establishes units of uniform lithic character, was essentially the same scheme given unofficial based entirely on rock type instead of con- sanction by the U. S. Geological Survey tained faunas. Rock units (members) are (Bridge, 1937, p. 234). The use of member separated from biostratigraphic units (faunal names for faunal units is carried in the Cam- zones), and both are recognized as important brian Correlation Chart of the National Re- but distinct entities. When the stratigraphic search Council (Howell et al., 1944). and geographic characteristics of the two Uniformity between Minnesota and Wis- are established, they are superimposed to consin usage was established in 1939 when the produce a better understanding of depositional Minnesota Geological Survey adopted the history. Conference Classification (Stauffer, Schwartz, For the Franconia, this procedure requires and Thiel, 1939). The Minnesota classification the rejection of previous member names. The differed only in the use of "Taylors Falls former members do not now designate, nor were member" for beds containing the Conaspis they intended to designate, rock types in the fauna. Upper Mississippi Valley. They were defined Attempts to recognize lithic members in by faunal criteria and have become established Minnesota were begun in 1946 under the direc- by usage as biostratigraphic terms. tion of W. C. Bell. Nelson (1951) and Bell, Feniak, and Kurtz (1952) describe persistent A cknowledgments rock types along the St. Croix and Mississippi rivers, and in the present study the rocks were Special acknowledgment is due Professor W. traced from these regions into the main Fran- C. Bell, who originally recognized the problem conia outcrop area. and method of solution, and under whose guid- ance the study was completed at the University Proposed Nomenclature of Minnesota. Field expenses were defrayed by the Geological Society of Minnesota, a group The inherent difficulty in the member whose enthusiastic and generous response made nomenclature of the Conference Classification the project financially possible. In September is apparent. The use of geographic member names implies the definition of rock units, but 1950 a number of geologists concerned with the authors (Twenhofel, Raasch, and Thwaites, Cambrian