Abstract

The upper 2 members of the middle Croixan Franconia Formation were sampled in detail at 7 localities along Lake Pepin, Wabasha County, Minnesota, in order to test the validity of supposed sharp faunal breaks and distinct zonal units of previous literature. The Tomah Member, lower, is sparsely glauconitic, highly fossiliferous sandstone alternating with beds of sparsely or unfossiliferous mottled sandstone that appear to have been burrowed. A few beds within this member are more glauconitic, lithically transitional between the Tomah and the overlying Reno members. The Reno Member is highly glauconitic sandstone with undisturbed fossiliferous beds similarly alternating with unfossiliferous burrowed beds. Thorough sampling of burrowed beds that previously were thought to be unfossiliferous discovered many specimens that filled out the known range of species. The ranges of all species for which zonal units were named were found to overlap one another. Assemblages at all levels contain species that are more abundant at other levels or in other assemblages. The established zonation is shown to be based upon epiboles (beds containing maximum abundances of species), rather than upon teilzones, first occurrences, or assemblages as asserted in previous literature. Lowest occurrences of assemblages, normally found in sparsely fossiliferous beds, provide the most satisfactory bases for zonal units. Traced laterally, zonal units transgress lithic units; and epiboles occur at various levels within zonal units. The faunal succession records successive migration's of faunas through the area, concurrent with evolutionary modification of species. Large sample produced by detailed collecting provide basis for study of variation and phylogenetic relationships of species. Trilobites of the 2 members belong to the Ptychaspis-Prosaukia zone of the Cambrian Correlation Chart. Lowest is the Psalaspis zonule, expressed by epiboles of several species whose numbers decrease upward. Species of the superjacent Ptychaspis granulosa zonule begin as rare constituents of the Psalaspis faunule, increase in abundance to their epiboles, then decrease higher in the section, with a few remaining as rare constituents in the overlying Ptychaspis striata zonule. Species of the P. striata faunule likewise appear below their epiboles and extend above them. Species of the uppermost Prosaukia subzone show a similar gradational distribution pattern. Epiboles of succeeding assemblages, separated from one another by sparsely fossiliferous beds, are central to the local zonation and account for seemingly radical changes within the Ptychaspis-Prosaukia fauna. Deposition of sediments is interpreted as having been essentially continuous, with partly contemporaneous Tomah and Reno environments shifting back and forth through the area as trilobite species evolved. The member boundary crosses faunal units, and is interpreted as time-transgressive from E. to W. Glauconitic beds within the Tomah are thought to reflect early encroachment then retreat of an intermediate sedimentary environment. Many species in each faunal unit appear to be descendants of species in the immediately lower unit. New taxa are Drumaspis subulosa, n. sp., D. tanycodia, n. sp. , Ellipsocephaloides pepinensis, n. sp. , and Ptychaspis granulosa angaria, n. subsp. .

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