David K. Larue
Previous studies have interpreted the Paleoproterozoic Enchantment Lake Formation in the Lake Superior region (northern Michigan, U.S.A.) as a glacial deposit. In the current investigation, detailed stratigraphic studies were made to characterize depositional processes and environments, describe lonestones, and assess whether diamictite units represent tillites or instead reflect mass-flow or other depositional mechanisms. The Glacial Trifecta , tillites, glacial striae, and dropstones, form the traditional basis for inferring pre-Pleistocene glaciation. Each of these features must first be interpreted as glacial in origin, a non-unique inference that introduces circular reasoning. In the Enchantment Lake Formation, none of the inferred Trifecta indicators withstands close sedimentological scrutiny. A new classification system is introduced to more precisely characterize lonestones, identifying five distinct types. Type 1 lonestones are outsized clasts resting in and disrupting the layering of laminated mudstone beds. Type 2 lonestones are outsized clasts residing in structureless mudstone beds. Type 3 lonestones are chains of outsized clasts present in mudstone intervals, sometimes showing a basal erosional contact. Type 4 lonestones are isolated clasts occurring immediately above pebbly mudstone units in laminated muddy sediments. Type 5 lonestones are outsized clasts residing in tectonically foliated mudstone. None of these lonestone types can be uniquely attributed to glacial dropstone origins. Notably, lonestone types 2 through 5, and possibly type 1, are also present in the non-glacial, deep-water Cretaceous Pigeon Point Formation and Miocene San Onofre Breccia of California. Throughout most of the study area, depositional environments include fan-delta and associated marine deposits. The key area in the Wewe Hills, previously cited as containing glacial deposits, is shown here to contain deep-water slope deposits of non-glacial origin. Deposition of the Enchantment Lake Formation is interpreted to have occurred in a syn-sedimentary, fault-bounded basin, resulting in a complex facies mosaic. Although lonestones are present throughout the study area, none meet criteria to be described as dropstones. Conglomerates with muddy matrices are interpreted as debris-flow deposits rather than tillites, based on their association with fan-delta and slope deposits. Additional arguments against a glacial origin include the presence of quartz-pebble conglomerates, quartzites, and a single dolomite bed above the lower part of the Enchantment Lake Formation, which, by analogy with Phanerozoic examples, indicate warm, humid environments of deposition. There is no compelling evidence that the Enchantment Lake Formation was deposited by glacial processes, but there is clear evidence for syn-sedimentary deposition in a fault-bounded basin.