The Glacial Trifecta: A critical look at pre-Pleistocene glacial interpretations using a Paleoproterozoic example

Abstract

Sedimentology and stratigraphy can offer essentially unique insights into key depositional environments and processes, including tidal, wave-driven, deep-water, and fluvial systems. However, the interpretation of pre-Pleistocene glacial deposits is rife with non-unique and questionable interpretations, yet the implications of glacial events for earth history are much more profound.

Assignments of glacial environments are typically based upon interpretation of what is termed here the Glacial Trifecta, the group of three key interpretations used to recognize glaciogenic deposits: glacial till, presence of striated basement and striated and faceted clasts, and the interpretation of lonestones as dropstones. Briefly stated, uniquely distinguishing pre-Pleistocene tillites from debris flow deposits is extremely difficult or almost impossible. Striated basement and clasts can form by non-glacial mechanisms, including emplacement of olistostromes and other mass movements. Lonestones greatly resembling dropstones can form in non-glacial deep-water slope environments. When any of the features of the Glacial Trifecta appear in a geologic time period previously considered glaciogenic, they reinforce the interpretation that glaciation occurred during that time period, even though the interpreted glaciogenic period may have been defined a century ago prior to better understanding of debris flow and other sediment gravity flow mechanics, sequence stratigraphy, tectonics and sedimentation, and deep-water depositional environments.

Challenges interpreting glacial deposits in the Paleoproterozoic of the Lake Superior region are presented. Here, prior interpretations of tillite are shown to be likely examples of foliated debris flow deposits. A classification scheme for lonestones is presented, and a number of lonestones that could be misinterpreted as dropstones are described from the non-glacial Cretaceous Pigeon Point Formation and the Miocene San Onofre Breccia. Lonestones in the Paleoproterozoic of the Lake Superior region are interpreted to represent sediment gravity flow deposits.

Some have argued that the geochemistry of mudstones (the Chemical Index of Alteration, CIA) can be used as an indicator of glacial environments, but counter examples are provided from arid, tectonically active settings. An additional criterion against recognition of glacial environments is the presence of quartz arenites and quartz-rich detritus in purported glacial deposits. Future studies would benefit from focus on more rigorous testing of the Glacial Trifecta. More non-glacial analogues to the Glacial Trifecta, such as those discussed here (the Pigeon Point Formation and the San Onofre Breccia), would help future discussions concerning uncertainty in the interpretation of glacial deposits.