Drifting Within Ice: Revealing the Glacial Origin of Mid-Pennsylvanian Sediment Pellets Through Micromorphology

The sedimentary record of late Paleozoic glaciation is mainly composed of sedimentary products formed in glacially influenced subaqueous environments, including ice rafted debris (IRD). IRD is particularly abundant in the Campo do Tenente Formation, a mid to basal portion of the Itararé Group (Paraná Basin, Brazil), and appear predominantly in the form of pellets. Despite the importance of IRD as significant components of the geological record of the late Paleozoic ice age (LPIA) and their potential as paleoclimatic proxies, there is limited information to support their genesis by seasonal sea/lake ice or icebergs. This study aims to address this gap by characterizing the micromorphology of 257 pellets derived from three distinct facies of the Campo do Tenente Formation in order to investigate formative mechanisms that could help to define the rafting agent. Microscopic examination identified circular arrangements of grains (turbates), plasma and skelsepic fabrics, and clay reorientations associated with shearing. SEM analysis of quartz surfaces also suggests shearing, indicating that the diamicton from which the pellets were derived were subjected to stresses typical of subglacial environments. The presence of skelsepic fabrics and turbated structures supports the interpretation of subglacial meltwater movement. Therefore, the Campo do Tenente pellets likely originated from poorly consolidated subglacial sediment beneath wet-based glaciers, later incorporated into icebergs and released during melting events.