SEDIMENTARY PETROLOGY OF THE SAUK SEQUENCE ON THE TRANSCONTINENTAL ARCH: TESTING THE SMOKING GUN

The stratigraphic base of the Phanerozoic in many regions is characterized by the transgressive onlap of continental crystalline basement rocks by shallow marine sediments of the Cambrian-Early Ordovician Sauk Sequence. The Sauk transgression, therefore, marks the termination of a protracted period of widespread continental denudation that exhumed crystalline basement rocks and exposed them to physical and chemical weathering at the Earth’s surface. The paleogeomorphic surface separating continental crustal rocks from overlying sedimentary cover, known as the Great Unconformity (GUn), is geographically widespread and stratigraphically unique. This study is motivated by the hypothesis that shoreface erosion of crystalline basement during the Cambrian and Early Ordovician Sauk transgression affected sea water chemistry by increasing the flux of continental crustal weathering products to the ocean. To evaluate this hypothesis, we executed a stratigraphic and petrographic analysis of the Sauk Sequence, as expressed in several outcrops in southern Wisconsin and recovered in a core from the subsurface of northern Illinois. In addition to producing a detailed sedimentological log of the core, we documented the occurrence, stratigraphic distribution, and petrographic textures of glauconite, carbonate cements, intraformational conglomerates, and other sedimentary features indicative of early authigenesis and seafloor cementation. Multiple stratigraphic intervals within the Sauk Sequence, representing a wide range of marine shelf environments, preserve evidence of early seafloor cementation and mineral authigenesis. The common occurrence of carbonate-cemented quartz sandstone flat pebble conglomerates in subtidal environments and other unusual patterns of chemical sedimentation, including rapid glauconite authigenesis, is readily interpretable as a sedimentological expression of enhanced continental weathering during the formation of the GUn. Thus, many previously documented sedimentological features of the Sauk Sequence, including non-analogoue glauconite formation (sensu Chafetz and Reid 2000) and sandstone cementation (e.g., Runkel et al. 2010) may be unique sedimentary signatures of the GUn.