Paper No. 50-7
Presentation Time: 9:00 AM-5:30 PM
SEDIMENTARY PROVENANCE AND WEATHERING PROCESSES IN THE 1.1 GA MIDCONTINENTAL RIFT OF THE KEWEENAW PENINSULA, MICHIGAN, USA
Sedimentary rocks associated with the failed 1.1 Ga North American Midcontinental Rift outcrop on the Keweenaw Peninsula (Michigan, USA). Outcrops of syn- and post-rift fluvial, lacustrine, and debris flow lithologies result from the erosion, weathering, and redistribution of effusive and plutonic rocks during periods of volcanic quiescence. Three units comprising the sedimentary/igneous-mixed Portage Lake Volcanics, the sedimentary-mixed Copper Harbor Conglomerate, and the exclusively sedimentary post-rift fill Jacobsville Sandstone, are geochemically characterized here to examine weathering and provenance processes. Provenance indicators (Ti/Zr and Ti/Al ratios and La-Th-Sc diagrams) reveal that the Portage Lake Volcanics conglomerates are near-source, and that the Copper Harbor Conglomerate is primarily derived from uplifted basement and recycled orogen lithologies. The source of the Jacobsville Sandstone is diverse and varies from the top to the bottom of the stratigraphic section: 1) felsic arkosic arenites sourced from the uplift and erosion of basement orogenic igneous rocks (top), 2) felsic arkosic-subarkosic arenites sourced from reworked sedimentary and metasedimentary rocks eroded from uplifted basement and recycled orogen (middle), and 3) quartz arenites eroded from the cratonic interior (base). This work can contribute towards constructing a stratigraphic nomenclature for the Jacobsville Sandstone. Both petrographic data and weathering indices, such as the chemical index of alteration (CIA) and the paleosol hydrolysis metric (Al/(Ca+Na+Mg+K)), indicate low degrees of weathering during syn-rift deposition at 1.1Ga, but more intense weathering in post-rift units towards the Late Mesoproterozoic/Early Neoproterozoic. Less mature units generally show little evidence of oxidative weathering, which supports recent inferences from marine Cr isotope studies that suggest atmospheric oxygen levels remained low through the early Neoproterozoic. CIA values from this work agree with the proposed trend of atmospheric CO2 concentrations derived from paleosol data through the Mesoproterozoic, demonstrating that characterizing the weathering trends of floodplain deposits can be a useful addition to deciphering past paleoclimatic conditions in the Precambrian.