Sedimentary provenance and weathering processes in the 1.1 Ga Midcontinental Rift of the Keweenaw Peninsula, Michigan, USA

Abstract

Fluvial, alluvial, lacustrine, and debris flow sedimentary rocks from the Keweenaw Peninsula (Michigan, USA) are geochemically characterized to examine weathering processes and provenance. These sedimentary rocks result from the erosion, weathering, and redistribution of effusive and plutonic igneous lithologies during periods of volcanic quiescence associated with the failed 1.1Ga North American Midcontinental Rift. Three units comprising the sedimentary/igneous-mixed Portage Lake Volcanics, coarseness and source-varied Copper Harbor Conglomerate, and the quartz-rich post-rift fill unit of the Jacobsville Sandstone were studied over a geographically-wide and lithologically-varied range of localities across the Keweenaw Peninsula, Michigan. Provenance indicators such as Ti/Zr and Ti/Al ratios and La-Th-Sc ternary diagrams reveal that the Portage Lake Volcanic conglomerates are near-source, and that the Copper Harbor Conglomerate is primarily derived from felsic uplifted basement and recycled orogen lithologies to the east. The Jacobsville Sandstone is a combination of pulses of sources from the top to the bottom of the stratigraphic section: (1) felsic arkosic arenites sourced from the erosion and uplift of basement orogenic volcanic and plutonic rocks (top), (2) felsic arkosic-subarkosic arenites sourced from the erosion of uplifted basement, recycled orogeny and reworking of primary sedimentary and metasedimentary rocks (middle), and (3) quartz arenites eroded from the cratonic interior (highly mature). This work can consequently aid in structuring 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 increases with evolution of the rift in post-rift units toward the Late Mesoproterozoic/Early Neoproterozoic. Values for the CIA throughout the Mesoproterozoic from this and other localities have the same trend to that of proposed atmospheric CO2 concentrations derived from paleosol data, demonstrating that characterizing the weathering trends of floodplain deposits can be a useful addition to deciphering past paleoclimatic conditions in the Precambrian geologic record.