The Precambrian nonesuch formation of the North American mid-continent rift, sedimentology and organic geochemical aspects of lacustrine deposition

Abstract

Abstract The Precambrian Keweenawan Trough in the Lake Superior region of North America is part of a failed intractonic rift that is filled with a thick sequence of volcanic and sedimentary rocks. One unit in this sequence, the None-such Formation, is composed of black-to-green siltstones and shales and is conformable with the underlying alluvial Copper Harbor Conglomerate and the overlying fluvial Freda Sandstone. The formation is exposed as a lens in northern Michigan and Wisconsin and attains a maximum thickness of ∼ 200 m. Based on the integration of outcrop and core data, nine facies have been recognized in the formation; each has been assigned to one of three genetic facies assemblages. A marginal lacustrine assemblage, characterized by interbedded lithic sandstones, siltstones, mudstones and sandstone-shale couplets, is interpreted as a record of deposition on a sandflat-mudflat complex. A lacustrine assemblage is characterized by massive to well-laminated dark shaly siltstones, carbonate laminites, shales, siltstones and mudstones. These sediments were probably deposited in a progressively shallowing perennial lake. Bottom conditions were anoxic and became increasingly more oxic as lake regression progressed. A gradual transition from a lacustrine environment to a fluvial environment is represented by the red to brown, massive, horizontally laminated and rippled fine sandstones, siltstones and mudstones of the fluvial-lacustrine assemblage. Interactions among rates of subsidence, rates of sedimentation, fluctuations in lake level, possible changes in climate and differences in tectonic setting controlled sedimentation in the Nonesuch Lake and resulted in variable vertical facies sequences. Total organic carbon (TOC) analyses show a strong correlation between organic richness and the shale facies (lacustrine assemblage) and mudstone facies (marginal lacustrine assemblage). Detailed petrographic analysis using incident white light and reflected blue light fluorescence revealed two prevalent organic petrographies which could be distinguished using combined pyrolysis-gas chromatography-mass spectrometry (PY-GC-MS) and, to a limited extent, by stable carbon isotope mass spectrometry. The geographic and stratigraphic distribution of rocks containing the two organic petrographies suggests that differential preservation (e.g., microbial degradation) accounts for the difference rather than maturity or variable source input.