Abstract

The Upper Cambrian Mt. Simon Formation (0-65 m thick) is a basal quartz arenite exposed in westcentral Wisconsin. A detailed field investigation of the physical and biogenic sedimentary structures of the Mt. Simon has led to the recognition of three distinct lithofacies. The lower one unconformably overlies Precambrian basement rocks. It consists of medium- to very large-scale sets of tabular and trough cross-bedded, medium- to very coarse-grained sandstone and pebbly sandstone with minor intercalated horizontal beds of very fine- to medium-grained sandstone, siltstone, and shale. Sparse examples of Skolithos and Arenicolites are present. This facies consists of a very thin sequence of possible braided-fluvial and marine foreshore deposits, overlain by probable marine shoreface and tidal channel deposits. Much of the facies seems to represent shallow subtidal deposition in a relatively high-energy regime. The middle lithofacies consists of two distinctly different subfacies, which probably were deposited in a low tidal flat setting. The higher-energy subfacies consists of small- to medium-scale sets of tabular and trough crossbedded, fine- to coarse-grained sandstones containing distinct zones dominated by Skolithos and Arenicolites . This subfacies probably represents deposition in meandering tidal channels. The lower-energy subfacies consists of thinbedded, horizontally-laminated and ripple cross-laminated, very fine- to medium-grained sandstone, siltstone, and shale, with common specimens of Cruziana, Rusophycus, and Planolites . This subfacies probably represents deposition on lower-energy tidal flats adjacent to the tidal channels. The upper lithofacies consists predominantly of structureless, densely bioturbated, very fine- to coarse-grained sandstone containing abundant specimens of Skolithos . The upper few meters of the facies consists of small- to medium-scale sets of trough cross-bedded, very fine- to coarse-grained sandstone with layers of disarticulated valves of the brachiopod Obolus . The upper facies probably represents deposition on tidal flats, perhaps in a midtidal flat setting, characterized by slower sedimentation rates, a correspondingly higher degree of bioturbation, persistent reworking of shelled macrobenthos, and periodic subaerial exposure. The Mt. Simon Formation is interpreted as a largely progradational (regressive), shoaling- and fining-upward tidal sequence. A marine interpretation is supported by the widespread occurrence of marine trace fossils within this unit. Evidence for a tidal origin is seen in the presence of unimodal cross-strata associated with reactivation surfaces, compound cross-strata, numerous scour and truncation surfaces lined with intraformational conglomerates, common clay drape laminae separating sets of cross-strata, interference and flat-topped ripple marks, and desiccation cracks. Sedimentation continued without apparent interruption as the overlying Eau Claire Formation was deposited. also under tidal influence. Recent reinterpretations of other basal Cambrian cratonic quartz arenites, together with this new interpretation for the Mt. Simon Formation, suggest that the long-held concept of basal transgressive sandstones deposited as blankets across the craton may be too simplistic, for deposition in braided-fluvial, marginal marine (tidal flat-tidal channel), and marine foreshore and shoreface environments seems indicated.

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