Environmental Analysis of the Swan Peak Formation in the Bear River Range, North-Central Utah and Southeastern Idaho

The Swan Peak Formation in the Bear River Range of northern Utah and southeastern Idaho varies in thickness from 0 feet to over 400 feet. It consists of three units: (1) a lower unit of interbedded quartzites, shales, and limestones; (2) A middle unit of interbedded quartzites and shales; (3) An upper unit of nearly homogeneous quartzites. The different sedimentary structures, ichnofossils, body fossils, and mineral compositions of each unit represent different environments of deposition. The lower unit probably was deposited in a shallow-shelf environment, and its sediments grade upward into probably shoreface-, tidal-flat-, and lagoonal deposits of the middle unit. The upper unit is believed to be a shallow-marine sand deposited by south-flowing currents. The lower and middle uits of the Swan Peak Formation consist of a progradational suite of nearshore lithologies formed during the regression of the sea that terminated the early Paleozoic Sauk Sequence. The formation lies disconformably beneath the Ordovician Fish Haven Dolomite, and rests conformably on the underlying Ordovician Garden City Formation. The upper and middle units thin eastward and south-eastward to a feather edge, whereas the lower unit is thickest along an east-west trending belt and thins northward and southward. The lower unit could be time-equivalent to the upper and middle units in the north. Possible estuarine deposits containing detrital hydroxyapatite suggest a local fluvial source in the southeast. The immediate source for much of the sand in the middle and upper units lay northward in Idaho. Fucoidal markings within the middle unit appear to be feeding burrows filled with reworked sediment that was consumed or searched for the organic content by littoral to sublittoral benthonic predators or scavengers, probably orthoconic cephalopods