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

Abstract

The Swan Peak Formation in the Bear River Range of northern Utah and southern Idaho ranges in thickness from 0 to more than 400 ft (120 m). It consists of three units: (1) a lower unit of interbedded quartzite, shale, and limestone, (2) a middle unit of interbedded quartzite and shale, and (3) an upper unit of nearly homogenous quartzite. The different rock types, sedimentary structures, ichnofossils, body fossils, and mineral compositions of the units represent different environments of deposition. The lower unit accumulated in a shallow-shelf environment, and its strata grade upward into shoreface, tidal-flat, and lagoonal beds of the middle unit. The upper unit is believed to be a shallow-marine sandstone deposited by south-flowing currents. The lower and middle units of the Swan Peak Formation consist of a progradational suite of nearshore deposits 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 conformably overlies the Ordovician Garden City Formation. The upper and middle units thin eastward and southeastward to a featheredge, whereas the lower unit is thickest along an east-west-trending belt and thins northward and southward. The lower unit may 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 had been searched for organic content or consumed by littoral or sublittoral benthonic predators or scavengers, probably orthoconic cephalopods.