EARLY ORDOVICIAN STROMATOLITE MOUND AND REEF INTERVALS IN THE IBEX AREA OF WEST-CENTRAL UTAH

The Ibex area in west-central Utah has a series of stromatolitic mound units and patch reefs in the very thick Lower Ordovician Fillmore Formation and the thin Wah Wah Limestone, just below the Lower-Middle Ordovician boundary. These occur at an early stage of the Great Ordovician Biodiversification Event when stromatolites were gradually being supplemented by lithistid sponges and attached echinoderms, and then partly replaced by frame builders such as receptaculitids. Mound layers are widely spaced in these units, vary in thickness, may extend for several kilometers along strike, and are often well exposed. In the lower Fillmore Formation (lower Zone E, Tremadocian), the first extensive mound interval occurs about 61 m above the base and consists of 0.5-1.5 m thick, bluish-gray, stromatolite columns 10-40 cm wide; this has been called the Miller Mound layer. A second even larger mound and reef interval occurs about 124 m above the base of the Fillmore (upper Zone E, Tremadocian) where 2 layers of 1-2 m thick mounds occur, forming a distinctive gray band that can be followed for several kilometers. This layer was originally noted by Lehi Hintze, and has been designated as Hintze’s Reef. Another mound layer about 1 m thick having reddish, case-hardened, and scalloped top surfaces is found in the middle Fillmore (Zone G-2, Floian) about 240 m above the base mostly NW of Skull Rock Pass and has been called Church’s Reef. Hintze noted another 3 m thick mound sequence high in the Fillmore about 502 m above the base (Zone I, Floian) that he designated as the Calathium Reef after a conical reef-building receptaculitid. A 2 m thick mud-mound horizon occurs in the middle Wah Wah Limestone (Zone J, uppermost Floian), and is here designated as the Wyatt Mound layer.