Very well preserved Lower Ordovician graptolites are found in the Ibex region of Millard County, Utah. They are preserved in the shale that is in terbedded with limestone of the Pogonip Group throughout much of its 1 ,000+-m thickness. The graptolites are an important element of a shaly fauna that interbeds uniquely with an extensive, well-developed shelly fauna.In this region, 45 species from 13 genera are recognized. Of these, 15 are new and are described and named; 26 were collected in great enough numbers to be accurately identified and statistically analyzed, and 12 have excellently preserved ontogenetic and (or) early astogenetic developmental stages, de scriptions of which add considerably to our present knowledge of graptolites of Early Ordovician age.Seven graptolite zones are established for biostratigraphic correlation. The zones are the first to be established in the Great Basin of the western United States. These zones will serve as a much-needed reference for correla tion of strata within and possibly beyond this region.l
StratigraphyThe Ibex region comprises the southern ends of the House Range and Confusion Range approximately 80 km southwest of Delta, Utah, and covers the southern part of the Notch Peak and northern part of The Barn topograph ic quadrangles (Fig. 1).Both the House and Confusion Ranges are raised fault blocks that are Figure 1. Form ations of Low er O rd ovicia n Pogonip G roup at Ibex, U tah , and location of Ibex in relation to the m iogeosyncline of the western U n ited States. 3 4 L. F. BRAITHWAITErelatively undeformed in spite of nearby thrust faulting of late Mesozoic age. Lower Ordovician beds are slightly more than 1,000 m thick and are very well exposed because of the present semiarid climate. Nearly every gully in the region contains distinct limestone ledges regularly interbedded with shale. Intervening slopes have unweathered shale near the surface, thus affording excellent conditions for collection of fossils. Virtually every limestone ledge and shaly slope is suited to fossil collection.Six formations compose the Pogonip Group at Ibex (Fig. 1); these are, from oldest to youngest, the House Limestone, Fillmore Limestone, Wahwah Limestone, Juab Limestone, Kanosh Shale, and Lehman Formation.The House Limestone is 157 m thick (Hintze, 1973) and consists mainly of bluish-gray siliceous (up to 50 percent) calcilutite and calcisiltite with thin beds of calcarenite. The fine-grained nature of the siliceous deposits indicates that the formation most likely was deposited in moderate water depths some distance offshore. Conodonts, trilobites, gastropods, and brachiopods are com mon, and fragmentary dendroid graptolites occur infrequently in the for mation.The Fillmore Limestone consists of 550 m (Hintze, 1973) of predominant ly intraformational conglomerate composed of calcareous siltstone in a cal carenite matrix. The conglomerate is interbedded with numerous thin beds of olive-gray to greenish-gray shale, minor fine-grained limestone, and inter spersed lenticular algae-and-sp...
