MACRO- AND MICROFACIES INTERPRETATIONS OF THE MICROBIALITE-SPONGE HINTZE'S REEF IN THE LOWER ORDOVICIAN FILLMORE FORMATION, HOUSE RANGE, WESTERN UTAH

Most reef-like structures built during the Early Ordovician were small (1-2 meters in height/diameter) isolated mounds. The lower portion of the Fillmore Formation (Ibexian) in western Utah contains a meters-tall wave-resistant reef, known informally as “Hintze’s Reef” (Miller et al., 2012), that was constructed by microbial communities and sponges and extended laterally for tens of kilometers. Notably, Hintze’s Reef is the stratigraphically-highest occurrence of conspicuous reefal stromatolites in the Ibex-area succession (Miller et al., 2012). The establishment of this reef coincided with both the most significant biodiversification event in the history of life (the Great Ordovician Biodiversification Event) and the Cambro-Ordovician microbialite resurgence (Rowland and Shapiro, 2002), ~40 million years during which reef building was primarily carried out by microbial communities rather than animals.

Hintze’s Reef is more than three meters thick at Section C (Hintze, 1951) in the House Range and is made up of interconnected mounds: steep-sided structures up to 2.5 meters wide that were built upon a stabilized grainstone substrate. Each mound within the reef is composed of a stromatolitic core surrounded by non-laminated (possibly dendrolitic) microbialite that contains fragmentary metazoan fossils and carbonate mud. This non-laminated fabric likely stabilized the reef, making continued upward growth possible. Lithistid demosponges, many in life position, are preserved throughout the reef. Individual mounds within the reef are bounded by shelly grainstone-filled channels that vary in width, reflecting episodes of high current energy that deposited larger grains into the topographic lows between the rigid mounds. In outcrop, the grainstone-filled channels resemble lenses between the larger interconnected mounds; thus, the reef has an egg-carton-like morphology in three dimensions. Polished slabs and thin sections of the non-laminated microbialite reveal a variety of microbial structures, including Wetheredella and representatives of the Girvanella and Renalcis groups, as well as branching, clotted, peloidal, and fenestral fabrics.