CRYPTIC MICROBIAL BUILDUPS AND HARDGROUNDS IN A LOWER ORDOVICIAN SHALLOW SUBTIDAL CARBONATE SUCCESSION, SOUTHERN HOUSE RANGE, UTAH

The southern House Range, Utah, is well known for its superbly exposed Lower Ordovician succession, and is the type area for the Ibexian Series. The Skullrockian Stage at the base of the Ibexian is represented by the House Formation. The Red Canyon Member of the upper House is a 79 m thick interval of subtidal carbonates that yields abundant and diverse trilobite faunas; peritidal indicators, such as planar microbial laminites, fenestral fabric and desiccation cracks are absent. Previous workers have noted that the Red Canyon is dominated by lime mudstone. However, detailed examination indicates that much of the lime mudstone is in the form of cryptic microbial buildups that lack the laminated or clotted mesoscopic textures typical of stromatolites or thrombolites. The mesoscopic fabric is bioturbated lime mudstone, so that buildups are typically recognizable only from the geometry of the associated coarse sediment (usually intraclastic pack-, grain- or rudstone) that infills topographic lows and narrow, steep-sided channels. Associated facies include heterolithic units of cm-scale interbeds of lime mudstone, intraclastic and bioclastic packstone and intraclastic rudstone. Lime mudstone layers are commonly capped by cryptic planar, undulating or micro-scalloped hardgrounds overlain by packstone or rudstone. Cryptic buildups are also features of correlative strata in the Garden City Formation of southern Idaho, about 350 km to the north of the House Range. Indeed, the importance of these features in the study area suggests that they may be pervasive but previously overlooked characteristics of Lower Ordovician shallow water carbonate successions.