THROMBOLITE-STROMATOLITE-SPONGE MOUNDS DURING THE CAMBRO-ORDOVICIAN MICROBIALITE RESURGENCE: INSIGHTS FROM THE LOWER ORDOVICIAN FILLMORE FORMATION, HOUSE RANGE, UTAH

Microbial communities superseded metazoans as the primary reef builders in marine environments following the Cambrian extinction of the archaeocyaths. Reef frameworks dominated by skeletal metazoans did not return to prominence until the Early–Middle Ordovician transition, ~40 million years later. The Lower–Middle Ordovician succession near Ibex in western Utah records the later portion of this Cambro-Ordovician microbialite resurgence (Rowland and Shapiro, 2002). Most organic buildups in this succession can be classified as reef mounds based on their dimensions: 1-2 meters in height and 1-3 meters in diameter. Lithistid demosponges, the primary metazoan reef constituents, become increasingly abundant up-section.

Although some of the Ibex-area mounds are primarily stromatolitic, exhibiting well-defined laminations, most mounds that contain numerous lithistid demosponges do not have distinct macroscopic microbial structures. Instead, the non-skeletal portions of these mounds appear, in outcrop, to be composed of homogeneous micrite. An exception to this occurs in exposures of the lower Fillmore Formation (Lower Ordovician) in the House Range. Here, each of several laterally-adjacent reef mounds is made up of a well-laminated stromatolitic core, ~70 centimeters in height, surrounded by thrombolitic to possibly dendrolitic material. Sponges occur throughout these mounds but are particularly abundant within the thrombolitic portions. Thin sections of the thrombolite reveal numerous irregular fenestrae, clotted and peloidal fabrics, and other features indicative of microbialite microstructures, as well as abundant metazoan skeletal debris. Understanding the conditions that promoted the development of these thrombolite-stromatolite-sponge mounds—and inhibited the development of similar mounds elsewhere in the succession—is a goal of our ongoing research.