Although phylloid algal mounds have been studied for 50 year, much remains to be determined concerning the ecology and sedimentology of these Late Paleozoic carbonate buildups. Herein we perform a digital outcrop study of the well-known Middle Pennsylvanian Lower Ismay mound interval in the Paradox Basin because outcropping mounds along the San Juan River are cited as outcrop analogs of reservoir carbonates in the Paradox Basin oil province of Utah and adjacent states. The principal field area is the Eight Foot algal field located at river mile 19.2 on the San Juan River, approximately 14 miles SSW of Bluff, Utah. The Lower Ismay section is exposed on both sides of the river for 1.4 miles. Mechanisms for mound formation are still a heavily debated topic and even now aren't fully understood. While this study does not seek to solely answer this question, it does shed some light on the argument. A combined total station-LIDAR survey of the exposed Eight Foot mounds indicates that the mound field is comprised of 83 individual and composite mounds that have an average height of 10.9 meters and peak spacing of 48.8 meters. Further, statistical examination of survey data reveals a correlation between mound height and east-west alignment, showing that shelfward mounds were slightly taller than their more basinward counterparts.. However, other shape parameters do not appear to vary systematically across the algal field. Curve-fitting indicates that the overall mound morphology does not differ significantly from a Gaussian surface indicating that mounds are conical in shape. This suggests that mounds did not form under the influence of directional currents such as waves or tides. Yet, Ivanovia-fragment packstone and grainstone facies typical of the mound interval suggest a high-energy depositional setting.

Phylloid-algal mounds form heterogeneous hydrocarbon reservoirs in the southeastern portion (Blanding sub-basin) of the Paradox Basin. Well-studied Lower Ismay mounds exposed along walls of the San Juan River gorge in the vicinity of Eight Foot Rapids, the west limb of the Raplee Anticline, and at the classic Honaker Trail locality (southwestern Paradox Basin) have often been cited as outcrop analogs of productive subsurface mounds. Until now, however, there has not been a complete description of the distribution, size, and spacing of outcropping algal mounds at the classic Eight Foot Rapids locality. The Lower Ismay sequence was analyzed in the context of a build-and-fill model of deposition. There are three facies associations within the sequence: 1) a basal lowstand to middle highstand pre-mound facies association, 2) a late highstand to middle falling stage phylloid algal-dominated relief-building facies association, and 3) a late falling stage, post-mound relief-filling facies association. Above the basal maximum flooding surface (Gothic Shale), the facies succession displays a distinct shallowing upward trend through the Lower Ismay sequence. Mound dimensions and facies stacking patterns permit evaluation of two depositional models. The first is a traditional, moderate- to low-energy model of vertical and radial mound accumulation of phylloid algal plates. The second is a high-energy, tidally influenced model of accumulation wherein mounds become hydrodynamically elongate. Outcrop data indicate that algal-dominated buildups are domal in shape with no preferred axis of elongation. These patterns do not support a hydrodynamic accumulation of loose algal plate fragments. The absence of in-situ algal thalli in all but the upper few tens of centimeters of the mounds, however, argues against a purely biological/ecological origin of mounds. A down-stepping ramp model is proposed wherein a muddy algal facies was deposited at the base of the mounds in the low energy of the outer ramp, followed by a grain-rich algal core in the mid-ramp environment. Mounds tops accumulated in an algal bafflestone facies in the inner ramp setting. Restriction of energy due to basinward algal buildup may have also contributed to deposition of algal bafflestone. Mounds accumulated radially at differential rates and were influenced by these variations in energy. This differential deposition of microfacies and subsequent diagenetic alteration have produced heterogeneities in algal reservoir rock, producing algal mound reservoirs that have a high potential for compartmentalization.