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Abstract

Muleshoe Mound is a composite Waulsortian buildup that forms a distinctive promontory along the western escarpment of the Sacramento Mountains (New Mexico, U.S.A.). Excellent exposures allow detailed study of the mound's fades, paleontology, geometry, internal architecture, and off mound stratal relationships. Recent work suggests that many Waulsortian mounds may have originated and grown in more agitated, shallower water environments than previously thought. The previously 'enigmatic' character of these Lower Carboniferous buildups may largely be due to differences in basin stratification, oxygen and possibly nutrient levels.

Muleshoe Mound can be separated into at least five distinct stratal packages or 'growth phases'. Successive growth phases differ in fades, geometry, and symmetry, reflecting different environmental conditions (energy, carbonate production, oxygenation and accommodation space). Intervening surfaces represent periods of hiatus and erosion. The architecture of this mound growth phase pattern strongly suggests that accommodation space was a critical control on mound growth.

Field study, serial slabbing, and petrographic examination of the mound facies indicate that Muleshoe grew in appreciable currents and intermittent high energy. Much of the micrite in the buildup originated as a microbial precipitation within an organic (algal?) host, rather than as depositional carbonate mud. This process of microbial precipitation, combined with extensive marine cementation, built the mounds' depositional relief and created a relatively rigid growth framework.

These features appear to conflict with the common interpretation of Waulsortian mounds as deep, quiet water buildups and Muleshoe may have formed in depths and energy conditions not dissimilar to many modern reefs. These features are not unique to Muleshoe, but occur in many Waulsortian mound suites. Differences between these ancient buildups and modern reefs suggest the modern ocean is an imperfect model for Early Carboniferous seas.

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