On the origin of carbonate nodules in the Bursum Formation at Cibola Spring, Socorro County, New Mexico

A single half-meter-thick bed in the upper part of the Bursum Formation near Cibola Spring contains abundant carbonate nodules with distinctive knobby surface texture. These structures are shown to consist of the remains of a diverse assemblage of organisms, mainly foraminifers, which encrusted skeletal substrates and built finger-like columns with intervening uncolonized areas. The nodules are similar to previously described ones of comparable age from Kansas, Texas, southern New Mexico and other areas (variously termed “Osagia”, “Ottonosia”or “osagid grains”). However, they differ greatly from most “oncolites” or “algal biscuits” in lacking through-going laminations as well as lacking predominant macroscopic or microscopic algal contributors. The Bursum nodules are inferred to have formed in moderately deep waters (below normal wave base) in an open shelf setting. 369