Borings in an oomoldic rockground, Pennsylvanian of southeast Utah

The upper surface of an ooid grain- stone within the Upper Pennsylva- nian Hermosa Formation of south- eastern Utah bears abundant borings and is encrusted by syringoporid coral colonies. Aragonite-dominated ooids were cemented on the sea floor by high magnesium calcite and were later leached by meteoric waters to form abundant oomolds. Two lines of evidence show that the bored, en- crusted, marine-cemented surface is not a simple synsedimentary hard- ground: 1) The margins of the mi- crite-filled borings are mammillate: micrite fills truncated oomolds along the perimeter of the boring. Oomolds (and a leaching episode) therefore predate borings. 2) Rhizoliths within the ooid grainstone (previously in- terpreted as penetrating from a higher unconformity) are also bored along the encrusted surface, provid- ing corroborative evidence of an ex- posure event that predated boring and postdated marine cementation. Rather than a sediment-starved ep- isode within a transgressive se- quence, this subtle subaerial expo- sure surface requires an additional cycle of relative sea level change within the marine portion of a Penn- sylvanian cyclothem. Borings, long known to provide evidence on the timing of cementation, can also yield important evidence on the timing of dissolution events.