Abstract

Porous algal limestones of a type important as petroleum reservoir rocks in the Paradox basin crop out as cliffs along San Juan Canyon in the Monument upwarp of southeastern Utah. These limestones and associated facies are accessible for study along Honaker Trail about halfway down from the canyon rim. They are of Middle Pennsylvanian age and occur in a part of the section believed correlative with the Desert Creek and Ismay zones of the subsurface.

A section about 120 feet thick which includes the marker unit locally known as the “Horn limestone” is here described and interpreted on the basis of observations of the accessible outcrops along Honaker Trail and on relationships visible on adjacent, inaccessible canyon walls. This predominantly carbonate stratigraphic sequence displays cyclic repetition.

Two contrasting limestone facies with abundant leaflike or phylloid calcareous algae occur in the section. In each the porosity can be related to these algal fragments. The “muddy algal facies” is composed mostly of medium gray, somewhat cherty, mud-rich limestone (mostly algal wackestone) and possesses solution porosity of a moldic type. The “sparry algal facies” is characterized by lighter colored, grain-supported limestones composed largely of coarse fragments of the phylloid alga Ivanovia and of sparry calcite cement. The “sparry algal facies” possesses appreciable interparticle and intraparticle primary porosity, and is comparable to reservoir rocks in many Paradox basin oil and gas fields.

The “sparry algal facies” occurs as a widespread, biostromal deposit with a thickness ranging from a few feet to about 25 feet. Thickness variations are abrupt and some broad-scale cross-bedding is evident. This unit has a conspicuously flat base. The broadly undulating upper contact parallels the underlying cross-bedding, and is believed to be a time surface. This deposit is interpreted to have originated by the lateral coalescing of more abrupt individual algal build-ups that formed by essentially in situ clastic accumulation of algal fragments within a relatively turbulent marine environment.

Carbonate facies that are associated with, but do not appear to be contemporaneous, at least in this locality, with the “sparry algal facies” include sponge-rich, cherty, dolomitic limestones (mudstones and wackestones); lighter colored limestones (mostly fossiliferous wackestones) with an abundant assemblage of “normal marine” invertebrate fossils; and foraminiferal (largely ophthalmidid) limestones ranging in depositional texture from mudstones to grainstones. The depositional environments are interpreted to have ranged from anaerobic to well-oxygenated, and from “normal marine” to somewhat more saline conditions. Bottom turbulence was generally lower during deposition of the associated facies than during deposition of the “sparry algal facies.”