Abstract

A large part of the Middle Pennsylvanian Butterfield Peaks Formation of the Oquirrh Group is very well exposed at Mt. Timpanogos, Utah County, Utah. The nearly 1,200 m of stratigraphic section measured for this study consists mostly of very fine- to fine-grained sandstone, sandy carbonate, and fossiliferous carbonate rocks, in which the non-skeletal carbonate is largely dolomite. Trace fossils of the Cruziana facies are found in some units of most lithofacies. Twelve facies are distinguished, but only about half of these are prominent.

Tabular cross-bedded sandstone (approximately 20% of section) is thought to have been deposited in large part by wind in eolian dunes, on the basis of cross-set geometry and inferred depositional bedforms, internal translatent stratification, association with possible interdune units (crinkly-laminated sandstone, some with evaporite casts) and mudcracked units, lack of shale, and paucity of trace fossils. Most of the same arguments also apply to contorted cross-bedded sandstone.

Most fossiliferous carbonate rocks (approximately 10% of section) represent deposition in various shallow, normal marine environments. Diverse marine faunas in these rocks include the following fossil groups: brachiopods, bryozoans, echinoderm ossicles, mollusc fragments, corals (solitary rugose and Chaetetes), phylloid algae, sponge spicules, fusulinids, other Foraminifera, conodonts, ostracods, and trilobite fragments. The rarer spiculitic dark mudstone probably represents a less hospitable, possibly deeper and less-oxygenated marine environment.

A variety of “coastal” environments are represented by other facies; open shore (sandstone with ripples and other sedimentary structures, approximately 10% of section), quiet waters (structureless sandstone(?) and sandy carbonate, some with small oscillation ripples, approximately 20% of section), and restricted, euryhaline environments (peloid packstone and light mudstone, green shaly dolomite). Many of the facies changes were apparently the result of migration of coastal features and environments, and show little order. A few transitions identified by Markov chain analysis as occurring more often than they would in a random sequence suggest that rapid deposition and subsidence had great influence on facies changes, and must have complicated any effects of repetitive eustatic sea level changes.

The overall depositional setting for these rocks is pictured as a shallow sea with a few isolated “deeps”, and windblown, migrating sandy islands, bars, and spits, from which sand was also blown into the water. The sea probably had a comparatively small (less than 2 m(?) or microtidal) tidal range, as tidal features are absent, and several facies suggest deposition behind long bars or barrier islands.

The mean paleocurrent direction from 471 measured indicators, mostly cross sets, is from north to south (180°, present coordinates), which is consistent with the paleomagnetically-inferred trade wind direction for the Utah region during Pennsylvanian time.