VARIATIONS OF FLAT-PEBBLE CONGLOMERATE STRATA IN HINTZE'S SECTION C AND MOUNT LAW

Flat-pebble conglomerates (FPCs) are deposits that contain various tabular clasts of carbonate facies. They are commonly observed in strata of shallow marine successions during the Cambrian and Ordovician time periods. They are rarely found in post-Ordovician strata. Previous interpretations have suggested they are storm deposits, cycle caps, or products of sea level change. Detailed analysis of FPCs may allow better understanding of their origins, including events and processes associated with meteorite impacts, tectonic activity, superstorms, slope failures, and mass wasting events.

This study examines FPCs based on intrinsic features and documents patterns of secular variation in selected Late Cambrian and Early Ordovician intervals from west-central Utah. The principal stratigraphic units include the Steamboat Pass Shale Member of the Orr Formation (Furongian Series), upper and lower parts of the Fillmore Formation (Stairsian and Blackhillsian series), and Kanosh Shale (Mohawkian Series). Clast heterogeneity, sorting, normal or reverse grading, apparent dip and imbrications were measured though descriptions of clasts lithology, matrix lithology, lateral distribution of clasts along selected horizons, and observations of stratal architecture. Statistical analysis, clast-size frequency analysis, serial sectioning of oriented samples and petrography supplements field collections.

Observations from Hintze’s Section C show various grading patterns of three distinct lithological clasts in a grainstone matrix. Clast frequency analysis and scour marks indicate a storm deposit. The features in this stratum are consistent with tsunamis; however, initial geochemical data favors a storm deposit. In contrast, the FPC from Mt. Law indicates a debris flow. This stratum tapers out, clasts features show a flow direction, and clasts are monomictic, probably derived from single strata. The differences in these two FPCs strata show multiple processes and origins.