PROVENANCE, SORTING, AND SECULAR VARIATION IN LATE CAMBRIAN AND EARLY ORDOVICIAN CARBONATE FLAT-PEBBLE CONGLOMERATES IN WEST-CENTRAL UTAH

Flat-pebble conglomerates (FPCs) are deposits that include tabular clasts of carbonate facies in shallow marine successions. They are common features in Cambrian and Ordovician successions and are more rarely preserved in post-Ordovician strata. Traditional interpretations suggest they are storm deposits, but more recently, they have interpreted as cycle caps and products of sea-level change. Examining FPCs in detail may further our understand of possible origins, including major Earth events such as meteorite impacts, seismic events, superstorms, slope failures, and mass wasting events.

This study, which is in its initial phase, examines FPCs based on intrinsic features and seeks to document 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 are key components to consider. The principal field techniques include descriptions of clasts lithology, matrix lithology, analyses of clast-size frequency distributions, lateral distribution of clasts along selected horizons, and micro-sequence stratigraphy (identification of scour surfaces that help to define the stratal architecture). Serial sectioning of oriented samples and petrography will supplement field collections.

Preliminary observations from the upper Fillmore Formation, in road cuts along US Hwy 50, show a bi-directionally imbricated bed, composed of laminated dark lime mudstone clasts in silty limestone matrix. The imbrications show onshore- and offshore-directed flows, which is consistent with storm deposition, where surge and return flow are dominant forces. Such features also could be associated tsunamis, although tsunami deposits conceivably would include multiple horizons. In contrast, features associated with slope failure and mass wasting would include tapering out of the strata, clast imbrications along flow direction, and clasts generated from nearshore in an offshore setting, where clasts can be inversely or normally graded.