CAMBRIAN SHEET SANDSTONES OF THE CRATONIC INTERIOR OF NORTH AMERICA: USING CLASSIC METHODS TOWARD A "MODERN" STRATIGRAPHIC UNDERSTANDING

Lower Paleozoic, cratonic sheet sandstones are the siliciclastic epitome of “layer-cake” strata and the challenges they pose for interpretation of depositional origin. Contrasting models that alternatively depict an anomalously widespread sand facies spread by shallow marine processes, a blanket of sand spread by terrestrial processes, or exceptionally large lateral migrations of more typical facies belts, have been proposed, abandoned, and revived many times over the past 100+ years. The perpetuation of these disparate models perhaps reflects more than anything else limited understanding of how time is packaged in sheet sandstones.

Our recently completed, regional-scale chronostratigraphic characterization of Upper Cambrian sheet sandstones in the cratonic interior of North America reveals that they are remarkably similar in many important respects to much younger stratal packages that are not of the classic layer-cake motif. The key to deciphering stratal architecture is regional investigation, use of high-resolution biostratigraphy, core and petrophysical logs, and deployment of multidisciplinary techniques to identify subtle sequence-bounding unconformities. For example, high-resolution timelines and subsurface geometry demonstrate that sheet sandstones internally consist largely of a relatively ordinary arrangement of shingled, progradational parasequences that show typical and predictable stacking patterns. Stratigraphic attributes of sheet sandstones differ from those of most younger nearshore successions largely in that the systems tracts of the former are stretched laterally great distances in proportion to their thickness, reflecting a low subsidence rate.

The investigative methods that provide our recent insights into the origin of cratonic sheet sandstones of this area can be traced back to 1950’s work of W.C. Bell and his students, Robert Berg and Clemens Nelson, who rigorously kept lithostratigraphic and biostratigraphic units separate, and interpreted depositional origin based on “...relationships between the two.” They identified most of the major sequence boundaries and maximum flooding surfaces, depicted parasequence geometries, and interpreted transgressive/regressive systems tracts, largely in a manner consistent with our recent results.