LOWER PALEOZOIC SHEET SANDSTONES IN THE CRATONIC INTERIOR OF NORTH AMERICA: SOLVING THE SHEET GEOMETRY PUZZLE

As Bob Dott eloquently conveyed in several publications, the origin of quartzose sheet sandstones has long been a seemingly intractable problem. One outstanding question has been how such homogeneous, thin layers of sand were spread over thousands of square kilometers. Models that have been proposed, abandoned, and revived over the past 100+ years include spreading via marine currents across an enormous shallow shelf; a blanket of sand spread by terrestrial processes; and exceptionally large lateral shifts of typical nearshore facies belts.

Sporadic bursts of research on the Lower Paleozoic sheet sandstones in the cratonic interior of North America have now greatly clarified their origin. This began in the 1950s by W.C. Bell and his students, who used litho- and biostratigraphic techniques to identify what are known today as some of the major sequence boundaries, maximum flooding surfaces, and parasequence-scale clinoforms. To some degree they practiced modern sequence stratigraphy before it was developed. However, sedimentary processes remained poorly understood. This was remedied about two decades later when Bob Dott began to make his landmark contributions. In sheet sandstones previously regarded as homogeneous, Dott and his colleagues and students discriminated facies indicative of tidal, wave, combined-flow, eolian, and fluvial processes.

Our current understanding was achieved by integrating these classic stratigraphic and facies practices at regional scale, and incorporating higher-resolution biostratigraphy, core, and petrophysical logs. This resulted in a sequence stratigraphic framework that revealed sheet sandstones as remarkably similar to other stratal packages not of the classic sheet motif. They consist of ordinary parasequences arranged in predictable patterns in response to changes in relative sea level, and largely differ from most other nearshore sedimentary packages only in having falling stage and transgressive systems tracts that stretch laterally great distances in proportion to their thickness. This reflects a low subsidence rate and nearly flat shelf gradient. Thus, rather than enigmas, sheet sandstones should be viewed as part of a continuum of stratigraphic packages that differ from one another because of variable controls such as tectonic and physiographic setting.