Mounds that are Not Mounds: Build-and-Fill Architecture in Glacioeustatic Sequences

Julienne Emry, Robert Goldstein, and Evan K. Franseen
University of Kansas, Lawrence, KS

Pennsylvanian strata in the US Midcontinent were deposited in association with high-amplitude glacio-eustatic sea-level fluctuations. Many such sequences are thin and maintain similar thickness throughout wide geographic areas. The facies both build and fill relief. Many of those that fill relief are commonly, but incorrectly ascribed to carbonate mounding. Missourian strata were studied in a 3,670 km2 area of eastern Kansas to evaluate the controls on build-and-fill architecture.

Lowermost strata are those of the Liberty Memorial Shale which created lobate positive topography. Shale facies changed laterally to phylloid algal and possible microbial carbonates, but no mound-like topography was built. A subsequent relative rise in sea level resulted in a condensed section. Phylloid algal and other carbonate facies were deposited after a minor relative fall in sea level. Strata were deposited preferentially in low areas, onlapping preexisting topography. Although these lithologies are generally ascribed to carbonate buildups, geometries filled topography. After a minor relative sea level fall, channels were eroded into underlying carbonates, creating topography commonly misidentified as mounds. With stillstand in sea level much of the topography was filled again with grainstone. Results from this study show that creation of relief in high-frequency glacioeustatic sequences can occur after falls in sea level, with deposition of lobate siliciclastics and erosion of preexisting strata. High or moderate sea levels result in carbonate deposits which fill relief and even out topography. This mechanism of building and filling of relief is a predictable mode of internal architecture for carbonate reservoirs.