ISGS Seminar | The Lower Mississippian carbonate ramp of North America is not quite what we thought for 50 years – the rocks, paleogeography, and modern-day oceans tell a different story

C. Robertson (Robert) Handford received his Ph.D. in geology from Louisiana State University in 1976. He has spent most of his professional career in petroleum industry research chiefly within the field of sedimentology and stratigraphy. He has worked for the Texas Bureau of Economic Geology, and the research laboratories of Unocal, Amoco, and Arco. In addition, he taught geology at the University of Arkansas and was a senior advisor with HessCorporation. With more than 20 years of consulting experience, Robert is currently conducting independent research from his home base in Bella Vista, AR. Although his primary interest is in sedimentology and sequence stratigraphy and application to hydrocarbon exploration and production, Dr. Handford has conducted research and published papers on the sedimentology and stratigraphy of carbonates, evaporites, mixed clastic-carbonate systems, fluvial to deep-marine carbonates, siliciclastics, and karst processes and facies. His experience base ranges from North America, Central America, and South America, to Africa, Europe, Russia, the Middle East, Australia, and Southeast Asia. An invited lecturer to numerous geological societies and universities, Dr. Handford was selected as AAPG Distinguished Lecturer for 1995-96 on carbonate sequence stratigraphy. He has received the SEPM Excellence of Oral Presentation Award on two different occasions, the SEPM Excellence of Poster Presentation Award, second runner-up for the AAPG Jules Braunstein Memorial Award, and SEPM Excellence of Presentation Honorable Mention. He also received Honorable Mention for Best Paper published in SEPM's Journal of Sedimentary Research in 1995. Dr. Handford is a member of the AAPG, IAS, and SEPM.

Carbonate-ramp depositional models, initiated in the 1970s, emphasize wave, storm, tidal, and sediment-gravity flow processes, which predominantly transport sediment downslope forming low-angle clinothems. During the past decade, however, studies utilizing multi-beam bathymetry and shallow seismic reflection technology in modern slopes and ocean floors have shown that along-slope bottom currents (contour currents) commonly interact with or dominate over downslope processes. These studies are prompting a re-evaluation of depositional models for Lower-Middle Mississippian carbonates in North America. 

Convergence of Laurussia and Gondwana during Early Mississippian time narrowed the Rheic Ocean into deep seaway of east-to-west equatorial throughflow. Warm seawater flowed westwardly from the Paleotethys through a narrow gateway, which separated southeastern Laurussia (N. America) and western Gondwana, and into the southwest Panthalassa Ocean. Outflowing currents mixed with Panthalassa water along the southern margin of Laurussia, setting the stage for the construction of the Early Mississippian platform by mixed downslope and along-slope currents. 

Ongoing studies show that a wide variety of contourite and mixed depositional systems comprise the carbonate low-angle clinothems, which formed along the southern margin of Laurussia of North America (Handford, 2025; Janson et al., 2025). Large- and small-scale features suggestive of mixed processes along the outer ramp and slope include: linear carbonate drifts (10s km), sediment waves (100s m), sheeted drifts, delta drifts, turbidites, and mass transport deposits. Today we will review examples from three key areas: (1) Tennessee-N. Alabama, (2) Illinois Basin, and (3) Ozark region. The Fort Payne Formation in SC Kentucky is mainly composed of clinothem foresets (downslope processes) that prograded southward into Tennesee where they transition into bottomsets comprised mainly of contourite deposits such as sheeted drifts and large sediment waves. The Fort Payne and overlying Ullin formations make up a shelf-to-basin clinothems up to 250 m thick in the Illinois Basin, where water depth is estimated to have exceeded 300 m during Early-Middle Mississippian time. Regional isopach maps and correlations indicate that the Ullin clinothems are present as several delta-shaped bodies up to 200 m thick extending across >10,000 km². Each body is dip-oriented with apex directed upslope into a gateway channel extending across the shelf.  We interpret these as large delta drifts formed by persistent shelf currents, which were focused through gateway channels along the platform margin. Sheeted and elongate, mounded or separated drifts and sediment waves dominate the Lower Mississippian St. Joe-Boone Interval in the Ozarks. However, an isopach map of the 0-30 m-thick St. Joe drift deposits across the Ozark region indicates that thickening and thinning trends follow regional basement-fault zones. Mass transport features are also most common near faults. These strongly suggest that Late Devonian-Early Mississippian tectonic activity (Evans et al., 2011) along basement weakness zones created seafloor bathymetric perturbations, which affected contour-current depositional and erosional patterns and triggered slope failure and mass transport deposition.

To attend online, contact info@isgs.illinois.edu for a Teams link.